Definition of NPSHr (required)

It is the energy in the liquid required to overcome the friction losses from the suction nozzle to the eye of the impeller without causing vaporization. It is a characteristic of the pump and is indicated on the pump's curve. It varies by design, size, and the operating conditions. It is determined by a lift test, producing a negative pressure in inches of mercury and converted into feet of required NPSH.

An easy way to understand NPSHr is to call it the minimum suction pressure I necessary to keep the pumped fluid in a liquid state.

According to the Standards of the Hydraulic Institute, a suction lift test is performed on the pump and the pressure in the suction vessel is lowered to the point where the pump suffers a 3% loss in total head. This point is called the NPSHr of the pump. Some pump manufacturers perform a similar test by closing a suction valve on a test pump and other manufacturers lower the suction elevation. Know and Understand Centrifugal Pumps

Basic Pump Principles (Part 2)

Pressure measurement


Force (F) is equal to Pressure (P) multiplied by the Area (A):

F = P x A.

Pressure is equal to the Force divided by the Area:

P = F/A

If we apply pressure to the surface of a liquid, the pressure is transmitted uniformly in all directions across the surface and even through the liquid to the walls and bottom of the vessel containing the liquid (Pascal’s Law). This is expressed as pounds per square inch (lbs/in2, or psi), or kilograms per square centimeter (k/cm2).


Atmospheric pressure (ATM)

Atmospheric pressure (ATM) is the force exerted by the weight of the atmosphere on a unit of area. ATM = 14.7 psia at sea level. As elevation rises above sea level, the atmospheric pressure is less.

Absolute pressure (psia)

Absolute pressure is the pressure measured from a zero pressure reference. Absolute pressure is 14.7 psia at sea level. Compound pressure gauges record absolute pressure.

Basic Pump Principles

Pumps are used to transfer liquids from low-pressure zones to highpressure zones:

Without a pump in this system, the liquid would move in the opposite direction because of the pressure differential. Pumps are also used to move liquids from a low elevation into a higher elevation, and to move liquids from one place to another. Pumps are also used to accelerate liquids through pipes.