Evans Water Engineers - tangenital flow turbine


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Tangenital Flow Turbines


In this section I will describe the main features of ‘tangential flow turbines’, the sizes and types that we make and the main advantages and applications for the type. The best-known turbine in this group is the Pelton wheel. I will initially describe the basic hydraulics of these turbines and then go on to describe the layout and enclosure that we manufacture and its particular advantages.

Tangenital flow turbineGENERAL

Water turbines of this type receive high-pressure water from single or multiple jets that either point at a tangent to the runner along the centre line, or point at the side of the runner near the periphery. In each case the water must meet the bucket inlet with as little ‘shock’ as possible and discharge tangentially with an equal and opposite velocity so that the water falls to the bottom of the casing having extracted as much energy as possible.

Conventional Pelton wheels have a bucket that is divided along the centre line so that the flow discharges from both sides of the runner, resulting in no axial thrust. With vertical arrangements we offset the jet slightly so as to reduce the axial loading on the bearings by putting in an upward thrust from the jet. The ratio between the jet diameter and the pitch circle PCD ranges from as low as 4:1 to as high as 300:1.

Evans Engineering - pelton wheelMost Pelton turbines are designed in the range of 8:1 to about 20:1. In this range it is necessary to have a ‘cut-out’ in the centre of the bucket inlet so that the jet does not strike the back of the bucket first, causing loss of energy. The first point to meet the jet should be the point of the ‘splitter ridge’ and the ‘cut-water’ at the base of the bucket ‘cut-out’. The hydraulic efficiency of the runner itself can be in excess of 90% but for the complete turbine efficiency you have to deduct manifold losses, jet losses, splash and windage losses. A realistic ‘water to shaft’ efficiency for a small turbine will be about 82%.

Runners with a very high jet to PCD ratio are used where the working head is very high and it is necessary to keep the turbine R.P.M. down to safeguard the generator. We manufacture a unique CNC machined version that can operate with pressures in excess of 100 Bar (approximately 1000 metres of head). This is by no means the limit, but the selection of the material and the cleanliness of the water are both important, since even small amounts of silt will destroy the runner rapidly.

Evans Engineering - Ultra High Pressure TurbinesUltra high-pressure turbines are a very economic alternative to hydraulic motors in water-hydraulic power transmissions of the type that are now being used in some ‘marine renewable energy systems’ such as wave and tidal generators. These turbines, having only one moving part have a considerably longer maintenance interval than a motor that has dozens of intricate parts. Being manufactured entirely in stainless steel and ceramic, there are no problems operating on seawater.

Ultra high-speed turbines that have very low jet to P.C.D. ratios are used as a substitute for ‘Turgo’ type turbine runners. Like the conventional Pelton there is no end thrust from the water jets because they enter the runner on the centre line. The amount of ‘hydraulic compromise depends on a number of factors including the target efficiency, ease of manufacture and turbine layout. We call this type of turbine a ‘Pegasus’ because of the winged bucket shape, but Pegasus was also responsible for creating the fountain of Hippocrene to flow on Mt Helicon.

Evans Engineering - other advantagesOther advantages of the ‘tangential flow turbine’ are its easy flow control with a variable ‘spear valve’ and the facility to reject energy very rapidly by interposing a ‘deflector’ between the nozzle and the runner. This system is used for hydraulic governing of the turbine speed in larger machines and for automatic shutdown if the load is rejected for any reason. We have a unique deflector system that is totally balanced and can operate in less than one second, preventing any turbine ‘runaway’. This type of turbine can be fitted with a very simple speed limiter to prevent over-speed or to give basic speed regulation for low-cost agricultural units.

Evans Engineering - very small runnersVery small runners, from as little as 10mm in diameter are CNC machined. Larger runners up to 365 P.C.D can be ‘Investment Cast’ in one piece. Still larger units are sand cast or shell moulded. Our maximum runner weight to date is 1250 kg. These runners were used in the construction of the Owenbeg power station in Co Cork Ireland.

The ‘Armstrong’ or ‘Turgo Impulse’ type runner can conveniently be used with one or two jets controlled by spears and deflectors if necessary. Where more flow is required the full admission multi-nozzle layout is required and the runner is installed in a circular case instead of a flat-sided one with separate pipes feeding the jets.

There are many possible layouts for tangential flow runners, but our standard layout is with a vertical shaft and belt drive to a vertical alternator. The efficiency is good and the access for maintenance is through the tailrace. Where access from below is not possible because the tail-water enters a small culvert or the turbine is physically too small to get underneath, then a removable top plate is required, or the layout needs to be horizontal with an access door in the side. These options are a little more expensive, but they allow the use of standard alternators and make it easier to drive other types of plant such as heat-pump compressors or agricultural processing equipment. In the PRODUCTS section you will find more photographs of complete turbines. These are grouped under ‘Output’ and ‘Type’. My advice is to start from the simplest standard option and add features or modifications only if it is necessary.

Rupert Armstrong Evans

Managing Director