PUTTING SENSORS 
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Table of Contents

Monitoring Bearing and Gear Failures in
Aircraft Gas
Turbine Engines

Part 2: Removing Air and Wear Debris Particles from the Scavenge Lube Mixture

How do aircraft engines work? What you need to know to monitor wear of engine components

David J. Aslin, Eaton Corp.

photo

Both hot and cold tank scavenge lube systems used in aircraft gas turbine engines have a common problem: how to remove entrained air from the scavenge lube mixture. Entrained air at the lube (pressure) pump’s suction port causes cavitation and reduces the oil flow to the bearings and gears. Standard air-to-oil ratios vary from 1:1 to 4:1, but the maximum amount of entrained air by volume normally allowed in the lube oil is 10%. This means that lube oil leaving the reservoir should contain <10% air by volume, based on the reservoir’s outlet port pressure and temperature.

Deaeration Techniques
You can use several techniques to remove the air from the scavenge mixture that enters the oil reservoir inlet. These include:

  • Incorporating residence time to allow air to separate by way of buoyancy
  • Using in-tank horizontal baffles to slow the flow of incoming air-oil mixture prior to it entering the main body of fluid (the horizontal baffles spread the incoming air-oil mixture into a wide, thin stream, which allows air to escape more readily)
  • Using vertical baffles to prevent short-circuiting of the incoming mixture with the lube pump suction flow
  • Applying cyclonic phase separators

Residence Time. This is the volume of oil in the reservoir (which is measured in gallons) divided by the lube pump flow out of the tank’s oil outlet port (gallons per second). The residence time should be sufficient to allow the scavenge mixture’s air to migrate via buoyancy to the tank’s vapor space. Depending on the internal geometry of the tank and the method used to introduce the oil into the lube tank, re-entrainment of air is possible.

The use of residence time alone to allow the air in the scavenge mixture to escape increases the oil volume requirements of a lube oil tank. The buoyancy of air bubbles in the air-oil mixture is related to the mixture’s temperature. At high temperatures, the air becomes more buoyant because of the expansion of the air bubbles, the natural convection forces, and a decrease in the oil’s viscosity. Oil viscosity creates drag that retards the speed of an air bubble migrating to the vapor surface of the mixture.

In-Tank Horizontal Baffles. These devices are used to slow the incoming fluid velocity to allow time for the air to separate from the oil. These baffles are usually 10°–15° off horizontal. The incoming fluid is funneled over the baffle, where buoyancy causes the air to separate from the fluid. The end of the baffle is submerged in the reservoir’s fluid. This prevents splashing and subsequent re-entrainment of air.

Vertical Tank Baffles. Hydraulic tanks also use vertical tank baffles. The incoming mixture is forced to travel the length of the tank and back to the hydraulic pump’s suction strainer. This allows the air to escape via buoyancy and prevents short-circuiting of the incoming fluid with the hydraulic pump’s suction strainer.

photo
Photo 1. Centrifugal phase separation is an efficient way of separating air from lube oil. Pressure in each of the two stage cyclonic separator chambers drives the fluid vortices, requiring no additional mechanical or moving parts.
Cyclonic Phase Separators. The most efficient way of removing air and debris from a scavenge air-oil mixture is by using a cyclonic phase separator. These devices are so named because they handle the three phases of matter—gas (air), liquid (oil), and solid (debris). Cyclonic separators usually consist of a constant diameter cylinder with a tangential inlet.

You can integrate inline, three-phase separators (see Photos 1 and 2) into the scavenge lube system’s oil reservoir. Inline and in-tank separators operate in the same manner. The scavenge air-oil mixture tangentially enters the constant diameter chamber, which causes the
photo
Photo 2. Lube oil must be deaerated before returning to the bearing compartments and gearbox. Inline separators (pictured here) separate gas (air) from the liquid (oil) and solids (debris). A magnet on the debris sensor retains ferrous wear debris.
mixture to spin (rotate), creating centrifugal force. Because of the large density difference between the air and the oil, the higher density oil displaces the air to the center of the chamber. A central baffle at the bottom of the chamber prevents the air from escaping through the bottom of the separator. A vortex finder tube in the center of the top of the chamber provides a means for the air to escape. The oil annulus surrounding the central air column passes down the sides of the central baffle and into the lube tank. An exit cup at the bottom of the separator chamber ensures that the oil flowing out of the separator and into the lube tank is directed upward to the oil-vapor space interface.

As wear debris moves into the separator’s chamber, it’s forced to the outer chamber wall by its own high density (the heaviest phase of matter contained in the scavenge lube oil mixture). A debris exit in the chamber wall lets particles escape in a small slipstream of lube oil. The slipstream with concentrated wear debris is directed to either a debris sensor pocket or into a flow-through-type debris sensor.

