Common Pumping Mistakes
by Jim Elsey
July 17, 2017

This article is based on the assumption that the flow measurement is accurate. Unless there is metering and/or transfer of custody situation, most flow measurement is not accurate. This issue is usually not the fault of the flow meter manufacturer. Incorrect flow meters are installed and/or correct meters are installed incorrectly. Empirical measurement of tank levels or containers is another method that will yield satisfactory results for flow rates if conducted properly.

Sometimes an end user will install a pump and note the discharge head does not correctly match where the pump should be on the curve. In that case, some likely issues to look at are:

  • The suction side (absolute) pressure has not been taken into consideration. You must know the differential pressure across the pump, not just the discharge pressure.
  • The gauges are not accurate due to age, are out of calibration or the gauge range is incorrect. Additionally, the gauge location/position must be compensated for elevation differences above or below the pump centerline. Gauges can only read pressure and do not read velocity head or the elevation difference.

The fluid velocity in the piping is often ignored or misunderstood. If the system is properly designed/operated, the velocity head can usually be dismissed because it is such a small component. In recent years I have noticed an increase in issues where the wrong size pipe is installed (usually an issue on the suction side). As a general rule, the suction pipe should be one size larger than the pump suction, except in cases of self-primers (suction lift conditions), where they may be the same size. The suction pipe size will depend on length of the run and other friction losses. A general rule for flow rates and velocities is that the pipe diameter squared, multiplied by 20 will yield the normal flow rate for an 8 foot per second velocity. If you know the flow rate, you can use the same equation to solve for the proper pipe size.

The Pump Curve must be corrected for fluid properties like viscosity. Keep in mind that viscosity changes directly with temperature, and many fluids are also reported at the wrong viscosity. In the event of a higher specific gravity, the horsepower will also increase.

Field test the pump by operating at a few different places on the curve. You can accomplish this by manipulating the discharge valve. Carefully operate the pump at shutoff (maximum head at zero flow) just long enough to record a discharge pressure. Operate at a few more points of increasing flow. If the measured performance does not meet curve expectations at shutoff or anywhere else on the curve, then the issue is likely wear, opened clearances, wrong size impeller or incorrect speed. If the performance meets curve expectations at shutoff but drops off from the curve as the flow increases, then the issue is likely a starved suction, insufficient NPSHa, inadequate submergence or air binding.

Most pump manufacturers in North America test and report pump performance in accordance with Hydraulic Institute / ASME specification 14.6, where depending on the selected test grade the specification will offer some small acceptable tolerance between the published curve and actual pump performance.

To read other articles in the 'Common Pumping Mistakes' column, go here.