Operating on a Pump Curve

As I have often said, the pump curve is the most important document for a fluid piping system. All centrifugal pumps are designed to operate around a Best Efficiency Point (BEP). Operating a pump away from its BEP for extended periods adversely affects the pump’s operating and maintenance cost while causing a reduction in system reliability. 

In this month’s blog, we’ll review the ANSI/HI 9.6.3 Standard for Rotodynamic (Centrifugal and Vertical) Pumps – Guideline for Allowable Operating Region. In addition, we will see how to incorporate pump operating data into manually entered centrifugal pumps to take advantage of these guidelines.

Pumps should be selected so the specified flow rate through the pump should be close to its BEP flow. The Guideline for Allowable Operating Region (AOR) provides some clarity on best practices. Figure 1 shows a typical manufacturer’s supplied pump curve. I have annotated the AOR and Preferred Operating Region (POR) on the pump curve for a 6.125-inch diameter impeller. In addition, the BEP is indicated on the drawing.

Figure 1 – Annotated pump curve showing the various operating regions called out in the ANSI/HI 9.6.3 – Guideline for Allowable Operating Region.

The key landmark used for all pump regions is the pump’s BEP. The design of the pump for both performance and length of service life is optimized for a flow rate close to the BEP flow. At BEP flow the liquid traveling through the pump suction, impeller, and diffuser section has minimal losses. This is because the flow through the pump is relatively uniform and matches the pumps design geometry. 

When the flow rate through the pump moves away from the BEP, the flow through the pump is no longer uniform, resulting in areas of flow recirculation and hydraulic separation. This causes excessive turbulence, hydraulic loss, and increased vibration. All of the items above increase the wear and tear in the pump. 

The guidelines define the POR on each side of the BEP in which the hydraulic efficiency of the pump is not degraded. The POR for most centrifugal pumps range from 70 percent to 120 percent of the BEP flow. Looking at the pump curve shown in Figure 1, with a BEP of 400 gpm, the range of its POR is from 280 gpm to 480 gpm. 

The pump can still be operated outside this range of flows, but its efficiency and hydraulic operation will be adversely affected. The farther the pump is operated away from its BEP, the worse it is for the pump.

The standard has some guidelines for pumps with higher specific speeds as indicated in Table 1. 

Specific Speed		Preferred Operating Region (POR)
Metric	US Customary
≤ 87	≤ 4500	Between 70% & 120% of BEP
> 87	> 4500	Between 80% & 120% of BEP

 

For example, if the specific speed of the pump is greater than 4,500 the guidelines recommend a narrower flow range of 80 to 120 percent of BEP flow. The manufacturer’s provided value of Pump Specific Speed for the pump shown in Figure 1 is 1,490 so it can operate in the wider flow range.

Allowable Operating Region

The AOR provides a wider range of flow outside the POR where the service life of a pump is acceptable.  The limits of the AOR are determined by the pump manufacturer and are based on factors other than efficient operation. The guidelines also state that the pump should not be operated outside regions that are not fully defined by the pump curve (head, efficiency, and NPSH) without consulting with the pump manufacturer. Looking at the pump curve in Figure 1 the shaded yellow area is the Allowable Operating Region for the pump. The red vertical line on the left side of the AOR represents the minimum flow, and the area past the 70 percent iso-efficiency line represents the maximum flow. 

The AOR does not have clear cut guidelines like the POR. Instead the AOR is based on the manufacturers experience and knowledge about their pump operation. Items that the manufacturer may consider when establishing an AOR consists of the hydraulic loads on the bearing life, shaft seal life, internal mechanical contact, shaft fatigue life, thrust reversal, process fluid temperature rise, vibration, noise, power limits, liquid velocity, Net Positive Suction Head available, suction recirculation, and pump size. 

There are no requirements in the guideline for the manufacturer to state the reasons for selecting the pump’s AOR, but it should be well understood that operation of the pump outside the AOR will dramatically affect pump operation. The guidelines provide discussions of each of the above-mentioned concerns for establishing the AOR, but this information is only provided for an understanding of why the limits are established.

Conclusion

The manufacturer’s supplied pump curve is an important document and provides a wealth of information on how to operate the pump efficiently.  It is one thing to have a copy of the pump curve, but the most important information is knowing where each pump is operating on its pump curve.

In PIPE-FLO®, users have the ability to manually enter pump performance data into the program, and it will accurately model pump performance in the system. To learn more about this process, please read our “Using PIPE-FLO® To Simulate and Analyze Data from a Paper Pump Curve” article in the May ESI News Brief.

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