Source: Steam Turbine Major
1. The main reasons for steam turbine overspeed:
Steam turbine overspeed is caused by the failure of the steam turbine in the speed regulation and protection system and its own defects, and it is also directly related to the operation, operation and maintenance.
1. Defects of the speed control system:
The task of the steam turbine speed regulation system is not only to ensure the normal operation of the steam turbine, but also to ensure that the speed increase does not exceed the specified allowable value after the steam turbine is loaded, so the speed regulation system is the basic guarantee to prevent the steam turbine from overspeeding. If the turbine is not able to run no-load after the load has been shed, it can cause overspeed. The causes of steam turbine overspeed are as follows:
(1) The speed control valve cannot be closed or the amount of steam leakage is large
(2) The exhaust backstop door is not strict or refuses to act
(3) The sluggishness rate of the speed control system is too large or the adjustment component is stuck
(4) The operation mode is unreasonable or improperly adjusted
(5) The unequal rate of the speed regulation system is too large
(6) The dynamic characteristics of the speed control system are not good
(7) The speed regulation system is improperly adjusted, such as the adjustment range of the synchronizer and the expansion gap of the steam distribution mechanism do not meet the requirements.
2. Failure of steam turbine overspeed protection system:
(1) The emergency safety device does not operate or the operating speed is too high
The operating speed of the critical safety device is generally specified at 10 - 12% higher than the rated speed, which is the main protection equipment to protect the steam turbine from excessive speed, and different units are also equipped with additional protection or electric overspeed devices that the specified operating speed is slightly higher than that of the critical safety device. When the speed of the steam turbine increases, the main reasons for the failure or slow action of the critical safety device are:
(1) The heavy hammer or flying ring guide rod is jammed
(2) The dynamic and static parts of the hammer or flying ring are not concentrious, and they hold back their strength during operation
(3) When the spring is stressed, the radial deformation is too large, so that it rubs with the hole wall
(4) The gap between the kissing buckle or the striking plate is too large, and the spool valve of the critical safety device cannot be activated after the impactor flies out
(2) The slide valve of the critical security device is stuck and cannot be operated
(3) The automatic main valve and speed control valve are jammed.
3. Problems in operation adjustment:
(1) Poor oil quality management, inferior oil or oil with different grades and untested oil is added to the main oil tank for operation; The maintenance quality is poor, and the shaft seal clearance adjustment is not within the standard range; During operation, the shaft seal pressure is adjusted too much, and the steam leaks to the oil gear, resulting in water ingress in the oil, causing speed regulation and protection parts to jam;
(2) When the synchronizer is manual or electric during operation, the load addition and subtraction exceed the specified adjustment range, which will not only cause the speed of the unit to fly up after the load is shaken, but also cause the adjustment sleeve to lose pulsation, resulting in jamming and overspeed accidents;
(3) Poor steam quality during operation, chemical monitoring does not implement the ministerial standards, and long-term operation will cause the main valve and speed control valve to jam;
(4) When doing the overspeed test, the speed rise is not implemented according to the standard, and the operation is improper, resulting in the speed soaring too fast.
Second, the software and hardware settings of overspeed protection
The overspeed protection of the digital electro-hydraulic control system of the steam turbine is an important part of the steam turbine safety, and the overspeed protection function of the DEH is realized by the software and hardware protection loops respectively.
1. Software Settings
As can be seen from the logic diagram of DEH software overspeed protection, the overspeed protection OSP signal occurs in the following four situations:
(1) 103% speeding; (2) 110% speeding; (3) The speed acceleration exceeds the limit; (4) The load is greater than 15% when the main switch of the generator is disconnected.
Logic block diagram of DEH software overspeed protection
The Overspeed Protection (OSP) action energizes the two overspeed solenoid valves of the high-voltage blocking module, so that the overspeed oil pressure is quickly removed, the four high-pressure valves and two medium-pressure valves are closed, and the speed is controlled in time to continue to soar. Condition (4) is not limited by the actual speed and is a preparatory protection set up to prevent overspeeding. In addition to quickly removing OPC (over speed protection control) oil, the 110% overspeed signal action also controls the main valve blocking solenoid valve, high-pressure blocking module and mechanical shutdown solenoid valve, that is, finally stopping the steam turbine.
