Discussion on the utility of monitoring line in the judgment of air compressor equipment
1 Troubleshooting
According to the surge mechanism of the centrifugal air compressor and for the surge control of this type of unit, we analyze the reason from simple to complex step by step.
1.1 Judging authenticity
Judging whether the authenticity is really caused by the surge or the anti-asthmatic protection malfunction.
According to the historical trend of online monitoring, it can be seen that the vibration of the first, second and third axis suddenly increased beyond the stop value. We detected that the vibration sensor and the corresponding line were normal, and the duty officer said that he actually heard the sound of breathing. From this, it is judged that the crew has actually surged, and the anti-wheeze protection normally sends a stop command.
1.2 Analysis of the causes of general surge
According to two possible situations, the first step is that the unit cannot overcome the back pressure of the system, and the compressed air cannot be discharged smoothly. In another case, the suction port is not smooth, so that the unit does not have enough suction, and the corresponding pressure cannot be established, resulting in surge. .
For the first possible situation, we checked the outlet check valve, the outlet bypass valve was all normal; checked the air circuit of the aftercooler, and found that the blockage was more serious. The highest pressure loss recorded at the time was 1 bar, exhaust Poorness may be the cause of surge, and we replaced the spare cooler core.
For the second possible case, we checked the air filter, suction line and inlet guide vane control valve, all normal.
During the start-up test, the unit was operating normally, and the post-cooler pressure loss was 0.2 bar, and the operation was observed. As of August 5, the machine was suddenly unloaded automatically, and the crew record indicated that it was anti-asthmatic and unloaded.
1.3 Analysis from the control aspect
We analyzed the surge control of the Atlas ZH series centrifugal air compressor. The surge control of the unit mainly refers to two parameters Pdt19 and Pdt20, which are the secondary nozzle differential pressure and the secondary differential pressure, respectively. These two parameters constitute the two axes of the surge control (for example).
The calculation formula of the surge control is S=(1000Pdt19×Gain)+Bias]-Pdt20/0.01 GainDDD surge line slope.
The offset of the BiasDDD surge control line on the Y-axis.
SDDD is the distance from the operating point of the unit to the surge control line.
The Gain value and the Bias value are fixed values ​​that are experimentally completed during commissioning. The main reference is Pdt19 and Pdt20 during operation. When the S value is close to 0, the anti-surge protection of the unit starts to operate, and the air intake and air release are adjusted. The unit's operating conditions allow the unit to operate in a safe area. We observed the historical curve of the differential pressure between the secondary nozzle and the differential pressure between the two stages. It was found that before the unit was unloaded, the differential pressure of the secondary nozzle suddenly dropped from 132 to 0, which was abnormal here. (eg) Because the unit has been inspected both in the suction line and the discharge line, it is mainly suspected that there is a problem with the control system, possibly the module or the computer sends an error signal. We have adopted the replacement method to confirm that the specifications of the four units on the site are exactly the same. We replace the sensors, modules and unit control computer boards of the secondary nozzle differential pressure and the secondary differential pressure of the four air compressors with the machine. And the pressure pipe is unblocked. After the change, the boot is normal, observe the operation
On August 8th, the 4 air compressors were automatically unloaded again, and the query record was still a surge protection action. This online monitoring system has collected an important piece of information. It can be seen from the historical curve that the differential pressure of the secondary nozzle has a significant fluctuation before the surge protection action.
We analyze the significance of the differential pressure of the secondary nozzle, which is the parameter of the reaction flow evolved from the curved pipe flowmeter. There must be a reason for the change of the suction flow. Looking at the curve of the inlet guide vane opening at this time, there is no change, that is, the inlet guide vane valve is not adjusted. It is generally considered that it will not move, but we analyze the characteristics of this unit, although the control computer can display the entrance. The vane valve opening and the outlet bypass valve clearance, but the two displayed values ​​are the output signal and not the feedback signal, then we can not rule out the action of the inlet guide vane valve, which needs further verification. We have adjusted the electric actuators of the inlet guide vane and the outlet bypass valve accordingly, and if the actuator will act, the outlet bypass valve will be opened to allow the unit to ventilate.
Boot to observe the operation.
On August 23, 4 air compressors were automatically unloaded again. This time we query the historical curve and verify the above inference. The three-stage impeller outlet pressure is 3.9 bar, and the unit outlet pressure is 6.4 bar. At this time, the secondary nozzle pressure difference does not change. The inlet guide vanes are fully open, and the bypass valve should be in the fully closed position. From the three-stage impeller outlet pressure and the unit outlet pressure, it is judged that the bypass valve is open and the wind is released, and the control computer is not sent to the inlet guide vane and the outlet bypass valve. Adjust the signal. The cause of the automatic unloading of the unit was finally found.
2 problem handling
We replaced the electric actuator of the air compressor outlet bypass valve, and then there is no problem after starting the operation. Due to the confidentiality of the internal procedures of Atlas's executive agency, it is necessary to return to the factory for repair.
3 Conclusion
For the diagnosis of centrifugal air compressor surge, we generally analyze the two directions of suction and discharge from the perspective of working conditions. The application of real-time monitoring system can play a role of compass for unit operation and maintenance. Our engineers and technicians can quickly find the correct diagnosis direction through the analysis of the monitoring curve and judge the unit failure through step-by-step verification. The method solves the problem and shortens the diagnosis time. At the same time, it can also predict the possible failures by observing the change of the curve, and deal with the fault in the bud state to create benefits for the enterprise.
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