Centrifugal Pumps

This work horse of the process industries offers one of the largest opportunities for energy saving.

The design process is usually conservative when specifying pumps, especially when calculating the required discharge pressure. As a result, the pressure drop across the discharge control valve is commonly higher (sometimes much higher) than needed for process control.

Many, if not the majority, of centrifugal pumps in the oil, gas and petrochemical industries are specified to meet the Code API - 610 or its equivalent. The prime function of the Code is to ensure safe and reliable operation. Whether the pump operates efficiently is mainly the responsibility of the engineer making the selection.

Using a typical single stage centrifugal pump as an example, a selection chart looks like Figure 8.2.

Obviously, the ideal situation would be for the pump to have the maximum impeller size and operate at a flow corresponding to the best efficiency point (BEP). However, it is usual to select a smaller impeller (typically between 1/3 and 2/3 of the size range). Also, the design operating point is chosen to the left of BEP. Both of these decisions are taken to provide for possible future upgrading in both head and flow, associated with plant de-bottlenecking-without having to replace the pump. Note also that the pump head includes an allowance for pressure drop across the discharge control valve.

Taken altogether, the energy consumed can easily be 20% more than the achievable minimum.

The best way to minimize pump energy consumption of an existing unit is to fit the maximum impeller size for the casing and convert to variable frequency

Curve for maximum impeller size

Curve for maximum impeller size

recommended impeller size

Flow

Figure 8.2 Pump selection chart.

Figure 8.2 Pump selection chart.

Vfd Control Valve

drive (VFD) . Pump speed is then determined by the required process control parameter -flow, drum level, pressure etc. This eliminates the control valve pressure loss. Speed is also lower without a control valve and so bearing and seal life is improved. Some operators leave a control valve in place-full open but set to operate if the VFD system defaults to full speed if a fault is detected.

Application of VFD must be done with care. If pumping into a system where pressure drop is defined by friction (for example a transmission pipeline), VFD is ideal. However, if pumping into a fixed pressure system, then depending on the shape of the pump headflow curve, small changes in speed can have a large impact on flow and care must be taken in designing the control system. In this case, VFD can be linked to the pump absorbed power to avoid instability (Figure 8.3).

There is a further problem to avoid. If the speed control is set within too tight a range, then sudden and continuing changes in transmitted torque can result in coupling failure or even shaft failure. Though rare, this has occurred leading to a reluctance among engineers who have experienced the problem to apply VFD-thereby losing the energy saving opportunities.

Where two pumps are run in parallel, then VFD can only be used in conjunction with flow metering of the individual pumps to ensure load sharing. This becomes complicated if flow is not the control parameter and so VFD of paired running pumps is rarely used.

There is the potential for considerable energy saving when the system is applied to boiler feed pumps linked to steam drum level. For a long time, the 'rule ' was that the pump discharge pressure should be high enough to get water into the drum with the drum relief valves blowing. Pump discharge pressure in this case is therefore often 50% higher than necessary with VFD.

Centrifugal pumps are generally specified to have a continuously rising head-flow curve from operating point to cut-off (zero flow). This is done for stability since a rising-falling curve can have two operating points at the same pressure (Figure 8.4).

For high head, low capacity pumps, the most efficient impeller design results in a rising-falling curve. To get a continuously rising curve a lower exit angle from the impeller vanes is needed. This reduces efficiency and head due to higher velocities in the impeller passages, and so a larger diameter impeller is required. (N.B. This refers to conventional impeller plus volute design, not the Barske type which uses a different principle.)

It is possible to adopt the more efficient design by ensuring the control system maintains flow to the right of the peak head of the curve. Since this type of pump needs a smaller impeller there is the added benefit of cost saving since the casing size is also smaller.

For new installations, there is a case for increasing impeller diameter and running at lower speed. The reduced fluid velocity in the impeller passages increases efficiency.

Two possible flows at the same discharge head

Two possible flows at the same discharge head

Best efficiency point
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