Questions regarding double-flange capillary differential pressure level gauges
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The level gauge for one of the distillation towers in the plant is a double-flange type gauge with a capillary. I’ve only been working at this chemical plant for half a year, and my supervisor asked me to calibrate this level gauge. Here’s how I did it: I closed the pressure-taking valves on both the lower and upper flanges, then removed the screws from those flanges. I allowed air to enter the system; I left one screw on each flange, without removing them, keeping them vertically in place. The transmitter uses the HART protocol, and I used a BT200 data logger to communicate with it. After that, I checked the lower and upper measurement limits – the lower limit was -14.340 KPA, while the upper limit was 0.6981 KPA. Thus, the range of measurement was 15.0381 KPA. I then set the lower limit to 0 KPA and the upper limit to 15.0381 KPA, after which I performed the zeroing procedure. After zeroing, the transmitter displayed 0%. I then restored the original measurement range, and the gauge showed over 80%. I was so scared that I didn’t dare to put the system into operation. Isn’t this the correct way to calibrate a level gauge? ? The master scolded me mercilessly. I’m asking fellow netizens for help. What exactly did I do wrong?
The zero point should be negative (flange spacing multiplied by the density of the silicone oil inside the capillary), and the range is the measured liquid level range multiplied by the density of the medium being measured.
The upper and lower measurement limits are calculated based on the range; the actual upper and lower limits are determined according to the installation on-site. What is measured for the atmosphere represents the lower limit of the range, and adding this value to the range gives the upper limit of the range
Bro, after I changed it to positive-pressure zeroing, I set the zero point back to negative pressure, but the gauge still shows a reading – shouldn’t it display 0 instead? Do the inlet and outlet valves need to be opened for it to work properly?
When zeroing, the lower limit of the range is negative pressure; it needs to be changed to a 0-to- range in order to perform calibration, right? No need to change it? Can it be set to 0 directly?
Bro, to zero it out, remove the two flanges and place them at the same level; then the gauge will show a differential pressure of 0. If it doesn’t show 0, perform a zero-point calibration using the handheld controller. After installation, before putting it into use, check whether the differential pressure is correct; it should be negative. Depending on the type of silicone oil used, calculate how much the zero point should shift based on the measurement height, and use this value as the lower limit of the range. Check what KPA value is set as the process zero point in the gauge, and if it does not match the value you calculated, adjust it accordingly. Based on the density of your medium, calculate the differential pressure value for the range; adding this to the differential pressure at zero point gives you the upper limit of the range.
Thanks, bro. I saw someone say that it’s not necessary to remove the two flanges; you can just shut off the manual valve, loosen the bolts, and allow air to flow in order to perform calibration. If that’s the case, how should calibration be carried out then?
Close the pressure tapping valves on the lower and upper flanges, then remove all the screws securing those flanges, allow air to flow in, and use a handheld controller to check the actual pressure value. Change the LRV lower limit to the actual pressure value, and modify the URV after calculating it based on the range value.
If the master can say you’re wrong, that proves he definitely knows how to do it. You can just ask your master to handle it. Don’t be upset if your teacher scolds you; sometimes it’s really a necessary part of the growth process. Hahaha
When the two flanges are at the same horizontal level, the pressure is 0 kPa. If you change the measurement range, reset it, and then change it back to the original range, you end up ignoring the amount of displacement that has occurred. Be more careful next time; generally, there’s no need to adjust the parameters unless the double-flange diaphragm is damaged. Check for any damage, oil leakage, or dirt accumulation, and be careful when cleaning to avoid scratching or damaging the diaphragm