Low viscosity – In most fluids, viscosity decreases with increasing temperature. Maximum allowable pump or fluid temperature set point can prevent a pump from operating beyond minimum allowable conditions.
Low volumetric efficiency – When internal recirculation or slip in a rotary pump approaches 50 percent of its displacement (typically from low-speed operation, internal wear or both), the pump differential temperature will not stabilize, and the pumped liquid temperature can exceed maximum allowable pump or fluid temperature.
Vapor pressure – Vapor pressure increases with increasing temperature. For volatile fluids, increasing temperature may cause the pumped fluid to vaporize in the pump inlet and cause the pump to cavitate.
Relief valve, system recirculation or bypass valve open – Rotary pumps require relief valves for overpressure protection. Recirculation valves are used to maintain fluid system pressure. Bypass valves are often used to control flow to the system. If the relief valve or bypass valve is partially or fully open, then the temperature of the exhaust increases proportionately to the oil horsepower bypassed. If the exhaust recirculates to the inlet side of the pump, then the pumped liquid temperature can quickly exceed the maximum allowable pump or fluid temperature, resulting in catastrophic pump failure.
Dry run – With no liquid to dissipate heat, temperature will rise rapidly. Pump failure is likely.
See HI’s new standard, ANSI/HI 9.6.9 Rotary Pumps – Guidelines for Condition Monitoring.
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