Brief Analysis on Fluctuation of High-Pressure Regulating Valve of Steam Turbine in a Thermal Power Plant

introduction

The Tenglong Aromatics (Zhangzhou) Co., Ltd. thermal power plant has 4×670t/h+3×150Mw units. The main engine distributed control system (DCs) uses the NT6000V3 system provided by Nanjing Keyuan Smart Technology Group Co., Ltd. The steam turbine is an ultra-high pressure impulse, single-shaft double-cylinder double-exhaust, extraction condensing steam turbine manufactured by Nanjing Steam Turbine and Electric Motor (Group) Co., Ltd., model C150-12.5/4.3. The steam inlet parameters of the steam turbine are: 12.5MPa, 534℃: the first stage extraction load is 4.3MPa/382℃/282t/h. The combined heat and power supply method is adopted to improve the energy utilization rate and the economic efficiency of the unit.

The DEH control system is mainly composed of two high-pressure main steam valves, four high-pressure regulating valves to control high-pressure steam inlet, and a low-pressure extraction steam regulating valve for low-pressure extraction steam heating. Speed ​​control, power control, main steam pressure control, overspeed control, extraction steam control, online testing, etc. in the DEH control system require the direct participation of high-pressure regulating valves, which play a decisive role. The high-pressure regulating valve is the core actuator of the DEH control and ETs protection system, and has a significant impact on the safety of the unit. When the steam turbine exceeds the safety limit, all steam inlet valves of the steam turbine can be closed through the system to achieve emergency shutdown.

1. Fluctuation of high pressure regulating valve

1. The high-pressure regulating valve of steam turbine No. 12 vibrates due to signal interference. On October 11, 2020, when the maintenance personnel were dealing with equipment defects in the DCS electronic room, they found that the high-pressure regulating valve of steam turbine No. 2 fluctuated within a certain range. After checking the monitoring, no personnel were working in the DEH cabinet and the ETS cabinet. The maintenance personnel were checking the control signal line of the newly added electric valve in the control cabinet opposite the DEH cabinet (about 1m away), and intermittently used the intercom during the process. The preliminary judgment is: the intercom is used in the DCS electronic room, and due to signal interference, the four high-pressure regulating valves fluctuate to varying degrees. In order to verify the above speculation again, the operating personnel and maintenance personnel used the intercom to directly interfere after being fully prepared. As a result, similar fluctuations occurred in all four high-pressure regulating valves.

1. Fluctuation of No. 2 high pressure regulating valve of No. 21 steam turbine due to valve stem falling off

2020-12-05T17:19, the load of turbine No. 1 was 100.5Mw, the main steam pressure was 8.2MPa, the main steam temperature was 533℃, and the vacuum was -93.5kPa. The high and low pressure were put into normal operation and the valve was running smoothly. It was found that the main steam pressure of turbine No. 1 suddenly increased to 9.5MPa, and the load dropped to 64.5Mw. The report value was long and the boiler was contacted to pay attention to the change in main steam pressure. It was found from the historical records of the DCs system that the command and feedback of the No. 2 high-pressure regulating valve were both 100%, but the pressure after the valve suddenly dropped from 7.3MPa to 4.4MPa (minimum 2.75MPa). It was initially judged that the valve stem of the No. 2 high-pressure regulating valve fell off, causing the fluctuation.

1.31 Steam Turbine High-Pressure Control Valve Stem Axially Rotated Steam Turbine No. 1 was put into operation in March 2013. Intermittent axial rotation of the valve stems of the four high-pressure control valves occurred. The axial rotation of the valve stems changed with the valve position command, the one-way valve, etc., and it was difficult to grasp its specific rules. The axial rotation of the valve stems of the high-pressure control valves No. 1 and No. 2 was more obvious, followed by the high-pressure control valve No. 3, and the high-pressure control valve No. 4 basically had no such phenomenon (it may also be closely related to the one-way valve and the valve opening size). The axial rotation of the high-pressure control valve has existed for a long time, which can easily lead to the fracture of the bolt-type roller needle bearing behind the high-pressure control valve operating seat, thereby aggravating the axial rotation.

