Fuel contamination in a car or truck is seldom fatal. In an aircraft it’s a different matter. Water can freeze and block fuel delivery lines, which will stop the engine or engines. Other types of contamination can be just as problematic. When particulates block fuel system components pilots lose control over the engines and an emergency landing is usually required.
Two-stage coalescer-separators are designed to prevent this problem. These rely on cartridge-style filters made from perforated tubes. Here’s more detail about the role played by perforated tubes in aviation filtration.
The Contamination Problem
The biggest concern with aviation fuel is water. Jet fuel can dissolve water and hold it in solution. This isn’t what causes problems. Problems arise when the water is held in suspension in the fuel as what’s called “free water.” Temperatures at elevation are often below 32°F (0°C), which is when “free water” can freeze and block delivery lines.
“Free water” gets into aviation fuel in three ways. Temperature changes in storage tanks cause condensation. Water may enter through a poorly sealed filler neck during a rainstorm or when the aircraft is being washed, or it may have entered further upstream during processing.
Deicing fluid and particulates can also cause problems. In 2010 for example an Airbus A330 flying from Indonesia to Hong Kong experienced problems with both engines. The subsequent investigation found small particles from filters used on the ground had caused the fuel metering units to seize.
The Role of Filtration
Aviation fuel passes through many filters on its journey from refinery to aircraft. These comprise a mix of coalescing and separation filters and fuel filter monitors. At each stage the goal is to take out contaminants that may have got into the fuel.
A point to remember is that filters clog over time as contaminants are trapped and this increases pressure drop. Eventually the filters must be replaced to avoid the risk of collapse.
Two Stage Coalescing and Separation Filters
These use cartridge-style filters to remove water and particulates from the fuel. In coalescing, which takes place first, fuel is pumped through a multi-media filter enclosed between two concentric perforated tubes. Microscopic water droplets are forced to commingle and as they become larger are drawn down to the bottom of the coalescer by gravity. At the same time, the filtration media traps particulates as small as or even smaller than 1 micron.
Cleaned fuel then passes through the second stage separator cartridge filter. This repels water droplets which drop to the bottom of the assembly, and strips away any remaining contaminants.
Coalescing-separating filter units are produced under various categories or Types. (Type S, for example.) Selection depends on factors such as fuel type, the absence or presence of anti-icing additives, and the extent of contamination anticipated.
Aviation Fuel Filter Monitors
These are single stage cartridge-style filters. They are typically used in mobile refueling trucks. Filter element construction is as in the coalescer-separator units: filter media enclosed between concentric perforated tubes.
Perforated Tubes in Aviation Filtration
The cartridge design used in aviation filtration puts filtration media – glass fiber or a combination of materials – between two concentric tubes. These hold the media in place while allowing fuel to flow with the lowest possible pressure drop. In addition, the first tube spreads the fuel flow out over a wide area. This prevents damage to the filter media that would result from impingement on a small area and improves overall filtration efficiency.
Optimizing perforation size, pattern and total area is a complex problem. Smaller holes reduce pressure on the filter media but create a larger pressure drop. Densely packed holes, (larger “open area”,) improve flow but reduce overall tube strength. It takes experience, supported by sophisticated design tools, to arrive at a design that best satisfies all the performance requirements.
Perforated tube material is another design/engineering decision. The material chosen must resist attack by jet fuel while providing the strength required at the target cost. Stainless steel is a common choice but tubes can also be fabricated from superalloys like Hasteloy and Inconel as well as mild steel or even Titanium. As flanges may be welded onto the ends of the tubes, weldability and the risk of galvanic corrosion are additional considerations.
Last, there’s the joining method. Longitudinal-welded seams are common in aviation filtration but the alternative is to use a spiral welded seam.
If Perforated Tubes Keep People Safe in the Air, What Could They do for You?
Perforated tubes have a host of uses, but one of the most interesting is in aviation filtration. Clean fuel is essential for aircraft safety and perforated tubes are at the heart of the filters used. That’s something to think about during your next flight.
Could perforated tubes improve the way your products or processes perform? Contact us and let’s find out.