1-Decrease in Economiser inlet feed water temperature
Upon decrease in economiser inlet feed water temperature, fuel consumption in furnace increases & hence main steam temperature increases more than desired.
2-Load on Boilers is more than design
Main steam temperature increases if the steam generation is more than actual designed
3-Leakage in super heater (SH) coils
If there is any leakages in SH coils, steam flow to SH coil decreases & hence main steam temperature increases.
4-Choking in Super heater coils
Super heater coils Choking due to foreign materials & internal scaling due to poor water chemistry leads to less steam flow through coils resulting
into higher main steam temperature due to poor heat transfer.
5-Passing of start-up vent control valve
Passing of start-up control valve loads more on Boiler steam generation, hence main steam temperature increases
6-Passing or pop up of drum safety valves : Lifting of drum safety valves lead to less steam flow to SH coils resulting into higher main steam temperature.
7-High moisture fuel: If the fuel moisture is more than required, then water present in the fuel absorbs heat from furnace & transfer heat convectively to SH coil.
8-Low GCV fuel: Burning of low GCV coal in furnace releases more heat & hence MS temperature increases.
Note: Burning of high GCV fuel with high moisture also leads to temperature shoot-up.
9-Failure of attemperator control system or wrongly tuned attemperator control valves.
10-Operating the Boilers at more negative draught: This causes more convection heat transfer in SH, (& also at economizers & APH) leading into high main steam temperature
11-More excess air also can lead to fast heat transfer, which can result into high main steam temperature
12-Wrongly designed SH coils: If there is provision of more heating surface in SH coils, then eventually steam temperature always maintains on higher side.
13-Very Clean & scale free surface of SH coils: Leads to faster heat transfer in SH zones causing more steam temperature
14-High moisture content combustion air leads to increase in MS temperature
15-Lesser turbulence in furnace leads more convective heat transfer at SH zone & eventually results high main steam temperature.
16-Wrongly designed SH & furnace zone: High velocity of FG in furnace & SH zone leads to fast heat transfer, this causes high main steam temperature.
17-Mixing of fuels in furnace: Causes uncontrolled excess air & draught which causes main temperature to increase.
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