aThese parts are called out in Figures 3, 4, and 8.

aThese parts are called out in Figures 3, 4, and 8.

wall dividing the initial section and the discharge nozzle portion of the casing is called the tongue of the volute or the "cutwater." The diffusion vanes and concentric casing of a diffuser pump fulfill the same function as the volute casing in energy conversion.

In propeller and other pumps in which axial-flow impellers are used, it is not practical to use a volute casing. Instead, the impeller is enclosed in a pipe-like casing. Generally, diffusion vanes are used following the impeller proper, but in certain extremely low-head units these vanes may be omitted.

A diffuser is seldom applied to a single-stage, radial-flow pump, except in special instances where volute passages become so small that machined or precision-cast volute or diffuser-like pieces are utilized for precise flow control. Conventional diffusers are often applied to multistage pump designs in conjunction with guide vanes to direct the flow efficiently from one impeller (stage) to another in a minimum radial and axial space. Diffuser vanes are used as the primary construction method for vertical turbine pumps and singlestage, low-head propeller pumps (see Figure 4).

Radial Thrust In a single-volute pump casing design (see Figure 5), uniform or near uniform pressures act on the impeller when the pump operates at design capacity (which coincides with the best efficiency). At other capacities, the pressures around the impeller are not uniform (see Figure 6) and there is a resultant radial reaction (F). A detailed discussion of the radial thrust and of its magnitude is presented in Subsection 2.3.1. Note that the unbalanced radial thrust increases as capacity decreases from that at the design flow.

For any percentage of capacity, this radial reaction is a function of total head and of the width and diameter of the impeller. Thus, a high-head pump with a large impeller diameter will have a much greater radial reaction force at partial capacities than a low-head

FIGURE 4 Vertical wet-pit diffuser pump bowl (the FIGURE 5 Uniform casing pressures exist at number refers to parts listed in Table 1) (Flowserve design capacity, resulting in zero radial reaction.


FIGURE 4 Vertical wet-pit diffuser pump bowl (the FIGURE 5 Uniform casing pressures exist at number refers to parts listed in Table 1) (Flowserve design capacity, resulting in zero radial reaction.


pump with a small impeller diameter. A zero radial reaction is not often realized; the minimum reaction occurs at design capacity.

Although the same tendency for unbalance exists in the diffuser-type pump, the reaction is limited to a small arc repeated all around the impeller. As a result, the individual reactions cancel each other out as long as flow is constantly removed from around the periphery of the diffuser discharge. If flow is not removed uniformly around its periphery, a pressure imbalance may occur around the diffuser discharge that will be transmitted back through the diffuser to the impeller, resulting in a radial reaction on the shaft and bearing system.

In a centrifugal pump design, shaft diameter as well as bearing size can be affected by the allowable deflection as determined by the shaft span, impeller weight, radial reaction forces, and torque to be transmitted.

Formerly, standard designs compensated for radial reaction forces encountered at capacities in excess of 50 percent of the design capacity for the maximum-diameter impeller of the pump. For sustained operations at lower capacities, the pump manufacturer, if properly advised, would supply a heavier shaft, usually at a much higher cost. Sustained operations at extremely low flows without the manufacturer being informed at the time of purchase are a much more common practice today. This can result in broken shafts, especially older designs, on high-head units.

Because of the increasing application of pumps that must operate at reduced capacities, it has become desirable to design standard units to accommodate such conditions. One solution is to use heavier shafts and bearings. Except for low-head pumps in which only a small additional load is involved, this solution is not economical. The only practical answer is a casing design that develops a much smaller radial reaction force at partial capacities. One of these is the double-volute casing design, also called the twin-volute or dual-volute design.

The application of the double-volute design principle to neutralize radial reaction forces at reduced capacity is illustrated in Figure 7. Basically, this design consists of two 180° volutes, and a passage external to the second joins the two into a common discharge. Although a pressure unbalance exists at partial capacity through each 180° arc, the two

FIGURE 6 At reduced capacities, uniform pressures do not exist in a single-volute casing, resulting in a radial reaction f.

FIGURE 7 Transverse view of a double-volute casing pump.

FIGURE 6 At reduced capacities, uniform pressures do not exist in a single-volute casing, resulting in a radial reaction f.

FIGURE 7 Transverse view of a double-volute casing pump.

forces are approximately equal and opposite. Thus, little if any radial force acts on the shaft and bearings. Subsection 2.3.1 also covers this topic.

The double-volute design has many hidden advantages. For example, in large-capacity medium- and high-head single-stage vertical pumps, the rib forming the second volute that separates it from the discharge waterway of the first volute strengthens the casing (see Figure 8).

