Steam drums in CCGT's 1

[woody81]

Hi all. As a relatively new starter in the Power industry can you help? I have been on a large coal station (500MW, 160bar), where the boiler has a single steam drum and reheat systems to operate HP, IP and LP steam turbines. I am now at a CCGT where the HRSG operates with three drums, a HP, IP and LP drum. Why are three seperate drums preferred at the CCGT? I am guesing it is because of the low GT exhaust temperature, i.e the HRSG will not have a temperature gradient sufficient to operate purely on a steam reheat system?

 

[Xianglu]

Your guess is correct.
Because water evaporates at constant temperature, HRSG is not able to generate enough high-pressure steam and thus heat energy in the GT exhaust gas is utilized in stages, ie HP, MP and LP.

 

[davefitz]

right. Nearly all CCGT systems use 3 steam drums plus a reheater bank. There are some exceptions:

a) low pressure , non-reheat, steam turbines, typically with HRSG downstream of a small aeroderivative gas turbine,might be only 2 pressure levels

b)For parallel powered combined cycle plants, there will be 1 steam drum ( IP steam), a reheater bank, and a feedwater heater (HP + LP ) circuits.

c) for Kalina cycles, there will be no steam drums- the ammonia water mixture may be seperated in external seperators, and only one evaporator circuit would be needed, since the binary fluid has a varying boiling temperature. Problems with system cost and nitriding of superheater tubes implies a grand total of one of these were built.

d) Siemens KWU HRSG uses a once thru evaporator ( HP circuit ) plus an IP and LP drum- the HP circuit uses small seperators.

e) Once thru single pressure HRSG ( IST- toronto)does not use a drum nor a seperator, but uses incolnel tubes and a very high inlet orifice DP for flow stability and flow equalization.