Impulse Turbine / Pelton Turbine

Pelton Turbines The only type of impulse turbine that is in use these days is Pelton turbine. It is also called a free jet turbine as it uses a free jet of water under atmospheric pressure. It works under high head and uses less quantity of water. Water from the reservoir is brought to the turbine through penstocks, at the end of which a nozzle is fitted. The nozzle converts whole of the available head into the kinetic head in the form of a high velocity jet.

Pelton Turbine

Pelton Turbines The jet strikes the buckets mounted on the rim of a wheel called runner. A shaft passes through the runner. The force of jet causes the runner to rotate and mechanical power is produced. In the end water goes into the tail race. Number of nozzles depends upon specific speed. However, maximum number of nozzles can be 6.

Components of a Pelton Turbine Runner with Buckets The runner of a Pelton turbine consists of a number of double cupped buckets fixed to the periphery of the wheel. Each bucket has a sharp edge at the centre called the splitter The jet strikes each bucket at this splitter and is divided into two sides, thus avoiding any unbalanced thrust on the shaft. As the splitter takes the full impact of the jet, so it has to be quite strong.

Components of a Pelton Turbine Runner with Buckets

Components of a Pelton Turbine Nozzle with guide mechanism The function of the nozzle of a Pelton wheel is to convert the available pressure energy into high velocity energy in the form of jet. The quantity of water required is proportional to the load on the turbine. Therefore, to control the flow through the nozzle, some sort of a regulating or a governing mechanism is necessary. This is generally done by using a spear inside the nozzle. The movement of spear inside the nozzle changes the area of flow through it, thus varying the discharge.

Components of a Pelton Turbine Nozzle with guide mechanism The nozzle is usually made of either cast iron or cast steel. Sometimes, a small brake nozzle is also used in case of large turbines. When the wheel is to stopped, besides cutting off the supply of water through the main nozzle, the brake nozzle also directs the water on to the back of buckets to bring the wheel quickly to rest.

Components of a Pelton Turbine Casing The casing is not to perform any hydraulic function. However, a casing is necessary to avoid accidents, splashing of water, to lead the water to the tailrace and to support the hosing for the bearing and the nozzle. Material for the casing is usually cast iron.

Dimensions of bucket The minimum possible width B of the bucket is approximately 2d, where d is the diameter of jet. This dimension is usually taken as 2.8 to 3.2d to avoid losses. Some prefer to take it as 3 to 4d. The depth of the bucket (T) usually lies between 0.8 to 1.2d. The height (H) of the bucket is 2.4 to 2.8d. Other dimensions of the bucket are taken as below: E = 1.2d F = 0.4d G = 0.9 to 1.0d βi = 3 – 6 degrees βo = 10 - 20 degrees 10

Dimensions of bucket 11

Number of buckets The number of buckets is decided such that the frictional loss is minimum and the path of the jet is not disturbed and the jet must be fully utilised, i.e. all the water particles strike the buckets and impart kinetic energy to the wheel. Taygun gave the following relation for the calculation of number of buckets. where m = D/d = Jet Ratio D=dia. of runner, d=least dia. of the jet 12

Diameter of Jet The velocity of jet out of a nozzle is given by, where, Vi = Velocity of jet Cv = Coefficient of velocity for the nozzle; value lies between 0.96 – 0.99 d = diameter of the jet Discharge through the nozzle, 13

Speed Ratio Speed ratio (Ku) of a Pelton turbine is defined as the ratio of the tangential velocity of wheel u to the theoretical velocity of the jet. where in which N is the speed in rpm 14

Jet Ratio Jet ratio is defined as the ratio of the diameter of runner to the least diameter of jet, 15

Number of Jets Number of jets may be increased, usually upto 4, but never more than 6 because higher number of jets makes the governing complicated. 16

Number of Jets 17

Example-1: Each turbine at a hydro-power-house works under a head of 940m and produces 8000kW. If the turbine runs 600rpm, find a) Least diameter of the jet b) Mean diameter of the jet c) Jet Ratio d) Number of buckets Take Cv = 0.98, Ku = 0.46, and ηo=0.89

Example-1 Solution: 19

Example-1 Solution: 20

Example-2: Calculate the number of jets required for a Pelton wheel to develop 10,000 kW under a head of 370m when running at 500 rpm. Ratio of the wheel diameter to the jet diameter is 12. Take Cv = 0.98, Ku = 0.46, and ηo=0.88 Solution: H = 370m P = 10000 kW N = 500 rpm D/d = 12

Example-1 Solution: 22

Example-1 23