Atomizing Air (Motor) - Ms9001

System Description

ATOMIZING AIR (MOTOR) – MS9001

Atomizing Air GENERAL The atomizing air system provides sufficient pressure in the air-atomizing chamber of the fuel nozzle body to maintain the ratio of atomizing air pressure to compressor discharge pressure at approximately 1.4 or greater over the full 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 water/dirt. Major system components include: the main atomizing air compressor, starting atomizing air compressor and atomizing air heat exchanger.

FUNCTIONAL DESCRIPTION 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) HX1 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.

Bharat Heavy Electricals Limited, Hyderabad

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System Description


Thermocouple AATI-1A senses the high atomizing air temperature and sounds an alarm when the temperature of the air from the atomizing air pre-cooler entering the main atomizing air compressor is excessive. When the atomizing air reaches this high atomizing air temperature setting, as per the control system 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 cooled reaches the main atomizing air compressor. This is a single stage, flange mounted centrifugal compressor driven by an inboard shaft of the turbine accessory gear. It contains a single impeller mounted on the pinion shaft of integral input speed increasing gearbox driven directly by the accessory gear. Output of the main compressor provides sufficient air for atomizing and combustion when turbine is at approximately 60 percent speed. Differential pressure switch 63AD-1, located in a bypass around the compressor, monitors the air pressure and annunciates an alarm if the pressure rise across the compressor drops to a level inadequate for proper atomization of the fuel. 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 delivering sufficient air for proper fuel atomization. During this period, the starting (booster) atomizing air compressor, driven by its own motor is in operation supplying the necessary atomizing air. The starting atomizing air compressor at this time has a high-pressure ratio and is discharged through the main atomizing air compressor, which has a low-pressure ratio. The main atomizing air compressor pressure ratio increases with 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


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System Description


input line to main atomizing air compressor begins to open allowing air to be supplied to the main compressor simultaneously from both the main air line and starting air compressor. The pressure ratio of the starting atomizing air compressor decreases to one and when turbine reaches its operating speed (> 95%) the starting air compressor drive motor is stopped. Now all of the air being supplied to the main compressor is directly from the pre-cooler through the check valve, bypassing the starting air compressor completely. At this time, the 20AB-1 (starting atomizing air solenoid valve) is energized and the isolation valve VA22-1 is closed preventing any air from getting to the booster compressor. During the fired shut down of the unit the starting air compressor drive motor is again commanded to start when the turbine speed reduces below its operating speed (< 95%). Also 20AB-1 (starting atomizing air solenoid valve) is de-energized and the isolation valve VA22-1 is opened. The starting air compressor drive motor continues to operate till the unit reaches zero speed. In the event of a turbine trip while operating on fuel oil also the starting air compressor drive motor is commanded to start. 20AB-1 (starting atomizing air solenoid valve) is de-energized and the isolation valve VA22-1 is opened. The starting air compressor drive motor continues to operate till the unit reaches zero speed.

DUAL FUEL OPERATION (GAS AND DISTILLATE OIL) Restriction of Atomizing Air when on Gas Fuel Operation

On dual fuel (gas and distillate oil) machines, the discharge of the accessory gear driven atomizing air compressor is restricted by the piping design of the bypass line of VA18-1 atomizing air system when the gas turbine is operating on 100 percent gas fuel; except for a small amount of air flow that is bled off to purge the oil passages in the oil fuel nozzle. Air operated bypass valve (VA18) is controlled by the operation of solenoid valve 20AA-1. When the fuel system is set for 100 percent gas fuel operation, solenoid valve 20AA-1 is energized and operating air from the turbine compressor discharge is passed through filter FA4-1 and then admitted to the piston of bypass valve VA18. This causes the valve to close allowing


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System Description


the atomizing air to be routed to the suction of main atomizing air through the bypass line piping of VA18-1.

Purge Air System

The small flow of air through the atomizing air passages of the oil fuel nozzle also prevents entry of any combustion products that could foul this section of the oil fuel nozzle. When solenoid valve 20 PL is energized, operating air (from the turbine compressor discharge) is admitted to the diaphragm of purge air valve VA19-1 which opens, allowing purge air (atomized) to flow into purge air manifold on the turbine. A porous filter FA3-1 is installed in the purge air piping ahead of the purge valve. The purge air check valves, one for each nozzle, are connected into the oil feed lines to the nozzles. These purge air check valves prevent oil fuel from entering the purge air system when the machine is operating on oil fuel. Similarly, the oil fuel check valves installed in the oil piping to the oil fuel nozzles ensure that the purge air is directed to the nozzles and not into the oil fuel system. If any oil leakage should occur through the purge air check valves when operating on oil fuel, it will drain out of the purge air manifold through the normally open port of the purge air valve. “Tell-Tale” leak off piping connected to the purge valve vent port provides a visual means for determining the general condition of the check valves. There should not be any leakages.

Water Wash Provision

When water washing the gas turbine compressor section and turbine’s sections, it is important to keep water out of the atomizing air system. To keep water out of the atomizing air system, the system inlet and discharge are equipped with vent valves and isolation valve or full area spacers. The vent valves are used to avoid completely throttling or deadening the compressors and drain valves are used to remove any leakage past the isolation valves. During normal operation of the gas turbine, the vent and drain valves must be closed and the isolation valves and spacers must be open. Before initiating water wash, the isolation valve must be closed or blank area spacers installed to keep water out of the atomizing air system. The vent valves must be opened to allow air to pass through the atomizing air compressors.


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System Description


CAUTION

Running the atomizing air compressors completely throttled or deadened may result in overheating and damage to the compressor.

At the conclusion of water wash, drain water from the atomizing air piping by opening low point drain valves. When all the water has drained from piping, the drain valves and vent valves must be closed and the isolation valves opened or the blanks replaced with full area spacers.

CAUTION

Running the atomizing air compressors with water in the piping will result in damage to the atomizing air compressors.

NOTE : Project specific scheme shall be referred for applicable devices.


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Atomizing Air (Motor) - Ms9001

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