According to the National Safety Council (NSC) flying commercial is just about the safest way to travel. Even so, many passengers have a fear of flying and for one rather obvious reason. Unlike in a car or on a train or boat, if a problem should occur you can’t just coast to a halt. This is why quality standards for aerospace components are so demanding: every component has to perform as expected and there’s no margin for error.
Perforated tubes perform a vital role in aviation. You’ll find one or more at the core of practically every filter used for fuel, lubricating oil or hydraulic fluid. Commercial airliners are a big market for such filters, but so too is the defense industry. You’ll even find them used in hardware that’s launched into space! Here’s a look at how the aerospace industry uses perforated tubes, and some observations on their particular expectations.
Aerospace filtration
Most filters for fluids are constructed in much the same way. Inside an external housing you’ll find layers of filter material wrapped around a perforated tube. The ratio of total hole area to surface area of the tube is called the “Open Area” or OA.
Fluid is pumped into the housing where it flows through tube and filter material. Together these remove any particulates that could otherwise wear surfaces, damage seals and block small passageways. Aerospace filters use either this “micronic” cartridge design or a disk-type filter that uses spacers and screens around a perforated tube.
The main fluid systems on an aircraft are those for fuel delivery, engine lubricating oil and hydraulic systems. Arguably, the most important hydraulic system is the one that raises and lowers the undercarriage, though flight control surfaces and brakes are perhaps just as critical. Each of these fluid systems has one or more filters, protecting the pumps and actuators from particulates. (Accidents have been caused by contaminated fluids.) In addition, micronic filters are used through the fuel distribution system, from refinery to airport.
Unlike the filters used in cars, which are replaced on a time or mileage basis, aerospace filter replacement intervals are often specified in terms of pressure drop. To assist with this, many filter housings are designed with some form of pressure differential monitor that indicates when the filter is nearing end-of-life.
Material Selection
The aerospace environment is exceptionally demanding for filters. Temperatures can go as low as -65°F and around the engine reach 1,700°F. Some liquids are quite corrosive and internal operating pressures can go as high as 5,000 psi.
Only a few metals can withstand such conditions, which is why the perforated tubes are often produced from specialized nickel alloys or even titanium. These retain their properties over temperatures ranging from cryogenic to over 1,000°F, offer excellent corrosion resistance and are not prone to cracking. Particular alloys often used for aerospace filtration include Inconel, Hastelloy, Waspaloy and Monel. Each is around 50 to 60% nickel with additions for chromium, molybdenum and in the case of Monel, copper.
Titanium and nickel alloys are challenging to work with. The biggest issue with nickel alloys is work hardening, which is addressed through careful process planning and optimization. Tool wear can also be a problem and the materials are sometimes difficult to shear when punching. Titanium is less ductile than stainless steel and is sometimes worked in a warm condition.
Quality and Consistency
Applications less sensitive to safety concerns can tolerate a degree of part-to-part variation, but that’s not the case with aerospace filtration. Here every filter must provide the same flow or pressure drop when new. Cleanliness is also a priority, and the perforated tube manufacturer must ensure an absence of sharp edges and partially attached chips that might separate in service. Only manufacturers with robust processes and equipment and high levels of expertise can meet such exacting expectations.
Dependable, High Quality Perforated Tube
Flying is the safest way to travel, and that’s due in part to the exacting requirements aerospace companies place on their suppliers. Filters and filtration are essential components on every aircraft, whether for civil or military use, and at the heart of most filters you’ll find a perforated tube made to stringent specifications. If the manufacturing and quality processes used to make and inspect a perforated tube are good enough for the aerospace industry they’ll probably be good enough for you.