Pump Basics

This page outlines a few fundamental concepts you should find useful when working with our pumps. We’ll cover the main pump types, their various performance characteristics and factors, and look at how horsepower requirements are calculated.

Types of Pumps

Engineers describe pumps based on the kind of motion they use: displacement; mechanism, cylinders, and pressure.

Positive Displacement pumps are used for liquids or to compress gases. They operate on a “what goes in, must come out” principle.

Reciprocating pumps convert rotary motion to linear motion. The rpm determines the
pump’s output volume for any given combination of cylinder bore and piston stroke.

Reciprocating Positive Displacement pumps may use pistons with dynamic seals, plungers with static seals, or liquid diaphragms to achieve their pumping action.

Output may also be a function of the number of cylinders in a certain pump. Pumps usually feature up to six cylinders. Terms like ‘simplex’ (for ‘one’) and ‘triplex’ (for ‘three’) indicate the number of cylinders a particular pump features.

When fluid is forced through a restriction, the fluid’s pressure is increased. When that pressure exceeds 150 psi, that pump is deemed “high pressure.”

Performance Characteristics

When looking at pump applications, it is essential to mention three performance characteristics: crankshaft bearing life, volumetric efficiency, and inlet flow curve.

Crankshaft Bearing Life is determined by lubrication, speed, and load. Load is calculated as follows:
Plunger Area (in square inches) x Pressure (psi) = Plunger Load (in pounds)
OR
(( Plunger Diameter in inches)2 x p) x Pressure (psi) = Plunger Load (in pounds)

Volumetric Efficiency compares the pump’s actual output to its theoretical output. If there is no loss in the pumping system, the flow would equal:
Area x Stroke x Number of Cylinders x Crankshaft Speed
231 in3 /gallon = gpm

Volumetric efficiency is the value (in percentage form) of the actual flow divided by the theoretical value, as calculated above.

Valve timing is a major factor in achieving good volumetric efficiency, and is affected by plunger speed, plunger diameter, poppet weight, spring rate, and poppet travel distance.

Inlet Flow Curve relates volumetric efficiency to specific inlet pressures. General Pump provides inlet flow curve charts for all its pumps on request.

Horsepower Requirements
The horsepower required to achieve a given output depends on the energy source, because different types of prime movers vary in their pumping performance capabilities.

Hydraulic power is more efficient than either electric power or internal combustion.
Various requirements may be calculated using these relationships:
Hydraulic horsepower required = Pressure x Flow / 1714
Brake horsepower (electric) required = Pressure x Flow / 1457
Gas engine horsepower required = Pressure x Flow / 1100

To learn more basic pump theory, be sure to attend one of General Pump’s training classes. Sessions are offered throughout the year on a first-come, first-served basis; and are free-of-charge to our customers!