In the event of an unsuccessful start, the accumulation of combustible fuel oil is drained through false start drain valves provided at appropriate low points in the combus-tion/turbine area. The false start drain valve, normally open, closes as the turbine accelerates during start-up. Air pressure from the discharge of the unit’s axial- flow compressor shutdown sequence, the valve opens as compressor speed drops (compressor discharge pressure is reduced).
1.5.4 ATOMIZING AIR SYSTEM:
Atomizing Air Schematic:
The atomizing air system provides sufficient pressure in the air atomizing chamber of the fuel nozzle body to maintain the proper ratio of atomizing air pressure to compressor discharge pressure at approximately 1.2 or greater over the full
GAS TURBINE (PG-9171E)
operating range of the turbine. Since the output of the main atomizing air compressor, driven by the accessory gear, is low at turbine firing speed, a starting atomizing air compressor provides a similar pressure ratio during the firing and warm-up period of the starting cycle, and during a portion of the accelerating cycle. Continuous blowdown to atmosphere is also provided to clear the main gas turbine compressor of accumulated dirt.
Major system components includes :
the main atomizing air compressor, starting atomizing air compressor motor driven(88AB), atomizing air heat exchanger(s) and an air filter.
When liquid fuel oil is sprayed into the turbine combustion chambers it forms large droplets as it leaves the fuel nozzles. The droplets will not burn completely in the chambers and many could go out of the exhaust stack in this state. A low pressure atomizing air system is used to provide atomizing air through supplementary orifices in the fuel nozzle which directs the air to impinge upon the fuel jet discharging from each nozzle. This stream of atomizing air breaks the fuel jet up into a fine mist, permitting ignition and combustion with significantly increased efficiency and a decrease of combustion particles discharging through the exhaust into the atmosphere. It is necessary, therefore, that the air atomizing system be operative from the time of ignition firing through acceleration, and through operation of the turbine.
Air taken from the atomizing air extraction manifold of the compressor discharge casing passes through the air-to-water heat exchanger (pre-cooler) HXI to reduce the temperature of the air sufficiently to maintain a uniform air inlet temperature to the atomizing air compressor.
The atomizing air pre-cooler (heat exchanger), located in the turbine base under the turbine compartment, uses water from the turbine cooling water system as the cooling
medium to dissipate the heat.
Thermocouple AAT-IA,-2A is sensitive to the temperature, thermoswitch provided to sound an alarm when the temperature of the air from the atomizing air precooler entering the main atomizing air compressor is excessive. When the atomizing air reaches this temperature setting of this switch, the alarm is activated. Improper control of the temperature may be due to failure of the sensor, the pre-cooler or insufficient cooling water flow. Continued operation above 275°F (i.e. 135" C) should not be permitted for any significant length of time since it may result in failure of the main atomizing air compressor or in insufficient atomizing air to provide proper combustion.
Compressor discharge air, now cleaned and cooled reaches the main atomizing air compressor. This is a single stage, flange mounted, centrifugal type compressor driven by an inboard shaft of the turbine accessory gear. It contains a single impeller mounted on the
pinion shaft of the integral input speed increasing gear box driven directly by the accessory gear. Output of the main compressor provides sufficient air for atomizing and combustion when the turbine is at approximately 60% speed.
GAS TURBINE (PG-9171E)
alarm if the pressure rise across the compressor should drop to a level inadequate for proper atomization of the fuel. A quick connection with check valve allows reading of the pressure with a differential pressure gauge.
Air, now identified as atomizing air, leaves the compressor and is piped to the atomizing air manifold with "pigtail" piping providing equal pressure distribution of atomizing air to the 14 individual fuel nozzles.
When the turbine is first fired, the accessory gear is not rotating at full speed and the main atomizing air compressor is not outputting sufficient air for proper fuel atomization. During this period, the starting (booster) atomizing air compressor, driven by the starting motor 88 AB-I is in operation supplying the necessary atomizing air.
The starting atomizing air compressor at this time has a high pressure ratio and is discharging through the main atomizing air compressor which has a low pressure ratio. The main atomizing air compressor pressure ratio increases within increasing turbine speed and at approximately 60 % speed the flow demand of the main atomizing air compressor approximates the maximum flow capability of the starting atomizing air compressor. The check valve in the air input line to the main compressor begins to open allowing air to be supplied to the main compressor simultaneously from both the main air line and the starting atomizing air compressor. The pressure ratio of the starting atomizing air compressor decreases to one and it is shut down at approximately 95 % speed (14 HS pickup). Now all of the air being supplied for atomizing purpose is directed to the main atomizing air main compressor, bypassing the starting air compressor completely. At this time, the 20 AB-1 solenoid is energized and the isolation valve VA 22-1 is closed preventing any air from getting to the booster compressor.
This valve VA 22, actuated by regulated air with the pressure regulating valve VPR 68, will be actuated only if the solenoid valve 20 AB-1 is energized.