Aircraft Systems - Chapter 1.5 - PDF

ME3531 Aircraft Systems Chapter 1.5: Types of Pressure Control Valve and Indication Components For Training Purpose Only Hydraulic Power Objective Describe the functions of different types pressure control valve 1. Relief valve...

ME3531 Aircraft Systems Chapter 1.5: Types of Pressure Control Valve and Indication Components For Training Purpose Only Hydraulic Power Objective Describe the functions of different types pressure control valve 1. Relief valve 2. Thermal relief valve 3. Pressure regulator 4. Pressure reducer Describe the function of each individual component for a closed center configuration that is used on aircraft. Describe the hydraulic system indicating system Page  2 For Training Purpose Only Hydraulic Power Valves Pressure Control Valves They are used to ensure safe and efficient operation of fluid power systems. System components and related equipment and requires a means of controlling the pressure. The following are examples of flow control valve: 1. Relief valve 2. Thermal relief valve 3. Pressure regulator 4. Pressure reducer Page  3 For Training Purpose Only Overview Aircraft Fuel Tank Fuel Pump Fuel & Hydraulic Oil Reservoir Return Filter Heat Exchanger Case Drain Filter Aircraft Combustion Chamber Relief valve Engine Driven Pressure Filter Selector Valve Actuator Hydraulic Pump Page  4 For Training Purpose Only  Hydraulic Power Fundamental laws of hydraulics (Recap) Resistance to flow create pressure Passage returning to Bore side reservoir is Gear Pump closed Linear Actuator What happens to the pressure line with the gear pump delivering oil and the actuator is unable to extend because the returning line is closed? Burst Pipe Page  5 For Training Purpose Only Hydraulic Power Relief valve A pressure relief valve is used to limit the amount of pressure being exerted on a confined liquid. This is necessary to prevent failure of components or rupture of hydraulic lines under excessive pressures. The pressure relief valve is also known as a system safety valve. The pressure drop across the valve is normally converted into heat energy Reservoir Passage returning to Bore side reservoir is Gear Pump closed Relief Valve 3500 PSI Selector Valve Linear Actuator 3000 PSI Page  6 For Training Purpose Only Hydraulic Power Relief valve Cracking Pressure The pressure at which the valve just begins to let oil escape through the relief valve. Full Flow Pressure The pressure needed to pass all of the output flow of the pump through the relief valve. Pressure Override The difference between full flow pressure and cracking pressure for any particular valve. Page  7 For Training Purpose Only Hydraulic Power Open Center Hydraulic Power System Double Actuating Hydraulic Cylinder A B A B 4 way and 3 Position Selector Valve (Open center) P T P T Tubing Connection M Constant displacement Pump Typical Hydraulic system set up M Electric Motor with open center selector valve Reservoir Page  9 For Training Purpose Only Hydraulic Power Closed Center Hydraulic Power System Double Actuating Hydraulic Cylinder A B 4 way and 3 Position Selector Valve (Closed center) A B P T P T Tubing Connection constant displacement M Relief Pump Valve Typical Hydraulic system set up M Electric Motor with closed center selector valve Reservoir Page  10 For Training Purpose Only Hydraulic Power Closed Center Hydraulic Power System The hydraulic system is in idle state: 1. The selector valve is a 4 way 3 position valve. A B 2. The electric motor will be powered up and operates the constant displacement pump. P T 3. The selector valve is in neutral position (no input from the operator). 4. The pump will deliver the oil to the selector valve. As the M Relief flow is unable to flow through, the pressure is build up Valve and subsequently is the relief valve will open and return the fluid back into the reservoir. Typical Hydraulic system set up with closed center selector valve 5. There will be no movement from the actuating cylinder. Page  11 For Training Purpose Only Hydraulic Power Closed Center Hydraulic Power System Initial Actuator Position Final Extended Actuator Position A B The operator intends to extend the actuating cylinder to perform work: 1. The electric motor will be powered up and operates the constant P T displacement pump. 2. The user operators the lever on the selector valve to the intended position as shown in the diagram. M Relief Valve 3. The oil will flow through the selector valve to the bore end of the actuating cylinder. The system will extend the actuating cylinder with selector 4. The oil from the piston end of the actuating cylinder will flow out and valve in the position as shown back to the reservoir via the selector valve. 