Can you afford to shutdown the GTG whenever you have problem in HRSG/STG or whenever you don't need the steam (if it is cogen)!
The bypass stack enables yo to continue generator power even without HRSG (Ofcourse you may not be able to continue for long if the efficiency or environmental controls happen to be overriding issues).
The principal criteria are pressure drop, temperature and sealing. The device has to live in a very hot atmosphere and survive to work the next time. You don't want it constructed so that it adds back pressure to the turbine and you don't want it leaking either to the atmosphere when in straight through mode, nor into the HRSGS when in bypass mode.
Another criterion worth mentioning is actuation. Pay attention to that. There is a good possibility for unequal expansion, corrosion, and all kinds of ills that will quickly overcome a cheap actuator if you skimp in that area.
Yes Raghun, we can afford to shutdown the GTG whenever we have problem with HRSG/STG. We have so much spare on the power i.e. about 35% spinning reserve. For steam we have auxiliary boiler.
pressure drop, temperature & sealing another criteria.
In a big combined cycle power plant, it is quite common to see bypass stack but again it depends on the above criteria...
Then you don't need bypass stack but during starting of gas turbine, you will have to simultaneously start your steam cycle as well and any trip from steam turbine side also needs to go to the gas turbine.
Most modern installations do not have a stack damper or bypass... and the ones that have them... are still regretting them.
The only advantage of a bypass is the ability to accelerate the start-up of the combined cycle and also allowing the GT to run by itself.
The leaks, problems and repairs needed by the by-pass system are far more expensive than the benefits...
This arrangement is ONLY somewhat successful in units that start and stop everyday and the bypass is exercised everyday... if the unit starts and the bypass is left in the same position for several hundred hours (even thousands) it is almost guaranteed to have problems when shutting down.
