Monday, December 15, 2008

Hydraulic Start System

The starter, or hydraulic starting motor, drives the LM6000 gas turbine high pressure rotor through the accessory gear box (AGB) starter drive pad which has a 0.9562 gear ratio to the high pressure rotor. The starter is required to crank the gas turbine for starting, water wash, and inlet/exhaust duct purge.

The hydraulic start equipment is located in the auxiliary module and consists of the reservoir, filters, air-oil heat exchanger, charge pump and motor, a SOV-actuated valve providing pressurized hydraulic fluid to the variable displacement main pump, and a hydraulic starter motor mounted on the turbine auxiliary gear box (AGB).

The hydraulic starter consists of a variable displacement type hydraulic motor. Piston stroke controlled by a wobble (swash) plate. Displacement is controlled by varying the angle of the wobble (swash) plate by means of a pressure compensator. The starter is equipped with an over-running clutch to prevent the motor from being driven by the high pressure rotor when the hydraulic supply pressure and flow are reduced to zero.

Hydraulic Starter Operation
For starting a minimum pressure drop of 4,200 psi is applied. As starter speed increases, the flow will increase from 0 to 55 gpm. At 55 gpm flow, an internal pressure compensator in the starter maintains starter inlet pressure and accelerates the starter while the hydraulic supply system maintains the 55 gpm flow. When the gas turbine is fired, it will eventually attempt to drive the starter above above its maximum speed. This is referred to as self-sustaining speed. At this point an internal over-running clutch allows the gas turbine to continue to accelerate while the starter continues to run at its maximum speed. The hydraulic supply system for the hydraulic starter will then be shut down.

If the gas turbine is unfired, such as for purge or compressor cleaning, the starter speed will approach a steady 2200-2400 rpm, depending upon ambient conditions.