Turbine Water

The Pelton Wheel.

This is usually built as a horizontal shaft machine and consists of a runner carrying a series of buckets around its periphery on which impinge one—or in exceptional cases two—high velocity jets from a nozzle or nozzles at the end of the supply pipe line (fig. 5). The buckets are spoon shaped and have a central sharp ridge which divides the impinging jet into two halves which are deflected backwards by the buckets through about 165°. The modern Pelton wheel is always fitted with a circular nozzle, with an axial needle or spear for regulating the size of the jet. The maximum diameter of jet as yet adopted is about 12 inches. The axial position of the needle in the nozzle is regulated by the governing mechanism in all important installa tions.

The Pelton wheel, being essentially a high head turbine, is usually supplied through a comparatively long pipe line, and any rapid closing of the nozzle such as might occur if the load were suddenly thrown off, would set up severe surges of pressure in the pipe line which would not only prevent close speed regulation but might be dangerous. To prevent this, modern Pelton wheels are also fitted with a jet deflector, consisting of a curved plate pivoted between the nozzle and the buckets, which is operated by the gov ernor and which when in action cuts into the jet and deflects it either wholly or partially into the tail race. The governing mech anism is so arranged that when load is thrown off the wheel, the deflector at once comes into play, deflecting the jet from the buckets. The needle then begins to move slowly towards the closed position, and at the same time the deflector moves slowly back towards its idle position. Ultimately both come to rest with the deflector just clear of the jet, and with the diameter of the latter so adjusted as to give the required supply of water to the wheel.

Hydraulics of the Pelton Wheel.

If H be the pressure head behind the nozzle, in feet, the velocity of efflux is C,,,V 2gH ft. per second, where the coefficient of velocity, in a formed needle nozzle is approximately .99. Calling this velocity the horsepower of the jet is equal to The loss due to rejection of kinetic energy in the discharge =— ft. lb. per pound, where 2g Tests show that in an average wheel 21), may be as low as from .5 to .6 In a well-designed bucket, however, having a ratio of bucket width to jet diameter not less than about 3.3, this ratio approximates to .75 or even .8. If the angle of deflection were 18o°, and if the buckets were frictionless, the value of the peripheral speed of the wheel for maxi mum efficiency would be V1 cos ce-1-2, or approximately V1÷2, since a is small. When account is taken of the various losses and of the fact that y is less than 18o°, the best peripheral speed lies be tween .44 and Comparison of Impulse and Reac tion Turbines.—The peripheral velocity

of a Pelton wheel for maximum efficiency is slightly less than one-half the velocity of the jet (usually approximately .46V 2gH, where H is the head), while that of the reaction turbine varies from about .65V2gH to I.05V2gH, depending on the design. Because of this, the Pelton wheel is well adapted for very high heads, which may then be utilized with moderate speeds of rotation. On the other hand the relatively high speed of the reaction tur bine enables reasonably high rotative speeds to be obtained with low heads.

The Pelton wheel cannot well be designed to utilize efficiently more than two jets on a single wheel, and as the maximum prac ticable jet diameter is not large, the volume of water which can be handled and the output of the turbine become small under low heads. The reaction turbine with its full peripheral admission on the other hand is well adapted for large volumes. It is not suited for small powers under high heads, since the volume of water is then small, the waterways are of very small sectional area and easily become choked by floating debris, and the fluid friction losses become relatively high.

The Pelton wheel cannot easily be adapted to the use of a suc tion or draft tube, and, where the tail-race level may vary appre ciably, must be installed above the highest probable tail-water level with some sacrifice of head. The efficiency of the reaction turbine is not so sensitive to changes of head as that of the Pelton wheel, but if operated under constant head and at constant speed, the efficiency of the Pelton wheel does not fall off so rapidly at part loads as that of the reaction turbine. On the other hand, the modern reaction turbine has a slightly higher full-load efficiency, :o that the average efficiency from half to full load is sensibly the same in a well-designed machine of either type. The following table shows typical values of the part-load efficiencies of modern turbines of both types of large size, installed under equally f a vourable conditions.

The possibilities of accurate speed regulation are about equal in the two types.

For large units the reaction turbine is generally preferable for heads up to 400 feet. For heads above 75o ft. the Pelton wheel is more suitable, while between these limits the choice depends largely upon local circumstances and on the power required. The greater simplicity and accessibility of the parts requiring replace ment due to natural wear and tear renders the Pelton wheel more suitable when the supply is taken from a stream carrying an appreciable amount of grit.

The reaction turbine has been built in units capable of develop ing 65,00o H.P. The most powerful Pelton wheel yet constructed develops about 30,00o H.P., but these outputs could be largely extended if necessary.