PNEUMATIC TUBE SYSTEMS
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POST OFFICE ENGINEERING DEPARTMENT. TECHNICAL INSTRUCTION, X.
TABLE OF CONTENTS
SECTION I - GENERAL 1. The pneumatic services of the Department are in general made use of as a means of conveying telegraphic messages. 2. In practically every office where important telegraph traffic is dealt with and where the Counter, Delivery, and Instrument Rooms are not adjacent, some form or other of pneumatic despatch is used for connecting up the points and in many large towns tubes are laid in the street for connecting the Head Office with other buildings. 3. Tubes are, therefore, divided into two classes, known as House and Street Tubes. 4. The tubes may be hand or power worked. In the former case a reciprocating air-pump operated by hand through a lever is generally used, and in the latter case reciprocating pumps or rotary air blowers operated by electrical or mechanical power are employed. 5. In most cases compressed air is used for sending carriers from, and rarefied air for drawing carriers to, a central station from which the tubes radiate and are worked. In house installations where short power-worked tubes only are used it is often found preferable to loop the tubes and make use of one of these methods of working only. 6 For house tube installations, the principle connections are between the Instrument Room and:-
In the case of (1) and (2) the loaded carriers practically all flow in one direction, but in the case of (3) the telegraphic work is in both directions. 7. The authorised practice is to return empty carriers by hand for hand-operated installations, and by tube when power is used. 8. Street tubes are worked up, down, and in both directions. If the traffic between two offices is sufficiently large, two tubes are installed to deal with the Lip and clown work respectively. Where the traffic is smaller the work is carried by one tube worked alternately up and down. Where the traffic is in one direction only, and the power provided can only deal with carriers moving in that direction, the empty carriers are returned in batches by hand. 9. Tables XI. and XII., giving the most important particulars of power-worked installations, will be found on pages 63 to 72.
SECTION II - CARRIERS 10. The carrier which is used to contain the messages consists of a light hollow cylinder, the head of which is closed by a circular felt pad fitting the tube sufficiently tightly to prevent air passing, but at the same time offering as little friction as possible. 11. At present the carrier in general use for street tubes is made up of a gutta-percha tube covered with baize or felt and closed at one end by means of a wad of felt. The messages are inserted at the open end and are held in by means of an elastic band stretched across the opening, which can be pulled to one side to allow of access to the carrier. The felt pad which forms the head is made of the same diameter as the tube, but the body of the carrier is slightly smaller. The felt covering the body is extended beyond the gutta-percha tube at the open end, and is made in the form of a skirt, the idea being that the current of air in the tube shall swell out this skirt to fit the tube and so assist the felt pad at the head to prevent air leaking past. 12. For house tubes fibre carriers are principally used, these consisting of a fibre cylinder open at one end and fitted at the other with a thoroughly shrunk felt pad which fits the tube. A fiat spring is fitted internally to retain the messages. 13. For both these carriers it is essential that the felt pad at the head should not be worn down more than one-eighth of an inch in diameter. When this limit is reached the carrier should be withdrawn from service. Both types of carrier are shown in Fig. 1.
FIG. 1
TABLE I 14. It is essential that all carriers shall be so inserted into the tube that they travel buffer foremost. If not, the open end takes the shock on arrival, and the carrier is likely to be buckled up or otherwise severely damaged. 15. Coloured carriers Nos. 1À and 3A are used in a few instances for tubes connecting three stations and fitted with a switch at the intermediate station arranged to intercept the carriers. It can be ascertained by means of an inspection window in the switch whether the carrier should be allowed to go on or be withdrawn from the tube at this point.
16. Tubes are made of lead and brass, the former being used for street tubes and the internal part of street tubes in offices where the distance is so inconsiderable that no advantage would be gained by jointing a short length of brass tube to the lead street tube. Brass tube is otherwise always employed for pneumatic tubes (street or house) within buildings. 17. Tubes are in three sizes, namely, of 1.5inches, 2.25inches, and 3inches diameter. In all cases the section is made as truly circular as possible. 18. Further details and information regarding the various sizes and kinds of tube stocked are given in the following table:-
TABLE II
HOUSE TUBES 19. Brass tube is now the standard for house tubes. 20. Where possible, horizontal runs of the tube are laid between the floor joists, but where this is not possible and also in the case of vertical runs, special supports are necessary. These supports can be any of the standard fittings employed for fixing hot water or steam pipes, the 1inch and 2.25inch brass tube requiring 1.25inch and 2inch standard Wrought Iron (W.I.) fittings respectively. Supports or brackets are also stocked by the Controller, Stores Department. They consist of a circular iron plate which is fixed to the wall or ceiling into which is screwed an iron pipe, the length being made to suit the local conditions. To the other end of the pipe is screwed one-half of a split clamp in which the tube is held. A sketch of these showing some of the methods in which they are employed is given in Fig. 2. Clips, brass, for 1.5inch and 2.25inch are also stocked, and these are shown.
FIG. 2 In many cases where girders are available, the tube may be supported by clips or hangers fixed to the girders. One method is illustrated in Fig. 2, but as these vary so much in size and shape they should be made or purchased locally. Supports should, in all cases, be fixed not more than 8feet apart. It is the standard practice. to terminate the tubes at the operating station in a downward vertical direction, as this gives the best results. 22. Where the tube terminates vertically it should be kept polished bright for the first 3feet, the remainder, where exposed, being painted. 23. Where tubes pass through floors they must be protected against
damage for a height of 4feet from the floor by means of wood or other
suitable casing.
FIG. 3 24. For hand-worked tubes a half-split coupling, shown in Fig. 3, is used for joining the lengths of tubes together, and this forms a ready means of connecting up. The two tube ends are inserted in the coupling, which is then drawn 'up tight by means of two screws. The stock title is "Coupling, Pneumatic," No, 5. 25. For power-worked tubes this coupling, not being air-tight, is unsatisfactory, and a close-fitting sleeve, Fig. 3, into which the two ends of the tube are sweated, is used. For straight rims of tube on the low vacuum system, joints may he made by means of a mixture of red lead and gold size. 26. For coupling the lead street tubes to the internal tubes in terminal offices, flanges pneumatic. Nos. 1 to 6 are used. These are supplied with liners bored and tinned to take the brass or lead tube. Care should be taken that the tubes are joined up so that the bore internally makes a smooth joint. 27. Standard bends of 3feet radius and 6feet in length are stocked for use in the transmission part of 1.5inch and 2.25inch tubes, and these should, if possible, be used where bends are. required. 28. If bends of smaller length are employed they should be made up in the case of 2.25inch tubes with parts of standard bends and special short sleeves. STREET TUBES 29. Lead tube is used for street work in all cases, and the method of laying, is as follows:- 30. As soon as each length of lead tube is manufactured it is placed in a wooden trough (supplied by the Department), which serves as a packing case and enables the tube to be handled easily and without fear of damage. 31. When the tube is cold a tightly fitting polished steel mandrel, 1foot in length and of the exact standard gauge of the tube, is drawn through from end to end of the tube, with the object of ensuring that the tube is smooth, cylindrical and uniform throughout. The mandrel is lubricated with soft soap. 32. The cast-iron pipes used for encasing l,.5 2.25 and 3 inch tubes are the standard 2, 3 and 4 inch pipes respectively. 33. A trench will be excavated either ih the carriageway or footway (sec T.I., XIV., Section H. (b), page 1) to a depth of 2feet.. The bottom of the trench should be well punned and depressions made to receive the sockets of the pipes, so that the barrel of each pipe may rest on a firm foundation. Where solid slides are to be fixed, the trench will be deepened so that the slides may pass freely over the ends of the pipes.. If a straight run is not obtainable the line of trench must he set out so that the bends of the tube have a radius of not. less than 8feet 6inches. Any alteration of level that may be necessary must be effected gradually. 34. While the trench is being excavated, the tubes and pipes will he prepared for jointing. In the case of a 2inch tube (standard length 28feet), a 3inch spigot and socket pipe, 9feet long, will be passed over the tube until it occupies the middle position. Another spigot and socket pipe will be passed over one end of the tube and a double spigot pipe over the other (Fig. 4). These will be jointed to the first pipe with yarn and lead in the usual manner. The tube will project about 6 inches at each end.
Fig. 4 35. Three 6foot lengths of 2inch spigot and socket pipe, and one 2 inch double spigot pipe will be required for each 25feet length of 1.5inch tube. One 4inch spigot and socket pipe, 9feet long, and one 4inch double spigot pipe for each 19feet length of 3inch tube will be necessary. 36. A wire (G.I. 400lb.) will be threaded through the tube, and by means of this a mop of yarn moistened with water and having a sash line attached, will be draw through so as to clean the tube thoroughly. The sash. Tine will be threaded through a "Bobbin, Boxwood," which is 9inches long; and of diameter equal to that of the tube, and then secured to a "Weight, driving," as shown in Fig. 5.
FIG. 5 37. The bobbin and weight will be drawn through together. If any obstruction prevents the passage of the bobbin, it will be drawn back and then pulled forward quickly, so that the weight may strike the bobbin a' smart blow, and press out the bulge. 38. When 'the bobbin passes freely, the ends of the tube will be trimmed perfectly true by means of a "Cutter, Pneumatic Tube," 14inch, 21inch, or 3inch, as the case may be (Fig. 6).
FIG. 6 - CUTTER, PNEUMATIC TUBE 39. This tool consists of' a short mandrel of a diameter approximately equal to the internal diameter of the tube. To the end of the mandrel is affixed a milling cutter, the outer portion of which is in a plane perpendicular to the axis of the mandrel, while the inner portion is inclined towards the mandrel, as shown in Fig. 6. When the cutter is turned, the edges of the tube are cut quite true, and a chamfer of about one-quarter the thickness of the tube is cut on the inside edge. The surface of the tube at the ends will then he carefully cleaned for a suitable distance with a file brush, coated with plumber's black, and scraped bright. The bright surface will he protected by a wrapping of cloth or stout paper, and each end of the tube plugged. 40. The pipes and tube will then be carefully lowered into the trench by means of slings, placed not more than 5feet apart. A. "Slide, Solid, C.I. 2feet," for the size of pipe concerned, will be passed over the forward end of the pipe, under which the ground has previously been sufficiently excavated to allow the elide to be readily moved. 41. Another length of pipe and tube which has been similarly Prepared will lie lowered into the trench, with its end sufficiently clear of the first pipe to allow a wire to be pushed through and grasped at the gap. To the end of the wire a mop of yarn and a 0.25inch chain will be attached. When the mop has been drawn through, it will be detached, and a "Mandrel, Steel, 5 or 6inch," which has been heated to a suitable temperature, will be connected to the chain and drawn for half its length into the second section of tube (Fig. 7). The end of the tube containing the mandrel will then be gripped by a pan of " Tongs, Draw, for Mandrel, 1.5inch," and held firmly while the tube is pushed forward, so that the projecting, half of the hot mandrel enters the length of the tube already laid. The ends of the tubes should now butt closely together, and there should be a space of about 12inches between the ends of the cast-iron pipes. The pipe and tube will be held in position by a crowbar pressed against the rear end of the pipe, and the edges of the tube will be gently hammered down over the mandrel. A wiped joint will then be made. If, after inspection, the joint he found to be satisfactory, the mandrel, which should have ensured perfect alignment of the tubes will be withdrawn by means of the chain, when it has become cool.
