The steam condenser in power plant is designed to condense the turbine exhaust steam by circulating water and to reduce the back pressure of LP turbine as much as possible. The lower back – pressure gives the higher heat energy to LP turbine from steam.
Condensed clean water is used as boiler feed water.
Components of Steam Condenser in Power Plant
1) Exhaust Hood
LP turbine exhaust hood is welded to the condenser upper shell.
2) Upper shell
Upper shell refers to the section between lower shell and exhaust hood LP turbine steam bypass pipes are installed in the upper shell. Reinforcing ribs are welded on the upper shell inside wall against external pressure.
3) Lower shell
Lower shell consists of lower shell plates, support plates, tubesheets, tubes, baffle plates and many reinforcement pipes and angles. The lower shell is also built as a pressure – resistant structure. Hotwell has a 3 minutes retention capacity of normal condensate flow.
Carbon steel with Epoxy coating on Water box side which has highly reliable corrosion resistance is applied to the tube sheet.
Welded stainless steel thin walled tubes are used for condenser tube. Tubes are installed on tubesheets with both ends fixed by means of an expansion and is installed inclining toward the back side, which facilitate draining at shut-down.
6) Support plate
Support plates are installed in the LP & HP condensers. The functions of support plate are preventing resonance of tube natural frequency and turbine revolution, contact of each tubes and abnormal vibration of tubes raised by turbine exhaust flow.
7) Tube arrangement
Tube bundle is designed to optimize the steam flow into the bundle and the outer rows where exhaust steam makes direct contact are protected by using t0.889mm tubes.
8) Water box
The water boxes are designed that circulating water flows smoothly and manholes are installed on the water boxes for inspection. To prevent the corrosion, epoxy resin lining is applied to the internal surface of the water boxes.
The flashbox is provided at LP and HP condenser. The function of the flashbox is to receive heater drain and miscellaneous drain temporarily before entering condenser to protect tubes from being damaged. The flashed steam in the flashbox will be led to condenser shell through the opening made upper side while the drain flows into condenser shell from the bottom side. The internal of the flashbox is protected with stainless steel plate where necessary.
Start-Up of Steam Condenser in Power Plant
1) Prior to start-up, the following systems around the condenser are kept in start-up condition in accordance with each system operation manual.
- Condensate system
- Condenser make-up water system
- Circulating water system
- Condenser air evacuation system
- Other systems
2) The closed cycle of cooling water system is filled and is in-service. The condenser hotwell is filled automatically to NWL (normal water level) via condenser hotwell make up line. Ensure the circulating water and condenser vacuum venting equipment are already in-service and the condenser vacuum is established. A condensate pump is started.
3) At a turbine load of 100%, check whether or not the condenser satisfies the requirement of the prescribed performance on that condition. In case the performance is not satisfied, trace the cause and make sure that the prescribed performance is satisfied. If the auxiliary unit adjusting device is perfectly adjusted, the condenser will work automatically.
4) Do not enter inside of Condenser during circulating water system is in service.
Shutdown of Steam Condenser in Power Plant
To stop the condenser, take the sequence of steps in reverse order to start up:
1) Even if the flow rate of steam flowing into the condenser decreases, the water level of the condenser is kept constant by means of water level adjusting device. When the condensate quantity has decreased, the condensate is recirculated to secure the minimum water quantity. This step will protect the condensate pump and secure the cooling water for the other cooler and gland steam condenser.
2) When the steam flow to the condenser has stopped and the turbine has tripped, open the vacuum breaker valve (In case of restarting after a short halt, the breaker need not be opened sometimes.) and stop the vacuum venting equipment.
3) After turbine steam flow to the condenser has stopped, shutdown the condensate pumps. In deciding the timing for the shutdown, the cooling water system which is feeding water to other plant items must be taken into consideration.
4) Even after the turbine steam flow to the condenser has stopped, it is desirable to allow the water to circulate in the condenser for some time and shut down the circulating water pump after it has become known that the temperature within the condenser will not rise. Also take into consideration the case in which the circulating water is fed to the other cooler. Condenser water box vent and drain valves have to be opened and circulating water in tubes is drawn to prevent the fouling inside tubes.
