Thus, in the case of the delivery air chamber, when the pump piston is travelling at a speed above its average for the stroke, the water accumulates in the air chamber, and the air is more compressed, while, when the pump is on the dead centre, or at the end of its stroke and the delivery valve closes, the air compressed in the air chamber continues the delivery or discharge, thus maintaining a more uniform flow.
Pumps sometimes have an air or vacuum chamber on the suction side, from which the air is exhausted when the pump starts, leaving a vacuum which causes a steady flow of water up the suction pipe.
Both these chambers are more effective as the speed of the pump increases. The chamber on the delivery side is apt to lose its air, which is gradually absorbed by the water, which should be let out when the pump is standing still.
Feed escape valves or feed relief valves are fitted to the feed pumps, so that in case all the feed water cannot pass into the boiler it may pass back to the hot well.
The construction of a feed escape valve is as follows:
It is an ordinary mitre valve held to its seat by the compression of a spiral spring, whose pressure upon the valve may be regulated by an adjusting screw, whose end abuts upon a stem provided for the purpose.
In proportion as the valve is relieved of the pressure of this spring, a greater proportion of the water delivered by the feed pump will pass back into the hot well, hence the amount of boiler feed may be regulated by the feed escape valve, which also acts as a safety valve, preventing undue pressure in the feed pipe.
When no feed escape valve is employed, the delivery water from the feed pump must pass unobstructed to the boiler, or the feed pipes may burst from over pressure, and it follows that the feed check valve on the side of the boiler must not be restrained in its amount of lift, hence it must not have a lift adjusting screw.
The amount of the boiler feed must, in this case, be regulated from the suction side of the pump, the suction pipe being fitted with a cock or valve whose amount of opening may be adjusted so as to regulate the amount of water drawn per pump stroke from the hot well.
If the feed valve on the suction side, or the escape valve on the delivery side of the pump, as the case may be, is adjusted to permit of a proper amount of boiler feed, and yet the feed is insufficient or ceases altogether, it may occur from the following causes: