August 19, 2008

Flow Device Upstream or Downstream of Control Valve

Often when you prepare a P&ID, a question is raised by my young engineers where to place any flow orifice or any other flow measuring device when there is a control valve in the system. And of course, Why????

What do you think? Should it be upstream or downstrem of control valve.
The answer depends on many factors e.g. the condition of fluid, type of fluid, operating parameters, purpose of flow mesurement etc. but most practical situations permit it upstream of the control valves.

The first important thing is that what type of flow meter you are going to use & what are the possible conditions of flow upstream as well as downstream of control valve.

It is easier to put it in upstream of any type of control valve because of two simple reasons. One is that you always know the flow condition (whether it is single phase or two phase flow) at upstream but you never know all possible variations downstream of the control valve.

Second thing is that due to variations in flow conditions your flow element may not be suitable or may give you erratic results due to larger variations in P & T parameters compared to updtream condition which is almost fixed & can be envisaged for different scanrios.

If we list these possibilities then probably it will be a list like this.
  • Variation in pressure downstream of CV may change FE reading, may lead to phase change, may lead to flashing etc.

  • Downstream of CV may have problems of full bore flow also in case of extreme limit operation

  • Downstream of CV may have expansion & sudden cooling in case of vapor as well as liquids e.g. in Cryo processes

  • Downstream of CV may have vibrations problem & therefore type of meter is important (Vortex meters may not work)

  • In case of compressible fluids the low pressure side may or may not have large flow element requirement. As density goes down DP across FE goes down & may result in higher bore size which may lead to larger inaccuracies (Depending on beta ratio).

You may also add more reason to the above list.

The evolution of good practices is a result of large experience of many & every process engineer over a period of time. There is no written rule for them, but logical common sense is required. To develop the habit of good reasoning I ask lot of step by step questions to my team engineers to give them right direction so that they themselves feel the practical situation & learn these small things.

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valves and actuators said...

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Anonymous said...

Interesting post as for me. It would be great to read a bit more concerning that theme. The only thing that blog misses is some pictures of any gadgets.
David Trider
Cell jammer

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