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- Drum Filling - Thursday, July 15, 2010 - Gas turbine Ops [1079]
  Hi to All.
  
  We are operating GE 7FA gas turbines with HRSGs as a cogeneration plant. whenever we do the Offline washing of Gs turbines we are doing the 6 hours of natural cooling + 4 hours Forced cooling of the gas turbines.As we dont have any diverter damper the GT cooling will also show effect on the HRSG drum metal temperatures.Forced cooling will be done at a cranking speed of 849 rpm.
  
  We have a limitation of Max. 40 Deg C of difference between the Top and bottom metal temp.During the forced cooling sometimes we are reaching 45 Deg C and we stop the forced cooling that time.
  
  There are two practises we adopted and their results are
  
  1 To fill the drum once to Maximum level and start the forced cooling activity. During natural cooling we will not fill the drum. -- This causes the drum metal temp difference to reach 60 Deg C during forced cooling and wasting time.
  
  2 To fill the drum immediately after reaching the machine barring gear and to maintain in a very close margin during the natural cooling & Forced cooling periods.If we maintain the drum level in a very close range near to the maximum level then this causes the drum metal temp difference to be maintained @ 42 to 46 Deg C during forced cooling.
  
  My doubts are :
  
  1 If we fill the steam drum immediately after the shutdown and maintain the drum level at high levels will there be any effect on HRSG tubing.
  2 Some people asked to fill only once and to leave like that.But it causes very high differential temp.
  3 Our feedwater incoming temperature will be at 150 Deg C.
  
  Can anyone please explian which is a good practice.
  
  Thanks,
  pvrk
  
  Post a Reply...
- - Re: Drum Filling - Thursday, July 15, 2010 - Ross [390]
    For clarification, when you say this -
    
    We have a limitation of Max. 40 Deg C of difference between the Top and bottom metal temp.During the forced cooling sometimes we are reaching 45 Deg C and we stop the forced cooling that time.
    
    do you mean -
    
    there is an expected Max 40C between the bottom of the drum and the top of the same drum or the lowest header or drum to the companion steam drum?
    
    Post a Reply...
  - - Re: Drum Filling - Thursday, July 15, 2010 - gas turbine ops [1079]
      Hi,
      
      It is the difference high alarm limit for the same drum. High limit for difference between the Top and bottom metal temp.of the same drum during forced cooling.
      
      Thanks,
      
      pvrk
      
      Post a Reply...
  - Re: Drum Filling - Thursday, July 15, 2010 - Jim Watts [764]
    There are two reasons for topping up the drum when cooling after shutdown.
    
    One is to allow for shrinkage and prolong the time before the water needs to be topped up again so that the drum always has water in it.
    
    Two is to ensure the total circumference of the drum is in contact with the water and will cool evenly from top to bottom.
    
    The second only works if you continue to top up as the level tries to drop with shrinkage.
    
    The 40 degree limit top to bottom is very important as it prevents the drum bending like a banana if the length of the top of the drum is longer than the bottom due to differential contraction.
    
    It seems to me you basically know what to do.
    
    The drum top to bottom must not be exceeded.
    Keeping the water level high ensures this is ok.
    
    The tubes are able to deal with much higher temperature changes hundreds per hour as they are thinner than the drum.
    Bending the drum stresses the tubes most.
    
    Of course the top to bottom is not the only rate to be respected as the drum inner to outer metal temperature difference is important also so your maximum cooling rate should not be exceeded either.
    
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