IMPULSE TURBINE :
Diagram (a) and (b) illustrate the principle of impulse.
To make an impulsive force a high velocity jet should strike a blade. If series of blades are mounted on the wheel (rotor), each blade in turn receives the impulsive force resulting in high rotational speed.
In Impulse Turbine the high velocity jet is produced by expanding steam in a nozzle. Steam from the boiler is at high pressure and high temperature. When steam expands in a nozzle, pressure drops which is followed by enthalpy (heat) drop. This heat energy is converted into kinetic energy which appears in the form of high velocity steam jet. This jet or number of jets directed towards a wheel with blades fixed on its periphery produces motive force. Steam flows over the blades and velocity gradually reduces. One conspicuous thing is that pressure remains constant as steam passes over the blades in the impulse turbine and it is almost atmospheric.
Impulse turbine is classified as :
(i) single stage turbine or simple impulse wheel (Ex., De Laval turbine)
(ii) multiple velocity stage turbine (Ex., Curtis turbine)
(iii) multiple pressure stage turbine (Ex.. Rateau or Zoelly turbine)
(iv) pressure - velocity compounded turbines [combination of (ii) and (iii)]
De-LAVAL IMPULSE TURBINE:
De-Laval (1881), a Swedish engineer worked on the idea given by Giovanni Branca in 1629 and devised a simple impulse wheel.
De-Laval turbine is the simplest form of a single stage impulse steam turbine. Diagram shows the turbine in action. Main components of a De-Laval Turbine are
The runner is a circular disc mounted on a horizontal shaft. Number of blades are fixed uniformly on its periphery. The blades are symmetrically curved and active surface is polished. They are made of steel alloy to resist corrosion and impact strength.
Casing that houses the runner is an air-tight metallic chamber. Nozzles are located around the inner periphery of the casing inclined at about 20° to the wheel tangent. Steam issuing out of these nozzles strike the set of blades at a number of points.
The smallest De-Laval wheel has a diameter of 125 mm and speed of 30,000 r.p.m. It is suitable for low pressure steam supplies. Steam expands down to atmospheric pressure in the nozzle and pressure remains constant as it flows over the blades. The blades are made symmetrical with angles of about 30° at inlet and exit. The power developed is about 3.7 kW and the blade speed is about 220 m/s.
Demerits : The high speed of rotation will, for mechanical reasons, such as centrifugal force, stresses etc., restrict the size of the wheel. It also needs a reduction gear box unit when coupled to an electrical generator to run the machine at practical speed limits.
COMPOUNDING OF IMPULSE TURBINES :
OR STEAM TURBINE STAGING :
OR METHODS OF REDUCING ROTOR SPEED :
Necessity : The absolute velocity of steam entering the turbine, if expanded in a single stage, would be as high as 1500 m/s. For maximum efficiency blade speed will be correspondingly very high (about 700 to 800 m/s.). This results in abnormal rotational speeds of rotor (20,000 to 30,000 rpm).
Such high rotational speeds demand large rotor diameters. Moreover at high speeds, the centrifugal stresses become excessive and there is a risk of structural failure. Besides, with single stage, there would be considerable (10 to 12%) loss of kinetic energy. These considerations limit the blade speed to a maximum value of 400 m/s.
In view of the above, compounding of turbine (reducing rotor speeds) has become essential, ofcourse, making full use of kinetic energy st in the steam. By increasing the stages higher outputs are also posalc. Methods of compounding are
1. Velocity compounding
2. Pressure compounding
3. Pressure - Velocity compounding
Velocity Compounding (Curtis Stage):
This involves conversion of total enthalpy into kinetic energy in one stage and dividing this K.E. into mechanical energy in subsequent stages of blades. Quite a number of wheels are mounted on the shaft. Alternatively the wheels are keyed or press fitted to the shaft and the blades carried by them are called moving blades, as they move with the shaft. The wheels which are simply mounted' on the shaft coaxially carry the blades called fixed or guide blades. Diagram. shows variation of pressure and velocity graphically.
Velocity Compounded Turbine was first devised by Curtis and is called Curtis stage Impulse Turbine. Its features are
• lower speed ratios
• higher utilisation of K.E
• large pressure drop in one set of nozzles
• velocity drops while steam flows over moving blades and remains almost same on fixed blades
• axial discharge and maximum efficiency is possible.
Advantages : Low initial cost; Compact; Reliable; Easy to operate; Simple casing.
Disadvantages : High frictional losses; Blade Speed Ratio is less than optimum value; Maximum efficiency is relatively lower.
Applications : Curtis turbines are used to drive centrifugal pressors, pumps, small generators, feed pumps in boiler plants.
Pressure Compounding (Rateau Stage) :
A number of simple impulse turbines arranged in series form a pressure compounded (Rateau) turbine. In this method the entire pressure drop is distributed over number of stages; each stage consisting of nozzle ring followed by a ring of moving blades. The velocity is absorbed completely in the moving blade ring. The pressure of steam is constant during its flow over moving blade rings.
In the nozzles (N) only a small pressure drop and so enthalpy drop occurs giving limited increase of K.E.
Since pressure gradually reduces, volume of steam will increase; hence, blade height has to be increased towards the low pressure side. Rateau and Zoelly turbines are pressure compounded turbines.
This is the most efficient method of compounding since speed ratio remains constant. But it is expensive owing to a large number of stages.
Pressure - Velocity Compounding :
This method involves combination of pressure compounding and velocity compounding. The total pressure drop (p1 — pb ) is divided into stages and the velocity obtained in each stage is also compounded. Pressure drop in each stage being considerably high, less number of stages are necessary for a given output. Thus size of the pressure - velocity compounded turbine is comparatively smaller.
The diameter of the turbine increases after each stage so as to accommodate increase of volume of steam.
This method of compounding is used on Curtis and Moore turbine.