2. Hardware settings
THE HARDWARE OVERSPEED PROTECTION LOGIC IS IMPLEMENTED BY THE TPS-TURBINE PROTECTION SYSTEM, WHICH CONSISTS OF THREE TPS02 MODULES AND A TPSTU02 TERMINAL UNIT CONNECTED BY CABLES. All functions related to electronic overspeed protection are monitored and performed by the modules and terminal units. These protection functions are independent of the control system's data bus and control processor. The TPS uses three redundant inputs, two-of-three protection logic, and the ability to test online to improve reliability. All three protection functions have four relay outputs to the hydraulic manifold, and the contact outputs of the four relays are connected to the DEH's solenoid valve control relay circuit. The TPS module interfaces with the control processor through the expansion bus (SLAVEBUS), transmits the signals such as engine speed, power, brake status and grid-connected status to the processor module, and receives test-related logic instructions.
１． The TPS offers the following overspeed protection features:
(1) Overspeed Protection (OSP)
The OSP protection output controls the overspeed of the unit by controlling the OSP solenoid valve of the high-pressure blocking manifold and the solenoid valve of each regulating steam valve. This feature is initiated by two conditions:
A. The speed of the steam turbine exceeds the OSP set value of overspeed protection (generally 103% of the rated speed).
B. When the power of the steam turbine exceeds a minimum set value, the generator oil switch is turned on, and the field is set to 15% of the rated power.
( 2)低压遮断保护( TRIP)
Control the low-pressure blocking solenoid valve to be electrified, the crisis safety device to trip (trip), and quickly close all the main steam valves and regulating steam valves of the steam turbine. The operating speed is set at 110% of the rated speed.
(3) High Voltage Blocking Protection (EHC)
Operates in conjunction with low-voltage blocking. The protection action causes the high-voltage blocking solenoid valve to lose power, and quickly closes all the main steam valves and regulating steam valves of the steam turbine. The operating speed is set to the same as the low-voltage shut-off setting value (i.e. 110% of the rated speed).
(4) Power Unbalance Protection (PLI)
The overspeed protection of the steam turbine is concerned when the mechanical input power of the steam turbine is greater than the output power of the generator, and the TPS02 steam turbine protection module uses the actual power of the generator and the exhaust pressure of the medium pressure cylinder to determine whether the power imbalance exists according to the following formula:
Power Imbalance (Percentage) = Medium Pressure Cylinder Exhaust Pressure (Percentage of Rated Pressure) - Generator Load (Percentage of Rated Load).
When the mechanical power input of the steam turbine exceeds the output of the electric load and reaches a set amount, it will show that the steam turbine may overspeed and will temporarily close the medium pressure control valve. This protection is also a precautionary action to prevent overspeeding. The on-site protection setting value is 30%, and the operating time is 2 seconds.
２． The TPS offers the following overspeed test capabilities:
(1) Electrical overspeed test
This test function provides a test switching switch interface, according to the setting of the overspeed test speed fixed value, during the test, the electrical overspeed test switching switch is put into the operator station, and a digital signal is sent out, which can make the steam turbine speed increase to a fixed value, and send a high-voltage blocking signal after reaching the fixed value. This test is set to 105% of the rated speed, and the action value is lowered to ensure that the shutdown is done by the electronic equipment rather than the mechanical shutdown mechanism. The test was withdrawn, and the overspeed value was still restored to 110% of the rated speed.
(2) Mechanical overspeed test
As with the high-voltage blocking overspeed test, a test switching interface is still provided. In the test mode, the high-voltage blocking overspeed action value can be increased to 114% of the rated speed, which can be used as a backup protection for the mechanical shutdown mechanism to ensure the safety of the unit, and to ensure that the steam engine shutdown is achieved by flying ring attack rather than electronic equipment action.
