Hydraulic Starting of Marine Gas Turbine Engines

Transcription

1 68-GT-29 The Society shall not be responsible for statements or opinions advanced in papers or in discussion at meetings of the Society or of its Divisions or Sections, or printed in its publications. $1.50 PER COPY 750 TO ASME MEMBERS Discussion is printed only if the paper is published in an ASME journal or Proceedings. Released for general publication upon presentation Copyright 1968 by ASME Hydraulic Starting of Marine Gas Turbine Engines L C. JENNINGS Chief Engineer, Aircraft Equipment, New York Air Brake Company, Unit of General Signal Corporation, Watertown, N. Y. E. V. MISULIS Senior Applications Engineer, Aircraft Equipment, New York Air Brake Company, Unit of General Signal Corporation, Watertown, N. Y. Hydraulic starting is currently being used on several major ships employing gas turbine power. The system designs, as well as the basic principles of hydraulic starting, are discussed in this paper. Contributed by the Gas Turbine Division of The American Society of Mechanical Engineers for presentation at the Gas Turbine Conference & Products Show, Washington, D. C., March 17-21, Manuscript received at ASME Headquarters, January 4, 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 Hydraulic Starting of Marine Gas Turbine Engines L. C. JENNINGS E. V. MISULIS INTRODUCTION The inherent adaptability of hydraulic starting for gas turbine engines has long been recognized and accepted by the aircraft industry. In that field, it has provided self-contained starting systems which do not necessitate use of ground support. It has facilitated low temperature starting capability and also utility by combining both starting and pumping functions in a single unit. This diversification has resulted from integration of the starting equipment with the aircraft system, rather than recognizing it only as a necessary accessory. Now, since the advent of gas turbine usage in this country for marine applications, uniquely new concepts of requirements are being revealed for starting equipment. The basic components have remained unchanged; however, their adoption is distinctly different in this field. Also, many heretofore unimportant capabilities of hydraulic starting are now being disclosed as very desirable attributes. Since its recent introduction into marine applications, hydraulic starting is being used on every major U. S. ship employing gas turbine engines. A listing of these ships is shown in Table 1. SYSTEM DESIGN A basic marine system for hydraulic starting consists of a hydraulic power source (pump), an engine starter (motor), and associated components and controls. A typical single engine starting system such as used for the PG Gunboat applications is shown in Fig.l. The system utilizes two pumps, either of which is capable of starting the single engine. This dual pump design is necessary since the ship's diesel generator sets, which drive these pumps, are usually operated alternately during service. Consequently, a single set cannot be always depended on for engine starting power. Use of the redundant pump also provides the secondary benefit of increased system reliability. In order to relieve load from the pump during normal sustained operation of the generator set, the pump is either declutched or depressurized. Both designs have proven to be equally efficient in providing long reliable life. It should be noted that a closed system is utilized wherein return fluid from the starter is routed directly to the inlet of the pump, rather than the reservoir. A charge pump integrated in Table 1 Current Use of Hydraulic Starting for Marine Applications ' High Endurance Cutter U. S. Coast Guard Pratt & Whitney Avondale Shipyards. Inc. WPG Class FT-4 (2) ' Patrol Gunboat U. S. Navy PG Class General Electric LM-1500 (1) Tacoma Boatbuilding, Inc. also Peterson Builders, Inc. 3. Destroyer - DDH-280 Class Royal Canadian Navy Pratt & Whitney FT-4 (2) Pratt & Whitney FT-12 United Aircraft of Canada, Ltd. (Propulsion Machinery) Boat Builder Not Selected 4. Roll-On/Roll-Off Cargo Ship U. S. Navy Military Pratt & Whitney Sun Shipbuilding & Sea Transportation FT-4 (2) Dry Dock Co. Service 2

