Gas performance heater of a combustion turbine - Design issue

[alcor30]

Dear all,
my colleagues and myself are working on the design of a combined cycle powerplant. It is a repowering project. The gas turbine is a state-of-the-art model but the bottoming cycle will re-utilize existing equipments inherited from an older thermal power station from the 70's (steam turbine, pumps, etc...)

Gas module of the combustion turbine will include a so-called gas performance heater.

Function of this gas heater is to raise gas temperature upstream of the injection in the combustor nozzles, basically to optimise Combined Cycle overall efficiency. The gas heater is a heat exchanger fed by hot water bleeded from the HRSG.

Design of the gas performance heater appears to be a challenging issue because, in some situations, the pressure of the hot IP water available from our HRSG may exceed the "design pressure" of the gas performance heater currently proposed by the turbine manufacturer.

Hot IP water available from the HRSG may reach up to 200 bar(g) = 2901 psi ... which is significantly higher than the design pressure of the gas heater (currently 110 barg = 1015 psi).

According to the gas turbine manufacturer, the gas performance heater may be redesigned to withstand high pressure conditions imposed by our project. But, in any case, the GT manufacturer warns that a such a project specific re-design would have a significant cost impact...

I would really appreciate to have advises/recommandations to best handle this design issue...

especially if you have already been facing similar problems with gas heaters on previous projects.

What is the typical cost augmentation we should expect if we were to redesign the gas performance heater to withstand high pressure up to 200 barg (2900 psi) ?

Thank you.

Best wishes to you all.

 

[rmw]

Does the "old thermal power station" turbine have any extraction points for feedwater heating that might be utilized for this fuel gas heating steam? Normally combined cycle designs with HRSG's don't require the feedwater heating stages that the old steam boiler did so this extraction steam is not utilized.

rmw

 

[chief]

Consider using a secondary reheater. That is use the 200bar from the HRSG to circulate another separate heat exchanger. Then tap off and circulate both the heat exchanger and your gas combustor.
Use a steam coil arrangement for the high press 200 bar supply within the heat exchanger, because of the high pressure rating. In marine circles this used to be called an Evaporator. The heat exchanger vessel can then be rated at low pressure , with obviously the necessary PSV fitted.
Some extra expense involved of course, reheater vessel, circulating pump,flow controls.
The re-heater is an off the shelf item,or can be adapted from an existing heat exchanger.

Offshore Engineering&Design

 

[abeltio]

From the project evaluation point of view, the way i would look at it is considering the life cycle of the project and not the short sighted initial cost only.

What is the profit lost by limiting the pressure to the current design (100 bar) over the design life of the plant
(penalty for not upgrading the heater)
vs.
The profit + cost of the mod over the same period of time

Looking just at the initial cost of modifying the heater may not be beneficial for the plant in the long run.


saludos.
a.