Comm Failure in a Mag Meter

[Noway2]

I am experiencing some trouble with an ABB Mag Master, electromagnetic flow meter and I am hoping that someone with some experience might have some ideas as to what could be the trouble.

The meter is measuring the flow of water (ranging from 45-75 deg F - well within operating range) in the condenser side of a water chiller. On a couple of occasions over the last few months, the meter has started reporting strange values including the positive and negative rails and finally declares an "installation fault" asking if the coil is installed or something similar, but I forgot exactly what.

Prior to the fault condition, monitoring the 4-20mA output, with the connection to the DCS input open circuited, it shows about 30Vdc with a 1V pulse AC that I am reasonably certain is the HART signal. When the unit goes into failure mode, the HART signal all but disappears, though the 30Vdc remains.

We performed an experiment today where the meter was correctly reading zero flow and was reading correctly until the flow reached a certain level at which point the meter stopped reading and the HART signal disappeared. Stopping the flow and closing the valve did not cause the signal to return.

The only installation spec violation that I can see is that the meter was placed too close to a set of elbows / hard bends. Normally, you want 5x pipe diameter before and 2x after. What effect, if any would this violation cause and could it cause the problem seen?

Is there anything else that anyone can think of that could cause the described failure?

 

[robertjo24]

#1- I would look for poor electrical connections in the meter hardware that could be intermittent, or check the coils themselves when in a failed condition. They could be temperature sensitive and fail when too cold or too hot.

#2- this question might get a quicker/more acurate answer if asked in the “Programmable Logic Controllers (PLC)” forum located within this group

 

[itsmoked]

For the HART to disappear too I'm inclined to bet the unit is just flat failing. It certainly seems like the output driver is having problems. I suppose it could be the field wiring but would expect the HART to still exit the unit.




PS You never responded to my last email!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[djs]

Could you be getting an empty pipe? "Empty Pipe" with mag flowmeters is a big no-no. The pipe does not have to be completely empty, just enough so that one or more sensors are uncoverd.

 

[Noway2]

I am starting to think that it may just be a hardware failure. I have been told that the tech is waiting to hear back from ABB regarding when they can come and look at this. It does seem as though the electronics are malfunctioning in that they lost both the Hart and apparently sensing of the coil.

As far as air in the line, unfortunately, the coil is in the high part of that local portion of the system, but it isn't the highest part of the system. I know that the pipe isn't empty, but I don't know if there is enough air the line to uncover a sensor. The plant was under renovation until a couple of months ago and it is possible that trapped air may have accumulated in this portion post start up. This is something we will need to look at.