Steam turbine impulse

EDDY CURRENTS (STEAM TURBINES) - Impulse Turbine - As the steam passes through convergent nozzles, if the exit pressure is less than 0.577 x inlet pressure (the critical pressure for nozzles), eddy-currents are developed and the exit velocity will be less than calculated. 

Figure 2-1. Steam turbine using impulse blading. (Source Mitsubishi Heavy Industries.)...

A range of industrial steam turbines with a ehoiee of reaetion and impulse blading are available to satisfy these needs. They virtually guarantee an optimal solution to the various problems eneountered when eombining eompressors, expanders, and turbines to form an effieient, reliable nitrie aeid train. A typieal train is depieted in Figure 4-26. 

TTiere are two main types of expansion turbines axial flow and radial flow. Axial flow expansion turbines are like conventional steam turbines. They may be single-stage or multistage with impulse or reaction blading, or some combination of the two. Turbines of this type are used as power recovery turbines. They are used where flow rates, inlet temperatures, or total energy drops are very high. 

Figure 14-16G. Impulse type single stage steam turbine with shaft type governor. (Used by permission Westinghouse Electric Corp., Steam Div.)...

Figure 14-16H. Impulse type multistage, multivane high speed steam turbine. (Used by permission General Electric Co.)...

From an internal design perspective, the steam turbine is either an impulse-or a reaction-type design. In the United States, almost all turbine designs are of the axial flow variety, and only a small number are of the tangential flow variety. In Europe, a significant number of turbines are of the radial flow... 

Liquid turbines are large, slow-moving devices, and gas and steam turbines are small, fast-moving devices. This is easiest to see for an impulse turbine, for which we previously showed that the optimum velocity of the blade is one-half that o f the jet. So the speed of a single-stage impulse turbine rotor is set by the available jet speed. At Hoover Dam the fluid drops about 700 ft. Applying Bernou can solve for the... 

The first successful steam turbine, developed by de Laval [4], was a simple one-stage impulse turbine, as described above, and it turned at about 20,000 rpm. 

BLADE CLEARANCE (STEAM TURBINE) - Reaction turbine - the gap between blade and casing. Impulse turbine - the gap between the stationary and moving blades. 

EQUALIZING HOLE (STEAM TURBINE) - A hole in the turbine disc designed to equalize axial thrust with impulse bladed turbines. 

Turbines are classified according to their principle of operation and the type of fluid that turns them. All turbines respond to impulse or reaction movement. The four main types of turbines are steam turbines, gas turbines, wind turbines, and water turbines. The primary function of a turbine is to convert steam, gas, wind, or water energy into mechanical energy that can be used to drive rotating equipment. 

Steam turbine stage designs are generally either of an impulse or reaction type. 

The Curtis and the Parsons turbine designs are based on different fundamental principles of fluid flow. The Curtis turbine has an impulse design, where the steam expands through nozzles so that it reaches a... 

Figure 14-17C. Steam vapor flow through impulse and reaction turbines. (Used by permission Skrotzki, B. G. A. Power, p. 170, Sept. 1959. McGraw-Hill, Inc. All rights reserved.)...

Impulse turbine design employs a stationary, circular diaphragm onto which a large number of fixed-position, tear-shaped nozzle blades (vanes) are mounted. High-velocity steam moves across the vanes and produces steam jets that are directed into waterwheel-type buckets, mounted onto discs around the turbine rotor. The pressure of the steam in the buckets forces the shaft to rotate. The kinetic energy of the jets is translated into mechanical work as the shaft turns. 

Turbine A machine for generating rotary mechanical power from the energy of a stream of fluid (such as water, steam or hot gas). Turbines convert the kinetic energy of fluids to mechanical energy through the principles of impulse and reaction or a mixture of the two. 

Compounding.—For maximum efliciency, the peripheral speed of the wheel at the bucket pitch line should approach the value of one-half the steam speed. In simple impulse turbines, this leads either to very high rotative speeds or very large diameters. If the peripheral speed is reduced one-half, the efficiency decreases about one-third. Small capacity turbines of this type must therefore have ridiculously large wheels or else run very fast. Thus the original De Laval turbines used speeds of 10,000 to 30,000 r.p.m., with reduction gears. The present De Laval simple wheel uses peripheral speeds up to 1,300 ft. per second, implying 24,900 r.p.m. at 1 ft. diameter. 

Size and Capacity.— The wheel diameter of an impulse turbine is determined by speeds rather than by capacity, the buckets not being filled with steam. 

Overload capacities of impulse turbines are very high. Additional nozzles may be used, or in compound machines steam at boiler pressure may be admitted to the secondary nozzles. 

In addition to the impulse utilized on the forward edge of the buckets, most commercial pressure turbines are built in combination with an impulse wheel. The objectionable features of superheat, if used, are then confined to stationary nozzles provision for overloads is readily made and end thrust may be eliminated by causing the steam to flow both ways from a central impulse wheel through two sets of pressure elements (double flow turbine). 

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