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.
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.