FIG. 7 - TUBE PREPARED FOR JOINTING 42. It is important that the diameter of the mandrel shall not vary by more than 1/64inch from the standard diameter of the tube. 43. The minimum, quantity of metal to be used for each wiped joint is as follows:- 1.5inch tube - 1.75lbs. 44. The joint must be of uniform section, and must be carefully made so that it is absolutely air-tight. 45. When the mandrel has been withdrawn, a carrier of the proper size, attached to a stiff iron wire, will, be pushed up the tube past the newly made joint to prove that the passage is free. 46. The Solid Slide will now be drawn over the joint so as to overlap each pipe-end 6inches, the spaces between the solid slide and the pipes being Caulked with yarn and completed by a 11inch filling of molten lead, well caulked. 47. When it is necessary to form bends in the lead tube, special care must be taken to ensure that the circular section of the lead, tube is maintained. The work of bending should be carried out by an experienced plumber. The bend must be made above ground and must not be regarded as satisfactory until the boxwood bobbin referred to previously will pass through freely. The use of an Ash Bending Stick (Fig. 8) will assist in avoiding damage to the tube during this operation.
FIG. 8 - BENDING STICK, ASH 48. The bent tube will be placed in "Bends, C.I., Split," no bend being used which is of less radios than 8feet 6inches. The standard sizes of split bends are shown in T.1., X IV. 49. After the bend is placed in the trench the bobbin will again be passed through to ensure that no alteration of shape has taken place during handling. 50. In no case should joints be made on bends. 51. Testing - On completion of a street tube., or, if the length exceeds three-quarters of a mile, on the completion of each half mile, the tube will be tested with air pressure. Each end of the tube will be stopped by means of "Stopper, Brass for Pneumatic Tube" (Fig. 9).
FIG. 9 - STOPPER, BRASS, FOR PNEUMATIC TUBE 52. This stopper has a tube t passing through it, the end of which has a 3/4inch Whitworth gas thread. When the butterfly nut 'n is tightened, the pieces f, e compress the rubber washer c, and cause it to spread and fit the tube tightly. To one stopper will be connected a "Gauge, Pressure, 0-60lbs., 4inch dial," and to the other stopper a similar gauge and a "Connection, Flexible, for Desiccator," will be connected by means of a "Cock, Air, G.M. 3-way." Air will be pumped in until the gauges indicate 20lb. pressure per square inch, and when the compressed air has had time to cool to atmospheric temperature there should be no fall after a reasonable interval. A test of the whole length will be made when the tube is completed. The fittings will be arranged as indicated above, and after raising pressure to 20lb., the tube will be left for two hours in order that the air may cool and the pressure equalise; a reading of the pressure gauge will then be made. A second reading will be taken after the lapse of 24 hours. The second reading should not be appreciably less than the first reading. In the event of a leakage being detected the fittings will be carefully tested by coating with soap suds and watching for the formation of soap bubbles. If the defect is not discovered in the fittings, the solid slides will be removed and the soap test applied at each plumber's wipe until the fault is found. 53. A special tool "Chisel, Cold, Mortice," is supplied for removing the lead from the joints of solid slides. When the lead has been removed from one end of the slide it will generally be found possible to drive the slide back by putting a block of wood against the other end and striking the wood with a heavy hammer. If the slide is fixed tightly it may be necessary to remove the lead from both ends. 54. If the portion of a street tube inside a building is short, or if it is to be laid horizontally, it is often convenient to continue the lead tube up to the despatching or receiving apparatus, but for all other cases it is necessary to use brass tube, the joint being made by means of brass flanges fixed near the point of entry of the tube into the building. 55. As mentioned previously, the traffic on street tubes can be dealt with by providing one tube between two offices for both sending and receiving, pressure and vacuum being switched on for the two services as required.
WORKING CAPACITY OF TUBES 56. If, however, the traffic is too heavy or the tube too long for the work to be dealt with in this manner, two tubes are provided - one for the "up" work and one for the "down." If it is necessary to increase the carrying capacity, service regulators described in Section XII. can be fitted. By means of these regulators it is not necessary to wait for one carrier to be received before the next is despatched, and considerable delay in the case of very long tubes can thus be avoided. The pressure or vacuum is in such cases kept continuously on, and the tube is said to be worked "continuously." 57 Tubes on which the traffic during the busy hours of the day warrants continuous working, should be provided with a pneumatic tube service regulator and worked continuously during the busy hours. The intermittent working should be reverted to during the slack hours to avoid waste of power. A diagram of the connections for block instruments in such a case is shown in Fig. 41. A change from intermittent to continuous working is of value when the traffic is being delayed considerably on account of having to wait two or three minutes while a carrier is in the tube. 58. The size of the tube to be used on an installation is determined by the volume of traffic, the length of the tube, and the transit time allowable. From a return of all provincial power-worked tubes the following figures were obtained (see Table III). 59. As pointed out earlier, the capacities of 1.5inch and 2.25inch carriers are 5 and 20 messages respectively, and from Table III, it would appear that in the great majority of cases a 1.5inch tube would suffice from a capacity point of view. There is, however, another very important determining factor, viz., the transit time permissible. 60. When estimating for new tubes the traffic in telegrams for each month of the year and during the busiest hour of the day should be obtained from the Postal Department, and, if necessary, a percentage added to this figure to allow for future growth. It should also be ascertained beforehand if the messages to be conveyed will he enveloped. If the hourly figures are not available, it is safe to take one-fifth of the total daily traffic to represent the busiest hourly traffic. From the length of the tube the transit times for the various sized tubes may be obtained from Tables VIII. and IX., and to this must be added, in the case of tubes worked in both directions, about 30 seconds for each switching operation, i.e. 30 seconds for despatching the carrier and 30 seconds for removing it, together with an amount approximately equal to one-tenth of the transit time to allow for the period which must elapse before the tube is ready to work in the opposite direction. This information will then show how many carriers per hour can be sent, and this, divided into the message traffic previously obtained, will. give the messages per carrier, and so show which size should be provided, or if two tubes are necessary to deal with the work.
TABLE III *There is only one of each ff these types of tubes.
62. The, relative transit times and powers required for tubes of the same length and working at the same pressure are given in the following Table:-
TABLE IV It will be seen that at the same effective pressure a 2.25inch tube would give a speed only 18 per cent. higher than a 1.5inch tube with an expenditure of more than two and a half times the engine power; whilst a 3inch tube would give a speed only 14 per cent. higher than, a 2.25inch tube, and would require more than double the engine power. It is, therefore, obvious that any increase in diameter beyond that necessary to fulfil the requirements of the service would he attended by a waste of power. 63. Generally speaking, 1.5inch tubes will deal with the
traffic in smaller offices. In larger offices 2.25inch house tubes are used.
The 2.25inch size is in most general use for street tubes, although several
1.5inch tubes have been laid. The 3inch size is used only for long heavily
worked street tubes. Faults 64. Faults in working tubes may be divided into two classes, viz., those clue to leakage of air and those due to obstruction in the tube. Faults are sometimes caused by the operations of excavators;, and when such is the case they can generally be found by an inspection of the the tube in those places where the ground has been recently disturbed. A frequent cause of leakage, however, is an imperfectly made plumber's joint, and the position of this is more difficult to locate. The existence of a leak can be proved and its extent roughly determined by fixing a stopper and gauge at the distant end of the tube and applying pressure until the gauge indicates between 10 and 15lb., when time air supply will he shut off and the indications of the gauge noted. The existence of a leak sufficient to interfere with the working will be indicated by a rapid fall in the pressure, and means must be taken to localise the position of the leakage. To show the importance of such leakages, it has been found that under 10lb. per square inch pressure a circular hole inch in diameter will allow 5 cubic feet of free air per minute to escape. Unfortunately there is no method of locating absolutely the position of a leak. Unless it is known that the ground has recently been opened at certain points on the route, and damage possibly caused by excavators at these points, the only course. available is to expose the tube at a convenient spot near the centre of the route, and ascertain in time following manner on which side of the opening the leak exists.. 65. A hole will be opened in the ground over a joint, and the slide driven back as previously explained in para. 53. The annular space between the. tube and the pipe on each side of the opening will be filled with clay, through which a. very small hole will be made. Air pressure will then be applied to the tube, and a lighted taper held near the hole. If air is escaping from the hole it will cause the flame to flicker and indicate upon which side the leak is. Another opening will then be made in the faulty section and a further test made, This method is not, always. successful, as the air sometimes escapes through fractures iii the east-iron pipes or at the joints of solid slides, especially when the slides have been jointed with cement. 66. If the fault cannot be located by this means, a slot about 3inches long and 1inch wide will be cut in the tube itself, and a "Stopper, Intermediate, Steel," (Fig. 10) inserted.
FIG. 10 - STOPPER, INTERMEDIATE, STEEL 67. The constituent parts of this stopper are easily taken apart so that the portion b, e, e, f can first be inserted through the slot and turned to the correct position, and then the other parts, a, d, n hitched on. By turning the butterfly nut n the rubber washer c becomes compressed and spreads out between the metal plates e, f (the part a being, connected to the plate f) and fits the tube tightly. Air pressure will then be applied as before, and the gauge will indicate the presence of a leak or otherwise. When the operation is finished, the slot will be closed by soldering over it a stout piece of sheet lead. 68. The tests will be repeated at various points until the position of the leak is located. 69. A very convenient method of locating leaks in street
tubes has been used in London and can with advantage be used generally. Two " Stoppers, Rubber" fitted with rubber bags are held at Birmingham for issue on loan when required.
FIG. 11 - STOPPER, RUBBER 70. If a carrier is unable to pass an obstruction, or is delayed in its passage through the tube, the position of the defect may sometimes be localised approximately as follows:- A carrier is inserted and blown down the tube with
reduced pressure until it is stopped by the obstruction. The standard vacuum
should then be applied and the carrier drawn back. The time occupied in its
return journey should be noted, and from this and the known normal rate of
travelling, the distance of the obstruction can be calculated 72. After a slot has been cut in the tube, "Sweep's Rods, thin" will be pushed gently into the tube until the stationary carrier is felt. The rods will then be withdrawn and carefully measured or counted, and the ground opened at the point where the carrier remains fast. 73. The faulty section will be cut out and repairs
executed, as follows:- If no inspection trough is fitted within a distance of 100yards, the opportunity should be taken of fitting one in place of repairing the tube as above described.
