Why an Engine Air Particle Separator (EAPS)?

Transcription

1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47 St., New York, N.Y GT-297,r. The Society shall not be responsible for statements or opinions advanced in papers or in dis. `S cussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. [t^ Discussion is printed only If the paper Is published in an ASME Journal. Papers are available 3^^ from ASME for fifteen months after the meeting. Printed in USA. Copyright 1990 by ASME Why an Engine Air Particle Separator (EAPS)? JOE THOMAS POTTS, JR. Headquarters, Army Material Command Aviation Division (AMCDE-HE) Alexandria, Va ABSTRACT The purpose of this technical paper is to describe how an Engine Air Particle Separator (EAPS) removes contaminant particles before they enter the gas turbine engine. Gas turbine engines perform poorly in air containing sand, volcanic ash, industrial pollutants, etc. Typical dirt related gas turbine malfunctions include: Erosion of the engine and air cycle machinery rotating components. Clogging and fouling of turbine section. Wear of oil wetted components caused by contaminated lubricants. Contaminated air entering an EAPS is sent through a swirling motion induced by the vortex generator. This swirling motion causes the heavier dirt particles and water droplets to be thrown radially outward by centrifugal force so that they may be scavenged from the engine air intake. This report will provide test results of helicopters with and without EAPS and describes the steps necessary to design an EAPS for various air vehicles and engines. INTRODUCTION In March 1989, Headquarters, WESTCOM (Army headquarters in the Pacific Theatre) requested that a team consisting of members of the CH-47 Project Manager Office (PMO), U.S. Army Aviation Systems Command (AVSCOM) Directorate of Engineering, Textron Lycoming, Corpus Christi Army Depot (CCAD), Hamilton Standard and Howell Instruments be sent to Hawaii to investigate starting problems with four engines on two CHINOOK (CH-47) helicopters stationed in Hawaii. The location of the helicopters was Pohakuloa Training Area (PTA) at Bradshaw Army Air Base ( ft altitude), in a dried volcano base full of lava cinders and very dirty. During starting attempts compressor stalls occurred commencing at about 28% of N1 and continuing through the stagnation point, 40-43% N1. Test data from a Howell Instrument Portable Engine Analyzer Test Set (PEATS) determined that compressor deterioration was causing the engine start difficulty. Similar results had occurred in 1981 in Egypt. Joint U.S. - Egyptian maneuvers were in progress. Helicopters from both countries filled the air. The maneuvers had pleased each country's military command but many of the helicopters' engines began to lose power. Performance fell far short of expectations. Sand and dust from the desert environment were eroding engine parts, causing abnormal wear. This was particularly prevalent on the U.S. Army Cobra gunships. The Government of Israel was also aware of the expensive damage and reduction in readiness that dust can cause to helicopters and worked with the U.S. Army, Bell Helicopter and Pall Corporation to develop an improved engine air filtration system for the Cobra. Similar systems were already in service on the helicopters with the U.S. and foreign milit`ary'governments. For example, EAPS are installed on the following military and commercial heli- 'Presented at the Gas Turbine and Aeroengine Congress and Exposition June 11-14, 1990 Brussels, Belgium

2 copters: Bell: 205, 206, UH-1 Huey, AH-1 Cobra, OH-58, OH-58D; McDonnell Douglas: 500, 530; Sikorsky: CH-53; MBB: B0105, BK117; Augusta: A109; Aerospatiale: Lama, A Star, Twin Star, Dauphine, Gazelle, Super Puma, Super Frelon; Westland: Sea King, Commander, Lynx; Boeing Helicopter: CH-47 Chinook in Egypt. Engine erosion became a particularly serious problem with the U.S. Army at Ft. Bliss, Texas where the same sand and dust environment exists. The average time between major overhauls on helicopter engines at Ft. Bliss, Texas was 262 hours vs. the planned life of 1200 hours. Some of the Cobra and UH-1H Huey engines required an overhaul after only 30 flight hours; only a few went to 350 flight hours. Of all the Ft. Bliss helicopter engines sent to Corpus Christi Army Depot, Texas for overhaul, 90% were returned because of engine erosion. Following a test program at Ft. Bliss and at the Army Aviation Engineering Flight Activity, Edwards AFB to evaluate the benefits of improved engine