Turbocompound raises diesel generator output19 October 2001
In a period of comparatively high fuel oil prices more expensive options such as diesel exhaust fired turbines come back into the economic frame. J. Bucher, MAN B&W Diesel AG, Augsburg, Germany
Turbocompound systems consisting of a large diesel engine coupled with a power turbine driven by its exhaust are a proven way of improving the fuel economy of marine engines and stationary diesels. The first applications of such systems appeared about 20 years ago, but the idea itself is much older.
Power turbine development
While working on the turbocharging of large 4-stroke engines, Karl Zinner, head of the engine research at MAN Augsburg, saw a possible way of extracting additional power from the turbocharging system. The prerequisite was, and still is, a power surplus in the exhaust turbine compared to the power requirement of the turbocharging compressor. The resulting patent described two connection possibilities (Figure 1). The top half of the figure shows the power turbine connected upstream of the turbocharger turbine; the lower half shows the connection downstream of the turbocharger turbine. Connections for a power turbine cut-off at partial load are provided in both cases. Toothed wheels and a shifting coupling are used to transmit the output of the power turbine to the crankshaft. The power turbine is connected and disconnected automatically depending on the engine load.
A subclaim of the patent refers to the series connection of turbocharger turbine and power turbine. With the engine power levels discussed at that time a power turbine running in parallel to the turbocharger turbine would have led to such a small gas flow that this arrangement would not have been expedient. The theoretical and experimental investigations of turbocompound systems are described in Theoretical and Experimental Investig-ations of an Operational Procedure Involving the Use of a Coupled Exhaust Gas Turbine Compound System (Karl Zinner, CIMAC 1962, Copenhagen). But although it describes the most important characteristics of turbocompound applications executed for many years the patent has remained unused.
The idea was revived after competition forced up turbocharger efficiencies to values which the unaltered engine could reach only partially. Mikkelsen and Woods (MC Engines, Service Experience and Further Development, Nordisk Skibs Teknisk Möde 1986) showed that with increasing turbocharger efficiency in the unaltered engine improvement of the specific fuel consumption decreases. The exhaust gas temperature after the turbine drops simultaneously, limiting the utilisation of exhaust heat. A separate turbine converting the energy surplus of the turbocharging system into mechanical power promised a higher overall efficiency of the plant and thus a more favourable specific fuel consumption. Since these considerations were applied to large 2-stroke engines generally equipped with several turbochargers, a power turbine connected in series would have meant an indefensibly high expenditure. For that reason a power turbine connected in parallel to the turbocharger turbines was chosen. This turbine has to process a partial stream of around 4 to 10% of the entire exhaust gas stream at the full pressure gradient.
For the largest (then) up-to-date systems a power turbine flow rate resulted which was approximately in the range of the smallest turbocharger type with axial-flow turbine available at that time. All applications for smaller engines were in the flow rate range which required the use of turbochargers with radial-flow turbines. According to contemporary state of the art (described in MAN Turbo chargers Special Publication on the Occasion of the Commissioning of the Test Stand for Large Size Turbochargers, October 1982) a higher efficiency of radial-flow turbines was to be expected for the smaller flow rates. Moreover, the rugged design, with fewer components and lower production costs, favours a power turbine of radial design. Figure 2 shows the principle of the power turbine structure. The turbine casings, which are uncooled, consist of high-grade grey cast iron. The gas enters radially into a volute where it flows into the impeller by means of a nozzle ring. It leaves the impeller in the axial direction and is retarded in the adjoining diffuser, recovering part of the exit loss. The turbine shaft is carried by two bearings – a radial and an axial. A toothed coupling which accommodates a sun pinion gear is mounted on the free shaft end. Another gear rim located next to the toothed coupling triggers non-contact process impulses for speed measurement via a transducer. A second transducer can be arranged in the gear opposite one of the gear rims for a second independent measuring system. The turbine is flanged directly to the welded gear casing. Turbine speed is reduced in two steps to 1800 or 1500 rpm at the driven shaft. Turbine bearings and gears are lubricated with the same oil as the engine. A labyrinth seal is located between the radial bearing and turbine wheel; air admitted to this seal prevents exhaust gases from penetrating the oil room. For various technical reasons only a few of the possible power turbine arrangements, (partly discussed by H G Bozung, Yearbook of Schiffbautechnischen Gesellschaft 1986, and Manfred Apel, MTZ Motortechnische Zeitschrift No 50 1989), could be realised. Some of the systems used for stationary applications are described below.
The mechanical power released by the power turbine is usually employed in one of two ways, either by directly supplying power to the engine shaft or by converting it into electrical power. In either case a gearbox is required to reconcile the shaft speed differences. Either a synchronous or asynchronous generator can be used for power generation. The asynchronous generator is a simple, insensitive machine which can be connected to the mains without requiring precise synchronisation. It should be connected to a strong stable mains supply which is always able to absorb the full power of the turbine and to provide the necessary reactive power.
Power turbine/gearbox/4-stroke engine
After being successfully applied to 2-stroke engines, power turbines were also coupled to 4-stroke engines. In this case only parallel connection of the power turbine was employed – the deciding factor was the significantly higher design and construction cost of a series connected turbine. Figure 3 shows the arrangement. Exhaust gas is admitted to the turbine via a control flap valve and an emergency flap. A bypass valve (spill flap) with orifice is arranged parallel to the turbine. This spill flap serves to divert the all the exhaust directly into the stack when the power turbine is disconnected. The turbine is equipped with a speed transmitter and a speed relay which trigger an emergency shut-down and simultaneously open the bypass when a turbine overspeed of 5 per cent is exceeded. Depending on the charge air pressure the turbine is connected and disconnected automatically. Power output is a function of the exhaust gas condition, so it is not controllable.
