Vane tip

Fig. 16. Gaps A (between impeller shrouds and diffusor wall) and B (between impeller and diffusor vane tips) corrective option where D = diameter and is diameter reduction (a) overall diameter reduction (b) vane diameter reduction (c) angle-cut, single-suction impeller and (d) angle-cut,...

Semi-enclosed—used for general purpose applications, has open vane tips at entrance to break up suspended particles and prevent clogging. 

The impeller is the working part of a centrifugal pump. The function of the impeller is to increase the velocity or kinetic energy of the liquid. The liquid flows into the impeller, and leaves the impeller, at the same pressure. The black dot shown at the top of the impeller in Fig. 23.6 is called the vane tip. The pressure at the vane tip is the same as the pump s suction pressure. However, as the high-velocity liquid escapes from the impeller and flows into the volute, its velocity decreases. The volute (which is also called the diffuser) is shaped like a cone. It widens out in the manner illustrated in Fig. 23.7. As the liquid flows into the wider section of the volute, its velocity is reduced, and the lost velocity is converted—well, not into pressure, but into feet of head. 

The impeller itself may be worn, or the vane tips at the edge of the impeller might be improperly machined. 

Consider Figure 8-26 showing a closed impeller with pump-out vanes on the back shroud. The impeller main vane tip radius is i 2- but the pump-out vanes extend only to the radius A shaft sleeve behind the impeller has R as a tip radius. In the front shroud of the impeller, another set of pump-out vanes extend to the radius Rf and provide dynamic sealing between the impeller and the throatbush to repel any solids that may tend to slip toward the suction (where the pressure is obviously lower). As the impeller rotates, a pressure field develops on the front shroud of the impeller due to the front pump-out vanes, and another pressure field develops behind the impeller due to the back pump-out vanes. In an ideal world, both fields should balance each other. In reality, wear of these vanes and the difference of clearance between the front and the back vanes with respect to the casing or its liners tend to create an unbalance. 

The problem can be corrected by realigning the fan assembly. Alternately, vane tip clearance mesh can be attached to the interior of the shroud, which will eliminate excessive blade-to-shroud clearance and thus push more cooling air through the finned tube bundle. 

Alternately, the shroud itself may become distorted with age. This will also cause the clearance between the blade tips and the inside of the shroud to increase. This vane tip clearance mesh is also used to correct this problem, which promotes air recirculation and diminishes air flow through the bundle. 

The nozzles, used to aeeelerate the flow toward the impeller tip, are usually straight vanes with no airfoil design. The vortex is a vaneless spaee and allows an equalization of the pressures. The flow enters the rotor radially at the tip with no appreeiable axial veloeity and exits the rotor through the exdueer axially with little radial veloeity. 

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.

Figure 5-24. Vector tip triangle without slip, showing the effect of different exit vane angles.

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