Air Systems (15.12) Learning Objectives PDF

Air Systems (15.12) Learning Objectives 15.12.1 Describe the main operational features of engine air systems designed for engine internal cooling (Level 2). 15.12.2 Describe the main operational features of engine air systems designed for engine internal sealing (Level 2). 15.12.3...

Air Systems (15.12) Learning Objectives 15.12.1 Describe the main operational features of engine air systems designed for engine internal cooling (Level 2). 15.12.2 Describe the main operational features of engine air systems designed for engine internal sealing (Level 2). 15.12.3 Describe the main operational features of engine air systems designed for engine external air services such as customer bleed (Level 2). 15.12.4 Describe the main operational features of engine air systems designed for engine anti-ice (Level 2). 2024-07-23 B1-15b Gas Turbine Engine Page 130 of 290 CASA Part 66 - Training Materials Only  Engine Internal Cooling Systems Engine Internal Air/Cooling Internal Air System The engine internal air system is de ned as the air ows which do not directly contribute to engine thrust. The system has several important functions to perform for safe and ef cient engine operation. These functions include internal engine and accessory unit cooling, bearing chamber sealing, prevention of hot gas ingestion into the turbine disc cavities, control of bearing axial loads, control of turbine blade tip clearances and engine anti-icing. Internal Cooling System The purpose of the engine internal cooling system is to ensure that certain parts of the engine do not absorb heat to the extent that it is detrimental to their safe operation. The main areas that require internal cooling air are the combustors and the turbines. Rolls Royce SPEY / F28 Engine Diagram 2024-07-23 B1-15b Gas Turbine Engine Page 131 of 290 CASA Part 66 - Training Materials Only  Combustor Cooling You may recall from Gas Turbine Principles that of the total air ow introduced into the combustor, only about 20% - 30% was used for combustion. The remaining 70% - 80% was bypassed around the snout area. In this example, of the air that is bypassed, 40% is used for ame stabilisation and dilution. This allows us to pass the remaining 40% of the air ow around the ame tube for cooling to ensure that its safe operating temperature is not exceeded. Combustor Cooling Example 2024-07-23 B1-15b Gas Turbine Engine Page 132 of 290 CASA Part 66 - Training Materials Only  Turbine Cooling High thermal ef ciency depends on high turbine entry temperatures, and the maximum turbine entry temperature is limited by the ability of the nozzle guide vanes and the turbine blades and discs to withstand the hostile conditions encountered. Continuous cooling of these components allows the temperature of the environment in which they operate to exceed the melting point of the component materials without affecting their integrity. Conducted heat transferred from the turbine blades into the discs means the discs must also be cooled so that thermal fatigue and uncontrolled expansion rates do not occur. Blade and Nozzle Guide Vane Cooling Turbine vane and blade life depends not only on the shape of the blades, but on the methods used to cool them. Therefore, the design of the internal air passages is important. The methods used to cool the components has undergone steady improvement over the history of the gas turbine engine. Single-pass internal cooling, as used during the 1960s, was of great practical value, but development has led to multi‑pass internal air lm cooling of both vanes and blades and impingement cooling of vanes and blades with external air lm cooling. Turbine Blade Cooling Designs The ‘pre-swirl nozzles’, as illustrated below, reduce the temperature and pressure of the cooling air fed into the disc for blade cooling. The nozzles also impart a swirl velocity to the air to assist ef cient entry of the air into the rotating cooling passages. 2024-07-23 B1-15b Gas Turbine Engine Page 133 of 290 CASA Part 66 - Training Materials Only  Pre-swirl nozzle cooling system Hot Gas Ingestion It is important to prevent the ingestion of hot mainstream gas into the turbine disc cavities as this causes overheating and results in unwanted thermal expansion and fatigue. Prevention of hot gas ingestion is achieved by continuously supplying the required quantity of cooling air into the disc cavities to oppose the inward ow of hot gas. The ow and pressure of the cooling air are controlled by the interstage seals. 