3. Steam turbine overspeed test
Prerequisites and operational points for steam turbine overspeed testing
1. The following situations must be tested for overspeeding:
(1) After the new installation or overhaul of the steam turbine
(2) The unit will be started after one month of shutdown
(3) Before the load rejection test
(4) After the dismantling or adjustment of the critical security device.
2. The overspeed test shall not be carried out under the following circumstances:
(1) The local or remote shutdown is abnormal
(2) The high and medium pressure main valves and adjusting valves are not closed tightly
(3) At the rated speed, the vibration of any bearing is abnormal
(4) The temperature of any bearing is higher than the limit value.
3. Key points of operation:
(1) Before the test, the steam turbine should run for more than 4 hours with 10%~25% rated load, and the steam parameter value should be stable during this period;
(2) The mechanical overspeed test and the 103% and 110% electrical overspeed test should be carried out separately;
(3) The mechanical overspeed action speed is 110%~111% The rated speed should be tested twice, and the difference between the two action speeds should not exceed 0. 6％。 For the newly installed or overhauled unit, the overspeed test should be carried out three times, and the difference between the third action speed and the average action speed of the first two tests should not exceed 1% of the rated speed; If the speed reaches 3330r/min, the critical safety device still does not operate, it should be stopped immediately;
(4) The test time should be controlled within 15 minutes.
Overspeed test of a steam turbine
In order to ensure the reliable operation of the overspeed protection device, after the installation of the steam turbine, before and after each overhaul, or after 2000 hours of operation, the overspeed protection device test should be carried out in accordance with the relevant regulations, that is, manual test, oil injection test and overspeed test.
1. Manual testing
The manual test is to check whether the critical cut-off throttle, automatic main valve and regulating valve are operational. The tests are carried out in the static state and no-load state of the steam turbine, and are generally carried out in combination with the tightness test and closing time test of the automatic main steam valve and the regulating steam valve.
2. Oil injection test
The high-power steam turbine is equipped with a critical safety device oil injection test device, which can be tested at no load and under load respectively. The no-load oil injection test can measure the oil injection speed of the critical safety device, and the oil injection test with load is used to carry out the emergency safety device activity test under normal load operation conditions. The test method is to remove the tested critical security device tripping device from the normal operation system (another critical security device and its tripping device are in normal operation and still play a protective role), and make it connected to the test system, and then put it into the normal operation system after the test is completed, and then use the same method to test another critical security device.
3. Overspeed test
The overspeed test is used to measure the actual operating speed of the critical security device. The test is carried out when the steam turbine is idling, and the method is to increase the speed to the upper limit with a synchronizer, and then use an overspeed test device (overspeed test wrong throttle, auxiliary synchronizer or other device) to slowly increase the speed to make the critical safety device act and record the action speed.
4. Test precautions:
(1) For the unit with two critical safety devices, the oil injection test device should be used to switch the test handwheel to the critical safety device with a lower action speed (it has been determined in the no-load oil injection test), so that the action speed of two critical safety devices can be measured at the same time in a test, the first one uses the blocking indicator to judge whether the action is or not, and the second is only judged by the automatic main steam valve closure.
(2) For high-power units, the overspeed test should have sufficient warm-up time to prevent the rotor metal temperature from falling below the brittle conversion temperature and affecting the safety of the unit. This is particularly important for units with slip parameter starts. For this reason, some manufacturing factories have made special provisions for this. For example, Hitachi TCDF-33.5 steam turbine stipulates that all tests exceeding 3000r/min must be carried out after the unit has been running for three hours at 25% of the rated load. At present, there is no unified regulation for domestic large-scale steam turbines, but there are corresponding measures for each operating plant, and attention should be paid to compliance during the test.
