Flow measurement and valve position


Thread Starter



After observing several P&ID from my engineering department where do you normally locate the flow instrument in a pipe? Before or after the valve?

I have seen it both way but I would like to understand what are the reasons for it.

I understand that there must be enough distance between the primary element (orifice plate, venturi, etc.) and the valve in order to have a better flow measurement but what is the difference between installing the device upstream or downstream the valve?
The difference between installing the flow device upstream of the valve and downstream can be enormous (I've seen errors of as much as 25% of rate, with significant nonlinearity). I recommend that you NEVER install a flow meter downstream of a control valve. Typical estimates suggest between 15 and 25 pipe diameters downstream of the control valve for installation of a flow meter, but those are estimates, and do not take into consideration piping geometries such as reducers, ells, multiple ells, tees, thermowells, and other protrusions into the pipe. Anything that can cause swirling in the pipeline flow can perturb the flow measurement accuracy for over 100 pipe diameters and some control valves can do that.

If you must install the flow meter downstream of the valve, you should install an advanced flow conditioner such as Vortab between the valve and the meter.

Walt Boyes

Walt Boyes, FInstMC, Chartered Measurement and Control Technologist
Life Fellow, International Society of Automation
Editor in Chief, Control and ControlGlobal.com
[email protected]