Measuring Pump Operating Points

This guide is intended for use where the pump operating curve may have been lost after years of use; modifications have been made to a pump, or a pump from a new supplier requires verification of their pump curve.

1. Equipment

For any testing of a pump’s operating point, you will need to record the following:

• Pump Suction Pressure
• Pump Discharge Pressure. The pump pressure is the discharge minus the suction pressure.
• Flow rate
• Fluid Temperature during testing
• Electrical Current (for Power-Flow rate Curve)

This requires the following equipment:

• 2 pressure gauges
• 1 flow meter
• 1-2 calibrated temperature probe/s
• Power meter for measuring electrical power. This isn’t needed for the Pressure-Flow rate Operating Points but for the Power curve against flow rate. Alternatively, the current can be recorded and by using the standard voltage supply for the pump, the electrical power can be calculated.
• Water valve (can be gate or ball valve) to control the flow rate.

2. Setup

For setup of the test rig for the pump, it is necessary to do the following:

• Place the pressure gauge for reading discharge pressure immediately after the pump.
• Place the pressure gauge for reading suction pressure 100-200mm upstream from the pump. This minimises pre-rotation of the flow, which gives lower than true readings.
• The flow meter should be placed either before the suction gauge or after the discharge gauge. Unless otherwise specified, place flow meters horizontally with entrance and exit lengths equal to five times the diameter of the flow meter inlet.
• Ensure that the system has enough pressure on the suction side to prevent cavitation.
• The water valve is recommended to be on the discharge side.

The image below gives a recommended setup for the test rig.

 

 

If running on a closed system with mains pressure, ensure that the gauges are able to read well in excess of the mains pressure to avoid maxing out the pressure gauges.

3. During Test

The number of points to test is limited only by the positions the water valve can be set at. However, there are some points that must be tested to provide a proper pump curve.

• Dead Head. This is the maximum pressure rise that occurs when there is no flow of water in the system. This is achieved simply by closing the water valve completely.
• Maximum flow. Water valve should be completely open.

The water valve should be adjusted to provide the flow rates between these points. Wait around a minute after adjusting the valve before recording values to let the flow rate.
Water temperature should be constant throughout tests.

4. Plotting Data

For the Pump curve, the following should be plotted:

• Flow Rate on the x axis
• Pressure rise on the y axis.
• The ‘head’ can be plotted instead. This is calculated by dividing the pressure rise by the water density (9.81 m/s²) which is accelerated due to gravity. 

For the power curve, plot the power input on the y axis. Flow Rate remains on x axis.

Written by Michael Adams BE (Hons)