Installation of Positive Displacement Flow Meter

Normally anything that introduces frictional losses should be places downstream of the pump. This is to avoid suction problems, especially where NPSHa is already close to NPSHr. Exceptions might be footvalves (For priming purposes), filters (usually on PD pumps, for pump protection) and closed circuits.

I assume it is a gear type flowmeter. Likely causes for the undercounting could be:

· Very low viscosity, introducing an element of 'slip'.

· The meter is not properly calibrated.

Another possibility for "undercounting" might be that the flowmeter was intruducing sufficient pressure drop upstream of the pump to initiate cavitation at the impeller. If that has happened, then the impeller isn't going to last very long.

Move the flowmeter downstream of the pump instead.

If flowmeter is on upstream side of the pump, due to suction, the pressure may be so low that instead of filling the cavities of flowmeter completely by liquid, may be some proportion of vapour is flowing. This will undercounting the flow.

Thank you all for your wonderful contributions, I do appreciate.

I think its all been covered - only put something in pump suction if it absolutely positively definitely has to go there otherwise you risk cavitation in the pump.

You dont say what sort of PD meter it is and what sort of pressure drop it has but could you be getting cavitation in the meter as the pressure drops and the liquid vaporises. This would unfortunately damage the meter AND the pump in short order

PD flowmeters shall be used for high viscosity fluids. When fluid has low viscosity, turbine type flowmeters give better accuracy, much lower drop pressure and are cheaper.

The reasons for undercounting may be as follows:

1. DP flowmeter orifices are designed for perticular diameter of pipe. Normally pump suction have higher diameter, so lower fluid velocity. If same dia, no issue.

2. Flowmeters need strait pipe of about (5XDia) length, before and after to sence correct value. Whether it is fulfiled in both cases?

3. In downsream, if flowmeter is installed after discharge valve, it may not give correct value because full pipe flow is needed. Check where it was installed.

Positive Displacement flowmeters measure volume. Velocity profile practically has no influence on readings.

But most other types of flowmeters, including turbine, measure velocity and hence equal fluid velocity through cross-section is required. In this case straighteners (at least strait pipe) before and after flowmeter are required.