Problem facing in O&G site


Thread Starter

Renjith philip

I've got 2 pressure signals coming from the field to a honeywell PLC and from there to ABB DCS. If i'm injecting 4-20mA from ABB marshalling cabinet or from the honeywell PLC cabinet i'm able to get correct value reading in the HMI of ABB. but when the loop is powered (when actual transmitter is sending 4-20mA) i'm not able to get correct values in HMI, one signal is showing over range and other one under range in the HMI while 4 mA is coming. How to solve this issue ??? pls give suggessions....
My first thought is that you have a ground loop in the analog power supply.

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Can it be assumed that you have one (or two) loop powered transmitter(s), each of which is sending its 4-20mA signal to both the H-W and the ABB?

If so, doing such assumes:
1) that the inputs are wired in series, along with the loop power supply and the transmitter.
2) that the inputs have a fair degree of isolation.
3) that the power supply can supply sufficient voltage to drive the total loop resistance (input resistance, voltage drop of the transmitter, plus wiring resistance)
4) that the inputs are configured/setup correctly to read the true values (match the transmitters' output).

Overranging or underranging can be symptoms of
1) miswiring
2) excessive common mode at non-isolated inputs.
3) transmitter fail safe mode, which can be set for either low scale or high scale on most DCS/PLCs.
4) sometimes, loss of a 4-20mA signal can cause the input to drive high or low, depending on the input's system.

Underranging can be symptomatic of an inadequate power supply or just too much loop impedance for the power supply. Is your power supply associated with one of the two devices, or independent of either?

You don't provide models or the age of the DCS/PLC, so non-isolated inputs are possible and even probable. A solution for excessive common mode causing pegged readings is to use a loop isolator: 4-20mA in, 4-20mA out, where the input is isolated from the output.

Your ability to get the correct value when "injecting" a 4-20 says that the inputs are scaled properly, but you need to establish if an input failure mode is available or turned on for each of the inputs? fail high or fail low?

Can you get a correct reading of the transmitter signal when the loop is wired up? Xma at Y pressure?

Is the high or low reading indicative of miswiring or even an open circuit?
Looks like the pressure transmitters are current source and not current sink type. Check with transmitters vendor if txs can be current sink.

I personally experience this in gas detectors.