Location of Instrumentation for Valve Control

[kdt]

Two questions on placement of instrumentation in relation to controlled valve. My collegues don't agree with one another so...

1) Temperature Transmitter (TT) to close a valve on High Temperature Alarm to protect downstream system. I have seen this control loop with the temperature transmitter sometimes located upstream and other times downstream of the Valve being actuated/controlled. I prefer the TT being upstream of the valve but some engineers I work with prefer the TT being downstream. What are the pros and cons for each?

2) Flow Transmitter to control a flow control valve. Same question as above, should the Flow element be upstream or downstream of the valve?

Thanks in advance,

 

[sshep]

For the temperature shutdown in non-flashing service upstream will give an earlier warning, but in a flashing service a downstream location is neccessary to get the actual temperature going to the equipment protected.

For a flow measurement orifice in flashing liquid service, upstream is absolutely neccessary. In all other cases upstream is generally desirable.

-sshep

 

[bewdley]

Agree in principle with sshep. Consider that sensor downstream gives feedback but increased delay, and vice versa.

 

[ProcsssDr]

One thing to consider with the temperature transmitter down stream of the control valve is that once the valve closes, the transmitter is isolated from the source of high temperature. You need upstream temperature indication also to ensure that it is ok to reopen the valve and a manual reset on the controls so that it will not automatically open the valve when the down stream temperature falls.

 

[jsummerfield]

More data please.

Most head type flow elements, the flow meters that use pressure differential transmitters, require a good flow profile. An upstream control valve is not helpful in this issue. So, normally place flow meters upstream.

The temperature measurement is poor when the flow is stopped. So what are you controlling? It is different if you ar controlling steam of cooling water to a reactor. The temperature element placement does matter for gas. Are you controlling the process fluid? The fluid pressure drop across the valve causes refrigeration in compressible fluids. Thus the downstream temperature decreases. You must know where you want to control the temperature, above or below the valve.

Data, data, ...

John

 

[itdepends]

Expanding on Jsummerfield

In general I would agree to put temperature transmitters and flow elements upstream of the relevant control valve.

However I am assuming that for temperature the stream you are controlling is steam or some other hot medium that is transferring heat to the downstream process and that there is no reaction that the stream is involved in downstream that can cause high temperature.