FIG. 12
Maintenance 74. Maintenance tests of all street tubes should he taken quarterly and recorded on Form T.E. 212. The leakage test is made by first examining the cocks and terminals at the central station end and removing as far as possible all leakages from these sources. The tube is then closed at the far end by a stopper (Fig. 9) and air under pressure admitted to the tube from the service main. When a steady pressure of a little over 10lb. per square inch has been obtained, the control cock is closed and the time noted when the gauge, after being tapped, shows 10lb. exactly. The duration of the test is 8 minutes for 3inch, 4.5minutes for 2.25inch, and 2 minutes for 1.5inch tubes, at the end of which time the pressure is again carefully noted. The drop of pressure in lb. per square inch after the time specified should then not exceed the value obtained from the fraction 1,000, dived by length in yards. This value has been found to be the satisfactory limit up to which a tube exceeding 500yards in length can be efficiently worked, and when a leakage above this value is found the tube should be kept under careful observation and repaired when opportunity permits. 75. The transit time should also be accurately taken by tests with at least three new carriers sent in the direction in which the tube is normally worked, and the average reading of the pressure or vacuum gauges while this test is in progress should be stated. If it is found that, under the same conditions of working, the speed on a tube is decreasing each quarter, and that increased leakage is not present, the warning thus given of an obstruction should not be disregarded, but steps should be taken, as previously described, to locate this and remove the fault if the working is seriously impaired. 76. Table X at the end of the instruction gives a schedule of tools, etc. used in connection with laying and repairing pneumatic street tubes. 77. Foreign substances at times get access to the tubes and cause trouble; the three most usually met with being water, oil and gas. 78. Water accumulates in tubes readily if the compressed. air is moist. For this reason the containers should be in a moderately cool place, if possible, and should be regularly drained; the intake to the pumps should be from a dry cool place, not from a heated engine room. 79. When oil from the compressors or from the D boxes and fittings gets into the tubes and becomes a nuisance owing to its dirt and smell, the tubes may be cleaned by means of an old carrier soaked in paraffin. The carrier should be attached to a G.I. wire and driven backwards and forwards through the last few feet of the tube; about 20feet has been found sufficient, as the oil does not penetrate further in ordinary circumstances. 80. The carrier should never on any account be soaked in Benzoline or other oil which has a low flash point, as the vapour of such oils when compressed with air (as occurs in the Pumping-through System) forms an explosive mixture. 81. Coal gas occasionally finds access to the tubes. Cases in which the presence of gas in the tubes is suspected should be reported specially to the Engineer-in-Chief.
SECTION IV 82. Various types of apparatus are in use for inserting the carriers and withdrawing them from the tubes. 83. For hand-worked installations the Pneumatic Feed Slide (Fig. 13) is always used at the pump end of the tube. This consists of a portion of the tube, in which a slot, is out of sufficient. size to allow of the insertion or withdrawal of a carrier. The slot can be coveted by a close-fitting sleeve sliding over the tube Block and noise from the sleeve are deadened by means of India-rubber rings placed at the top and bottom positions. Inside and at the bottom of the slide a grating is provided to stop; the carrier, and this end of the slide :is mounted direct on the hand Pump.
FIG. 13 - FEED SLIDE 84. The feed slide is also used for power-worked house tubes where pressure .or moderately high vacuum is provided, and in these cases slides are provided at both ends of the tubes. 85. For convenience in making the connection between the feed slide and service pipe, a "Thimble for feed slide" with union as shown in Fig. 13 is provided, so that a wiped joint can be readily made. 86. The feed slide can be fixed horizontally or vertically. If the former, a cradle is provided to support the slide at the tube end, and a special elbow connection for the air supply at the other end. When fixed vertically, the carriers must enter in a downward direction. 87. At the open end of the tube in hand-worked or Low Vacuum installations a carrier cage is usually fixed which consists of a square wire enclosure fitted with a wooden top (Fig. 14). The tube may enter at the top or through the table from below. The carrier is extracted through the hinged swinging door. The noise of the carrier's arrival is deadened by a rubber disc fitted immediately below the tube opening.
FIG. 14 - CARRIER CAGE 88. For power installations worked on a very low vacuum such as that provided by rotary blowers, the air supply is not switched off when the carriers are inserted or withdrawn and special terminals which open the tube direct to atmosphere are used. The inrush of air is so slight, due to the low vacuum, that little or no noise is created when the terminals are operated. 89. The terminals used on these installations are described as Flap Terminals, Door Despatch Terminals, and Funnel Despatch Terminals, each of which is illustrated in Fig. 15. 90. The Flap Terminal consists of a chamber larger in diameter than the tube, but narrowed down at each end to the same size as the tube. The air is drawn through this chamber, the vacuum being sufficient to keep closed the light aluminium door provided with a soft leather seating as shown When, however, a carrier reaches the grids fixed inside the chamber it is guided against the door which it forces open, and is delivered into the wire basket axed below. 91. This terminal is normally used for receiving purposes and it must then always be arranged so that the carrier enters it in a downward vertical direction.
FIG. 15 - LOW VACUUM SYSTEM FITTINGS 92. The Door Despatch Terminal consists of a flanged brass chamber provided with an opening large enough for the insertion of a carrier and is, as its name implies, only used for despatching carriers. Normally the opening is covered by an aluminium door hinged on one side and kept closed by a spring. The inside of the door is provided with a soft leather face to make an airtight joint when-closed. To fix new springs or repair the door the hinge pin can readily be withdrawn. 93. This terminal can be fixed in any position and can be attached to the bottom of a flap terminal or in any convenient position in series with the tube. In some cases it is convenient to attach it to a branch off the tube and the top end of the terminal is then fitted with the cover plate top shown, the bottom end being connected to the special "T" piece. 94. The Funnel Despatch Terminal is used at the point at which the air enters the tube. It is a despatching terminal and should only be used when the tube from it rises vertically. It must never be used on a descending tube as foreign materials are liable to be introduced. 95. Wire Receiving Baskets are used in connection with flap terminals for catching the carriers when discharged. They are made from wire as shown in Fig. 15 and are clamped to the top and bottom of the terminal. In the bottom of the basket a piece of felt is placed to deaden noise made by the falling carrier. A wire guard is provided opposite the discharge mouth of the terminal to prevent carriers bouncing out of the basket. The baskets are made sufficiently rigid to be fixed on the terminal direct without additional support.
FIG. 16 - ARRANGEMENTS FOR CONNECTIONS OF SWITCH PNEUMATIC DOUBLE SLIDE No. 6
FIG. 17 - EMERGENCY CHAMBER, SLUICE VALE AND FLANGES 97. The switch proper (shown in the Fig. 16) is made up of two tubular sections a1 and a2 joined together and provided with a handle h, so that they can be moved between two horizontal platforms b1 and b2 which act as guides, and in each of which three holes c1, d1, e1, and c2, d2, e2 are provided. The tube is connected to the centre hole d1 in the top platform and the air supply to the corresponding hole d2 in the bottom platform, in which latter hole a grid is fixed for intercepting the carrier. The platform holes c1, e1, c2, e2 at each side are intended for inserting and withdrawing the carriers, and interchangeable funnels f1, f2, are provided. In order to permit of the interchange of the funnels as required for sending or receiving only, suitable blanking pieces are provided and fitted. The slide may be used for sending only, receiving only, or both sending and receiving, and Fig. 18 shows diagrammatically how it should be arranged for each of these services. inspection windows L1, L2, and m1, rn2 are provided at front and back respectively of each tubular section, so that it can be readily ascertained if the slide is empty or not. 98. Receiving only - When arranged for receiving only, an emergency chamber (Fig, 17) with hinged door is also provided to he fixed at the top of the switch to allow of the removal of the second carrier if two arrive simultaneously and so block the switch. A sluice valve as shown is provided at the top of this chamber so that the tube can he cut off from the switch while the door is opened, and by this means the necessity for destroying the vacuum in the tube, in order to open the door, is avoided. A test plug is fitted at the back of the emergency chamber to provide a connection for joining up a pressure gauge for test purposes. 99. The blanking pieces are fitted to the top platform side holes and the funnels to the bottom ones. Holes are drilled in the blanking pieces to allow of the insertion of an iron clearing rod (Fig. 16) to remove the carrier if it should accidently jam. 100. Sending only - For sending only, the emergency chamber, sluice and by-pass are not required. 101. The funnels are fitted to the side holes in the top platform, the blanking pieces to the bottom ones.
FIG. 18 - SWITCH, DOUBLE SLIDE, PNEUMATIC 102. To operate the switch a carrier is dropped into the funnel, below which is one of the moveable sections, care being taken to put the carrier in the funnel, open end first, so that it may travel with the buffer foremost. The switch is then moved to the opposite side and the pressure switched on, if this has not already been done. 103. Sending and receiving - When arranged for both sending and receiving, the emergency chamber, sluice and by-pass are again not required, as street tubes for such services are worked on the block system which only permits the insertion of a carrier when the previous one has been received and acknowledged. 104. The funnels are fitted to the top platform right hand and bottom platform left hand holes respectively, and the blanking pieces to the other side holes. 105. The switch, when used for this purpose, has one normal position only, which is with the handle on the right band side. When a carrier arrives, it is therefore received in the left, hand section a1 and its arrival is signalled. The switch is then moved to the left and the carrier removed, after which the switch should be at once returned to the right hand position in readiness to receive another carrier, or to he charged in section a2 for despatching one. It is essential that when empty the switch should be kept in the right hand position, as otherwise, if a carrier is received in the right hand section a2 it could not be readily removed. 106. The Switch proper consists of the moveable section fitted between the top and bottom platforms and provided with two funnels, two blanking pieces and a clearing rod. The standard arrangement for controlling the air supply is by a 3-way valve (see Fig. 21). If the tube is worked continuously, the air supply on the service which is not required should be shut off at the throttle cock on the main. The whole of this apparatus is shown in Fig. 16. 107. The Emergency Chamber, Sluice Valve and Three-way Test Cock required when the switch is used for receiving only are shown grouped together in Fig. 17. In the same illustration are shown the two flanges No. 3 and No. 4 for joining up the pneumatic tube, the former being suitable for brass tube and the latter for lead. The adaptor and plug shown on the right hand bottom corner of Fig. 17 are intended for connecting the test pressure gauge when the switch is arranged for sending only or both sending and receiving. 108. The switch and the various accessories are stocked in parts, of which the following should be requisitioned for the various duties for which the switch is required. For Receiving only:
For Sending and Receiving:-
For Sending only:-
109. At the out station on a street tube a wooden receiving box is usually fixed. This, as shown, in Fig. 19, consists of a box provided in front with a door.
FIG. 19 110. The tube enters at one side of the box and a rubber pad is fixed directly opposite to take the blow of the carrier. The exhaust is also taken out-at the opposite side but at a higher of lower level. When the box is being used as a receiving terminal the air, which in some cases may have oil fumes mixed with it, is conducted outside the building by means of the exhaust pipe. 111. The receiving box is provided with a Signaller, Pneumatic No. 3 (Fig. 19). The carrier drops on a hinged flap and its weight is sufficient to operate the contact of a bell circuit. 112. Intermediate Stations are sometimes provided on street tubes and in such cases an intermediate switch (Fig. 20) is required to intercept the carriers so that they can be removed when desired.
FIG. 20 - INTERMEDIATE SWITCH 113. The switch is made up of two horizontal sections of tube mounted on two circular end plates as shown in Fig. 20. The plates can be revolved so that on the arrival of a carrier in the bottom section this is turned round to the top position and the carrier removed by means of a rod. Sluices are arranged to intercept the carrier at the switch, these being opened when the carrier is to be allowed to proceed on its journey. Spring catches are provided to hold the revolving portion of the switch so that the tubular chambers are directly opposite the tube openings. These tubular chambers can be provided, with inspection windows so that a carrier on arrival may be examined without removal, and allowed to proceed if by its colour it is ascertained that it should do so. 114. In order to connect the. compressed or rarefied air to the tube terminals, control cocks are provided. 115. The Three-way Valve which is shown in Fig. 21 is used generally on installations where both vacuum and pressure are available, these services being connected to two of the ways and the tube terminal to tile third. It consists of a casing containing a hollow piston valve operated by a handle at one end. The casing has on the one side a flanged connection for the receiving and. sending apparatus and on. :the other two flanged connections for the vacuum and pressure service pipes respectively. Ports are cut in the cylindrical valve body so that the receiving or sending, apparatus is connected to one service pipe when the handle is pushed in, and to the other service pipe when it is pulled out. When pressure and vacuum are both. supplied from one cylinder or blower (see Fig. 21A) the valve can also be converted to a four-way valve by means of a flanged connection on. the end of the valve casing opposite the handle (see Fig. 21B). A blank flange is provided for tile latter opening when the valve is to be used for three-way working and is thus issued from stores. A guide is fitted to the handle to prevent the rotation of the valve.
