A combined cycle plant recovers the heat a gas turbine throws away and uses it to raise steam for a second turbine. The efficiency gain is large. So is the parts list, because there are now two machines and a boiler between them.
What the arrangement adds
A simple cycle plant is a gas turbine and a generator. A combined cycle plant adds a heat recovery steam generator, a steam turbine, a condenser and the water and steam circuit tying them together. Every one of those brings its own valves, actuation, filtration and instrumentation.
Where the consumables sit
On the gas turbine, the familiar list. Combustion liners, transition pieces, nozzles and buckets on the hot side; filters, seals, bearings and fasteners across the balance of plant.
On the steam side the emphasis shifts. Control and stop valves governing steam admission. Relief devices. Feedwater and condensate valves. Strainers protecting pumps. Expansion joints absorbing movement a rigid connection would not survive.
Cycling is the modern problem
Combined cycle plants were designed to run steadily. Many now follow demand instead, starting and stopping far more often than the design case assumed.
Thermal cycling is harder on hot section parts than continuous running, and it is harder on valve seats, packing and expansion joints too. A plant that has moved from baseload to cycling consumes spares faster than its original schedule predicts, and the schedule is usually the last thing to be revised.
What that means for a spares strategy
- Hold the long lead hot section items. They decide outage length.
- Hold the cheap items that stop a job: gaskets, fasteners, seals, filters.
- Know the frame and the steam valve makes before the outage, not during it.
- Review consumption against actual starts, not the original operating profile.
Gas turbine parts are searchable by frame in our parts catalogue, and steam side valve work is covered under services.
