Last updated on 06/03/2020
Combined cycle power plants require the use of pressure and temperature switches to ensure maximum efficiency and safe operation. From determining what mixture of air and fuel is combusted to how much pressure is allowed to build up within the steam turbine, pressure and temperature switches can be placed at every stage within the system. Read on to learn more about the specific functions of these devices throughout combined cycle power plants.
Combined cycle power plant schematic diagram
The Fundamentals of How Combined Cycle Power Plants Work
In combined cycle power plants, the gas turbine (shown upper left) generates electricity using natural gas fuel. Then, the steam turbine (shown center-middle) generates electricity using waste heat from the gas turbine. This process is up to 60% more efficient than other types of power plants since exhaust heat is re-used to heat the steam turbine when it would otherwise be lost through the smokestack.
How Pressure and Temperature Switches Keep Combined Cycle Power Plants Operating Safely
To understand where pressure switches or temperature switches are generally used within combined cycle power plants, we will go through the above diagram one step at a time. At the beginning of the process is a gas turbine that 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 requiring the pressures on entry to be precise. After the fuel mixture is burned, a temperature switch (T1) is used to monitor the temperature of the exhaust gas which 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-pressure, high-temperature steam. This steam is used to drive a steam turbine connected to a second generator. Unlike in other types of power plants, this generator provides additional electricity that otherwise would have been lost out of the smokestack. 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 the required 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 (P5) measures 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.
As combined cycle power plants are typically classified as hazardous areas, CCS 646GE or 6900GE series switches are commonly used in these plants. Learn more about CCS pressure and temperature switches designed for use in power plants here.