Glossary 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.