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Glossary of Pump Terms: H

Elements that attack carbon. Fluorine, bromine, chlorine, astatine and iodine.

Hard face

A seal face either rotating or stationary. The most common materials are silicone carbide, ceramic, tungsten carbide, Stellite, Ni-resist. The hard face must be the wider seal face.

Harmonic vibration

Vibtrating in harmony with something near by. This can be a big problem for bearings in stationary or non running equipment.

Hastelloy "C"

A nickel rich, corrosion resistant metal used for mechanical seal springs and metal bellows because it is not sensitive to chloride stress corrosion.


The equivalent height of the liquid. 20? C. water is used as the standard where 10 meters (33.9 ft.) of water equals one atmosphere (14.7 psi. or 1 bar). The term head is used instead of pressure in the centrifugal pump business.

Helium Light Band

A method of measuring seal face flatness. One helium light band equals 0.0000116 inches or 0.3 microns. Seal faces are normally lapped to within three helium light bands of flatness.

Horse power

33,000 foot pounds per minute. A common method of measuring work.

Hydraulic balance

A method of reducing mechanical seal face loading by reducing the seal face closing area.

Hydraulic force

Occurs any time pressure acts on a seal face area. Force times distance divided by time is a measurement of work done.


A petroleum product consisting of hydrogen and carbon.

Hydrodynamic force

Generated at the seal faces because, for all practical purposes, liquid is not compressible.

Hydrodynamic seal

Special geometric features on the seal face that provide lift by taking advantage of the rotation of one seal face upon the other.

Hydrogen embrittlement

A premature fatigue of metal caused by the presence of free hydrogen. This is a major cause of ceramic breakage in hot water seal applications, and bearing fatigue if moisture penetrates into the bearing case.

Hydronium ion

An ion formed by the transfer of a proton from one molecule of water to another. Used to determine the pH of a substance. Each step increases by a factor of ten.

Hydrostatic seal

Maintains a controlled gap between the seal faces by balancing the open and closing forces. There is a small amount of leakage across the faces when the shaft is rotating. Used in some compressor applications, but not very practical for the chemicals found in the process industry.


The delay or lag that causes seal faces to open.

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