Answer Posted / sam

In any industrial plant there will always be good records
and documentation on all equipment as well as all
instrumentation installed on the plant. The easiest in your
case is to find the FT tag number on the P&ID drawings or
the CCR faceplate and look at the data sheet of the missing
transmitter. The original calibration in differential
pressure will be on there as well as the model number of the
transmitter you need.
All you then have to do is calibrate the new DPT with the
diff press as per the data sheet and it will work as before.
In most cases the squire root extraction is done in the DCS
and not in the transmitter so try it just like this first.

The faceplate in the CCR will normally be set up to display
the min and max flow so that will not help you to find the
calibration diff pressure of the transmitter.

You can also try and find the original transmitter that was
removed and put power on it ans see what it was calibrated
for before. Even if the transmitter is faulty it might still
be able to display information. The is just as a last resort
since you cannot be sure if this calibration is what it
should be and it is better to double check it with the
documentation. This is just a method to double check
information.

As a desperate measure you can also measure the upstream and
downstream pressure as it is now during normal operation and
calibrate your transmitter to 1,5 that differential pressure.
For example:
Say your upstream pressure before the orifice is 300Kpa and
downstream it is 270Kpa you therefore have a DP over the
orifice of 30Kpa during normal operation.
Calibrate the transmitter for 0 to 45Kpa.
The flow indication inside the CCR will indicate the correct
flow since the extraction and the display is already setup
as it was originally. Have a look at what the % the flow is
indicated on the CCR faceplate. Since it is normal
operations it should be about 50% of the scale on the
faceplate. If it is not, modify the DPT calibration until
the current flow display about 50% of the full scale in the
CCR. You should be pretty close then.
This is not a very good or accurate way to do it and you
should keep on looking for the original design specs and
then re-calibrate the DPT when you found it.
Good luck

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