A company's No. 1 660MW supporting steam turbine generator set, the generator is QFSN-660-2-22 type hidden pole, two-pole, three-phase synchronous steam turbine generator, using water, hydrogen, hydrogen cooling mode. The sealing pad of the generator sealing oil system adopts the form of a single flow ring, and the system is composed of 2 AC main oil pumps, 1 DC accident oil pump, sealing oil return expansion tank, float oil tank, air extraction tank, smoke exhaust fan, differential pressure regulating valve, oil filter, vacuum oil tank, vacuum pump, recirculation pump, instrument box and other components.
Analysis of the accident and its causes
1) How the accident happened.
At 13:30 on August 25, the load of Unit 1 rose to 660MW, the operating parameters of the unit were stable, the hydrogen pressure was 436kpa, the generator sealing oil system was running normally, and the differential pressure of hydrogen oil was 83kpa. At 14:15, Unit 1 gave an alarm of "hydrogen leakage from the generator", and the hydrogen leakage content of the oil return at the steam end and the excitation end of the generator quickly rose from zero to 39%, when the alarm value is reached, the alarm is alarmed. The hydrogen pressure of the generator drops rapidly, and the steam engine room is 13There was an open flame at the steam end and the excitation end of the 7m generator, and there was also an open flame at the smoke exhaust port of the sealed oil explosion-proof fan, the operator immediately destroyed the vacuum shutdown, and the generator emergency hydrogen discharge, and the hydrogen pressure of the generator was reduced to about 48kpa, and the open flames at both ends of the generator were extinguished by the on-site personnel. Due to timely detection and appropriate measures, this accident only burned part of the thermal cable and measuring probe, and did not cause the fire of the steam turbine oil system and the major equipment damage of the hydrogen system.
2) Analysis of the causes of large hydrogen leakage and ignition of generators.
The inspection found that the float valve of the sealed oil float tank was stuck in the full open position, and the "oil seal" at the float tank was damaged, resulting in a large amount of hydrogen leakage from the oil return expansion tank to the air extraction tank in the generator. Due to the large amount of hydrogen leakage, the explosion-proof fan has no time to discharge the hydrogen, and the hydrogen in the air extraction tank is returned to the two ends of the generator steam and excitation through the oil return pipeline at both ends, and then leaks from the shaft end through the large end cover of the generator, which causes the oil return of No. 7 and 8 watts to be unsmooth, and a large amount of hydrogen carries lubricating oil and oil return oil to be sprayed out from the large end cover of the generator, which eventually causes hydrogen and lubricating oil leakage at No. 7 and 8 watts. The path of large amount of hydrogen leakage from the generator is shown in Figure 1.
Under high flow rate, hydrogen is easy to rub against the pipeline to produce static electricity, and hydrogen is very easy to catch fire or ** after encountering static electricity. GB 50177-93 Code for the Design of Hydrogen and Oxygen Stations requires that "the working pressure of hydrogen in the pipeline shall be 01～1.6MPa, the maximum flow rate can be 25m s in stainless steel pipes". The hydrogen leakage rate of the generator during the hydrogen leakage shutdown accident of Unit 1 was calculated, and the hydrogen leakage rate reached about 25m min in some periods, which was converted into a maximum hydrogen flow rate of 150m s at the smoke exhaust port of the sealed oil explosion-proof fan, which greatly exceeded the safe hydrogen flow rate stipulated in the national standard. It can be seen that after the oil seal at the float tank of the generator set operated at rated hydrogen pressure is damaged, if the float valve is stuck in a large opening at this time, hydrogen ignition will occur, and it is almost inevitable that the accident must be stopped immediately.
Analysis of the causes of seal oil float float float valve jamming
After the accident, the float valve of the sealed oil float tank was inspected, and it was found that the float valve was stuck in the open position, and the hand did not move, and no foreign matter was found in the inspection valve body, and the rubber sealing ring of the valve core was intact. There are obvious signs of jamming and wear at the spool guide and bonnet guide pistons. The inspection found that there were more non-metallic particles such as rubber and paint skin at the bottom of the fuel tank. After understanding with the on-site operation personnel, it was learned that before the failure of the float valve of the float tank of Unit 1, the fuel tank had periodic abnormal sounds, and at the same time, the fuel level of the float tank fluctuated periodically, and the maximum fluctuation range of the oil level could reach 30mm. Compared to Unit 2, which has the exact same configuration as the seal oil system, the Unit 2 seal oil float tank has no abnormal sound, and the fuel level of the tank only fluctuates by a few millimeters. In March 2015, the float valve of the float tank of the two units was disintegrated by taking advantage of the simultaneous shutdown of units 1 and 2, and it was found that there were obvious differences in the float valve in key parts after comparison, so as to judge the main reasons for the jamming of the float valve of unit 1 as follows:
(1) There are deficiencies in the manufacturing process of float valves
The No. 1 unit sealed oil float tank float valve spool is made of stainless steel, after comparing the No. 2 unit float valve, it is found that there is a layer of copper inner bushing with a thickness of about 5mm on the joint surface of the valve body hole and the valve core, because the float tank float valve of the single oil ring sealing oil system has been in a state of frequent adjustment for a long time, the metal surface of the valve core and the valve body bore is frequently abrasive, and the copper texture is soft, which can effectively protect the valve body and the valve core. The bore of the float valve of Unit 1 is made of the same material as the surface of the spool, which is made of stainless steel, and the metal of the same hardness is repeatedly worn for a long time, which will inevitably cause the surface of the spool to be worn, and finally cause the float valve to jam in the fully open position, which is the main reason for the shutdown of a large amount of hydrogen leakage of the No. 1 generator.
