Free Range Technology

Like the Sands of Time Through an Hourglass

A jet turbine is a carefully engineered piece of rotating machinery, with close tolerances being essential to safe and efficient operation. Many military and civilian applications of turbine engines, however, require operations in sub-optimal conditions. Sand, dust, and even salt water can impinge on the rapidily rotating blades inside the engine and quickly cause severe erosion.

The solution to this problem may surprise you with its simplicity, and yet is a quite effective means of cleaning intake air.Prior to the 1960s, initial testing indicated that sand and dust had a minimal wear impact on gas turbine engines. Test cell engines were subjected to what was thought to be very heavy sand concentrations without catastrophic failure. But in the mid 60’s, the military in particular began to notice much heavier erosion problems than had been anticipated, with the average to overhaul being 300 hours. Even as late as Desert Storm, sand was causing engine overhauls with around 100 hours of engine operation in some cases.

Typical malfunctions that were dirt related were the prviosly mentioned component erosion, oil contamination, pneumatic control jamming, clogging of small ports and fouling of heat exchange surfaces.

The answer to these problems? Sand and other contaminant particles just happen to be heavier than air. Taking this simple fact into account, and that intake air is being sucked into an engine at a rapid rate, inertia can be used to separate the dirt from the air.

Some inertial separators are as simple as a tight turn in the intake pathway. The heavier particles cannot make the turn, and pass along to an ejection port. These intakes can be integrated into engine design, and offer 85-95% filtering of course sand and 65% for fine sands.

Another way to accomplish the same task is with intake tubes that contain a small set of vanes. As the air passes through the tubes, the vanes impart a swirl to the air/dirt mixture, and centrifugal force pushes the heavier dirt to the outside of the tube. At the opposite end of the intake tube is a cone shaped tube with a center opening.

Clean air passes through the center of the cone, while the dirt is directed along the outer sides into a middle chamber that vents the contaminants overboard.

These strata tubes or vortex tubes can be arranged into banks in order to supply required airflow.

Scavenge airflow is usually imparted by means of electric fans of through tapping of engine bleed air, and these centrifugal tube separators can be capable of 93-98% filtering efficiency.

This type of engine air particle separator has found its was onto many different helicopter platforms, and while some designs are not exactly aerodynamically efficient drag-wise, this issue is less critical at the relatively lower airspeeds present in helicopters.

During the Iran hostage crisis, a rescue was attempted by the Carter administration in 1979. The mission had to be aborted due to loss of helicopters–two were lost entering Iran in a sandstorm, one crashed and the other was forced to turn back. Another suffered a flight control hydraulics malfunction, likely due to sand contamination. This limited the force to 5 helicopters, too few to safely execute the rescue. Then in a tragic hover taxi incident one of the remaining helicopters crashed into one of the C-130s used on the rescue. The end result was that in addition to the crash losses, 5 usable helicopters were abandoned in Iran. These sand related failures are the sort of thing that the engine air particle separators (EAPS) are designed to prevent, and if you look carefully, in the picture above there are no screens over the engine intakes on the helicopters used on that rescue. Compare that with the image below, with the current H-53 versions having EAPS present on all engines.

These filters have also been adapted to wide ranging applications, not simply just jet turbines.





Sources for information and images were: Donaldson, Pall, NATO Research and Tech, the DoD, MD Helicopters, the RAF, and the ever ubiquitous Wikipedia.

  • Cavemanengineer

    It's sad that the 1979 chopper incident was on watch of the only engineer president we've had. EAPS were invented in 1966 so there's nothing but shame there. Shaaaaaaame.

    • The only engineer? I'm pretty sure Herbert Hoover wasn't on watch in 1979….

      • Cavemanengineer

        Yep my bad forgot about hoover, who was an actual mining engineer. Turns out Carter was an engineer only according to his 1976 campaign staff, he resigned his commision in the middle of nuke school to go mind the farm after his father died, reportedly.

      • OA5599

        A pretty strong case could be made that Thomas Jefferson was an engineer, among his many talents.

    • Hueys had pretty decent filtration. My old Army aviation unit, based at Ft. Lewis, WA, had a hell of time keeping their Hueys in the air in the aftermath of the Mt. St. Helens eruption. A good friend of mine was doing medivac flights and they carried several sets of intake filters with them, which is not typically done. Throughout their missions, they were routinely auto-rotating down with no flame. They'd pull out the clogged intake filters, slap in some new spares, and take off again. Those birds' engines were replaced pretty quickly afterward, however.

  • The Professor

    That's pretty cool. I've wondered how the armed services managed dust filtration for their desert equipment. It's not something that you can get by with by stopping every so often and knocking the dirt out of the filter by banging it on the hull. Those inertial air filters are surprisingly efficient.
    Good one, Hyco!

  • highmileage_v1

    These particle control systems are essential in the sandbox. With the C-130's we used to see a 2% – 5% drop in power output after 6 months in theatre. The funny part is that a lot of turbine engines require some sort of internal cleaning while in service, and this is done by shooting a mild abrasive (walnut shells?) down the throat of the engine while it is running.

  • CruisinTime

    Thank you for keeping the Atomic Toaster archive operational.