inlet dust protection, the U.S. Army is now retrofitting their AH-lF Cobras and UH-1H Hueys with high efficiency Improved Particle Separators (or EAPS). The Egyptian Government was quick to recognize the benefits of protecting their helicopter engines from sand and dust ingestion and had engine air particle separators (EAPS) manufactured by the Pall Corporation under the trade name, Centrisep Air Cleaner, installed on their CH-47 Chinooks. The U.S. Army, Boeing Helicopters and the British Royal Air Force have also tested this EAPS. Comparisons were made between the EAPS and the existing engine protection which is an all-weather screen (AWS), intended primarily to prevent ingestion of large CH-47 soa MIDDLE EAST EXERCISE OCT/NOV 1986 items that would cause foreign object damage to the compressor. A CH-47 in the desert with only the AWS could achieve only 12 landings before both engines were changed. With EAPS it did 13 landings with no change in horsepower. In another desert test CH-47 engines with AWS deteriorated to an unacceptable level in only 30 hours. With EAPS there was no degradation (Figure 1). The installation of the EAPS on the CH-47D helicopter was shown to be compatible. No airframe or engine related operations were significantly affected by the EAPS. The ground and flight test qualification showed: - Overall performance of the EAPS was superior to AWS in icing, sand, dust, and other foreign object damage conditions. - There is no significant difference in power required between the EAPS and the AWS. Sand and dust can cause damage to a turbine engine compressor (Figure 2) in as little as 20 hours. This will cause failures to start that have been known to happen to cargo aircraft in Vietnam, Washington state (near Mt. St. Helens), the 1988 Yellowstone fire, and at Hawaii in As well as eroding compressor blades, dust will clog turbine cooling passages and blade attachment, cause sulphidation, corrosion, and over-heating. WJ M EAPS V FOD SCREENS EAPS No. 2 ECU IPTIT "C N.. I ECU FIG. 2 TYPICAL AIR COOLED TURBINE 650 ENVIRONMENT - MUD FLATS AND SOFT SAND 600 SOREENS t HOUR IN VERY ^\ HE HOU SAND \\\ 0 s t0 ^s m 1s ENGINE OPERATING HOURS FIG. 1 CH-47 EXERCISE Sand and dust will also contaminate an No. 1 ECU engine's oil system leading to blockage of oil system passages, oil seal erosion, NO.YECU filter blockage and failure of pumping elements that can cause internal sump fires. Sand and dust can also enter air bleed systems, leading to cockpit contamination and avionics cooling passage blockage and avionics failures. The `i

3 problems that dust can cause aero engines was highlighted dramatically during the erruption of Mt. St. Helens. Aircraft and helicopters flying in the vicinity of the dust cloud experienced so much engine erosion that engine changes were required upon landing. A telegram from Pratt and Whitney to commercial operators in 1980, warned of a problem stating that experience indicates that repeated or severe ingestion of volcanic ash will accelerate compressor airfoil erosion and contaminate the engine oil system. Volcanic ash is both abrasive - since it is a fine powder, and corrosive - since it contains acid-forming components. Both turbine engines and reciprocating engines will be affected by volcanic ash. Turbine engine oil contaminated by volcanic ash has led to engine failures in as few as 20 hours after exposure. The acids associated with the ash are soluble in oils and as such attack engine parts, resulting in rapid deterioration. Saudi Arabia has been buying a number of American-made helicopters. Current experience among pilots in Saudi Arabia is that: 1. Sand abrasion is greater than anticipated. 2. Sand contains corrosive elements. 3. The very fine sand particles cause wear on bearing and moving parts. 4. Seals need replacing earlier. 5. The high altitude of many airfields (4000 ft) and the high ambient temperatures, (95 degrees is common) tend to reduce helicopter capabilities. Performance becomes a major problem, which is compounded when engine erosion further reduces engine power. Now that we have learned why EAPS is needed, let's take a look at the background of EAPS's development. BACKGROUND Early experiences with gas turbine powered helicopters brought the problem of sand and dust ingestion into focus. The problem became particularly serious during the Vietnam conflict when the need for engine protection from dust ingestion became urgent. Typical dirt related malfunctions were: - Erosion of engine and air cycle machinery