15.2 Chemical injection pumps
These are used to pump a measured quantity of chemical into the boiler feedwater. The pump must retain repeatable accuracy over long periods and positive displacement designs are used for this purpose, built with suitable corrosion-resistant materials.
The principle of operation of a normal metering pump is that a plunger displaces a certain amount of liquid with each stroke. Liquid is drawn into the pump head through a non-return valve and displaced out of another non-return valve. The gear drawing the plunger has a mechanism for altering the stroke length to effect a variation in pump delivery, thereby allowing close control of the amount of chemical injected. The positive displacement action results in a series of pulsations of flow rather than a steady feed. This can be smoothed out, if necessary, by the use of an air vessel.