The individual stages of a multistage pump can be made double volute, as illustrated in Figure 9. The kinetic energy of the pumped liquid discharged from the impeller must be transformed into pressure energy and then must be turned 180° to enter the impeller of the next stage. The double volute therefore also acts as a return channel. The back view in Figure 9 shows this as well as the guide vanes used to straighten the flow into the next stage. An alternative to the volute design for multistage pumps is the diffuser and its return vanes that channel the flow from the discharge of the diffuser vanes back into the impeller of the next stage.

Solid and Split Casings Solid casing implies a design in which the discharge waterways leading to the discharge nozzle are all contained in one casting or fabricated piece. The casing must have one side open so that the impeller can be introduced into it. Because the sidewalls surrounding the impeller are actually part of the casing, a solid casing, strictly speaking, cannot be used, and designs normally called solid casing are really radially split (refer to Figure 1 and see Figures 11, 12, and 13).

A split casing is made of two or more parts fastened together. The term horizontally split had regularly been used to describe pumps with a casing divided by a horizontal plane through the shaft centerline or axis (see Figure 10). The term axially split is now preferred. Because both the suction and discharge nozzles are usually in the same half of the casing, the other half may be removed for inspection of the interior without disturbing the bearings or the piping. Like its counterpart horizontally split, the term vertically split is poor terminology. It refers to a casing split in a plane perpendicular to the axis of rotation. The term radially split is now preferred.

End-Suction Pumps Most end-suction, single-stage pumps are made of one-piece solid casings. At least one side of the casing must be open so that the impeller can be assembled in the pump. Thus, a cover is required for that side. If the cover is on the suction side, it becomes the casing sidewall and contains the suction opening (refer to Figure 1). This is called the suction cover or casing suction head. Other designs are made with casing covers (see Figure 12) and still others have both casing suction covers and casing covers (refer to Figure 8 and see Figure 13).

FIGURE 8 The sectional view of a vertical-shaft, end-suction pump with a double-volute casing (the numbers refer to parts listed in Table 1) (Flowserve Corporation)

FIGURE 9 The double volute of a multistage pump, front view (left) and back view (right) (Flowserve Corporation)

For general service, the end-suction, single-stage pump design is extensively used for small pumps with a 4- or 6-in (102 or 152 mm) discharge size for both motor-mounted and coupled types. In these pumps, the small size makes it feasible to cast the volute and one side integrally. Whether or not the seal chamber side or the suction side is made integrally with the casing is usually determined by the most economical pump design.

For larger pumps, especially those for special services such as sewage handling, there is a demand for pumps of both rotations. A design with separate suction and seal chamber heads permits the use of the same casing for either rotation if the flanges on the two sides

FIGURE 11 End-suction pump with semi-open impeller (Flowserve Corporation)

are made identical. There is also a demand for vertical pumps that can be disassembled by removing the rotor and bearing assembly from the top of the casing. Many horizontal applications of the pumps of the same line, however, require partial dismantling from the suction side. Such lines are most adaptable when they have separate suction and casing covers.

Casing Construction for Open- and Semiopen-Impeller Pumps In the open- or semiopen impeller pump, the impeller rotates within close clearance of the pump casing or suction cover (refer to Figure 11). If the intended service is abrasive, a side plate is mounted within the casing to provide a renewable close-clearance guide to the liquid flow-

FIGURE 12 End-suction pump with semi-open impeller, inducer and renewable side plate (Flowserve Corporation)

removable removable

FIGURE 13 End-suction pump with removable suction and stuffing box heads (Flowserve Corporation)

ing through the impeller (refer to Figure 12). One of the advantages of using side plates is that abrasion-resistant material, such as stainless steel, can be used for the impeller and side plate, while the casing itself may be of a less costly material. Although double-suction, semiopen-impeller pumps are seldom used today, they were common in the past and were generally made with side plates.

In order to maintain pump efficiency, a close running clearance is required between the front unshrouded face of the open or semiopen impeller and the casing, suction cover, or side plate. Pump designs provide either jackscrews or shims to adjust the position of the thrust bearing housing (and, as a result, the axial position of the shaft and impeller) relative to the bearing frame.

Pre-rotation and Stop Pieces Improper entrance conditions and inadequate suction approach shapes may cause the liquid column in the suction pipe to spiral for some distance ahead of the impeller entrance. This phenomenon is called pre-rotation, and it is attributed to various operational and design factors in both vertical and horizontal pumps.

Pre-rotation is usually harmful to pump operation because the liquid enters between the impeller vanes at an angle other than that allowed in the design. This frequently lowers the net effective suction head and the pump efficiency. Various means are used to avoid pre-rotation both in the construction of the pump and in the design of the suction approaches.

FIGURE 14 Possible positions of discharge nozzles for a specific design of an end-suction, solid-casing, horizontalshaft pump. The rotation illustrated is counterclockwise from suction end.

