Last month we explored the effect of the system curve on the output of two identical centrifugal pumps operating in parallel. This month we will review several other factors that can also impact their performance.
Unstable Pump Curves
In a stable head/capacity (H/Q) curve, head rises continuously as flow decreases and reaches its highest level at shut off. This curve type is typical of most centrifugal pumps. There are times, however, when a pump design may push the efficiency envelope and result in an unstable curve similar to Figure 1 (below).
This curve reaches its maximum head between 50-gpm and 75-gpm and then falls substantially as it approaches shut off. Even the head produced at BEP (125-gpm) is higher than that produced at shut off. Theoretically, small changes in system conditions could cause this pump to oscillate between higher and lower flows if it is operated in the highest head region of the curve.
Although pumps with unstable curves can work well in many applications, they are not suited for operation in parallel. Why? If the primary pump is operating at a head that is higher than shut off, the secondary pump may not be able to produce enough head to come online. This is especially true for larger pumps that may be started against a closed valve. Several agencies recommend that all pumps should have stable H/Q curves. API mandates them for parallel operation and recommends a minimum head rise from rated capacity to shut off of 10 to 20 percent.
Non-Identical H/Q Curves
Figure 2 (below) shows two pumps with unequal flows but identical shut off heads operating in parallel. This example is typical of a duplex pressure booster system that pairs a smaller jockey (lead) pump with a larger lag pump. A similar configuration is sometimes seen in sewage lift stations where infrequent but unusually high flows may occur. Even though the individual flows vary substantially, the equal shut off heads and stable, continuously rising curves allow them to function properly.
As with identical pumps, each will contribute to the parallel flow based on individual flows at a particular head point. Just like any parallel pumping system, the system curve must be factored into the equation.
Often the "as built" system can cause the pumps to operate well to the left of BEP, and prolonged operation in this area can adversely affect pump life. For example, if the system conditions will allow the pumps in Figure 2 to operate at 163-ft of head, both will operate near BEP. If the system curve forces them to operate at 171-ft, both will run much less favorable portions of their individual curves. In Pump FAQs (Pumps & Systems, November 2007), the Hydraulic Institute states, "For reliable pump operation and maximum energy savings, both pumps must be operated at or near their BEP."
Figure 3 (below) shows the result when pumps with unequal head and flow are operated in parallel. The red (parallel) curve shows that the lower head pump will not begin contributing to parallel flow until the flow of the higher head pump exceeds 100-gpm. As flow increases and head decreases, both pumps will contribute to parallel flow in the same manner as in the previous example.
HI and other agencies discourage using pumps with unequal shut off heads in a parallel application. I agree that this is a good general rule, but in applications where elevation is the primary system component, they can operate successfully. The pumps must be started in proper sequence, and the system conditions must allow both pumps to operate at or near BEP.
Variable Speed Control
There are three basic ways that a VFD can be used to control two identical pumps operating in parallel. The first uses a single drive sized to operate both pumps simultaneously. Both run at identical speeds, and changes in system conditions cause identical speed changes. This configuration, more often seen in industrial applications, can result from application growth and the addition of a second identical pump. This control scheme is best suited for applications where minimum flow always exceeds the capacity of a single pump. Both pumps can also take advantage of soft start and stop via the shared drive.
The second utilizes a single drive sized to control the primary pump. If flow reaches some preset maximum or pressure falls to some preset minimum, the secondary pump is started across the line and runs at full speed. The VFD reduces the speed of the primary pump and then attempts to maintain the required conditions. Although this control scheme has a lower initial cost, the savings may not be worth some of the resulting negatives. For example, in some constant pressure systems, a control valve (PRV) will be required to prevent overpressure by the secondary pump. The primary pump may also operate well to the left of BEP during parallel operation, and only the primary pump can take advantage of soft start and stop.