Category: Blogs December 7, 2022
by Peter Gaydon, Deputy Executive Director
The terms net positive suction head (NPSH), preferred operating region (POR) and allowable operating region (AOR) are common in the pumping industry, but many pump users do not fully understand the concepts or their impact on pump reliability. This article is aimed at defining the terms so that pump users understand their meanings. Users can also learn about resources available to better understand the requirements and why it is important to operate a pump within its POR and with adequate NPSH margin.
To address these two important issues of required NPSH margin and operating regions, the Hydraulic Institute published and recently revised two American National Standards covering the topics, providing recommendations on the NPSH margin required for pumps by market used design and the POR for pumps of different designs.
Pressure & NPSH Terminology
To understand these points, the terminology used to describe energy in a system is critical. In the rotodynamic pump industry, energy is described by total head in meters (m) or feet (ft) added to a system. There is a direct relationship between head and pressure. If we focus on U.S. customary units, 1 pound per square inch (psi) is equivalent to 2.31 feet of water at 68 F (specific gravity = 1.0). To adjust for other liquids or water at a different temperature, specific gravity is used and 1 psi is equal to 2.31/(specific gravity) (feet).
This is described in Figure 1, showing that a tank with a 23-ft level of water will have a pressure of 10 psi gauge (psig) at the bottom of the tank when there is 0 psig at the water surface. The pressure units are psig, indicating that they are “gauge pressure,” meaning they are in addition to the atmospheric pressure around us. The atmospheric pressure varies slightly day to day and changes predictably with elevation, but at sea level the standard atmospheric pressure is 14.7 psi absolute (psia). The 10 psig at the bottom of the tank is equivalent to 24.7 psia (10 psig +14.7 psia) or 57 ft absolute (24.7 psia * 2.31). This is an important distinction to consider as we start to discuss NPSH.
The NPSH available (NPSHA) describes the total head at a point in a system—measured in meters or feet absolute—greater than the vapor pressure of a liquid at its operating temperature, essentially indicating the relative closeness of a liquid to vaporizing. In Figure 1, the NPSHA at the bottom of the tank is 57 ft minus the 0.34 psia (0.78 ft absolute) vapor pressure of the water resulting in 56.2 ft of NPSHA. To cause the water to vaporize at the measurement point, the NPSHA would need to be reduced by 56.2 ft. In a static illustration such as this, it could occur by any combination of reducing the pressure at the surface of the water, reducing the level of the water or increasing the temperature of the water, which increases vapor pressure. In a pumping application, the piping friction head loss between the pressure measurement and the pump will also contribute to a reduced NPSH at the pump location.
When applied to a pumping system, it is important to evaluate the NPSHA at the pump suction. When the liquid enters the impeller, there is a point of reduced head in the impeller eye before the energy imparted by the impeller increases the head. The reduced head in the impeller eye can result in the fluid falling below its vapor pressure, causing it to vaporize and suddenly change back to liquid in a higher pressure region of the impeller. The formation of vapor bubbles blocks flow area in the impeller, which results in reduced pump performance, and the repeated collapsing of the vapor bubbles results in erosion wear of the impeller. This phenomenon is referred to as cavitation. To avoid cavitation and maximize the reliability of a pump, it is imperative that proper NPSH margin exists.
Pump impellers are designed to operate at a specific flow rate where there is a zero incidence angle between the inlet impeller vanes and the approaching liquid at a particular rate of flow, called shockless entry flow. The flow of shockless entry is typically near the pumps best efficiency point (BEP), and BEP is used as a proxy because it is more easily determined by people who did not design the impeller.
Higher or lower rates of flow relative to the BEP cause a mismatch between the angle of the approaching liquid and the impeller vane inlet tips. The greater the incidence angle, the greater the potential for flow separation and cavitation to occur. Design characteristics for both performance and service life are optimized near the BEP, and the pump operates with maximum hydraulic efficiency. Therefore, operating as close to BEP as possible will result in maximum pump reliability.
The Hydraulic Institute defines two important pump regions of operation. The preferred operating region (POR) is the range of rates of flow to on either side of BEP within which the hydraulic efficiency and operational reliability of the pump are not substantially degraded. The allowable operating region (AOR) is a wider range of flow, outside the POR, over which the service life of a pump is acceptable. When pumps are operated outside their POR, the flow through the pump is no longer uniform. This results in areas of flow recirculation and separation that can cause additional loading, flow induced vibration and local areas of cavitation, all of which result in reduced reliability. The AOR is impacted by factors such as hydraulic loads, temperature rise, vibration, noise, power limits, liquid velocity, pump and potential for clogging, NPSH margin, head flow curve shape, suction recirculation, and pump size. The user should consult with the pump manufacturer to determine this value.
NPSH Margin & Operating Region
To understand the NPSH margin required, it is important to understand the NPSH requirements (NPSHR) of the pump. NPSHR is defined as a minimum NPSHA, provided by the manufacturer, which is required of a pump to achieve a specified performance at a specified rate of flow, speed and pumped liquid. This definition requires engineering judgement and is not easily documented. Therefore, as early as 1932, the Hydraulic Institute implemented a 3 percent reduction in head at a constant flow rate caused by a reduced suction head as the NPSHR of the pump. This is because this value was the smallest head drop that could be consistently and practically measured (see Figure 2). Historically, and still today, manufacturers’ pump curves show plots of NPSH3 and pump flow rate.
If a pump is applied with an NPSHA equivalent to the NPSH3, the pump is operating with reduced head due to cavitation. The general consensus in 1932 was that pumps operating under conditions of 3 percent head drop would achieve generally acceptable service life. This was probably true at the time, when pumps for a given application were typically larger and slower than pumps for the same application today. Today’s higher speed, higher energy density pumps may not achieve acceptable service life under suction conditions without an adequate NPSH margin above the NPSH3. For this reason the guidance provided in ANSI/HI 9.6.1 should be followed to ensure reliable and efficient operation.
The topics of operating regions and NPSH margin go hand in hand because when a pump is operated away from BEP and outside the POR, the required NPSH margin is increased. See Figure 3, which illustrates the amount of NPSHR required needed to limit the head drop to specific levels. Additionally, Figure 4 illustrates the POR and AOR for a pump along with the NPSH3 curve and the NPSH margin. To limit the potential cavitation damage that can occur, the NPSH margin must be increased at flows beyond the POR. Reference ANSI/HI 9.6.3 to determine the POR.
This article was originally printed in Pump & Systems Magazine, 07/19/2017.
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