The main objective of the gas turbine controller is to maintain the speed of the gas turbine. The speed of the gas turbine is maintain by varying the fuel flow. The speed variation depends upon the load factor. The fuel regulator takes the electrical and pneumatic control system, to control the fuel flow rate to the gas chamber of gas turbine.
The gas turbine controller as several features to control the machine. The gas turbine controller also ensures to reduce the damage and maintenance.
The control system Limits the Fuel-
- To Maintain Normal operating of gas turbine
- To prevent Exceeding limit
The required fuel is ensured to obtain the good air fuel ratio. Too much fuel supply result in over temperature. The Less fuel will results the loss of combustion.
Now we know about startup control loop
What is startup control loop?
Startup control loop controls the rate of fuel addition by
- Establish flame in the combustors
- Control acceleration rate
- Control rate of temperature change
During the startup the air flame temperature is low. If there is enough fuel introduced in combustion chamber without enough air, then high temperature may damage internal hardware of turbine. Therefore the rate of temperature change must in the limit.
Now we see how the internal hardware assembly protected?
- The shaft speed is controls the fuel regulation supply. It protects the shaft from high speed, temperature and over pressure.
- The thermal stress on the engine is reduce due to set point depression function.
- The vary of acceleration reduce the under flame and over flame.
- protection of surge tank of compressor.
- Alarm system to shutdown all control element in case of deviation.
- Over speed protection of gas turbine by Open loop and closed loop.
- Limiting over temperature and pressure protection.
Now we see one by one how these controls protects the gas turbine assembly?
1) shaft speed
The fuel regulation supply from surge tank controls the shaft speed. The shaft speed is the critical factor for protecting the hardware assembly of the turbine. The excess speed of the shaft may introduce the fatigue stresses in shaft. If there is manufacturing defects in the shaft, then the axial fatigue stress pick up quickly and it will damage the shaft assembly.
2) Thermal stress control on the engine
The analysis of thermal stresses is important while designing the gas turbine. It is important to analyse the thermal stress stress in first stage rotor blade of two stage gas turbine. Since the allowable temperature on turbine blade depends on the hot gas temperature from combustion chamber. The temperature has the significant effect on the stress on the rotor blade.
3) Reduce the over flame and under flame
The over flame in the combustion chamber due to over supply of the fuel. The over flame cause the excessive rise in the temperature in the combustion chamber which is may form the thermal stress. The excessive temperature rise may damage the gas turbine components. So the it is important to avoid the over flame inside the combustion chamber.
The under flame in the combustion chamber due to under supply of the fuel. The under supply of fuel may creates excessive smoke in the chamber. The excessive smoke affects the quality of air fuel ratio. The under supply of fuel may affect the efficiency of the gas turbine. The fuel supply issue is controlled by varying acceleration.
4) Protection of Surge tank
The one way surge tank has non return connection to the system. It allows the flow out and refill the supply of water when required. The surge tank is the H2S service pressurized vessel for the shortage of hydrocarbons after separation. Most tank has working pressure of 50 PSI. Surge tank are used to measure the liquid flow rate and the combined shrinkage and meter factor. They are also used as second storage separator which hold constant back-pressure by using an automatic pressure control valve on gas outlet. Surge tank are also used for storage.
The bladder surge tank serve as important component of combined surge protection system which protects both positive and negative pressure surges. The tanks internal and flexible bladder separates the fluid from air that is pre-compressed the metal tank and housing bladder.
5) Alarm System to shut down all control element
The electronically controlled fire and combustible gas detection alarm system installed in gas turbine. The fire and gas system provides the system supervision. It initiate the alarm system. It release the fire suppression agent. The suppression system activated by push button.
6) Over speed protection
Over speeding in turbine can be catastrophic. It result in failure of turbine shaft and blade. The high speed may off balance of and fling its blades and any other material parts. The over speed protection system provides the safe, simple and realistic testing.
7) Over temperature and pressure protection
The over temperature and pressure may cause damage to the rotary part of the gas turbine. The relief valve provides the function to discharge water. It is essential that discharge line be piped from the valve in order to carry the overflow to a safe place disposal. The discharge may indicate an unsafe temperature or pressure condition.
The efficiency of gas turbine can be achieved by running the turbine to the closure limit of safety horse power of engine. The gas turbine can fully function by automation in sequence. The fuel density control the fuel out put control valve. The out put signal require to obtain variation in fuel density.