Radial turbine

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[edit] Concept

The difference between axial and radial turbines consists in the way the air flows through the components (compressor and turbine). Whereas for an axial turbine the rotor is 'impacted' by the air flow, for a radial turbine, the flow is smoothly orientated at 90 degrees by the compressor towards the combustion chamber and driving the turbine in the same way water drives a watermill. The result is less mechanical and thermal stress which enables a radial turbine to be more simple, more robust and more efficient (in a similar power range as axial turbines). When it comes to high power ranges (above 5 MW) the radial turbine is no longer competitive (heavy and expensive rotor) and the efficiency becomes similar to that of the axial turbines.

[edit] Advantages compared to axial turbines

Thanks to lower thermal and mechanical stress on the turbine tips, it is possible to boost power quite significantly by increasing the turbine entry temperature (increasing fuel input) which results in an improved mechanical efficiency. The lower mechanical stresses also enable radial turbines to handle single stage compression and expansion. As a result, the radial turbine does not need to be air cooled, which means that all the air entering the compressor is used only to drive the turbine which gives the radial design a strong advantage for cogeneration applications. Another result of avoiding air cooling is that power and efficiency are kept almost constant during the lifetime of the radial turbine whereas an axial gas turbine needs to be washed often to maintain ISO performance standards. The other advantage of such a simple rotor is that the bearings can be placed at the front, in the cold part, so less lubrication oil is needed, and there are no thermal losses due to lubrication of the hot parts of the rotor.

[edit] Constructors

1963, Norway, Jan Mowill initiated the development at Kongsberg Våpenfabrikk, a Norwegian company founded by Jan Mowill. The turbine had a unique, all radial configuration, originally rated at 1,200 kW. The turbine proved very successful and was generally sold in electric generating packages. The major markets for the units were in the maritime, offshore oil and gas and communications industries. During the following years, more than a thousand units were delivered world wide. Kongsberg Våpenfabrikk was privatized, split up and sold off in the late nineteen eighties and development of the original turbine business was discontinued under the new ownership.

As a result, Jan and Hiroko Mowill founded OPRA in Hengelo in 1991, leading to the manufacturing of a the OP16 advanced radial gas turbine in 2003, rated at 1900 kW.

Consequently the first 1.6 MW OP16 was designed as a single shaft, all-radial machine. NOx emissions were developed to a very low level for both diesel fuel and natural gas. This was achieved with a unique, patented fuel and air pre-mixer in connection with an annular combustor.

The current production model, OP16-3 features both single and dual fuel operation as well as low emissions on natural gas. For improved maintenance and serviceability, a four can combustion systems was favored rather than the annular combustor used on the prototype.

For a single stage radial turbine the pressure ratio of 6.7: 1 is relatively high, which entails a high turbine impeller tip speed of 700 m/s (equal to the velocity of a rifle bullet).

Since this is nearly the same as the velocity of the gas entering the impeller tip from the nozzle guide vanes, an "impact" between the hot gas and the turbine impeller is avoided.

It could be said that this phenomenon constitutes "dynamic" cooling gaining about 100°C compared to a temperature increase in an axial turbine. OPRA’s radial turbine is able to take this high tip speed due to its "Eiffel Tower" shape with a strong base and a thinner blade tip region with low stresses. Having low stresses in the hot tip region and higher stresses in the cold, "fat" hub region makes OPRA work with nature rather than against it.

The OPRA radial turbine stage has an advanced aerodynamic design with an efficiency of 90% from the inlet of the guide vanes to the exhaust diffuser exit.

The efficient centrifugal compressor has a very good "match" with the turbine as their optimal running speeds are similar.

Since both compressor and turbine are close coupled via a Hirth-type teeth connection, an overhung rotor suspension is possible. This system provides balance integrity despite the differential thermal expansions between the compressor and turbine.

A ball bearing is placed in the front of the rotor support housing taking the combined thrust- and radial load. The rear, tilting pad bearing takes the main radial load. The cantilever, or overhung suspension of the rotor places the bearings in the cold section of the engine, avoiding oil supply to hot bearings. This system has considerable positive impact on engine reliability and maintenance.

A flexible coupling connects the turbine to the two stage planetary gearbox, reducing the turbine speed from 26000 to 1500 or 1800 rpm, depending on generator speed requirements

The OP16-3 has an ISO rating of 1.9 MW. The engine efficiency of nominally 27% is at the highest level in the below 2 MW power range. Past competitors (no longer active) in this range have been at the 23–25% level.

Utilising proven radial gas turbine technology, the OP16 gas turbine is a compact, efficient and reliable industrial gas turbine designed for supplying power generation applications to both the Oil and Gas and Industrial markets.

The OP16 generator sets can be provided in a variety of configurations to meet customer specific requirements. The engineering design, component selection and maintenance accessibility of the generator sets enhance high reliability and long product life. The generator sets can be provided with low emission and dual and multifuel capabilities.

Single or multiple OP16 units can effectively cover installations from 1.5 to 10 MW electric power demand.

OPRA provides gas turbine generating sets for customers world-wide within the oil & gas and industrial sectors. OPRA's 2 MW class OP16 gas turbine is of an industrial, all-radial design which provides robustness, reliability and class leading efficiency and emissions. Dual fuel and off-specification fuel options are also available. Complete gas turbine generating sets are engineered to meet customer specific requirements both for land based and offshore applications.