Am I doing right if I'll place a gate valve or butterfly valve or any valve before a magnetic flowmeter? Just to stop the flow of the process in particular operation? I mean there will be no spill of fluids whenever time comes the instrument is replaced/serviced? Or much worst case... a pressurized fluid comes out, if not closed during replacement/servicing.

Thanks,

jan michael albaran

wraith1523 at yahoo. com

 

Yes, it's right if you place a valve before a magnetic flowmeter.
It's better to place a ball valve with same pipe diameter.

 

If you place the valve upstream of the meter, be sure it is a dead minimum of five times the diameter, measured from the electrode plane. If you can possibly do it, put the valve ten diameters away. Also, whenever possible, provide a valved bypass... meters break.

Walt Boyes
Editor in Chief
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Read my blog SoundOFF!! At www.controlglobal.com/soundoff

 

The problem with putting a valve in front of the magnetic flowmeter is the turbulence it creates will effect the flowmeter's accuracy. You normally want about 10-20 pipe diameters between the valve and flowmeter.

The butterfly valve will have a worse effect than a ball valve or plug valve. The flowmeter will be very reliable so you really don't need to provide a special valve for servicing, simply isolate the source at pump or tank.

Hope this helps.

Roy

 

Well, yes and no.

Because the field of a magnetic flowmeter extends from pipe wall to pipe wall, the voltage at the electrodes is the average of all the velocity vectors impinging on the field. Remember, Faraday didn't care what the moving conductor _was_.

What this means is that with the exception of large amounts of voids, i.e., bubbles, the signal from the magmeter primary IS the average velocity in the pipe. This permits a relatively short straight run upstream of the meter...much shorter than would be required with, say, a turbine meter, or a vortex meter. Most manufacturers specify 3 diameters upstream of the electrode plane (which is the centerline of the meter housing, end to end), but I recommend that if you can go longer, you should do so.

Because of this, as long as your valve is as far upstream as it can go, you should be okay. Yes, I really prefer pinch or ball valves to other valves, but if you do it right, it shouldn't matter.

EXCEPT FOR ONE THING. Mounting a mag meter less than 100 diameters
downstream from a source of spiraling flow (2 elbows out of plane, for example) will cause an error of up to as much as 40%.

And I still recommend bypasses. It's fine when the meter works. But oh, when it doesn't...

Walt Boyes
Editor in Chief
Control and Controlglobal.com
www.controlglobal.com
Mailto:[email protected]
Read my blog SoundOFF!! At www.controlglobal.com/soundoff