5. The actuating cylinder will extend as shown. Page  12 For Training Purpose Only Hydraulic Power Closed Center Hydraulic Power System Initial Actuator Position Retract to Original Actuator Position The operator retract to extend the actuating cylinder to perform work: A B 1. The electric motor will be powered up and operates the constant P T displacement pump. 2. The user operators the lever on the selector valve to the intended position as shown in the diagram. M Relief 3. The oil will flow through the selector valve to the piston end of the Valve actuating cylinder. The system will retract the actuating cylinder with selector 4. The oil from the bore end of the actuating cylinder will flow out and valve in the position as shown back to the reservoir via the selector valve. 5. The actuating cylinder will retract as shown. Page  13 For Training Purpose Only Hydraulic Power Thermal Relief valve The thermal relief valve is used to relieve excessive pressures that may exist due to thermal expansion of the fluid. It is used where a check valve or selector valve prevents pressure from being relieved through the main system relief valve. Page  14 For Training Purpose Only Hydraulic Power Pressure Regulator The pressure regulator is installed after a constant displacement type pump. One purpose of the pressure regulator is to manage the output of the pump to maintain system operating pressure within a predetermined range. The other purpose is to permit the pump to be unloaded through the regulator and back to the reservoir. Initial Actuator Position A B P T M Pressure Regulator Page  15 A constant displacement pump in a closed center selector valve with a pressure regulator as shown For Training Purpose Only Hydraulic Power Pressure Regulator Initial Actuator Position Return line A B P T M M Pressure Regulator Pressure line A constant displacement pump in a closed center selector valve with Check Valve a pressure regulator as shown Page  16 For Training Purpose Only Hydraulic Power Pressure Regulator Return line Return line M M Pressure line Pressure line Check Check Valve Valve open Pressure Regulator spring is compressed Page  17 For Training Purpose Only  Hydraulic Power Pressure Regulator Return line Return line Pressure Regulator spring is compressed M M Pressure line Pressure line Check Check Valve Valve open When the system pressure is just above p (psi) at which the The system will continue to unload until the system pump unloads, is called the kicked-out pressure. pressure line drops to a value where the spring is able to push the piston down and the ball seats. This pressure is During unloading [kick-out position], the pressure on the prior called the kicked-in pressure. to the check valve drops to almost zero (near reservoir pressure) and the check valve seats (closes) and pressure in the system to the downstream of check valve is maintained. Page  19 For Training Purpose Only Hydraulic Power Pressure Regulator Return line When the system pressure just below P (psi) Downward F acting on the piston > Upward F acting on the piston (Since Fspring = Constant) Where, The spring is initially in the extended or uncompressed M Downward F acting on piston = P x Aball + Fspring Upward F acting on piston = P x Apiston Pressure line Check The ball remains seated and hence there is no pump output Valve flow through the valve to the return manifold. Page  20 For Training Purpose Only Hydraulic Power Pressure Regulator Return line When the system pressure (P) is 3000 psi Upward F acting on piston = P x Apiston = 3000 x 1 = 3000 lbf Downward F acting on piston = P x Aball + Fspring = 3000 x 1/3 + 2000 = 3000 lbf. Upward F acting on piston = Downward F acting on piston Where M P = 3000 psi Pressure line Ap = 1 square inch AB = 1/3 square inch Check Fspring = constant Valve Page  21 For Training Purpose Only Hydraulic Power Pressure Regulator When the system pressure is just above p (say 3001 psi) Downward F acting on piston,3000.33 lbf < Upward F acting on Return line piston, 3001 lbf When the system pressure is just below P (assume 2999 psi) Pressure Regulator spring is compressed Downward F acting on piston, 2999.67 lbf > Upward F acting on piston, 2999 lbf Ball remains seated and hence there is no pump output flow through the valve to the return manifold. M Where Pressure line