SERVO VALVE CALIBRATION / Null Bias
Servo valve calibration
(Null bias) Formula
During the initial setting, the turbine will drop to launch speed. The ignition sequence will begin and fuel will be injected into the turbine at a fixed firing rate.
Once ignition is achieved, the fuel flow will be withdrawn to the warm-up level for one minute to allow the hot gas passage to warm up.
Waheed u'llah Marwat
I&C Engineer
Any negative bias error in the bypass valve causes a certain amount of delay in the actual flow of the ref and may result in insufficient fuel flow to stop the light.
There is a procedure in the AOIO control specification drawing that describes the correct procedure for adjusting and checking the current setting on the bypass valve.
FAISAL AHMED
DURING HELTH CHECK OF SERVO COIL
Basically, this involves checking that the bypass actuator will close (flow to the turbine) if
The FQR flow bookmark overrides FQLI and will open if the FQLI notes are less than FOR.
Typically, an error of +0.2% causes the valve to close slowly and an error of -0.2% should cause the valve to open slowly. To achieve this the required null-bias current setting should be 2.67% or a total of 0.8 111A on a new clean servo (total current across all 3 windings).
Some newer units use the Mark VI's null bias correction algorithm, which may reduce the effects of this problem somewhat. This does not obviate the need to periodically check for zero bias and take appropriate action.
3. FSR Table (Fuel Stroke Reference)
Sometimes it is necessary to modify some of the fuel stroke signals to compensate for various factors. These changes should only be made in consultation with GE Product Service. Bypass valve and spark plug zero bias should always be checked first as possible causes of ignition failure. Excessive fuel flow during ignition can cause heavy lighting and cause problems with the spark plugs.
:::
Operations and Maintenance
Air system
Irregular fuel flow (as read as Flow Divider Speed FQLI or FQLM) can indicate air in the system. This signature must be searched on every startup/change. Every effort should be made to ensure that air is purged from the liquid fuel system. See GEK-111540 for the currently supported method. Air can accumulate from two main sources.
Backflow through check valves/three-way valves to turbine chamber during gas fuel operation.
Air is drawn through the high-point leak holes during times when the system is idle/disturbed, or during system maintenance, such as filter changes.
The use of redirection systems to compress and drain liquid fuel downstream of the stop valve is currently prohibited. The main reason for this is that there is currently no safe way to monitor the small amounts of flow that can leak through the check valves and three-way valves in the combustion system. Flow separators have a minimum flow limit and can accumulate potentially dangerous amounts of fuel over time. TIL-1250 and drawing 368A6343, identified by several P&lD liquid fuel system drawings, have been withdrawn due to safety concerns.
Valve lineup
A common cause of trip is manual valves not running in the system before starting. It is recommended to manually compile all system P&lD drawings and compile complete valve configuration procedures for check-out after interruptions or long intervals. A short water washing checklist and routine pre-start checks are also recommended. Because of the variety of units and configurations that exist, each procedure must be site-specific or even unit-specific.
:::
3. Preventive maintenance
Follow GE and vendor recommendations for maintenance intervals on critical components such as spray compressors, fuel pumps, flow separators, check valves, three-way valves, purge valves, limit switches and other components.
R011 Check valves and systems are recommended to maintain high performance. These shutdowns and shutdowns should be planned for based on operational experience and history.
Exhaust air and other turbine chamber leakage must also be kept to a minimum. They are known to have a significant effect on fuel line and valve/three-way valve temperatures and therefore coke.
4. Servo mechanism
3.Relating to A.1. Cleanliness and correct operation of the hydraulic system is very important. Monitor all hydraulic valves for position/flow malfunctions. On Dry Low NOX (DLN) systems, a fresh clean system will typically operate with hydraulic valves after Ref with an error rate of less than 0.5%. Maintaining a clean system is very important. Hydraulic system filters should be changed as recommended in the manual. Varnishing is a separate issue and can also cause problems with hydraulic valves.
5. Weekly transfers.
Routine weekly 30-minute fuel oil changes are recommended to keep the system lubricated/air-free and to help identify minor system problems before they become serious or transfer problems. Intervene as needed. This practice has been seen to greatly improve the reliability of the dual-fuel fleet (as measured by the number of flights per hour of launch).
This does not apply to liquid fuel recirculation or automatic nitrogen purge units.
C. On the hardware drive
1. Fuel nozzles
The condition of the fuel nozzle indicator is critical to reliable starting. Adequate fuel mist is essential for reliable ignition. The cartridge must be inspected by GE personnel when the power is turned off.
2. End cover distributor valves
Distributes fuel evenly between nozzles during operation. During start-up, the fuel pressure is relatively low and fuel is delivered through the nozzles to only a few holes to improve ignition and crossfire. Clogging the nozzles with coke/carbon will not catch fire. Broken fuses should be sent to a GE service store for repair.
3. Check the valves.
Check valve problems can be caused by high differentials, trips, high differential pressures observed in the flow-splitting limiter valve, and poor lighting. Reverse leakage can force purged air into the liquid system.
A basic bubble test can also be performed by applying air or gas pressure to the lower vaporizing portion of the valve and immersing the upper portion of the valve in a bucket. Specific test pressures and results will vary by valve unit and type.
The crack pressure will vary somewhat depending on the type of combustion unit/system. Adequate testing of crack pressure is difficult to do on site because crack pressure is usually defined as the pressure required to drive a fixed amount of flow through the valve. A special test stand may be required to do this correctly. Return the valves to the vendor for crack pressure testing.
Make sure the check valves are sloped toward the can. Refer to liquid fuel piping drawing for details. This is necessary to prevent residual liquid fuel from accumulating on the valve seal.
Comments
Post a Comment