How to prevent turbine overspeed and shafting failure accidents?_

This mainly depends on the reliability of the turbine’s speed regulation and control systems, and relevant tests should be conducted regularly. Shaft system fractures generally occur less frequently.

Turbines are all equipped with overspeed protection devices. Generally, stopping will occur once a certain speed is reached. This post was last edited by pauer on 2008-4-14 20:41]

Turbines are equipped with overspeed protection devices, so accidents like yours generally do not occur. The control system of the turbine is the first protective device designed to prevent the turbine from speeding up. The overspeed safety device is the second protective mechanism; it comes in two types: flyweight type and flyring type. When the operating speed of the turbine exceeds a certain limit, this overspeed safety device quickly cuts off the air supply to the turbine.

Under normal conditions, the governor controls the speed of the turbine; when overspeed occurs, a safety device comes into action and stops the turbine! As for shaft fracture, it generally does not occur, unless the surge is extremely severe; it’s unlikely that the shaft will break just because normal operation cannot be restored immediately after shutdown! Under normal circumstances, when the shaft temperature is too high, the vibration is excessive, or the displacement is too large, the system will shut down automatically before those values reach the allowable limits; therefore, such a situation does not occur!

Measures to prevent turbine overspeed and shafting failure l Preventing turbine overspeed 1.1 Requirements for the control system Under normal steam parameters and condenser vacuum, the control system should be able to maintain the turbine operating stably at its rated speed. When the load is stable, the unit load and steam inlet control valve should not fluctuate. When the load changes, the control system operates smoothly. When the unit loses load, the control system should be able to maintain the turbine in idling operation, with the speed not exceeding the speed at which the emergency shutdown device activates. 1.2 The speed variation rate of the speed variation rate control system shall be 7% of the rated speed (adjustable between 2 and 10). 1.3 Requirements for overspeed protection: All overspeed protections of the turbine must be properly activated. When the overspeed protection cannot operate reliably, the unit is prohibited from starting and operating. The operating value for the general emergency stop device is 109 to 112% of the rated speed, while the operating value for the electrical overspeed protection is 114% of the rated speed. 1.4 Requirements for important monitoring instruments: If the display of important operational monitoring instruments such as the turbine PROCONTROI control system and turbine speed fails, the unit is prohibited from starting up or operating. 1.5 Strengthen the supervision and management of oil-related operations: The installation, maintenance procedures, and oil circulation flushing requirements for the turbine oil system must be carried out in strict accordance with ABB’s standards as well as those set by relevant authorities. The turbine oil used in the main engines should meet NAS7 standards, while the fire-resistant oil used in auxiliary engines should meet NAS5 standards. The quality of turbine oil (such as particle size, kinematic viscosity, flash point, acid value, moisture, liquid-phase corrosion, etc.) and the inspection frequency shall meet the requirements of chemical supervision. The automatic adjustment of the turbine shaft seal steam is operating properly, and the shaft seal steam pressure should be maintained at 103–105 kPa. The oil filtering devices and oil purification devices in the turbine oil system should be in normal operation. 1.6 Static characteristic tests: Static characteristic tests and adjustments must be carried out after the system has been newly installed, overhauled, or modified, as well as before dynamic tests. Additionally, based on any abnormalities in the operating control system, it is determined whether a static adjustment test is required. After the static adjustment test, it is necessary to organize and plot the throttle characteristic parameters and characteristic curves, ensuring that all technical parameters meet ABB’s requirements. 1.7 Load shedding test: A load shedding test must be conducted on newly commissioned units or those whose turbine control systems have undergone major modifications. After the unit is put into operation, in the event of load shedding or similar situations, it is necessary to promptly record and collect data such as the turbine speed variation curve. 1.8 After the unit undergoes Class A maintenance, a test should be conducted to determine the valve closing time. If any abnormalities are detected in the control system during operation, measurements should be carried out immediately. The valve closing times shall comply with ABB standards: the closing time for the high-pressure main steam valve is 75 ms, the closing time for the high-pressure control valve is 120 ms; the closing time for the medium-pressure main steam valve is 90 ms, and the closing time for the medium-pressure control valve is 130 ms. 1.9 Oil filling test for emergency safety devices: The oil filling test for the turbine’s emergency safety devices is carried out every 3 months. Tests are generally conducted after maintenance, when the turbine is started up to 3,000 r/min, through the 110% and 112% protection channel devices. 