In Closed-loop Flow Control, What is Better? FM Before CV or CV Before FM

Flow meters are affected by turbulence caused by valves , fittings or obstructions.

What is the distance between your CV and FM?

Typically 3-4 metres. basically bends for vertical FM orientation & straight runs.

As liquids are practically incompressible it shouldn't matter. However, flowmeters generally do need to be placed in low-turbulence zones and following their manufacturer's installation instructions.

PWSlack is right-on: assuming an incompressible fluid, with completely filled pipes both upstream and downstream of the control valve, there should be no difference in lag time for the flowmeter's response to valve positioning. A change in valve position should cause a corresponding change in flow throughout the piping system. The only real lag time for an enclosed liquid is the speed of sound through that liquid.

However, control valves are some of the worst offenders for creating the kinds of large-scale turbulence (swirl, eddies) that tend to mess up flowmeter accuracy. For this reason alone, it is inadvisable to place a flowmeter downstream of the valve unless it is a type of flowmeter that is unaffected by flow profile (e.g. Coriolis, positive displacement).

Right you are and stay away (5 pipe dia.I think) from 90's.

I agree with Tornado.

Agree with Tornado's simple and effective answer.

One thing that many people fail to realize about flow meters (it's usually printed in the Manufacturers literature), is that a certain length of straight unobstructed pipe/tube must be installed before the flowmeter to calm down any turbulent flow into semi-laminar flow before any accurate reading can be taken. Sometimes it is recommended to have a flow straightener before the meter if turbulent flow is present.

It's early... Where is my coffee?

Hi,

Turbulance is a big factor when meassuring flow. Flow needs to be as laminar as possible.

Flow elements need to be installed upstream of the final elements, the distances upstream and downstream of the flow element away from pipefittings or any other restrictions vary depending on the type of flow elements being used.

For example restriction type flow elements require greater distances compared to mag meters.

Thank you

I agree with almost all the posts, but feel some clarification is needed.

There are many different types of flow meters that operate via different principles, paddle wheel, Magflo, Vortx, or the simple oriface etc

For the specific style of meter you have, you need to understand the working principle, then design your system around that principle. In general manufacturers will give you the information you need to adhere to. Follow these instructions accurately.

As others have said, in general the flowmeters need stable flow for a certain lengths upstream and downstream. Note stable flow, not Laminar flow. For example MagFlo typical rule of thumb is straight lengths of pipe for 5 pipe diameters upstream, and 10 pipe diameters downstream. No valves, T's, elbows, or any other flow interuption within this space (including chemical dosing). And MagFlo's are typically accurate over 0.1 - 10 m/s velocity - and 10 m/s is well into turbulent flow (not laminar).

Note: this rule of thumb should be checked against the manufacturers recomendations.

Where I disagree with most of the posts is, I don't believe there is any significant difference in tuning the control loop between the control valve and flow meter wether it is placed upstream of downstream of the CV. Water doesn't compress, so the flowrate change is virtually instantaneous, therefore it shouldn't matter. If the distance between FM and Cv was large, then stressing/flexing of the pipeline may cause a lag in the control.

There is one other consideration that in reality will be a very significant driver for this decision. If the pressure loss over the valve is large, positioning the FM downstream of the CV will advantagous for pressure rating of this instrument (ie the cost of a lower pressure instrument vs a higher pressure rated instrument can be very significant).

My 10 cents.

Anthony

Generally the Flow Meter is installed upstream of the control valve so that fluctuations of the flow due to valve control action do not give a false reading.

Also upstream if the upstream and downstream sizes are maintained then the flow recovery is good enough that the flow element is not affected.

Any instrument donstream of a valve will be signalling the pressure and flow drop caused by the control valve.

So unless something very specific and special is needed then always place the flow meter upstream of the control valve.