3 the variable displacement pump provides makeup oil into the closed circuit to replace the oil removed through the case cooling flow lines of the starter and pumps. This system eliminates the need for a pressurized reservoir or a relatively large size charge pump to provide the entire system flow demand. The flow regulator prevents overspeed of the starter in the event both pumps should be on the line simultaneously. For applications requiring simultaneous starting of multiengines, regulators are also used as flow dividers. Fluid in accordance with MIL-L is generally used in these systems. This fluid is used in the marine field for other hydraulic requirements and, consequently, is preferred because of its availability. It is a petroleum base fluid and available in three viscosities. Type 2075T-H, having characteristics similar to those of Type A Transmission Fluid, is recommended for systems having a fluid temperature range between +20 F and +200 F. The flexibility of the basic starting system design is shown in Fig.2. A system sililar to this is utilized on the WPG Class Coast Guard Cutters. In this system, the same hydraulic power source (pumps) are utilized; however, two or more engines may be started sequentially without a significant change to the basic system. An electric motor for driving a pump, in lieu of a diesel generator set, has been effectively utilized in some starting system installations. The design provides flexibility for locating the hydraulic power source at any desired location. A 100-hp motor is required to drive a variable displacement pump capable of starting an engine such as the Pratt & Whitney FT-4. Although simultaneous starting of engines has not been a requirement for marine applications, it can be provided with the basic system design described. Appropriate provisions for pump hydraulic horsepower requirements must be made. Also, line sizes and components require the capability for transmitting the increased hydraulic flow. A photograph of a hydraulic starter for the Pratt & Whitney FT-4 engines appears as Fig.3. For a size comparison, it is shown with an aircraft starter as used for the General Electric T-58 engines. The FT-4 starter provides 280 lb-ft of torque in a 4000 psi system as compared to 12 lb-ft for the T-58 starter. It is of variable Fig.l Basic single engine starting system DIESEL- GENERATOR SET PUMP PUMP DRIVE FLOW REGULATOR START VALVE FILTER DIESEL- GENERATOR SET PUMP STARTER GAS TURBINE HIGH PRESSURE 44. RELIEF VALVE HEAT EXCHANGER FILTER LOW PRESSURE RELIEF VALVE RESERVOIR 11*,J I PRESSURE (Transmission) M RETURN (Transmission) CHARGE CASE & SYSTEM RETURN === 3

4 DIESEL- GENERATOR SET PUMP STARTER GAS TURBINE PUMP DRIVE FLOW REGULATOR FILTER START VALVE DIESEL- GENERATOR SET PUMP FILTER ---"" U NM At RELIEF VALVE HIGH PRESSURE 4 HEAT EXCHANGER STARTER II LOW PRESSURE RELIEF VALVE GAS TURBINE RESERVOIR PRESSURE (Transmission) RETURN (Transmission) 1.1== CHARGE CASE 8 SYSTEM RETURN === Fig.2 Basic multiengine starting system Fig.3 Starters for FT-4 and T-58 engines displacement, pressure compensated configuration having a displacement of 6.0 cu in./rev. The starter weight is 79 lb with an overhung moment of 580 lb-in. The pumping and motoring units are of conventional design for high pressure service. A typical cross section of a starter is shown in Fig.4. It is a pressure compensated unit of the rotating cylinder block design. The pistons are forced against a variable cam angle which provides a proportionate change in pump displacement and torque output. The pumping unit is similar in design with the exception that an overrunning clutch is not utilized and servo control functions are slightly different. The overrunning clutch consists of a caged sprag type assembly supported in antifriction bearings. This allows the engine to accelerate from starter cutout speed up to rated engine speed and operate at rated speed without rotation of the main starter parts. Lubrication is accomplished by flooding the clutch assembly with oil from the starter case which is vented to the nonpressurized reservoir. 4

5 SERVO CONTROL FOR POSITIONING CAM ROTATING CYL. BLOCK ASSY. DRIVE COUPLING 'ATT+ Y- 1-7(71' T TUBE CONNECTIONS OVERRUNNING CLUTCH Fig.4 Cross section of hydraulic starter The hydraulic reservoir and valves for these systems are provided in preassembled modules. This facilitates final installation since only connections are required from the module to the starters and pumps. A typical module as currently used for the Coast Guard and PG Gunboat Applications is shown in Fig.5. In addition to the hydraulic components and electrical relay panel, it includes an oil to water heat exchanger, where required. SYSTEM CONTROL The starting system is controlled automatically by integration with the gas turbine controls. In order to initiate a start, it is only necessary to close an electrical control circuit at the appropriate control station. This signal actuates the pump control to allow a buildup of system pressure and simultaneously opens the hydraulic start valve to the engine which is being started. A pressure switch at the starter provides a holding circuit for maintaining full pump output until starter cutout speed is attained. At this speed, the pressure switch senses the reduction of pressure and automatically opens the circuit to the pump control and start valve. This stops rotation of the motoring portion of the starter allowing the engine drive shaft to automatically overrun the starter. During water washing cycles when salt deposits are washed from the gas turbine, the complete starting cycle will not be performed automatically. The engine is not ignited during this mode of operation; consequently, it will accelerate only to a speed where engine resisting and starter output torques are in balance. At this speed, sustained motoring will occur. Upon conclusion of the washing requirements, the electrical circuit to the start valves and pump control 5

