15 Appendix F Lubrication system
15.4 LUBRICATION SYSTEM CONTROL
As previously mentioned, a lot of signals from the lubrication system have to be interpreted and controlled to ensure the reliability of this system. Some of the warning signals should shut down the system immediately, while others should be noted to be corrected on a later stage. All of these signals have to integrate into the main control, but it will be discussed in a later section. The lubrication control and the effect it have on the complete system will be discussed here:
15.4.1 Warning signals
The warning signal will be addressed in two stages. Stage one: the system is in pre-start mode.
Stage two: the system is in running mode.
Reservoir low level indicator. This switch will be activated when 25% of the total working oil level is reached. Stage 1 : Add lubrication oil immediately. Stage 2: Allow safe shut down and fill reservoir.
Return oil temperature is too high. Stage 1: Do not start (should be impossible. thus investigate, instrumentation failure). Stage 2: Allow save shut down, and investigate (check cooler).
Proximity valves in the wrong position. Stage 1: Open valves to the turbo unit. Stage 2:
Immediate shut down and open valves right away!
Feed line pressure to high/low. Stage 1: Stop start procedure and investigate (line blocked). Stage 2: Shut down immediately, investigate and repair failure
Flow rate to high/low. Stage 1: Stop start procedure, and investigate (line blocked).
Stage 2: Shut down immediately, investigate and repair failure
Filter clogged. Stage 1: Replace filter before restart. Stage 2: Save shut down and replace filter before restart is attempted.
Back-up solenoid valve's power source fail. Stage 1: This should be repaired immediately. Stage 2: Save shutdown and repair. Important to note is that since the valve was open, compressed air has entered the lubrication system. Air is compressible, thus will the pressure control react to the air and the pressure will be uncontrollable. The pressure will be lower, for the pumps won't be able to pressurize the system, while the control will run the variable speed motors at full speed. It is important to bleed the back- up accumulator system before start up is attempted when ever the solenoid valve was opened.
Cooler clogged. Stage 1: Investigate the cooler and the primary filter. Do not start before the failure is repaired. Check the primary filter and its clogging device. Stage 2:
Save shut down and investigates.
The conceptual design for development 8 f a micro gas turbine generator.
Appendix F: Lubrication system
All of the sensors have an allocated number which the operator can follow to check each instrument's condition at any time. This also give the maintenance crew an indication as to where a failure or fault has occurred (by saving the instruments that tripped the alarm's number).
15.4.2 Control signals
The most important requirement of the lubrication system is for lubrication oil under pressure.
For this a control system was designed with pressure control in mind. In principle, the system varies the speed of the motors driving the pumps, resulting in variable flow rates through constant sized orifices. By setting a wave band in which the pressure may fluctuate, and reacting on input from the sensors to change the speed of the motor to compensate pressure.
For instance, pressure is set on 4 bar. Start up. Sensing that line pressure is zero, the speed of the motors accelerates, pressure is picking up until the line pressure reaches the set pressure. The motors will be running at a constant speed. If the line pressure falls due to a valve opening or a leak, the motor will speed up until the pressure is equal to the pre-set pressure. If the valve closes, the motors will slow down until the line pressure is again equal to the pre-set pressure.
The set pressure can be changed by the operator during operating and the motors will follow the set point. A simulation is shown in Figure 14.5 where the pressure following can be seen.
X- X-X
*
3C X-
b-- -
Pre-set Pressure -System Pressure-
A
--Figure 15.5: Simulation of the pressure following for the lubrication system While the operator can set the operating pressure, there is also a high and low pressure region in
0 1 2 3 4 5 6 7 8 Time [sec]
--
/ ~ s y s t e m Pressure
-++-
Pre-Set Pressure-1
Max pressure - - -
- -
- Min pressureI_zi
Figure 15.6: Simulation of the upper and lower level of lubrication oil pressure
15.4.3 Control modes of operationStart mode
Before start up there are some crucial inputs from the lube system that need to be checked. The level of the reservoir (1 I ) , the condition of the proximity valves (6) and the back-up solenoid valve (8) status. During this mode the operator has to make a visual check of the lubrication system to look for any obvious failures like oil spills, leaks, ext.
If any warning signal is received the system will go into the fault mode and start up will be terminated. This will continue until the operator resets the fault mode to standby mode. If start up is initiated while a fault is still persist, the system will again go to fault mode. Start up will only go into running mode if no more faults are detected.
Running mode
Start mode completed its checks, thus no warnings have been sound, the system will initiate running mode. The control will sense that the oil pressure in the feed lines is lower than the set pressure (line pressure should be equal to zero). The motors will start and the control process will continue like previously explained. During the running mode all sensors will be monitored and if any fault is detected, warning shall be sound and save shut down mode will commence.
This is also the time where the operator should check whether the accumulators are charged (full of oil), by opening the bleed valve fractionally and checking that the oil is bubble free.
Bleed mode
If the operator suspects that the accumulators have air in them, he should initiate the bleed mode. Most of the times this mode will be ignored only activated when the solenoid valve was opened or if the operator expects that air has entered the system.
Once the system is in the bleed mode, it will allow the operator to switch the proximity valve (6) (in front of the turbo unit) to close position, while the bleed valve is open. If the operator closes the other proximity valve (following the turbo charger), the system will go into the fault mode. Note that while you are in the bleed mode, the control still receives signal from the system, and in case of any fault (like low oil level) the system will trip back to fault mode.
The conceptual design for development o f a micro gas turbine generator.
Appendix F: Lubrication system Save shut down mode
With all the systems running, that is all the auxiliaries and main components, shut down are not as simple as cutting the fuel feed. The turbo chargers are running at high speed, and the combustion chamber is at an elevated temperature. The fuel has to be cut gradually, to make sure that the temperature gradient is not to steep while the combustion chamber cools down. The turbine shaft and bearings are also cooling down. But the shafts are still running and need constant lubrication. Even after the shafts have stopped rotating, the shaft bed and bearings have to be cooled by the lubrication oil. Thus, in any event where a fault is detected while the system is running, a shut down mode has to be initiated, to ensure fast, but save shut down of all sub-systems. The save shut down mode of the lubrication system will be discussed here, while all the sub-system's shut down procedures will be discussed in there own allocated sections in this study.
In the event of a fault occurring in the lubrication system while in running mode, save shut down mode will be initiated. This means that the system will shut down, but the lubrication system will be running as no fault has occurred until five minutes after the last shaft has stopped turning. Then the motors will stop and the lubrication system will go into fault mode. Since every instrument has a number, the fault as well as the number of the instrument who tripped the system will be saved to assist with the repair and also the maintenance of the system.
Fault mode
When ever the system comes into the fault mode, the operator has to reset the control before start up can commence once again. If the fault persists the system will go into fault mode again and this will be repeated until no warning or faults occur. (Only then will start up be possible and go into running mode). It is not dependent on which mode the system was before it went into the fault mode. The system will always start with the start up mode after it has been reset by the operator. As previously mentioned, the fault and the part responsible for the alarm and trip will be saved and the operator will be able to see where the problem occurred.
The software is designed in such a way that it is impossible to override the warning s ignals or the trips.