rotating components Wear of oil wetted components caused by contaminated lubricant Jamming of pneumatic controls Clogging of small ports Fouling of heat exchange surfaces Similar problems were later experienced on other aircraft as well as vehicular and industrial gas turbine engines. Initially, conventional barrier filters were used in an attempt to solve these problems on turbine engines. Clean engine intake air has always been a problem. Piston engines have pretty well solved the problem with the advent of porous paper barrier filters. These filters are about 99.9% efficient in removing dust from the air. Because they are so good, paper filters tend to plug up rather rapidly. This does not cause them to be less efficient in terms of dirt and sand removal, but it dramatically affects the amount of air flowing to the engine. For this reason they have to be designed with an over-capacity in order to compensate for the reduced airflow volume. A turbine engine needs about eight times as much airflow as an similar piston engine. To make a paper-type air filter for one of these air gobbling machines would be a superhuman job. In order to last for any length of time, it would have to be so big that the airplane couldn't carry it. The barrier filters were effective in producing dust free air, but proved to be unsatisfactory for aircraft engines for the following reasons: - Filter media providing adequate protection clogged rapidly. Some lasted only a few hours. - High pressure drop produced by the use of barrier type filters caused an adverse effect on the performance of the protected system. - Use of large air filters in order to obtain practical service life resulted in bulky installation as well as difficulties with spares support. - Filter servicing increased the cost of operations and decreased the reliability. Based upon field reports and inquiries made by airframe and engine manufacturers, it became apparent that the sand and dust problem could not be solved by using barrier filters. The Pall Corporation initiated a major program directed toward the development of a high efficiency, compact, self-cleaning inertial air cleaner which could be used to protect gas turbine engines. The result of this extensive development effort was the Centrisep Air Cleaner. 3

4 HOW DOES AN ENGINE AIR PARTICLE SEPARATOR WORK The Centriseo Air Cleaner main components consist of the body, vortex generator, scavenge arrangement, and outlet tube (Figure 3). V ORTE% GFNER ATOR BODY S AVENGE AIR ITH CONTAMIN ENT OUTEFT TUBE DIRTY AIRg^ CLEAN AIR is several times the inlet air velocity. These tubes are designed with a small radius of curvature and therefore produce greater radial acceleration for the same tangential velocity. This fact together with a shorter radial distance through which the dirt must travel to reach the tube outer walls makes these inertial separators very efficient. Increasing inlet air velocity or flow tube increases radial acceleration and hence improves separation efficiency. Radial acceleration may also be increased by changing the pitch of the vortex generator. However, this is accomplished with increased pressure drop. FIG. 3 VORTEX TUBE Contaminated air entering the Centrisep Air Cleaner Tube Assembly is given a swirling motion induced by the vortex generator. This swirling motion causes the heavier dirt particle and water droplets to be thrown radially outward by centrifugal force toward the wall of the tube. The contaminant passes through the body of the air cleaner where it is directed to the overboard port and scavenged overboard with a small portion of the inlet air. Clean air at the eye of the vortex generator proceeds straight through the outlet tube. Thus clean air is delivered and dirt is continuously removed, making the air cleaner self-cleaning. The principal of inertial separators is based on the fact that particles dispersed in air have greater mass or inertia than the air carrying them. These particles travel along the air stream lines with a velocity equal to the local air velocity. Thus the velocity of a given particle relative to the air is zero unless a force is applied to the particle. If such a force F is applied, it will accelerate (A) FA particle which is a function of the mass of (M) the particle (A = F g/m). The particle accelerates until the dragforce balances the applied force. Thereafter, the particle moves at a constant velocity called terminal velocity. The terminal radial velocity in a centrifugal field is based on the radial