Diaphragm pressure gauge involved in chemical reactions of a variety of media compositions is in the entire production process of complex processes, long process. It has a high temperature, corrosion, flammable, explosive, easy to crystallize, and other characteristics. Therefore, more than 90 diaphragm pressure gauges are used in the foreign device community. These diaphragm pressure gauges are supplied with foreign devices; it is more expensive, about 200 U.S. dollars per gauge.
In my plant’s water intermodal commissioning and trial production, 39 of these diaphragm pressure gauges have failed, affecting the regular operation of the process and production, to solve the problem of diaphragm pressure gauge maintenance and calibration, to ensure that the needs of commissioning and production, combined with our practice, to discuss the specific methods of diaphragm pressure gauge maintenance and calibration.
1. There are differences between diaphragm pressure gauges and standard pressure gauges in terms of pressure measurement principle and structure.
The pressure measuring principle of the standard pressure gauge is to be calculated into the pressure measuring spring. The spring tube is under pressure, and it will produce elastic displacement and drive the connecting rod and the central gear deflection. The pointer is mounted on the main gear shaft to show the pressure measurement value. Diaphragm pressure gauge is in addition to the regular pressure gauge part, but also increase the connection flange, isolation diaphragm (isolation diaphragm to adapt to different corrosive media or temperature, can use other materials), it is in the isolation diaphragm and pressure measurement between the spring tube, filled with pressure transfer medium silicone oil.
Diaphragm pressure gauge and standard pressure gauge are different, a diaphragm pressure gauge in the process of pressure measurement, the medium to be measured only contact isolation diaphragm. This can prevent corrosive press and easy to crystallize media into the spring tube caused by corrosion and blockage of the spring tube. The simple pressure measurement principle and structure are shown in Figure 1.
2. Analysis of the causes of faults in the use of diaphragm pressure gauges.
2.1 The diaphragm pressure gauge is distributed in the TEC boundary zone, and the failure rate
2.2 Analysis of the causes of several common faults.
2.2.1 When there is pressure, its pointer does not indicate.
This failure is usually because solid vibrations loosen the connection between the pointer and the central gear shaft, and the spring tube is displaced by the pressure and cannot drive the pointer to deflect. Secondly, the silicone oil in the spring tube is leaking too much, and the force cannot be transmitted.
2.2.2 When the diaphragm pressure gauge is not measuring pressure, the low end of the pressure indication changes, the pressure rises, the pointer does not change.
2.2.3 The diaphragm pressure gauge has a jumping pointer during pressure measurement.
2.2.4 When the diaphragm pressure gauge is in the vibration of the occasion, (such as the pump outlet pressure) due to fluctuations in pressure changes, the pointer also swings back and forth, resulting in serious wear of the center gear of the pressure gauge core, the center gear, and sector gear is not a good fit, thus causing the pointer is jumping change.
3. Diaphragm pressure gauge servicing
We have overhauled more than one diaphragm pressure gauge, and we found that most of the failures are caused by the loss of silicone oil in the spring tube. So it can be said that the spring tube filled with silicone oil solves the key problem of diaphragm pressure gauge maintenance. But to the spring tube filled with silicone oil is not easy. In general, the manufacturer has a special silicone oil-filled apparatus the uses vacuum pumps to pump out the air in the spring tube, in the case of negative pressure, so that the silicone oil is automatically in the spring tube.
In the early days of the factory, we did not have such conditions and lacked the corresponding special apparatus. To solve the diaphragm pressure gauge maintenance problem, after many experiments, we have worked out a method of using injection syringes to fill the spring tube with silicone oil. In the process of filling the silicone oil, the spring tube should be placed with the pressure port facing upward, lightly knocking vibration so that the bubbles in the spring tube can overflow—this method of filling the silicone oil, to repeat the operation many times.
We deal with the jumping of the pointer. Small files and scrap steel can be used to file and scrape the worn central gear area for repair. When the shaft hole of the pointer becomes large, a small hammer can be used to tap it to shrink it so that the pointer can be closely combined with the central gear shaft.
4. Calibration of diaphragm pressure gauges
4.1 Production of special calibration tools
Our diaphragm pressure gauges are supplied from abroad with the equipment and have a standard 1/2 NPT thread to connect the pressure gauge head. In addition, each diaphragm pressure gauge is provided with a connection flange. Therefore, we want to calibrate the diaphragm pressure gauge. Consequently, we have to make particular connection joints and match calibration flange, unique connection joints as shown in Figure 2, Figure 3 is a sketch of the connection flange for calibration.
4.2 Diaphragm pressure gauge calibration procedure
4.2.1 First, we should remove the head section of the diaphragm pressure gauge from the flange. Using the special connector shown in Figure 2, connect the pressure gauge to the calibrator and adjust it to pass the calibration according to the standard pressure gauge calibration method.
4.2.2 We have to fill the silicone oil method as described in section 4 to the spring tube filled with silicone oil (no silicone oil, can also be temporarily replaced with 20 # transformer oil). After we have filled it, wrap the PTFE sealing tape around the 1/2NPT joint, pour silicone oil into the membrane with the flange, quickly twist the pressure gauge head and the flange membrane together, and make it tight.
4.2.3 We use the hexagonal plate hand to open the air release hole on the membrane box to release the residual pressure in the membrane, and then we observe the meter head pointer to make it indicate at the zero scale line.
4.2.4 We should connect the diaphragm pressure gauge to the fabricated calibration method, and we should fit it to the pressure calibrator to make it pass the calibration.
The diaphragm pressure gauges are widely used in oil transmission stations for the primary measurement of operating pressure. Still, they also have the problem of being damaged and expensive to purchase new gauges. As a result, we have to repair faulty or damaged diaphragm pressure gauges ourselves to reduce the purchase of new meters and reduce production and operating costs.