Pump Flow

Closing the valve to 50% is more likely to reduce the flow only to 65% or so, but it depends on the specific valve construction. Some manufacturers may give data that includes C_v (flow coefficient) at partial positions of the valve, but that is a bit unusual. If the pump has inlet and outlet pressure gauges, you can estimate the flow by consulting the pump curve.

Other factors that would affect pump flow would be pipe friction (internal diameter and length), as well as other valves downstream in the system.

If it's a positive displacement pump, the flow will remain at 100% as the valve closes, up to the point where the line upstream of the valve ruptures. At that point, the flow will still be 100%, though very little will be going through the valve.

The actual figure depends upon two things: the pump characteristic curve and the system characteristic curve. The operating point is where these two curves intersect.

Pump characteristic curves come from the manufacturer.

System characteristic curves are determined by the user, either by theoretical analysis or, more likely, from flow/pressure measurement.

That's why it is good practice to have pressure gauges each side of a pump; using the gauges, it is possible to determine the operating point.

sir , if i have pressures at the inlet and outlet then how i can proceed further for flow calculation?

The difference in pressure between the two gauges equates to the "total dynamic head", which is one of the axes on the pump curve. From that you read across to find the flow rate. This depends on having the pump curve available, and may not be precise if there has been impeller wear. Alternatively, as Lyn mentioned, you could use a flowmeter.

sir , how can i develop system charecteristic curve? what are the factors should i consider ?

Use the pressure gauges upstream and downstream of the pump, and a clamp-on ultrasonic flowmeter, which can be hired readily if none is available at the facility.

This may prove to be rather complicated. The system curve depends on "pipe friction", which in turn depends on the internal diameter, roughness, fitting types, and lengths of various segments of the piping system, as well as the flow coefficient (C_v) of valves and other flow restrictions. For this purpose, I would probably use the Darcy-Weisbach formula for calculating pressure drop, using various flow rates. By graphing the flow rates versus pressure drops for a few conditions, you can construct a system curve that correlates the flow with the total pressure drop through the system. In general, pressure drop will vary with the square of flow rate.

Then compare with the pump curve, and see where the two curves intersect.

[This may not be easy to understand until acquiring experience with several examples, which would be done best person-to-person with a mentor.]

Apparently, you don't have a flow meter in the system? Why not? You can rent a clamp-on flow meter, or use a calibrated valve. Otherwise, you are not going to know how much the flow is reduced.

Some more meaningful information from you may yield an answer. Information like what you are trying to do.

Banu....

Get the pump flow curve for your particular pump and your particular impeller.

Read the pressure from the gauge on the discharge and find the flow from the curve.

Ohhhhh...... Wait .... The pump curve has been discarded by the previous plant owner !!

There is no steel tag identifying the pump....It has been painted over...!!!

Good luck