2024-07-23 B1-15b Gas Turbine Engine Page 134 of 290 CASA Part 66 - Training Materials Only  Turbine Disc Cooling Cooling air for the turbine discs enters the annular spaces between the discs and ows outwards over the disc faces. Air ow over the faces is controlled by the interstage seals. When the air has completed its task, it is discharged into the main gas stream. The diagram below illustrates the ow of cooling air through the turbine. Turbine Disk Cooling 2024-07-23 B1-15b Gas Turbine Engine Page 135 of 290 CASA Part 66 - Training Materials Only  External Cooling The engine bay or pod is usually cooled and ventilated by atmospheric air being passed around the engine and then vented overboard. An important function of the air ow is to purge the nacelle of any ammable vapours. Keeping the air ow to a minimum correspondingly minimises engine drag. A typical external cooling system is depicted below. External Cooling System 2024-07-23 B1-15b Gas Turbine Engine Page 136 of 290 CASA Part 66 - Training Materials Only  Engine Internal Bearing Sealing Bearing Sealing The purpose of the seal pressurisation system, or bearing sealing, is to supply air to the seals that require it at the pressure that they need to prevent leakage of oil or air from the various bearing chambers. It also controls the cooling air ow to prevent the ingress of mainstream gases into the turbine disc cavities. The choice of sealing method depends on the surrounding temperatures and pressures, wearability, heat generation, weight, space available, etc. The types of seals used are: Labyrinth seal Carbon seal Ring seal Hydraulic seal Brush seal. Air for pressurising these seals is supplied from various stages of the compressor. Air is tapped from the compressor by air transfer ports in the disc or drum, and travels down the inside of the shaft to the turbine. The compressor stage that the air is tapped from determines the air pressure to the seal. rommma/stock.adobe.com used with permission An example of a labyrinth seal 2024-07-23 B1-15b Gas Turbine Engine Page 137 of 290 CASA Part 66 - Training Materials Only  Labyrinth Seals A labyrinth seal consists of a nned rotating member with a static bore which is lined with a soft abradable material or a high-temperature honeycomb structure. Initially, the ns rub on the liner and cut into it to give minimal clearance between the two. Across each n in the seal, there is a pressure drop which gives a restricted ow from one side of the seal to the other. This seal is used for sealing bearing chambers and prevents oil leakage by providing air ow from outside the chamber to the inside. This positive pressure in the chamber also assists the scavenge oil system. The three types of labyrinth seals, shown below, are: Fluid and abradable lined seal Continuous groove interstage air seal Thread type oil seal. Fluid and Abradable Lined Seal Continuous Groove Interstage Air Seal 2024-07-23 B1-15b Gas Turbine Engine Page 138 of 290 CASA Part 66 - Training Materials Only  Thread type oil seal Carbon Seals Carbon seals consist of a static ring of carbon which constantly rubs against a collar on a rotating shaft. They rely on a high degree of contact and do not allow oil or air leakage. Any heat generated by friction is quickly dissipated by the oil system. Due to their construction, and carbon’s heat resistance, carbon seals can be used in ‘hot areas’ of the engine, i.e. the turbine. Aviation Australia Carbon seal 2024-07-23 B1-15b Gas Turbine Engine Page 139 of 290 CASA Part 66 - Training Materials Only  Ring Seals A ring seal comprises a metal ring housed in a close- tting groove in the static housing. The normal running clearance between the ring and the rotating shaft is smaller than that obtained by the labyrinth seal. This clearance is obtained because the ring is allowed to move in its housing whenever the shaft comes into contact with it. Ring seals are used for bearing chamber sealing, except in areas of extreme heat where oil degradation due to the heat could cause the ring to seize within its housing. Aviation Australia Ring seal 2024-07-23 B1-15b Gas Turbine Engine Page 140 of 290 CASA Part 66 - Training Materials Only  Hydraulic Seals Hydraulic seals are often used between two rotating members to seal a bearing chamber, but unlike a labyrinth or ring seal, they do not allow a controlled ow of air across the seal. Hydraulic seals are formed by a seal n immersed in an annulus of oil which has been created by centrifugal force. Any air pressure