(2) After the installation and overhaul of the unit, the large flow flushing of the oil return pipeline of the sealing oil system is not thorough
Check the oil quality monitoring report of steam turbine lubricating oil and generator sealing oil in the past 3 years, and there has been no excessive oil quality. At the same time, the sealing oil supply pipeline of the unit is provided with 20 filter screens, and it can be seen that the solid impurities do not come from the sealing oil supply system and pipelines. Tracing back to the latest inspection of the sealing oil system, it was found that the particles such as rubber and paint skin mainly came from the 7,8 watt lubricating oil return, from the sealant and the outer end cover of the generator after the installation of the sealing pad and the installation of the inner and outer end covers of the generator and the outer end cover of the generator.
(3) The arrangement of the internal sealing oil inlet of the float oil tank is unreasonable
In the process of internal inspection of the sealed oil float tank, it was also found that the arrangement of the seal oil inlet of the float tank of unit 1 was unreasonable, the oil inlet of the oil tank was facing the float valve ball, and the oil flow caused continuous impact force to the float valve during the operation of the sealed oil system, and the direction of the impact force was perpendicular to the activity direction of the valve core, which increased the friction between the valve core and the valve body, thereby accelerating the wear of the valve core. Compared with the internal design of the sealed oil float tank designed by Alstom, it can be found that Alstom has installed an oil baffle at the oil return port, which effectively avoids the impact of the oil inlet on the float valve.
Operational measures to prevent the float valve from jamming and abnormal expansion of the float tank float valve
In view of this weak link in the single oil ring sealing oil system, before the system transformation and upgrading is not completed, in order to guide the operators to find the abnormal operation of the sealed oil float valve in time and prevent the accident from expanding due to the stuck float valve of the float tank, the following measures can be taken:
1) Focus on the selection and daily maintenance of the float valve of the sealed oil float tank to ensure the reliable operation of the key valves of the sealed oil system. Seal oil float tank float valve in the single oil ring seal oil system is a very core important valve, the float valve is stuck, will inevitably cause a large amount of oil into the generator or a large amount of hydrogen leakage serious consequences, such valves in the selection, maintenance, daily operation need to focus on treatment, found that does not meet the specifications of the valve should be replaced as soon as possible, the choice of high reliability, not easy to occur the key to prevent major accidents.
2) Strengthen the management of spot inspection, regular repair and maintenance. Pay attention to the installation and maintenance of generator sealing oil system, sealing pad and other components, clean up the dead angle of generator bearing lubricating oil and generator sealing oil return system, and do a good job of large-flow flushing of sealing oil inlet and return system, so as to avoid a large number of impurities left in the system causing the float valve to be stuck. When the unit is stopped, the float tank float should be inspected. When the unit is overhauled, the vacuum oil tank, float oil tank, air extraction tank, and filter screen of the sealed oil system should be inspected and cleaned.
3) Strengthen the supervision and management of oil quality. Regularly inspect the lubricating oil and sealing oil supply of the steam turbine to ensure that the oil quality of the oil return system is qualified.
4) For the unit with abnormal sound or large fluctuation of the float tank liquid level during the operation of the float valve of the sealed oil float tank, it is recommended to throttle the outlet door of the sealed oil float tank to avoid major abnormalities. Through the case of a large number of hydrogen leakage from the No. 1 generator, it can be found that after the "oil seal" of the float fuel tank is broken, the hydrogen leakage rate of the generator is very fast, even exceeding the hydrogen discharge rate after the accident hydrogen discharge valve is fully opened.
In order to avoid a large amount of hydrogen leakage caused by the float valve stuck in the full open position again, the following tests were carried out on site: under the rated hydrogen pressure of the generator and the differential pressure of the sealing oil hydrogen oil, the outlet door of the sealing oil float tank was gradually closed and the opening was as small as about 10%, and the float valve adjustment could still meet the needs of normal operation and have a certain margin. The test shows that the design of the float valve itself considers the adjustment ability of the float valve under different sealing and return differential pressures under various hydrogen (wind) pressure conditions of the generator. During the normal operation of the unit, the wind pressure of the generator is basically maintained at 430 450kpa, so during the normal operation of the unit, the outlet door of the sealed oil float tank can be properly throttled, which can effectively avoid a large amount of hydrogen overflow of the generator under the condition that the float valve is stuck in the full open position, effectively avoid the hydrogen ignition caused by excessive hydrogen leakage in the instant, and gain time for the operation personnel to deal with the accident. However, when operating in throttling mode, it should be noted that tracking adjustment is required when the wind pressure of the generator is not rated.
5) Install a liquid level gauge that can be remotely monitored on the sealed oil float tank, and set a large light word alarm for the generator hydrogen leakage rate, so as to provide a reliable basis for the operator to judge the abnormality of the float tank. At the same time, the liquid level disturbance test of the float valve of the float tank is carried out regularly to verify the adjustment ability of the float valve under variable working conditions.
6) In view of the potential safety hazards in the float tank of the single oil ring seal oil system, regular training, drills and summaries are carried out.
Conclusion
Because the float valves such as the vacuum tank and float tank of the sealed oil system are arranged inside the oil tank, it is easy to ignore their importance during the operation of the unit, but it can be found from the on-site accident cases that once these float valves fail, the impact is great. The selection, maintenance and operation mode selection of key valves of the sealing oil system should be supervised as an important part of the production process, and some small abnormalities in the sealing oil system should be highly valued by on-site technicians, because there are often major safety hazards hidden behind the small abnormalities. In the event of a large amount of hydrogen leakage and ignition accident of the generator, the operators of power generation enterprises should strictly implement the regulations and countermeasures, and immediately stop the unit to prevent damage to the main equipment.