Engineering Training



Assignment Sheet Number 64B7-205


This topic discusses the purpose and operation of the Bleed/Start Air system. As an OOD or Combat Systems watchstander, you must have a working knowledge of the Prairie and Masker air systems for conducting Anti-submarine Warfare (ASW) operations. In addition, the EOOW and the OOD must understand the capabilities and limitations of the various types of start air available for the gas turbine engines.


Terminal Objective:

7.0 DESCRIBE the principles, function, construction, components, control and monitoring systems, and operation of a gas turbine propulsion plant and associated auxiliary support systems. (JTI:A)

Enabling Objectives:

7.16 DESCRIBE the purpose and operation of all major components in the Bleed Air System and systems supplied by the Bleed Air System to include:

a. Heat Exchangers

b. Pressure Regulators

c. Temperature Regulators

d. Isolation valves

7.17 DESCRIBE the location as well as the operation of the Bleed Air System and systems supported by the Bleed Air System including:

a. Masker Air System

b. Prairie Air System/Fin Air

c. Anti-icing System

d. Cross Bleed Starting System

7.18 LIST and DESCRIBE the location as well as the operation of all major components of the Start Air System.

a. Heat Exchangers

b. Pressure Regulators

c. Temperature Regulators

  1. LIST Start Air Compressor (SAC) control and monitoring stations and describe capabilities and operational limitations.


1. Read Information Sheet 64B7-205

2. Outline information sheet 64B7-205 using the enabling objectives for lesson 64B7-205 as a guide.

3. Complete study scenarios.


1. While conducting a walk-through of the MER, you stop to talk with the watch. Discuss the uses of customer bleed air.

2. As OOD, you give the EOOW permission to conduct a cross bleed start on 1B GTE. What is the minimum Gas Generator (Ngg) speed required for cross bleed starting?

3. During an intake inspection you notice the anti-icing piping in front of the combustion and cooling intakes. What is the purpose of Anti-icing air?



Information Sheet Number 64B7-205


This topic discusses the purpose and operation of all major components in the bleed air system. This lesson also covers the systems supplied by the bleed air system. These systems include prairie, masker, and GTE cross-bleed start air. We will discuss the various types of start air available for the gas turbine engines including the SSDG start air compressor (SAC), High Pressure (HP) air, and GTE cross-bleed air.


(a) SIB Vol. 2, Part 2, NAVSEA S9FFG-AM-SIB-022/(U) FFG-21

(b) PPM Vol. 1, S9234-BL-GTP-010/FFG-7, Propulsion Plant Manual

(c) Applicable FFG7 Class Advisories



  1. Customer Bleed Air
    1. Customer bleed air (Figure 1), extracted from the compressorís 16th stage, provides gas turbine anti-icing, prairie and masker air, and start air for the other Gas Turbine Engine (GTE). Customer bleed air passes through the bleed air valve located inside the module. The Local Operating Panel (LOP) or Propulsion Control Console (PCC) controls the bleed air valve. Bleed air exits the module through the base penetration plate. The anti-icing system pipes the hot bleed air directly into the intakes to prevent ice formation. Bleed air used for cross bleed starts, masker air, and prairie air passes through the bleed air reducing valve. This valve reduces bleed air pressure from 250 to 75 psig. Bleed air then passes through the bleed air cooler. This cooler uses sea water from the firemain to lower the bleed air temperature to below 400o F. A Resistance Temperature Device (RTD) at the discharge side of the cooler provides remote monitoring. The RTD generates an alarm at the Auxiliary Control Console (ACC) when the cooler discharge temperature exceeds 400o F. A temperature switch located on the discharge side of the bleed air cooler automatically closes the reducing valve when the cooler discharge temperature reaches 425o F. After air passes through the bleed air cooler it splits off into two branches, one for starting air and the other for prairie/masker air.
    2. Figure 1 Gas Turbine Bleed Air System

    3. Icing conditions occur at 410 F at 70% relative humidity and create Foreign Object Damage (FOD) hazards for the GTE. The anti-icing air system pipes the hot bleed air directly to the gas turbine intakes to prevent ice formation. Both combustion and cooling air inlets upstream and downstream of the demister pads receive anti-icing air reduced to 38 psig. The PCC operator normally controls the anti-icing system.
    4. The Masker Air System (Figure 2) uses air from the ship's bleed air system via the bleed air cooler for discharge through emitter belts located around the underwater girth of the ship. The masker regulator valve reduces masker air pressure from 75 to 28 psig. After leaving the reducing valve, the air supply divides into two branches supplying air to the forward and aft emitter belts. Masker air forms an air bubble screen around the hull of the ship, reducing transmission of machinery noise to the surrounding waters. The ACC operator normally controls the masker air system.
    5. Figure 2 Masker Air System


    6. The Emitter Belts (Figure 3) are located at frames 177 and 253. Each belt is divided into port and starboard halves. Each belt has a separate air connection. Each emitter belt uses a solenoid operated valve to control air flow. The ACC controls these solenoid valves. Masker air discharges through each connection at a rate of 425 squared cubic feet per minute (SCFM) at approximately 12 psig. Perforations in the emitters allow discharge of Masker air from the keel to the water line. An orifice plate in the port side emitter belt balances air flow.
    7. Figure 3


    8. The Prairie Air System (Figure 4) supplies air along the propeller blade leading edge to reduce the hydrodynamic noise originating at the propeller. Prairie air flows at 400 SCFM from a branch of the bleed air system through the prairie air cooler. The cooler uses seawater from the Firemain system as a cooling medium. From the cooler, prairie air flows through a flow meter into the roto-seal at the Oil Distribution Box (OD Box) and into the prairie air tubing to the propeller. At the propeller hub after end, the air enters drilled passages in the hub body. The passages direct the air to the base of each propeller blade. Air reaches each blade through a bushing connection between the blade base and the hub body. Air then flows through an air channel in the blade leading edge and discharges through 302 orifices. Two check valves prevent entry of water when the air supply is secured. The Fin Stabilizers use prairie air supplied directly from the discharge side of the prairie air cooler. The air passes through a network of apertures along each stabilizerís leading edge. The air suppresses flow noise and cavitation. The ACC operator normally controls the prairie air system.

