I have a six inch Rosemount Magnetic flow meter in service on a caustic scrubber. The meter measures the pump around from the sump back to the top of the scrubber. In normal operation it reads 200GPM. When the unit is started there are no problems, as it runs over time the meter starts to swing and lose the upper-end say between 20-150 GPM. After a few days the top end drops off further and a few days after that it just stops reading flow altogether. The meter is fine, no alarms no problems reported...it just can't see anything. Ther is still flow through the pipe, no plugging. We recently bought a conductivity meter for the lab and I tested samples for as far back as I could go and got the following.
Meter swinging @ 215 millisemens on 8/12

Meter barly regeistering flow @ 225 milliseamens 8/20

Meter flatlined @ 285 milliseamens on 8/22

It seems that the higher the conductivity the more it effects the meter.

When we take the unit down and clean, the meter comes back and works fine for awhile (6-8 weeks) then starts to swing/drop-off until no flow signal
At this time I have no conductivity numbers for a "clean" system.
I have stated that this seems to be more of a process/application issue than a meter issue but for some reason I still get to try to make it work.

There is an unknown amount of Hydrocarbon in the sample. I say unknown because the samples don't seem to have the seaperation I was expecting to see so I could judge an amount.

I know Hydrocarbons are bad for Mag meters but what about very high conductivity?

 

VERY high conductivity, like that found in sea water must be compensated for with special circuitry and sometimes a special transmitter. What I suspect is that as the system gets dirty, the electrodes are getting coated, and once the coating reaches the point where it breaks the circuit through the fluid, the meter stops registering. I could be wrong. But that's my first guess, based on what you said.

Walt Boyes

Walt Boyes
Spitzer and Boyes LLC
3433 Connecticut St
St Louis MO 63118
[email protected]

 

Perhaps checking the Emerson corrosion guide may be useful. If the caustic is at a high temperature it can be quite harsh on flow meters.

http://www.documentation.emersonpro...ic_mmisami/documents/data_sheets/gi-00415.pdf

Chris Jennings

 

Magnetic flowmeters have two electrodes in contact with the process fluid, the caustic stream in this case. The behavior you describe is common for processes that coat the surface of the electrodes. The coating forms a high resistance barrier that eventually becomes high enough resistance to reduce current flow between the electrodes to near zero. The solution is to keep the electrodes clean. There are options for most magnetic flowmeters to install ultrasonic cleaning circuits on the electrodes. These are commonly used in the pulp and paper industry where pulp gradually coats the electrodes. Call your magnetic flowmeter supplier and determine if they can install ultrasonic cleaning on the electrodes.

Dick Caro

 

The scaling on the electrode may be the cause of the problem. Also, check if excitation frequency can be increased.

Regards
Sanjeev

 

I couldn't find any magnetic flow meter with ultrasonic cleaning circuit. Could you/anyone please list some of the Magnetic model/Brands which have ultrasonic cleaning options?

I will thank for your favor.

 

Ultrasonic cleaning has kind of fallen out of favor. ABB used to make versions, as did some others. But, "Ultrasonic cleaning is like prayer, Walter," the late great Christian Dubbick of Krohne said to me. "You have to believe it is working."

I recommend either hot-tap removable electrodes or a bypass line so the primary can be cleaned without stopping flow. I believe Krohne still makes the removable electrodes.

Walt Boyes

 

You didn't describe the meter orientation. Flow down seldom works because air bubbles seem to collect around the electrodes isolating them from the process. I assume your electrodes are horizontal for a horizontal pipe.

We measure Sulphuric acid all the time, at 100,000 microsiemens. If you suspect the electrodes are coating up you can power the meter down and measure between electrodes with a multimeter.

Measure between each electrode and the grounding electrode/ring also they should be the same.