DIFFERENT METHODS OF STEAM TEMPERATURE CONTROLS IN POWER PLANT
DIFFERENT METHODS OF STEAM TEMPERATURE CONTROLS IN POWER PLANT
today we discussed about a very important topic which is different methods of steam temperature controls in steam power plant .During the Operation of boiler super-heat temperature control is very important task for a desk engineer during boiler Operation. Super-heated Steam temperature is required to maintain within permissible limit. The temperature is not allowed to fall below and rise more than certain value. This range is very close when steam is used to drive a turbine. It is essentially to maintain temperature to within limit to minimize thermal stress. When steam demand decrease suddenly, flow in super heater reduced so the temperature of Steam increase.
In similar way when flow increased in super heater due to sudden load demand, steam temperature decrease. To maintain outlet temperature some control is required. In power plant industries Steam temperature is generally controlled by spraying water into Steam between the first and second stage super heater to cool it down. Feed water which is used for spraying is taken from boiler feed pump discharge. Water injection is done in a device called an attemperator or de- super heater. The spry water comes from either an intermediate stage of the boiler feed water pump or from the pump discharge. Other methods of Steam temperature control include flue gas recirculation, flue gas bypass, and tilting the angle at which the burners fire into the furnace. Various methods are adopted to control outlet steam temperature. Some of these methods are discussed below.
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY GAS BYPASS METHOD
in this method flue gas path is by passed by the help of a damper, so that some portion of flue gas does not flow through super heater.During low load, when steam flow is less in super heater bypass damper is kept open. Some portion of flue gas is by-passed. As less volume of hot flue gas flow through super heater temperature of Steam can be maintained constant during low load condition. During higher load damper is closed and either flue gas is allowed to flow through super heater.
According to the desired temperature at certain load, damper is set at certain position.
The damper has to operate at high temperature and environment. So high temperature corrosion, erosion and failure may be take place. Draft loss in flue gas path is not always constant in this case. When damper is in closed condition flue gas flows through super heater so draft loss is more in this condition. When the damper is at open condition, flue gas path is bypassed and draft loss in this case less.
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY EXCESS AIR CONTROL METHOD
Air supply to the furnace can be increased or decreased to increase or decrease superheated Steam temperature. When it is required to increase Steam temperature more air is supplied to the furnace than normal requirement. Due to more excess air heat absorption at furnace water wall decrease. So total heat content in flue gas increase. More heat transfer takes place at super heater tube and Steam temperature increase.
To decrease steam temperature, air is reduced so heat absorption at water wall increases. Heat content in the flue gas decrease and heat transfer to super heater decrease.
This type of control is mostly used, where convective short heater is used.
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY UTILTING / ADJUSTING BURNER CONTROL METHOD
In this method burner’s position is adjusted to control super heated Steam temperature. In this method the burners are designed in such a way that it can be tilted upward or downward when burner is tilted downward water walls receive more heat. So less heat is available in flue gas entering to the super heater zone. So temperature of Steam comes down.
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When the burners are tilted upward less heat is absorbed at water wall and more heat is absorbed at superheater. So Steam temperature increase. Depending upon requirement burners may be tilted upward or downward.
In some boiler burners are placed at different elevation of the furnace. During load down when furnace temperature is less upper tire burners are taken into service. So that the flue gases temperature at furnace exit increase and Steam temperature increase. During higher load lower burners are taken into service along with upper burners, so that the furnace exit temperature is maintained constant.
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY SEPARATELY FIRED SUPER HEATER METHOD
In this method there are two furnaces. The radiant super heater is placed in another furnace and convective superheater is placed between common flue gas path of both main boiler furnace and super heater furnace. Superheated Steam temperature can be controlled by adjusting the firing rate of both the furnace
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY COIL IMMERSION IN BOILER DRUM
In this method some portion of superheated Steam is passed through a coil immersed in boiler drum. A bypass valve is provided to control flow of steam to drum.
When temperature of steam is high control valve closed so more steam is pass through the coil immersed in boiler drum. Temperature of Steam is controlled. When temperature of steam decrease by pass valve is opened. Most of the steam passes in the bypass in the bypass line and temperature increase.
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY DESUPERHEATING OR ATTEMPERATION METHOD
this is a commonly used method to control superheated steam temperature. In this method cold condensate is spryed directly to the superheated steam. Some time superheated steam is cooler in a heat exchanger where stem flow in coil and feed water flows in the Shelly. Both these methods of temperature controlling are very effective.
The super heater is divided in to two sections called as primary and secondary super heater. Attemperator is placed between primary and secondary superheater heater. So the temperature control can be done very effectively. Also chance of moisture in Steam is eliminated as after attemperator the steam passes through secondary superheater. These two methods of temperature control is discussed below
STEAM TEMPERATURE CONTROLS IN POWER PLANT BY SPRAY TYPE ATTEMPERATOR
in this case feed water from boiler feed pump is sprayed directly to the steam coming out from primary super heater through a nozzle. Temperature of the steam reduces. This steam then passes through secondary super heater. If out let temperature of secondary superheater is high then the control valve is put in auto mode so that spray water quantity can be adjusted automatically.
In spray type attemperator, the spray nozzle is fitted to steam header with suitable pipe connection feed water is connected to this pipe. A metallic protective jacket is provided at the header. Length of this jacket is around 4 to 5 meters. Feed water is sprayed to steam with the help of nozzle. Spray water evaporate completely at the end of the jacket. So hot may steam header does not come in contact with this water and thermal shock is avoided.
Some times when purity of steam is important at that condition feed water cannot be sprayed directly as it is mixed with Steam and chances of steam contamination is there. In this case instead of directly feed water spray, saturated steam from boiler is condensed and this condensate is used for spray. This method minimizes steam contamination chances.
B. SURFACE TYPE ATTEMPERATOR
In this system, feed water is not mixed with super heated steam. Temperature of Steam is controlled by varying feed water flow in a heat exchanger. Feed water flows in the shell of exchange and super heated steam flows inside the tube. When it is required to increase temperature of steam, feed water is bypassed with help of a control valve. When it it required to decrease steam temperature more water is allowed to flow through heat exchanger by closing the control valve. The main advantage of this system is, feed water does not mix with Steam. So the purity of Steam is maintained.…
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