Pneumatic Starting Systems

Transcription

1 $1.50 PER COPY 75C TO ASME MEMBERS The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its 67-GT-15 Divisions or Sections, or printed in its publications. Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation Copyright 1967 by ASME Pneumatic Starting Systems ROBERT J. VON FLUE Senior Project Engineer, Turbine Starters, AiResearch Manufacturing Division, A Division of The Garrett Corporation, Phoenix, Ariz. Pneumatic starting systems for gas turbine engines are discussed. A general description of various energy sources, system components and arrangements is included. Systems are reviewed from the standpoint of energy utilization, versatility and reliability. It is concluded that the auxiliary gas turbine compressor/air turbine starter combination is the optimum arrangement, and, without a doubt, it will continue to be the primary starting means in gas turbine engine applications. Contributed by the Gas Turbine Division for presentation at the Gas Turbine Conference and Products Show, Houston, Tex., March 5-9, 1967, of The American Society of Mechanical Engineers. Manuscript received at ASME Headquarters, December 20, Copies will be available until January 1, THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS, UNITED ENGINEERING CENTER, 345 EAST 47th STREET, NEW YORK, N.Y

2 Pneumatic Starting Systems ROBERT J. VON FLUE It is intended that this discussion provide an understanding of pneumatic starting systems when employed in gas turbine engine applications. Possible system arrangements and actual operating experience will display the versatility and reliability of this type of system. The basic systems may be energized with high or low-pressure gas depending on the means of energy utilization. Generally speaking, highpressure gas is available from storage vessels at pressures from 250 to 3000 psi. The output of a gas turbine compressor at the usual pressures of 30 to 6o psi is representative of low-pressure gas. STARTER METHODS Two methods are applicable for utilizing pneumatics to rotate the gas turbine engine for starting -- the impingement system and pneumatic (air turbine) starters. The impingement system is comprised of a duct and a set of nozzles which directs the air supply on either the compressor or turbine for rotation of the engine to be started. The impingement system is generally restricted to low-pressure operation. The air turbine starter, on the other hand, produces rotation by converting the pneumatic energy and mechanically rotating the gas turbine engine rotor. This type of starter can be designed to use either high or low-pressure air or both. PNEUMATIC SYSTEMS AND THEIR OPERATION The gases used for pneumatic starting systems are numerous. Air, natural gas, nitrogen, and steam are presently in use. Obviously, only air is desirable for use in impingement systems. Steam, nitrogen, and natural gas can be applied in conjunction with turbine starters, although their use is not common. Steam has been employed in isolated instances in Naval applications; nitrogen has been used interchangeably in high-pres sure air (cold gas) systems because of its commercial availability; natural gas is being used in oil-field applications at remote pumping stations as a matter of convenience. Air, by far, is the most commonly employed gas in pneumatic systems. It is also true that most of the larger aircraft gas turbine engines in service today, including the CJ805 (J79), JT3 (J57), JT4 (J75), JT8 (J52), TF30 and 501 (T-56), utilize pneumatic starting systems. In light of this, subsequent discussion will concern itself with systems possible with various air sources when adapted for impingement or turbine starting of the aircraft gas turbine engine. Table 1 summarizes the pneumatic starting methods used commercially and in the majority of military aircraft. It will be noted that a pneumatic starting system or some variation is used on every large jet-powered commercial airliner in the United States. It is most significant that each application listed has the capability of utilizing a gas turbine compressor available as ground equipment. A typical gas turbine compressor (GTC) is pictured in Fig.l. This type of unit can be adapted as ground equipment or in onboard installations. The particular unit shown is the Ai- Research GTC85. It is an electrically started self-sufficient unit with a two-stage radial compressor, and the turbine is driven by the exhaust products of a single, tangentially located combustor. Air is bled from the compressor section and supplied to the starter from this unit at a pressure ratio of approximately 3 to 1 and a temperature of 350 F. The unit is approximately 38 in. long, 18 in. dia, and weighs 275 lb. An air turbine starter is shown in Fig.2. It consists basically of a turbine section, gear system, and engaging-disengaging clutch. The gear system of the starter converts the high speed, low torque of its turbine to the low speed, high torque required by the engine. The clutch disengages the starter at the end of a start cycle and automatically re-engages during engine rundown to permit subsequent starts. This unit is generally about 10 in. long, 8 in. dia, and weighs 24 lb. Figs. 3, 4 and 5 display the most common pneumatic starting system arrangements. In Fig. 3, the engine-mounted air turbine starter (ATS) is energized by relatively low-pressure air supplied by an auxiliary gas turbine compressor unit or by some other controlled air source. As shown, starts can be accomplished by a gas turbine compressor available either on the ground or mounted aboard the aircraft. In the case of multi-engine aircraft, it is also possible to perform starts 2

