Gas Turbine Lubrication System

NOTE: Lubricating oil recommendations are included in the "Gas turbine subcontractor’s Literature chapter".

The lubricating requirements for the gas turbine power plant are furnished by a common forced-feed lubrication system. This lubrication system, complete with tank, pumps, coolers, filters, valves and various control and protection devices, furnishes normal lubrication and absorption of heat rejection load of the gas turbine. Lubricating fluid is circulated to the three main turbine bearings, generator bearings, and to the turbine accessory gear and fuel pump. Also, lubricating fluid is supplied to the starting means torque converter for use as hydraulic fluid as well as for lubrication. Additionally, a portion of the pressurized fluid is diverted and filtered again for use by hydraulic control devices as control fluid.

Major system components include:

• Lube reservoir in the accessory base;

• Main lube pump (shaft driven from the accessory gear);

• Auxiliary lube pump and emergency lube pump;

• Pressure relief valve VR-1 in the main pump discharge;

• Lube fluid heat exchanger;

• Lube filters;

• Bearing header pressure regulator VPR-2-1.

Lube fluid temperatures are indicated on the thermocouples which may be located in the bearing header, bearing drains, and the oil tank. For turbine starting, a maximum of 800 SSU is specified for reliable operation of the control system and for bearing lubrication. A thermocouple, LT_OT-1A, prevents turbine start-up if the temperature of the lubricant is lower than the switch setting (only if applicable).

Lubricating fluid for the main, auxiliary and emergency pumps is supplied from the reservoir, While lubricating fluid used for control is supplied from the bearing header. This lubricant must be regulated to the proper, predetermined pressure to meet the requirements of the main bearings and the accessory lube system, as well as the hydraulic control and trip circuits.

Regulating devices are shown on the Lube System Schematic Diagram Figure LS-1. All lubricating fluid is filtered and cooled before being piped to the bearing header.

The reservoir for the lubrication system is the 3300 gallon (i.e. 12 491 l) tank which is fabricated as an integral part of the accessory base. Lubricating fluid is pumped from the reservoir by the main shaft driven pump (part of the accessory gear) or auxiliary or emergency Pumps at a pressure of 25 psig (i.e. 1.75 bar) to the bearing header, the accessory gear and The hydraulic supply system. After lubricating the bearings the lubricant flows back through various drain lines to the lube reservoir.

All lubricant pumped from the lube reservoir to the bearing header flows through the lube fluid heat exchanger(s) to remove excess heat and then through the cartridge type filters providing five micron filtration. The dual heat exchangers are connected in parallel.

Filtration of all lube oil is accomplished by a 5 micron, pleated paper filter installed in the lube system just after the lube oil heat exchanger. Two filters are used with a transfer valve installed between the filters to direct oil flow through either filter and into the lube oil header.

 Lubricating Oil Pumps 

Lubrication to the bearing header is supplied by three lube pumps:

1-The main lube supply pump is a positive displacement type pump mounted in and driven by the accessory gear.

2 -The auxiliary lube supply pump is a submerged centrifugal pump driven by an A.C. motor.

3 -The emergency lube supply pump is a submerged centrifugal pump driven by a D.C. motor.

Main Lube Pump

The main lube pump is built into the inboard wall of the lower half casing of the accessory gear. It is driven by a splined quill shaft from the lower drive gear. The output pressure to the lubrication system is limited by a back-pressure valve to maintain system pressure.

Auxiliary Lube Pump

The auxiliary lube pump is a submerged centrifugal type pump driven by an A.C. motor. It provides lubricant pressure during start-up and shut-down of the gas turbine when the main pump cannot supply sufficient pressure for safe operation. Operation of this pump is as follows:

The auxiliary lube pump is controlled by a low lube oil pressure alarm switch (63 QA-2). This low pressure level alarm causes the auxiliary pump to run under low lube oil pressure conditions as is the case during start-up or shut down of the gas turbine when the main pump, driven by the accessory drive device, does not supply sufficient pressure. At turbine start-up, the A.C. pump starts automatically when the master control switch on the turbine control panel is turned to the START position.

The auxiliary pump continues to operate until the turbine reaches approximately 95 per cent of operational speed.

At this point, the auxiliary (cooldown) lube pump shuts down and system pressure is supplied by the shaft-driven, main lube pump.

During the turbine starting sequence, the pump starts when the start signal is given. The control circuit is through the pressure level of pressure switch 63 QA-2. The pump will run until the turbine operating speed is reached (operating speed relay 14 HS picks up), even though the lube oil header is at rated pressure and the discharge pressure level (63 QA-2) is above alarm level setting.

When the turbine is on the shut-down sequence, this pressure transmitter will signal for the auxiliary pump to start running when the lube oil header pressure falls to the point at which pressure level alarm setting is reached.

Emergency Lube Pump

The emergency lube pump is a D.C., motor-driven pump, of the submerged centrifugal type. This pump supplies lube oil to the main bearing header during an emergency shutdown In the event the auxiliary pump has been forced out of service because of loss of A.C. power, or for other reasons. It operates as follows:

This pump is started automatically by the action of pressure transmitter 96 QA-2 whenever the lube pressure in the main bearing header falls below the pressure switch set ting.

Should the auxiliary pump fail during the shut-down sequence, because of an A.C. power failure or any other cause, the emergency lube pump will be started automatically by the action of low lube oil pressure transmitter 96 QA-2 and continue to run until the turbine shaft comes to rest.