Engineering Training



Assignment Sheet Number 1.16


–In this lesson we will cover the LM2500 design and operating characteristics, and discuss how these features affect overall operation of a gas turbine platform.


Terminal Objective:

–1.0 Execute the responsibilities of the Junior Officer of the Deck in a nuclear-powered surface combatant, taking into account the impact of conventional propulsion plant operations and casualties on a battle group.

Enabling Objectives:

–1.55 Describe, using the Brayton Cycle as a basis, how the following engine subsections interact to produce propeller shaft rotation:

•a. compressor

• b. combustion

• c. high pressure turbine

• d. low pressure turbine

• e. accessory gear box

• f. support system

–1.56 Describe the following support systems required to start and operate an LM2500 Gas Turbine Engine under normal and abnormal conditions:

•a. lube oil storage and conditioning assembly

• b. fuel oil service system

• c. air system

• d. electric power

–1.57 Describe combustion and cooling air flow through the intake and exhaust system in terms of:

•a. hull entry and exit points

• b. moisture removal capabilities

• c. particulate removal capabilities

• d. normal and abnormal modes of operation

• e. inherent infrared signature reduction capabilities

–1.58 Describe installed Gas Turbine Module fire containment and firefighting systems and their capabilities.

–1.59 Explain the uses and limitations of Sixteenth Stage Bleed Air in terms of source, distribution and customer uses.

–1.60 Describe how the LM2500 Gas Turbine Engine is maintained in terms of impact on ship operations:

•a. routine inspections and maintenance

• b. non-routine inspections and maintenance

• c. engine change out

• d. recurring deficiencies


–1. Read Information Sheet 1.16.

–2. Outline information sheet 1.16 using the enabling objectives for lesson 1.16 as a guide.

–3. Answer study questions.


–1. Name the four major units of the LM2500 gas turbine module.

–2. List seven services/systems that the LM2500 module base penetration plate provides interfaces for.

–3. Describe the EOP procedure used for entering the LM2500 gas turbine module, and what this procedure is designed to accomplish.

–4. What temperature can the LM2500 gas turbine module withstand and for how long?



Information Sheet Number 1.16


–In this lesson we will cover the LM2500 design and operating characteristics, and discuss how these features affect overall operation of a gas turbine platform.


(a) DD 963 propulsion plant manual

(b) DD 963 ship information book

(c) LM2500 technical manual


. Basic Gas Turbine Theory (Figure 1.16-1)

. Today's operational gas turbine engines use the open Brayton cycle.

. Atmospheric air provides the working mass.

. The mass is drawn from the atmosphere and compressed.

. Heat is added by mixing fuel with the compressed air and igniting the mixture.

. Work is extracted by expanding the hot gases through a turbine.

. The working substance (products of combustion) is then exhausted to the atmosphere.

. The LM2500 uses the following components to accomplish these processes:

. 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.

. LM2500 CONSTRUCTION (Figure 1.16-2 LM2500 Component Breakdown View)

•. Compressor

–. The compressor front frame directs air flow into the compressor via the bellmouth and bullet nose, and supports the stator vane casing and forward end of the compressor rotor shaft.

•) Five struts support the bearing housing/sump

• ) The sump area houses the inlet gear box, which is connected to front rotor shaft and provides a power take-off point to drive engine support accessories (fuel and oil pumps, etc.)

–. The compressor stator casing provides mounting for the stator vanes which are used to direct airflow from stage to stage through the compressor

•) The vanes consist of one set of Inlet Guide Vanes (IGV) and 15 sets of stator vanes.

• ) The IGVs and the first six stages of stator vanes are variable in that they pivot on their mounting points to control the amount of air flowing through the compressor.

–. The compressor rotor provides mounting for 16 stages of rotating blades and consists of spools and discs supporting the blades.

– . The compressor rear frame supports the aft end of stator casing. It also supports the aft end of rotor shaft (a hub supported by radial struts and containing a bearing assembly), and provides a mounting area for the combustor and the high pressure turbine.

•. The combustor is of the annular type and consists of an inner and outer liner with the flame centered between the two.

–. The outer case of the compressor rear frame supports the fuel manifold, 30 fuel nozzles, and two spark igniters.

– . The fuel nozzles protrude into swirl cups where combustion air combines with the fuel spray and the mixture enters the combustion area.

– . Most of the air provided by the compressor is used to center the flame and keep the combustor section cool. Only approximately 30% of the air is used for combustion.

•. High Pressure Turbine is housed in the compressor rear frame and the forward portion of the turbine mid frame.

–. It consists of a two stage axial turbine similar in design to a reaction type turbine.

– . Thermal energy produced in the combustor is converted to mechanical energy in the high pressure turbine to drive the compressor and the accessory drive gear box.

•. The turbine mid frame bearing assembly supports the aft end of the HP turbine shaft and the forward end of the LP turbine shaft.

• . The Low Pressure (LP) turbine is often called the power turbine and is a six stage axial flow turbine. Output energy of the LP turbine is coupled to the reduction gear through a flexible coupling.

• . An accessory gear box (Figure 1.16.3) is suspended beneath the compressor front frame and provides mounting and/or drive power for the following components:

–. The fuel pump consists of both a centrifugal and a gear element with an output that is piped to the main fuel control for metering.

– . Main fuel control meters fuel based on engine horsepower required, Compressor Discharge Pressure (CDP), Compressor Inlet Temperature (CIT), and compressor speed (Ngg).