5) When draining off hotwell, one circulating water pump shall be operated for safety.
6) When operating condenser tube ball cleaning system at one circulation water pump operation, half tube bundle operation shall be carried out (i.e. one pump operates one side of the tube bundle. As for half tube bundle operation,
Cautions to be Taken When Vacuum Falls – Steam Condenser in Power Plant
1) Check whether or not the indication of measuring instrument is correct. Check whether or not vacuum reading has been corrected with atmospheric pressure and temperature.
2) Check whether or not the temperature of cooling water is correct.
3) Cross-reference check with the performance curves.
4) Compare readings with the curves taken at the beginning of installation or immediately after tube cleaning.
5) Check the coefficient of overall heat transfer by calculating back from above data.
6) Check whether or not the air leak quantity has increased by outlet flow meter of vacuum venting equipment.
7) Check whether or not the cooling water level in the water box has lowered by the water box level indicator.
Cause of Fall in Vacuum and its Countermeasure – Steam Condenser in Power Plant
1) Condenser condensing capacity
a) Fouling of cooling tube
If scale or foreign matter stick to the tube, a marked fall will be observed in performance even though the deposit layer may be very thin. Therefore, clean and remove scale or foreign matter. The property of it can be made clear to some extent with naked eyes.
It is necessary to analyze a sample of the scale or foreign matter and clearly determine the property.
The temperature of circulating water as cooling water is subject to seasonal variation. Accordingly, it is necessary to measure the inlet and outlet temperature of cooling water and calculate the performance. Fall in performance is, in most cases, attributable to tube contamination.
In places where cooling water quality is bad, it is necessary to record the vacuum for temperature of cooling water and graph it to confirm the purification of the cooling tube. If the cooling tube is fouled with foreign matter too much, it will be impossible to remove it with a rubber-headed nylon brush. As a result, pickling cleaning or high pressure water jet cleaning may be required in some cases.
b) Shortage of cooling water
If the difference in temperature of cooling water outlet and inlet is high, there may be a shortage of cooling water.
If the pressure drop in tubes is high, there may be a clogging trouble at the tube inlet. If the pressure drop in tubes is comparatively low, there may be a drop in pump performance and clogging of the water passage.
In any case, it is necessary to check a drop in performance of the circulating water pump, pipe passage and tube, siphon effect from the outlet pressure of condenser and faulty outlet valve opening.
c) Air accumulated in upper part of water box
If air is accumulated in the upper part of water box, there will be some cooling tubes through which no water passes, resulting in decrease in cooling surface area. Accordingly, it is necessary to discharge air fully at the time of starting the circulation water pump.
d) Half tube bundle operation
In the case of half tube bundle operation, be sure to close the air outlet valve to the vacuum venting equipment at no cooling water flow side. If not, steam will flow out, resulting in exceeding the capacity of the vacuum venting equipment and falling vacuum.
2) Air leakage through the apparatus in the vacuum zone
a) Air leakage at the turbine
Air leakage at the turbine is attributable to loosening the low pressure casing joint at the vacuum zone, the connecting pipe joint, a shortage of gland steam, poor drain valve or drain pipe.
b) Air leakage at condenser and vacuum venting equipment
Air leakage at the condenser and vacuum venting equipment is attributable to loosening of the joint or gland for condenser shell trap, vent and drain pipe with valve, and water level controller etc.
c) Deterioration of capacity of condensate pump
If the capacity of the condensate pump has deteriorated, the level of condensate in the condenser will rise and result in a decrease of the cooling surface area. Accordingly, always check the level indicator. Attention must be paid to the air leakage of gland and equalizing pipe clogging in case the condensate pump is operated.
d) Vacuum venting equipment malfunction
See vacuum venting equipment instruction manual.
e) Instrument defects
Instrumentation should never be incorrectly calibrated, have any air leakage of the pipes, or be set to stay drain. The gauge should be replaced with new one or calibrated every so often by comparison with a high accurate manometer. Additionally, the gauge and manometer must be corrected with atmospheric temperature.