ASME TURBO EXPO 2001
June 4–7, 2001, New Orleans, Louisiana, USA
U.S. NAVY GAS TURBINE ELECTRIC START SYSTEM
Glenn McAndrews, GE Marine Engines Rick Worth, NSWCCD-SSES Robert Smith, Ultra Electronics Power Systems
Today’s marine gas turbines are started using pneumatic or hydraulic systems. The weight, space, and reliability drawbacks of these systems is well known to ship systems designers who increasingly look to new technology for better solutions. It is not surprising then that the push towards an all-electric ship platform would eventually prompt the development of a suitable electric motor to replace fluid based systems. Lately, research efforts sponsored by the U.S. Navy have intensified to do exactly that. This paper will discuss the background and current efforts to introduce an electric start system for future marine gas turbines.
Since the first days of marine gas turbines engines, the starting systems of those engines have relied on fluid means. This is not surprising since the vast majority of the world’s gas turbines use fluid power for starting. Invariably, marine engines are cousins of some industrial or aircraft engine. It follows therefore that marine engines would adopt the same starting systems used by the “parent” engine. Indeed, the air turbine starter, designed for low weight in aircraft applications, has become the predominant starter in marine gas turbine applications. It is interesting to note however, that the former Soviet Union has employed electric starters in their surface ship gas turbine applications. Unfortunately no details are available at this writing.
Pneumatic systems used by aircraft engines and hydraulic systems used by industrial engines are quite satisfactory for those applications, although there is continued research in the area of integral electrical starter-generators for aircraft engines (1). Neither system is optimal for shipboard gas turbine use however. The comparison of hydraulic versus pneumatic starting systems is well documented in the literature (2). That comparison will serve as the foundation for identifying the
advantages of an electric start system in a later section. First a description of the system is warranted.
ELECTRIC START SYSTEM (ESS)
The Navy’s ESS is comprised of two sub-systems, namely the electric start motor (ESM) and an electronic start controller (ESC). This paper will concentrate on the ESM because of its direct impact on the gas turbine. A general outline drawing of the ESM is shown in Figure 1.
For reference, pneumatic and hydraulic starters are shown in Figures 1a and 1b respectively.
Figure 1a – Pneumatic Starter
Figure 1b – Hydraulic Starter
Without a production unit in place today, the ESM will be described by looking at the development requirements that guide the design effort. Foremost among the requirements is torque output and power. To maximize the use of an ESM, consideration must be given to the broad range of naval gas turbines, both current and future models. The ESM should be designed for the highest expected load, yet not be overly heavy or overly voluminous. The goal here is to provide a unit small enough, that it can be mounted on engine using the conventional cantilevered approach.
The Navy’s specification requires a 165 horsepower (123 kW) motor capable of a maximum torque limit of 500 lb-ft (678 Nm). How does this compare against the performance of a pneumatic or hydraulic starter? Using the GE LM2500 as an example, an engine’s starting requirements are conveyed in a graph of unbalanced torque versus speed as shown in Figure 2. This torque is commonly labeled “pumping torque” because work is being expended to not only overcome inertia and friction but also pump air through the compressor.
The output characteristics of pneumatic and hydraulic starters are also expressed as a function of torque and speed. Again using the LM2500 hardware as an example, the output characteristics for a pneumatic starter and hydraulic starter are shown in Figures 3 and 4 respectively. Where the output curves of Figures 3 and 4 intersect the load curve of Figure 2 is where steady state operation can be expected. Evaluating a case where
the inlet air temperature is 59 degF (15 degC), the steady state speed is approximately 2200 rpm. Indeed, this speed is exhibited in actual practice. The steady state torque is approximately 250 lb-ft (339 Nm). Using the formula:
Torque (lb-ft) = Power (hp) x 5250
Where torque = 250 lb-ft and speed = 2200 rpm A power of 105 hp (78 kW) is calculated. As a check, power can also be calculated from the formula,
Power = mass flow x change in enthalpy
using the properties of air as it changes state from the starter inlet to the starter outlet. The starter airflow at an ambient temperature of 59 degF (15 degC) and a regulated inlet pressure of 53 psia (3.73 kg/cm2) is approximately 190 lbm./min (86 kg/min). The air turbine starter exhausts to ambient pressure resulting in a calculated 168 isentropic horsepower (125 kW). The starter manufacturer indicates the unit is approximately 65% efficient resulting in a delivered 109 horsepower; (81.3 kW) approximately equal to the previously calculated horsepower from the torque-speed relationship.