differential, inside and outside the bearing chamber, is compensated by a difference in oil level on either side of the n. Hydraulic seal Brush Seals Brush seals comprise of a static ring of ne wire bristles. They are in continuous contact with a rotating shaft, rubbing against a hard ceramic coating. This type of seal has the advantage of withstanding radial rubs without increased leakage. Brush seal internal diagram 2024-07-23 B1-15b Gas Turbine Engine Page 141 of 290 CASA Part 66 - Training Materials Only  Brush seal physical component 2024-07-23 B1-15b Gas Turbine Engine Page 142 of 290 CASA Part 66 - Training Materials Only  Control of Bearing Loads Engine shafts experience varying axial gas loads which act forwards on the compressor and rearwards on the turbine. The shaft between them is therefore always under tension, and the difference between the loads is carried by the location bearing, which is xed in a static casing. The internal air pressure acts on a xed-diameter pressure balance seal to ensure the location bearing is adequately loaded throughout the engine thrust range. The air balance chamber aids the compressor thrust bearing (No. 2) in combating high gas-path pressures, which try to push the compressor forwards. The balance chamber and thrust bearings help restrain the compressor against the axial pushing force. Some engines do not need an air balance chamber because the opposite (rearward) thrust load, at the turbine, adequately cancels out the forward pushing loads on the compressor. Control of Bearing Loads 2024-07-23 B1-15b Gas Turbine Engine Page 143 of 290 CASA Part 66 - Training Materials Only  Engine External Air Systems Customer Bleed Air Customer bleed air is air that is tapped from a turbine engine compressor and used for such airframe functions as anti-icing, air conditioning and pressurisation of the cabin, and other services. Depending on engine con guration, bleed air is tapped from the compressors at different locations to suit the aircraft requirements and maintain optimal engine ef ciency. Low-pressure (LP) bleed air is taken from the early stages of the compressor at high rpm, while high-pressure (HP) bleed air is taken from the later stages of the compressor at low rpm. Rolls Royce SPEY engine - dual spool engine example 2024-07-23 B1-15b Gas Turbine Engine Page 144 of 290 CASA Part 66 - Training Materials Only  Pre-cooler To reduce the temperature of the hot engine bleed air going to aircraft services, it passes through a pre-cooler. As a cooling medium, cold air ow from the engine’s fan area is ducted and controlled through a heat exchanger. The fan air exhausts overboard after passing through the heat exchanger. A fan air temperature sensor controls the Fan Air Modulating Valve position, allowing more or less cooling air ow. The bleed air temperature is cooled to 200–250 °C, depending on engine type. Bleed air pre-cooler Installation 2024-07-23 B1-15b Gas Turbine Engine Page 145 of 290 CASA Part 66 - Training Materials Only  Bleed air pre-cooler at low power Bleed air pre-cooler at high power 2024-07-23 B1-15b Gas Turbine Engine Page 146 of 290 CASA Part 66 - Training Materials Only  Bleed Air Pre-cooler with Air Supply Overheat 2024-07-23 B1-15b Gas Turbine Engine Page 147 of 290 CASA Part 66 - Training Materials Only  Pressure-Regulating Valves A simpli ed schematic of a bleed air Pressure-Regulating and Shut-Off Valve is shown below. These valves have many uses in both engine and airframe pneumatic systems. They utilise: A solenoid valve to signal ON or OFF A comparison of upstream and downstream air pressures to modulate a butter y valve to deliver a constant pressure to the aircraft system. Bleed Air Pressure Regulating and Shut-off Valve (PRSOV) Engine Ice Formation Ice can form on aircraft surfaces and engines when the Outside Air Temperature (OAT) on the ground or the Total Air Temperature (TAT) in ight is 10 °C or below and visible moisture in any form is present. It can also occur when operating the engine at high speeds during ground runs or where surface standing water is being ingested. 