NPSH is one of the most important variables to be considered when it comes to pump selection and system design. Not having a complete understanding of this hydraulic principle can lead to pump failure and poor system performance. In this post, we’ll define NPSH, and layout the ways changes in your system, can affect NPSH and pump performance.
WHAT IS NPSH?
NPSH stands for Net Positive Suction Head. Simply stated, NPSH is the measurement of liquid pressure at the suction nozzle of the pump. There are a couple variations on this measurement to point out as well, NPSHR (required), and NPSHA (available).
NPSHR is related directly to the design of the pump, where NPSHA is a calculation of the head the system is able to deliver to the pump. As expected, if the amount of NPSHA is less the amount required, the suction system will not perform properly, resulting in cavitation and damage to the pump.
CHANGES THAT AFFECT NPSH
- Source Liquid Level – Changes in the liquid level feeding to the pump will affect NPSH. A raised liquid level in the tank will increase NPSHA, while a lower than expected liquid level will cause NPSHA issues.
- Height of Source – Flooded suction applications, where sources located higher than the pump will increase NPSHA, while suction lift applications will lower NPSHA.
- Changing Pressure On Surface Of The Liquid – Thinking of going from a pressurized tank to an open one? Or changing pressures in a pressurized tank? Keep in mind that reducing the amount of pressure on the fluid in a tank will decrease the NPSHA. Before doing this, make sure other factors in the NPSH calculation will make up for the loss in NPSHA.
- Vapor Pressure – Vapor pressure is the amount of pressure required for a specific liquid to stay in liquid form. As the temperature of the liquid increases, the vapor pressure increases, decreasing the amount of NPSHA. If consideration is not taken into the fluid temperature and vapor pressure met, the liquid could flash to a gas state near the pump suction.
- Suction Piping Diameter – Increasing the piping diameter will increase NPSHA, while decreasing will obviously decrease NPSHA. This change is due to the amount of piping friction losses prior to the pump suction.
- Poor Suction Piping Design – Suction piping with a lot of valves, fittings, and elbows will have higher friction losses than straight pipes with minimal valves and fittings. Consider the amount of friction your liquid will incur, as too much will adversely affect NPSHA.
- Increased Fluid Velocity – Speeding up the flow (increasing pump speed for instance) will impart more friction on the fluid, ultimately lowering NPSHA.
- Dirty or Restricted Pipes – Allowing dirt, minerals, or other solids to build up on pipe walls will effectively impart points 5 and 6 on your NPSHA. If the walls of your pipes are closing in, it may be time to implement a filter of some sort to prevent it.
- Changes in Fluid Properties – If the properties of a fluid change, or the pumped fluid is changed completely (i.e. switching from pumping water to wax), this can have an effect on the NPSHA requirements of the system. These fluid properties would be viscosity, specific gravity, and vapor pressure.
NPSH should be calculated any time a new system is being designed, or during revisions to existing systems. Not doing so puts your pumps at risk of cavitation, causing decreased flow and head, increased noise and vibrations, and permanent damage to the internals of the pump.