Changing/Increasing GT Ramp Rate for warm start of a CCGT Powerstation 1

[daviwy]

I am currently loking at increasing the ramp rate of a Gas Turbine on a CCGT system, and have a temperature gradient limit.

Can people please just give me what sort of concerns / advice / look outs / issues I should be on the look for as I am not experienced at this.

the only concern I can think of is:
- the HRSG units associated to it
- temperature cycling and low cycle corroison fatigue

any comment will be greatly appreciated

 

[GTstartup]

Speed ramp or load ramp?

 

[daviwy]

Speed ramp

 

[GTstartup]

Ramp rates are restricted by the OEM. It's not just a matter of keeping withing the temperature limits. If you could tell us the manufacturer and type it might help

 

[daviwy]

it's GE's MS 6000 GT

 

[davefitz]

In general one can double the startup rate of the CTG/ HRSG if one utilizes a properly designed and operated final attemporator ( between boiler outlet and STg inlet) and install a boiler stress analyzer to limit CTG ramp rate to the max permitted by the boiler steam drum and outlet headers ( thermal stress limits). Usually the startup limitation is thermal stress at the steam turbine ; this is sidestepped by using a final attemporator.

Further improvements are available in the design phase, by minimizing drum wall thickness and SH/RH header thickness, full penetration welds at tube stube to header welds ( superheater and reheater), and properly desing spray atemporators ( wide range spray nozzles + long straight run of downstream piping to allow 0.15 sec steam residence time before downstream bend or header.

 

[stgrme]

I would be very cautious about installing final attemperators in the main steam and hot reheat steam piping as suggeste by davefitz. These attemperators present a real threat of water induction to the steam turbine. ASME TDP-1-2006, "Recommended Practices for the Prevention of Water Damage to Steam Turbines Used for Electric Power Generation", specifically discourages the use of such attemperators. However, TDP-1 does give specific recommendations if final attemperators are installed.

Best of luck!

 

[GTstartup]

The question is about speed ramp not load ramp. During the start up of the GT (0-100% speed) the steam turbine is on turning gear, so I don't understand the comments on the steam turbine being the limitation.

Basically you can increase the speed ramp rate but it puts additional stress on the GT hot gas parts. The OEM should be consulted and they will give the maximum allowed speed ramp, with probably an addition to the operating hours which, in turn, will shorten the time between inspection and overhauls. The HRSG may be an issue also, but this may be warmed in advance, depending on your configuration.

 

[daviwy]

Guys, please do give your comments on both speed and load ramps. As no knowledge is wasted...

Please do drop more information or knowledge you have on this subject they will all be taken in and appreciated.


David O.
Mechanical Engineer
Graduate as of: 2007
Oil, Gas & Power

 

[rmw]

You asked for concerns. My concern is that the ramp rate is determined by being arrived at by a group of engineers who are familiar with all aspects of what is affected by changing a parameter such as ramp rate. There are turbine implications. You can break that down into several categories, hot gas path parts, shaft growth, etc. There are hrsg implications, hot gas path parts, thermal growth, circulation issues, etc. There could be generator issues.

That is to name a few but it covers most bases. Now you want to change it. I think you should get an opinion from each OEM with parts affected by the ramp rate change and then make up your mind if they are all on board and if you have met all their requirements.

rmw

 

[davefitz]

Of course each component and subcomponent needs to be evaluated to determine the limiting factor for ramp rate. But on a much simpler level, some of these limitations are well known.

For a large frame gas turbine in simple cycle mode, a cold start to full load is feasible within about 15-20 minutes. So the CTG limits are more or less defined by the simple cycle ramp rate lmitations.

For a drum type HRSG with a 4" thick HP steam drum and 1.5" thk P91 SH outlet header and no concerns regarding downstream equipment ramp rates, full HRSG load could theoretically be reached within 1 hr.

When the CTg's and HRSG's are mated to a 200 MWE x 2000 psig STG without a final attemporator, one has a typical startup time from cold to 50% MCR load in about 4 hrs, on a very good day.Most environmental permits for these units are also predicated on achieving this load within 4 hrs.

This 4 hr plant cold startup period could theoretically be reduced to about 2 hr if a final attemporator were provided, and HRSG thermal stress limits were monitored by a BSA boiler stress analyzer. STG prewarming system and aux boiler would help also.