3 Table 1 Pneumatic Starting System Application No. of Can Utilize GTC Available as Interengine Aircraft Engines Ground GTC Airborne Equipment Bleed Utilized Remarks Commercial a. Boeing x x See Note x x See Note x x x x x x x x x b. Douglas - DC x x See Note 2 DC-9 2 x x x c. General Dynamics x x x x x x x d. Grumman - Gulf stream II 2 x x x e. Lockheed - L188 4 x x See Note 3 Military a. Boeing - B52A/H 4 x x KC - 135A 4 x x b. Douglas - TA-3B 2 x A4E 1 x - B66 2 x C133A/B 4 x x x F6A 1 x c. General Dynamics - B58 4 x x d. Grumman - F111B 2 x x E2A 2 x - x e. Lockheed - C x x x C-141A 4 x x x F x - - P3A 4 x x See Note 3 C5A 4 x x x f. LTV - F8U 1 A7A 1 g. McDonnell - F4B 2 x See Note 4 F4C 2 x F101A/C 2 x x h. North American - A5A 1 x See Note 4 F100 1 x J. Northrop - F5 2 x See Note 4 j. Republic - F105 1 NOTES: 1. Source of Information: Aerospace Information Report AS912, published by the Society of Automotive Engineers. 2. High pressure pneumatic system is available on some models. 3. Some conversions have been made to incorporate an airborne GTC. 4. Starting is accomplished via impingement start arrangement. 3

4 Fig. 1 AiResearch gas turbine compressor (GTC85) with air bled from the main engine, which is started first. The same approaches are applicable on either military or commercial aircraft. The military GTC is generally incorporated in a ground cart, the most common units in Air Force inventory being the MA-1A and M32-A60. Navy carrier applications employ gas turbine compressor units which are mounted in transportable pods, low-silhouette tractors, or used as permanent installations paralleled to a hydrant system. The wide usage of gas turbine compressors as ground equipment is readily explained by the large number of single-engine aircraft in military service. It is also noted in Table 1 that most of the fourengine military aircraft use the auxiliary gas turbine onboard the aircraft. These include the C-130, C-141, and C5A. The Navy's P3A is being modified to incorporate the same type of installation. Auxiliary gas turbine power units (APU) which supply air for starting can also supply electrical power and air conditioning. Here again, they are becoming the rule on the latest commercial aircraft, such as the Boeing 727, 737 and 747. Air turbine starters are being used exclusively in that they are an extremely efficient means of converting the pneumatic energy to shaft power. AiResearch starters, for example, have peak efficiencies between 70 and 75 percent, which provides the major element in requirements for an optimized pneumatic starting system, particularly when designing for an airborne installation. IMPINGEMENT SYSTEM LIMITATIONS As noted in Fig.4, an impingement arrangement can be utilized interchangeably with the air turbine starter. This method no doubt appears to be an excellent way to rotate the engine and eliminate the need for some moving parts, i.e., the air turbine starter. The impingement arrangement has some shortcomings however: 1 Starting characteristics of the engine must be predicted to permit incorporation of the impingement nozzle in the original design. 2 The position of the nozzle with respect to the engine rotor is critical and, therefore, design freedom is limited. 3 Weight and cost that are attributed to the impingement nozzle and ducting are an appreciable percentage of the weight and cost of an air turbine starter. 4 Impingement starting requires three to five times the pneumatic energy required to start the engine with an air turbine starter. The most significant of these shortcomings is the amount of pneumatic energy required. In fact, a large gas turbine compressor unit, the AiResearch Model GTCP105, had to be built to provide satisfactory starts for impingement starting of the 4

5 GROUND GAS TURBINE COMPRESSOR CONNECTION STARTER CHECK CONTROL gilli NBOARDCAS Mr61 TURBINE COMPRESSOR ENGINE BLEED AIR?4 I N AIR TURBINE STARTER SHAFT POWER MAIN ENGINE Fig. 3 Schematic: low-pressure pneumatic starting system supplying an air turbine starter GROUND GAS TURBINE COMPRESSOR CONNECTION r-, CHECK CONTROL Fig. 2 AiResearch air turbine starter (ATS100) ONBOARD GAS TURBINE COMPRESSOR ENGINE BLEED AIR AIR TO IMPINGEMENT START ARRANGEMENTS ON MAIN ENGINES A5A and F4B aircraft. Had these aircraft utilized air turbine starters, the smaller and lighter AiResearch Model GTC85 would have started their J79 engines. A system weight savings of over 100 lb would result from using the smaller GTC. It is also interesting that the GTCP105 provides sufficient output to achieve two simultaneous starts of the J79 engines on the B58 aircraft with air turbine starters. The limitations of impingement starting have become quite apparent and this type of arrangement is being avoided in the newer engines. MAIN ENGINE Fig. 4 Schematic: low-pressure pneumatic starting system supplying an air impingement start arrangement HIGH PRESSURE COMPRESSOR AND RELATED EQUIPMENT STORED AIR REGULATING AND SHUTOFF AIR TURBINE STARTER HIGH PRESSURE AIR INLET SHAFT POWER OPERATIONAL EXPERIENCE Fig.5 depicts a high-pressure pneumatic starting system that is used on two commercial aircraft, the Boeing 720B and the stretched DC-8. Besides requiring a dual-purpose starter (capable of utilizing both high and low-pressure air), the related equipment has demonstrated relatively low reliability as compared to that being experienced with the low-pressure pneumatic systems. Such a system has merit, however, and is expected to find usage in isolated instances when dictated by a particular requirement. The key is system reliability. Accurate records have been kept on commercial equipment, and LOW PRESSURE AIR INLET (OPTIONAL) Fig. 5 Schematic: high-pressure pneumatic starting system these records are drawn upon to verify the reliability now being experienced with the low-pressure pneumatic starting system of the type depicted in Fig.3. Table 2 summarizes the operational experience on air turbine starters in commercial service. It will be noted that this 5