5. Adjust the speed of the critical security device
Fourth, measures to prevent steam turbine overspeed accidents:
1. In the shutdown, the emergency safety device is used to close the main valve and the speed regulating valve to ensure that the active power of the generator is zero, and then the generator is disconnected after the current is reversed, so as to avoid the generator unloading time is too long, because the time is too long, the exhaust temperature will increase under the action of the blast, and the low differential expansion will increase, resulting in the difficulty of the unit when starting again
2. Strengthen the quality supervision and testing of soda to prevent the quality of steam from being unqualified, and the steam with salt in operation will make the main valve rod, speed regulating valve lever, and each section of the steam extraction backstop door rod scaling, and the opening and closing are inflexible and stuck
3. Strengthen the supervision of oil quality, and regularly sample and test to prevent water or sundries in the oil from causing the adjustment part to be stuck and corroded
4. Automatic main valve, speed control valve should be opened and closed flexibly, the tightness is qualified, and the automatic main valve lever should be regularly moved every day. The load is changed by 30% every week in order to activate the speed control valve and the gasoline spool valve. After the overhaul of the unit, before the load rejection test, the tightness test of the main valve and the speed-regulating valve must be carried out to ensure that the technical requirements are met. When the main valve and the speed-regulating valve are closed separately, under the rated parameters, the maximum stable speed shall not exceed 1000r/min, and the test steam pressure should be carried out under the rated steam pressure to obtain the tightness of the closure of the main valve and the speed-regulating valve when the load is thrown or after the accident is thrown. During operation, when the quality of steam and water is unqualified, it is necessary to appropriately increase the number and range of activities of the main valve and speed control valve, and do a good job in safety measures.
5. When the main valve, speed control valve, and steam extraction backstop door are found to be stuck in operation, they should be eliminated in time, and there should be safety measures to prevent overspeed before elimination, and a quick shut-off valve should be installed if necessary.
6. When there are changes in the regular meter reading, patrol inspection and microcomputer monitoring instrument during operation, close attention should be paid to the relationship between the opening of the speed control valve and the load, and the door seat should be prevented from rising or moving down according to the pressure change of the speed control valve, so that the speed control valve can not be closed tightly, and the load is overspeeded.
7. The operator should be familiar with the signs of overspeeding (load to zero or only with factory electricity, abnormal sound abrupt change, continuous rise in tachometer, increase in oil pressure, increase in vibration, etc.), when encountering overspeed, decisive measures should be taken, and strict implementation of the regulations to stop the brake to prevent the expansion of the accident.
8. When the unit is out of operation for a long time, do a good job of maintenance during shutdown and dry it once a month to prevent steam or other corrosive substances from entering or remaining in the steam turbine and regulating oil supply system to cause corrosion of the valve and adjusting part sleeve.
9. In the process of starting the sliding parameters of the unit, or the unit running under sliding pressure, the opening of the speed regulating valve should be left to be rich, and it should not be opened too much, so as to prevent the synchronizer from exceeding the normal adjustment range or increasing the flying speed when the load is thrown.
10. Adhere to the static characteristics test of the speed regulation system, after the overhaul of the steam turbine or the defects of the speed control system, the replacement of the speed control sleeve and the retuning should be done after the steam turbine speed regulation system test, the speed change rate and the slow rate of the speed control system should meet the technical requirements, the general speed change rate is 3% ~ 6% of the rated speed, and the slow rate is not greater than 0.5% of the rated speed.
11. Reasonably adjust the adjustment range of the synchronizer, the upper limit of the rich stroke should not be too large, generally require the high limit to increase the speed change rate = (1 - 2) % of the rich stroke, in the low limit can reduce the rated speed of 3 - 5%, the cam clearance of the steam distribution mechanism, to ensure that the steam turbine can tightly close the speed regulating valve in the hot state.
12. After the technical transformation of the newly installed unit and the speed regulation system of the unit, the dynamic characteristics of the speed regulation system should be tested to ensure that after the steam turbine is thrown off, the dynamic ramp-up speed does not exceed the specified value, and the steam turbine should be able to maintain no-load operation after the load is shaken, and the ramp-up speed after getting rid of the rated load is generally required not to exceed 8% of the rated speed, and the equipment defects should be eliminated in time.