1. The high-pressure regulating valve of the No. 43 steam turbine cannot be opened or closes suddenly under hot conditions

The No. 3 steam turbine entered the full set of startup and commissioning in October 2018. The pull valves of each high-pressure regulating valve were normal in the cold state, and the valve linearity and overlap were good. During the unit's run-up, some high-pressure regulating valves could not be opened or some high-pressure regulating valves suddenly closed during the medium-speed warm-up process. The parameters before the run-up were main steam pressure 2.5MPa, main steam temperature 533℃: the parameters during medium-speed warm-up were main steam pressure 7.0~9.0MPa, main steam temperature 533℃. Because our plant is a main pipe heating unit, the steam turbine is run-up through the start-up pipeline (there is no start-up boiler, and it is impossible to start with sliding parameters). The start-up pipeline mother pipe is connected to the main steam mother pipe, and the pressure during the turbine run-up is controlled by interception.

2 High-pressure regulating valve fluctuation analysis and treatment

2.12 Steam Turbine High-Pressure Control Valve Signal Interference Processing

(1) After confirming that the use of intercoms in the DCs electronic room caused the four high-pressure regulating valves to fluctuate to varying degrees, the maintenance personnel rewired the high-pressure regulating valve cable tray based on the on-site equipment and the original cable tray layout in order to completely solve the problem of high-pressure regulating valve signal interference.

(2) During the full shutdown and maintenance period in January 2021, the DCs system performance test was carried out to check the shielding and grounding of the cabinets in the thermal power plant area. The grounding resistance was measured using the voltage and current method (three-pole method). The test found that the terminal part from the main grounding junction box to the ground grid had insufficient tightening force, and the DEH1/DEH2/ETs cabinet had multiple grounding points. After the grounding cable was replaced, the test was normal.

2.21 Steam Turbine No. 2 High-Pressure Regulating Valve Stem Falling Off

During the shutdown inspection, it was found that the direct cause of the accident was that the locking nut of the No. 2 high-pressure regulating valve of the No. 1 unit loosened due to the valve shaking, causing the valve stem to fall off, and the No. 2 high-pressure regulating valve suddenly closed, resulting in steam pressure fluctuations and sudden load drops. The treatment measures were to reassemble the valve stem, install the valve stem positioning pin and add angle steel for spot welding reinforcement.

2.31 Steam Turbine High-Pressure Regulating Valve Stem Axial Rotation Treatment

When shutting down for maintenance, check the internal structure parameters of the high-pressure regulating valve, as shown in Figure 1.

It was found that the direct cause of the accident was that the clearance of the fan-shaped teeth in the middle section of the valve stem was too large during the valve assembly process, as shown in Figure 2. The actual measurement was 1.2mm, compared with the required axial clearance of not less than 0.5mm. The installation clearance was too large, causing the valve stem to rotate axially under the action of the airflow, causing the bolt-type roller needle bearing behind the high-pressure regulating valve operating seat to break, which in turn led to a series of problems such as fluctuation of the high-pressure regulating valve, severe wear of the LVDT rod, and rapid wear of the two joint bearings on the LVDT connecting rod.

Figure 1 Internal structure of high pressure regulating valve

Figure 2 Sector tooth structure

2.43 steam turbine high pressure regulating valve cannot be opened and suddenly closed during the run-up

2.4.1 Analysis and treatment of the reasons why the high-pressure regulating valve cannot be opened during the flushing

After analysis, the high-pressure regulating valve operates normally under cold conditions, indicating that there is no problem with the DEH control system and control circuit. The difference between cold and hot (rushing or running) conditions is whether steam enters the high-pressure regulating valve and forms a certain pressure difference before and after the valve. If the power of the high-pressure regulating valve oil motor is not enough to overcome the resistance formed by this pressure difference, the high-pressure regulating valve cannot be opened. Obviously, the power design of the high-pressure regulating valve oil motor must be able to match it. The formation of pressure difference is also related to the diameter of the pre-start valve flow hole in the high-pressure regulating valve. The pre-start valve in the high-pressure regulating valve is not opened or has been opened but the pre-start valve flow hole is too small, which may cause a large pressure difference before and after the valve. Upon inspection, the high-pressure regulating valve cylinder diameter is 125mm, the piston rod diameter is 55mm, the oil motor cylinder distance is 10mm, the oil motor maximum working stroke is 40+10=50mm, the regulating valve stroke is (40±2)mm (of which 4mm is the pre-start valve stroke distance, i.e. the first 10% opening), and the pre-start valve flow hole diameter in the regulating valve is 8mm, as shown in Figure 3.