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Glossary of Pump Terms: V
Variable speed motor

Used to control flow in a system by varying the frequency of the motor. Recommended for circulating systems and any other system where the main head is friction losses in the piping system.


A German standard for mechanical seals.


Any pressure less than atmospheric. Can present a problem for the elastomer in many seal applications.

Vane passing syndrome

A type of cavitation caused by the impeller/ cutwater clearance being too small.

Vapor pressure

Below this pressure, the liquid at this temperature will vaporize.


The fluid passes from a liquid to a gaseous state. If this happens at the seal faces the seal faces will be blown open.


A measurement of the speed of the liquid in the system. Measured in feet or meters per second. The pump is a constant velocity device.

Velocity head

Part of the total head calculation. Derived from the formula h = V2/2g


To remove air or gas from the system. It is important to vent the stuffing box in vertical pumps to prevent the seal faces from running dry.

Vibration Damping

Important in metal bellows seal designs. The elastomer acts as a vibration damper in the other seals. The vibration can chip carbon faces, destroy anti-rotation drive lugs, and open the lapped seal faces.


Resistance to pouring. Higher viscosities can restrict seal movement. Centrifugal pumps can handle a maximum viscosity similar to 30 weight oil at room temperature. Above this viscosity a positive displacement pump should be used.


An E.I. Dupont Dow manufactured elastomer widely used in the sealing industry. The generic name is fluorocarbon. Many of these compounds are attacked by water and steam.

Volute casing

Derives is name from a spiral shaped casing surrounding the pump impeller. It converts velocity energy to pressure energy.

Vortex Pump

A type of pump used for excessive solids. The impeller is recessed into the volute. A very low efficiency design, but practical in many applications.

Vortexing liquid

Creating a "whirlpool affect" that can draw air into the suction of the pump.

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