Pumps

Lesson 1 — ABOUT PUMPS

Lesson Introduction
All pumps are broadly classified into two general categories: kinetic and positive-displacement. Within these categories, there are six common classes of pumps.

In this lesson, the six classes of pumps are described and identified according to the method used to create pressure. Within each class, in turn, are several types of pumps that vary from each other by their pumping method, and consequently their operating technique. For you to work safely and efficiently in a facility, you must know about the pumps your facility uses and how they operate.

In this lesson, you will learn the categories, classes, and types of pumps. You will also learn the functions of each of the common classes of pumps.

Categories of Pumps
Pumps can be broadly classified into two categories: kinetic pumps and positive-displacement pumps.

Kinetic Pumps
A kinetic pump builds pressure by first creating high fluid velocity with a rotating element. Then the fluid velocity is converted to pressure by the shape of the discharge passage. Because the fluid1 in a kinetic pump is always free to spin unrestricted within the pump casing, the discharge may be restricted or blocked without building up excessive pressure.

Positive-displacement Pumps
A positive-displacement pump is one in which a trapped amount of liquid is forced (or displaced) from the pump as the pumping mechanism moves. In theory, pressure is limited only by the power available to move the pumping element. In practice, if the discharge of a positive displacement pump is blocked, either the pump driver will stall, or the pump will rupture to relieve the pressure

NOTE
In general practice, a relief valve is installed on the discharge of a positive-displacement pump to protect against over pressure if the discharge becomes restricted or blocked. The relief valve outlet is usually routed back into the suction line. It is a dangerous practice to intentionally block the discharge valve of a positive-displacement pump and depend on the relief valve to relieve the pressure.
1 A substance that conforms to the outline of its container and has a tendency to flow. Liquids, gases, and vapors are fluids.
2 The measure of a liquid’s force per unit area.

Six Common Classes of Pumps
There are six common classes of pumps used in the process industry:
• Centrifugal
• Axial flow
• Turbine propeller
• Reciprocating
• Rotary
• Metering

The first three classes are categorized as kinetic pumps, while the last three are positive-displacement pumps. Each of the six classes applies different methods to create pressure, and these differences significantly affect pump operation.