HRSG Stack Temperature

[Rpsfinest]

We currently operate 4 ABB HRSGs in combined cycle orientation. On one of the four HRSGs the stack temperature is around 60 degrees higher than the average of the other three. What would be some of the causes for this? What should I look into in getting the heat transfer across the boiler tubes back? Any ideas? If it is indeed true that I al loosing thermal effiecency in this particular HRSG how can I determine if it is real? Would steam flows be less in this HRSG as compared to the others?

 

[imok2]

Sounds to me that a boiler inspection is required to determine if the tube fins are sound and or a deflection plate is not damaged. Yes steam flows should be less and are you maintaining the proper steam pressure at the steam turbine. You might also consider a partial restriction on the turbine gas flow through the boiler

 

[davefitz]

could be several reasons:
a) the LP economizer recirc pump may be
b) the LP economizer bypass valve is open
c) excessive buildup of ammonium bisulphate- due to firing high sulphur fuel with excessive ammonia slip past the scr- time for a washing
d) LP drum pegging steam was left open- check pressure of LP drums for the 3 units
e)other possiblities depending on how the total plant is integrated together
f) LP steam block valve to stg is closed
g) gas thermocouples in error.

 

[toothless]

Is the operation of all 4 HRSGs identical? If not, you can not compare the measured parameters. For instance, if on this one HRSG your are NOT firing duct burners but you are on the other 3, then you can expect a higher stack temperature from the unfired unit.

 

[rmw]

Rpsfinest,

There are sooooooo many variables that could cause that it would be impossible to reasonbly determine the cause without a whole lot more information from you.

rmw