Because cyclonic separators operate at g (acceleration) levels that exceed the force of gravity, the mounting attitude of the separator has no effect on its ability to separate air or debris from oil. Therefore, these devices function efficiently in negative and micro-gravity (zero g) environments. Cyclonic separators also make ideal pulsation dampers.

Experiments have been conducted on air-oil separators to measure the effect of various two-phase flow regimes (e.g., slug flow and annular flow) on the separator’s performance. Static inline mixers were placed at the separator’s inlet, and air-oil separation efficiencies were compared with separation efficiencies recorded prior to the mixers’ installation. The results were identical. This proves that irrespective of the two-phase flow regime, the air separates from the oil immediately upon entering the separator’s chamber.

The amount of acceleration the mixture experiences upon entering the separator is calculated as follows (the equations in Part 2 are numbered consecutively from those in Part 1):

equation (5)

where:

Acc    = fluid acceleration (ft/s2)
V = mixture inlet fluid velocity (ft/s)
R = chamber radius (ft)

Fluid acceleration is normally expressed in multiples of gravity (32.2 ft/s2) and calculated as follows:

equation (6)

Normal fluid acceleration required in a cyclonic phase separator to remove air from oil is between 50 and 100 g. The g level is directly related to pressure drop. The higher the fluid acceleration in the separator’s chamber, the higher the associated friction loss. The friction loss results in a higher separator inlet pressure. The higher the inlet pressure, the smaller the actual air-to-oil ratio of the scavenge lube oil mixture at the separator’s inlet. New low-pressure drop, three-phase cyclonic separators have been developed that let you remove wear debris from the scavenge lube oil mixture with a minimal fluid acceleration increase.

The frictional loss through a cyclonic separator includes inlet and exit losses, and the friction loss associated with the fluid-to-chamber wall interface. The higher the fluid velocity at the fluid-to-wall interface, the higher the corresponding friction loss. These devices are efficient at separating air and wear debris from oil, providing the oil viscosity is <30 cp. Above this point, a liquid vortex is destroyed.

Typical air and oil separation efficiencies over the operating envelope of an aircraft gas turbine engine are 95% by volume. Wear debris capture and/or indication efficiency depends on the fluid acceleration level; the transport fluid viscosity; and the size, material density, and shape of the wear particle. Typical capture efficiencies are:

  • 100% for 1000 and 500 micron steel particles
  • 90% for 250 micron steel particles (For particle size definition, see “Trending Wear to Predict Engine Failures” in Part 1 of this series in the October issue of Sensors.)

Air separation efficiency is calculated as follows:

equation (7)

where:

Airsep% = air separation efficiency expressed as a decimal
SCFMairvent    = standard cubic feet of air per minute of air (14.7 psia, 70°F) flowing out of the oil tank air vent
SCFMinlet = standard cubic feet of air per minute flowing into the separator

Oil separation efficiency expressed as a decimal is calculated as follows:

equation (8)

where:

Oilsep% = oil separation efficiency expressed as a decimal
GPMoilout    = gallons per minute of oil exiting the separator’s oil outlet port
GPMinlet = gallons per minute of oil entering the separator

Three-Phase Separation
In a hot tank system, you can achieve three-phase (gas, liquid, and solid) separation by substituting a three-phase cyclonic separator in place of a two-phase device. New three-phase designs use the same energy normally required to separate air from oil. Using an in-tank debris sensor with a three-phase separator inside a scavenge system lube tank conserves energy in terms of friction energy loss. Further, cyclonic separators can concentrate the wear debris particles in a small slipstream.

Concentrating wear particles into a small slipstream in an oil reservoir cyclonic separator has advantages:

  • For retaining-type debris detectors that use a magnetic probe to catch and retain debris, the probe-to-flow conduit cross-sectional area ratio increases. The effect of this ratio increase is an improvement of the sensor capture efficiency.
  • For flow-through-type debris sensors, the flow area requirements based on minimum particle transport velocity (~7 fps for particles up to several thousand microns) gets much smaller. A 1/4 in. diameter flow-through sensor passage would result in a flow rate of 1 gpm, to achieve a fluid velocity of 7 fps.
  • Cyclonic separators are ideal pulsation dampers because they separate the air from the oil. Most debris sensors function better in the absence of fluid pulsation.
  • You achieve high capture efficiency with no additional increase in pressure energy.
Next
Next month, Part 3 of this series discusses the various types of metallic wear debris collectors, detectors, sensors, and systems predominately used in the aerospace industry for monitoring metallic bearing and gear wear.

David J. Aslin, Ph.D., is Group Leader, Engineered Sensors, Systems Engineering Group, Eaton Corp., 7370 College Pkwy., Ft. Myers, FL 33907; 941-849-5762, fax 941-939-0152, davidjaslin@eaton.com.

MORE!
For further reading on this and related topics, see these Sensors articles.

"A Mobile Test Stand for Aircraft Wheels and Brakes," June 1998




 
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