A steam turbine in power plant is a rotating machine that utilize high pressure steam to move the moving blades that are mounted onto the rotor of the turbine.
In the boiler, feed water at a certain pressure and temperature is heated to the saturated liquid state, vaporized to the saturated vapor state, and saturated steam then passes through the superheater and is heated to its desired temperature. Superheated high pressure steam is then piped to the steam turbines which drive the generators.
The steam turbine in power plant consists of three stages:
- High Pressure (HP) Stage
- Intermediate Pressure (IP) Stage
- Low Pressure (LP) Stage
After the steam has passed through the HP stage, it is returned to the boiler to re‐heat to its desired temperature although the pressure reduced substantially. The reheated steam then passes through IP stage and finally to the LP stage of the turbine through interconnecting steam piping.
After driving the steam turbine, the exhaust steam from the low-pressure turbine is passed into the condenser where hot steam is condensed into water and sent back into the boiler again by condensate extraction pumps (to deaerator) & boiler feed pumps.
Equipment of Steam Turbine in Power Plant
- Turbine rotor
- Turbine Blade
- Turbine Casing
- Turbine Nozzle
- Journal and Thrust Bearing
- Rotor Coupling
- Turning Gear
Turning Operation of Steam Turbine in Power Plant
Turning operation refers to the continuous or intermittent low-speed operation of the steam turbine rotor system, auxiliaries, and equipment, including the turning unit and its auxiliaries.
Turning Operation before Steam Turbine Start-up
To prevent abnormal vibration during turbine start-up, especially while increasing speed, the rotor eccentricity must be stable and within normal values. Generally, after the unit trip, there is an unavoidable difference in temperature between the upper and lower halves inside the casing, with or without turning operation.
During suspended turning operation, the rotor bends because of a deformation generated by this temperature difference within the limit of elasticity. The primary purpose of turning operation prior to turbine start-up is to prevent or correct this bending.
Turning Operation at Steam Turbine Start
When steam is admitted to the turbine, the rotor automatically parts from the turning unit, and the speed increases continuously from the turning speed. Lubrication of the bearing at this time shifts smoothly from boundary lubrication to fluid lubrication. This is another purpose of the turning operation during turbine start.
If rotor speed increases because of steam only, without turning operation, it may damage the bearing and rotor because the static friction will shift to dynamic friction between the inner surface of the bearing and journal. Turning operation stops at the stopping of the turning motor, when it has been confirmed that the rotor is away from the turning unit, and at the start of the turbine
Turning Operation at Steam Turbine Shutdown
The purpose of continuing the turning operation at the time of stopping the turbine is to minimize deformation of the rotor and casing and to cool them uniformly. Conspicuous deformation of the rotor may cause contact between the stationary part and the rotating part, with resulting damage.
The turning operation tends to make the temperature distribution inside the turbine shell and on the rotor more uniform. However, it should be noted that some temperature difference between the upper and lower casing is unavoidable.
On the other hand, because of the construction of the turbine, the bearing bottom side is the contact part between the stationary and the rotating part. When the turbine stops at a high temperature, the heat is conducted in the axial direction of the rotor from the bearing surface to the stationary part. Oil circulation is necessary to prevent the temperature of the bearing metal from rising.
Turning operation at turbine stop is based on when the turning unit and/or oil pump should be stopped.
Turning Operation During Long Term Steam Turbine Shutdown
When long-term turbine shutdown of 2 weeks to several months is scheduled, the turning system should be maintained according to the following:
(1) Once a week, the oil pump should be operated for about one hour to renew oil in the system. During that hour, the turning gear should be operated about 10 minutes continuously to renew the oil film in the bearings.
(2) Instructions for operation of lubricating system auxiliaries should be exactly followed for each item of equipment.
(3) For long-term turbine shutdown, the orientation of the poles of the generator should be set vertically. At each subsequent turning operation stopping point, the pole orientation should be alternated between top and bottom settings
Operation Philosophy of Steam Turbine in Thermal Power Plant for Start-Up
The Start‐up modes of steam turbine in thermal power plant are defined as follows:
- Cold Start up : shut down period > 72 hours
- Warm Start up : shut down period > 10 hours and < 72 hours
- Hot Start up : shut down period < 10 hours.
