LM2500 Gas Turbine Engine
As the Navy proceeds into the twenty first century, gas turbine ships will become more and more abundant.
GE's highly-successful, well-proven LM2500 aeroderivative gas turbine is directly derived from GE's CF6 family of commercial aircraft engines and GE's TF39 military engine. The CF6 family is used on DC-10, MD-11, A300, 747 and 767 aircraft, which have logged millions of flight hours. The TF39 military engine is employed on the U.S. Air Force's Galaxy C-5A/B transport aircraft. Currently more than 870 LM2500 gas turbines are in service on a variety of cruisers, frigates, destroyers and patrol boats for more than 24 international navies.
In early applications, the LM2500 was rated at 21,500 bhp for the U.S. Navy's DD 963 and DDG 993 class destroyers, the CG 47 class cruisers, and the FFG 7 class frigates. General Electric uprated the LM2500 to 26,250 bhp for the DDG-51 destroyers and the AOE 6 class fast combat support ships. Each DDG-51 ship is powered by four LM2500 gas turbines which are in a combined gas and gas (COGAG) configuration. Each reduction gear combines the input of two LM2500 engines rated at 26,250 brake horsepower (bhp) for a total output of 105,000 bhp per ship. The LM2500 was again uprated in 1993 for use on the US Navy's newest Sealift gas turbine-powered ships. The uprate increased the LM2500 U.S. Navy military specification rating to 29,500 bhp, with the engine actually being rated at 32,000 bhp in the Sealift ships.
The LM2500+ is an uprated version of LM2500 which provides lower installed dollar per horsepower and life-cycle costs than the LM2500. The 3600 rpm LM2500+ has been designed for 39,000 brake horsepower (bhp) with a simple cycle thermal efficiency of 39% at ISO conditions. The LM2500+ achieves increased power over the LM2500 primarily by increasing the compressor airflow 23%, with a minimal increase in combustor firing temperature by adding a compression stage (zero stage) to the front of the LM2500 compressor. The temperature capability of the hot section was also increased by adding a thermal barrier coating to the combustor, upgrading turbine airfoil materials and by improving internal cooling designs.
The LM2500 gas turbines drives controllable pitch propellers through a conventional main reduction gear, shaft and clutch. A 16 stage axial flow (air travel is parallel to the shaft) compressor increases the pressure and temperature of the working mass (air). In the combustor section, fuel is added through fuel nozzles and combustion occurs. A two stage high pressure turbine on the exhaust end of the combustor drives the compressor. The combination of the compressor, combustor, and high pressure turbine are often called the gas generator. The exhaust from the high pressure turbine passes through a low pressure turbine which extracts the work required to drive the main reduction gear and ultimately the ship's propeller. The LM2500 module totally encloses the LM2500 to provide thermal and acoustic insulation. A top hatch and a side door (DD/DDG/CG) or two side doors (FFG) provide access to and viewing of the engine. The module is shock mounted to reduce hull-borne noise transmission.
LM2500 Gas Turbine Engine has a 16 stage axial flow compressor made up of a 16 stage Compressor rotor of moving blades driven by the high pressure turbine. The Compressor stator casing contains one stage of Inlet Guide Vanes (IGV), six stages of Variable Stator Vanes (VSV) and 10 stages of stationary stator vanes. The IGVs and stator vanes 1-6 are variable, meaning they are variable geometry. The angle of attack of the blades can be changed to prevent compressor stall. Bleed air is extracted from the compressor for use in the ship's bleed air system and for internal use in the engine.
The combustor is an annular type with 30 fuel nozzles and 2 spark ignitors. Of the air from the compressor approximately 30% is mixed with fuel to support combustion. The other 70% is used to cool and center the flame within the combustion liner.
The ignition system produces a high intensity spark to ignite the fuel/air mixture during the start sequence. Once the engine is started the ignitors are no longer needed and will be de-energized.
High pressure turbine section extracts enough energy from the hot expanding gasses to drive the compressor and accessory drive. The HP turbine is a two stage axial flow type which is mechanically coupled to the compressor rotor. The HP turbine uses approximately 65% of the thermal energy from the combustor to drive the compressor and engine mounted accessories.
The accessory drive assembly is driven through the compressor rotor shaft via the inlet gearbox, radial drive shaft, and transfer gearbox.
The Accessory gearbox provides mounting for the fuel pump, lube oil pump, air/oil separator, and pneumatic starter.
The power turbine is a six stage axial flow type turbine. The power turbine extracts the remaining 35% of useable energy and uses this to drive the main reduction gear. The power turbine drives the reduction gear through a high speed flexible coupling shaft and clutch assembly. The high speed flexible coupling absorbs the radial and axial misalignment between the GTM and the main reduction gear.
Foriegn Object Damage (FOD) is a major concern of gas turbine engines. FOD is simply debris of one form or other which can damage the internal components of a gas turbine engine. Propulsion and electrical generation turbines are no different than aircraft engines and the hazard FOD represents to the operation of these engines demands the attention of all personnel. On a flight deck periodic "FOD Walkdowns" keep the operating areas clear of debris. For shipboard engines FOD screens, wire mesh moisture separators, and if operating in a high air particulate environment "Scott" foam pads are used to filter incoming air and keep the engine operating.
An advantage of a gas turbine engine is its ability to be removed from the ship for repair in a short period of time (approximately 72 hours). A set of rails is permanently installed in the intakes of each engine and a set of temporarily installed transition rails allow the disconnected engine to transit from its mounting attachments in the module onto the rails in the intakes. Crane service capable of extending over the soft patches in the intakes is required to pull the engine up the rails and out of the ship.
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