FIG. 21 - THREE-WAY VALVE 116. The Four-way Valve is used where a single tube is supplied with both pressure and vacuum by connecting the tube to the delivery or suction of the blower as shown in the key diagram in Fig. 21. 117. Each tube on a power system is fitted with a throttle cock. These are made in 1.25inch, 1.5inch, and 2inch sizes; the figures indicate the diameter of the bore (see Fig. 22). The 1.25inch size is all brass; the other two sizes have G.M. plugs in C.I. bodies. Each of these is a plug cock designed for connection to the air main or common connection box and each has a union for making a wiped joint to the lead service pipe. The 1.25inch cock is suitable for all 1.5inch tubes. The 1.5inch and 2.25inch cocks are for use on the pressure and vacuum services respectively of street tubes. 118. All apparatus at the tube table should be thoroughly examined at least once every six months.
FIG. 22 - THROTTLE COCKS
SECTION V 119. The service pipes and connections include the whole of the pipework cocks, etc., containing the rarefied or compressed air between the pump and the controlling apparatus on the tube table used for operating the tubes. 120. For hand-worked installations the feed slide is usually fitted direct on to the pump, hut in cases where this is not convenient a lead service pipe is run between the feed slide and the pump. 121. For power-worked installations on the Low Vacuum System with a single loop the service is provided by extending the brass tubework to the blower, so that all resistance to the flow of air is removed as far as possible. For this reason standard bends are also to be preferred on this pipework. 122. Where there is more than one loop on the same service main a common connection box is to be used and connected to the blower by W.I. pipe, the brass tubes being terminated on to the common connection box. 123. For all other power-worked installations the main service pipes to the tube table should be of wrought iron, and the smaller connections to the controlling cocks, etc., of lead or wrought iron, preferably the former, on account of increased flexibility. Practically all the table apparatus is now provided with brass union fittings or flanges suitable for wiping the lead service pipes on and so making these latter easily removable for cleaning purposes. Cleaning should be carried out at least once a year. 124. The wrought-iron pipes and bends should be of gas quality, lap-welded, and free from internal projections, the inner surface presenting a clean and smooth bore. Socketted joints are made, the screwing being Whitworth standard gas thread. Elbows and sharp bends should be avoided as much as possible, and inspection doors should he provided on bends of 3inches diameter and upwards whose radius is equal to at least twice the bore of the pipe, to allow the pipework to he inspected and cleaned out. 125. The method adopted for fixing and supporting the wrought-iron pipework is exactly similar to that employed by the Post Office for heating systems, and needs no special description. This work is usually carried out by a local firm of heating engineers. 126. The weights, exclusive of socket, for the various sizes of wrought-iron pipe and the weights of the lead. pipes in use are as follows:-
127. The lead exhaust pipe is used for connecting street tube terminals at the remote stations to atmosphere and is therefore made of light gauge. To avoid putting back-pressure on the tube, these exhausts are made as large as is conveniently possible. 128. The size of the air main should be such that loss of power through friction and leakage is kept at a minimum. The velocity of the air between the compressor and container should not exceed 30feet per second under full load conditions. 129. The volumes of compressed or rarefied air passed, at the velocity stated, by the various sizes of wrought-iron pipes are given in the preceding table: these volumes can be reduced to volumes of free air by multiplying by
where X represents the pressure at which the air is delivered by the compressor, or Y the vacuum maintained by the pump.
SECTION VI 130. Containers are provided for power-worked street tube systems worked by reciprocating pumps. 131. A container on a pneumatic installation acts similarly to an accumulator, storing up energy when the demand on the pumping plant is small and giving it up when the demand exceeds the capacity of time plant. It will, therefore, be seen that in the case of continuously running plant, such as is used for street tubes, it is advantageous to have as large a container as possible, the size being determined, from a consideration of the probable fluctuation in the demand. 132. The containers used for the smaller installations are built tip of wrought-iron plates not; less than 3inch in thickness and riveted together. They are usually fixed vertically, and are 6feet; high and 3feet in diameter, and are raised I loot from the ground by an iron stand or by an extension of the sides of the container itself. The latter is shown in Fig. 23.
Fig. 23 Fig. 24 Such containers are cylindrical in form with both ends domed. A. drain pipe fitted with a cock is connected to the lowest; point so that the oil. and water deposited may be drawn off. A through-way valve fitted near the bottom of the container is provided to connect to the compressor, and to a similar valve near the top the service main is attached. The remainder of the fittings consists of a pressure gauge, a spring loaded safety valve and a manhole door of sufficient size to allow of the inspection and cleaning of the inside. These containers are galvanized inside and out after completion and are tested at the makers' works up to 60lb. per square inch hydraulic pressure. 133. For the larger street tube installations the containers are, as stated above, much larger and are used both for pressure and vacuum. The plates are made of ample strength For the container to withstand an hydraulic pressure of 80lb. per square inch, and, if necessary, internal stays are provided. They are painted internally and externally after erection, the size of the container being usually too large to galvanise conveniently. The only fittings consist of two flanged stools with flanges to provide connection for the pipework, a drain cock at the lowest point, a manhole large enough for a man to enter, and in some cases a safety device to open the container to atmosphere immediately, should an explosion occur due to the presence of coal gas or other mixture. 134. The two flanged stools are rivetted to the container, one near the top and one near the bottom.. If for vacuum working, the connection to the pump is made to the top lifting and the service main to the tube table is joined to the bottom. The reason for this is to allow carrier fluff and dirt drawn in from the tubes to settle at the bottom of the container instead of passing through or clogging the valves of the pump. For a similar reason the pipe connections on the pressure container are joined up in an opposite manner, the pump delivery in this case entering at the bottom and depositing oil and water there, while the clean air is taken off at the top to the tube table. 135. The safety device is shown in Fig. 24 and is simply a weak spot in the container wall, designed to give way at once if the pressure at any time exceeds 40lb. per square inch. It consists of a flanged stool bolted or riveted to the container and covered normally by plate glass tested to burst at the above-mentioned pressure. A wire dome is fitted over the glass to catch any particles which might otherwise be discharged into the room. A clear space to allow for the free discharge of the gases should be arranged for opposite the fitting. 136. All containers should be cleaned out every twelve months and thoroughly overhauled at least every two years; this biennial inspection should include an hydraulic pressure test for pressure containers. The test pressure should be twice the maximum working pressure and should be applied by the most convenient means. The pressure should be maintained on. the container for 15 minutes and there should be no sign of leakage. Small leaks which may be revealed should be stopped by means of red lead; more important leaks should he removed by caulking in the ease of seams or re-seating in the case of fittings. At the same time the container should be repainted inside and out if necessary and the safety valve reset, if required, to the proper maximum working pressure. A careful record of these tests should be kept in the Superintending Engineer's Office, and in the event of weakness or excessive leakage being observed at the test, the container should be put out of use at once, and the matter reported to the Engineer-in-Chief. 137. Where a large number of tubes have to be supplied from the pressure or vacuum main, common-connection boxes (Fig. 25) provide a ready means of connection for the purpose.
FIG. 25 - COMMON-CONNECTION BOX They are fixed under the tube table and are made up from special castings, fitted at each end with removable flanges. Along the top a level surface is provided and holes are drilled and tapped in this for the pneumatic fittings. Usually spare boles stopped with plugs are provided for possible extensions. The end flanges can readily he removed so that the boxes can be inspected and cleaned out when occasion demands. Such inspection should be made at least every 12 months, Suitable feet which raise the boxes from the floor permit of any dirt which may accumulate underneath being removed.
SECTION VII 138. The hand pump generally in use for departmental installations is of the reciprocating double-acting type fitted with a cock to enable it to be used either as a Pressure or vacuum pump. 139. Two sizes are provided, the smaller having a cylinder 6inches in diameter and a piston stroke of 6.5inches, and the larger a cylinder of 8inches in diameter and a piston stroke of 6.5inches. The smaller pump is made for horizontal or vertical working, the larger for vertical working only. The smaller pump is suitable for operating up to 100feet of 1.5inch or 50foot of 2.25inch tube and the larger pump up to 200 feet of 1.5inch or 100 feet of 2.25inch tube. 140. The pumps should work quite freely, the effort when moving the handle in either direction being the same. For the small size the, force required should be about 8lb. and for the larger size about 9lb., the length of stroke measured at Ike end of the handle being about 18.5inches for the small horizontal size20.5inches for the small vertical size, and 26inches for the large size. Steady full length strokes give better results in working than short jerky strokes. 141. The smaller sized pumps are usually mounted on a wooden base or table and the larger ones on a cast-iron base. The various pumps are similar in detail; one of the large vertical ones, fitted on a base, is shown in. Fig. 26.
FIG. 26 - PNEUMATIC HAND PUMP AND 3-WAY SWITCH 142. The pump is operated by means of the handle H of the lever L which moves about the fulcrum F, which itself is allowed limited motion through the link R. To this lever the piston rod M is connected, the lever itself being extended beyond the fulcrum. A weight W is fixed on an extension of the lever in such a manner that it balances the combined weights on the other side of the lever.. 143. A delivery and a suction valve are fitted at the top end of the cylinder and two similar valves at the bottom end. The top delivery valve DV and the bottom suction valve SV only are shown in the sectional view. 144. On the upward stroke of the piston the air in the top part of the cylinder is expelled through DV and air is dawn into the bottom part through SV. On the downward stroke the action is reversed, the valves, then in use being the bottom delivery and the top suction. 145. The four valve chambers are connected by passages to four holes spaced equidistant round the outer shell of the controlling cock E, by means of which the suction or the delivery valve passages can be connected to the tube terminal S at will. The passages from similar valves are led to opposite holes, the top suction facing the bottom suction and the top delivery the bottom delivery. 146. The body of the cock is provided with two distinct through passages J and K, terminating in four holes spaced similarly to those in the shell, so that the two suction valve passages are always connected together and the two delivery passages together. The passage J is also connected to atmosphere through the hollow space A at the top of the valve body, and the passage K is connected to the tube terminal S through the lower half of the valve body and the pipe P. Thus, with the controlling cock lever L1 in one position, the two suction valve passages are connected to the tube and the pump discharge to atmosphere, the pump then being arranged for "Receiving." By turning the lever to the opposite position the action of the pump is reversed, it then being arranged for "Sending." 147. When requisitioning hand pumps the following particulars should be given:-
148. In order to operate more than one tube by means of one hand pump, two or three-way switches are provided when required. The switch is shown in Fig. 2, and consists of a cast-iron plug cock, the shell "G" of which is provided with a flanged opening "O" on one side for connecting to the pump and two or three similar openings, N1, N2, N3, with a common flange on the opposite side, to which the tube terminals are connected. 149. The plug body "Q" is hollow and tapered, and is open to the passage leading to the pump flange "O" above mentioned. In the body of the plug an opening "T" is cut and this can be moved opposite to either of the tube terminal openings in the switch shell. 150. The switch is operated by depressing the handle "U," and so releasing a spring catch which fits in the notches XI, X2, X3, so ensuring that the opening in the plug body is directly opposite the corresponding tube opening. The switch is kept air-tight by means of the brass ring "Z" which is pulled towards the shell by screws, so preventing the tapered body from becoming loose. Only one tube can be operated at a time by this switch.
