Unstable Flow Problem in Magnetic Flowmeter

It strikes me that it's not a problem, it's to be expected.
A sudden disterbance causes a sudden disturbance.
Two Three solutions:-
1. Close the valve slowly.
2. Fit a load of smaller bore pipes inside the pipe upstream (maybe downstream too?)of the flow meter to help laminarize the flow (did I just make up that word?)
3. Ignore the 'problem'
my preference is for number 3.
Del

Has the response indication of the flowmeter been slugged in any way? Some details of the flowmeter would assist.

<groan>
You haven't answered any of the other questions.
If you want good answeres you need to respond and interact.
The fundamental question seems to be, are you trying to stop the flow or regulate it?
It you are trying to regulate it, you shoul n't be slamming the valve shut.
post#1 point#1 probably covers it, but in the absence of comment I shall unsubscribe from this discussion.
Del

>control valve, valve closes rapidly

When the downstream control valve slams shut the flow rate goes to zero or near zero.

The flow meter probably has a low-flow cutoff setting. Any flow below the low-flow cutoff setting is set to zero.

>flowmeter does not show the value of flow

Is this the local indicator display value? What does the 4-20ma analog output value show when this situation happens?

What is the relationship between the flowmeter measurement and the change-in-flow pulses?

"When flowmeter gives pulse to control valve, valve closes rapidly."

At this point flow meter should show zero flow. That's normal isn't it?

My question is, is the flowmeter supposed to regulate the flow or stop it?

The flowmeter output response time could also be affecting the signal that you "see". We have megnetic flowmeters in multiple locations with response times adjusted to suit the application. Some are set at miliseconds while others are at seconds. We datalog the lines to then get minutes/hours averaegs and so on.

MAg flow meters work on velocity, this then is converted to flow when you enter your pipe size.If you do not achieve the min. velocity the meter will not read correctly.You can prove this by setting the output to measure velocity instead of flow and check if you are getting above min. value(manual).If this is the case ,use a smaller flow tube with reducers.

When flowmeter gives pulse to control valve, valve closes rapidly.

If this the intended design, the flowmeter is operating correctly.

If you don't want the valve to slam shut, you can adjust the positioner/ fit it with a valve positioner to slow down the action, or limit the valve travel for 'fully shut' to remain 10% open.

When you close suddenly a valve you can have water hammer effect, drop of flow so sudden it upsets the measurement and most importantly what are the response characteristics of the mag meter electronics.

Even though you are measuring a flow, the electronics are set up so that minute changes are eliminated by filters, these same filters are trying to read your flow changes even if they are abrupt and will show the last stable indication prior to the change till it stabilizes.