On-site instrument failure analysis (one question per week) – Question 11

First, determine the scope of the fault: 1. Both the DCS indications and the on-site gauge readings are low； 2. The on-site gauge readings are normal, while the DCS readings are low. In case 1, the reasons are as follows: a. The pressure guide tube on the positive-pressure side is blocked by slag or dirt, resulting in low pressure in the positive-pressure chamber. Solution: Clear the pipes. b. There is leakage on the positive pressure side, and the balance valve has internal leakage; solution: fix the source of the leakage and tighten the balance valve. c. Due to improper installation, the positive pressure port of the orifice plate is located too low, or the positive pressure side of the diaphragm box of the differential pressure transmitter is too high; solution: recalibrate it. d. Neither the differential pressure transmitter nor the DCS performs any square-root processing on the differential pressure signal. Solution: Either the transmitter or the DCS can take the square root of the differential pressure signal. e. Due to the power distribution in the DCS card, the +24V voltage is either too low or too high, resulting in reduced signal strength. Solution: Replace the DCS safety barrier (or surge protector) to restore normal power supply. If it is case 2, it may be due to an imbalance in the ratio between the transmitter’s output signal and the differential pressure signal; the solution is to replace the output board or the transmitter. b. Problems with the safety barriers and surge protectors in the DCS control cabinet result in a decrease in the signal current. Solution: Use the short-circuit method to check step by step and replace it. c. Neither the transmitter’s output signal nor the DCS takes the square root of the signal. Solution: Take the square root of any one of the steps.

The positive pressure gauge tube is blocked or leaking, or both the positive and negative pressure gauge tubes are blocked (resolve the blockage, clear the pipeline, or fix the leak by tightening the connections)； The pressure tapping valve is not fully open, the bypass valve is open; the balance valve equipped with the three-valve assembly is not sealed properly and is leaking – (address the leakage issue with the balance valve) ； The density of the (liquid) medium in the pressure guiding tube changes, or the instrument pipeline filled with water contains gas due to improper installation (remove the gas from the pipeline and reinstall it properly) ； The range of those equipped with secondary meters or DCS does not match the design requirements; (change the range of the secondary meter or DCS) ； No square root function is built into the differential pressure transmitter or the DCS; (intelligent transmitters have this function built in, ordinary differential pressure transmitters can be connected to a square root converter, or the DCS can be used to perform the square root operation – but it is sufficient that one of these devices performs the square root function, as both doing it at the same time is not allowed.) These are just my personal opinions for reference only. We learn from each other*.

The reasons for this are as follows: 1. The pressure guide tube on the positive pressure side may be blocked； 2. There is a leakage point on the positive pressure side, resulting in low pressure on that side ； 3. Internal leakage in the balance valve ； 4. Air is accumulated in the positive pressure chamber of the differential pressure transmitter, or there is air in the pressure guiding tube on the positive pressure side. Solution: 1. For issues 1 and 4, they can be addressed by means of waste discharge. That is, first open the balance valve, then close the valve on the negative pressure side, and next open the drain valve in the positive pressure chamber to allow drainage, checking whether any substances or air are discharged until the drainage process proceeds normally ； 2. Handling the issue related to 2 is relatively simple: identify the leakage point and either weld or tighten it until the problem is resolved ； 3. For the issue related to 3, the balance valve can be replaced directly ； It is estimated that the possibility of internal leakage in the balance valve is relatively low.

1) The transmitter has zero drift; replace the transmitter； 2) The orifice plate is installed in the wrong direction; restore it to the correct installation orientation ； 3) Internal leakage in the balance valve; replace the three-valve assembly ； 4) Positive pressure valve is leaking; replace the three-valve assembly ； 5) Positive pressure tube is leaking; eliminate the leak ； 6) There is vaporization inside the positive pressure pipe; eliminate this vaporization phenomenon ； 7) No square root calculation was performed; the square root function should be set in the transmitter or DCS ； 8) The safety barrier’s zero point is too low; calibrate the safety barrier or replace it.

Check from the installation: whether there is a static head, and whether the positive and negative pressure conduits are connected in reverse. From the perspective of operation: There is air leakage in the negative pressure chamber; has a solution been found?

Possible reasons: 1. Bubbles or dirt present in the pressure tap tube； 2. There is a leakage point on the positive pressure side ； 2. The inconsistency between the transmitter’s calibration direction and installation direction leads to a shift in the initial zero point after reinstallation ； Solution: 1. Remove impurities through continuous discharge ； 2. Fix the leakage points in the fastening process ； 3. Close the positive and negative pressure valves, open the balance valve to equalize the pressures in the positive and negative chambers, and then recalibrate the zero point.

Consider the following aspects: First, on the positive pressure side, from the pressure tapping point of the orifice plate to the transmitter, check for any leaks； The second is to check whether temperature and pressure compensation has been performed ； Third, check whether the signal is attenuated ； Finally, check whether a square root has been taken in the program or table.

Judging from the title, there is no problem with the transmitter itself； Then the problem must lie in the installation ； 1. The positive-pressure pipe and the negative-pressure pipe are not on the same plane (i.e., the negative-pressure pipe is higher than the positive-pressure pipe). 2. There is a leakage point in the positive-pressure pipe. 3. There is gas inside the positive-pressure pipe ； 4. Blockage on the positive pressure side

1. Blockage in the positive pressure chamber 2. Leakage in the positive pressure chamber 3. Internal leakage in the balance valve 4. The pressure sampling port of the pressure sampling tube is above the horizontal line centered on the pipeline 5. Is temperature compensation functioning properly? 6. Is square root calculation enabled?

Installation issues: positive pressure chamber blocked, positive pressure chamber leaking, air in the pressure sensing tube, leakage in the balance valve; the pressure sensing port is located above the horizontal line of the pipeline center