Turbine 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.

De Laval Impulse Turbine-Single Stage

Optimum efficiency occurs when the blade is moving at half the speed of the jet stream. To achieve this very high rotational speeds would be required ( in the order of 15000 rpm). High centrifugal stress, high journal speed and excessive gearing requirements prohibits the use of such system for propulsion by itself.
This system is often found as the first stage of a HP turbine were a large pressure drop is required to allow for a smaller turbine. Only the nozzle box has to cope with full boiler pressure and temperatures simplifying design especially of gland boxes. Special material requirements are again restricted to nozzle box. Reduced pressure within the following stages reduces tip leakage
The steam leaving the blades has a high kinetic energy indicating high leaving loss.

Pressure Compounding (Rateau)

The overall heat and pressure drop is divided between the stages. The U/Ci ratio is 0.5 for each stage. By careful design the rotor mean diameter may be kept to a minimum.
Excessive number of stages produces an overly long rotor, these leads to problems of critical vibration, increased rotor diameter, increased stage losses due friction and windage and increased gland leakage both at the main glands and the diaphragm plate glands. This due to the increased number of glands and the increased rotor diameter.
Stage mean diameter and nozzle height are increased at the LP end as the steam expands to the limits of centrifugal stress. Nozzle and/or blade angles may be altered to accommodate the increase in volume reducing the requirement to increase blade height excessively.This is referred to as taper-twisting
The blade height increase towards the LP end means that the rotational velocity also increases. Hence for the same value of U/Ci they can deal with higher inlet steam velocities and hence higher enthalpy drops p>The design produces a short lightweight turbine used where size, weight and strength are more important than efficiency. E.G. feed pumps , astern turbines and the inlet portion of HP turbines where it provides a large initial drop in temperature and pressure lightening the rotor and reducing the need for high grade alloys for remaining stages

Velocity Compounded (Curtis)

For a two stage system U/Ci = 1/4, for a three stage system U/Ci = 1/6
There is no pressure drop except in the nozzle ( although in practice some drop occurs due to losses as the steam passes over the blade). Dividing the velocity drop across the stages leads to a loss of efficiency but gives a more acceptable blade speed reducing centrifugal stress and simplifying gearing arrangement.
For a three row system, the steam speed at inlet to the first row is 6 times the blade speed, reducing the velocity makes the conditions at the final stages close to ideal.
To maintain the same mass flow for the reducing velocity, blade height is increased to the limit of centrifugal forces. Taper-twisting and flattening of the blade angle is then given to the final stage blades.
Some reheating occurs due to friction of the fixed blades associated with a loss of velocity of about 12%
Theoretically efficiency is independent of the row number. However in practice efficiency and work done in final stages reduces and therefore overall efficiency drops with increase rows.

• Typical values for efficiency are
  • two wheel curtis 68%
  • three wheel curtis 50%
  • Single wheel rateau 85%

Pressure-Velocity Compound

This system gives the advantage of producing a shortened rotor compared to pure velocity compounding. In addition it also removes the problem of very high inlet steam velocities and the reduction in efficiency and work done in the final stages.
In this design steam velocity at exit to the nozzles is kept reasonable and thus the blade speed (hence rotor rpm) reduced.
Typical applications are large astern turbines

Reaction

U=Blade speed
Ci= velocity of steam at inlet to blade, i.e. leaving nozzle( giving nozzle angle)
Ci rel= velocity of steam relative to the blade( giving blade inlet angle)
Co= Velocity of steam at outlet of blade

Parsons Impulse-Reaction

The original blade design was thin section with a convergent path. Blohm & voss designed blades similar to bull nose impulse blades which allowed for a convergent-divergent path. However due to the greater number of stages the system did not find favor over impulse systems
U/Ci = 0.9
If the heat drop across the fixed and moving blades are equal the design is known as half degree reaction.
Steam velocity was kept small on early designs, this allowed the turbine to be directly coupled to the prop shaft.
Increased boiler pressure and temperature meant that the expansion had to take place over multiple rotors and gearset.
As there is full admission over the initial stage, blade height is kept low. This feature alone causes a decrease in blade and nozzle efficiency at part loading. In addition, although clearances at the blade tips are kept as small as practical, steam leakage causes a proportionally higher loss of work extracted per unit steam
Blade tip clearances may be kept very tight so long as the rotor is kept at steady state.
Manoeuvring, however, introduces variable pressures and temperatures and hence an allowance must be made.
End tightening for blades is normally used. This refers to an axial extension of the blade shroud forming a labyrinth. When the rotor is warmed through a constant check is made on the axial position of the rotor. Only when the rotor has reached its normal working length may load be introduced. Alternatively tip tightening may be used referring to the use of the tips of the blade to form a labyrinth against the casing/rotor. This system is requires a greater allowance for loading and is not now generally used.
To keep annular leakage as small as possible these rotors tend to have a smaller diameter than impulse turbines.
To keep the mass flow the same with the increasing specific volume related to the drop in pressure requires an increase in axial velocity, blade height or both -see above. Altering the blade angle will also give the desired effect but if adopted would cause increased manufacturing cost as each stage would have to be individual. Generally the rotor and blading is stepped in batches with each batch identical.
The gland at the HP end is subjected to full boiler conditions and is susceptible to rub. The casing must be suitably designed and manufactured from relevant materials.
A velocity compounded wheel is often used as the first stage(s) giving a large drop in conditions allowing simpler construction of casing and rotor and reducing length. Special steels are limited to the nozzle box.

Dummy piston arrangement on Parsons Turbines

In parsons reaction turbines there is always an end thrust due to the steam at inlet being higher than the exhaust. This leads to high thrust bearing loading. The dummy piston arrangement is a wheel or drum integral to the rotor. Forces are balanced by the drum offering a greater surface area to the low pressure balancing steam than to the HP steam.Note the drawing above is not to scale. A labyrinth arrangement is fitted to seal the drum.

Double Flow Turbines

These are found mainly on large LP turbines. Here steam enters mid rotor and passes axially towards both ends. The advantages are;

• End thrust is balanced removing need for dummy pistons or cylinders on reaction turbines . Reduces the size of the thrust on impulse-reaction turbines
• As steam flow is split the final stages blade height and angle is reduced allowing for increased efficiency and reduced centrifugal stress. Greater power per unit size may be absorbed.

The main disadvantage of this system is increase rotor length leading to increased risk of sagging

Blade Sealing

May be end or tip tightening

End Tightening
This is seen particularly on reaction turbines. It requires accurate positioning of the turbine rotor and is normally associated with lengthy warm up perios during which the position of the rotor is carefully monitored. Operational limitations on rapid power changes may be in place. The author has seen this system in use on very large but compact turbo alternators which required a warm up period consisting of increaseing the rotor speed in stages over one hour

Tip Tightening
Clearance is governed by maximum blade centrifugal stretch