Process Design Process Equipments

Pump Performance Curves

Pump performance curves are important drawings produced by the pump manufacturer. Pump performance curves are primarily used to predict the variation of the differential head across the pump, as the flow is changed. But in addition variation of efficiency, power, NPSH required etc, as the flow is changed, can also be represented on the pump performance curves by the manufacturer.

Typically a pump performance curve will carry information about the following points.

Variation of differential head Vs flow

This is the primary information reported in the pump performance curves and very important information regarding most of the pump calculations related to differential pressure across the pump. As shown in the sample performance curves, usually 3 curves of differential head Vs. volumetric flow are reported.

Differential head Δh is related to differential pressure ΔP by the equation, ΔP = ρgΔh.

1. Curve of differential head for Rated Impeller Diameter represents the variation of differential head with volumetric flow for the impeller with rated diameter which will actually be provided with the pump.

2. Variation of differential head with volumetric flow for Maximum Impeller Diameter is plotted for the impeller with the maximum diameter that can be accommodated within the pump. This impeller can be used in case flow through the pump is increased or if more differential head is required in the future, with the same pump.

3. Variation of differential head with volumetric flow for Minimum Impeller Diameter is plotted for the impeller with minimum possible diameter. If the flow or differential head requirement is reduced in future, this impeller can be used with lower power consumption.

Although the 3 curves are plotted for a wide range of volumetric flow rates, the actual operation is to be limited within the Maximum and Minimum allowable flow rates as indicated in the sample pump performance curve. Values of the maximum and minimum flow limits are given by the pump manufacturer.

The point on differential head axis (Y-axis) where each of these 3 curves terminates, represents the shut-off differential head for that particular impeller diameter. For normal intended operation, the shut-off differential head for rated impeller diameter is important.

It should be note that the pump curves for differential head Vs. volumetric flow rate are plotted for a particular liquid density. If in the future the process liquid or even just liquid density is changed, that effect has to be considered to finally determine the differential pressure. In such as case, revised volumetric flow should be calculated and located on the pump curve and corresponding differential head should be then determined from the curve for the appropriate impeller diameter. This differential head should then be used along with the changed liquid density to determine the differential pressure across the pump.

Pump Efficiency

As indicated in the sample pump performance curve above, the plot of pump efficiency against volumetric flow rate is also commonly reported on the pump performance curves. When the theoretical pumping power requirement is divided by this efficiency for the corresponding flow, the result is pump shaft power requirement. For more information on pump power calculations using efficiency, refer to EnggCyclopedia's solved sample problem. The calculated pump shaft power has to be provided by an electric motor.

The efficiency curve typically has a maximum within the allowable operating range. This maximum is also known as the Best Efficiency Point (BEP) as indicated in the sample curves. The normal operation should be preferably done close this best efficiency point for minimum power requirements.

Sometimes a plot of Pump Shaft Power requirement is also done against the volumetric flow rate on the performance curves. This curve readily gives the value of power requirement for a particular flow rate.

NPSHR (Net Positive Suction Head Required)

Every pump requires certain Net Positive Suction Head (NPSH), for safe and smooth operation and to avoid cavitation in the pumps. The pump manufacturer provides these values by plotting them against the volumetric flow. As seen from the sample performance curves, NPSH requirement increases for increase in volumetric flow. When designing a pump system and positioning the pump, it must always be ensured that NPSH Available is higher than the NPSH requirement as per pump curves. For details about calculation NPSH Available, refer to EnggCyclopedia's solved sample problem.

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