Figure 2 – Typical Gas Turbine Starting Torque (CIT = Compressor Inlet Temperature)
Figure 3 – Pneumatic Starter Output
Figure 4 – Hydraulic Starter Output
The torque curve of the ESM will be similar to the torque curve of the hydraulic starter; flat throughout the speed range of unfired acceleration. In fact, in the speed range up to engine ignition, the torque can be limited to a constant value by the ESC. The final torque-speed profile will reflect whatever control strategy the engine manufacturer requires. For example the ESC may be called upon to produce a specified timed rate of change of core speed.
The Navy has specified that the maximum size of the motor is 9.65 inches (245 mm) in diameter and 21 inches (533 mm) long. This footprint is easily accommodated by most gas
turbines. Surprisingly, there is no weight requirement. It is interesting to note that a solid iron object with these dimensions would weigh 435 pounds (197 kgs), suggesting a worst case upper limit. Fortunately, an early ESM prototype, designed for higher loads, weighed a mere 167 pounds (75.8 kgs). It is necessary to consider both weight and overhang moment in order to gage the supportive capability of the engine’s gearbox. The gearbox mounting capability should not only be a static evaluation but also an assessment of the mount pad’s capability during a shock event.
Again using the LM2500 equipment as a benchmark, the pneumatic and hydraulic starters weigh 52 and 92 pounds respectively (23.6 and 41.7 kgs). A shock load of 25g was used to evaluate the suitability of the hydraulic starters on military applications. The analysis examined stud pullout loads and gearbox housing parent metal (aluminum) bending stresses. Results show that the starter weight could more than double and adequate design margins would remain. It is expected that a final production ESM will weigh approximately 120 pounds (54.4 kgs). Therefore, provided the ESM itself is shock qualified, the installed ESM will satisfy shock requirements, if the study of shock qualification for the LM2500 hydraulic starter is a good harbinger.
The ESM is a permanent magnet, eight-pole rotor design. It relies on Samarium Cobalt material boasting energy levels of 32 MGOe. Because the unit achieves high power density, the thermal design is critical. Adequate steady-state temperatures are achieved by a careful selection of materials and cooling schemes. To lower armature winding current density, vanadium permendur stator and rotor laminations are being used. Cooling is accomplished through a combination of natural convection from the aluminum housing, and cooling airflow around housing ducts and winding end turns.
The engine’s start cycle should not be considered the worst duty cycle for design purposes. Gas turbines require motoring for water-washing or cool-down following an emergency shutdown. In the case of the LM2500, it is recommended to motor the engine for 5 minutes at full starting speeds (2200 rpm) or seven minutes at reduced water-wash speeds (1500 rpm). The authors have reviewed a pending gas turbine specification for the LHD8 program requiring 10 minutes continuous motoring. It is expected that the ESM will have a steady state temperature of less than 300 degF (149 degC) during these steady-state conditions (electric starting motors are not being considered for LHD8).
Other features of the ESM include a sprag-type over-running clutch lubricated by Mil-L-23699 lubricating oil. It will be self-contained splash-type similar to today’s pneumatic starters used in military service. The starter output shaft will utilize a shear section to protect the gearbox in the event of over-torque.
MERITS OF ESS
Shown in Table 1 is a comparison of pneumatic, hydraulic, and electric start systems. The table is derived from a similar
table found in the literature (2). The details of that comparison, evaluating available production hardware, will be repeated here to the extent that the merits of an ESS can be introduced. Since a production ESS is not yet available, a qualitative assessment is offered for some of the listed attributes.
