FLOW ELEMENT AND CONTROL VALVE 2

[Instr13]

Hi,
Could anybody brief me about why we normally put Orifice plate upstream of the associated flow control valve in the flow loops. Is this because of upstream requirement of the orifice plate?

 

[hacksaw]

generally yes, but there are are exceptions

 

[shobi]

As a general rule "control valves and abrupt enlargements must be installed an adequate distance downstream of the flowmeter to minimize downstream pipe effect"

 

[jmw]

Another reason for putting control valves on the downstream side of any flowmeter is to provide back pressure.
Pressure drop across any flow element can lead to gas breakout. If the control valve is on the downstream side this helps prevent bubble formation.

JMW

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[Scotsinst]

Another reason is that in most of the process industry, the upstream pressure is normally more stable than the downstream of a control valve which is constantly adjusting its opening to maintain the controlled flowrate.

 

[bmsg]

I think the reasons for putting an orifice type flowmeter UPSTREAM of the control valve are all of the above replies:

1)The control valve causes major dislocation in the flow profile in the pipeline, and to minimise the effects of these disturbances, a much larger straight length will be required between the valve and the flowmeter.

2) On the other hand, too much of distance between the two equipment might cause significant transportation lags to make flow control more difficult to implement.

3)the fluid downstream of the control valve might experience cavitation / flashing. Two phase flow through the orifice plate will complicate the measurement.

4)The pressure downstream of the valve will fluctuate, depending on the throttling of the valve (and the flowrate).
So, if you put the orifice plate downstream, the (upstream) pressurefor the orifice plate will vary - and so will the accuracy of the flow measurement.