STATIONARY ENGXNES.

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larly at full speed, and therefore a reservoir to receive the pumpedwater should be provided if possible.

When pumping under a heavy pressure it is usual to have an airvessel to each engine on the delivery pipe beyond the pumps;and sometimes a larger one is placed on the main pipe into whichthe others deliver. Of course care must be taken that each airvessel has its full complement of air; sufficient is usually drawn in

Fig. 142.—Air Vessels.

a a, Pumps, b b, Air vessels. C C, Sluice valves. d, Main air vessel. e, Main pipe to the town.

by the pump, and a very small hole or tap is sometimes inserted tosupply it.

When more than one pump is arranged for pumping into an airvessel, stop valves must be fitted on the delivery pipe, to preventthe return of the water when either or both pumps are not atwork. The air vessel is of great importance, as it equalizes the flowof the water through the main,

and less weight is required onthe top of the plunger for thedown stroke. The capacity ofthe air vessel should be aboutten times the volume of waterdelivered by each stroke of thepump.

A relief valve should be placedon the delivery pipe to preventundue pressure; it is fitted witha lever and weight. In someexamples a solid plunger isadopted, having a piston androd at the top, the piston work-ing loosely in a cylinder connected to the main by a small pipe.The plunger A rises when the pressure increases, being larger at Athan at C, and allows the water to flow through the slots into the

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Fig. 143.—Relief Valve.

A, Solid plunger. b, Piston. C, Slotted pipe leadingto reservoir. d, Pipe connecting the cylinder withthe main. E, Waste pipe leading to the reservoir.