15 Appendix F Lubrication system
15.2 LUBRICATION OF THE TURBO UNIT
Figure 15.1 shows the flow of lubrication oil through the turbo charger. Note that the lubrication oil in the feed side of the turbocharger experiences a higher oil pressure than the drain side. There are small orifices in the turbine hosing that allow only a certain mass flow through the turbo charge. The lubrication pressure is a function of the drain pressure, and not depended on the inlet pressure. Figure 15.3 also shows that the return side of the shaft is a sump, under atmospheric pressure only, and that the back pressure to the pumps is done through the small channels itself and not the return lines to the reservoir. For this reason no backpressure may occur in the return lines.
The two seals in Figure 15.2 and Figure 15.3 show all the internal oil channels in the housing of the turbocharger.
Another factor to consider is the fact that a certain amount of lubrication oil is going to leak through the seals on the shaft. This is dynamic seals, and most of the leakage occurs when the lubrication system is running and the shaft stationary. The leakage could be minimized by synchronizing the shaft movement with the lubrication system.
The conceptual design for development of a micro gas turbine generator.
ADDendix F: Lubrication svstem
Figure 15.1: Lubrication of a typical turbo unit
any event. This includes secondary auxiliary failures like a power cut or instrumentation fault.
This indicates that a dedicated back-up system has to be in place.
The lubrication system needs a filtration system to ensure that no foreign objects could enter and damage the shaft or bearing bed. This system has to have a clogging indicator to warn the operator that the filter should be replaced.
The system needs to be controlled to ensure that a pre-set pressure will always be maintained independent of the oil flow rate.
A heater should be included to heat the lubrication oil to the working temperature before the oil enters the turbo charger, thus preventing thermal shock on the bearings and shaft. It will also need a lubrication oil cooler, because once the system is running; the temperature of the oil will rise because of the elevated temperature of the turbo.
A reservoir to supply the pumps and to contain the drained oil will be needed.
A simplified system with only one of the three feed lines is shown in Figure 15.4.
1) Reservoir
2) Variable speed motor and pump 3) Filter
4) Flow indicator 5) Pressure indicator 6) Proximity switch valve 7) Turbo charger
8) Solenoid valve 9) Back-up accumulator
10) Pressure regulator 11) Low level switch 12) Heater
13) Temperature transmitter 14) Sight glass
15) Filler cat and breather 16) Cooler
17) Mechanical valve
18) Constant speed motor and pump 15.2.2 Reservoir
A reservoir in which the oil could be stored from which the feed lines suck. The pumps may never suck air, thus there has to be a surplus oil, not only to keep all the lines filled, but also for the heat transfer of the oil. It takes time for the oil to change its state and with that its temperature. All so in the event of a failure, there is a surplus of oil. When oil is returned into the reservoir from the drain it may include air and these bubbles need time to reach the surface. If oil with bubbles is sucked into the pumps, it will result in capitation. It is a process where the air bubbles are flung to the pump casing, under high speed and pressure, and hit the wall with little explosions. These explosions will damage the pump wall and gears and will result in pump failure.
The reservoir will also house the heater. It will be discussed later in this section, and can be seen in Figure 15.4.
The conceptual design for development of a micro gas turbine generator.
Appendix F: Lubrication svstem
Figure 15.4: Simplified lubrication system The reservoir will comply with the following:
Pumps, coolers, or filters will not be mounted on top of the reservoir.
The top of the reservoir will be sloped at least 1: 100. [If flat top].
All oil return flow streams shall be located as far as possible from the pump suction connections as possible.
All atmospheric return lines will be located above the oil level.
Pump suction connections shall be located near the high end of the sloped reservoir bottom.
The reservoir's wall-to-top junctions may be welded from the outside if a full-penetration weld is used.
All welds will be continuous.
Internal joints will be made smooth by grinding or other suitable means as necessary to eliminate pockets and provide an unbroken finish.
15.2.3 Feed lines
Since each of the turbo units need oil at a pre-set pressure that is independent of the flow rate of the oil through the unit, each of the units will have its own feed line. This feed line includes a pump coupled onto a variable speed motor (2), a pressure regulating valve (10) and a filter (3).