2024-07-23 B1-15b Gas Turbine Engine Page 148 of 290 CASA Part 66 - Training Materials Only  Visible moisture can be in the form of: Clouds Fog with visibility of 1 mi or less Rain Snow Sleet Ice crystals. Anti-icing The pneumatic anti‑ice system provides surface heating of the engine and/or power plant where ice is likely to form. Protection of the rotor blades is rarely necessary due to their centrifugal action, but the rst-stage inlet guide vanes and nose cone may require protection, as may the inlet cowling. The hot air for the anti-icing system is usually taken from the HP compressor. It is ducted through pressure-regulating valves to the parts requiring anti-icing. Spent air from the nose cowl anti-icing may be exhausted into the intake or vented overboard. The system illustrated below, a typical anti-icing system, contains two electric motors that drive air shut-off valves. The air shut-off valves are both opened at the same time when the cockpit switch is actuated. Engine anti-icing is required to eliminate the formation of ice in the intake, which would effectively change the air ow characteristics of the intake. Engine Anti-icing System 2024-07-23 B1-15b Gas Turbine Engine Page 149 of 290 CASA Part 66 - Training Materials Only  When the anti‑ice switch is actuated, a light illuminates in the cockpit and a slight rise in exhaust gas temperature and rpm are noticed. Engine Anti-icing Schematic When the anti-ice valves are opened, bimetallic spring coils inside the air regulator valves control the amount of air ow according to the temperature of the air. Anti-ice Bimetallic Regulator Valve 2024-07-23 B1-15b Gas Turbine Engine Page 150 of 290 CASA Part 66 - Training Materials Only  The bimetallic spring controls the air ow in the following manner: As the temperature of the air passing through the valves increases, the bimetallic spring, which is attached to a valve disc, expands. This expansion is translated into rotary movement of the valve disc. Rotating the valve disc partially covers the ports through the stator plate in the valve and reduces the air ow. The air ow through the valve needs to be controlled because air that is too hot may adversely affect the material strength of the inlet components and engine performance as the anti‑ice air is ingested into the engine. 2024-07-23 B1-15b Gas Turbine Engine Page 151 of 290 CASA Part 66 - Training Materials Only  Bleed Air System Construction Due to the high pressures, temperatures and moisture content involved with engine bleed air, the ducts, or plumbing, are usually constructed from stainless steel to resist fatigue and corrosion. To prevent the hot ducts from transmitting heat through the duct walls to the aircraft structure, they are covered with an insulating material that varies depending on the manufacturer. Flexible elbows are also incorporated into the system to allow the ducting to be bent and retain exibility. Flexible Elbow Flexible Elbow 2024-07-23 B1-15b Gas Turbine Engine Page 152 of 290 CASA Part 66 - Training Materials Only  To join all components, including plumbing, a crush type seal is placed between the mating faces of each component before the joint is clamped with a V-band type clamp, which brings the joints tightly together. These joints are covered with an insulation blanket which is lock-wired around the bare metal, effectively insulating it. Bleed Air Duct, Seal and “V” Clamp Temperature Compensators Due to the length of the ducting in larger aircraft and the wide temperature variation to which the ducts are subjected, thermal expansion could cause problems. To relieve this problem, expansion- absorbing units called temperature compensators are used. A typical temperature compensator is shown below. The compensators provide a slip joint to allow the ducts to ‘grow’. Temperature compensator 2024-07-23 B1-15b Gas Turbine Engine Page 153 of 290 CASA Part 66 - Training Materials Only  Check Valves One-way check valves are incorporated in the system to control air ow from the pressure sources to the various pneumatic components and/or systems. These valves could be located in the ducts from the: APU External air source connection. They prevent the loss of manifold pressure into the APU or out the ground air connection when these sources are not in use. Check Valve Diagram 2024-07-23 B1-15b Gas Turbine Engine Page 154 of 290 CASA Part 66 - Training Materials Only  Shut-Off Valves Shut-off valves are usually located aft of the rewall or in the pylon on which the engine is mounted. They are also located in the ducting leading to the starter system if a pneumatic starter is used. They control air ow, in the case of the main engine shut-off valve, to prevent or allow air ow into or from the engine, or in the case of the starter valve, to allow air to ow to the starter for engine start, then cease when the starter is no longer required. These valves are normally motor-driven butter y valves