ARTICLE REF – ART226
Intro / Overview
The lubrication system is responsible for the cooling and lubrication of the Accessory Gearbox, Gas Generator bearings, Power Turbine bearings and the driven equipment bearings.
As all the principal bearings are tilt pad type bearings, there will be a large volume of oil required. With ball and roller bearings there isn’t as much oil required and therefore a small oil tank.
Typically on a Mars unit there will be a lube pump driven by the AGB, an AC motor diven pump for pre and post lube, and a DC motor driven pump for emergencies. There are units without mechanically driven pumps, that use two AC driven pumps, normally referred to as Auxiliary Pumps.
Oil will be regulated to the required pressure and kept at the required temperature by the oil cooler. Then passing through a filter before being delivered to the different areas of the equipment to lubricate and cool. Oil then falls by gravity directly to the oil tank.
The oil tank will contain approximately 5,000 liters of oil when full.
Normally viscosity VG-32 oil is use, but in locations where the ambient temperature are very high some operators opt for VG-46 oil.
This selection is not arbitrary and Solar Engineering need to evaluate this change if desired. This will be discussed a little further on.
Oil level
The oil level is displayed locally on a level gauge and also by a level transmitter which sends a 4 to 20 milliamp signal to the PLC.
Four milliamps is equal to 8.8 inch level of oil and 20 milliamp is equal to a level of 32.6 inches.
There is a low alarm at 19.32 inches and shutdown at 17.3 inches.
There is also a high alarm at 23.74 inches.
Oil heater
A three phase heater is used to heat the oil while on standby if necessary. While on standby (below lite off T5 temperature) the oil heater will be commanded on if the oil temperature drops to 75 degrees F, and cuts out when the temperature is 80 degrees F. It uses the Oil Tank RTD to control the temperature.
The heater will be inhibited if the lube oil pressure is less than 8 psi, the tank level is low or fire is detected.
This heater has a thermostat which will kill the power to the heater should the oil temperature reach 125F. This is an additional safety built into the control of this heater. If the PLC does not trip the heater, the thermostat operating independently of the PLC will.
Oil tank pressure:
The oil tank can become pressurized if one or more of the oil bearing seals are damaged or worn. Each seal on the Gas Generator and Power Turbine is pressurized with PCD or Eleventh Stage Air to prevent oil leaking into the airflow.
Bearing one at the front of the gas generator and bearing four of the PT is pressurized with 11th stage air. A small quantity of this air will flow through the labyrenth seals and return to the oil tank, as will the other seals.
On a compressor set sealing gas returns with the oil, which not only has pressure, it alos is a fire or explosion hazzard.
To prevent the tank from pressurizing and removing any explosive gases, the tank is vented to a safe place outside the enclosure.
The tank has a vent pipe connected to the top of the oil tank. Mounted alongside the vent pipe is a pressure differential transmitter that monitors the tank space pressure.
Alarm is set at 8.5 inches of water.
Shutdown is set at 10 inches of water.
Solar Oil Viscosity:
The most common oil viscosity used on Solar turbines is VG-32 with VG-46 being used in very hot climates. The viscosity of the oil is important, because it will determine the oil film thickness between bearing and shaft. The oil film thickness is important for heat transfer from the metal to the oil as well as lubricating qualities to reduce wear on the bearing and preventing elevated oil temperatures which will lead to oil oxidation which degrades the oil. The viscosity is measured in centistokes (cSt) which is a measure of a fluid’s flow.
If you think that a change of viscosity might help your operation, you would need to contact Solar Engineering, as there is much to consider. The basic thinking behind moving to VG-46 oil would be because it obtains its optimum oil film thickness at a higher temperature. The thicker the oil, the higher the VG number.
A Solar oil system is designed to run at a specific temperature to take advantage of the optimum oil film thickness. This is achieved by sizing the oil cooler, fan speed, temperature control valve to get the oil temperature where it needs to be.
Some items Solar Engineering will consider:
1) The thermal control element for the lube oil cooler needs to be changed.
2) The control set points, alarms etc. in the software will change.
3) Going from C32 to C46 is okay as far as the oil cooler is concerned. The other way, the oil cooler may be too small.
