[Case study] What are the common faults of differential pressure transmitters in the coal industry?

Everyone knows that the differential pressure transmitter is widely used in many industries, especially in the coal industry, measuring the differential pressure of the pipeline and measuring the liquid level are inseparable from it, so it is very affirmed by the coal industry. Differential pressure transmitters will inevitably have problems of one kind or another during long-term operation. Many people don’t know anything when they see the problem, and they don’t know how to fix it. This is also a common phenomenon, especially for new users. Use case analysis to find out what are the common faults, so that the majority of users can have the experience of visiting the scene. Case Study Shanxi Datong Coal Industry Group Co., Ltd. Steam Power Plant '30 CFB boiler desalinated water main pipe flow, measured with orifice throttle flowmeter and differential pressure transmitter'. On March 23, 2019, the process of Shanxi Datong Coal Industry Company reflected the fluctuation of the flow meter of the desalinated water main pipe. The on-site inspection of the pressure-inducing pipeline showed no leakage, no damage to heat preservation, and no leakage of heat tracing. The display on site was consistent with that in the main control room. . According to the inspection steps, first confirm the working status of the differential pressure transmitter. Zero check. Close the positive and negative pressure shut-off valve, open the balance valve, and the transmitter output indicates 0‰. @Change trend check. After the zero point inspection, each valve returns to the original open meter state, and the negative pressure chamber exhaust blowdown valve is opened. At this time, the transmitter indicates the full scale. If only the positive pressure chamber blowdown valve is opened, the output maximum value is small. Through the above steps, it can be confirmed that there is no blockage or internal leakage in the pressure guiding pipeline and each valve, and there is no problem with the electrical measurement circuit of the differential pressure transmitter. In the 3# boiler deaerator feed water thermal control system, the desalinated water header flow meter is used to measure the desalted water flow from the urea plant as shown in Figure 1. Figure 1 Demineralized water feed deaerator from a urea plant. Deaerator liquid level regulator L1075102 adjusts the opening of the deaerator liquid level control valve LV·75102 to control the flow of desalinated water. PIT-75103 and LIT-75102 monitor the pressure and liquid level of the deaerator in real time. As can be seen from Figure 3, when the deaerator liquid level LI does not fluctuate, and the PID parameters of the deaerator liquid level regulator LIC·75102 are unreasonably set, LV·75102 will fluctuate, so that the PIT-75108 indication fluctuates. In the same way, the change of the measured value of the deaerator pressure PIT·75103 will also cause the indication fluctuation of the desalinated water main pipe flow meter. After checking with the technicians, it is confirmed that the indications of L 75102 and PIT-75103 are stable, and the opening of the valve position of LV-75102 on site is stable. In the process of elimination method, almost all the possibilities of problems are eliminated, but the working conditions of the process medium in the pressure-inducing pipeline are ignored. When the demineralized water in the pressure pipe is at a high heat tracing temperature, it will vaporize to generate bubbles, and the bubbles will burst due to the rise of buoyancy, thereby disturbing the measured pressure. If the heat tracing temperature is constant, This disturbance exists regularly, and this process will cause the transmitter output to fluctuate. Solution In order to verify the rationality of this reasoning, it is necessary to find out the cause of the high temperature of the medium in the pressure guiding pipe and try to eliminate it. If the distance between the heat tracing pipe and the pressure guiding pipe is too close, the temperature of the pressure guiding pipe will be too high. Use a cotton board to effectively isolate the heat tracing pipe from the pressure guiding pipe; check that the heat tracing (low pressure steam) pressure is low and the temperature is high, check that the trap bypass is in a straight-through state, and the trap is not working, close the bypass valve to the trap After being put into use, the indication of the desalinated water main pipe flow meter gradually becomes stable and the crux of the problem is eliminated. In fact, there are many reasons for the fluctuation of the differential pressure transmitter. For example, the fault of the isolation barrier, the fault of the valve positioner, and the environmental vibration may affect the indication fluctuation of the differential pressure transmitter.

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