Mercedes Mullins
The type of fuel-booster pump that requires a pressure relief valve is an aircraft positive-displacement fuel pump. This is in contrast to an aircraft centrifugal fuel boost pump, which does not require a pressure relief valve.
| Characteristics | Values |
|---|---|
| Type of pump | Aircraft centrifugal fuel boost pump |
| Pressure relief valve requirement | True |
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What You'll Learn
Aircraft fuel pumps
Fuel pump systems are more complex than gravity flow systems and often include a primary fuel pump, also known as an engine-driven fuel pump, and a backup auxiliary pump. The engine-driven fuel pump is typically a rotary-vane-type or diaphragm-type pump bolted directly to the engine crankcase. It delivers fuel under pressure directly to the carburetor or fuel injector. To prevent the build-up of excessive fuel pressure, a relief valve is necessary. This valve, often spring-loaded, allows excess fuel to flow back to the inlet side of the pump or the fuel tank return line.
The backup auxiliary pump, also known as a boost pump, serves multiple purposes, including priming the engine before start-up, restoring fuel flow in case of primary pump failure, and suppressing vapour formation at high altitudes. The boost pump can be electric or hand-operated and must have an internal bypass valve to allow fuel to pass through even when turned off.
The complexity of aircraft fuel pump systems is further highlighted by the need for a fuel pressure gauge or fuel flow meter to monitor the performance of the pumps. Additionally, the fuel lines must be carefully routed and protected from heat sources to prevent issues like vapour lock. Overall, the fuel pump system plays a critical role in ensuring the reliable operation of the aircraft's engine by delivering fuel under the required pressure and flow rate.
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Pressure relief valves
A pressure relief valve (PRV) is a type of safety valve used to control or limit the pressure in a system. It is designed to open at a predetermined set pressure to protect pressure vessels and other equipment from being subjected to pressures that exceed their design limits. When the set pressure is exceeded, the valve opens and diverts a portion of the fluid through an auxiliary route, relieving the excess pressure. This is particularly important in systems containing flammable fluids, where excess pressure could lead to an explosion or fire.
In the context of fuel-booster pumps, a pressure relief valve is required on an aircraft positive-displacement fuel pump or an aircraft centrifugal fuel boost pump. These pumps are used to deliver fuel under pressure to the engine, and the relief valve helps ensure that the fuel is delivered at the correct pressure. Without a relief valve, excessive pressure could build up, leading to potential equipment failure or safety hazards.
The specific type of pressure relief valve used can vary depending on the application and the fluid being handled. For example, piston-style designs are often used for higher relief pressures or when ruggedness is a concern, while diaphragm-style valves are preferred for low-pressure applications or when high accuracy is required.
In addition to their role in fuel-booster pumps, pressure relief valves are also commonly used in various other applications, such as downstream of pressure-reducing valves, pumps, and pipelines to maintain safe working pressures and protect equipment.
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Vent valves
The Role of Vent Valves in Aircraft Fuel Systems
During engine shutdown, the inlet port to the ecology valve opens, and fuel is suctioned from the flow divider manifold to the sump chamber. This process helps prevent fuel from draining into the combustion chamber and mitigates the formation of smoke during the next startup.
How Vent Valves Work
Selecting the Right Vent Valve
When selecting a vent valve for an aircraft fuel system, several critical factors must be considered:
- Cracking or Shut-off Pressure: It is essential to ensure that the valve is precisely calibrated to open and close at the appropriate differential pressure.
- Leakage: Minimizing leakage is critical to valve performance, whether it is sealing against air during engine operation or sealing in fuel during shutdown.
- Flow Rate: When the vent valve is open, it must have sufficient flow capacity to discharge the necessary volume of fuel. Insufficient flow can lead to valve instability and damage.
- Envelope: Aircraft components, including valves, should be as lightweight as possible to minimize fuel consumption.
- Fluid and Ambient Temperature: The valve must be constructed with materials that can withstand a wide range of temperatures, from cold starts to the heat generated during operation.
- Vibration and Life: The valve must be qualified to operate under high levels of vibration and metal fatigue, as the engine generates the highest levels of vibration in an aircraft.
- Resistance to Contamination: Contamination is the number one failure mode for miniature components. Therefore, the valve design must be robust and adequately protected from debris and dirt in the fuel system.
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Fuel system shutdown
Shutting down a fuel system can vary depending on the type of system in use. For example, the procedure for shutting down a fuel cell system involves first shutting down electrical generation and then supplying air to the water gas shift reactor to oxidise any remaining CO and H2. This controls CO poisoning of the fuel cell and prevents the emission of flammable and poisonous gases.
In the case of a fuel pump emergency, such as a fire, gas leak, sparks, or static electricity, it is important to have an emergency switch that allows users to safely shut down the fuel pump function. This can limit damage and make it safer for personnel to respond to the crisis.
For turbine engines, such as the Pratt & Whitney PT6, a dump valve is incorporated into the fuel system to prevent fuel from dribbling through the nozzles during shutdown. This valve is held closed by positive fuel pressure but opens due to spring force when fuel pressure drops, allowing fuel to be diverted and dumped harmlessly.
For aircraft, a fuel system shutdown may involve the use of an auxiliary or boost pump to prime the engine before startup or to restore fuel flow in case of an issue with the engine-driven pump.
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Fuel flow during aircraft shutdown
During an aircraft shutdown, the fuel flow is terminated, causing a rapid drop in pressure in the fuel manifold. This leads to a loss of the force required to push the fuel through the nozzles and achieve atomization. As a result, the remaining fuel dribbles through the nozzles and enters the combustion chamber as a liquid instead of atomized vapor.
This can cause the lighter weight "hydro" part of the hydrocarbon fuel to boil off, leaving behind heavier weight carbon residue, known as "coking" of the fuel nozzles. This results in a poor distribution of fuel and non-uniform temperature distribution in the combustion chamber, potentially leading to premature and costly repairs.
To address this issue, turbine engine engineers have incorporated a Dump Valve in the Pratt & Whitney PT6 engines. This valve, located between the Fuel Control Unit (FCU) and the manifold, remains closed during normal operation due to positive fuel pressure but opens during shutdown due to spring force when fuel pressure drops. By providing an alternative path, the Dump Valve prevents fuel from dribbling through the nozzles and redirects it to a vent tube, avoiding the issue of coking and ensuring a more complete combustion.
Regarding the initial question, a fuel-booster pump that requires a pressure relief valve is an aircraft positive-displacement fuel pump.
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Frequently asked questions
A fuel-booster pump that is an aircraft positive-displacement fuel pump requires a pressure relief valve.
A pressure relief valve is used to prevent issues caused by residual fuel remaining in the engine's combustion chamber following shutdown, such as turbine damage due to 'hot starts' caused by rich fuel, increased maintenance due to fuel coking inside combustion nozzles, environmental and safety concerns due to fuel leakage, and negative financial impacts on the airline due to wasted fuel.
Yes, vent valves can be used as an alternative to pressure relief valves. Vent valves are normally open and allow relief flow during low-pressure conditions, closing when the differential pressure across the valve reaches a specified level.
Some key considerations include cracking or shut-off pressure, leakage, flow rate, envelope (weight and size), fluid and ambient temperature, vibration and life, and resistance to contamination.