Lube system instrumentation:
Let’s take a look at the instruments of the lube system that interface with the PLC. First let’s look at temperature monitoring using Resistance Temperature Detectors “Called RTDs”
The temperature of the oil is very important because it needs to be kept at the design temperature for maximum cooling and lubricating, and to prevent oil degradation due to oxidation. The temperature of the oil at the different locations gives us an indication of pending or current problems.
The three bearing drain RTD monitor the oil leaving the different bearing areas. These bearings have labyrenth seals and the seals are pressurized with PCD or eleventh stage air. This air is very hot and when the seals are damaged or worn a high flow of this air will heat the oil. That is why you monitor these drains.
These are, “bearing one” which drains via the accessory drive gearbox. Oil from bearing two and three drain together and oil from the power turbine bearings four and five drain together.
There is an alarm on each RTD set at 250 Degrees Fahrenheit.
Also each sensor failure is annunciated if the temperature is less than 160 or greater than 400 Fahrenheit. The signal is considered out of range at these values.
There is also a differential alarm and differential shutdown on each RTD.
The Header RTD is used as a reference, and what the control is looking for is differential from this value. The temperature of the “Header RTD” is subtracted from the value of each drain RTD.
For bearing one and bearing four dash five (PT drain), there is an alarm if the differential temperature is greater than 45 degrees Fahrenheit, and there is a shutdown if the differential is greater than 50 degrees Fahrenheit.
For bearing drain two dash three there is an alarm if the differential temperature is greater than 110 degrees Fahrenheit, and there is a shutdown if the differential is greater than 125 degrees Fahrenheit.
LUBE OIL TANK RTD:
The oil tank RTD is used as one of the start permissives. Often the tank heater is locked out for maintenance and when it comes time to start back up, all the locks are removed only to realize that they have to wait for the oil to heat up before a start permissive is available.
The temperature for the start permissive depends on whether the oil is viscosity grade 32 or 46. Grade 32 is set to 68 degrees Fahrenheit and Grade 46 is set to 85 degrees Fahrenheit.
The tank oil temperature has an alarm set to 160 degrees Fahrenheit and shutdown at 165 degrees Fahrenheit for Grade 32 oil. Grade 46 is sent to 175 and 180.
The oil tank RTD also controls the tank heater “on off”.
With Grade 32 oil the heater comes on at 70 degrees Fahrenheit and off at 75 degreed Fahrenheit. Grade 46 is 90 and 95 degreed Fahrenheit.
LUBE OIL HEADER RTD:
The oil temp needs to reach 125 degrees Fahrenheit for C32 oil, 30 minutes after starter dropout speed.
Failure of lube header RTD means a Cool Shutdown.
The Lub oil cooler is controlled “on/off” by the lube oil header RTD.
With Grade 32 oil the cooler is turned on at 110 degrees Fahrenheit and off at 100 degrees Fahrenheit.
The header alarm is set to 175 degrees Fahrenheit and the shutdown is 180 degrees Fahrenheit.
Cooling Fan RTDs
Temperatures are only for display and are not found in the logic. They are used by the operator to evaluate the efficiency of the oil cooler.
NEXT LETS LOOK AT THE PRESSURE INSTRUMETS:
Pressure Switches:
There are different configurations of the Mars lubrication system, so therefore there will be different control logic and instrumentation. In this example there are three pressure switches.
The pressure switch on the main lube oil header is subject to bearing oil pressure. The switch activates at 8 psi decreasing, but it is not an input to the PLC. It’s wiring is connected to the turbine backup system. The backup system is a relay logic system which is used in case of PLC failure, to safely control the shutdown of the turbine and the post lubrication cycle of the lubrication system. If the PLC fails, the turbine will shut down. The Main Pump driven by the Accessory Drive Gearbox will loose pressure. The Pre-Post pump would normally be commanded on by the PLC, but if it has failed, it wont be commanded on. The pressure switch will change state and completes the backup pump loop command to start. At this stage the lubrication system is being controlled by relays and mechanical timers.