Via an automatic coupling the turbine drives a gear that reduces turbine shaft speed to crankshaft speed. A highly elastic coupling is connected between gear and crankshaft. This coupling protects the gear and turbine against the irregularity of the crankshaft speed. 4-stroke engines require significantly lower gearing compared to 2-stroke engines because of their lower capacity, lower power, higher speed configuration. MAN B&W Diesel have built four plants with the 9L 52/55B engine and the smallest available power turbine, the PTG18, for a diesel power station in Nouakchott, Mauretania. The power turbine achieves 230 kW at engine rated power.
Turbine/asynchronous generator/4-stroke engine combination
For two power stations in India MAN B&W delivered turbogenerators with PTG18 power turbines and asynchronous generators together with 9L 58/64 engines. Figure 4 shows the arrangement of the turbogenerators.
Exhaust gas is supplied to the turbine as described above and the turbine is protected against overspeed. The connection process is controlled according to a patented method originated by the author. First, the control flap opens slowly until an adjustable exhaust pressure upstream of the turbine is achieved. This pressure is chosen so that with decreasing acceleration the turbine runs up to synchronous speed in approximately 5 to 10 seconds. If this speed is reached a speed relay closes the main switch. Not until the generator is connected to the mains is the gas supply fully opened. The mechanical power rating of the turbine amounts to 310 kW as measured at the site.
Turbine/asynchronous generator/2-stroke engine combination
The economy of Mauritius is founded mainly on sugar production, light industry, tourism and the recently opened free port. The Central Electricity Board (CEB) covers the growing energy demand with several diesel power stations (some also equipped with four-stroke engines built by MAN B&W Diesel and S.E.M.T. Pielstick), a few water powered turbines and a gas turbine. The backbone of the power supply is the Fort George power station to the north of the capital Port Louis, which also supplies the base load. Two 30 MW diesel generating sets were put into operation as engines 4 and 5. Two-stroke engines of type 9K80MC were built and delivered by Hyundai Heavy Industries under a MAN B&W licence. In addition, MAN B&W supplied exhaust-gas turbo-generators including the control and monitoring equipment.
Exhaust-gas turbo-generators can be used so long as the turbocharger efficiency is high enough to tap off a partial stream of the exhaust gas without risking thermal overload of the engine. MAN B&W also provided two turbochargers for each engine, of type NA70/TO9, which comply with this requirement, a fact that had been proved during the factory test of the engine. In these tropical site conditions, the plant can tap off an exhaust stream of about 6% of the exhaust-gas volume. The power turbine and the generator are arranged on a solid foundation frame. The power is transmitted by means of an elastic coupling. The frame also supports the exit elbow for the exhaust gas in order to protect the turbine from external forces.
Figure 5 depicts the plant diagramatically. Two control flap valves, actuated by compressed air motors with high gearing, control the exhaust gas stream. This type of drive was selected for several reasons: it is very rugged, it does not need to bring a combustible medium close to the neighbouring exhaust pipes, it has a very large torque reserve and it makes possible extremely sensitive flap control. Moreover, an emergency flap that is opened by compressed air and closed by spring loading is arranged in the inlet pipe. An emergency venting valve arranged directly at the actuator closes the flap within the shortest possible time. A second spill flap, also spring loaded and closed by means of compressed air, is connected in parallel to the first spill flap. This actuator also has an emergency venting valve. The branches of both spill flaps run into a common outlet pipe with an orifice, ensuring that each flap can release the outlet branch independently. In case of an emergency stop signal or if the auxiliary power, compressed air or direct current fails, the emergency valve closes and the second spring loaded outlet flap opens independently of all other control commands.
The control valves for the flaps are accommodated in a control cabinet located close to the turbogenerator. This cabinet also contains pressure switches and relays which control the flaps for engine operation without turbogenerator if the control is disconnected or not operating. The flaps in the inlet pipe to the turbogenerator are kept closed while the spill flaps open if the charge air pressure exceeds the limit value and close if the pressure drops below the second limit value. The programmable control for automatic start, stop and monitoring of the turbogenerator is accommodated in a switch cabinet in the engine control room.
Automatic operation of the turbogenerator is controlled by reference to the charge air pressure. The spill flaps open first. If an engine power increase occurs the emergency flap opens fully and the control flap operates slowly until the turbine has reached the stipulated speed. The control flap remains in its position while the turbogenerator runs up to just below the synchronous speed with increasing deceleration. Then the control flap is opened further in small steps until the synchronous speed is reached. Afterwards the main switch is closed. Resistors, which are overridden after a short period of time, are brought into the circuit to attenuate the unavoidable inrush current when switching on asynchronous machines. Then the control flaps are opened fully and the spill flaps closed. If the engine power drops, the turbogenerator is disconnected, depending on the charge air pressure. Since the turbogenerator is operated fully automatically all essential data such as oil pressure, control air pressure, voltage, speed, winding temperature, bearing temperature, vibrations, overload, reverse power and flap position are monitored by the control. The system is disconnected if the limit values are exceeded or if a signal fails. A safety circuit is connected independently of the control to trigger an emergency stop in the event of a manual emergency stop, overspeed, low oil pressure or in case of an error in the high-voltage system. All safety functions including sudden load release were demonstrated during the commissioning of the plant. At engine rated power (30 MW) the turbogenerator provides 580 kWe to the mains. One of the two systems has been running for about two years and the second for more than a year without any service problems arising.
Exhaust driven power turbine technology has reached a degree of maturity that can support commercial sales. In the last few years, however, no new power turbines have been ordered other than for the installations described in this article. The reasons are:
• the drop in oil prices after both oil crises
• customer aversion to complicated systems
In the meantime, fuel prices have increased dramatically compared to the early nineties. Under these conditions it may be worth while reconsidering the application of turbo compound systems.