Drop Tests

Drop tests have traditionally been used to assess the capacity of pumping stations. They can produce reliable results, but rely on three assumptions:

1. The rate of change in level in the sump can be directly related to the rate of change of volume in the sump.
2. The rate of inflow during pump operation is the same as the rate of inflow between pump operations.
3. The pumping rate is constant.

Each of these assumptions may be incorrect: The depth/volume relationship may be affected by sediment or benching in the sump, by surcharging of the connecting sewer network or other storage volume. In some cases, this may make a drop test impracticable.

The rate of inflow may vary considerably and unpredictably, particularly if the upstream catchment is fed by further pumping stations or point source of effluent. The pump rate may vary, either as a consequence of faults in the pump or in response to the pump characteristics as the sump level varies. To improve the reliability of drop tests, we measure and record the level in the sump continuously. This allows variation in pumping rate and inflow rate to be identified, together with any non-linearity of either sump configuration or pump characteristics. (We have seen pumps where the output at the start of the pump cycle is 50% higher that the output at the end of the cycle - but there was a fault in the pumps!)

The pressure in the outlet main is also logged continuously. This permits identification of any anomalies in the output characteristics of the station and allows identification of surge pressures within the system.

The profile of the rising main must be considered in the analysis. Measurement of the static lift by measurement of pressure at the pumping station is only adequate for a continuously rising pipe with no intermediate high point or air valves.

These test results, interpreted by a qualified and experienced engineer, greatly improve the reliability of the hydraulic assessment of the pumping station. The characteristics of the pumping station pipework and rising main can be established together with the pump performance.

On occasion, the layout of the pumping station may make measurement of pump capacity by drop test impracticable. In this event, the flow would be measured by non-invasive flow meter attached to the pipework or rising main.

The test results can be used to compare the actual performance of the pumping station with the intended design values. The effects of modifications to the pump system or rising main can be predicted with more certainty, and limitations on potential capacity can be identified.

The asset management planning of pumping systems, including upgrading or renovation, can be carried out with greater confidence when the actual and potential performance of the pumping station is known.

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