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Pumps 101
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Glossary of Pump Terms: P
P.D. Pump

Positive displacement pump. It can pump a high pressure or head, but at a low volume.

P.V. Factor

An attempt to correlate the relationship between the pressure and the velocity at the mechanical seal faces. Unfortunately carbon graphite is sensitive to P, but not to V so the correlation does not work too well.


The soft rings that mechanical seal replace to stop leakage. Packing must leak because it works on the theory of a series of pressure drops to reduce the stuffing box pressure to the point where the leakage is acceptable. A minimum of five rings of packing is required to do this.

Parallel operation

The pumps are discharging to a common header. It is important that the impeller speed and outside diameters be the same or one of the pumps will throttle the other.


A metric unit for pressure. 100 kPa = one atmosphere.


The protective oxide layer (ceramic) has formed on the metal surface.


A measure of the acidity or the alkalinity of a fluid. The scale ranges from 0 (acid) to 14 (alkali) with 7 considered neutral.


Same as the specifications.

Pipe strain

The strain on the pump volute caused by the piping. It will cause excessive mechanical seal movement and can cause contact between rotating and stationary pump and seal components.


Surface voids caused by corrosion, erosion or cavitation.

Plasma spray

A method of putting a hard coating on a base material. Not recommended for mechanical seal faces.

Plastic range

If you stress metal enough, it stretches through the elastic range, its yield point and ends up in the plastic range where it loses its memory.

Positive displacement pump

Called a PD pump. Gear, sliding vane, progressive cavity, lobe etc. the capacity determined by the pump speed. The maximum head is determined by the horsepower available and the casing strength.

Power end

The end of the pump that attaches to the power source and does not get wet from the pumpage. The bearings are in this part.

Power factor

A measure of how the voltage leads or lags the amperage.

Plastic range

The metal has been stressed beyond its yield point and will not return to its original shape.

Precision bearing

Ball or roller bearing as opposed to a sleeve or babbitt bearing.

Pressure drop

Referring to the loss of pressure from the outside to the inside of the mechanical seal faces or across the individual rings of packing.

Press fit

The best way to insert a carbon/graphite face into a metal holder. The carbon will shear to conform to the holder's irregular shape. Usually done with an arbor press.

Pressure gradient

The pressure drop between the seal faces. Usually illustrated by a wedge.

Pressure head

The pump head exerted by atmospheric pressure or any additional pressure that might be in the vessel.

Pump curve

A diagram supplied by the pump manufacture to describe the relationship between the head and the capacity of a particular pump using various size impellers. The curve also include information about efficiency, horse power consumption, N.P.S.H. required, etc.

Pump out vane

Located behind the impeller shroud in some impeller designs to lower stuffing box pressure. Should no be used in hot well condensate pumps or any pump running with a negative stuffing box pressure.

Pumping ring

Used with a convection system to get circulation between two mechanical seals. Absolutely necessary if oil is used as a barrier fluid because of oil's poor specific heat..

Pusher seal

A design that has a spring loaded dynamic elastomer or rubber like part. A very poor design that should be avoided.

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