We have installed two electromagnetic flow meters on two parallel pipe lines with common water source. sometimes current output of both these flow meters fluctuates at the same time. few times only one flow meter shows this fluctuations. This fluctuation can only be observed on the DCS as this fluctuation lasts for only few milliseconds. What could be the reason for this fluctuation?

 

You say they are on parallel lines. How close together are the meters? Are they side by side?

Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC

**Spitzer and Boyes LLC publishes the Industrial Instrumentation INSIDER (www.iainsider.com)**

 

The reason for this fluctuation is possibly due to system hydraulics, also maybe electrical noise, heavy vibration, air in the system. Try increasing the dampening in the transmitters.

icenta Controls
[email protected]

 

Thanks for replying .

Dear Walt sir,
Flow meters are besides each other. Distance between two pipe lines is around 2 feet.

@ icenta controls

There is no air and vibration in the system. Line pressure is 5 bar and line size is 1 inch. Didn't understand 'System hydraulics'?
Electrical noise could be the reason but how to eliminate it as finding the source of this noise would be quite difficult.

Please suggest

 

There is a magnetic field surrounding the flow tube. It extends outward from the flow tube outside the meter body. Two feet should be enough distance but it might not be. Put a steel plate upright halfway between the two meters and see if the problem goes away.

Walt

Walt Boyes, Life Fellow, ISA; Fellow InstMC
Chartered Measurement and Control Technologist
Spitzer and Boyes LLC

**Spitzer and Boyes LLC publishes the Industrial Instrumentation INSIDER (www.iainsider.com)**

 

You need to provide some additional information regarding the fluctuation (e.g. amplitude, shape, frequency, polarity, etc.). This data can provide clues as to the source of the fluctuations. System hydraulics refers to how the fluid flow through the pipe (e.g. Bernoulli's equation) based on its piping configuration, flow rate, its pressure source, etc. If your parallel runs separate upstream with a right angle connection for example and then shortly make another right angle turn, you will introduce different piping flow resistance and a flow disturbance effect in the flow pattern and which may serve to filter or modify out some flow fluctuations giving you two slightly different frequency responses to the same flow disturbance. By its nature, flow is noisy and catching real life fluctuation in the small millisecond range on a DCS can be difficult and a question could arise as to whether these flow "fluctuations" are real or some how they are a system noise artifact (e.g. noise generated by the system and not the flow). A good start in reducing any electrical noise is to make sure that your grounding and shielding is done correctly and that you have followed the manufacturer's recommendations in this regard, including the use of grounding rings if appropriate. Second, make sure that no AC supply line run parallel to your signal wires. If your two meters signal wires run in separate return paths, this can be a clue as to how electrical noise might enter one meter's reading and not the other's. If they are run in steel conduit so much the better from a noise perspective.


William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.

 

Thanks for the detailed suggestion. Earthing provided to the flow meter is proper. In Fluctuations amplitude of the o/p gives positive spikes. Both the flow meters gives such fluctuations near about same time. Fluctuations occur after around every hour and last for 2-3 mins. We have set 5 seconds damping in o/p yet spikes are there. I will check the electrical noise points that you have suggested.

 

you might try using an electrical jumper between the piping flanges if you have metal piping, or adding grounding rings if you do not.

 

Is it possible to check the flow with a portable ultrasonic meter to rule out any hydrodynamic disturbance?

Any heavy electrical machinery starting in intervals?

Geir

 

To Mr Geir,

Yes, we are considering to check it with ultrasonic flow meter. If there is change in water composition then electromagnetic FM can detect it, but ultrasonic FM may not be able to detect it properly as impurities may be in dissolved form or as small particles in the water.

Yes, heavy machines get started near around the flow meter periodically and we are observing its pattern too. We have simulated FMs electronic set by providing external signal through simulator. We did not get any fluctuation during this simulation period .

To d

Flow meter has grounding electrode so it does not require grounding ring.

 

you may need a ground strap so as to to connect the two ends of the adjacent pipe together, this prevents the currents in the pipe from flowing through the meter bore.