I work on a 400MW CCGT with an HRSG and an ACC.

I am looking at the possibility of loading the GT up on bypass to 200MW, for example, without temperature matching on to establish a time frame for doing so. If this is possible, can I then reduce GT load and go back into temperature matching?

If so; I plan on running the plant up in auto sequence and holding it after the GT has synchronized, then bringing the GT out of Remote into Auto/Preselect, where I plan to run the GT up to 200MW before reducing the GT load back down to 50MW and placing the unit back in to Remote before continuing the auto sequence, which should continue with temp matching.

I envision that the MKV1 will then reduce the GT exhaust temps in relation to the ST metal temps for temp matching.

Can anyone see any concerns with this?
Does anyone have experience with this?

 

Yes, this should be possible. We have been doing this for years with our HRSG's. I cant really comment for GE (MKVI is GE...isn't it?), but I cant imagine why they would be any different to other companies on this matter.

Generally you should be able to run the GT up to max load in Single Cycle Mode, then if you want to open your Diverter Damper, then, as you stated, you will need to drop load in order to reduce the temperatures.
Importantly though, you may need to purge the HRSG for a certain time, as you are now potentially exposing the hot exhaust gases to an explosive environment.
This is the reason that you need to reduce below a certain temperature (take the exhaust gas below the ignition point of the fuel gas for purging).

I'm really surprised that this functionality is not built into your HRSG/GT controls.

 

First of all, you are lucky to have a bypass stack at your plant (newer plants don’t have one!). Usually the power plant owner specifies the mode of operation type for their CCGT and the intended design philosophy is operating the plant in most efficient (economical) way.

The following operating modes are standard:
1) LOCAL
2) REMOTE whereby the DCS became the master

1.1) In LOCAL mode you will be able to load your turbine as required (min to max power output of the GT coupled Generator ONLY-SIMPLE CYCLE MODE!). Make sure that the "GT HOLD" function from the DCS is NOT activated.

2.1) In REMOTE mode, post warming up cycle, you will be able to load your GT-GEN and ST-GEN as required.

Usually in the MKVI there are two temperature operating modes programmed. First temperature operating mode will be so called SIMPLE CYCLE IGV Fully open mode and the second COMBINED CYCLE IGV MODULATING mode. In the first mode the exhaust temperature will be following the fixed Isothermal line (400DEGC IGV 85DGA). Second mode will operate at higher exhaust temperatures (>400DEGC 85>=IGV>57 DGA) and modulating the amount of inlet air which results in exhaust flow.

Bear in mind that your proposed operating mode is possible and useful only for the GT-unit only. Sometimes the so called three element control to the HRSG is used. If you have this system, you will have unstable operation of your entire plant during this kind of transient conditions. In any case MKVI have many interlocks between the HRSG sequences. What every you do make sure that you select from the DCS the so called "GT HOLD' or Boiler warming up option prior going back to your normal operation. If you will not do that most likely you will not be able to close the damper.

Good Luck.

 

Hi, thank you for your time and help.

First I would like to apologise to you as I seem to have misled you a bit. When I said "I am looking at the possibility of loading the GT up on bypass to 200MW", what I meant by "bypass" was bypassing the ST into the ACC, not by a bypass stack.

I have however gained a lot from your responses, and have a better understanding of how to proceed.

I hope this does not change things to much?

Thanks.