Impulse blading

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. 

Impulse machines are used for small turbines, and the high-pressure section of large turbines use impulse blades. The low-pressure sections of large turbines tend to use reaction blades. 

The only energy available to an impulse blade is the kinetic eneigy at the outlet from the inlet nozzle, and we tiuy assume that a perfectly efflcient impulse blade would convert all this energy into work, so that... 

It is important to emphasize the limitations on the use of this equation. It is essentially a design equation and it represents the variation in blade efficiency with blade/gas speed ratio for a turbine impulse blade under the following conditions ... 

This fraction of the kinetic eneigy possessed by the mid-stage gas stream will be converted into additional enthalpy. There is no mechanism for reconverting this enthalpy into kinetic energy in an impulse blade, so there will be no recovery of any energy lost at the blade entry in an impulse stage. A reasonable estimate may be 1. The loss correction factor to be applied to the initial value of blade efficiency is simply (l-X ). The final calculation of blade efficiency for an operating impulse blade is thus... 

Off-design conditions in an impulse blade typical corrections for kinetic energy losses... 

Equation (15.88) contains a term not possessed by the equivalent equation (15.31) for the impulse blade, namely the enthalpy difference, h — A250. the magnitude of which depends on conditions upstream and... 

Meanwhile, the actual specific work abstracted, w, is given by the same equation as for the impulse blade, namely equation (15.31), repeated below ... 

This is the same equation as (15.47), derived in analysing the performance of the impulse blade.)... 

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

Single-row impulse stages have a maximiun efficiency of about 86 percent at a velocity ratio of 0.45. Figure T-69a shows a combination of impulse buckets with an expanding nozzle, and Fig. T-69d shows multistage impulse blading with nonexpanding nozzles. 

For the examination of the applied metallic or ceramic layer, the test object is heated up from the outside The heat applying takes place impulse-like (4ms) by xenon-flash lamps, which are mounted on a rack The surface temperature arises to approx 150 °C Due to the high temperature gradient the warmth diffuses quickly into the material An incorrect layer, e g. due to a delamiation (layer removal) obstructs the heat transfer, so that a higher temperature can be detected with an infrared camera. A complete test of a blade lasts approximatly 5 minutes. This is also done automatically by the system. In illustration 9, a typical delamination is to be recognized. 

The large temperature difference of the remarkable borehole, opposite other boreholes and their environment is significant. This high temperature difference is a typical feature for a small wall thickness between borehole and blade surface. For technical reasons, precise eroding of the boreholes is difficult. Due to this, the remaining wallthickness between the boreholes and the blade surface has to be determined, in order to prevent an early failure, Siemens/Kwu developed a new method to determine the wallthickness with Impulse-Video-Thermography [5],... 

Turboexpanders can be classified as either axial or radial. Axial flow expanders have either impulse or reaction type blades and are suitable... 

Because of the smaller blade angle the reaction stage is more efficient than the impulse stage, but it requires more stages for the same... 

Impulse Turbine The impulse turbine is the simplest type of turbine. It consists of a group of nozzles followed by a row of blades. The gas is expanded in the nozzle, converting the high thermal energy into kinetic energy. This conversion can be represented by the following relationship ... 

An example of a typical turboexpander is shown in Fig. 29-46. Radial-flow turbines are normally single-stage and have combination impulse-reaction blades, and the rotor resembles a centrifugal-pump impeller. The gas is jetted tangentially into the outer periphery of the rotor and flows radially inward to the eye, from which the gas is jetted backward by the angle of the rotor blades so that it leaves the rotor without spin and flows axially away. 

Variable nozzles produce a series of jets of gas entering the rotor, and these impulses add up to form a frequency equal to the blade-passing frequency the number of revolutions per second multiplied by the number of nozzle vanes, which is of the order of thousands of cycles per second. Frequently the rotor will resonate at this frequency, and if it does, it will be fatigued and crack and break up thus these frequencies must be avoided, and the manufac turer should be asked to supply information to the customer on this subject. 

As mentioned earlier, turboexpander are generally of radial reaetion turbine design beeause this geometry is often most effieient. In an ordinary impulse turbine the high veloeity stream from the nozzles makes a U-turn in the rotor blades, and this U-turn eonsumes 8%-10% of the energy. 

The two types of turbines—axial-flow and radial-inflow turbines—can be divided further into impulse or reaction type units. Impulse turbines take their entire enthalpy drop through the nozzles, while the reaction turbine takes a partial drop through both the nozzles and the impeller blades. 

Figure 9-6 shows a diagram of a single-stage impulse turbine. The statie pressure deereases in the nozzle with a eorresponding inerease in the absolute veloeity. The absolute veloeity is then redueed in the rotor, but the statie pressure and the relative veloeity remain eonstant. To get the maximum energy transfer, the blades must rotate at about one-half the veloeity of the gas jet veloeity. Two or more rows of moving blades are sometimes used in eonjunetion with one nozzle to obtain wheels with low blade tip speeds and stresses. In-between the moving rows of blades are guide vanes that redireet the gas from one row of moving blades to another as shown in Figure 9-7. This type of turbine is sometimes ealled a Curtis turbine.

Another impulse turbine is the pressure eompound or Ratteau turbine. In this turbine the work is broken down into various stages. Eaeh stage eonsists of a nozzle and blade row where the kinetie energy of the jet is absorbed into the turbine rotor as useful work. The air that leaves the moving blades enters the next set of nozzles where the enthalpy deereases further, and the veloeity is inereased and then absorbed in an assoeiated row of moving blades. 

The relative veloeity fV remains unehanged in a pure impulse turbine, exeept for frietional and turbulenee effeet. This loss varies from about 20% for very high-veloeity turbines (3000ft/see) to about 8% for low-veloeity turbines (500ft/see). Sinee the blade speed ratio is equal to (eosa)/2 for maximum utilization, the energy transferred in an impulse turbine ean be written... 

Figure 6-5(b) is a case for a reaction of unity, that is, all the pressure rise is in the rotor, with the stator blades acting only as guide vanes to deflect the gas. A reaction of unity is aerodynamically the equivalent of R = 0 or impulse, as shown at Figure 6-5(f) since it corresponds to an interchange of the moving and fixed blade row. 

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. 

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