1.10 Overspeed test of the emergency safety device: The overspeed test of the emergency safety device is carried out every 1–2 years. (1) Speed tests are generally conducted in the following situations: after a newly installed unit or after a major overhaul of a unit ； After adjusting the security system following disassembly and reinstallation ； Restart it after a 1-month shutdown. (2) For the overspeed test during cold start, it is necessary to connect to the grid under rated load and operate continuously for 1–2 hours before disconnecting; or operate at no load for 2–3 hours before proceeding. (3) Before the overspeed test, the ¨0 and ¨2 oil filling tests, as well as the control room tripping and local manual tripping tests, should be conducted first to confirm that the high-pressure and medium-pressure main steam valves and control valves are closing properly. (4) The overspeed test shall be conducted at a main/IG steam pressure of 8 MPa/1.6 MPa. (5) During the overspeed test, a dedicated person should closely monitor parameters such as the turbine speed and the vibration of each bearing. If the speed exceeds the specified value and the emergency shutdown device has not yet activated, manual tripping should be carried out immediately. (6) The turbine overspeed test is organized by the plant management, carried out by the operation department, with the support of the mechanical and instrument control teams from the maintenance department. 1.11 Sealing test of main steam valves and control valves: After the main steam valves and control valves have been newly installed, overhauled, or modified, a sealing test must be conducted on them. Additionally, based on any abnormalities during operation, it is determined whether a valve tightness test is required. The valve tightness test is generally carried out before the turbine is started up. During the test, the main/reheat steam pressures are maintained above 8 MPa/1.6 MPa, and the condenser vacuum is within normal limits. During testing, it is generally acceptable if, when one valve (the main steam valve or the control valve) is closed while the other valve is kept fully open, the turbine speed does not automatically rise above 300 revolutions per minute. After the turbine trips, it should be confirmed that the main steam valve, control valves, and all extraction check valves are tightly closed, causing the turbine speed to drop rapidly. 1.12 Valve looseness tests: A partial-stroke looseness test of the main steam valve is carried out every 2 weeks; a full-stroke looseness test of the high-pressure control valve along with a linkage test for the associated valves is conducted once a month; a linkage test between the high-pressure main steam valve and the control valves is performed every 3 months; and a looseness test of the extraction check valve is carried out every 2 weeks. (1) The valve looseness test should be conducted at low load as much as possible, and the linkage test of the high-pressure main steam valve and the control valve must be carried out when the control valve opening command is less than 65. (2) During the test, close monitoring should be carried out of parameters such as main steam pressure, reheat steam pressure, turbine speed, axial displacement, vibration, and load; if any abnormalities are detected, the test must be stopped immediately. (3) During testing, a dedicated person must be present on site to confirm that the valve operates properly and shows no signs of sticking. (4) If the test fails, the cause must be identified and eliminated. During the operation of the turbine, if sticking occurs in the high and medium pressure main steam valves, control valves, or extraction check valves, it must be resolved promptly. Before elimination, in the event of an emergency shutdown, it is necessary to pay attention to changes in the turbine speed, as well as to the status of closure of the main steam valve, control valves, extraction isolation valves, and extraction check valves. If needed, emergency shutdown of the boiler, pressure relief, or an emergency shutdown of the turbine due to a loss of vacuum should be carried out. 1.13 Strictly follow the operating and maintenance procedures: (1) During major repairs, it is necessary to carry out maintenance tasks such as cleaning and testing of the control components ； During operation, it is necessary to closely monitor its status to ensure there is no jamming, no leaks, and that the system remains stable; any issues detected must be addressed promptly. (2) During normal shutdown, reverse power protection is used to disconnect the system. Disconnection under load is strictly prohibited. (3) During the normal operation as well as startup and shutdown of the unit, the high and low pressure bypass systems should be able to operate properly. In the event of load shedding or an accident, the bypass system must operate properly; when the unit is restarted, the reheat steam pressure should remain below 1.6 MPa. (4) During major unit overhauls, the extraction check valve must be disassembled for maintenance. If there are defects in the extraction check valve while the unit is in operation, they must be addressed properly. (5) During major overhauls of the unit, it is necessary to check the lubrication and wear conditions of the gear drive between the main oil pump and the turbine shaft, and maintenance must be carried out in strict accordance with ABB’s specifications. (6) Strengthen technical training for relevant professionals and operators, so that they can master the control logic, functions, and operational procedures of the regulation system. Major modifications to the control system should be subject to comprehensive and thorough evaluation to ensure their practicality, safety, and reliability. 