It will, however, be more efficient with the addition of either of them, and most efficient with both.
A bucket pump with a foot valve and no discharge valve would, however, suffer more from a leaky gland than if it had a discharge valve and no foot valve, because the air would, on the ascent of the bucket and the closing of the bucket valve, pass to the suction side of the bucket and impair the vacuum.
Let the delivery valves be where they may, the foot valve will always have some water above it, and the pump bucket will dip into this water, and on lifting produce a vacuum that will cause the pump to fill with water. Notwithstanding that the gland may leak air on the other side of the bucket, this air will in a single acting pump be expelled with the water, but in a double acting pump it will impair the vacuum, and therefore the suction, on the gland side of the piston.
Bucket air pumps are provided with a valve or pet cock on the top or delivery side of the bucket and above the bucket, when the latter is at the highest point of its stroke. This valve opens on the descent of the bucket, admitting air to act as a cushion between the surface of the water and the delivery valve, when the water is about to meet the latter. It obviously reduces the effectiveness of the pump, and in a double acting pump is inadmissible, because of its impairing the vacuum and the suction.
This valve also enables the engineer to know whether the air pump is working properly.
A pet cock is also supplied to the feed pumps for this same purpose.
A bilge injection is one in which the injection water is taken from the bilge, which may be done when the ship makes more water than the bilge pumps can get rid of.
The fittings necessary for a bilge injection are a cock or globe valve placed on the side of the condenser, and at or near the foot of the exhaust pipe, with a spray or rose inside that pipe. From the cock a pipe, usually lead, leads to the bilge, having at its end a strainer or strum, and care must be taken that this strum does not get choked and let the condenser get hot from the exhaust steam not being condensed.
The water in the hot well of a surface condenser is usually kept at a temperature of about 100° Fahrenheit. A higher temperature than 100° Fahrenheit injures the rubber valves of the air pump, while lower temperatures cool the engine cylinders too much and cause waste from cylinder condensation. Moreover, it is obvious that, since the boiler feed is taken from the hot well, it is desirable to keep it as hot as the valves and as the desired degree of vacuum will permit.
An air vessel or air chamber is a vessel fitted to the delivery and sometimes also to the suction side of a pump. Its office is to maintain a steady flow of water through the pipes.