Impulse blading system design & Bull nosed blades

Impulse blading system design

Hence, maximum blade efficiency is when entrance angle is at 0^o and when the blade is rotating at 1/2 the speed of the jet stream
As the steam must enter at an angle a^o

Optimum value for U / Ci = 1/2 cos a ( 0.45 to 0.48 )
Maximum blade efficiency = Cos² a (14^o to 20^o)
Impulse blading may have up to 20% reaction effect at mean blade height.
Astern turbines generally consist of a single wheel on which are mounted a tow stage velocity compound followed by a single stage wheel

Properties required of the blade material
• Good tensile and fatigue strength
• Toughness and ductility at working temperature
• Resistance to corrosion and erosion
• Rate of expansion similar to both rotor and casing
• Machinability
• Low density
• Good vibration dampening properties
• Good crep resistance
• Weldability

Typical blade material is
• 11.5 to 13.5% Chromium
• 1% Nickel
• 1% Manganese
• 1% Silicon
• 0.12% Carbon
• Trace Sulphur & phosphorus

Low tensile stainless steel preferred to high tensile stainless iron due to better fatigue resistance. Where lacing wires are to be brazed in special care must be made as to the intergrannular penetration effects of the braze

Bull nosed blades

Standard blades have the same inlet and outlet angles.
Bull nosed blades are capable of accepting a wide range of steam angles without serious increase in blade losses.The cross sectional area is increases and hence the blade is stronger and better resistant to vibration. The increase thickness also allows a circular tang to be fitted for attaching a shroud. Non circular such as square tangs require the shroud to be punched rather than drilled which introduces residual stress, micro-cracking etc.