SECTION VIII 151. The power-worked pneumatic pumps used by the Department are driven by steam engines, internal combustion engines or electric motors. Gas engine and oil engine driven installations have only been provided in towns where no electrical supply is available, and consequently there are very few of these sets required, and engine-driven plant generally is being superseded by electrically-driven plant. For the purposes of this instruction it is sufficient to describe the electrically-driven plant only. 152. in the sets in use the power is transmitted from the motor to the compressor by means of belt, gear, or direct coupling. The first named is by far the most commonly used, and to obtain a smooth and even drive endless Balata belts have been found most suitable. 153. II the supply is on the direct current system, shunt-wound motors are used in all eases, and for the larger installations, where street tubes are operated, variable speed motors will be provided in future. For polyphase systems, induction motors fitted with slip rings are in most general use, and for single-phase installations either induction motors with special starting devices or some other design of single-phase motor which can be started without difficulty is adopted.
FAULTS IN ELECTRIC MOTORS 154. It is not proposed to describe motors in this instruction, and ordinary practical details of motors and starters will be found in Technical Pamphlets for Workmen Nos. G2 and G3. 155. Remote control starters - For installations which require to be started and stopped from a distance remote control is necessary. The starter is operated either by a powerful solenoid or a small motor. All starters, whether operated by hand or automatically, are fitted with overload and no voltage releases, so safeguarding the motor in case of overload or failure of the supply. 156. Compressor - Two types of compressors are in 'use, viz., rotary and reciprocating. The former at present is used only for installations worked on a low vacuum or pressure; in all other cases reciprocating plant is used. 157. The rotary pump, which generally is a Roots Blower, as shown in Fig. 27, is usually used to extract air from the tubes. It consists of a cast-iron casing, accurately bored and planed, in which two cast-iron rollers shaped in section like the figure 8 are fixed on parallel mild steel spindles and geared to one another at one end by equal sized cast-iron machine-cut spur wheels. The driving pulley is fixed to the other end of one of the spindles.
FIG. 27 - ROOTS BLOWER 158. The bearings are gunmetal hushes of ample length fitted in the end plates of the casing. 159. A pedestal hearing is provided outside the pulley. 160. The rollers are set at 90degrees to each other, so that the rounded top of one fits in the hollowed waist of the other. The clearances between the rollers and the casing and between the rollers themselves are kept as small as possible consistent with safe working, and to maintain a fairly efficient air-seal the interior is amply lubricated with engine grease. 161. The rollers rotate in opposite directions, as indicated, and the air is sucked into the casing at the top side .and delivered at the bottom. To avoid noise the speed is limited to 300 r.p.m., and at this speed it is possible to maintain a vacuum of 2lb. per square. inch wider favourable conditions. In practice it is necessary to adjust the speed to suit the demand of the installation for air. 162. The blower is driven in such a direction that the air is drawn in at the top flange, to which the tube installation is connected. To the bottom flange a wrought-iron bend is fixed to discharge the air into the room or to connect to a silencer if the noise of discharge is too great, although if the speed mentioned is not exceeded this should not be necessary. 163. Lubrication is provided for by means of "Stauffer" grease cups on the hearings and an oil bath for the. gear wheels. 164. Rotary blowers require very little maintenance. If the vacuum maintained falls below the required value a piece of engine grease about the size of a walnut dropped into the blower will usually remedy the defect. 165. The bearing bushes are usually the first parts to wear, and these can be quickly taken up if adjustable or replaced by new ones if solid. 166. The blower should be taken to pieces and thoroughly overhauled at least once every six months. 167. Centrifugal fans. For low vacuum systems centrifugal fans are also being employed, especially where a large volume of air is required for working a number of loops. These are similar in principle to the ordinary blowers used for ventilating purposes, but are constructed for a greater difference of pressure between the intake and delivery. These fans will deliver a varying volume of air at practically constant pressure ii the speed is maintained constant, and it is therefore an advantage to use them where considerable variation in the volume of air demanded is likely to occur. 168. Air compressors of the reciprocating type are used by the department for two purposes, Viz.:-
169. In all cases the desired pressure or vacuum is obtained by single stage compression, i.e. the air is compressed or rarefied to the required extent during each stroke of the piston. 170. The pressure and vacua are provided for house and street tube installations where the air, compressed or rarefied in the pump cylinder, is delivered to or withdrawn from the tubes through a container. A single cylinder pump is made use of in each instance. 171. Where street tubes are worked in both directions and pressure and vacuum both have to be, provided, these may be obtained from separate cylinders on one pump, or from one cylinder. In the former case, one cylinder provides pressure, drawing its supply from atmosphere, and the second cylinder provides vacuum, delivering its exhaust air after compression to atmosphere. 172. Where one cylinder provides both pressure and vacuum the air withdrawn from the vacuum main or container is compressed to the required pressure, and delivered into the pressure main.
PUMPS 173. For all the above methods of working the same type of reciprocating pump is made use of. This is of the water-cooled type with the cylinder or cylinders arranged vertically or horizontally. The valves are in most eases of the poppet type, but in a few instances mechanically-operated valves are used. These latter are, however, chiefly made use of on large compressors, such as those in London, and this description is confined to the compressors fitted with the former type of valves. 174. A vertical single cylinder pump and a horizontal double cylinder pump are shown in Fig. 26 and two sectional views of a cylinder in Fig. 29.
FIG. 28 - SINGLE CYLINDER VERTICAL COMPRESSOR, SIDE ELEVATION 175. The cylinder proper in each case consists of a cast-iron liner a, accurately bored for its entire length and finished smooth. In the end plates which close the cylinder are usually fitted the suction and delivery valves so that the clearance in the cylinder when the piston is at the end of its stroke may be reduced to a minimum. 176. An outer shell b and end covers c provide spaces for the air passages and the water circulation. Water jacketting is in all cases provided between the shell and the liner, and in most modern compressors the jacketting is :continued to the cylinder heads and valve chests. This is done in the first place to prevent the air being heated, and therefore expanded, as it enters at the suction valves; secondly, to remove the heat due to compression from the air in the cylinder; and thirdly, to cool the air as much as possible when discharged at the delivery valves. 177. The air chambers between the valves and the outer covers are termed valve chests, and two such chests are provided at each end, one for the delivery valves D to discharge into, and one for the suction valves E to draw from. The two delivery valve chests are connected together by the air passage f on one side of the cylinder, and the two suction valve chests by a similar passage g on the other side. At the mid point of each passage flanged openings h and j provide connections for the delivery or pressure pipe and the suction or vacuum pipe respectively. 178. Typical examples of poppet type valves for suction and delivery are given in Fig. 29. The valve plate k is a disc of special steel or phosphor bronze.
FIG. 29 - SECTIONAL VIEWS OF CYLINDER AND DETAILS OF VALVES A hole is cut at the centre to allow the plate to move along the spindle I. The valve seat m is of cast iron with a carefully ground surface on which the valve plate beds. The valve is normally kept on its seat by a light spring n, but when the air is drawn through or driven out of the ports in the valve seat this spring is compressed by the valve plate The valve plate of the suction valve moves inwards towards the cylinder when admitting air and closes as soon as compression starts. The delivery valve plate works in the opposite manner. 179. The valves complete are kept in position by the valve stud p passing through the outer cover or by iron straps bolted to the inner cover. In the former case access to the valves may be obtained by means of the small cover plates q, but in the latter the outer cylinder cover must be removed. 180. The piston r is of cast iron and is provided with split packing rings 8 of the same material. These latter form an airtight joint with the inner surface of the liner. The piston rod t, on which the piston. is fixed, is of mild steel and passes through the outer cover at u, where it is made airtight by means of a gland which is packed with tallowed engine packing or packing-soapstone. 181. The motion to the piston is given through the crosshead V by means of the crank W on the main shaft, which runs in the main bearings Y and to which is fixed the driving flywheel Z. 182. The piston moves to and fro in the cylinder liner, compressing and driving out the air before it and drawing in a fresh charge behind it, which charge is compressed and delivered on the return stroke of the piston 183. Indicator diagrams showing the variation of pressure which takes place during the cycle of operations inside such a compressor are illustrated in Fig. 30.
FIG. 30 - INDICATOR DIAGRAMS 184. The illustrations show what takes place on one side of the piston only. For double acting pumps, such as are generally used by the Department, the diagram for the other side of the piston is similar but reversed. The examples given are actual working diagrams taken from. compressors used for (I) drawing air from atmosphere and delivering against a pressure of 50lb, per square inch above atmosphere; (2) drawing from atmosphere and delivering against a pressure of 10lb. per square inch above atmosphere; (3) drawing from a vacuum of 6.5lb. per square inch below atmosphere and delivering at atmospheric pressure; and (4) drawing from a vacuum of 6.5 lb. per square inch below atmosphere and delivering at a pressure of 10lb. per square inch above atmosphere. 185. The mean effective pressure shown in each case is obtained from the mean height of the diagram set to the scale of pressure to which the spring used is calibrated. The result obtained therefore represents the average or mean pressure in the cylinder during one stroke of the compressor, and the indicated horse-power can then be obtained from the following well-known formula:-
where or if the volume displaced by the piston in cubic feet per minute is known, the indicated horse-power can be obtained more readily from the formula:-
where V = the volume displaced in cubic feet per minute. 186. These calculations assume that the diagram on one side of the piston is the same as the other, but in practice these are usually slightly different. The two diagrams should then be treated separately and the results added together. The value for N taken in such case would be the revolutions of the compressor, not strokes of the piston. 187. The indicated horse-power of the compressor can also be calculated theoretically by assuming the law of compression. The compression is practically adiabatic, and in this case the formula connecting pressure and volume is given by:- PV to the power of n = K - - - - - - - (1) where P = pressure absolute; 188. From the indicated horse-power of the pump the brake horsepower of the motor may roughly be obtained by multiplying by four thirds, represents the reciprocal of the efficiency. This figure, however, varies for different makes of compressors and is, therefore, only an approximate or average value. 189. It may be well to state here the conditions which should be fulfilled by a perfect compressor so far as the work done in the cylinder is concerned:-
190. The lubrication provided on reciprocating pumps should be sufficient for a run of 16 hours without stopping, and is obtained, when forced lubrication is not employed, by means of fixed oil chambers on the stationary parts and either by wipers or large closed vessels on the moving parts. 191. The important parts requiring lubrication are indicated by numbers on Fig. 28, the key to the numbers being as follows:-
192. The waste oil from all the parts, with the exception of the cylinder, is drained, into a reservoir in the bedplate, from which it can be withdrawn and in some cases used over again. In some types of compressors a small auxiliary oil pump is provided. This draws the oil from the bedplate reservoir and delivers it into a cylindrical oil vessel fixed above the compressor, from which vessel it is taken through sight feed lubricators and brass tubing to the various points Abundant lubrication without waste can thus be provided, it being only necessary to remove the oil for filtering or renewing about once a week. 193. Generally speaking, over lubrication will not do harm except in the cylinder, but should not be carried so far that splashing or throwing the oil about takes place. 194. The cylinder lubrication should be cut down to an absolute minimum required for satisfactory running as this oil vaporises to a certain extent, and if the air is not sufficiently cooled in the container the oil is deposited in the tubes and fittings. In some cases special cylinder oil with a high vaporisation temperature is used to overcome this trouble, but it is usually advisable to make use of an oil similar to that recommended by the makers of the compressor. 195. A forced circulation of water is in most cases provided for by means of a small water pump fixed to the bedplate and driven off the main shaft. The water is taken from the bottom of a circulating tank and passed by way o the water pump through the bottom and out of the top of the cylinder jacket, being finally delivered into the top of the tank. The temperature of the water at the top of the tank should not be allowed to exceed 100 degrees Fahr. A water supply with ball cock is provided to admit cold water to replace any warm water drawn off. 196. At the lowest point of the water circulation pipes a drain cook is provided to empty the jackets and pipes in frosty weather. 197. Maintenance - The lubricating oil, if used over again, should be filtered at least once a week until a marked deterioration is evident; when it should be replaced by new oil. 198. Any knocking at the bearings should be removed by taking up the brasses as soon as it appears. If allowed to continue, the trouble will quickly become magnified, resulting in uneven strains on the other working parts and distortion of the brasses, which may then have to be refitted. 199. Thumping inside the cylinder may be caused through broken valves or foreign matter getting. between the piston and cylinder head or by the clearance between these parts becoming too small, due to the brasses on the crosshead or main shaft having worn. As soon as such thumping is noticed the plant should be stopped, and overhauled. 200. The valves should be removed and examined at least once a month, and if the valve plates do not make even contact with their seats they should be re-bedded. 201. The whole plant should be overhauled at least once every twelve months. This overhaul should be carried out even if the compressor appears to be working satisfactorily, as small faults can then be detected and remedied, which if left would later develop into more serious ones. 202. A spare set of small parts for renewal purposes is usually supplied with each compressor, and these should, as used, be replaced by new ones. A full set of spare valves should always be kept. 203. The belts in most general use for pneumatic plant are of the endless Balata type, as these, by reason of the absence of rough joints, give a more even load on the motor. 204. The thickness of the belt is rated in the number of layers of which it is made up. The thinnest Balata belt in use by the Department is the 3-ply. This is used for the small 1/2B.H.P. motors driving Roots' Blowers. For motors of 2 to 3B.H.P. 4-ply belts are usually used, and 5-ply belts for sizes of from 3 to 10B.H.P. For larger sized plant the power is transmitted by 6, 7 or 8-ply belts. The widths of belt recommended by the makers are shown in the following Table:-
205. As the above figures are intended to apply for drives where the pulleys. are nearly equal, it is advisable to allow a margin of 25 per cent. extra for unequal pulleys such as are used for pneumatic plant driving. 206. No belt composition of any kind, or resin, should be used on Balata belts. Should a belt become dry and smooth in work or show signs of slipping, a soft and tacky surface on the working face can be obtained by applying a 'few 'drops of castor oil to the face of one of the pulleys. Not more than six drops should he applied at one time. Once a year the belt should be dressed by rubbing both surfaces with a flannel moistened with castor oil. 207 Wherever possible belts should be arranged so as to run with what is known as a "bottom drive," that is, the driving or pulling side should be underneath, and the returning or slack side on top of the pulley. By this means the arcs of contact between the belt and pulleys are increased and a better grip is obtained. 208. The belt should never be run extremely tight, but should move with an undulating vibration on the returning or slack side, thus showing that there is no greater strain imposed than is necessary to transmit the power required. 209. The proportion between the diameters of two pulleys working together should not exceed 6 to 1, and both pulleys should be from 1inch to 2inches wider than the belt, so as to allow for sufficient play of the belt.
