Boiler - Turbine Cycle effeciency

[Rpsfinest]

I am trying to calculate the efficiency losses directly attributed to High Backpressure on the steam turbine due to an air in leak on the rupture disk. Can anyone help me out with a calc to perform, I want to see what the heat rate loss is and try to put a dollar figure on it.

Also I want to try and calculate the heat rate cycle efficiency loss directly attributed to another plant that has an Low Pressure steam valve failure that is not allowing us to admit LP steam to the Low pressure turbine. The only steam going to the LP section now is from the IP crossover. Because of this we have to vent that steam to atmosphere in prevention of over pressureizing the LP drum. The unit is a GE D11 steam turbine on a 2x1 Combined cycle unit.

Any help is much appreciated... Thanks guys this is a bit new to me...

 

[davefitz]

Its been a while, and I will double check after I dig up some old papers, but here goes:

For the exhaust loss, there should be available a turbine cycle diagram for the steam turbine. At design load and design back pressure, for the LP turbine, it provides inlet P, T,H, expansion line endpoint exhaust P +H, and also used energy end point UEEP exhaust H. The difference between the exp line endpoint and the UEEP is due to the lost energy due to the exhaust annulus velocity 0.5 rph v ^2, and this value can be obtained from a curve of exhaust loss vs exhaust velocity. The diagram should also list the exhasut annulus area, or the last satge blade diameter.

The apparent isentropic stage efficiency of the LP turbine can be calculated from this design cycle diagram, based on inlet P,H and exp line endpoint P,H; it may be around 92%. this is via effic= (Hi-Ho)/ (Hi- Ho*), where Ho* is the theoretical exhaust enthalpy if the expansion had been isentropic ( S0=Si). The higher backpressure will yield a lower Ho* for teh new conditions, and if you maintain the same effic, then the new Ho will be a higher value, leading to a lower amount of energy generated across the LP turbine.

This value of Ho must be further adjusted for exhaust losses, 0.5 rho v^2. This is best accomplished by calculating the new exhaust velocity and obtaining the exhaust loss from the manufacturer's generic loss curves.

After all the above calcs are doen, you can recalculate the turbine cycle heat rate,using the new used energy endpoint.