All three feed lines are identical, except for the flow rates though the turbo chargers. The pumps are submerged under the oil low level in the reservoir (1). The secondary filter follows the pump before it leads to the turbo unit (7) and back through the drain into the far side of the reservoir. A pressure indicator (5) is mounted on the feed line and enables the control system to read the pressure in the line, compare it to the pre-set pressure and send a pulse to the variable speed motor to correct if necessary.
By varying the speed of the pump, while the orifices stay constant, one could vary the pressure in front of the orifice. The turbo charger's orifice stays constant and will allow a certain oil flow rate through it under a certain pressure. Thus, the oil flow rate is a function of the pressure and orifice opening. If a turbo charger needs a bigger flow rate, the control will allow the motor to speed up until the pump is able to provide a constant pressure. This pressure will vary until all the air is out of the system.
Note that in front and just following the turbo charger (7), one would find two proximity valves (6). These valves enable you to isolate the turbo in an event like maintenance, but also protect the system. It sends a signal to the control, so the system could not be stated before both these valves are open.
15.2.4 Filtration and cooling
As previously mentioned each feed line will have its own filter (3), but this is used as secondary filtration units. The primary filtration system is in the cooling line (18). This line is not linked to a turbo charger, but merely circulates the oil back into the reservoir (1). It is used both as a filtration system as well as a circulating system to condition the lubrication oil to the optimum temperature. Situated in this line is the cooler (16). When the oil temperature rises, the cooler will be activated and the oil will be cooled. The cooler is passive until the temperature reaches a pre-set value. Then a valve (17) will open, allowing cooling water to enter the cooler, and the cooler becomes active. The cooler also has a clogging warning device. Important is that the oil pressure in the cooler will always be higher than the water pressure. This way, in the event of a failure, the oil will leak into the water and not water into the oil. Water can damage the turbo units if it gets pumped into the feed lines. The secondary system is driven by a constant speed motor and the pressure and flow rate of this line can not be controlled.
15.2.5 Heater
Under operation, the shaft will heat up with the turbine wheel, while the oil from the reservoir will be at ambient temperature. If this cold oil is pumped onto a hot shaft thermal shock will occur and will lead to failure of the unit. The thermal shock can be avoided by heating (12) the oil to near working conditions. A heater will be mounted inside the reservoir (I), and will be controlled by the temperature transmitter (13) measuring the reservoir temperature. While the
The conceptual design for development 157
of a micro gas turbine generator.
Appendix F Lubrication svstem
temperature of the oil is be neath working conditions the heater will be activated and once that temperature is reached the heater will shut down.
A thermostatically controlled, removable electric immersion-heating element will be provided for heating the charge capacity of oil prior to start-up in cold weather. The device will have sufficient capacity to heat the oil in the reservoir from ambient temperature to start-up temperatures. Heater elements in contact with oil will be sheeted in austenitic stainless steel, copper or copper-bearing materials will not contact the oil.
15.2.6 Back-up
The moment in which the gas turbine generator experiences some failure and seizes working the shafts will still be rotating and need lubrication and cooling. There has to be some system that will ensure that there will still be oil pressure and flow over the bearing in such an event.
Two back-up systems are considered. The first back-up system is an uninterrupted power supply (UPS) that stores power while the generator is providing power. The UPS charges a battery while the generator is operational, and will be able to drive the feed pumps. The second back-up system (9) will be using instrumentation air to produce pressure in the feed lines, if the UPS fails, or if it is not able to drive the pumps in the event of a failure. The control of this consists of a normally open, "energized to close solenoid valve (8)" that receives an impulse from the electrical feed line (and UPS). The moment this impulse fails the valve opens and allows the pressurized air into the oil feed lines. The air pressure will force the oil in the accumulator into the feed lines.
p = - F
By using the equation: A , where P the pressure is, F the force on the Area (A). Since the instrumentation air's pressure is maintained at a constant pre-set value, we can deduct that with a smaller area, a larger force could be obtained. For this reason, the back-up accumulators will be of cylinder type, small diameter, but with a big height. This way, you can get a stable force, with a long working time once the solenoid valve is opened. To ensure that the accumulator is full of lubrication oil, and not air, there is a bleed valve on the return line. To bleed the system, the proximity valve before the turbo unit must be closed, while the bleed valve is opened. Start the motor under bleed mode and let it run until clear lubrication oil without air bubbles pass through the bleed valve. Open the proximity valve before closing the bleed valve. The control should be in running mode again.