P = 2999 psi Check Valve Ap = 1 square inch open AB = 1/3 square inch Spring force = 2000 lbf Fspring = constant Page  22 For Training Purpose Only Overview Aircraft Fuel Tank Fuel Pump Fuel & Hydraulic Oil Reservoir Return Filter Heat Exchanger Case Drain Filter Aircraft Combustion Chamber Relief valve Engine Driven Pressure Filter Selector Valve Actuator Hydraulic Pump Page  23 For Training Purpose Only Hydraulic Power Variable Displacement Pump Piston pump This pump does not requires a pressure unloading valve. The fluid output can be varied to meet the pressure demands of the system automatically by a pump compensator within the pump. When the pump outlet pressure is below the operating level, no fluid will flow to the yoke actuator to change the swash plate angle hence maximum flow is delivered to the output. Page  24 For Training Purpose Only Hydraulic Power Closed Center Hydraulic Power System Double Actuating Hydraulic Cylinder A B 4 way and 3 Position Selector Valve (Closed center) A B P T P T Tubing Connection Variable Displacement Pump Relief Valve Typical Hydraulic system set up with closed center selector valve Reservoir Page  25 For Training Purpose Only Hydraulic Power Closed Center Hydraulic Power System The diagram shows a closed center hydraulic power system with the directional (or selector) valve that does not allow a continuous flow of system hydraulic oil through the valve when the selector is in a position (Neutral) to actuate the actuating cylinder. A B P T Relief Valve Typical Hydraulic system set up with closed center selector valve Page  26 For Training Purpose Only Hydraulic Power Closed-Center Hydraulic Power System 1. The selector valves are arranged in parallel. 2. The fluid is under pressure when the pump is operating. 3. The pressure is available the moment the selector valve is positioned. 4. There are two means of controlling the output pressure from within the pump or pressure regulator installing after the pump. Page  27 For Training Purpose Only Hydraulic Power Open-Center Hydraulic Power System The selector valves are arranged in series. The open-center is one having fluid flow, but no pressure in the system when the actuating mechanisms are idle. The pressure built up gradually after the selector valve is selected, there is less pressure surges hence with smoother operation of the actuating mechanisms. The pump circulates the fluid from the reservoir, through the selector valve and back to reservoir. Page  28 For Training Purpose Only Hydraulic Power Pressure Reducing Valve Pressure reducing valves are used in hydraulic systems where it is necessary to lower the normal system operating pressure by a specified amount. Pressure reducing valves provide a steady pressure into a system that operates at a lower pressure than the supply system. Page  29 For Training Purpose Only Hydraulic Power Indicating System The indicating system is important for the aircrew to know whether the parameters of the hydraulic system is functioning within limits. This is achieved through various sensors and switches which sense the pressure, temperature and quantity and these signals are sent directly to indication lights or gauges in the flight deck or cockpit. Quantity Transmitter The quantity in the reservoir will be displayed in the flight deck to ensure that there is sufficient fluid is present for safe operation prior the flight. Low Level Float Switch It will generate a warnings to inform the crew that the fluid level has reached the allowed minimum to supply the pump Page  30 For Training Purpose Only Hydraulic Power Indicating System Pressure Sensors This sensor is used to measure the actual system pressure delivered by the pump. The higher the pressure, the voltage on the output. Pressure Switch This switch will indicate whether the hydraulic pump is delivering the intended pressure. Temperature The temperature sensor allows the monitoring the hydraulic fluid temperature of the hydraulic systems. It is normally installed near the reservoirs on the return pipe. Page  31 For Training Purpose Only Hydraulic Power Recap List the functions of pneumatic systems in the aircraft and its comparison with hydraulic systems. List and describe the different sources of input such as air bottles, ground supply and compressor. Describe with the aid of diagram, the working principle/functions of the various components in a typical high pressure pneumatic system, showing the distribution and the interface with other systems. Page  32 For Training Purpose Only