2 Preventing breakdown of the turbine shafting: (1) All protective devices of the turbine must be in proper operation, such as those for overspeed, excessive axial displacement, high shaft vibration, low vacuum, low lubricating oil pressure, high thermal stress on the rotor, high blade temperatures, and lateral interlock tripping. (2) During normal operation of the turbine, the shaft vibration limit during the start-up process and at critical speed is 200 m, and attention should be paid to monitoring the trend of changes. The alarm value for shaft vibration is 200μm, and the shutdown protection value is 280μm. Turbogenerator sets should be equipped with shaft vibration protection. The vibration protection signal should generally not be generated through an \"AND\" operation of multiple signals, nor should there be any delay. If vibration protection triggers erroneously, the cause should be analyzed and addressed; the protection cannot simply be disabled. When the protection device is prone to false operation due to external interference, as a temporary measure, the two vibration signals can be combined using an AND gate or a trip delay can be applied. (3) Asynchronous grid connection of generators is prohibited. (4) As required by ABB, after the unit has been in operation for 8 years, a comprehensive inspection of the turbine rotor must be carried out. When the unit has been in operation for over 13 years, the turbine rotor should be inspected every 3 to 5 years; if its operating time exceeds the designed service life of 20 years, the inspection interval should be reduced accordingly. (5) During each major overhaul of the turbine, flaw detection inspections must be carried out. Metallographic and non-destructive testing should be conducted on the stress concentration areas in the high-temperature section, selecting those areas that do not affect rotor safety. (6) Defective rotors that have been put into operation should undergo a technical assessment, and corresponding safety operation measures should be established based on the specific conditions of the unit and the nature of the defects. Rotors that fail inspection must not be put into operation. (7) During the major overhaul of the turbine, it is necessary to check the tightness of the rotor balance weights, the generator hydrogen cooling circulation fans, the stator core supports, as well as the screws of all bearings and bearing housings. The expansion sleeve screws of all couplings must be properly tightened with no gaps, and the mechanical safety devices must be in good condition. (8) During the major overhaul of the steam turbine, the deformation of the cylinder should be checked to ensure that it is within the limits specified by ABB standards. 3 Establish a technical record book: (1) Equipment documentation such as the drawings and technical data provided by the manufacturer for the control and safety systems, information on the bearing structure and bearing type, critical speed, the locations where dynamic balancing weights are installed and the results of dynamic balancing at the time of manufacture, limits regarding shaft vibration and metal temperature, data related to the cylinder support and slide pin systems, as well as information on the generator and exciter; technical specifications of the vibration measurement system; material properties of the turbine rotor and any existing defects. (2) Installation and maintenance records, such as installation and maintenance as well as inspection technical records (e.g., foundation settlement, horizontal deflection of cylinders and bearing housings, load distribution on cylinders and bearings, original bending of the shaft, rotor alignment status, dynamic and static clearances, etc.), records of defect and fault resolution, maintenance reports, etc. (3) Commissioning and operation data such as the speed variation rate and adjustment range of the speed control system, static adjustment data, protection verification records, records of valve tightness and looseness tests, measurements of valve closing time, oil quality analysis reports, shaft vibration values during startup and under load, actual values of various critical speeds and the maximum vibration values during supercritical conditions, metal temperatures of each bearing pad and oil return temperature, bearing pad oil pressure for supporting the shaft (the elevation value of the shaft journal), metal temperatures and expansion values of various parts of the turbine, axial displacement and differential expansion, the turbine’s idle running curve, main operating parameters of the unit, cumulative operating time, number of startups and shutdowns in cold and hot conditions, cumulative time of over-temperature and over-pressure conditions, as well as the causes and handling methods for major accidents