Pelton wheel

The Pelton turbine is an impulse turbine ( " partial admission impulse turbine " ) for hydropower plants. It was designed in 1879 by an American engineer Lester Pelton and patented in 1880.

A Pelton turbine utilizes the kinetic energy of the water. This form of energy is produced by converting the potential energy of the water flowing out of a perched water bodies, such as a dam.

Pelton presented for the design of its turbine, the rediscovered by the German physician and physicist Johann Andreas Segner of reaction underlying principle of which was built on this basis, in 1750 the first Reaktionswasserrad. Before that time, the principle of the free turbine was realized in Horizontalrad water mills made ​​of wood, but with low efficiency.

Pelton turbine modified developed by Samuel Knight turbine and achieved this against a higher efficiency. This meant that the Pelton turbine prevailed as the industry standard.

• 3.1 Hydropower Plant
• 3.2 Measurement Technology

Operation

Pelton turbine in which the water flows in a jet with a very high velocity of one or more nozzles onto the vanes of the impeller. (Viewed in the direction of flow ) upstream of the nozzle, there is a high pressure (up to 200 bar), after leaving the beam itself prevails normal atmospheric pressure. Therefore, the classification based on the impulse turbine, the water is in front of the power output at the turbine wheel and then the same pressure. Teilbeaufschlagt called the Pelton turbine, because only some of the bucket cups are simultaneously acted upon by the beam. The back side that does not contribute to the power generation, moving through air or foam. This medium has a significantly lower density, and so the windage of the Pelton turbine to remain comparatively small. Since each blade only briefly transfers power to the wheel and then remains powerless, is very large because of the alternating stress in the root region of the cup, the risk of fatigue failure. There are different manufacturing processes to meet the high demands.

• Fully forged and then machined wheels, which are the best quality but also quite expensive solution.
• The wheel can be cast in one piece, there is a risk of undiscovered casting defects, which lead to the destruction of the wheel, respectively, of a large part of the power plant later.
• The variant Wurzelrad forged with welded cups.
• The patented " MicroGussTM method " in which the individual cups are constructed in a manner hardfacing method of the forged Wurzelrad.

The number of nozzles is determined by the flow rate, a nozzle can process a maximum of about 10 m³ / s. If the flow rate is higher, the number of nozzles must be increased, but with the shaft horizontal only two jets are technically meaningful because at higher number of nozzles, the waste water would fall back to the impeller. A higher flow rate is necessary, a second impeller is placed on the same shaft, or the entire turbine is constructed with a vertical axis. In this case, four nozzles are regularly used, but Pelton turbines have been already shipped with six nozzles (from Escher Wyss, Ravensburg ).

The circumferential speed of the blade circle should correspond exactly half the speed of the water jet. Since the water is diverted into the blades of almost 180 degrees, then there are almost his entire energy to the blades (for inviscid analysis). The diameter of the turbine depends on the generator rotational speed and the available water pressure or the speed of the water jet.

Each of up to 40 blades is divided into two half- blades, the so-called cup. In the middle of this half blades of the water jet from the nozzle strikes tangentially. The cups have a function to guide the water into the opposite direction, so that the kinetic energy can be better utilized. This was the innovation of Pelton. The cutting means at the first start almost razor sharp and a mug would quickly destroyed by the water pressure occurring if no funds cutting would be used for beam splitting and guidance. Furthermore, it is possible to protect the beaker from excessive wear, particularly from foreign bodies in the water in the form of small pieces, by applying a protective layer in the HVOF process.

At a height of 1000 meters, the water jet can reach a speed of almost 500 km / h. The largest unrealized impact velocity is about 185 m / s (666 km / h); at this value, it is understood that the cutting means is indispensable in every cup. The Pelton turbine consumes, depending on the type and case Height between 20 and 8000 liters of water per second. She has a very high speed: up to 3000 revolutions per minute. Their efficiency is between 85% and 90 %, with them, even if it is not running at full load, still performing well. One of the largest currently realizable case heights of 1773 m, at a flow rate of 6 m³ / s ( built by company Voith, Heidenheim, plant Reißeck Kreuzeckgruppe, Carinthia ). It was in 2000, surpassed by the plant Bieudron, Valais. There are three fünfdüsige Pelton turbines which provide 423 MW each at a record drop height of 1883 m. For the same reservoir, Lac des Dix, concerns also the older power plant Chandoline headrace, which is also one of five Pelton turbines at a height of 1748 m to the record-setting systems. The power plant Silz in the Inn Valley consists of two vertical-axis turbine sets, each with a sechsdüsigen Pelton impulse turbine and a fully water-cooled generator. Meet The water jets at a speed of about 500 km / h over six nozzle with a force of 350,000 N ( 35 t) 50 times per second on the turbine blades. The case height is 1258 meters here. In Naturns, South Tyrol, there is a power plant with three Pelton turbines (180 MW) with a height of 1150 m.

