The fundamentals of steam power plants

Steam turbine

Steam turbine life is generally extremely long. There are steam turbines that have been in service for over 50 years. Overhaul intervals are measured in years. When properly operated and maintained (including proper control of boiler water chemistry), steam turbines are extremely reliable.

They require controlled thermal transients as the massive casing heats up slowly and differential expansion of the parts must be minimized.

Regarding environment: Emissions associated with a steam turbine are dependent on the source of the steam. Steam turbines can be used with a boiler firing any one or a combination of a large variety of fuel sources, or they can be used with a gas turbine in a combined cycle configuration.

Steam power plants cycles

Rankine cycles describe the operation of steam heat engines commonly found in power generation plants as schematically shown here in Figure 1.

In such vapor power plants, power is generated by alternately vaporizing and condensing a working fluid (in many cases water, although refrigerants such as ammonia may also be used).

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in Figure 1.

Process 1-2s – First, the working fluid is pumped (ideally isentropically) from low to high pressure by a pump. Pumping requires a power input (for example mechanical or electrical).

Process 2s-3 – The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a saturated vapor. Common heat sources for power plant systems are coal, natural gas, or nuclear power.

Process 3-4s – The saturated vapor expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapor.

Process 4s-1 – The vapor then enters a condenser where it is cooled to become a saturated liquid. This liquid then re-enters the pump and the cycle repeats.

In actual situations, both the water pumps and the steam Turbines do not operate isentropically and losses result in more power demand for pumping and less power actually generated by steam to blades.