2. Efficiencies of an impulse Turbine
• HYDRAULIC EFFICIENCY :it is the ratio of work done on the
wheel to the energy of the jet.
work done/ Kinetic Energy of the Jet=ȵℎ= ……
• Maximum hydraulic efficiency= max ȵℎ=…..
• OVERALL EFFICIENCY: it is the ratio of actual power produce by
the turbine to the energy actually supplied by the turbine.
Power available at the turbine shaft/power available from the
water jet
• ȵ= shaft power/water power=P/wQH
2
3. 2. Design of Pelton Wheels
In general, a Pelton wheel is designed for the given head of
water, power to be developed and speed of runner. General
design parameters are:
• Diameter of the Wheel
• Diameter of the Jet
• Size of buckets (i.e. width and depth)
• No. of buckets
3
4. General assumptions:
• Overall efficiency between 80% to 87% (preferably 85%)
• Coefficient of velocity 0.99 (preferably 0.985)
• Ratio of peripheral velocity to velocity of jet as 0.46
4
5. • Size of buckets
The bucket of Pelton wheel have following dimensions:
Width of bucket = 5 x d
Depth of bucket = 1.2 x d
where d = diameter of the jet.
• Most vital component of Pelton wheel is its bucket.
Buckets are casted as single solid piece, in order to avoid
fatigue failure. 5
6. • No. of buckets is mainly decided on the basis of
following principles:
1. The number of buckets must be as few as possible
so there may be as little loss due to friction as
possible
2. The jet of water should be fully utilized so that no
water from the jet should go wasted.
6
• Number of buckets are find by the following formula.
Number of buckets = [
𝐷
2𝑑
+ 15]
D = mean bucket diameter
d = diameter of jet.
7. Example: Design a Pelton wheel for a head of 350 m at a speed
of 300 r.p.m. Thake overall efficiency of the wheel as 85 % and
the ratio of the jet to the wheel diameter as 1/10.
Example: 32.9, 32.10,
Exercise 1,2
7
8. 8
3. Governing of an Impulse Turbine
• It is the operation by which the speed of the turbine is
kept constant under all conditions of working load.
• This is done automatically by a governor which regulates
the rate flow through the turbines according to the
changing load conditions on the turbine.
• Governing of a turbine is absolutely necessary if the
turbine is coupled to an electric generator which is
required to run at constant speed under all fluctuating
load conditions.
9. • Servomotor or relay cylinder method is the commonly used
method. It consists of:
1. Centrifugal governor
2. Control valve
3. Servomotor
4. Gear pump
5. Oil pump
6. Spear or needle
7. A set of pipes connecting oil sump with control valves
and control valves with the relay cylinders.
9