The pressure switch just after the DC pump is used to verify that the backup pump is capable of making 12 psi pressure. It is isolated by a check valve downstream of the pump, therefore the pump can be checked while the turbine is in service. A daily check of the backup pump is carried out at midday if the turbine is running. The backup pump is checked as part of the pre start checks. For this check the transmitter mounted on the lube oil header is used for pressure verification.
The pressure switch just after the Pre-Post lube pump can be checked while in service similar to the DC backup pump.
Pressure Transmitter Main Lube Oil Header:
The solid red line is the lube oil shutdown schedule pressure, in psi versus NGP speed. You can see that there is an 8 psi minimum pressure for startup permissive.
There is a maximum delay of 5 seconds before shutting down.
The alarm limit in red dotted line is 2 psi higher.
This is checked during the pre start checks to verify the pre post pump can supply minimum 8 psi.
Note that the mechanically driven pump will not be capable of creating enough pressure until the speed increases and until that timn the electrical pre post pump is in service.
Some turbines have two electrical pumps and no mechanically driven pump, so the logic will change some.
Thermostatic Control Valve.
The Thermostatic Control Valve controls the ampount of oil that flows through the oil cooler. It is important to remember that unless the cooling fan is rotating there isn’t any cooling. As the oil heats up, the oil starts to be diverted to through the cooler.
The control of the valve is done automatically using an wax filled capsule. It expands when heated opening the cooler port and closing the bypass port progressively until it is in full cooler position. All the oil has to flow through the cooler at this temperature.
For example a Solar Mars Thermostatic Control Valve, part number 120337-25 has an internal capsule which is replacable. This has a capsule that starts to open at 110 deg F and is fully open at 130 deg F.
The sketch on the left shows the TCV in bypass. None of the oil is flowing through the cooler. The oil flow is represented by the red lines. The oil flows out the bypass connection of the vlave.
As the capsule in orange starts to expand oil will flow through both the bypass and the cooler connections. In our case with a 120337-25 valve, the oil starts to flow through the cooler at 110 degrees Fahrenheit and the flow through the bypass connection will be fully closed at 130 deg Fahrenheit.
The capsule used will depend on the viscosity of the oil used.
DC Contactor:
The purpose of the DC Pump starter is to safely control the DC backup pump. It was introduced around 1990 as some companies felt there was a risk of fire due to the pump not being monitored and protected.
Some people feel that this system is over protecting the pump and sarcrificing the turbine.
It is true that there are some weak links and more things that can go wrong to cause the backup pump to fail.
The components in the starter are:
The Disconnect Switch – this can be rotated / controlled from outside the box.
Line Fuses – protect from short circuits and high loads.
Relay M – this is the main motor contorl for the pump.
Relay CR – this is the on/off control relay from the PLC and backup system.
OL – these are thermal overloads – current heats the sensor and opens it if too hot. This is an indication of over load on the montor. Loose connections will have the same result!!!!!
1A is an acceleration contactor to give the motor a soft start.
Pump logic
Pre Start Logic: cold engine start or test crank.
Oil level and temp is checked.
Backup pump is turned on and pressure is checked and has to be greater than 8 psi for 5 seconds.
The pressure needs to decay within 30 seconds to indicate pump is turning off – otherwise an pump alarm will annunciate.
AC pump turned on and checked for min pressure of 8 psi for 5 seconds.
This pump stays on until the engine driven pump takes over or stays in service if there are only electric driven pumps.
The oil pressure while in servcie was allready discussed. The alarm values vary with speed.
Post lube 4 hours.
Post lube required once the NGP speed is greater than 65%.
In the event of a fire the lube system operates during the coastdown. Then it is turned off for a maximum of 20 minutes before resuming post lube.
If the pressure drops on the post lube below 8 psi the backup pumps comes on.
The backup pump comes on for one hour continuous and then cycles on off for the next three hours. 2.5 / 9.5 minutes on off.
If there is case pressure in the compressor the lube pump is commanded on. The reason being that the compressor may spin if the vent valve opened to depressurize the compressor.
Both the thrust bearings are also monitored. The maximum temperature allowed is 250 degrees Fahrenheit. There is also a differential shutdown. A maximum of 100 degrees Fahrenheit difference between the bearing supply temperature and the thrust bearing.
END