Design Temp for tubes... ASME SEC I 1

[Iyer S]

Dear Friends

Appreciate your help in the Design Temp for tubes for SEC I.
We have HRSGs (Heat Recovery Steam Generators), designed as per Sec I..
The Design Temp of Super Heater Tubes (SA 231 T22) are set at much less than the Gas Flow Design Temperature.

Design Temp of Gas Flow
838 & 810 C for Super heater sections....Design Temp of tubes 568 & 540 Deg C..

Whats the guide lines in Design Temp for tubes? Why its much lesser than Gas flow temp?

Thanks in advance
Iyer

 

[davefitz]

If the HRSG is designed for "dry firing" , meaning no steam flow during full load gas turbine operations without a gas bypass, then the tubes would have the same design temperature as the gas. That sort of design typically uses inconel tubes downstream of an aeroderivative gas turbine, which has a lower gas temperature than does a large frame gas turbine.

In the usual case of designing the HRSG for operating with steam flowing through the tubes when the gas turbine is at full load, then the superheater tubes are desinged for the maximum expected tube operating metal temperature, which includes the factors of gas flow and temperature unbalances and steam side flow unbalances and internal corrosion deposits. The calculated tube metal temperature includes the heat transfer of low temperature steam inside the tube and the higher gas temperature outside of the tube, and the inside tube convective heat transfer coeficient may be more than 10 times higher than the outside convective heat transfer coefficient, leading to a significantly lower tube metal design temperature.

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