This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
To provide for the easy removal of a damaged or broken pipe, in any case where there is a considerable run of pipe or where there are several fittings, a Connector (Fig. 103) should always be inserted. This consists of a short length of pipe with the ordinary screw at one end and a longer screw than usual at the other, together with a socket and backnut. This enables the socket to be run back beyond the end of the pipe. The end of the pipe to be connected is then brought into line with the long screwed end of the connector, the socket being screwed back over the joint between the two pipes, the backnut being afterwards screwed hard back against the inside end of the socket. To remove a portion of the pipe at any time afterwards, it is only necessary to run back the backnut and socket when the butt jointed ends of the two pipes are exposed, and either pipe can be easily unscrewed and taken out. Frequently considerable expense and trouble would be saved if this simple means of removal were provided more generally than is the case.
A slight inclination or fall should always be given from the service main to the street main, to allow any condensed water which may form in the pipes to drain away, and here attention may be drawn to the need of care being exercised in the work of laying in the service main. On no account should the pipes be laid so that they sag or dip. Great care should be taken that there is a continuous fall, for should a dip or sag occur it at once forms a place for the lodgment of water, and this will cause the lights to jump, and if the trouble be very considerable possibly form a trap which will stop the supply of gas passing through the pipe altogether. Where a fall cannot be obtained to the street main, the service main may be inclined to a siphon box such as is shown in Fig. 104, into which the condensed water can drain. This box must be easily accessible to enable the water to be removed from time to time by a siphon pump or other suitable means. Again, the service may be drained into a siphon formed close to the meter, as shown in Fig. 105 on the outlet side of the meter. This siphon is made up by means of a wrought-iron screwed tee piece, with a large centre branch into which a short length of pipe is screwed. The lower end of this pipe is filled with a screwed cap to enable any condensed water which might accumulate there at any time to be drained away.
The most generally used meter is that known as the Dry Meter, which is shown in Fig. 105, fitted up in position. Meters should be fixed as far as possible in a temperate position, and where the temperature is not too variable. The point to be observed in this respect is that gas contains more vapour at a low temperature than at a high one and the object should be to keep it as nearly as possible at the same temperature throughout the whole installation as when it leaves the main; for the lower the temperature of the gas falls, the greater the likelihood there is of trouble arising from water. The meter, too, should always be perfectly protected from the action of frost, and particularly should this precaution be observed in the case of wet meters.
The fixing of meter is generally done by the Gas Company, and the unions for the inlet and outlet are supplied by them attached to the meters; but should the gas fitter have to do the work he should not cramp the meter for room, but if possible allow sufficient space to enable it to be taken out at any time without disturbing any of the adjoining work; so that should it be required at any time to extend the system of heating or lighting a larger meter could be put in; and as the inlet and outlet pipes are of lead and easily bent and pliable, given sufficient room, this is not so expensive and troublesome a matter as it otherwise would be. The capacity of a meter is denoted by the number of lights it is calculated to supply. The accompanying table gives a list of meters which may be obtained from stock, and will be useful as a means of calculating for a meter.
Meters with a capacity exceeding 300 lights are not kept in stock by the makers, but are specially made to order. The makers as a rule give a guarantee with each meter for a specified time, usually 5 years.
The pressure at which gas is delivered into the mains varies from 2 to 3 inches head of water pressure. This is reduced considerably by the time it reaches the points where it enters the branches for the supplies to a building, where the pressure there would not be less than 14/10; and this would probably be reduced to as little as 5/10 by the time it reaches the burners.
The pressure decides the rate of the flow of gas in pipes, but for ordinary purposes it is not necessary to go into the velocity and volume of gas through a particular pipe, although the following is a formula by which this can be ascertained: -
Q = 1000
D = 0.063
G = 0.45 H = 1/2 to 1 inch.
Q = Quantity of gas in cubic feet per hour. L = Length of pipe in yards. D = Diameter of pipe in inches. H = Head of water pressure in inches. G = Specific gravity of gas.
As a general rule no pipe should be less than 3/8 inch internal diameter, although one or two lights can be and often are taken off a 1/4-inch pipe. A very important point to bear in mind in gasfitting is, that it is always better and more economical to have a good volume of gas at a lower pressure to supply a burner than to have a restricted area in your pipe and trust to the extra pressure to provide the amount of gas you require. With a fair ordinary gas supply the following may be considered a good practical table for the number of lights which can be safely taken off the respective pipes:- To supply up to -
internal diameter of pipe should not be less than
l 1/4 "
1 1/2 "
The size of the pipes may be reduced on the higher floors of a building, as the velocity is greater at high levels than lower ones.
Table Showing Sizes And Capacities Of Dry Gas Meters
Size of Meter
Cubic Feet per Hour.