Type of emergency in steam turbine;
In this article we discussed about the type of emergency in steam turbine occurred during the operation time of steam turbine . Steam turbine is a critical rotating equipment. High temperature and pressure is used to rotating the turbine at high speed. Mass of the rotating equipment is high. There is always chance of severe misshapen leading to fatal accident and damage to high cost equipment’s, in case any system goes wrong. Generation of power may be interrupted for a longer period, leading to heavy loss to the plant. So the power plant engineer should be trained enough to face any emergency situation at any time and properly Handel this emergency situation.
Some critical emergency we are discussed here with our readers here:-
1. Failure of lubrication oil system.
2. High bearing temperature.
3. High turbine speed
4. High vibration
5. Black out condition
6. Failure of barring gear device
7. High condenser level.
8. High axial displacement.
9. Steam, water, oil leakages.
11. Low and high steam parameters
12. Low condenser vacuum
13. Failure of cooling water system
FAILURE OF LUBRICATION OIL SYSTEM:-
Lubrication oil is used to be lubricate and cool down bearing metal temperature. In some incident the lubrication oil supply may interrupted due to failure of oli pumps, leakage in oil line , or chocking of oil filter. This condition may damage bearings and gear box of turbine. If such incident is happened for any reason the turbine is required to stop as soon as possible. Coasting down time my be reduced through by opening vacuum breaker valve .oil supply restored as soon as possible. After resume oil supply, if possible, turbine is to be rotated manually to find out any damage . bearing are to be inspected. If any damage found , same to be replaced.
Rotor of turbine is rotating at higher speed any deformation or unbalanced of the rotor produced high vibration. Some time deposits on blade and damage of any rotating part may create heavy vibration. Damage of journal bearing may also be productive vibration. The moving and rotating parts of turbine are closely spaced . due to the disturbance in rotor shift or differential expansion there is a chance of rubbing of these parts . rubbing create high vibration and abnormal sound.
So at any case high vibration of turbine is not to be overlooked. In case of high vibration, the turbine should be stopped immediately and turbine internals are to inspected to avoid further damage.
High bearing temperature:-
High bearing temperature occur due to inadequate oil flow in the bearing or metal to metal contact in between bearing and rotor . high temperature damages Babbitt material of the bearing .In case of high temperature of bearing the turbine is required to be stopped. Oil supply to the bearing is to be checked and if required bearing is to be opened for inspection. At any case high bearing temperature tripping is not be by passed.
FAILURE OF BARRING GEAR DEVICE:-
When turbine is stopped in hot condition, it is to be put on barring. In some situation stopping turbine, barring gear not found working properly. It is not recommended to keep the rotor in stand still condition in this situation. By any means rotor is to be rotated . the rotor position may be changed by 180 degree continuously at some interval.
HIGH CONDENSER HOT WELL LEVEL:-
Due to problem in CEP ( condensate extraction pump) and other equipment in condensate system condensate can not be evacuated from hot well. So hot well level become high. Also due to malfunction of level transmitter. Actual level may go high. In this situation there is a possibility that Water level in condenser will increase and enter in to the turbine through exhaust hood. Condenser vacuum reduced drastically in this condition. If at any case water enters in to a running turbine it creates serious condition and damage the internal of the turbine.
Turbine is not to be allow to run at high hot well level and transmitter not show the actual reading of hot well level . load is to be reduced in this condition and if situation is not controllable turbine is to be stopped . to avoid this situation local hot well level gauge glass to be checked regularly
HIGH AXIAL DISPLACEMENT
There is very small axial displacement between rotating part and stationery part of turbine .rotating part of turbine is to be supposed to be placed at exact center of the clearance of stationery parts. When the rotating center is changed it is reflected by axial displacement. At high axial displacement rotating part of turbine may come in contact with stationer part . so the turbine is to be stopped in case there is high axial displacement or axial shift . rotating float with respect to stationery parts to be checked if required
From the operation of turbine there are different pipe lines in which different fluids flow these are :-
-Main steam line
-Auxiliary or motive steam line
-Cooling water line
In case of failure of flange gasket or damage of pipe line the fluid conveyed may leakage affecting turbine operation. In some cases leakage is so serve that immediate stoppage of turbine is required. In some case turbine can be stopped in a planed way to arrest leakage . depending upon the severity of leakage and its effect on plant operation and safety action is to be taken.
Fire is always an emergency situation at any plant. But at turbine this situation is different. High temperature steam is used to drive steam turbine and also large quantity of lubrication oil is handled . so there is always chance of fire . precaution is to be taken to handle this situation to avoid damage to man and machine. Turbine is operated at higher temperature. So proper grade. Fire extinguisher and fire fighting equipment is to be used so that it will not damage turbine further. Operating personal working at turbine are to be trained to use appropriate technique to extinguish fire . fire hydrant lines and fire extinguishers are to be kept ready at proper location.
LOW STEAM TEMPERATURE
Due to problem at boiler side low temperature and low pressure steam may enter in to the turbine. This is to be avoided . at any case low temperatures and pressure steam below permissible level should not be allowed to enter in to turbine. This may damage turbine internals. Due to high water level in boiler, priming may take place . priming decrees steam temperature as boiler feed water enters in to steam line . if turbine is allowed to run in this condition, unsaturated steam enters to turbine and damages turbine severely . low temperature steam is condensed towards last stage blade of turbine. It also produced thermal shock on the hot turbine . in this case turbine stopped immediately.
High steam parameters:-
like low steam temperature and pressure. High temperature and pressure steam is not desirable for turbine operation.High steam temperature may damage turbine as the metallurgy of the turbine is designed for a particular temperature. High pressure steam may damage internal sealing fines etc.
Low condenser vacuum
Due to vacuum in condenser the steam from turbine is easily exhausted into condenser. If vacuum inside the condenser drops it restricted exhaust steam of turbine. Back pressure is created inside the turbine. Vacuum may drop due to failure in cooling water system failure of ejectors or leakage in condenser air line . the standby ejector or starting ejector is to be immediately take into line . leakage in the air line is to attested promptly or cooling water supply to be increased . if vacuum is not improved the turbine to be stopped immediately.
FAILURE OF COOLING WATER
due to the failure of cooling water pupms or chocking in cooling water circuit cooling water supply may be reduced or interrupted. In this cause turbine exhaust steam can not be condensed . pressure of the condenser will increase and the rapture disc of the condenser may rapture heavy back pressure will be created on turbine .
In this case load is to be reduced first and care is to be taken to normalize cooling water supply. If situation does not improve then the turbine to be stopped.
Failure of plant automation
In some incident it is experienced plant automation system fail due to control power supply failure of PLC or due to some other resin. In this situation it is difficult to keep the turbine under control. No parameter is visible nor any control system works . in this case the turbine is required to be stopped in a safe way. The operating engineer should apply the mind to act accordingly. Now days most turbine are design for fail safe operation. Turbine automatically Tripped in case of any malfunction or fault in any system.
due to failure of governing system , the turbine speed may become dangerously high. Rotor is designed to rotate up to the limit of 110% of rated speed. At higher speed the mass of rotating equipment is higher then in stationery position. Due to higher centrifugal force the blades which are fixed to the rotor may come out due to failure of blade root and causes to sever and damage to turbine. To avoid dangerous over speed , turbine is provide with mechanical and electrical over speed trip arrangements. Tripping limits are set in such a way that turbine speed does not exceed beyond 110% of rated speed . these over speed tripping limits are to be checked regularly. Mechanical over speed device is to be set within in safe limit and checked at suitable time of interval. At any circumstances over. Speed tripping limits is not to be by passed .
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