Graham Corporation
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C) Hard Water
If hard water is used as the service liquid, scale deposits caused by the precipitation of minerals
will occur. This will vary with the temperature of the water. Scale deposits on the internal
surfaces of the pump will cause an increase of the operating horsepower, wear on moving parts,
and may cause the pump to seize. If the hardness of the water is excessive, consider using a
water softening treatment.
3.4 Cavitation
Cavitation is identified by a characteristic metallic or grinding noise inside the pump. It is
caused when the pump suction pressure is too close to the vapor pressure of the service liquid.
If the service liquid temperature inside the pump rises such that its vapor pressure closely
approaches the suction pressure of the pump, cavitation will occur.
When cavitation takes place, vapor bubbles form and collapse within the liquid ring. This will
damage the surfaces of the impeller, side plates, and casing. Cavitation causes damage by
tearing away metal particles. The damage may be more severe in a corrosive situation.
Cavitation may be prevented by bleeding air into the pump to raise the suction pressure.
Vacuum relief valves can be fitted in the suction piping for this purpose. If the pump is
provided with an air attenuation valve, it can be opened to bleed air into the second stage
casing until the noise stops.
If the problem is not caused by a low flow of non-condensable gases, the service liquid
temperature should be checked. Ultimately, the vacuum at which the pump can be operated is
governed by the vapor pressure of the service liquid inside the pump.
3.5 Shut-Down Procedures
1) Shut off the service liquid supply and if used, the shaft seal coolant, and
immediately stop the motor.
2) If necessary, close all suction and discharge valves.
3) If necessary, drain the pump to protect it from freezing by removing all drain plugs.
4) Disconnect power from the motor if maintenance is to be performed.
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