This blog post will discuss where pressure or temperature switches can be used in gas or steam turbines for combined cycle power plants. This system diagram shows a combined cycle power plant:
The gas turbine (shown upper left) generates electricity using natural gas fuel. The steam turbine (shown center-middle) generates electricity using waste heat from the gas turbine. This process is up to 60% efficient since exhaust heat is re-used when it would otherwise be lost through the smoke stack.
To understand where pressure switches or temperature switches are generally used, we will go through the process at each step. At the beginning of the process is a gas turbine which compresses ambient air and mixes it with natural gas fuel in combustors. The fuel is burned and the resultant hot air-fuel mixture is expanded through turbine blades, making them spin about a shaft. The spinning turbine drives the initial generator that converts the spinning energy into electricity. Pressure switches here listed as P1 and P2 monitor the pressure of the compressed air and the pressure of the natural gas pipeline inlet prior to them being burned by the turbine. A correct mixture of air and gas is required for peak efficiency so the pressures on entry must be quite precise. After the fuel mixture is burned, a temperature switch, T1 is used to monitor the temperature of the exhaust gas heat. This gas is typically about 1000 degrees Fahrenheit. To ensure the gas mixture is burning properly and efficiently, this temperature should be consistent.
The next part of the cycle involves using the temperature of that burned exhaust to heat water added to the system to create high-pressured, high-temperature steam. This steam is used to drive a steam turbine connected to a second generator. This generator provides additional electricity that otherwise would have been lost out of the smoke stack. This secondary process uses so much of that heat that the original 1000 degree exhaust gas now leaves the smokestack at just 200 degrees. In this secondary process a pressure switch, P3 or temperature switch may be used to ensure the steam entering the steam turbine is at a certain pressure or temperature. Another switch P4 is used for the process steam which can also be routed to an optional co-generational thermal host. To measure the efficiency of the steam turbine, a vacuum switch, noted by P5, can measure the pressure of the turbine exhaust.
Additional pressure switches can be used to measure the water in-flows used to help cool the exhaust gas. A pressure switch, P6, can be used for the cooling water input that cools the turbine exhaust air for a condensate. Other pressure switches, P7-P10, can be used to measure the condensate inflows and additional water inputs used by the steam boiler to generate the steam input to the steam turbine.
Typically combined cycle power plants are considered hazardous areas, so 646GE or 6900GE series switches are commonly used in these plants.