SECTION IX 210. Information regarding the lay-out of power-driven pneumatic plant must necessarily be of a very general nature, as it depends on so many variables. The space available, the shape of the room, the type and make of the compressor, are a few of the many points which bear upon how the plant should he arranged, and frequently this has not been purchased when the space has to he decided upon. Rough information regarding the size of the room required must, however, be available at the early stages of any scheme, so that suitable accommodation can be provided. 211. Approximate figures only can be given, representing the outside sizes of the various items which are usually to be found in the power-room of small or medium-sized installations. For larger systems the running plant itself and the accessories vary to such a large extent that such cases must be treated as special and dealt with accordingly. 212. For the smaller house-tube installations, worked on the low vacuum system by a Roots Blowers, the problem is a simple one, as one size of blower has been found suitable when run at different speeds to deal with all such cases. The motor required is so small that in order to obtain a good mechanical article it has been decided that one size (1/2B.H.P.), which is larger than is actually required, shall be used in all cases. Information regarding the two main items is, therefore, definitely fixed and the lay-out can be readily settled. 213. The floor-space required for the blower referred to is approximately 24inches by 18inches, and that for the motor 18inches by 18inches, The pulleys are each 2inches wide, and the plant should be so arranged that their centres are about 36inches apart, making the total length required 5feet, with a width of 18inches. Both pulleys will clear whatever bed the plant is fixed on, and it has, therefore, been found convenient to mount both motor and blower on one foundation measuring 6feet by 2feet, and, in order to provide for easy examination of the plant, to make this foundation up to a height of 2feet above the floor level. 214. Such an arrangement can be situated in any corner of an existing room, preferably, of course, where other plant is also installed. To avoid complaints regarding the slight noise which must necessarily be created by this type of running plant, the room selected should be situated in the basement, or at all events not adjacent to a room. where the noise might cause inconvenience. If the noise from the blower is objectionable, the exhaust should be led to a silencer, or to the open air at a point as near to the blower as possible, to avoid any unnecessary length of pipework. 215. The problem of arranging reciprocating plant for larger installations presents many more difficulties, and it is desirable at. this stage to enumerate the various items which may have to be accommodated in the power room. These are as follows:-
216. For small house-tube or unimportant street-tube installations only one set of running plant is usually provided, as in the case of breakdown the service can in most cases be maintained by messengers or hand pumps. For large house-tube and the more important. street-tube systems it is necessary, however, to provide duplicate running plant to allow for overhauling and to guard against a complete shut-down if one set is out of commission. 217 The floor space required for the compressor varies to a very large extent. The type of compressor, vertical or horizontal, to be used, although making a difference to the size of the foundations required, need not necessarily make much difference to the poor space required for the complete set, as the compressor, if horizontal, can be arranged with the shaft beyond the body of the compressor (see Fig. 31). 218. Approximate figures giving the size, weight, speed, etc., for the usual sizes of air compressors used are given in Table VI., but it must be emphasised that the values shown are not correct for all cases, but are intended to represent outside figures of sufficient accuracy to enable a preliminary lay-out of the plant to be determined upon.
TABLE VI - PARTICULARS OF RECIPROCATING COMPRESSORS NOTE - A represents values for working pressure; B for vacuum; C from vacuum to pressure ; D small cylinder pressures, large cylinder vacuum. 219. The foundation should be of concrete covered with glazed bricks or tiles. It should be of such a height that the plant can be examined and attended to with ease. For small sets usually from 1foot to 2 feet above floor level is sufficient. 220. The floor space taken by the motor with Its fixing rails can be more accurately estimated as this type of plant is much more generally standardised. The necessary information is given in Table VII.
TABLE VII - PARTICULARS OF SHUNT-WOUND D.C. MOTORS 221. The foundation for the motor should be similar to that described previously for the compressor. 222. The floor of the engine-room, if a new one is being provided, should he tiled with dull red tiles. A guard consisting of two polished steel rails fixed in vertical stanchions should be provided round the running plant to prevent accidents. In the case of duplicate plant a clear gangway of at least, 3feet should be left between the two sets, and the belts should be arranged on the outer sides of the space occupied. 223. The starter may be of the pedestal type or of the ordinary type supported on a vertical switchboard or wall. In the former case, as the name implies, the starter proper is mounted on a east-iron pedestal which can be bolted to the floor. The space required is about 3feet by 2feet, and this can usually be found in the unoccupied space between the motor and compressor. Wall-supported starters vary A good deal in size but, as they do not encroach on the floor space, dimensions need not be given. 224. For large plant the cable conduit and the water pipes should be laid in trenches cut in the floor and covered by iron chequer-plate. For smaller installations the conduits and pipes can be fixed to the walls. 225. The circulating tank is usually 6feet high by about 3feet in diameter, and in the case of large installations two such tanks are generally provided. It is an advantage to mount these tanks so that the outlet at the bottom is at a higher level than the inlet to the cylinder jacket, and a wooden base or a concrete foundation should be provided for this purpose. 226. The containers are placed in the engine-room if there is sufficient space but, if not, they can be fixed outside.
FIG. 31 - LAY-OUT FOR DOUBLE CYLINDER SETS FOR STREET AND HOUSE TUBE INSTALLATION 227. The small accessories usually required in a power room consist of the following:-
228. Two typical lay-outs of reciprocating plant for street tubes are given in Figs. 31 and 32. For house tubes normally worked by blowers the conditions are less elaborate and are quite sufficiently illustrated in Section X. (a) Low Vacuum System.
FIG. 32 - LAY-OUT FOR DOUBLE SET FOR STREET A1D HOUSE TUBE
INSTALLATION
SECTION X 229. Power-worked tube systems may be roughly divided into two classes, viz., these in use for house tube installations, and those for street tube installations. 230. The various systems in use are as follows:-
231. Slight modifications of these are sometimes made, but the systems above mentioned and described hereafter, are the principal ones on which all others are based.
SECTION X 232. This system is now the standard system for house tube installations. Its operation is based on causing a sufficiently rapid current of air in the pneumatic transmission tubes by applying a low vacuum of 0.5lb to 1lb. per square inch below atmospheric pressure at one end of the tube by means of a blower or centrifugal fan. The velocity of the carriers depends practically on the velocity of the flow of air maintained, and in order to keep this as high as possible in larger installations, care must be taken that the traffic on the different sections of the the loop do not interfere materially with the flow of air. Brass tube with standard bends is used throughout and special carriers are made use of. 233. The tubes are looped back on themselves between each pair of stations, the air flowing in opposite directions in the two tubes, so providing for carriers to be despatched in both directions. The whole of the tubes are then joined in series as shown in the various methods of connecting up given in Fig. 33, and one end of the whole system is then joined to a Roots Blower (Fig. 2.7), and at the other open end air is admitted.
FIG. 33 - LOW VACUUM SYSTEM 234. The maximum total length of tube which found convenient to work on this system is limited to 500feet, and the number of stations should be kept as few as possible. House tubes only should be installed on this system. 235. The efficiency of the system depends to a very large extent on the number of stations on the length of tube forming a loop, as it will be seen that, when a despatch door is opened for the insertion of a carrier, the flow of air on the "down" part of the tube is interfered with. The length of tube should of course be kept as small as possible. 236. Flap Terminals (Fig. 15) are fitted at each point where carriers are to be ejected from the tubes, and wire Receiving Baskets (Fig. 15) are provided for catching and retaining the carriers at these points. The system is therefore provided with an automatic means of ejecting the carriers from the tube, and for this reason alone has a decided advantage over other systems for house tube work. 237. At despatching points Door Despatch Terminals (Fig. 15) are used or in some cases a Flap Terminal is made use of both these terminals are usually joined in series with the tubes, the current of air normally flowing through them, but in some cases the Door Despatch Terminal may be more conveniently joined up as a branch to the tube route, in which case it is mounted on a special T-piece (Fig. 15) and a cover plate (Fig. 15) is used to close the top of the terminal. These methods of fixing and connecting up the fittings are illustrated in Fig 34.
