Main Steam Line attemperator 2

[Philhi2011]

Hi there,
I'm working on a test plan on one of our HRSG to measure the maximum temperature on the super heater inlet and outlet tubes. Our GT will be running from base load (~40MW).
We are planning to run in a combined cycle. Our main steam line outlet from HRSG has a temperature limit of 920 deg F. The design temperature is 950 deg F.
We have the steam attemperator at the main steam line prior to the stop valve to the steam turbine. However the attemperator is only design to activate during cold start. It will deactivate when reaches to 900 deg F.
I'm wondering as I push more than 950 deg to the steam turbine, what will be the maximum temp allowed in the attemperator and also on the steam turbine.
Appreciate all your input.
Thanks.

 

[hacksaw]

You don't tinker with the steam turbine.

 

[davefitz]

The design temperature of 950F indicates the design limitiation is creep damage. If the plant operates over the 950F design temperature then the life of the main steam line and the HP turbine inlet will be less than assumed by ASME. Severe accidents have happened in the recent past due to operation above the design temperature limit, such as the 6 fatalities at Mohave station in 1985, so it is recommended the plant modify the equipment to ensure the 950F is not exceeded for any significant period.

One thing that can cause such extended overheats would be fouling of the HP evaporator or HP economizer tubing, such as might be caused by ammonium bisulphate.The final attemporator could be designed to allow use at higher loads, but that ability needs to be checked by the plant design engineer.

In the past it was strictly prohibited to use a final attemporator upstream of the steam turbine, but the ASME turbine water damage protection guidelines had been modified perhaps 15 yrs ago to allow HRSG's to use them to manage startup overheats. There are strict requirements that must be addressed to prevent water droplets from entering the steam turbine, including drip legs, steam traps, top and bottom thermocouples on the downstream HP main steam line, and a properly designed attemporator. Recent advances in understanding the behavior of different spray nozzles via lab testing using laser raman scattering techniques have led to improved design methods, but they are only useful if you read them, understand them, and use them within their test limitations.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[stgrme]

Keep in mind that the operating temperature of the main steam system may be limited by the “rated” temperature of the steam turbine. Check the turbine’s nameplate for the “rated” temperature. Operating above the “rated” temperature will result in reduced creep life of the turbine.

Best of luck!

 

[Dhurjati Sen]

The turbine would trip. Manufacturers do take into account such playfulness.

DHURJATI SEN
Kolkata, India