FIGURE 14 Possible positions of discharge nozzles for a specific design of an end-suction, solid-casing, horizontalshaft pump. The rotation illustrated is counterclockwise from suction end.

FIGURE 15 A bottom-suction, axially split casing single-stage pump (Flowserve Corporation)

Practically all horizontal, single-stage, double-suction pumps and most multistage pumps have a suction volute that guides the liquid in a streamline flow to the impeller eye. The flow comes to the eye at right angles to the shaft and separates unequally on the two sides of the shaft. Moving from the suction nozzle to the impeller eye, the suction waterways are reduced in area, meeting in a projecting section of the sidewall dividing the two sections. This dividing projection is called a stop piece. To minimize pre-rotation in end-suction pumps, a radial-fin stop piece projecting toward the center is sometimes cast into the suction nozzle wall.

Nozzle Locations The discharge nozzle of end-suction, single-stage horizontal pumps is usually in a top-vertical position (refer to Figures 1, 11, and 12). However, other nozzle positions can be obtained, such as top-horizontal, bottom-horizontal, or bottom-vertical. Figure 14 illustrates the flexibility available in discharge nozzle locations. Sometimes the pump frame, bearing bracket, or baseplate may interfere with the discharge flange, prohibiting a bottom-horizontal or bottom-vertical discharge nozzle position. In other instances, solid casings cannot be rotated for various nozzle positions because the seal chamber connection would become inaccessible.

Practically all double-suction, axially split casing pumps have a side discharge nozzle and either a side- or a bottom-suction nozzle. If the suction nozzle is placed on the side of the pump casing with its axial centerline (refer to Figure 10), the pump is classified as a side-suction pump. If its suction nozzle points vertically downward (see Figure 15), the pump is called a bottom-suction pump. Single-stage, bottom-suction pumps are rarely made in sizes below a 10-in (254 mm) discharge nozzle diameter.

Special nozzle positions can sometimes be provided for double-suction, axially split casing pumps to meet special piping arrangements, such as a radically split casing with bottom suction and top discharge in the same half of the casing. Such special designs are generally costly and should be avoided.

Centrifugal Pump Rotation Because suction and discharge nozzle locations are affected by pump rotation, it is important to understand how the direction of rotation is defined. According to Hydraulic Institute standards, rotation is defined as clockwise or counterclockwise by looking at the driven end of a horizontal pump or looking down on a vertical pump. To avoid misunderstanding, clockwise or counterclockwise rotation should always be qualified by including the direction from which one looks at the pump.

The terms inboard end or drive end (the end of the pump closest to the driver) and outboard end or nondrive end (the end of the pump farthest from the driver) are used only with horizontal pumps. The terms lose their significance with dual-driven pumps. Any centrifugal pump casing pattern can be arranged for either clockwise or counterclockwise rotation, except for end-suction pumps, which have integral heads on one side. These require separate directional patterns.

Casing Handholes Casing handholes are furnished primarily on pumps handling sewage and stringy materials that may become lodged on the impeller suction vane edges or on the tongue of the volute. The holes permit removal of this material without completely dismantling the pump. End-suction pumps used primarily for liquids of this type are provided with handholes or access to the suction side of the impellers. These access points are located on the suction head or in the suction elbow. Handholes are also provided in drainage, irrigation, circulating, and supply pumps if foreign matter may become lodged in the waterways. On very large pumps, manholes provide access to the interior for both cleaning and inspection.

Mechanical Features of Casings Most single-stage centrifugal pumps are intended for service at moderate pressures and temperatures. As a result, pump manufacturers usually design a special line of pumps for high operating pressures and temperatures rather than make their standard line unduly expensive by making it suitable for too wide a range of operating conditions.

If axially split casings are subject to high pressure, they tend to "breathe" at the split joint, leading to misalignment of the rotor and, even worse, leakage. For such conditions, internal and external ribbing is applied to casings at the points subject to the greatest stress. In addition, whereas most pumps are supported by feet at the bottom of the casing, high temperatures require centerline support so that, as the pump becomes heated, expansion will not cause misalignment.

Series Units For large-capacity medium-high-head conditions, two single-stage, double-suction pumps can be connected in a series on one baseplate with a single driver. Such an arrangement was at one time very common in waterworks applications for heads of 250 to 400 ft (76 to 122 m). One series arrangement uses a double-extended shaft motor in the middle, driving two pumps connected in a series by external piping. In a second type, a standard motor is used with one pump having a double-extended shaft. This latter arrangement may have limited applications because the shaft of the pump next to the motor must be strong enough to transmit the total pumping horsepower. If the total pressure generated by such a series unit is relatively high, the casing of the second pump could require ribbing. Higher heads per stage are becoming more and more common, and series units are generally used in only very high ranges of total head.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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