acceleration which is a function of the tangential velocity (VTAN3 and the radius of curvature (RC) (A = VTAN /RC). Centrisep Air Cleaner tubes are designed in such a manner that the tangential velocities imparted by the vortex generator to the air particles The dirt is carried along the walls of the tube, while the air in the center (just aft of the vortex generator) is relatively clean. The dirty air is conducted overboard by a scavenge system. A tube picks up the clean air and it continues on to the engine (Figure 4). o a o J O AIR CLEANER PANEL Q0 o G o 0 0 LIQUID, ; ' o a owl DIRT & o o O G: AIR IN o- a 00J D; 0 ^ ^, a O 0 O p. O a o 0 O v o, 6, Legend a O 1 p{ AIR FLOW.c C, ). 7 p'o SMALL PARTICLES I? o ` a O LARGE PARTICLES o 10 0 LIQUID DROPLETS EJECTOR i o^eed 0 r.a INLET SCAVENGE AIR, LIQUID & DIRT OUT FIG. 4 PANEL ARRANGEMENT These air cleaner tubes (vortex generator tubes) are not much over a half an inch in diameter. In order to get enough airflow, a large number of them are arranged in a box-like panel that can be curved or contoured to any shape required. The interior of this box arrangement provides a place for the dirt that has been removed. To keep the inside of the panel from filling up with dirt, a scavenge system is installed in the low point of the panel. In helicopter applications this scavenge system is usually either an electrically powered scavenge fan or a o o 4

5 pneumatically powered eductor ejector. In the ejector system power is in the form of high pressure air taken from the gas turbine compressor (Figure 5). a AIR CLEANER PANEL ASSEMBLY DIRTY Ain FLOW ELEMENTS (^ y C LII II'Al N Ain To ENGINE Ain FIG. 5 EJECTOR SCAVENGE The ejector is lighter in weight and maintenance free but takes more horsepower to run than an electrically driven scavenge fan. As a rule of thumb about 0.5% of engine air flow is needed for an ejector system. that it cannot be removed. Later IPSs have been designed to be removable. These particle separators, because of their geometry are always less efficient at removing dust than the EAPS. Typical comparisons of AC Coarse test dust removal efficiences would be 85% for an IPS and 92% for an EAPS. This equates to the difference between 15% dust ingestion or only 8% (little more than half the dust). In addition, the 15% contains many more of the erosion-causing-size particles. Independent gas turbine industry tests and U.S. Army tests have shown that if particle separator separation efficiency is increased from 85% to 92%, engine erosion will be reduced to one-tenth of that at 85% in heavy dust environment (Figure 6). 32 2e 24 EFFECT OF AIR CLEANER EFFICIENCY ON GAS TURBINE ENGINE EROSION LIFE 1±_HH_III ENGINE EROSION LIFE INCREASES UP TO 10 TIMES Gas turbine engines are extremely sensitive to inlet air restriction. Because of this high flow, low restriction requirement the self-cleaning inertial type air cleaners are now almost universally used on helicopters. Now that we know how the EAPS works we need to compare the Engine Air Particle Separator (EAPS) vs. Integral Particle Separator (IPS) vs. All Weather Screen (AWS). w20 JI- _w o< ow 1 6 M In ww ZU CD? W 12 VORTEX TUBE SEPARATOR 92% EFFICIENCY DIFFERENCES BETWEEN EAPS, IPS AND AWS The all weather screen (AWS) as installed on the CH-47 Chinook is a wire mesh screen that has 14 squares per inch. The AWS is a relatively inexpensive means for keeping out large FOD objects that could cause foreign object damage (FOD) but it has its limitations. The AWS on CHINOOK is removed during icing conditions. Once ice accumulates and completely covers the AWS then the turbine engine cannot get enough air to sustain itself in flight. The AWS also cannot prevent sand and dust from entering the engine which can, over a very short period of time, lead to the problems previously described. The Integral Particle Separator (IPS) is designed as part of the engine. In the case of the General Electric T700, it is so completely integral with the engine 8 INTEGRAL PARTICLE SEPARATOR 85% EFFICIENCY I I i I i ^ I i EFFICIENCY-x ON A.C. COARSE TEST DUST FIG. 6 EFFICIENCY V ENGINE LIFE Perhaps even more significant is that it is the fine dust that gets through the compressor that causes problems in the engine hot section. By reducing the quantity of dust ingested the hot section 5

