Glossary of Terms

Malcolm Thompson Pumps provides a range of pump tools including Pumping Formulas, Conversion Tables and a Glossary of Terms which can be used to assist with pump selection. To best ensure your pump choice meets your requirements, please contact one of our experienced customer service team members on 1800 439 607 who can assist with pump selection, specifications and a competitive quote.


The distance that the water level in the well is lowered by pumping. It is the difference between the static water level and the pumping level.


The vertical distance between the level where fluid enters a pipe and the level where it leaves. It must be added to the total discharge head if the inlet is lower than the outlet and subtracted if the inlet is higher. As a rule of good installation practice, however, pipes should slope continuously upward from the inlet to the outlet to prevent entrapment of air.

Friction Loss

The loss of pressure or head due to the resistance to flow in the pipe and fittings. Friction loss is influenced by pipe size and fluid velocity, and is usually expressed in feet of head.

Horizontal Run

The horizontal distance between the point where fluid enters a pipe and the point at which it leaves.


Net positive suction head available is the absolute pressure of the liquid at the inlet of the pump.

NPSHA is a function of the elevation, the temperature, and the pressure of the liquid, and is expressed in units of absolute pressure (psia).

Any variation in these three liquid characteristics will change the NPSHA. Accurate determination is critical for any centrifugal pump application.

The net positive suction head required (NPSHR) remains unchanged for a given head. Flow, rotational speed, and impeller diameter, but changes with wear and liquids.

(P.S.I.G) Setting

The vertical distance from the level at which the discharge pipe leaves the well to the bottom of the pump or jet assembly in the well.

Pumping Level

The lowest water level reached during pumping operation.

Service Pressure

The range of pressure in the pressure tank during the pumping cycle, usually expressed in pounds per square inch gauge.

Specific Gravity

Ratio of the density of one substance to that of a reference substance at a specified temperature.

Water at 4ºC is used as the reference for solids and liquids. Air is used as the reference for gases.

The specific gravity of a liquid affects the input energy requirements (or brake horsepower) of centrifugal pumps. It also affects the energy required to more the liquid, and therefore must be used in determining the pumps horsepower.

Static or Standing Water Level

The undisturbed level of water in the well before pumping.


The vertical distance between pumping level and the bottom of the pump or jet assembly. Submergence must be sufficient to insure that the suction opening of the pump or jet assembly is always covered with water, while maintaining enough clearance from the bottom of the well to keep it out of sediment (at least 10 feet clearance is recommended).

Total Head Discharge

The total pressure or head the pump must develop. It is the sum of the depth to pumping level, the elevation, the service pressure, and the friction loss expressed in the same units, usually feet of head, before adding them together.

Vapour Pressure

Pressure at which a pure liquid can exist in equilibrium with its vapour at a specified temperature.

Fluids at temperatures greater than their specified (critical) temperature will exist as a vapour with no distinction between gas and liquid phases. This is a single phase liquid.

At less than the critical temperature, the two fluid phases can coexist; the denser fluid phase exists as a liquid and the less dense phase as a vapour.

At the critical temperature, the liquid phase is stable at pressures exceeding the vapour pressure and the gas phase is stable at pressures less than the vapour pressure.

For a fluid to be liquid its surface pressure must be equal to or greater than the vapour pressure at the prevailing temperature.

Higher pressures are needed to maintain volatile materials in their liquid states.

The surface pressure of a liquid must be greater than its vapour pressure to maintain satisfactory operation of a centrifugal pump.


Resistance of a fluid to sheer motion – its internal friction. The viscosity of a fluid varies with temperature.

The molecules of a liquid have an attraction for each other, and they resist movement. This resistance to flow is expressed as its viscosity.

Dynamic viscosity can be defined as the ratio of shearing stress to the rate of deformation.

As viscosity varies with temperature it is always expressed at a specific temperature.

A change in viscosity alters liquid handling characteristics of the pumping system; more or less energy may be required to perform the same amount of work.

In a centrifugal pump the increase in viscosity reduces the pressure energy (head) produced while increasing the rate of energy input.