Volute

As the air leaves the blade tips, it contains kinetic energy by virtue of its velocity. The directional component of this velocity is both rotative and radial. When the fan blades ate inclined forward, these components are cumulative. With backward-iaclined blades, the components are ia opposition. The purpose of the fan volute or scroU-shaped casiag is to convert a portion of the kinetic energy of the air leaving the blades iato static pressure. 

Frame-mounted ANSIB73.1 Wet-pit volute Axial flow impeller (propeller) volute type (horizontal or vertical)... 

An axially spHt pump typically has a volute, and offers the advantage of easy maintenance because of quick upper-half casing and rotor removal that is similar to a single-stage version. The piping, attached to the no22les located at the lower half of the casing, remains undisturbed. 

Volute casings take the form of a spiral increasing uniformly in cross-sectional area as the outlet is approached. The volute efficiently converts the velocity energy imparted to the liquid by the impeller into pressure energy. 

Multistage pumps may be of the volute type (Fig. 10-41), with single- or double-suc tion impehers (Fig. 10-42), or of the diffuser type (Fig. 10-43). They may have horizontahy split casings or, for... 

Bypassing is occurring between volute channels in a double-volute pump casing due to a casting defect or extreme erosion. 

Volute and cutwater area of casing is severely eroded. 

Centrifugal pumps also rec]uire that the fluid be available to the pump s suction nozzle with sufficient energy. Centrifugal pumps cannot suck or draw the liquid into the pump housing. The principal pumping unit of a centrifugal pump is the volute and impeller. (See Figure 1-3). 

Pitting marks on the impeller blades and on the internal volute casing wall of the pump. 

With the pump di.sa.ssemblcd in the shop, with open impellers, the damage is seen on the leading edge of the impeller blades toward the eye of the impeller, and on the blade tips toward the impeller s OD. With enclosed impellers, the damage reveals itself on the wear bands between the impeller and the volute casing. See the illustration (Figure 3-2). 

To correct this condition with an opened impeller, it s nece,s,sary to perform an impeller adjustment to correct the strict tolerance between the blades and the volute. Some back pullout pumps arc designed with jack bolts on the power end of the bearing housing to easily perform this adjustment without pump disassembly. 

With the pump disas.sembled on the shop table, the damage is seen on the blade tips at the OD of the impeller, and just behind the cutwater on the internal volute wall. 

The available areas in the impeller, and the available area in the volute determine the flow, gpm. There are Uvo eritical areas in the impeller, the exit area and the entrance area. For the volute casing, the most impeortant area is the cunvater . All fluid must pass this point. 

Head or pressure is developed in the pump when the impeller imparts rcotational energy to the liquid (increasing the liquid s velocity), and then the volute converts this energy (by decreasing the velocity ) into pressure. 

The relationship benveen the exit area of the impeller, and the cunvater area of the volute, generally determine the flow of the pump. 

This type of pump is similar to others exeept that the impellers diseharge into a diffuser hell ty pe housing instead of the volute. The diffuser has multiple veins or ribs that direct the pumped liquid through a eolumn or into the next impeller (Figure 6-10). 

RADIAL FORCE VS. DESIGN CAPACITY WITH A SINGLE AND DOUBLE VOLUTE... 

There is more information on this in Chapter 9. Dual volute casings tend to equalize the radial hydraulic forces around the pump impeller, thus expanding the operating window of the pump. The Ns is a guide in selecting the adequate volute design. 

The family curve shows the range of different impeller diameters that ean run inside the pump volute. They re normally presented as various parallel H-Q eurves eorresponding to smaller diameter impellers. 

Except for the curve of the PE) pump, the other pump curves show various diameter impellers that can be u.sed within the pump volute. And, on all the.se family curves, the effieieneies are. seen as eoneentrie ellipses. There is very little variation in the presentation of the BHp and... 

Let s begin with a discussion and explanation on how a volute centrifiigal pump works. 

The volute type pump has its impeller mounted eccentrically within the volute. The degree of eccentricity governs the pressure that the pump can generate. If the impeller were concentric inside the volute, or equidistant, the pump would generate flow, but not much pressure or head (Figure 9-1). 

The impeller throws the liquid against the volute wall at a constant speed, the speed of the electric motor. The internal diameter of the volute wall converts the velocity into head or pressure (Figure 9-2). 

See Table opposite for what is happening inside the pump around the internal volute wall. 

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