PIPES AND FITTINGS.--As a general rule it is unadvisable to use lead or composition pipe for permanent acetylene connexions. If exposed, it is liable to be damaged, and perhaps penetrated by a blow, and if set in the wall and covered with paper or panel it is liable to be pierced if nails or tacks should at any time be driven into the wall. There is also an increased risk in case of fire, owing to its ready fusibility. If used at all--and it has obvious advantages--lead or composition piping should be laid on the surface of the walls, &c., and protected from blows, &c., by a light wooden casing, outwardly resembling the wooden coverings for electric lighting wires. It has been a common practice, in laying the underground mains required for supplying the villages which are lighted by means of acetylene from a central works in different parts of France, to employ lead pipes. The plan is economical, but in view of the danger that the main might be flattened by the weight of heavy traction-engines passing over the roads, or that it might settle into local dips from the same cause or from the action of subterranean water, in which dips water would be constantly condensing in cold weather, the use of lead for this purpose cannot be recommended. Steam-barrel would be preferable to cast pipe, because permanently sound joints are easier to make in the former, and because it is not so brittle.
The fittings used for acetylene must have perfectly sound joints and taps, for the same reasons that the service-pipes must be quite sound. Common gas-fittings will not do, the joints, taps, ball-sockets, &c., are not accurately enough ground to prevent leakage. They may in many cases be improved by regrinding, but often the plug and barrel are so shallow that it is almost impossible to ensure soundness. It is therefore better to procure fittings having good taps and joints in the first instance; the barrels should be long, fairly wide, and there should be no sensible "play" between plug and barrel when adjusted so that the plug turns easily when lightly lubricated. Fittings are now being specially made for acetylene, which is a step in the right direction, because, in addition to superior taps and joints being essential, smaller bore piping and smaller through-ways to the taps than are required for coal-gas serve for acetylene. It is perhaps advisable to add that wherever a rigid bracket or fitting will answer as well as a jointed one, the latter should on no account be used; also water-slide pendants should never be employed, as they are fruitful of accidents, and their apparent advantages are for the most part illusory. Ball-sockets also should be avoided if possible; if it is absolutely necessary to have a fitting with a ball-socket, the latter should have a sleeve made of a short length of sound rubber-tubing of a size to give a close fit, slipped over so as to join the ball portion to the socket portion. This sleeve should be inspected once a quarter at least, and renewed immediately it shows signs of cracking. Generally speaking all the fittings used should be characterised by structural simplicity; any ornamental or decorative effects desired may be secured by proper design without sacrifice of the simplicity which should always mark the essential and operative parts of the fitting. Flexible connexions between the fixed service-pipe and a semi-portable or temporary burner may at times be required. If the connexion is for permanent use, it must not be of rubber, but of the metallic flexible tubing which is now commonly employed for such connexions in the case of coal-gas. There should be a tap between the service-pipe and the flexible connexion, and this tap should be turned off whenever the burner is out of use, so that the connexion is not at other times under the pressure which is maintained in the service-pipes. Unless the connexion is very short--say 2 feet or less--there should also be a tap at the burner. These flexible connexions, though serviceable in the case of table-lamps, &c., of which the position may have to be altered, are undesirable, as they increase the risk attendant on gas (whether acetylene or other illuminating gas) lighting, and should, if possible, be avoided. Flexible connexions may also be required for temporary use, such as for conveying acetylene to an optical lantern, and if only occasionally called for, the cost of the metallic flexible tubing will usually preclude its use. It will generally be found, however, that the whole connexion in such a case can be of composition or lead gas-piping, connected up at its two ends by a few inches of flexible rubber tubing. It should be carried along the walls or over the heads of people who may use the room, rather than across the floor, or at a low level, and the acetylene should be turned on to it only when actually required for use, and turned off at the fixed service-pipe as soon as no longer required. Quite narrow composition tubing, say 1/4-inch, will carry all the acetylene required for two or three burners. The cost of a composition temporary connexion will usually be less than one of even common rubber tubing, and it will be safer. The composition tubing must not, of course, be sharply bent, but carried by easy curves to the desired point, and it should be carefully rolled in a roll of not less than 18 inches diameter when removed. If these precautions are observed it may be used very many times.
Acetylene service-pipes should, wherever possible, be laid with a fall, which may be very slight, towards a small closed vessel adjoining the gasholder or purifier, in order that any water deposited from the gas owing to condensation of aqueous vapour may run out of the pipe into that apparatus. Where it is impossible to secure an uninterrupted fall in that direction, there should be inserted in the service-pipe, at the lowest point of each dip it makes, a short length of pipe turned downwards and terminating in a plug or sound tap. Water condensing in this section of the service-pipe will then run down and collect in this drainage-pipe, from which it can be withdrawn at intervals by opening the plug or tap for a moment. The condensed water is thus removed from the service-pipe, and does not obstruct its through-way. Similar drainage devices may be used at the lowest points of all dips in mains, though there are special seal-pots which take the place of the cock or plug used to seal the end of the drainage-pipe. Such seal-pots or "syphons" are commonly used on ordinary gas-distributing systems, and might be applied in the case of large acetylene installations, as they offer facilities for removing the condensed water from time to time in a convenient and expeditious manner.
