Instruments: [One Question per Week] First Week of 2011

The inaccurate flow indication of a orifice plate flow meter may be due to the following reasons: 1. The pressure guide tubes on the high-pressure and low-pressure sides are installed in the wrong direction, causing the flow reading to become zero. 2. The straight sections before and after the pipeline do not meet the requirements for orifice plate measurement, resulting in the medium failing to reach a turbulent state. 3. The inaccurate installation location of the transmitter results in a gas-liquid mixture at the measurement end, leading to inaccurate flow readings. 4. Common-mode interference in the signal transmission cable causes the flow rate to be either too high or too low. 5. If the measured medium requires temperature and pressure compensation, consider whether the calculation formula of the DCS temperature and pressure compensation module is appropriate. That’s all I can think of for now; please give me some advice, moderator! Grow together

The process indicates that the newly installed orifice plate flow meter is giving inaccurate readings (the secondary instrument is a DCS), and it is normal for the instrumentation technician to calibrate the transmitter. Please analyze what other possible causes there might be 1. Since the transmitter has been calibrated, its output can be measured to determine the flow rate value that should be displayed. By comparing this with the value shown on the DCS, if the calculated value matches the displayed value, any issues with the display section can be ruled out. Conversely, it is a problem with the display section; in this case, check: 1) whether there are any issues with the direct connection between the transmitter and the DCS ； 2) Check peripheral components such as isolation modules and I/O cards ； 3) For those with temperature and pressure compensation, check the temperature and pressure compensation circuits ； 4) DCS display section. 2. The DCS shows no issues; check whether there are any problems such as blockages or leaks in the pressure transfer tubes. 3. Since it is a new installation, check whether the orifice plate is installed correctly and that it has not been installed in the wrong direction ； 4. Check whether the parameters in the orifice plate design specifications match the actual operating conditions; otherwise, there is an issue with the orifice plate design ； Also, check whether the parameters set for the transmitter and DCS are correct. 5. Finally, it’s necessary to check whether there are any issues with the process; the flow meter shows correct values, but there are many cases of disputes between the instruments and the process itself~~~

The straight pipe section upstream of the throttle element is insufficient in length, with a 90-degree elbow in the same plane； The orifice plate thickness exceeds the specified value, resulting in a higher flow rate ； The length of the cylindrical portion of the orifice plate is too large, resulting in a high flow rate ； The distance between the negative pressure tap and the end face of the orifice plate is too large, resulting in a high flow rate ； The negative pressure tap of the orifice plate is blocked, resulting in an excessive flow rate ； The temperature and pressure compensation is not done correctly ；

The transmitter is fine, so the problem lies on the measurement side. I. Pipeline issues: changes in the density of the medium ； II. Malfunctions of orifice plate flow meters: 1. Blockage in the signal guide tube and needle valve; cleaning is required to remove debris ； 2. Faults in the orifice plate itself (orifice plate eccentricity, orifice plate bending, wear and dulling of the orifice plate edges).

1. Is the differential pressure range consistent with the calculation sheet?； 2. Ensure that the squaring is configured properly; do not apply squaring to both the transmitter and the DCS at the same time, nor skip squaring on both – choose one of them for squaring ； 3. Is the compensation setting appropriate? The pressure compensation value of the field transmitter should match that set in the DCS ； 4. Check whether the pressure tapping tube is unobstructed ； 5. Check whether the balance valve of the three-valve assembly is properly closed ； 6. Are there any mistakes in the DCS configuration? ； 7. Is the installation of the orifice plate in compliance with the specifications?

Reply to 1# cnooczhouyx: 1. Leakage in the balance valve. 2. Presence of square root operation. 3. Inconsistent range settings for the DCS and transmitters. 4. Changes in the density of the process medium. 5. The pressure regulator not being properly calibrated. 6. The orifice plate installed in the wrong direction. 7. Leakage in the pressure lead pipes or presence of bubbles. 8. Insufficient filling of the isolation liquid, resulting in a fluid column difference. 9. Leakage in the process bypass

The main reasons are: 1. Leakage in the balance valve; 2. Presence of secondary square root operations; 3. Inconsistent range settings for the DCS and transmitter; 4. Changes in the density of the process medium; 5. The safety device not being properly calibrated; 6. The orifice plate being installed in the wrong direction; 7. Leakage or bubbles in the pressure lead pipes; 8. Insufficient filling of the isolation fluid, resulting in a liquid column difference; 9. Leakage in the process bypass

First, it is necessary to check whether the installation method is correct. The orifice plate should be installed in a horizontal pipe; generally, the straight sections before and after it should be 10D in length ahead and 5D behind. It must also be installed in front of the control valve, with the pressure tapping points located at an angle of ≥45 degrees below the centerline of the horizontal pipe. If the installation is correct, the following checks can be carried out: 1. Check for any leakage on the positive-pressure side of the orifice plate. 2. Check whether the orifice plate is worn. 3. Verify that the orifice plate is not installed in the wrong direction. 4. Ensure that the actual operating parameters match those specified in the design documents. 5. Clean the orifice plate flow meter by removing debris from it. That’s all for now; I’ll add more details if I think of anything else.

Problems with the selection of the orifice plate itself, issues in the manufacturing process, blockages in the holes of the orifice plate, blockages or leaks in the pressure transfer tubes, or even incorrect connection of the positive and negative pressure sides, as well as overly dirty process media, are all possible causes