How To Identify Orifice Meter Accuracy


Thread Starter



Another engineer specified an orifice plate to measure natural gas flow. The orifice meter is installed in the process line per the installation detail of a gas measurement with orifice plate.

Process Engineer is stating the meter is measuring natural gas accurately. States its of by factor of 10x. He states the flowing pressure and temperature needs to be measured, or compensated.

How accurate is a orifice meter for natural gas measurements? My reading says between 2-4% of full scale. If that's the case, the current installation should work fine.

How do I prove the meter is measuring within a certain accuracy?

Thank you
Commissioning errors are not uncommon. You need to check to see whether the OP/xmtr is reading in the ball park, before you get all huffy about 'accuracy'.

Clamp-on ultrasonic meters can be used as a check against the orifice plate/transmitter, if the line pressure is above the minimum. But many sites do not have one.

If so, you need to get the orifice plate (OP) sizing sheet from the vendor who created it (most OPs have a manufacturer name and serial number engraved on the paddle).

Check the line pressure gauge and see what the line pressure is.

Borrow an IR thermometer and check the pipe temperature right at the OP. It should be close to the gas temperature.

Check the data on the orifice sizing sheet against the real world pressure and temperature values you discovered above.

Get a tech to show you that the transmitter URV setting corresponds to the calculated max DP on the sizing sheet.

Get the tech to show you whether the transmitter is configured for linear or square root (typically obscured as something like 'transfer function').

Have the HMI/SCADA/DCS guru show you whether the flow signal is linear or square root and what the scaling is. Compare the scaling to the maximum flow rate value on the OP sizing sheet. They should be the same. The square root function should be enabled on either the transmitter or the HMI/SCDA/DCS, but never on both.

If all the settings match up and you still doubt the transmitter output, watch the technician check the transmitter calibration by appling a pressure signal to the transmitter when it is isolated from the line.

0% DP = 0% flow
25% DP = 50% flow
50% DP = 70.7% flow
100% DP = 100% flow

Compensating for temperature and pressure deviations from design conditions is great idea if your process measurement is way off from design conditions and the temp/pressure conditions vary.

But if the temp and pressure remain relatively constant but aren't the same as the OP design conditions, you could easily re-calculate the DP/flow rate at the current OP bore size for the alternative temp and pressure and then re-range the transmitter accordingly.