Consider VFDs for Centrifugal Pumps

Variable frequency drives (VFD) offer a useful control option for the motors of many types of reciprocating and centrifugal equipment. For instance, VFDs can deal with centrifugal-pump suction recirculation problems; running at lower speed can help reduce damage and operating difficulties.
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To understand how to apply VFDs effectively, you must know the characteristics of the system and the equipment. So, let’s look at a general procedure that can serve as a good starting point for evaluating specific cases — realizing, of course, that it can’t cover every possibility.

To evaluate a VFD application, we need the following:
• system information on pressure drop versus flow;
• pump performance curves for the existing impeller;
• pump performance curves for the maximum impeller; and
• pump inlet eye dimensions.

Ideally, the performance curves should include pump head, pump efficiency and net positive suction head (NPSH) required versus flow rate. Pump heads and efficiencies at various speeds are useful.

Often, only incomplete information is available. So, you may need to extrapolate to fill in missing information.

ANALYSIS PROCEDURE
A VFD evaluation involves multiple steps, each looking at a specific item. Below are the basic steps. (Some cases won’t need every step while others may require additional ones. The information available and needed as well as the number of cases to be evaluated determine what must be done.)

1. Define the system curve.
2. Examine the pump curve for the existing impeller at fixed speed.
3. Plot a flow rate versus speed curve for the installed impeller.
4. Determine the best efficiency point (BEP) flow rate versus speed for the maximum impeller.
5. Examine NPSH versus flow rate.
6. Plot flow rate versus suction specific speed (N_ss) for using the VFD to control.
7. Define operating fraction of BEP flow versus N_ss.
8. Compare the operating flow fraction versus N_ss to good practice.
9. Evaluate pump suction energy to determine the NPSH margin.
10. Check power available.
11. If the results of operating flow fraction versus N_ssand pump suction energy evaluation are unacceptable, assess alternative solutions.

The system curve plots the required differential head versus flow rate. The system curve includes both static head requirements for elevation and pressure change and dynamic head requirements from pressure drop.

FLOW RELATIONSHIPS

Figure 1. Combined plot shows both flow and fraction of BEP flow versus Nss.

Most often, only a pump curve at a single fixed speed is available. This typically is a nominal 1,800 or 3,600 rpm for 60-Hz motors and 1,500 or 3,000 rpm for 50-Hz motors. (In such cases, you can generate curves for other speeds by using the affinity law: pump head varies as the square of pump speed.)Plotting the pump curve for fixed-speed operation shows how much savings a VFD might provide. If the pump curve and system curve don’t intersect at a higher flow than for the given pump, the pump is too small. A VFD can help here by raising the pump’s operating speed to a higher-than-standard rpm so the pump curve and system curve intersect at the desired flow rate — but at the expense of higher power requirements.Evaluating pump performance requires a BEP versus speed curve for the maximum diameter impeller. BEP flow typically varies linearly with both pump diameter and speed.An NPSH curve also is necessary. If only available at one point, NPSH can be extrapolated based on the flow rate squared. However, this will underestimate NPSH requirements at low flow rates. So, it’s always best to use an actual NPSH curve.Based on pump speed versus flow, plot a curve of N_ss versus flow [1]. Also plot a curve of fraction of BEP flow (at that speed for the maximum impeller). Figure 1 shows an example plot of flow versus N_ss and fraction of BEP flow versus N_ss.Good practice suggests a maximum NSS of roughly 8,500 for a turndown of 50% on a centrifugal pump. Consult ANSI/HI 9.6.3-2012 [2], which is the most commonly accepted good-practice definition for flow flexibility on centrifugal pumps, to get an idea of the reasonable operating range of a specific pump with a VFD.Suction energy [3] is another, more recent, measure of suitability for running at low liquid rates. So, compare the pump performance to suction energy values. The lower the suction energy, the more likely the pump will operate successfully.The pump efficiency curve varies with the percent of BEP. Use the efficiency curve to compute power. At very low turndowns, savings might not be as great as expected. When operating above standard speeds, power consumption may be very high.If the VFD analysis shows problems may persist, consider changing either the process or adding additional features to protect the pump from low flow [4].

ANDREW SLOLEY, is a Chemical Processing contributing editor. You can e-mail him at [email protected]

REFERENCES
1. Sloley, A. W., “Cut Pump Speed to Cut Problems,” Chemical Processing, September 2009.
2. ANSI/HI 9.6.3-2012, “Rotodynamic (Centrifugal and Vertical) Pumps — Guideline for Allowable Operating Region,” Hydraulic Institute, Parsippany, N.J. (2012).
3. Burdis, A. R., “Improved Pump Hydraulic Selection Reduces Cavitation Risk,” p. 39, Hydrocarbon Processing, August 2004.
4. Sloley, A. W., “Properly Protect Centrifugal Pumps,” Chemical Processing, July 2007.