EXPULSION OF AIR FROM MAINS.--After a service-pipe system has been proved to be sound, it is necessary to expel the air from it before acetylene can be admitted to it with a view to consumption. Unless the system is a very large one, the expulsion of air is most conveniently effected by forcing from the gasholder preliminary batches of acetylene through the pipes, while lights are kept away from the vicinity. This precaution is necessary because, while the acetylene is displacing the air in the pipes, they will for some time contain a mixture of air and acetylene in proportions which fall within the explosive limits of such a mixture. If the escaping acetylene caught fire from any adjacent light under these conditions, a most disastrous explosion would ensue and extend through all the ramifications of the system of pipes. Therefore the first step when a new system of pipes has to be cleared of air is to see that there are no lights in or about the house--either fires, lamps, cigars or pipes, candles or other flames. Obviously this work must be done in the daytime and finished before nightfall. Burners are removed from two or more brackets at the farthest points in the system from the gasholder, and flexible connexions are temporarily attached to them, and led through a window or door into the open air well clear of the house. One of the brackets selected should as a rule be the lowest point supplied in the house. The gasholder having been previously filled with acetylene, the tap or taps on the pipe leading to the house are turned on, and the acetylene is passed under slight pressure into the system of pipes, and escapes through the aforesaid brackets, of which the taps have been turned on, into the open. The taps of all other brackets are kept closed. The gas should be allowed to flow thus through the pipes until about five times the maximum quantity which all the burners on the system would consume in an hour has escaped from the open brackets. The taps on these brackets are then closed, and the burners replaced. Flexible tubing is then connected in place of the burners to all the other brackets in the house, and acetylene is similarly allowed to escape into the open air from each for a quarter of an hour. All taps are then closed, and the burners replaced; all windows in the house are left open wide for half an hour to allow of the dissipation of any acetylene which may have accumulated in any part of it, and then, while full pressure from the gasholder is maintained, a tap is turned on and the gas lighted. If it burns with a good, fully luminous flame it may be concluded that the system of pipes is virtually free from air, and the installation may be used forthwith as required. If, however, the flame is very feebly luminous, or if the escaping gas does not light, lights must be extinguished, and the pipes again blown through with acetylene into the open air. The burner must invariably be in position when a light is applied, because, in the event of the pipes still containing an explosive mixture, ignition would not be communicated through the small orifices of the burner to the mixture in the pipes, and the application of the light would not entail any danger of an explosion.
Gasfitters familiar with coal-gas should remember, when putting a system of acetylene pipes into use for the first time, that the range over which mixtures of acetylene and air are explosive is wider than that over which mixtures of coal-gas and air are explosive, and that greater care is therefore necessary in getting the pipes and rooms free from a dangerous mixture.
The mains for very large installations of acetylene--e.g., for lighting a small town--may advisedly be freed from air by some other plan than simple expulsion of the air by acetylene, both from the point of view of economy and of safety. If the chimney gases from a neighbouring furnace are found on examination to contain not more than about 8 per cent of oxygen, they may be drawn into the gasholder and forced through the pipes before acetylene is admitted to them. The high proportion of carbon dioxide and the low proportion of oxygen in chimney gases makes a mixture of acetylene and chimney gases non-explosive in any proportions, and hence if the air is first wholly or to a large extent expelled from a pipe, main, or apparatus, by means of chimney gases, acetylene may be admitted, and a much shorter time allowed for the expulsion by it of the contents of the pipe, before a light is applied at the burners, &c. This plan, however, will usually only be adopted in the case of very large pipes, &c.; but on a smaller scale the air may be swept out of a distributing system by bringing it into connexion with a cylinder of compressed or liquefied carbon dioxide, the pressure in which will drive the gas to any spot where an outlet is provided. As these cylinders of "carbonic acid" are in common employment for preparing aerated waters and for "lifting" beer, &c., they are easy to hire and use.
TABLE (B).
Giving the Sizes of Pipe which should be used in practice for Acetylene when the fall of pressure in the Pipe is not to exceed 0.1 inch. (Based on Morel's formula.)
_________________________________________________________
| | |
| Cubic Feet of | Diameters of Pipe to be used up to |
| Acetylene | the lengths indicated. |
| which the Pipe |_______________________________________|
| is required to | | | | | |
| pass in | 1/4 | 3/8 | 1/2 | 3/4 | 1 |
| One Hour. | inch. | inch. | inch. | inch. | inch. |
|________________|_______|_______|_______|_______|_______|
| | | | | | |
| | Feet. | Feet. | Feet. | Feet. | Feet. |
| 1 | 520 | 3960 | 16700 | ... | ... |
| 2 | 130 | 990 | 4170 | ... | ... |
| 3 | 58 | 440 | 1850 | ... | ... |
| 4 | 32 | 240 | 1040 | ... | ... |
| 5 | 21 | 150 | 660 | 5070 | ... |
| 6 | 14 | 110 | 460 | 3520 | ... |
| 7 | 10 | 80 | 340 | 2590 | ... |
| 8 | ... | 62 | 260 | 1980 | ... |
| 9 | ... | 49 | 200 | 1560 | ... |
| 10 | ... | 39 | 160 | 1270 | 5340 |
| 15 | ... | 17 | 74 | 560 | 2370 |
| 20 | ... | 10 | 41 | 310 | 1330 |
| 25 | ... | ... | 26 | 200 | 850 |
| 30 | ... | ... | 18 | 140 | 590 |
| 35 | ... | ... | 13 | 100 | 430 |
| 40 | ... | ... | 10 | 79 | 330 |
| 45 | ... | ... | ... | 62 | 260 |
| 50 | ... | ... | ... | 50 | 210 |
|________________|_______|_______|_______|_______|_______|
TABLE (A).