It is anticipated that the ESM will weigh approximately 54.4 kgs and the ESC will weigh another 150 kgs. Compare that to 2777 kgs for a typical hydraulic start system and 9310 kgs for a typical pneumatic system (2) in order to appreciate one of the more promising benefits of an ESS.
Table 1 – Starter Evaluation by Type
Pneumatic Hydraulic Electric
Weight -- - +
Dimensions -- - +
Impact on Project -- - +
Life Cycle Cost -- - +
Operation -- - + Logistics - -- + Maintenance -- - + Reliability -- - + Black-out + - - Shock = = - EHS - - +
Key: + favorable in comparison = equally favorable in comparison - less favorable in comparison -- least favorable in comparison
Equally important are the space savings. Here again the benefits are tremendous. The dimensions of a hydraulic starter “skid” (outside of the gas turbine package) are approximately 2 x1.5 x 1.5 m (l-w-h). The space occupied by the pneumatic starter system is even more, considering that most ships use an average volume of 2400 liters for bottled air in addition to the air compressors. By comparison the ESS space requirements are trivial – the space required for power and control cables, and the ESC. Power will be normally drawn from the ship’s existing electrical plant. The minimal ESS ship interfaces result in valuable engine room “real estate” freed up for other services.
With all of the aforementioned hardware out of the way, it is easy to make the argument that the ESS competes more favorably than fluid systems in the areas of: impact to the project, cost, and logistics. Parts count reliability models (3) make the point that reductions in system-wide parts, improves reliability and associated maintenance costs. The same reduction in supporting auxiliary hardware results in improved combat readiness, meaning less susceptibility to combat damage.
In the area of environment, health, and safety (EHS) the ESS eliminates the need for hydraulic starting fluid (MIL-L-17672), high pressure fluid lines (up to 3000 psi (211 kg/cm2)),
and high pressure air flasks. Each of these items has the potential for adverse effects on personnel safety and the environment.
There is little starter control flexibility when using fluid systems. Pneumatic starters use a regulated air supply that heretofore has provided for only two levels of air starting pressure. The same holds true for hydraulic systems that have relied on a maximum of two regulated flow quantities (normally 55 and 22 gpm (208 and 83 lpm) in the case of the LM2500). Figure 5 shows the typical start cycle of the LM2500 engine. The actual curve is dependent on uncontrolled variables such as ambient air temperature, air or fluid pressure and turbine condition. With the ESS, the speed –time envelope can be tailored to any desired shape as long as the maximum torque and horsepower limits are not exceeded.
Figure 5 – LM2500 Start Cycle
In addition to the start cycle, the ESS will be called upon to motor the engine for the purposes of waterwashing or other miscellaneous maintenance activities. In all cases the ESM will be controlled via an ESC using existing control signals.
Another benefit of the ESS is that the ESC will provide selective control capability. For example the ESS will be able to slowly turn the engine during maintenance inspections.
The loss of ship’s electrical power is an area where the pneumatic starter offers advantages over the present ESS design. This fact has prompted parallel efforts to develop Internal Energy Storage Modules (IESM) in conjunction with ESS in order to provide enough energy for at least three start attempts. The IESMs will be based on leading edge energy storage technology; a typical example is discussed in the next section.
OPPORTUNITIES FOR SYSTEM PERFORMANCE ENHANCEMENTS
Battery Energy Storage for “Dark Ship” Start
Under normal conditions, the ESC would derive its power from the Ship Service Electrical System. For turbine starting when no electric power is available, an alternate source of energy is required. Our energy calculations assume a constant 350 ft-lb torque from zero to 2200 rpm, and a linear decrease of torque to zero from 2200 to 3200 rpm. Using these
assumptions and the rpm-speed curve of Figure 5, the total energy per start is estimated to be 3.2 MJ. Provided the ESS is 90% efficient, 10.7 MJ of stored electrical energy would be required for at least three start attempts. A notional battery pack was sized using a stack of 200 Saft model HP12 Lithium-ion batteries. The stack voltage was approximately 700 VDC, which would be compatible with a rectified 450 VAC source. The total energy of the battery pack was 40 MJ; the size was approximately 1m3 and the weight was approximately 250 kG.
