Category: Blogs March 21, 2022
By , contributing editor
Understanding these three terms will help you optimize your pump for efficiency and reliability.
The pump industry uses acronyms so often, we sometimes gloss over their true meaning and importance. So, let’s review the concepts behind three terms that have a huge impact on pump reliability: Net positive suction head (NPSH), preferred operating region (POR), and allowable operating region (AOR).
Net Positive Suction Head Margin
Net Positive Suction Head (NPSH) is the key to reliable pump operation. The key term here is NPSH available, a system characteristic defined as the absolute pressure difference between the liquid’s pressure at the pump inlet and its vapor pressure expressed in head (feet or meters).
To understand why this is important, consider how a pump works. Liquid flows from high-pressure to low-pressure. There needs to be enough pressure so that the liquid will enter the pump impeller eye, where pressure drops, without significant vaporization. Pressure increases as the liquid moves through the impeller.
It sounds simple enough, but there is a catch: Cavitation. Some cavitation occurs in most pumps, but the question is if it will be damaging or affect performance. The key to limiting damage and performance loss is to provide NPSH margin above the pump’s required NPSH. If there is insufficient NPSH margin, the liquid’s pressure will drop below its vapor pressure and vaporize. Vapor bubbles form that block the smooth flow of liquid through the impeller and reduces pump performance. Then, as the bubbles move to the higher pressure region of the impeller, they collapse and may have enough force to erode the impeller and cause other damage.
ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin provides NPSH margin values above a pumps NPSH3 to achieve reliable operation.
Preferred Operating Region
Before defining the preferred operating region (POR), let’s take a second look at pump impellers. They are designed for shockless entry, a flow rate that produces a zero incidence angle between the impeller vanes and approaching liquid. The more the flow rate diverges from this point, the greater the mismatch between the liquid’s entry angle and the impeller vane tips, and more likely you will find flow recirculation and separation. That can produce additional loading, vibration and cavitation, which reduce pump reliability.
The term “shockless flow” is not that useful to the pump user because it’s not commonly available in the pump information. It is, however, near a pump’s best efficiency point (BEP), which is commonly used as a proxy instead. By operating as close to the BEP as possible, users can maximize pump hydraulic efficiency and reliability.
How close? That is what the POR tells us. The POR defines the range of flow rates above and below BEP where pump efficiency and reliability are optimal. ANSI/HI 9.6.3 Rotodynamic Pumps Guideline for Operating Regions defines the POR for pumps based on their hydraulic design.
Allowable Operating Region
Sometimes, pumps need to operate outside their POR. They might, for example, need to handle a surge after a large storm or occasionally serve a secondary process that has different requirements. This will push them away from optimal operating conditions and push them into a range where flow is no longer uniform. Still, there is a range outside the POR where the pump will achieve acceptable service life. That is the allowable operating region (AOR).
Many factors determine AOR. These range from hydraulic loads, temperature, vibration, noise, power limits, liquid velocity, and potential for clogging to head flow curve shape, suction recirculation, pump size, and NPSH margin. To understand how these factors impact pump reliability, be sure to consult with the pump’s supplier.
Putting it All Together
NPSH margin has a strong relationship to the preferred and allowable operating regions (POR and AOR). To understand why, let’s revisit NPSH margin.
Dating back to 1932, the Hydraulic Institute defined an NPSHR test measuring a 3-percent reduction in head due to cavitation, as the NPSHR of the pump (see Figure 1). HI chose 3-percent because it was the smallest consistently measureable head drop and because the members believed it would still yield acceptable service life.
Today, manufacturers’ pump curves still plot NPSH3 or an NPSHR that could have some margin above NPSH3 as a function of flow rate. Yet pumps have changed. They are smaller, faster, and have higher head per stage than in the past. Running them with at the NPSH3 may not be enough to achieve acceptable service life; therefore, a recommended NPSH margin is applied.
Now let’s consider AOR, POR, and NPSH margin. When operating in the POR, a lower NPSH margin is needed, but when operating in AOR a larger NPSH margin is need.
To see what this might look like, consider the following two illustrations: Figure 2 describes the amount of NPSHR needed to limit the head drop to specific levels (0%, 1% and 3%) as a function of flow rate. Figure 3 shows a pump’s BEP, POR, and AOR plus how the NPSH margin is applied above the NPSH3 curve. Note that in the AOR, the NPSH margin increases.
The Hydraulic Institute publishes two American National Standards with recommendations for NPSH margin by application and POR by pump design. These guidelines will help ensure pumps achieve the best possible balance of efficiency and reliability. The are:
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June 20, 2022