Original article by Larry Bachus; edited by Pietro Perrone
The following information is based on an article by Larry Bachus published in the July 2006 issue of Flow Control magazine (www.flowcontrolnetwork.com). The graphics and portions of the text are sourced directly from that publication.
The information presented in this article highlights the critical role of net positive suction head (NPSH) in the proper operation of pumping systems. In a pumping system, the NPSH is the energy available at the suction nozzle of the pump. Smooth pump operation is contingent upon the NPSH. If liquid is leaving the pump faster than it can enter the pump, then the pump is starved. If a pump is starved, its output of pressure and flow will be reduced, leading process engineers to assume that the pump is running “off its curve.” A pump running off its performance curve is typically considered a maintenance problem but it is actually a design or operation problem, as will be explained below.
The NPSH takes two forms: the energy that the pump requires, or NPSHr, and the energy available to the pump, or NPSHa. There are factors on the source side of the pumping circuit that contribute to the energy present in the fluid and other factors that subtract energy from the fluid before it reaches the pump.
Referring to Figure 1, a tank of liquid supplies the suction nozzle of a pump through a length of pipe. The liquid’s elevation in the tank and pressure comprise the “gross” energy available in the system. We could call it the gross positive suction head available, or GPSHa. A pressure gauge, at the tank’s exit pipe, shows this energy.
When the pump is started and the liquid begins flowing from the tank, some of the “gross” energy available to the pump begins to get reduced by the friction between the liquid and the internal walls of the suction pipe and fittings. Additionally, energy is lost as the level drops in the tank. NPSHa expresses the energy available in the fluid as it arrives at the eye of the impeller. A compound pressure gauge on the pump’s suction nozzle shows this energy (see Figure 2).
The pump performance curve will indicate the pump’s requirement (the NPSHr) at different flow rates. Since the NPSHa (available energy) must be greater than the NPSHr (required energy) to avoid starving the pump, a compound gauge mounted at the suction of the pump is a key instrument for monitoring pump performance. Unfortunately, this gauge is often omitted. When the engineer is asked why, the typical response is: “We don’t use gauges on the suction nozzle of the pump. They’re too expensive. We go by the discharge pressure gauge at the previous process.” However, the gauge reading at the previous process would be equivalent to the gross positive suction head and is not indicative of what the pump is experiencing. The gauge reading at the pump suction nozzle is the net positive suction head available (NPSHa). It is the only head (available energy) the pump can use and may be considerably lower than the GPSH. When pump performance stops your plant’s operation, an investment of a few hundred dollars in a pressure gauge mounted on the pump suction line can save thousands in troubleshooting and auxiliary instrument costs to determine why a pump is operating “off the curve.” Incorporating the ability to monitor the suction pressure during the design phase will minimize the cost and maximize the return on your investment in terms of time and frustration when troubleshooting plant operations.
Larry Bachus is founder of pump services firm, Bachus Company, Inc. and is a regular contributor to Flow Control magazine. Pietro Perrone is a Business/ Projects Manager at Millipore Corporation. He has a degree in chemical engineering from Tufts University and over 20 years purification/separation technology experience in process development/optimization, equipment scale-up, and project management.
Page last updated: 5 March 2009