STATIONARY ENGINES.

221

STAND PIPES, ETC.

Stand pipes were originally introduced to equalize the weight onthe engines, and give the required pressure in the main pipes forthe town supply. The arrangement under notice was erected atTettenhall , and consisted of two pipes, one of them open at the top,inclosed in a tower of brickwork, as shown in Fig. 140.

We are indebted to Mt. Marten, C.E., for the following descrip-tion of its action:—The engine having only steam on the underside of the piston lifted the pump rods, and their own weightwas just sufficient to bring them down along with the plunger ofthe pump if the stand pipe was only full to the junction at the top,but it was not enough to let them force the water to the top of thetower. When the town required all the water the engine pumpedregularly, being worked by a cataract to give the requisite numberof strokes per minute; but if the town did not take the water theengine stood, because the rods were not heavy enough to make thedown stroke. Whenever the town drew off some water the enginesstarted again. The state of the water in the stand pipe was shownin the engine house by a mercury gauge, and the engines wereregulated to keep the stand pipe full up to the junction. Therewere no escape valves, because they were not needed; if by anychance the sudden shutting of the large mains in the town, or airreturning up the main, threw the water over the top of the standpipe, it filled the cap of the tower and ran out at small holes, fallinglike rain, but this very seldom happened. When there is muchdanger of overflow near a town an overflow pipe is fitted, whichallows the water to flow into the reservoirs from which it is pumped.At Tettenhall there was no provision made for brealdng the fall ofthe water in the descending leg of the stand pipe. This want causedmuch air to be carried into the mains, so that the water when firstdrawn-was often as white as milk with the minute bubbles of air,but it cleared in a very short time. The chief use of the stand pipewas to render undue pressure on the mains impossible, as there wasat first no reservoir. When a reservoir exists, however, always opent'o the pumping main, it serves the purpose of a stand pipe, andprevents any undue pressure.

In some cases, as the South Staffordshire water-works at Bromhills(Fig. 141), the stand pipe is placed on a hill on the line of the main,about half-way between the engine at Lichfield and the main reser-