21 Jun 2016
Think of aircraft and filters and you'll probably visualise devices for making sure that fuel and lubrication are kept pure for the engines. Of course, this is an essential element of the aircraft power systems to prevent failure and ensure the engines operate smoothly and efficiently. However, there's more to aviation filtration than engine systems, as explained by filtration specialist Porvair's Aerospace Market Manager, Andy Cowan during a recent visit I made to the company's facility in Segensworth, Hampshire.
According to Cowan, a typical twin-engine commercial airliner has around 150 filters installed and every modern commercial aircraft has Porvair products in it. This includes the latest offering from Airbus, the A350, which has six key components from Porvair providing protection to the doors, fuel tanks and control systems and even the coolant for the galley.
The Airbus A350 benefits from fuel tank inerting filters and strainers, filters for the fuel control system and filtration kits for the galley coolant system. Such a broad requirement for protection of the fluids flowing around the aircraft is very common and becoming more challenging as aircraft technology develops.
Cowan told us, “The increasing expectations of passengers for comfort and entertainment are placing more demands on supplementary aircraft systems. As a result, the power consumption on aircraft is higher and so there is a greater need for efficient cooling.”
Far from being a largely passive component, the filters within such complex systems are there to ensure reliable and safe operation, as well as to protect very expensive systems such as avionics and power systems and therefore prevent expensive failures.
To this end, Porvair's engineers engage at the very early stages within the design process to make sure that design requirements are correctly and adequately specified, based on real operational reliability requirements, rather than purely conformance to standard specifications or regulatory requirements.
In this way, filtration requirements are not examined as an afterthought or as a problem that needs to be solved once designs have already been optimised, but rather as an essential element in meeting expectations for reliability and performance that match the demands of airline passengers.
Demands differ by filter type
With so many filters having different functions and located in various positions within the aircraft, there is a range of environmental factors that need to be considered as well as the primary function of the filter itself.
Whilst most of the work done by aircraft filtration systems is in separating particles from liquid or air flow, there are others that perform aerosol separation tasks. An example is the separation of unburnt fuel or moisture in order to protect components.
For future applications, catalysts are also planned to be used for separating out gaseous contaminants.
Additionally, fuel inerting filters are a highly important aspect of ensuring the aircraft safety. These inerting systems eliminate flammable fuel vapours from the onboard fuel tanks by reducing oxygen from the engine bleed air and introducing nitrogen-enriched air to reduce the risk of flammability.
The air separation modules within the inerting system are extremely sensitive to contaminants and so the filtration system eliminates liquid and particulate contamination from the engine bleed air.
Other demands that need to be considered throughout the aircraft include: weight and space constraints; the very high temperatures to which the filters are exposed in engine lubrication applications; and both the heat and vibration experienced by filters which are mounted within the engines.
Whilst these factors are important considerations for commercial aircraft design, they take on even greater significance when designing products for military aircraft, which subject filtration components to much higher levels of heat, vibration and other loads.
Porvair's flexible manufacturing operation in Segensworth rarely supplies standard products and has capability to meet the huge variations in requirements for different mounting and housing requirements of the aviation industry.
Porvair has its own environmental test facilities comprising temperature soak and thermal cycling, as well as vibration and shock testing capabilities with which it verifies the resistance of its products to such climatic conditions.
All the filtration tests are done at Porvair and some of the climatic testing is subcontracted to specialist companies. Some of the environmental testing is done after the filters have been integrated into higher level assemblies, depending on the testing specification demanded.
The testing is performed to suit whatever the operational environment of the filter will be. For example, coolant systems are closed loop and so the fluctuations in temperature experienced by the fluid are not so high. However, for other hydraulic systems, this might not be the case. Landing gear hydraulics suffer from large variations in temperature and so thermal cycling is an important part of the verification testing process.
Thermal shock also features as an important climatic test with certain components of the aircraft being exposed to extreme thermal conditions, including the heat of fluids and gases, as well as extremely cold ambient temperatures.
To take account of such extremes, Porvair works within the supply chain in order to understand the environmental effects that a higher level assembly may have on the magnification of parameters against which the raw component is subjected.
For example, the filter level specification for vibration testing could be a figure of 20g but the actual figure to which it is exposed when mounted into a pump assembly could easily be 50% higher than this.
The same logic applies to temperature extremes. A specification for the filter might be an operational temperature of -31C but Porvair will take the component significantly beyond this figure. Such extremes may be transient conditions but Porvair nonetheless ensures that the component will still function, if not constantly at least for the duration of the transient exposure.