When centrifugal pumps stop operating at peak performance, there are several potential issues at hand. Seals, couplings, shaft alignment, and bearings are areas that need inspecting. Experienced pump operators also consider net positive suction head (NPSH), primarily when cavitation occurs.
Some in the pump industry joke that NPSH stands for “not pumping so hot,” because the pump just isn’t doing what it is supposed to. If minor problems are left ignored, and we don’t at least consider NPSH, catastrophic failures could occur. Understand NPSH and how to calculate it might save your pump system.
What is Cavitation?
Cavitation is often mistakenly used as a catch-all term for pump inoperancy. This problem is highly specific, and generally has a direct correlation to the net positive suction head. Cavitation happens in centrifugal pumps when the local pressure inside the suction eye of the impeller (or in the suction piping outside the pump) drops below the vapor pressure of the process fluid. The result is vaporized fluid and air bubbles that then hinder the suction power of the pump. Even worse, they’ll implode while passing through the impeller, causing a violent reaction inside the pump.
Cavitation is easy to detect when there is a decrease in suction pressure and the pump isn’t operating properly. Extreme cases of cavitation can lead to significant internal damage that will cause full system failures and will cost a great deal of time and money to repair.
There are two measurements to know to determine the NPSHa and the NPSHr. When the NPSHa is higher than the NPSHr, cavitation will not happen. NPSHr is easy to determine. It should be information that you can readily get from the pump manufacturer. Top manufacturers like KSB will have the NPSHr readily available for any pump based on duty conditions. The NPSHa, on the other hand, is a bit more complex to calculate–it rests on temperature, vapor pressure, absolute pressure, piping size and length, pipe fittings, and height of liquid in the pump system.
Here is the calculation for NPSHa:
NPSHa = Hps + Hss – Hf – Hvp
Hps is the absolute pressure on the surface of the liquid in the supply tank. It may be simple atmospheric pressure in an open tank, or vacuum pressure in an enclosed tank.
Hss is the static suction above or below the pump centerline. It will usually be a positive number, but is sometimes negative in suction lift conditions.
Hf is the piping entrance and friction losses between the supply tank and the pump’s suction flange.
Hvp is the vapor pressure (in absolute units) of the liquid at the pumping temperature.
This is a 10,000 foot view of the calculation and the factors to calculate NPSHa in a centrifugal pump. Understanding the overall NPSH will help you dial in and monitor your pump system for better performance and risk of cavitation.
Dialing in Your Pump System
KSB is a leading pump manufacturers, who understands the significant concerns of cavitation. KSB pumps are built to provide optimal performance and are glad to supply NPSH ratings for their pumps based on specific operating conditions and process fluids.
Still, no two applications in the field are the same, which is why operators need to understand NPSH, how to identify cavitation symptoms, and know the calculation for NPSHa.
For help with your pump system needs—including pump selection, custom system configuration, precision installation, preventative maintenance plans, and expert repair services—contact DXP Pacific today.