DISTRIBUTION OF GAS.
From the gas-holder the gas is conveyed to the consumers by means of main pipes, laid under the surface of the streets, from which branch or service pipes are led to the houses. The pressure on the mains, which varies in ordinary practice from to 4 inches of water pound to + pound per square inch ), is furnished by the weight of the gas-holder, and is regulated to meet the variation in the demand for gas by a governor on the holder outlet. This governor consists of a valve fastened to an inverted bell sealed in water, the weight of the valve and bell being supported by the pressure of the gas in the mains. If this falls, the bell falls, opening the valve, and so, by allowing more gas to pass, brings the pressure back to the proper point. The amount of pres sure can be varied by the use of removable weights to vary the total weight to be supported.
The main pipes vary in from 3 to 48 inches. They are usually cast-iron bell and spigot pipes, made in lengths 12 feet long, which are connected together with lead or cement joints, but wrought-iron pipe with screwed joints is sometimes used for the smaller sizes. The services are always made of wrought iron pipe. ( See PIPES.) The mains must be laid so as to drain to certain points, at which provision is made for removing the water and such hydrocarbon vapors as condense from the gas, and the services should drain into the mains.
During the past two or three years the use of high pressure (10 pounds to 20 pounds per square inch) has been advocated for the distri bution of gas in localities having a scattered population, and several distribution systems using this pressure have been installed, and are now being operated. In such systems wrought iron pipe is used exclusively.
The amount of gas supplied to each consumer is measured by means of consumers' meters, which are of two kinds, wet meters and dry meters. The wet meters are similar to the
station meter, and were the first ones employed; but on account of various difficulties connected with their use, the chief of which in cold climates is the danger of freezing, they have, in the United States, been almost entirely replaced by dry meters. A dry meter consists of a rectangular box, made of tin plate, divided into two main compartments by a horizontal partition. The lower of these compartments is also divided into two equal parts by a vertical partition. The measuring apparatus consists of two bellows, one in each of the divisions of the lower com partment, each formed by a circular metal disk, to the circumference of which is fastened one edge of a leather diaphragm having its other edge fastened to the central partition, the whole forming a gas-tight space. The alternate opening and closing of these bellows by the pressure of the gas as it is admitted, first into the spaces inside and then into the spaces outside of them, furnish es motion which, by suitable mechanism, is made to operate valves controlling the flow of gas into and out of the bellows and outer spaces in such a way that gas cannot pass simultaneously into and out of any given space, and also to work the train of gears which records the amount of gas passed through the meter. The mechanism also controls the extent to which the bellows can open and close, so that a fixed and definite vol ume of gas passes into and out of the meter each time one is filled and emptied. The house pipes, which are usually wrought iron, should drain to the meter, where any condensation can be run off if necessary.