Net positive suction head (NPSH) is an important measure used by centrifugal pump engineers and pump operators. It is a matter of the pressure energy within a liquid that enables the fluid to be fed into the eye of the first-stage impeller.
How to Calculate NPSH
NPSH is calculated by adding the total static and the kinetic pressure and then subtracting the liquid vapor pressure at the pump suction nozzle (or impeller entry). The final measure is expressed in terms of “head.” Ideally, the pump should be able to achieve higher net-positive suction head than what it actually requires. This helps avoid cavitation and ensures better overall pump performance and reliability. Cavitation happens when there is not enough suction pressure. Vapor bubbles can form and ultimately collapse the impeller and cause damage to the impeller blade, thus impairing the pump.
NPSHa vs. NPSHr
NPSH can be expressed two different ways in relation to a centrifugal pumping system:
- NPSHa—The “a” stands for “available.” NPSHa measures the amount of liquid pressure energy that is available at the pump impeller inlet.
- NPSHr—The “r” stands for “required.” NPSHr measures the amount of liquid pressure energy required by the pump system.
For optimal pump performance and dependability over its lifespan, it’s important for the NPSHa (the pumping pressure available) to exceed the NPSHr (the pumping pressure required).
Net Positive Suction Head Testing
Pump manufacturers will run pump performance tests during the engineering stage. These tests are carefully controlled and they utilize constant flow points and varying suction conditions. The data they collect under these controlled conditions allows them to determine NPSHr curves for any given centrifugal pump. Most manufacturers follow the NPSHr standards as set by the Hydraulic Institute.
What is NPSH3?
Pump manufacturers will also use a measure called NPSH3, which shows the relative NPSHr and NPSHa needed to create enough cavitation within the pump impeller to drop the developed head by 3 percent. The goal is to have a much bigger gap between the NPSHr and NPSHa, so that there is minimum risk of cavitation that dips below this 3 percent margin. Any reputable pump manufacturer will provide the NPSH numbers on any pump, so that you are able to make the right purchase and impeller size decisions.
If you are a pump operator, net positive suction head is a measurement you absolutely must understand to select the right pumps and make sure you are getting the overall performance and longevity you expect from your pump system. For help in finding the best pump(s) for your operation and all the NPSH data you need on any pump, contact Cortech DXP today!