The basic idea of an impulse turbine is that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward. The velocity of the steam is about twice as fast as the velocity of the blades. Only turbines utilizing fixed nozzles are classified as impulse turbines. The simple Impulse turbine - It primarily consists of: a nozzle or a set of nozzles, a rotor mounted on a shaft, one set of moving blades attached to the rotor and a casing. - A simple impulse turbine can be diagrammatically represented below. The uppermost portion of the diagram shows a longitudinal section through the upper half of the turbine, the middle portion shows the actual shape of the nozzle and blading, and the bottom portion shows the variation of absolute velocity and absolute pressure during the flow of steam through passage of nozzles and blades. Example: de-Laval turbine What so special about impulse turbine? Impulse turbine blades is shaped so uniformly that it curved uniformly along its symmetrical half line, which is like the exact image cut to halve by a mirror in the centre.So what it does as steam enters along one end of blade it is deflected , curled, without pressure drop, only change in vector , hence reduce in kinetic energy, ie velocity of steam .In theory, the entry speed and exit speed is the same, but in real, the speed slightly reduced due to friction and eddy losses. Lets have look at the diagram . Did you notice the pressure drop occur as the steam pass through the nozzle? Notice also the maintain as the steam pass through the blades or buckets?Now observe the steam speed , it increase sharply as it pass through the nozzle, then it drop equally as it pass through the blades. Now lets have a look at the rotor and stator arrangement of a turbine. A Rotor consists of buckets/movable blades and which turns and fixed blades which act as sets of nozzles. The Stator fixed consist of diaphragm plates or nozzles and attached to turbine casing. The turbine rotor is arranged so that movable blades and fixed blades is laid out one after the other. In real, pure impulse blade is only theoretical. Impulse characteristic of the turbine blade profile is always have raction profile to it, normally by 50 %, shaped like a tear drop An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. As the steam flows through the nozzle its pressure falls from steam chest pressure to condenser pressure (or atmosphere pressure). Due to this relatively higher ratio of expansion of steam in the nozzle the steam leaves the nozzle with a very high velocity. The steam leaving the moving blades is a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to this higher exit velocity is commonly called the "carry over velocity" or "leaving loss". Compounding of impulse turbine - This is done to reduce the rotational speed of the impulse turbine to practical limits. (A rotor speed of 30,000 rpm is possible, which is pretty high for practical uses.) - Compounding is achieved by using more than one set of nozzles, blades, rotors, in a series, keyed to a common shaft; so that either the steam pressure or the jet velocity is absorbed by the turbine in stages.
Posted on: Tue, 01 Oct 2013 01:16:34 +0000