Performance and efficiency

The velocity of the water with the density (usually 1000 kg / m³) can be derived from the energy conservation law.

The usable storage level (such as at the end of a pressure tunnel ) is reacted in the lossless velocity c of the exit of the nozzle of the turbine water ( g = acceleration of gravity = 9,81 m / s ²):

Performance of the water supply

The power from the supply of water is calculated from the product of the water pressure and the volume flow (in m³ / s) and taking into account the cross-sectional area of the nozzle aperture A ( in m ) and the above equation for the maximum water velocity c:

Performance of the Pelton turbine

The Pelton turbine works on the principle of the (double) momentum transfer (momentum conservation in the perfectly elastic collision), the impulse by a mass flow with the velocity c.

When impinging on the respective blade but can not be effective, the exit velocity c, since the vane on the axis of the rotation at the speed of the nozzle and hence moved away from the water jet. It can therefore act only the velocity ( cv ) for the impulse:

• From the nozzle or nozzles n, the total cross-section A, the mass flow is shot on the blades with the velocity c. The impulse would be ideal effective if the Pelton wheel would hit exactly perpendicular to the effective radius r forever. But this can not be the case, since shade the blades against each other. Therefore, the impulse is reduced in its effectiveness by a factor of.

• The Pelton bucket is formed so that the water jet divided in the middle and guided over the concavely curved blade as possible to 180 ° is thrown back to the nozzle, to reach the condition of perfectly elastic collision as close as possible with a substantially loss-free transmission of energy. Unfortunately, this can not completely succeed because boundary conditions must be observed: The neighboring blade is in the way. But it must be prevented that this blade is virtually hit from behind by the reflected water jet. Therefore, the blade has an opening angle > 180 °, the water is thus thrown back slightly deflected laterally.
• Even with a perfect deflection by 180 °, the nozzle would be taken. This would act on the beam and reduce its speed.
• Furthermore, the churning losses are to be observed: the returned hurled water is rubbed in a water mist, which increases the mean density of the trapped air in the housing and thus friction in the housing.
• Can on the way back, of course, only the act, which could be energetically effective when it hit the bucket, hence the second impulse, recoil, only the strength of the first force pulse taking into account the additional, just described, losses may have: .

The equation for the force acting on the turbine force is:

The equation for the force acting on the turbine torque is then:

The equation for the output of the turbine with the speed is then:

The performance of the Pelton turbine is optimal when it is operated at an optimum rotational speed and optimum effective velocity at radius r. This is the case, when the first derivative with respect to V of the power equation is set equal to zero and solving for V:

If the optimum speed is used in the equation of the power we have:

When is the ideal Pelton turbine, then from the above equation:

Efficiency of a Pelton turbine

Efficiency of a real Pelton turbine

For the Pelton turbine, the following equation can now be set up for efficiency:

The efficiency thus depends only on the factors, ie how favorable the geometry is selected and friction the water to flow over the blade and can be removed.

Efficiency of an ideal Pelton turbine

If the two factors are set, one immediately sees that the efficiency is equal to 1, so the water supply can theoretically be completely converted into kinetic energy. The reason for this is the " elastic momentum exchange " from the reflection of the water by the concave blades, which can be implemented without energy loss. In this respect, a Pelton turbine

• Not to be regarded as a heat engine, the only with a limited maximum efficiency - can be operated - the Carnot efficiency
• But classified as a transmission which can transform in the case of no friction losses without mechanical power (power from the potential energy of a reservoir into rotational energy for a generator ).

Areas of application

Hydroelectric power station

It is due to their characteristics predominantly (up to 2000 m ), used in hydroelectric plants with very high heads at relatively low amounts of water, especially in storage power plants in the high mountains.

The main disadvantage is the susceptibility to wear the Pelton turbine when in the mountains, the water has a high proportion of Schwemmpartikeln (sand, etc. ) has. This leads to a heavy wear of the blades.

Moreover, the Pelton turbine can not be operated as a pump, in contrast to the Francis turbine.

Measurement

In the flow measurement a system based on a Pelton turbine encoder principle is used as a highly accurate flow meter, with not only low volume flows of media, such as fuel, distilled water or hot fats are measured, but also high flow rates, as they normally found in power plants. By limiting temperatures of 135 ° C and nominal pressures up to 345 bar this Pelton wheel encoders fulfill almost all industrial requirements.

Heraldry

The Pelton turbine has been found in some recent municipal coat of arms collection, including:

Reißeck

Uttendorf