Fuel Control. Prior to connecting to the electrical grid the fuel control system is responsible for maintaining the turbine at the correct rotational speed (rpm), by positioning the fuel valve. Once the unit is connected to the grid control of the load on the unit's generator is essentially a function of the amount of fuel supplied to the turbine. To increase the load, the turbine fuel valve admits more fuel, and decreasing the fuel supply reduces the generator output. In this mode the control system controls the fuel supply to the turbine so that it can supply the mechanical energy needed to maintain the desired electrical energy output form the generator.
The fuel control system also has to be capable of reacting to sudden and drastic changes in load on the turbine such as during a full or partial load rejection. A load rejection occurs when the generator output breaker opens abruptly. When the breaker opens the load on the unit is instantaneously reduced and the turbine will overspeed and trip if the fuel supply is not quickly adjusted to maintain the turbine speed at less than the overspeed trip value.
Most gas turbines will use either natural gas or fuel oil, typically #2 or #6. The type of fuel used will depend upon the environmental requirements of the area where the plant is installed. Most units today are supplied with some manner of emissions control. In natural gas fired systems, this is accomplished through staged fuel control that lowers the overall flame temperature that in turn limits the formation of harmful emissions. This same technique is used when firing fuel oil, but must be accompanied by either water or steam injection. Water and steam injection is used to atomize and warm the oil droplets that will lead to a lower flame temperature and hence, lower emission levels.
Figure 9DĆ3 shows a typical staged combustion layout. Each stage will feed a separate portion of the nozzle that will admit a certain amount of fuel to that stage of the combustion process.
Service conditions for a typical fuel gas and fuel oil system can be found below. Valve selection will vary depending on the unit type and size. Many older installations used ball valves for fuel control, but issues with performance due to `sloppy' linkages caused the industry to change to sliding stem valves. The other reason was the increased need for emissions control.
Figure 9DĆ3. Fuel Gas Staged Combustion
Fuel Gas Fuel Oil Inlet pressure (psig) 600 1500 Pressure drop (psid) 105 Ć 550 105 Ć 700 Inlet temperature (F) 100 100
Valve type and size 2" Ć 4" ET/ES 2" Ć 3" ET/ES Actuation Electric Electric
The other valves noted in Figure 9DĆ3 are called the OST valves. These valves protect the turbine from overspeed when the fuel control system fails to react quickly enough to prevent an overspeed condition. Modern gas turbines use multiple electronic sensors to detect the turbine speed and trip the turbine when an overspeed condition exists. The ultimate result of the activation of this device is the closure of the fuel valves and isolation of the fuel system from the combustion chamber of the gas turbine. A line size butterfly valve with tight shutoff is used for this application. Power Augmentation. To increase the mass flow through the turbine, which increases the turbine output and efficiency, steam is injected into the turbine blades of the gas turbine. This is typically used in a combined cycle configuration where the steam is pulled off the cold reheat line, but can also be used in single cycle configurations if there is an available outside steam source.
The allowable load on the turbine blades limits power augmentation. Typically, injection rates can reach as high as two pounds per unit of fired fuel. When selecting a control valve, this typically means a range of 30,000 up to 150,000 pounds per hour.
Figure 9DĆ4. Power Augmentation
A control valve that can handle high temperatures and noise and still provide tight shutoff is required. In some cases, as noted in Figure 9DĆ4, an isolation valve is used in combination with the power augmentation control valve. This eliminates the need for tight shutoff at the control valve. However, the isolation valve must be able to provide tight shutoff in either direction. This is due to the possibility of combustion gases backing up in the steam line during operations where power augmentation is not required. This can lead to premature failure of the control valve components due to the high combustion gas temperatures. The follow recommendations have been made for the power augmentation throttle and isolation valves.
Throttle Valve Isolation Valve Inlet pressure (psig) 550 Ć 650 550 Ć 650 Pressure drop (psid) 50 Ć 200 1000 Ć 150050 Ć 200:
Inlet temp. (F) 550 Ć 680 800 Ć 1075550 Ć 680: Valve type WhisperFlo6" Ć 8" ED Line size butterfly
Actuation Pneumatic Ć spring return spring returnPneumatic Ć
Water and Steam Injection. The primary function of the injection system is to lower the flame temperature when firing fuel oils. By lowering the flame temperature, the formation of NOx
emissions is limited (Figure 9DĆ5). This also results is higher electrical output due to the increased mass flow through the turbine similar to power augmentation.
9D-5 Figure 9DĆ5. NOx Formation vs. Temperature
The steam injection application is very similar to the Power Augmentation application in terms of product selection. A steam control valve can be used to provide both control and tight shutoff or an isolation valve may be supplied in conjunction with the throttle valve. Steam is injected into the fuel nozzles to cool the flame temperature at the source of combustion. The product selection guidelines are below.
Throttle Valve Isolation Valve Inlet pressure (psig) 550 Ć 650 550 Ć 650 Pressure drop (psid) 50 Ć 200 1000 Ć 150050 Ć 200
Inlet temp. (F) 550 Ć 680 800 Ć 1075550 Ć 680 Valve type WhisperFlo6" Ć 8" ED Line size butterfly
Actuation Pneumatic Ć spring return spring returnPneumatic Ć
Water injection uses the same idea as steam injection, but is more common in single cycle configurations. This eliminates the need for an offsite steam source and can be readily used on peaking units. It is very uncommon to find two identical units in this configuration. The amount of water injection depends upon the emission
requirements, the temperature of the water and the ambient temperature. These factors lead to different valve selections for each site location. A valve with low flow and antiĆcavitation capabilities is usually required. The CAVIII or CAVIII/MicroĆFlatt can be used to meet the
requirements of this application. In cases where very low flows and cavitation control are required, the multistage MicroĆFlat can be used. A set of typical service conditions can be found below.
Fuel oil water injection Inlet pressure (psig) 1450 Pressure drop (psid) 250 Ć 1300
Inlet temp. (F) 60
Valve selection III/2ĆStage or CAVIII/MicroFlat1" Ć 2" ET with Cavitrol Actuation Pneumatic
Turbine Lube Oil. The turbine lube oil system is used to provide lubrication to both the gas turbine and gas turbine generator. This system consists of reservoirs, pumps, coolers, heaters, filters, piping and valves. A cooler rejects the heat that is absorbed from the equipment with two coolers typically being provided.
There are two main valves in the turbine lube oil skid. These are the bearing pressure regulator and the cooler temperature control valve. Turbine and generator bearing pressure is controlled via the bearing pressure regulator. Both valves have different functions, but rely on one another to provide lube oil at the correct pressure and temperature to ensure proper sealing between the shafts and bearings.
Pressure
Regulator TemperatureRegulator Inlet pressure (psig) 150 150 Pressure drop (psid) 20 Ć 50 20 Ć 50
Inlet temp. (F) 180 200 Valve selection 4" ED or 4" V150 4" ED
Actuation Pneumatic Pneumatic