– . A starter which is air driven

– . A manual drive pad used to manually jack over the engine for maintenance

– . A lube oil and scavenge pump which supplies oil to the bearings located in the four LM2500 bearing housings. The scavenge pumps sends oil to the Lube Oil Storage and Conditioning Assembly (LOSCA).

– . An air oil separator sends oil from the four bearing sump vents back into the lube oil system and discharges the air into the exhaust gas train.


•. Module Description (Figure 1.16.4)

–. The 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, and allows for penetrations to interface with the following systems:

•) Fuel

• ) Synthetic lube oil (MIL 23699)

• ) Bleed air

• ) CO2/Halon

• ) Waterwash

• ) Electrical connections for engine control and monitoring

•. Lube Oil Storage and Conditioning Assembly (LOSCA - Figure 1.16.5)

–. Since the bearing sumps in the LM2500 are "dry", a separate component is provided to store the oil (it acts as a "remote sump") and cool it.

– . The LOSCA is located on the deck above the modules and holds 32 gallons of 23699 synthetic oil.

– . The lube oil cooler uses main reduction gear lube oil (2190TEP) as a cooling medium.

•. Air Systems

–. Bleed air is extracted from the sixteenth stage of the compressor and used for starting LM2500s and gas turbine generators, the anti-icing system, and the prairie and masker air systems which help suppress engineering and propeller noise transmitted to the sea.

– . Start air is supplied to the starter on the accessory gear box, where it turns an air turbine. Bleed air, reduced HP air, or air from the start air compressors (SAC) located on the ship service diesel generators may be used.

•. Electrical Systems - The LM2500 and its support systems use 440VAC power source for the module cooling fans and the module heater, 110VAC power source for the igniters (This requirement prevents the starting of an LM2500 using emergency battery power from the consoles), and a 28VDC power source for solenoid power and various surveillance and control functions

• . FFG Intakes and Exhausts (Figure 1.16.6)

–. An air inlet plenum for each engine is situated between the O1 and O2 levels. A 1/2 inch screen inside each plenum prevents entry of large objects.

– . Moisture separators are a knit wire mesh that collect and drain water from the incoming air. They also act as a Foreign Object Damage (FOD) screen, and provide primary FOD protection to the engine.

– . The anti-icing system prevents ice from forming in the intakes during high humidity/cold ambient air conditions. Warm bleed air from the LM2500 compressor is admitted into the air stream to keep the intake surfaces from freezing.

– . Emergency combustion air inlet doors are opened by a pressure differential across the moisture separators.

•) If the moisture separators become clogged and sufficient air cannot enter the combustion air intake trunk, the emergency combustion air inlet door opens permitting air to bypass the moisture separators.

• ) The FOD protection offered by the separators is lost when the inlet doors are open.

–. The module inlet plenum attaches the compressor bell-mouth to the air duct via a flexible coupling. A FOD screen of approximately 1/4 inch mesh covers the bellmouth and provides secondary FOD protection.

– . The exhaust uptake vents the exhaust gases from the power turbine exhaust up through the ship to the atmosphere. An eductor effect is created by the exhaust gases going up the exhaust trunk that draws a vacuum on the module interior to draw cooling air from the enclosure.

•. DD/DDG/CG Intakes and Exhausts

–. The high hat assemblies located on the O4 level consists of electrically heated louvers that protect the air intake openings from freezing shut, both flat and chevron shaped filter pads to filter and coalesce water from the incoming air and to provide primary FOD protection, blow-in doors that are actuated by a differential pressure switch and open to allow combustion air flow if the filters become clogged, and anti-icing piping which admits bleed air into the air stream to prevent ice formation inside the intake duct.

– . The module inlet plenum is identical to that found on the FFG.

– . The exhausts are similar to those on the FFG with the exception of a device fitted to the very top of the stack. This device, called a Boundary Layer Infra-red Suppression System (BLISS) cap, reduces the infra-red signature of the exhaust gases as an anti-missile defense measure .


•. Bleed air is provided off of the last stages (16th on an LM2500 and 14th on the Allison SSGTG) of the compressor section of a gas turbine engine.

• . Distribution and Uses

–. The bleed air output pressure is dependent upon compressor speed and can be as high as 250 psig. A pressure regulator reduces the output to 75 psig.

– . Bleed air is piped to a common header to which the end users are connected.

•) Masker air via a masker cooler

• ) Prairie air via a prairie cooler

• ) Anti-icing air directly from the header

• ) Start air is a combination of masker air and air directly from the header to obtain temperature control for starts and motors of gas turbines


•. 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. Additional precautions are covered in the ship's PMS system. When a failure does occur the engine is typically replaced with an entire new engine.

• . PMS covers routine inspections of the engine, intake and exhaust systems. Several maintenance items require the Engineer Officer to close out the intakes prior to starting the engine to prevent FOD of the turbines and compressor. After maintenance inside an intake, a temporary FOD screen is placed over the bell-mouth of the compressor to further prevent engine FOD. The screen is removed before performing any high speed engine runs. A couple examples of engine PMS are a semi-annual borescope of the LM2500 and a waterwash of the LM2500 after 24 hours of operation or activation of the Counter Measure Wash Down (CMWD) system.

• . Engine Change Out - As previously mentioned, 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.

• . Recurring Deficiencies

–. Intake cracking

– . Starter failure

– . HP start capability (DD/CG)