Essay Sample on Pelton Turbine

Introduction

The Pelton wheel turbine is classified as an impulse type of hydraulic turbine. The total head is first converted into kinetic energy, and this is often accomplished in one or more nozzles. The issuing from the nozzles strike vanes attached to the periphery of a rotating wheel. As a result of the rate of change of angular momentum and motion of the vanes, work is done on the impeller by the fluid, and hence energy is transferred. Since the fluid energy which is reduced on passing through the runner is entirely kinetic, it follows that the absolute velocity at the outlet is smaller than the absolute velocity at the inlet (jet velocity). Furthermore, the fluid pressure is atmospheric throughout, and the relative velocity is constant except for a slight reduction due to friction.

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The Pelton wheel is an impulse turbine in which vanes, also called buckets, of elliptical shapes, are attached to the periphery of the rotating wheel. One or two nozzles project a jet of water tangentially to the vane pitch circle. The vanes are of the double-outlet section, as shown in the figure below so that the jet is split and leaves symmetrically on both sides of the vane. This type of turbine is used for high head and low flow rates. It is named after the American engineer Lester Pelton.

Components of the Pelton Turbine:

Runner with bucket: Also known as the impeller. Consists of a circular disc on the periphery of which a number of buckets are fixed.

Nozzle: Water coming from the reservoir through penstock is accelerated to a certain velocity by means of a nozzle.

Spear: Is a conical needle which is operated by a hand wheel or automatically in an axial direction upon the size of the unit. The amount of water striking the buckets of the runner is controlled by the spear in the nozzle.

Casing: Is used to prevent the splashing of the water and to discharge water to tailrace. It is made up of cast iron or steel plate.

Breaking jet: When the nozzle is completely closed by moving the spear in the forward direction the amount of water striking the runner reduce to zero. However, the runner due to inertia goes on for a long time. To stop the runner in a short time, a small nozzle is used which directs the jet of water on the back of the buckets. This jet of water is called breaking jet.

Governing mechanism: The speed of the turbine is required to be maintained constant so that electric generator can be coupled directly to the turbine. Therefore a device called governor is used to measure and regulate the speed of the turbine runner.

Results and Calculations

The following results were recorded during the practical:

The flow rate is given by; flow rate= volume of water recorded litrestime s=105.4flow rate=1.85 ls-1input power=508 WOutput power obtained=204W The obtained maximum efficiency obtained= output powerinput power=0.40=40%maximum available power=gh=1.859.8128 m=508.158 WThe maximum available power is equivalent to the total input power supplied to the generator.

Discussion

The Pelton turbine has been seen to be a kind of impulse turbine which converts gravitational potential energy to kinetic energy, where the velocity of water jets increases and hits the blades of a turbine thus converting to mechanical energy. The mechanical energy present in the shaft rotation is hence converted to electrical power.

From the graph of "Power output against turbine rotational speed," it was noted that the maximum power generated was at the speed of 700 rpm. The 700 rpm turbine speed is however not the maximum attainable speed of the turbine. This phenomenon can be explained by the fact that a Pelton turbine is designed to produce maximum power when the peripheral speed is half of the water jet speed. Therefore the power transmitted to the turbine wheel is 50% of the incoming jet power. This is because the water jet is often reversed by the wheel cup design 180 degrees back towards its source. The reversed water jet is not exactly in line so that the wheel itself has a real-world efficiency of 90% or better. From the graph of "Plant efficiency against turbine rotational speed" the rotational speed of 700rpm was found to provide the highest efficiency, which was in this case 40%. Maximum power and efficiency of the Pelton turbine can be achieved when the velocity of the water jet is twice the velocity of the rotating buckets. Other factors that determine the point of maximum operating efficiency of the Pelton turbine include the design of the vanes (buckets) and also the size of the nozzle. The smaller the size of the nozzle, then the higher the expected efficiency of the turbine due to the increased velocity of the water jet.

Conclusion

The experiment was conducted on the Centre for Alternative Technology (CAT) to determine the power output from the generator and a tachometer to detect the shaft speed at which the turbine was operating in RPM. A flow head of 28m was used in the experiment, which was a high head since the pump used, was operating at a constant speed unlike the generator. From the findings of the Pelton turbine experiment, it is important to note that this kind of turbine is used for high heads but low flow rates (discharge) like in the case of the experiment where the flow rate was 1.85litres per second. From the experiment, the maximum attainable power was found to be 508 W. In addition, the maximum efficiency was found to be 40% which is far below the theoretical efficiency of 90% or better. For a Pelton wheel turbine, the efficiency is noted to be highest at a rotational speed of 700rpm. Apparently, the maximum turbine speed is not 700rpm and this can be explained by the fact that maximum efficiency and power is recorded when the periphery speed of the buckets is half the velocity of the water jet. The low efficiency (40%) could be due to the losses incurred in the overall design of the system, such as mechanical losses and hydraulic losses in pipes.

Bibliography

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