Gas Turbine & Steam turbine in single shaft

[WALJA70]

We are planning to install first combined power station in our company compromise Gas Turbine & Steam Turbine with planned output of 105MW. We are planning to adopt Single Shaft methodology ( GT, ST & Generator) on single Shaft. Since this is new design for us (although well proven in the industry), I need your help in providing clarification for the following :
1- What studies, design criteria to be considered before ordering/float tender for such plant.
2- What are the risk/difficulties/complexity related with Single Shaft power pants.
3- Could some one share the operation experience for similar plant.

I’m looking forward for your feedback.
Thanks.
Walja

 

[racookpe1978]

I don't like it, and I don't think anybody in the GT or ST business will try to build one combined unit that way.

CT Control, Generator control (voltage and excitation), ST control for the main. Then add startup, shutdown, cooldown, auxiliaries, and exhaust gas flows that change as each different unit starts up. EPA reg's will be harder to meet also.

As a combined cycle unit, you cannot have steam pressure - even at low values - until the GT is running and has begun to stabilize. So your ST has to windmill at 3600 rpm (60 Hz assumed) over a forced vacuum over your condenser without the natural cooling of the steam flowing past and no condensation. (Vacuum pump is needed to avoid air friction from the non-condensed gasses) as the GT comes up to speed and assumes it part of the load.

You have only one generator, so it is at half-power until the steam HRSG comes up and warms up, but the ST turbine case, blades, and rotor have to be warm and ready to operate at full speed as soon as the GT begins. Then, when all systems are ready and hot and pressurized and drained down of condensate, etc, then you could begin cutting in steam to the ST to take up the other half of the load.

That's why you almost always see a CT + CT Genertor PLUS a ST and its own ST generator. The CT comes up first, runs a while as the steam comes up to pressure; then the ST is started.

 

[rmw]

Did you google "single shaft combined cycle"? Seems that there are plenty of folks offering it.

Things may have changed since I started up my first single shaft CC exactly 40 years ago, but as Mr. Cook mentions, windmilling the ST is a problem at 3000 or 3600 rpm, so what was done then was to slow roll the GT and build up steam pressure in the HRSG at reduced rpm and when there was sufficient steam pressure to provide flow to through the ST, the GT was accelerated to speed at which time the unit was ready to synchronize and load. It took about 45 minutes and most of that time was spent with the cranking motor supporting the GT rotation.

Cranking device for the GT has to provide for the WR^2 of the ST rotor as well.

If you clutch connect the ST, then you can diverter valve connect the HRSG.

If you don't intend to operate at full load, then you have to take into consideration the partial load efficiencies of the GT and the ST with respect to how it affects your heat rate.

The concept has been around a while and seems to be making a come back based on what I read upon googling. Footprint often trumps a lot of other factors.

rmw

 

[abeltio]

The Single Shaft Combined Cycle (SSCC) has been around for decades now.

From a GE Frame 5 with an ABB Gen and a Siemens GT delivering 38MW (the smallest i've seen) up to over 400MW.

The output ratio IS NOT 50/50%, the GT delivers approx 67% and the ST 33%. An easy way to evaluate the CC output is 1.5×GT.

There are many designs, with Generator in the middle, with Gen after the ST.

If there is a concern i would have when specifying a SSCC is to definitely have a dedicated lube oil system (for bearing lubrication and jacking oil - if used - only) and hydraulic oil supply (for servo and hydraulic actuation).

Another concern would be to have a single source for all three components, otherwise it will be a nightmare to make each supplier talk to each other. Also avoids the "not my fault" not if but when there is a problem.

I've been involved with design, operation and maintenance of SSCC since 1994 in Europe and Latin America and they are a well proven, very efficient technology.


saludos.
a.

 

[rmw]

Abeltio,

The one I started up way back then was 26 MW with 19 of that coming from the GT. The turbines were all of the same manufacturer (as was generator.) The heat rate, however, ranked better than the most efficient large central station units of the day.

rmw