DP transmitter for natural gas measurement


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we install a new DP transmitter for Natural Gas measurement.

flow is 400m3/hr., line size: 4 inch
we use 2 inch DP reduce line size 2 inch.
DP max.design: 5500mmwc
orifice: 30.52mm bore.

we face problem that dp always shows high dp pressure (6500mmwc) when no flow or flow passes through line.

the line pressure is 4 kg/cm2. when line pressure we reduce 0, dp shows zero.
Poking your data into my quick sizer orifice plate program, I get an orifice bore closer to 16.75mm for 400 m3/hr.
I get a bore of ~30.5mm for 1400m3/hr.

Did someone lose a digit somewhere?

Assuming that you mean that you get 6500mmwc DP with any flow in the line, then I have to ask where are you reading the 6500mm DP? on the transmitter's display? at the receiver (DCS/PLC)?
The first thing I would do is verify that all the impulse line valves, manifold, and root valves are in the correct positions and connected properly. The second thing I would do is verify with gauges or a differential pressure gauge that the differential pressure is present at the ports of the D/P cell or not. This would divide the system in half and determine if the problem is at the D/P cell or at the orifice and piping.

William (Bill) L. Mostia, Jr. PE
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.