Control.com - Nerds in Control

Hi,

In a flow control loop is it correct to locate the control valve between the pump and the flow measurement (DP transmitter across measurement orifice) or should the flow measurement be placed between the pump and the control valve?

Thanks in advance.

It's more common to see the flow element upstream of the valve but provided there is sufficient straight lengths between the valve and orifice plate it should work.

Another thing to think about is flashing, if the downstream pressure is low flashing may occur at the plate.

Regards
Roy

You want to put the measurement device as far away from sources of perturbation of the flow as possible. Both the pump and the control valve are serious perturbers of the flow. Since most people do it wrong, and most designers don't leave enough room in the piping design (clean piping design is not necessarily the best for flow measurement), the best thing to do is to locate the flow meter upstream of the control valve. You need at least 20 diameters between the pump and inlet to the flowmeter, and at least 10 between the flowmeter and the valve, unless your piping designer has given you turns in three dimensions (two elbows out of plane back to back). Then you need at least 100 diameters. Since none of this is usually possible, use flow straighteners, or accept that the flow reading may not be accurate (but it will be repeatable, probably).

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Hi,

Both the devices pump & control valve could lead to inaccuracy in the flow measurement provided it is not properly located. One has to maintain a great deal of straight length between a flow meter, pump & control valve. Placing flow meter before the control valve would ask for some shorter straight length rather than putting the flow meter downstream of pump & control valve. More the straight length is better when such devices are in line. Under any situation the minimum straight length required is not less than 24D for good accurate flow measurement.

With advancement of technology there are some DP flow meter which do not or need very less straight length. Those are the technology which I would recommend for use on this application. You can check out ACCELABAR from Veris, Inc. USA where NO STRAIGHT LENGTH is required.