FIG. 34 - METHODS OF FIXING LOW VACUUM SYSTEM TERMINALS 238. The open end of the tube where the air enters forms a despatching point at one station, and a simplified fitting termed a Funnel Despatch Terminal (Fig. 15) may be fitted here, provided that the tube rises vertically from it. If, how-ever, the tube descends, a Door Despatch Terminal must be used and the air should be admitted to the top of this through a tube extended below the table top, or by a special T-piece under the table as in Fig. 34. 239. In a few instances foreign matter has been wilfully or accidentally inserted in these tube installations and it has ultimately drifted into the blower, the rollers of which have become blocked. The last terminal before the blower must necessarily be a flap terminal, and the grids of this will prevent anything large passing on to the blower. Soft articles such as fluff and small pieces of paper will pass through the blower without harming it, but it is such articles as pencils, rubber, &c., which have to be guarded against. These may be caught in a metal or wooden box (Fig. 35) fixed near the blower to act as a trap, but in no case should wire gauze be inserted to intercept any articles, as this becomes quickly clogged with dust, so rendering the system inefficient. The necessity for fixing the trap referred to can only be determined by local conditions, but in practice it is seldom required. It has been described as a remedy only if the trouble referred to should arise. 240. The tube should be fixed to the top aperture of the blower and should ascend vertically for at least 4feet. About 1foot from the blower a V-shaped opening (Fig. 35) should be cut in the tube and a tight-fitting sleeve should be selected to cover this when required. The aperture can be used to decrease temporarily the vacuum in the tube, but if this should be found to be permanently too high, the speed of the blower should be lowered by fitting a smaller pulley on the motor. The opening may be conveniently used for dropping engine grease into the blower to lubricate the rollers.
FIG. 35 - LOW VACUUM SYSTEM TRAP 241. About a foot above this opening the brass tube should be divided and the two ends kept 6inches apart. They should be joined together by a piece of 4-ply 1.5inch rubber hose-pipe hound tightly to each end. This is inserted to prevent vibration from the blower being transmitted by means of the metallic tube to the rooms through which it passes. 242. The foundations required and the fixing of the plant are described in Section IX. 243. The blower should be run at such a speed that a small margin is obtained over the minimum vacuum required to draw the carriers through the tube. This speed is usually from 180 to 250 revolutions per minute and at such a speed the noise due to the discharge of air from the blower should not be heard from outside the room in which the plant is fixed. 244. The size of the electric motor which it has been found most suitable to use is 1/2B.H.P. for all installations. The speed at which it runs is usually about 1,000 revolutions per minute and, as the blower is always provided with a pulley 12inches in diameter, the motors are sent out from the manufacturers' works fitted with a 3inch pulley. When the plant is erected it should he experimentally ascertained by running the motor in series with a resistance in the supply mains if the installation can be worked with the blower running at a slower speed than the combination of pulleys provided would normally give, and if the difference is more than 20 revolutions per minute, a smaller pulley should be obtained for the motor. If, on the other hand, the speed of the blower is found to be too low with the motor running on the full voltage of the supply, the correct speed should be ascertained experimentally and a larger pulley for the motor should then be fitted. 245. As an illustration the following particulars are given. An installation at present in use has a total length of 500feet of 1.5inch tube with 21 bends and a vertical rise of 45feet. This is worked efficiently with a blower running at 250 revolutions per minute and the motor consuming 150watts. 246. During 2 the busy hours of the day the plant is kept continuously running and signalling is dispensed with, but when the traffic is slack the plant is started only when required. To do this a starter is fixed in the instrument room on the tube table and the attendant's attention is obtained by means of press buttons and bells. A 7-ampere coupled tumbler switch and two porcelain cut-outs of the same capacity are also provided in the electric supply circuit near the motor in the power room so that the leads to the motor and starter can be entirely cut off from the supply when, the plant is shut down for overhaul or other purposes. 247. The principle of the low vacuum system has been made use of for 2.25inch house tubes in the Central Telegraph Office, London, and other large offices. For this purpose, as stated in para. 234, the number of stations on a loop is kept as low as possible in order to avoid interference, and in order to effect this the various loops of tubes are connected to a common connection box which is maintained at a predetermined vacuum by means of a rotary blower or centrifugal fan. The latter piece of apparatus is found be more suitable to maintain a steady vacuum irrespective of the amount of air that has to be removed. The terminals are the flap despatch and funnel described earlier. The tubes are run bunched together as much as possible, and at each receiving station the flap terminals discharge into a common hopper which guides the carriers into a well cut into the table top, and provided with a string net bottom so that noise is avoided and the carriers do not bounce out. Where the funnel despatch terminals are used a set of wire baskets is fixed immediately below the terminals so that the messages for each particular tube can be sorted into the corresponding basket below it. Similar baskets are fixed on the top of each flap despatch terminal, and as the tubes descend from these the special T pieces described earlier are made use of. (NOTE.—No paragraphs 248 and 249 nor Fig. 36.)
SECTION X 250. If street tubes have to be provided for working in one direction only (usually in the "up" direction), a simple form of vacuum installation is provided as shown in Fig. 37.
FIG. 37 - SINGLE CYLINDER VACUUM SYSTEM DIAGRAM FOR STREET TUBES WORKED IN ONE DIRECTION 251. The plant consists of a single cylinder double acting reciprocating pump similar to that described under Pumps, paragraphs 173-174 and Fig. 28. The air main is attached to the suction side, and a container is usually joined up in this main to assist the compressor to deal with sudden heavy demands. The delivery from the compressor is liable to be rather noisy, and this may be taken outside the room by a full-sized wrought-iron pipe, or it may be terminated in a wooden or iron box filled with broken coke or loose rubble to break up the air currents and so deaden the noise. 252. The speed of the compressor should be proportioned so that the normal working vacuum is sufficient to cope with the greatest demand occurring during the day. In the case of a single tube, by assuming the maximum time of transit permissible the vacuum necessary can he obtained from particulars given in Tables VIII and IX.
SECTION X 253. This system provides both vacuum and pressure by means of separate cylinders for each service. In future, it will be used in large towns for street tube installations which consist of more than one tube worked in both directions. The system is illustrated diagrammatically in Fig. 38.
FIG. 38 - DOUBLE CYLINDER SYSTEM DIAGRAM 254. The street tubes are controlled by valves, pneumatic 3-way and terminate at the operating station on double slide switches for street tubes. A typical lay-out of the tube table for such a system is shown in Fig. 39, including common-connection boxes and throttle cocks.
FIG. 39 - LAY-OUT OF TUBE TABLE FOR DOUBLE CYLINDER OR PUMPING THROUGH SYSTEM 255. The compressor is usually of the tandem cylinder horizontal type (Fig. 28) with the small cylinder working on pressure and the large one on vacuum. Unless there is an abnormal demand for one of these services the most suitable ratio for the volumes of the cylinders has been found to be 1 to 2. 256. If duplicate compressors are installed stop valves are fitted on the outlet of each pressure cylinder, and the inlet of each vacuum cylinder and the two pairs of mains are then joined together, and each pair led by one common air main to the container. A small spring loaded safety valve should be fitted in delivery valve chest or delivery pipe of each pressure cylinder before the stop valve is reached, so that if this should be accidentally left closed when the plant is started the compressed air in the cylinder can be released before any damage is done. The small safety valves should be set to act at about 5lb. per square inch above the working pressure. 257. In the pressure main, and as close as possible to the point where the pressure cylinder delivery pipes join, it is necessary to fix a larger safety valve to allow a portion of the compressed air to escape should the working pressure be exceeded from any cause. In order to obviate a waste of energy from this cause it is necessary to provide some means to unload the pressure cylinder. This can be done by fixing an unloading valve in the supply delivery pipe, or in the inlet pipe of the pressure cylinder; the latter method is preferable as it does not involve any waste of compressed air when the unloading valve operates. Various types of mechanically and pneumatically operated valves have been tried with more or less success, but in all cases it has been found difficult to obtain a valve which would act with precision and without chattering, when the unloading limit was approached. An electrically operated valve has been constructed for this purpose which possesses the advantage that the range between the unloading limit and the reloading limit can be made as wide as required, and the limits themselves can be readily altered by simply moving contact fingers on a pressure gauge. 258. The electrically operated unloading valve consists of four essential parts:-
259. The unloading valve is made up of two pistons of unequal diameters joined together and moving in two cylinders formed in one casting. 260. The operation of the valve when inserted in the air, delivery pipe is as follows:- Around the top cylinder c, ports are cut, which open to a common chamber connected to the inlet d, to which the compressor delivery is joined. An opening e at the top of the valve forms the connection to the pressure container, and the lower opening f is connected to atmosphere. The lower and larger piston a is the operating one, and the normal position is as shown with the piston at the bottom of the stroke. The top piston b then allows the compressed air to flow from d to e and on to the container. When compressed air is admitted to the lower cylinder g at the point h. It the two pistons rise, and the top one then allows the air to flow from d to f and thence to atmosphere. At the same time the container pressure is out off by the top piston, and this therefore acts as a non-return valve. The pressure cylinder then runs light, the suction and delivery being both connected to atmosphere. To utilise the valve in the air inlet pipe the pressure cylinder inlet is joined to inlet d. When compressed air is admitted to the lower cylinder g at h, the top piston b closes the inlet at d, thus cutting off the air supply to the pressure cylinder and relieving it of load. A lubricating cup fixed on a hollow rod passes through the top cover of the valve to the top piston, so forming a means to. admit oil when necessary, and also acting as an indicator to show the position of the valve. 261. The solenoid operating valve consists of a gunmetal ball k kept normally against its top seating by the spring l acting on the spindle m. The top seating is permanently connected by the pipe shown, to the pressure on the container side of the unloading valve. When current is passed through the solenoid n the armature o is pulled down as shown, and with it the spindle m. The ball is then forced down on to its bottom seat and the compressed air is let into the cylinder g, so operating the unloading valve. When the current is stopped the spring and spindle return the ball to its top seating, so cutting off the compressed air from the cylinder g and allowing the quantity entrapped therein to escape to atmosphere through the holes p. The pressure on the top piston b, together with the weight of the valve, then returns the unloading valve to its bottom position. The solenoid is wound for working off a secondary battery at 40volts, the resistance being 310ohms and the current 0.13ampere. The current is, however, too large to make and break satisfactorily on a slow moving pressure gauge contact, and an electrical relay is therefore used. 262. The electrical relay consists of a solenoid, the armature of which, when attracted makes the circuit for the solenoid operating valve referred to. The current required for the relay solenoid is only 0.02ampere at 40volts, and the gauge contact can safely make circuit for this amount. To avoid sparking when breaking circuit, the gauge contact simply short circuits the relay solenoid, the armature then breaking circuit. While the needle of the gauge travels from its maximum to its minimum contact, the relay armature is held up by current through an auxiliary contact. 263. The contact pressure gauge consists of an ordinary Bourdon pressure gauge with maximum and minimum platinum contact needles mounted on the glass front. The gauge pointer carries a third platinum contact needle, and the two former can be set to make contact with this latter when the gauge indicates the desired pressures.
SECTION X 264. This system, Fig. 41, is similar to the Double Cylinder system, except that single cylinder compressors are used, the one cylinder providing both pressure and vacuum by drawing air from the vacuum mains and delivering it into the pressure mains. In reference to this system the following precautions must be carefully borne in mind:-
FIG. 41 - PUMPING THROUGH SYSTEM DIAGRAM 265. The electrically-operated unloading valve described in the double cylinder case is also required here in the pressure main, and in addition a valve of very similar construction in the vacuum main. This latter valve, termed the Auxiliary Suction Valve, is designed to admit air into the compressor cylinder when the pressure on the delivery side falls very low, due to a high vacuum being maintained, which consequently means a reduced supply of air for pressure.
