Make the Most of Centrifugal Pumps

Understand the factors and issues that can compromise performance

By Amin Almasi, rotating equipment consultant

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Centrifugal pumps usually handle most liquid pumping services at plants. Indeed, the pumps, which come in vertical and horizontal configurations (Figures 1 and 2), are ubiquitous at most sites. However, such pumps do not suit highly viscous liquids; these liquids require positive displacement pumps. Unfortunately, some engineers still specify positive displacement pumps for many water services and some medium viscosity liquids in the hope of, say, greater flexibility or operational advantages. In most such cases, though, variable-speed centrifugal pumps are better choices.

Ordinary centrifugal pumps usually can tolerate solids’ levels up to around 7% — with appropriate corrections on the pump curves. Specially designed centrifugal pumps can handle up to about 15% solids. For levels exceeding approximately 20%, specially designed positive displacement pumps, such as screw pumps, often are preferable.

These are rough guidelines. Pump selection depends upon many factors including the specifics of the service.

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If you opt for a centrifugal pump, a key to achieving optimum performance and reliability is to operate the pump as close as possible to its best efficiency point (BEP), which is its most stable and suitable operating point. The pump’s hydrodynamic unbalanced load usually is at its minimum at the BEP. If the pump operates at points far from the BEP, the resultant increased hydrodynamic unbalanced load can affect performance and reliability. This unbalanced load usually is at a peak at the shutoff point. So, avoid long-term operation near the shutoff point.

It’s sensible to target a relatively narrow operating range around the BEP. While the exact range depends upon the pump hydraulic design and the application, a good target for typical services often is to operate the pump within the range of 85%–107% of the BEP. Unfortunately, this ideal target frequently is not viable; so, select an optimized operating point (or operating range) that suits the service.

An important capacity-restricting factor could be the net positive suction head (NPSH) requirement. This is because a flow significantly greater than the BEP flow may increase pressure drop (within the suction passages and the suction piping) to a level above the NPSH margin, resulting in cavitation and pump damage.

Component wear and deterioration can open up pump clearances and cause a significant increase in liquid recirculation compared to new/healthy pumps. Liquid recirculation is one of the most important factors for pump efficiency. (For more on recirculation, see: “Fight Flow Recirculation.")

Operational Considerations

For a pump in a closed-loop service such as a recycle application, the operating point should be very close to the BEP or a little (say, around 5–10%) to the left of the BEP. Unfortunately, users often pay inadequate attention to the curve details of a pump for a closed-loop recycle service, sometimes even failing to check alternative operating points or recycle flow ranges on the pump curve. For some recycle services, the flow is fairly constant most of the time and the operating point won’t move much. However, in other applications, the recycle flow varies significantly — this requires locating and evaluating all possible operating points on the pump curve.

Many pump applications are known as batch transfer services. In these, pumps connect to vessels, tanks or other equipment with variable liquid levels and operate with varying liquid loads to fill the equipment (via the pump’s discharge) or empty it (via the pump’s suction). Such systems often include control valves. The operation of these control valves can alter the differential pressure significantly. In these services, the pump head constantly varies — sometimes at rate high enough to cause a sudden change in pump operation.

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