Calibration issues of differential pressure flowmeters for measuring steam
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I’m seeking advice from experts: one of the differential pressure flowmeters used for measuring steam at our site is giving inaccurate readings. We have checked the set parameters and the calculation documents, and everything seems to be fine; moreover, some water was drained from the pressure tapping tubes and no impurities were found. Do you have any other methods that could be used to calibrate this flowmeter?
First, calibrate the differential pressure transmitter to check whether it is within the allowable error range. Check the pressure guide tube and valves for any leaks or blockages.
The positive and negative pressure valves of the differential pressure transmitter are closed, the balance valve is fully open, so the reading is 0; there are no leaks in the balance valve. The pressure guide pipes have been drained of condensate and are not blocked. If there were a leak, it would be impossible to reach it at the base of the pipe. If steam were leaking, there should be condensate present, but none can be observed in this case
“\"It’s not allowed.\" Is it higher or lower than normal? If it is lower, it is necessary to check whether there might be internal leakage in the balance valve. Calibrating the transmitter is also a task that needs to be carried out.
Let’s take another look at the orifice plate to see if it has deformed
Open the drain valves on the two pressure tap pipes and allow drainage for 5 minutes; make sure it is at least five minutes. It may affect the measurement if it’s a bit clogged or not completely clogged. . After discharge, condensate can be manually added to the condensation tank, which helps to restore normal measurements more quickly; it is also possible to let it condense on its own, but this will take more than half an hour. If that doesn’t work, it’s necessary to remove the differential pressure transmitter and perform a pressure calibration to determine whether the transmitter diaphragm is damaged and whether it is within the allowable error range. This step is crucial; don’t assume there are no problems – only a pressure calibration can reveal the truth. Everything should be based on data. At the same time, check whether square root calculation is being performed and whether the compensation parameters have been modified; the steam flow rate must definitely be compensated for temperature and pressure – otherwise the values will not be accurate. The maintenance of differential pressure steam flow meters involves three key points: 1) whether the pressure transfer pipelines are unobstructed, 2) whether the transmitter is in good condition, and 3) the setting of compensation parameters. We can keep looking for reasons based on these three key points; this is something we can do on our own. Three points are sufficient to determine the issue; only then can it be determined whether there is wear on the orifice plate or a problem with the process pipeline.
If there is a discrepancy in the amount of condensate in the two pressure measurement lines, including that in the condensation tank, the readings will also be inaccurate; it is necessary to fill the tanks with condensate to establish a zero point through equal pressure
On-site, the signals from the voltage transformer, along with temperature and pressure data, are fed into a digital display unit where a compensation formula is used to carry out the calculations
“The fact that the balance valve is fully open and shown as 0” does not necessarily mean that the differential pressure transmitter is accurate; it still needs to be calibrated. Even the pressure transmitter should be calibrated, and it is also necessary to verify the accuracy of the temperature measurement