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Pump Terminology

Absolute Pressure

The absolute pressure is the sum of the gauge or dial pressure and the atmospheric pressure (14.7psi). If your pump gauge reads 150 psi, then the absolute pressure will be 164.7 psi.

Absorbed Power

This is the sum of the hydraulic power (work done by achieving the flowrate and head) and the frictional losses.


The rate of change of velocity. As velocity is measured in distance per unit time (e.g. m/s), acceleration is measured in change in velocity per unit of time (e.g. m/s2).


A device that measures proper acceleration. Not necessarily the rate of change of velocity, but the weight experienced by a mass at rest in the frame of reference of the device. An accelerometer at rest on the surface of the earth would measure an acceleration of 9.8 m/s2 normal to the surface due to gravity.


Anything pertaining to sound or to waves generated by sound.

Acoustic Resonance

Can occur in any closed or open pipe system due to pump excitation pulsation frequency. This happens when the return of the reflected pressure wave coincides with the generation of the next pulse of pressure. In this case a standing wave is formed inside the fluid filled chamber. This can cause catastrophic failure of systems.

Active Metal

Prior to passivation, the formation of a thin oxide on the surface of metals to protect them from corrosion, the metal is known as “active”.


A coupling that connects the pump to the rest of the system. Allows the pump to integrate with the downstream parts of the system.

Affinity Laws

They are a set of laws that relate the variables of performance, (impeller shaft speed, head, flow rate) to power. They are useful to compare the performance of different pumps.


The gap between the tips of the blades on an impeller and the inner diameter of the case bore. If the A-gap is too small, this can be a factor in high pump vibration whereas, if the A-gap is too large, recirculation may occur, leading to pump inefficiency.

Air Ingestion

Air leaks into the pump, causing cavitation and pump performance degradation. Air ingestion can be due to leaking flanges, valves located above the fluid line, porous intake lines, faulty shaft seals among others.


In the pump world, alignment refers to the alignment of the centerline of the pump shaft with the centerline of the driver shaft. This is critical, because a small amount of misalignment on the power end, results in a large amount at the wet end, where the seals are usually located.


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