6 problems associated with dust are also reduced (Figure 7). engines were also eroded, losing too much power to continue the mission (Figure 8). o, lox COMPARISON OF ENGINE INTEGRAL PARTICLE SEPARATOR AND VORTEX TUBE PANEL SEPARATOR TYPICAL VORTEX TUBE PANEL SEPARATOR TYPICAL INTEGRAL I PARTICLE SEPARATOR Mt. St. Helens erupted in Washington state in 1981, and the CH-47 Chinooks at Ft. Lewis had random hung starts after flying several missions over the volcano. Then there was the 1985 Bright Star desert exercise with U.S. and Egyptian helicopters. Eight (8) helicopters had to abort and the U.S. and the Egyptians again realized how vulnerable helicopter engines are to desert conditions. b 60\' ADDITIONAL DUST REMOVED BY ^.^ VORTEX TUBE SEPARATOR So yn _.._ _.. _-. _ I PARTICLE 5I2E - MICRONS FIG. 7 INTEGRAL V VORTEX TUBE An additional advantage is that the EAPS, when correctly configured relative to the air flow, will continue in operation even in an icing or snow condition. This eliminates the need for expensive, heavy and power thirsty engine inlet anti-ice systems. The engine air particle separator (EAPS) is in most cases considerably less expensive than the IPS. WHY EAPS IS NECESSARY FOR READINESS Remember the Iranian hostage rescue in 1979 leading to the loss of a helicopter and the rest of the fleet of helicopters having to abort the mission? A primary cause was sand from the desert winds clogging the helicopter components. The In Hawaii, in 1989, two CH-47 Chinooks and four T55 engines were grounded because of hung starts. The CH-47s conduct missions near the Island of Hawaii's volcanos. None of these helicopters had EAPS. What if these aircraft had had EAPS? SUMMARY An efficient EAPS will: Increase aircraft availability Reduce defect rates Reduce support costs Increase aircraft capability in icing conditions Maintain lift capability in erosive environments. Helicopters cannot fight in the desert without an efficient engine air particle separator on each engine. The AWS gives no protection from sand and dust and only partial protection from icing. The IPS removes most compressor erosive particles but still allows some compressor erosion and clogging, fouling and contamination of the engine hot section. The EAPS eliminates compressor erosion and reduces dust ingestion to the turbine to a level at which it no longer causes problems. It also works in an icing and snow environment without the aid of anti-icing devices such as hot air or electricity. With all the improvements that can be made to aircraft, we must constantly remember that an aircraft can fly only as far as its engines; then we realize the importance of engine air particle separators. ENGINE LIFE FIG 8 USEFUL POWER 6

7 BIBLIOGRAPHY Maj. Sheenan, J., 1978 Update of CH-47C/ T55-L-11 Fine Mesh Screen Kits, DA 180th Aviation Co., 19 December "Air Cleaners," U.S. Army Tank-Automotive Report, June Weston and Schilke, 1980 "Volcanic Ash Hazards," Federal Aviation Administration, "Army Test New PLM Particle Separator," Washington, D.C. 21 May Defense News, August Bell, P.G., 1987 AVCO Lycoming Engines in Chinook Aircraft - Trial Report on Engine Protection Against Erosion Due to Sand/ Grit by Use of Engine Air Particle Separators (EAPS) dated 3 April 87 by PG Ball. Boeing Vertol Co., CH-47 Engine Air Particle Separator (EAPS), Ground & Flight Test Report, dated 27 October "Centrisep Servicing," Aviation Mechanics Journal, March Roach, C., 1983 "Centrisep Air Cleaner Systems," Field Service Report FSR 10, Pall Land and Marine Corporation. "Clean Engine Air," Aviation Week & Space Technology, September 1980 Smith, W., 1986 "Depot Introduces New Filter," Aircraftsman dated 17 July Col. Shallcrops, G. "Desert Surprise!," Army Aviation, 31 December Lemmo, A., 1988 "Evaluation of the Turbodyne II Self Cleaning Air Filtration System on a Known Military Engine," U.S. Army Tank- Automotive Command RD&E Center Technical Report No , dated May Pettigrew, J., 1989 Field Engineering Trip Report - T55 Engine Hot Restart Problems in Hawaii, April 18-28, Blount, A. "Firms Filters Keep Army Choppers Running Clean, "Pasco Times. Collie, R.L., 1980 "Hazards Associated with Operating in Areas Where Volcanic Ash is Encountered," Federal Aviation Administration, Washington, D.C., dated 19 May "Huey Possibilities," Defense Helicopter World, June-July "Manufacturers Issue Volcano Advisories," Aviation Week & Space Technology, dated 9 June Watkins, J., 1981 "Oil Proves a Mixed Blessing," U.S. News, 12 June Cardinate, R., T55-L-11ASA, T55-L-11D and T55-L-712 Reliability Evaluation for the Period 1 January December 1987, RCM Report No Broad, W.J., 1981 "Threat to U.S. Air Power: The Dust Factor," Science, Vol. 213, 25 September