    Figure 4

  2. GTE Start Air
    1. The GTE Bleed/Start Air System (Figure 5) provides compressed air to the GTE pneumatic starter for starting, motoring and water washing. The system can use air from the Ship Service Diesel Generator (SSDG) Start Air Compressor (SAC), the High Pressure (HP) air system via eight storage flasks or customer bleed air supplied by a GTE operating above 7500 Ngg. NR 2 and NR 4 SSDGs are equipped with SACs. NR 3 SSDG is SAC capable, but is not normally equipped with a SAC. When using the SAC or the bleed air system, air flows to the start air temperature regulating valve. This valve divides flow through or around the start air cooler maintaining start air temperature below 200o F. Air then continues on to the motor pressure regulating valve. This valve has two positions and is Low Pressure (LP) control air regulated. For starting, the valve completely opens. System pressure ranges between 45 and 75 psi depending upon the air source. Full starting pressure is reduced to between 35 and 41 psi. For motoring (including water wash) the valve discharge pressure is regulated to 22 psi. From the pressure regulating valve air passes through cut out valves for each gas turbine enclosure, then through the module base penetration plate to the start air regulating valve inside the enclosure. The "Starter On" command, at the PCC or LOP, opens this valve admitting air to the pneumatic starter.
    2. Figure 5 GTE Bleed/Start Air System

    3. The High Pressure air system (Figure 6) uses eight 3000 psi HP air storage flasks to provide a source of HP start air. Five flasks are located in the MER. Two flasks are in Auxiliary Machinery Room (AMR) 2, and one flask is in AMR 3. A watchstander initiates a HP air start by opening the three solenoid valves that connect the eight storage flasks to the HP air system. These solenoid valves are wired in parallel and controlled from Central Control Station (CCS) or the MER. Two pressure reducing manifolds reduce air pressure from 3000 to 45 psi. The air travels directly to the inlet side of the start air pressure regulating valve.
    4. Figure 6 HP Starting Air System

    5. The SSDG output shaft drives the SAC (Figure 7). The gear hub of the input speed increasing gear box drives the oil pump. It provides pressurized MIL-L 23699 oil for lubrication and control. To engage the compressor, a 28 volt signal opens a solenoid valve permitting lube oil flow into the fluid coupling, allowing the compressor to be driven by the gear box. Upon depressing the engage push button, the fluid coupling has 15 seconds to fill and bring the compressor to required discharge pressure. Otherwise, the solenoid valve closes, disengaging the coupling and activating a "Fail To Engage" alarm at the ACC. While engaged, a continuous flow of oil passes through the fluid coupling to dissipate heat. De-energizing the solenoid valve disengages the compressor by securing oil flow to the coupling. Oil remaining within the coupling drains back into the oil sump. Filling or draining the fluid coupling requires approximately 15 seconds. The SSDG Local Control Panel (LCP) provides local control of the SAC. The ACC provides remote control for the SAC operation. The SSDG electrical load must be less than 666 KW to engage the SAC. If the load exceeds 666 KW during SAC operation, the solenoid valve automatically closes, disengaging the SAC. Regardless of the operating station, the minimum cycle times for the SAC are three minutes engaged and three minutes disengaged to allow for proper heating and cooling of the lube oil.

    Figure 7

  3. Associated Parameters
    1. HP Start Air
      1. Starting Air Flasks Capacity 6 Cubic Feet Each
      2. Primary Branch 10 Flasks
      3. Backup Branch 8 Flasks
      4. Starting Air Manifolds 2
      5. Manifold Inlet Pressure 3000 psig
      6. Manifold Outlet Pressure 45 psig
      7. Manifold Relief Valve Opens 78-82 psig


    2. SAC Start Air
      1. Starting Air Compressors 2 Centrifugal Type
      2. NR 2 SAC Location NR 2 SSDG
      3. NR 4 SAC Location NR 4 SSDG
      4. Discharge Capacity 2160 SCFM
      5. Discharge Pressure 45 psig
      6. Windmill Speed 0-6100 rpm
      7. Operating Speed 51000 rpm
      8. Low Lube Oil Pressure Alarm 65 psig
      9. Sac Fail To Engage Air <45 psi In 15 Secs
      10. Sac To Engage <666 KW
      11. Engaged/Disengaged Delay Time 3 minutes
      12. L/O High Temperature 177o F


    3. Common Start Air Parameters
      1. Starting Air Cooler Inlet Temperature 400o F
      2. Maximum Starting Air Cooler Outlet Temp 200o F
      3. Motoring Air Pressure 22 psig
      4. Starting Air Pressure 45 psig
      5. System Low Pressure Alarm 35 psig

    4. Bleed Air
      1. Compressor Discharge Air Temperature 925o F
      2. Maximum Compressor Discharge Air Pressure 250 psig
      3. Bleed Air Pressure Regulating Valve 250 / 75 psig
      4. Bleed Air System Relief Valve Opens 85 psig
      5. Bleed Air Cooler Inlet Temperature 925o F
      6. Maximum Bleed Air Cooler Outlet Temperature < 400o F
      7. Cooler Discharge High Temperature Alarm 400o F
      8. Bleed Air Regulating Valve Closes 425o F