1). All circuits have sufficient Instrument air pressure, Main Cooling Water (MCW) System is in service, Make up Water System is in service, Chemical Dosing System is in service, Closed Cooling Water System is in service, Condensate Water System (incl. Condensers, Condensate Extraction Pumps (CEPs), Condensate Polishing Plant (CPP), Gland Steam Condenser (GSC), LP Heaters) is in service, Condenser curtain spray water system, Steam Turbine LP casing spray water system, Power supply for all the valves, Turbine Bypass valves and spray water system is in service, Turbine Bypass Control Oil Systems are in service, Feed Water System (incl. Deaerator, Boiler Feed Water Pumps (BFPs), HP Heaters & Desuperheater) is in service, Boiler Super Heater & Reheater Spray Water Systems are available, Auxiliary Steam System is in service and the auxiliary steam is supplied to ST Gland Steam System and Deaerator warming from the auxiliary steam headers (HT ), Steam Turbine Lube Oil System & Control Oil System are in service, Steam Turbine Generator Shaft Seal Oil System and Stator Cooling Water System are in service, Steam Turbine Generator H2 gas filling is completed, STG is on turning gear, Turbine seal system is in service, Vacuum Pumps are in operation and condenser vacuum is established, GSC pressure is normal.
2). After condenser vacuum being established to the predetermined level then Turbine warming shall be started depending on the Turbine inner metal temperature.
IP Turbine warming valve will also be opened during HP Turbine warming period through HP Turbine warming valve. After Turbine start permissive conditions are established, Turbine can be started by pressing “TURBINE START PB” from DCS HMIPS either in manual start up or auto start‐up mode.
Main steam is supplied to the HP Turbine and Reheat steam is supplied to the IP Turbine to increase Turbine speed with the predetermined speedup rate. Turbine “RUB CHECK” at local shall be confirmed at the 400 rpm. Then low speed and high speed heat soak action is performed. High speed soak operation is performed to balance the turbine thermal stress at 3000rpm and further synchronization to grid system and then load up operation starts
3). Prior to start‐up, following drain valves shall be fully opened.
- Main steam/Reheat steam system drain valves
- MSV/CV & CRV seat drain valves
- Extraction steam system drain valves
- HP Turbine casing drain valve.
4). In case of Initial Cold Start up (when the Steam Turbine 1st metal temperature is less than 150 deg. C), Steam Turbine pre‐ warming is conducted in order to shorten the start‐up duration & reduce thermal stress by supplying the high temperature auxiliary steam (285 deg. C), which enters from HP Turbine Exhaust end through HP Turbine warming line through motorised inching HP turbine warming valve.
Warming steam enters the HP Turbine from exhaust end through CRH line and flows in reverse direction inside HP Turbine thereby increasing the turbine metal temperature. HP Turbine warming valve is gradually opened and adjusted by inching operation automatically in order to keep HP 1ststage shell pressure around 4.9 kg/cm2 .g until the 1ststage inner metal temperature reaches to 150 deg. C and stabilized.
To reduce the surplus HP 1st stage pressure, this MOV is only allowed to open partially. The non‐return valve is provided to prevent the steam back flow from HP Turbine. Steam after passing through the HP Turbine, enters the Main Steam lead pipes and flows towards MSV/CVs. Steam is finally dumped into the condenser HP flash box through Main Control valves after seat drain valves which are on AUTO and opened
5). To warm‐up the IP Turbine before starting operation in order to shorten the starting time and reduce the thermal stress of IP rotor at start‐up, IP Turbine warming is done by supplying the steam from the Gland Steam Seal Header (SSH) through the motorised ON‐OFF type IP turbine warming valve HSFV.
Warm steam (temp. 285 deg C) from Gland Steam Seal Header (SSH) is fed into the IP Turbine through HSFV and flows in forward direction through the IP Turbine section thereby increasing the turbine inner metal temperature.
Steam after passing through IP Turbine flows into the cross‐over pipe and further enters the LP Turbine and finally dumped into the condenser via LP Turbine exhaust end. IP Turbine warming HSFV will be Auto closed on HP Turbine warming valve closed.
6). At Turbine start‐up, the control for the HP and LP turbine bypass systems are used to facilitate matching Boiler steam parameters to turbine metal temperature for reducing start‐up time when the Boiler pressure is established to the predetermined level.
7). Turbine side drain valves are kept opened until the extraction steam system in‐service is completed, which is more than 10% turbine load.
8). The load change rate during the Turbine start‐up will be in accordance with the mismatch chart, which is automatically selected based on the 1st stage inner metal temperature. After ST load reaches the initial load or completion of initial load holding, ST load increases with Loading up rate by the operator or DCS with Loading up rate in accordance with ST Start up Curves.
9). According to the increase in opening of CV/ICV (Intercept Control valve), the HP and LP turbine bypass valves and its spray valves are automatically closed.