and are electrically actuated from the cockpit. The illustration below shows the valves in the open and closed positions. Shut-off Valve 2024-07-23 B1-15b Gas Turbine Engine Page 155 of 290 CASA Part 66 - Training Materials Only  Engine Accessory Cooling Systems The electrical generator and some other engine accessories can produce a considerable amount of heat. During ight, these accessories are cooled by atmospheric ram air. During ground operation, when there is minimal external air ow, some engine accessories require additional cooling air. This can be achieved using an ejector that ports a small volume of compressor bleed air through nozzles in the cooling air outlet duct of the accessory. The air velocity through the ejector nozzles create a low-pressure area that draws atmospheric air through the intake louvres and around the component that requires cooling. To ensure the system only operates while the engines are running and the aircraft is on the ground, the ow of air is controlled by a pressure control valve. Typical generator cooling system 2024-07-23 B1-15b Gas Turbine Engine Page 156 of 290 CASA Part 66 - Training Materials Only  Active Clearance Control Active Clearance Control is used to improve compressor and turbine ef ciency by heating the HP compressor drum and cooling the HP turbine case. HP compressor air is ducted to the inside of the compressor rotor cavity to heat and expand it. Expanding the rotor reduces the compressor blade tip to compressor case clearance, which improves compressor ef ciency. Active Clearance Control also applies to compressor cases in regard to Tip Clearance Control, but it is most common on the turbine cases. Both systems are operated at the command of an Electronic Engine Control (EEC), often as part of a Full Authority Digital Engine Control (FADEC) system. Active Clearance Control System A method of controlling the clearance between the tips of the turbine blades and the turbine case/shroud ring is Active Tip Clearance Control. Active means the tip clearance is controlled by varying the amount of cooling air to vary the thermal expansion rate of the turbine outer case. This in turn minimises tip losses at all power settings. 2024-07-23 B1-15b Gas Turbine Engine Page 157 of 290 CASA Part 66 - Training Materials Only  Active Clearance Control Cooling Tubes Relevant Youtube link: Bleed Air (Video) 2024-07-23 B1-15b Gas Turbine Engine Page 158 of 290 CASA Part 66 - Training Materials Only  Air System Maintenance and Faults Routine Maintenance As with all aircraft systems, periodic routine maintenance tasks must be performed. These are normally component changes, ducting inspections and leak checks. Typically components are changed when overhaul is due, but as we covered previously, not the entire system is changed. Items such as ducting remain in place for greater periods due to the absence of moving parts. mehmet/stock.adobe.com used with permission Engine pneumatic system ducting and valves When changing any component, care must be taken to ensure a tight and complete seal prior to system operation. As previously mentioned, a seal or gasket is placed between the mating faces of the components. The faces are then joined with a V-band coupling which is normally torqued progressively until the nal torque is achieved. A progressive torquing method is to set the torque wrench at a lower predetermined setting and gradually increase the torque until it reaches the nal setting required. Progressive torquing is used to ensure the faces are ‘pulled’ together by the V-band coupling, thus providing an effective seal. When any system component is removed and replaced, close attention must be paid to foreign object damage (FOD) control as any small foreign object may cause blockage of components. Also, after the task is complete, leak checks must be carried out. 2024-07-23 B1-15b Gas Turbine Engine Page 159 of 290 CASA Part 66 - Training Materials Only  Faults External bleed air problems can include pressure and temperature faults, which are monitored and controlled by a system of sensors and a bleed air computer. If air temperature is excessive, then damage could occur to components in user systems. If an over-temperature occurs, the Pressure-Regulating and Shut-Off Valve is signalled to close, shutting down the defective engine bleed system. Physical evidence of bleed air faults in below diagram. Bleed air monitoring computer 2024-07-23 B1-15b Gas Turbine Engine Page 160 of 290 CASA Part 66 - Training Materials Only