5 replies to this topic

[sreekanthtm]

why flow transmitters are placed upstream of control valve?

[Santoshp9]

Dear Sreekanthtm,
Control valve is placed for controling the flow of line and its possible by varying the opening of valve by varying pressure drop across the valve.
the flow variation takes place after the valve so obviously measurement should be taken after control valve rather before it.

If you have any doubt, I will reply you other way to understand easily.

[Shivshankar]

Hi

Hope this helps

http://profmaster.bl...nstream-of.html

http://www.cheresour...e-arrangements/


Regards
Shivshankar

[Art Montemayor]

Sreekanthtm:

I believe that in writing:
“why flow transmitters are placed upstream of control valve?

You really meant to ask:
“why are control valves placed downstream of flow transmitters?

If so, then this is a query I have often had to explain to young engineers and I will repeat my response here;

The reason is that basically, one is required to have a control valve in flow-controlled circuits – and that raises the questions of –

• WHERE to locate the control valve?; and,
• is the location important or not?

The answers are that the location for the control valve is obviously DOWNSTREAM of the flow measurement device in order to maintain a constant pressure (and sometimes temperature) across the flow measurement instrument. This is critical for the measurement since the flow calculation is based on keeping the temperature and pressures constant – and steady. The control valve does that, as well as throttling the flow in accordance with requirements. What happens downstream of such a control valve is irrelevant for the purpose of the flow process because the flow has already been measured and controlled.

If you were to place the control valve UPSTEAM of the flow measurement device, then the stream that the device would be trying to measure would be subject to measurement fluctuations due to changes in temperature, flow turbulence, varying conditions, etc. etc. all due to the needs of the control valve to throttle the flow. This is NOT a very good situation to put a control valve under – especially when trying to measure a gas stream that is subject to the Joule-Thomson Effect. The more the control valve throttles the flow, the greater the pressure drop across it and the greater the temperature drop. The greater the temperature drop, the greater the increase in fluid density. The greater the increase in fluid density, the greater the mass flow rate for a given volume of gas. This, as you can imagine, can drive you and the control valve crazy trying to maintain a constant mass flow rate.

[fallah]

I would like to add one point to Art's complete explanations:

Control valves would normally create higher pressure drop than the flow elemants (orifice plates), hence they tend to be sources of flashing and turbulency. Then, if they would be installed upstream of the flow elements can affect the functionality of them which normally need especial flow profiles.

[sreekanthtm]

Thank you for your reply.....