Cross connection of ESC units for redundancy
For systems with two or more turbines, the ESCs and the Battery packs could be cross-connected to eliminate single-points-of-failure. For systems with three or more turbines, cross-connecting ESCs could reduce the number of controllers required.
ESM for power generation
By eliminating the ESM overrunning clutch, the ESM could operate as a generator to recharge the battery (through the existing controller) or to provide a limited amount of backup electrical power to external loads.
Finally, the ESS is in keeping with the DoD’s More Electric Initiatives. It is envisioned that the ESS will join with magnetic bearings in creating a “more-electric” gas turbine.
OTHER DESIGN REQUIREMENTS
Naval electrical motors and their controllers differ from their commercial counterparts chiefly in their robustness and redundant features. Driving this robustness and redundancy are military requirements that essentially: 1. Stipulate a more onerous operating environment, and 2. Avoid single-point failures that would result in unavailability of the equipment.
Chief among these military requirements is MIL-S-901 shock, which was touched upon in a previous section. To date there are no test results to report regarding the shock integrity of the ESS.
MIL-STD-167 Type I (Environmental) and Type II (Self-Induced) tests address vibration susceptibility. Type I testing is used to evaluate the propeller passing frequency effect on equipment. The source of problems with this testing is usually low frequency resonance. The ESM, because of its relatively low weight and high stiffness is not likely to have a problem, nor is the substitution of the ESM for a hydraulic or pneumatic starter likely to alter the resonance of the turbine itself. It must, however, withstand any resonances of the turbine and its mounts, which are most likely to occur in mechanical drive trains where the turbine is solidly coupled to the propeller shaft. The ESC would likely be solidly mounted to a bulkhead and should suffer no peculiar resonances at typical propulsion frequencies.
The Navy’s ESS began in 1996 with the issuance of a SBIR (Small Business Innovation and Research Program) contract (4) to Axiom Corporation in Melbourne Florida. The SBIR Phase I program produced a preliminary design. Phase II resulted in prototype hardware that now resides with the Naval Surface Warfare Center Carderock Division (NSWCCD) in Philadelphia.
Extensive testing will be performed by NSWCCD during 2001. The Navy engineers in Philadelphia will first bench test the ESM on a water brake or dynamometer platform. The goal will be to correct any deficiencies, including minor vibration problems uncovered by Axiom during prototype evaluation. The hardware will also be tested in support of the Navy’s Dark Ship Study Demonstration. Finally, engine integration work will be conducted in order to operate the hardware on a GE LM2500 gas turbine engine.
The Gas Turbine Electric Start System offers numerous significant advantages over pneumatic or hydraulic start systems. These advantages include cost, size and weight, reliability and environmental impact. For some ship classes, high pressure air storage is required only for turbine starting and use of electric start would allow the high pressure air system to be eliminated altogether. Most significant, however, is the increase in flexibility in system design enabled by the Electric Start System. The only obstacles to realizing these benefits are the completion of the hardware testing and the system reengineering.
The authors would like to express their gratitude to Ed Struble and E. Robert Lee (Anteon Corp Machinery Group). Their support and feedback helped to make this paper possible.
1. E. Richter, et. al., “An Integrated Electrical Starter/Generator System for Gas-Turbine
Application, Design and Test Results”, Conference record ICEM-94, Paris, September 1994, p286ff 2. Capt. Massimo Maggini & Cdr. Michele Giuliano,
“Italian Navy Evaluation Concerning the Use of Electronic Fuel Control and Hydraulic Starters for Fiat-GE LM2500 Gas Turbine”, ASME GT-329, May, 2000.
3. Mil-HDBK-217F, “Reliability Prediction of Electronic Equipment”
4. Navy Contract N00024-97-C-4106 in response to SBIR Topic # N95-170