SECTION XL 266. In the case of power-worked house tubes, the transit time is a matter of a few seconds only and the flow of air is usually throttled down until a condition is arrived at which, while making sure that the carrier travels without sticking, also prevents it from reaching the switching apparatus with a momentum which would damage it or make unnecessary noise. 267. For street tubes the usual pressure provided is 10lb. per square inch above atmosphere and the vacuum 6.5lb. per square inch below atmosphere. 1-Tigher values than these are sometimes used where faster speeds are required, but economy in power is then much reduced. 268. For a given effective pressure, carriers travel most rapidly in pressure tubes when the air supply is intermittent, i.e., when the tube is connected to the mains during such times only as carriers are actually traversing the tubes. 269. In the case of carriers propelled by vacuum, if the exhaustion is not commenced until the carrier is inserted at the further end, then the carrier commences to move as soon as the exhaustion at that end reaches a few ounces only, and the fall exhaustion effect is not felt until the carrier has been moving for some time. When, however, the exhaustion is continuous, the carrier, on its insertion, practically starts off under the full pressure, hence its speed is greater than in the first case. The continuous system of vacuum working, where one tube is used for the "up" traffic only, is easily applied whether the traffic is heavy or light; but the intermittent system for "down," or pressure working, which, as pointed out, gives the quickest speed, cannot always be followed, as during the busy hours of the day the carriers have to be despatched in such quick succession that the current of air in the tube is practically, and in some cases quite, continuous. 270. The transit time of a carrier depends on the pressure or vacuum and the length and diameter of the tube. The theoretical figures which are closely borne out in practice, have been calculated and can be obtained from Tables VIII and IX, for all ranges of. pressure and vacuum, and for all lengths and sizes of tubes. 271. The theoretical indicated horse powers are also given, but these must be increased to allow for the losses in the compressor and belt. The efficiency is approximately 60 per cent., and the tabulated figures must therefore be multiplied by 1.66. 272. For a given transit time the actual horse rower required is much less in the case of vacuum than in the case of pressure working. Thus, for example, the transit time for 10lb. pressure is the same as for 6lb. vacuum, but the horse power required in the two cases would be as 1.83 to 1. Inasmuch, however, as the street tubes cannot in practice be worked by vacuum only, it is necessary to provide pressure in cases where outgoing traffic has to be dealt with.
SECTION XII 273. Signalling apparatus is provided on a tube of a pneumatic installation to draw the operator's attention when a carrier is being despatched and to signal its arrival at the receiving station. 274. On new hand-worked house tube installations electric bell signalling should be adopted. If speaking facilities are required house telephone instruments should be provided in addition. 275. Ordinary trembler bells, type 13A or 24A, with press buttons, type B, are now usually provided for hand-worked tubes, and if more than one tube has to be dealt with different toned bells or a multiple indicator can be used. 276. For power-worked house tubes on the low vacuum system, bell signalling is not required as the carriers are automatically ejected and the power is kept on continuously whilst the tube is in operation. 277. For house tubes worked from a street tube power system, "block instrument, pneumatic, for house tubes" is employed (Fig. 42). It consists of a mahogany wooden box, fixed vertically, in which two electro-magnets, a1 and a2 are arranged to attract a pivoted armature b carrying a needle c towards the coil through which current is passing. The index shows on one side "Carrier in Tube," and on the other" Tube Clear." When current passes through either coil a second armature d is attracted, and by this movement a bell dome e, mounted at the top, is struck. This block instrument, therefore, forms a combined means of signalling and indicating. Each electro-magnet coil is wound to a resistance of 150ohms, and it is intended to be connected on a 24volt universal battery. Two push buttons f1 and f2, are provided near the bottom of the instrument to give the necessary connections.
FIG. 42 - BLOCK INSTRUMENT, PNEUMATIC, FOR HOUSE TUBES 278. The block instrument for street tubes is larger and more sensitive. At present press buttons are not provided inside the street block instruments. 279. It is shown in Fig. 43 and the internal connections are also diagrammatically given. In this case the indicator electro-magnet coils a1 and a2, are separate from the bell coils b1, and b2, and consist of two 6ohm coils joined in series with a tapping taken off the centre point. The armature c, carrying the needle d, is of soft iron polarised by a permanent magnet e in the field of which it rotates. With a current passing through the bottom terminals of the instruments from right to left the needle shows "Carrier in Tube." The two bell coils are first wound to a resistance of 2ohms per coil and over this a second winding of 6ohms per coil is laid. By this movement of the armature of the bell electro-magnets the bell dome is struck in a similar manner to that of the house tube instrument. The two bell coil windings are joined up so that the outgoing current splits at the top right hand terminal and the currents in the two coils, by flowing in opposite directions, partially neutralise each other, and the bell is not rung at the home station. For incoming currents the two coils help each other and the bell signal is then given.
FIG. 43 - BLOCK INSTRUMENT, PNEUMATIC, FOR STREET TUBES 280. Secondary batteries, where available, are used both at the central and the out stations. Reverse currents are required at each station.
FIG. 44 - CONNECTIONS OF BLOCK INSTRUMENTS FOR STREET TUBES
FIG. 45 - CONNECTIONS OF BLOCK INSTRUMENTS FOR STREET TUBES, WITH INTERMEDIATE STATION 281. Figs. 44 and 45 give two wiring diagrams for block instruments for street tubes joined up for the following conditions:-
FIG. 46 - CONNECTIONS OF BLOCK INSTRUMENTS FOR STREET TUBES WITH
FIG. 47 - CONNECTIONS OF BLOCK INSTRUMENTS FOR STREET TUBES WITH 282, Figs. 46 and 47 show circuit arrangements in which, by means of power leads from secondary cells at the H.P.O., the use of primary batteries at the out offices can be dispensed with in the cases included under (44) in paragraph 281. The arrangements are in use experimentally at present and any proposals for extensions of the power leads principle should be referred to the Engineer-in-Chief for approval. 283. The signalling circuit provided for indicating the arrival of a carrier in a double slide switch, or on the grid signaller of a wooden receiving box, forms a separate circuit distinct from the block instrument circuit, the signal being given by a trembler bell, type 13A or 24A. 284. For street tubes worked by means of the above signalling apparatus it is essential for safe working that the "clear" signal should be received for one carrier before a second one is introduced. 285. Where a tube is worked in one direction only and it is necessary to increase the working capacity a Service Regulator is made use of. This instrument, working in conjunction with a block instrument, permits of a number of carriers passing through the tube at the same time, a definite time interval being preserved between the carriers, whilst means are also provided to prevent an accumulation of carriers should a block take place in the tube from any cause. 286. The Pneumatic Tube Service Regulator is illustrated in Fig. 48, and a wiring diagram showing the signalling circuit for a tube worked in this manner is also given.
FIG. 48 - PNEUMATIC TUBE SERVICE REGULATOR 287. When a carrier is inserted in the tube for transmission, the press button X is pressed. This causes a current from battery D (20volts) to pass through c (half coil of indicator), electro-magnet J, press button X, line, press Y, and bell SS (at the receiving office). This current (in passing through e) will move the indicator pointer to "Carrier in Tube," and in passing through electro-magnet J will cause armature C to be attracted; B will then strike against A, causing detent T to be disengaged from the teeth of wheel W, and allowing the arm L (to which the detent is loosely screwed) to be pulled up by spring a2 against stop S6. 288. The pulling up of L lowers the tail t correspondingly, and pin P1, which normally rests against the end of t, now drops between two of the teeth of wheel W, this being caused by the tension of spring a6, which pulls armature F forward, pressing pin P against the lower end of lever H (which is part of H); spring a1 has a weak tension, so that it is overcome by the stronger tension of a6. 289. Wheel W, which is kept in continuous slow rotation by a clockwork train, and which is geared to L by friction, now causes the right hand arms of levers L and L1 to move gradually down. 290. After a certain time, the length of which can be regulated by the position of stop pin S6 (against which L will be pulled by spring a2 when detent T is disengaged from the teeth of wheel W), pin P will touch contact m; this completes a circuit, causing a current from battery D to flow through d (half coil of indicator), 100ohm spark coil contacts b (which, when L is up, will be together), contact m, arm L, back to battery; this current (in passing through d) will move the indicator pointer to "Tube Clear." The arm L will continue its movement a slight distance beyond the point at which P makes contact with m; this causes contact to be eventually broken at b, so that, the circuit being interrupted, battery D ceases to send a current through ci. Arm L is finally arrested by stop pin S4. 291. Although arm L ceases to move, wheel W continues to rotate, as it is only geared to L by friction. The continued rotation of W causes the right hand end of L1 to continue to move gradually down, and eventually p (which is tipped with ebonite) will come against lever m, and will push it down until the contacts b separate. If, when this has taken place, the lever L should be moving down, then when p comes in contact with in, it will be unable to complete the circuit (as it is broken at b), and consequently a current cannot be sent through ci to move the indicator hand to "Tube Clear." 292. When L has moved to the position at which M is pressed down, then H1 will have moved up to the position at which the stop n comes opposite to pin P, spring a1 will then cause U to be pulled over to the left, so that n hitches over P, this left hand movement at the same time (since H1 and H are in one piece) pulling pin P1 out of the teeth of wheel W, which is thus left free to continue its rotation unimpeded. 293. If now electro-magnet N is excited, then armature F is attracted, and P becomes disengaged from n (for U comes against stop 55 and thus limits the left hand movement of H1, whilst P can move hard over to the left). This allows lever L, to fall back to its normal position against stop S, where it remains, for pin P1 (when F recoils and presses P against H1) comes against the tail t and cannot therefore drop between the teeth of wheel W. 294. Electro-magnet N is worked by a current sent from the Receiving Office by actuating press button Y, and this button is pressed every time a carrier is received; so that as long as the carriers continue to arrive, L1 is being continually disengaged from wheel W, and falls back to stop S1 and M, consequently, not being pressed down, the contacts b are not separated; p, therefore, under these conditions, when it touches m, is able to send the "Tube Clear" signal, which is not the case when M is pressed down. By this arrangement the "Tube Clear" signal can be given only so long as the carriers are passing freely through the tube, for if a stoppage takes place and carriers cease to be received, then the absence of the acknowledgment signal will in the course of a few minutes (about four) allow P1 to descend and (by pressing, down M) prevent the clear signal being given; thus indicating the existence of the stoppage. 295. The acknowledgment signal not only actuates the electro-magnet N but also rings the block instrument bell. If it should happen that, owing to inattention at the receiving office and consequent failure to give acknowledgment signals, arm L, has moved to the position at which it presses down M and separates contacts b, then the indicator pointer, if it were standing at "Carrier in Tube" when this took place, would remain in the same position, since although contact p of arm L may come against m, there is no circuit through d, contact being broken at b. Immediately, however, the receiving office commences again to send acknowledgment signals, the movement of armature F pulls the ebonite link Q and actuates K, bringing the contacts at b together, and by completing the circuit of d moves the indicator to "Tube Clear " again. 296. Instruction Cards T.E. 82, giving the Code of Signals for working pneumatic tubes, are stocked and should be fixed in a prominent position at each station close to the tube terminal. A reproduction of this card is given on next page. 297. A
smaller card, T.E. 509, is also issued
giving the special code of signals to be made by engineering staff when the
tube is taken for test or other purposes. |
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Last revised: June 05, 2023FM2 | |||||||||||||||||||||||||||||||||||||
