STEAM ATTEMPERATION PROCESS

STEAM ATTEMPERATION CONCEPT

• Superheating is a process in which stem is heated above saturation temperature & it is in the form of sensible amount of heat.
• For a unit he attemperation water, which is 2-5% of the total steam flow, may come from the boiler feed water pump outlet, or further downstream, just before the economizer inlet (After feed water heater).
• The attemperation system is called as Desuperheater used to control the steam temperature & pressure.
• From an energy efficiency standpoint, attemperation would be more effective and require lower flows if sprayed further downstream.
• To eliminate the possibility of water content entered in to the steam turbine attemperation is used before primary super heater.
• If any content of water particle is present in steam then it is super heated in primary & secondary superheater.

STEAM TEMPERATURE CONTROL METHODS

• Desuperheaters are used to control the temperature of superheated steam.
• Desuperheater decrease the degree of superheat of steam i.e steam is cooled.
• In a Desuperheater a calculated amount of water droplets injected in to the steam flow path.
• As a result saturated steam is obtained.
• An attemperator is a device that is used to spray water into the superheated steam between the primary, platen, and final superheaters and the reheat lines.
• The aim of the attemperator is to control the main steam temperature according to steam turbine inlet temperature.
• Basically attemperation is used for Main steam & reheater steam temperature control.
• Other means to control steam temperatures are;-
  • Tilting burners away from the superheater tubes.
  • Using bypass dampers in the back-pass or convection portion of the unit.
  • Using flue gas recirculation (FGR) to moderate combustion.
• The effect of these methods take longer time while attemperation is quick control of transition.
• Attemperation flows can be eliminated on supercritical boilers by controlling the firing rate to match the feed water flow.
• Steam temperature excursions are a result of an imbalance between the heat added to the steam by the firing rate and the heat withdrawn, by the steam flow rate.
•  A control strategy will control steam temperature for a drum boiler operating in either constant or sliding pressure mode.

BENIFITS OF STEAM TEMPERATION CONTROL

• As steam tapping is taken from the feed water so it does not goes for steam conversion so it has a negative impact on cycle efficiency.
• Improved temperature control on process transients caused by changing load, soot blowing, starting and stopping of mills in service.
• Improved steady state temperature control when operating under conditions of reduced pressure, slagging.
• Faster ramp rate.
• Reduce the need for attemperation, which improves plant heat rate.
• Flexibility to control main steam temperature on drum boilers in either a fixed or variable pressure operation.
• Avoiding unscheduled outages for superheater tube failure.
• The benefits of the higher temperature increased generation of backpressure power, better condensing power efficiency, and reduced steam use by the drive turbines, all due to the higher steam expansion work available in the turbines.
• Operating with higher energy content steam allows specific steam consumption to be reduced, with consequent reduction in auxiliary boiler feed pump power.

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