6 Table 2 Summary of Operational Experience on AiResearch Commercial Air Turbine Starters Through October 31, 1966 Total Number of Airplane Total Total Total No. Rate of Airplane Airplanes Flight Airplane Starter of Accumulation TypeIn Service Hours Departures Hours Startsper Month Hours Starts Boeing 707/ ,194,000 2,950,000 28,776,000 11,800, , ,000 Series Douglas DC ,096,000 1,606,000 16,384,000 6,424, , ,000 Series Convair 880/ ,137, ,000 4,548,000 2,896,000 87,300 60,000 Lockheed Electra 151 2,637,000 2,833,000 10,548,000 11,332, , ,000 Boeing ,059, ,000 3,177,000 2,886, , ,000 Douglas DC ,000 63, , ,600 23,400 27,000 TOTALS 1,383 16,190,000 9,138,300 63,567,000 35,464,600 1,315, ,000 summary involves experience with AiResearch air is demonstrating a MTBM (Mean Time Between Mainturbine starters up to the latter part of tenance Action) of 2000 hr or a MCBM (Mean Cycles This in fact is very representative of all com- Between Maintenance Action) of 4000 APU starts. mercial airline service, since AiResearch enjoys Over the period of some eight years, during the position of being the sole supplier of all the which the actual start history on Table 2 was air turbine starters required by the airlines in accumulated, the same type of coordination with the United States. Additional experience continues the user which built the APU reliability to what to accumulate at a rate well in excess of the it is today has brought about an even more impres- 26,000 starts per day occurring in late sive level of reliability for AiResearch air tur- Today, starts are occurring at a rate approaching bine starters and their related control valves. 29,000 per day, or 1200 starts every hour. Each of these components is exhibiting a MCBF As of the latter part of 1966, AiResearch had (Mean Cycles Between Failure) of approximately delivered a total of 12,500 gas turbine compressor 6000 starts. units for commercial and military use. Due to the varied usage, an estimate of operational ex- CONCLUSION perience related to the pneumatic start system is best made based on the summary in Table 2. Two assumptions are made which it is felt will yield Using this experience, the reliability of the a conservative estimate of run time to which re- low-pressure pneumatic starting system is deterliability indicators can be applied. First, con- mined by summing the failure rates of each considering an average of ground cart operating time ponent. For example, in the case of a multiand onboard usage, it is reasonable to assume engine aircraft with the onboard APU installation that the APU operates approximately one-half hour wherein a starter control valve is used, a syseach time it is started. Secondly, it is assumed tem MCBF of 1700 starts results for the initial that four main engine starts are made for each start (1/ / /6000 = 1/system MCBF). APU start (even in view of the fact that the 727 Further, it is evident that a MCBF of 3000 starts has three engines and the DC-9 has two). With is experienced on subsequent starts of a multithese assumptions it is calculated that auxiliary engine aircraft. In other words, one might exgas turbines in commercial pneumatic starting pect a starting system malfunction once in 800 systems have accumulated over 4,500,000 hr. One flights when considering a Boeing 727 aircraft of the most representative APU installations in with an average flight time of one hour. commercial service is that utilized by the Thus, from the standpoint of system efficiency, Boeing 727 aircraft. Records show that this APU versatility and reliability, it can be seen that 6

7 a pneumatic starting system employing an auxiliary gas turbine and an air turbine starter has proven itself the most desirable system for starting large gas turbine engines. All of this is being obtained in a system which today is approaching a cost per start of less than two dollars even when considering acquisition, operating, maintenance and overhaul costs. 1 1 I. ("Ike") Kennedy, "Braniff Cuts Starting Costs With AiResearch 'On Board' APU," Air Transport World, May