6 STARTER TORQUE C, Cd 0 +ENGINE RESISTING TORQUE 0 STARTER SPEED Fig.7 Torque relationship of fixed displacement starter and engine requirements Fig.5 Hydraulic component module must be opened manually to return the system to a standby attitude. This procedure would also be used to terminate a starting cycle in an emergency PERFORMANCE The primary consideration in designing an effective starting system is the matching of the torque-speed engine requirement curve and the output torque capability of the starter. This is ideally accomplished with the torque characteristic of a hydraulic motor. The torque output can be made to remain constant when resisting torque is high and can be reduced as the resisting torque decreases. In the simplest form, a fixed displacement starter provides the characteristics shown in Fig.6. The starter torque remains constant as the speed increases. It follows that hydraulic horsepower required to drive the starter consequently increases directly with an increase in speed. Adding a hypothetical engine torque require- 0 0 STARTER SPEED STARTER SPEED Fig.6 Performance characteristics of a fixed displacement hydraulic starter 6

7 FIXED DISPLACEMENT STARTER TORQUE VARIABLE DISPLACEMENT STARTER TORQUE FIXED DISPLACEMENT STARTER / VARIABLE DISPLACEMENT it STARTER 0 0 STARTER SPEED STARTER SPEED Fig.8 Performance of variable displacement hydraulic starter ment, the composite curve now appears as shown in Fig.7. It can be seen that a fixed displacement starter provides considerable more torque than required as the engine approaches cutout speed. Any superfluous torque, however, results in the penalty for increased horsepower requirements to drive the pump. The pump, as well as all components and lines, must be larger to handle the rela tively high flow rates. The ideal objective for attaining minimum pump horsepower and components is achieved by simply utilizing a variable displacement starter. The starter output characteristics are fitted to the engine requirements as shown in Fig.8. From this, it can be seen that by automatically reducing the displacement of the starter just above lightoff speed, a constant horsepower output is provided through the balance of the start cycle. In addition, the starter closely follows the engine requirement as both accelerate to the starter cutout point. Reliability and starter life are greatly extended by reducing the starter displacement as speed increases. ADAPTABILITY OF HYDRAULIC STARTING The specialized design of each ship propulsion system has required individual application considerations to assure best providing the system performance desired by the ship builder. However, an extraction of the main considerations for hydraulic starting, many of which are unique, follows: 1 Use of existing on-board prime movers such as diesel generators to provide motive power for starting. 2 Increase reliability economically by the use of a secondary prime mover for redundancy. 3 Capability for prolonged engine rotation during which salt deposits are water-washed from the engine. 4 Unlimited re-engagement of the starter for restart capability at any engine speed. This is directed at providing fast engine restart capability in the event of a misfire. 5 Flexibility for transferring power from essentially any desired location on the ship to the gas turbine. 6 Long life of components and minimum onboard repair parts. Longevity testing has demonstrated absence of deterioration after 10,000 starting cycles. 7 Fully automatic starting controls which are integrated with the gas turbine controls. 8 Design of the system components to the Hi Shock requirements of Specification MIL-S Absence of external lubrication. Problem areas in using hydraulic starting equipment have been primarily confined to the initial period of installation. The major problem in this regard has been fluid contamination in the first series of ships. Permanently installed piping within the hull which utilizes welded connections and numerous bends has not been conducive to system cleanliness. Stringent flushing procedures for newly installed piping and the indoctrination 7

8 of personnel to proper servicing has been effective in eliminating this as a problem. The flushing procedure also resolved early difficulties with system bleeding. A potential problem during the initial design and installation of the system is that of maintaining pressure loss within acceptable limits. Consideration of tube bends and length of lines must be compensated for by line diameter. Since relatively long lines are normally used, the economics do not permit the use of excessive overdesign or margin in these line sizes. Consequently, this requires particular consideration for proper sizing. SUMMARY Hydraulic starting is proving to be very adaptable for marine applications. Distinctive advantages exist; however, it must be recognized that considerable coordination between ship builder, engine manufacturer and starting system supplier is required in order that it may be effectively utilized. Desired performance is affected, for example, by connecting line design, orientation of components, cleanliness, compatible system components and controls. The system consequently is not one that can easily be fashioned in a "piecemeal" manner. The equipment has met all expectations during in-plant testing and field service. It is being service proven in several diversified applications which include the PG Class Patrol Gunboats in the Far East Combat Zone. Hydraulic starting is operational on the WPG Class High Endurance U. S. Coast Guard Cutters. It underwent sea trials in the latter part of 1967 on the Admiral William M. Callaghan roll-on/roll-off cargo ship. Starting systems are also being prepared for the Royal Canadian Navy DDH Class destroyers. As the advantages of gas turbines are more fully exploited, advances both in engine design and adaptability will undoubtedly be made. Paralleling this is the recognition by hydraulic starting system manufacturers that new development must complement future generations of marine gas turbines. Starters and pumps operating at higher speeds, higher pressures, and of larger displacements are currently being developed to assist in making this forecast a reality. 8