Pump Handbook by Igor J. Karassik, Joseph P. Messina, Paul Cooper, Charles C. Heald

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TABLE 20 Life of high-energy pump suction stage

2.92 CHAPTER 2

REFERENCES _______________________________________________________

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2.1 CENTRIFUGAL PUMP THEORY 2.93

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33. Bolleter, U. “Blade Passage Tones of Centrifugal Pumps.” Vibrations. Vol. 4, No. 3, Sep.

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5427, 1969.

43. Spring, H. “Affordable Quasi-Three-Dimensional Inverse Design Method for Pump

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Rotating Disks.” Transactions of the ASME. Series D, Vol. 82, Sep. 1960, pp. 553

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46. Graf, E. “Analysis of Centrifugal Impeller BEP and Recirculating Flows: Comparison

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Stokes Solutions.” Pumping Machinery

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2.1 CENTRIFUGAL PUMP THEORY 2.95

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Navier Stokes Calculations.” 1997 ASME Fluids Engineer-

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mance. Concepts, ETI, Inc., Wilder, Vermont, 1997.

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51. Keathly, W. C., Due, H. F., and Comolli, C. R. “A Study of Pressure Prediction Methods

for Radial Flow Impellers.” Final Report on NASA Contract NAS8

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nal of Fluids Engineering, Vol. 103, June, 1981, pp. 193

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53. Cooper, P. “Perspective:The New Face of R&D

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55. Brennen, C., and Acosta,A. J. “The Dynamic Transfer Function for a Cavitating Inducer.”

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Engineers. Vol. 184, Part 3N, 1970.

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American Perspective.” 1997 ASME Fluids Engineering Division Summer Meeting.

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3387, June 1997.

59. Bolleter, U., Frei, A., and Florjancic, D. “Predicting and Improving the Dynamic

Behavior of Multistage High Performance Pumps.” Proceedings of the First Interna-

tional Pump Symposium. Texas A&M University, College Station Texas, May 1984,

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60. Iino, T. “Potential Interaction Between a Centrifugal Pump Impeller and a Vaned Dif-

fuser.” Fluid/Structure Interactions in Turbomachinery. ASME, Nov. 1981, pp. 63

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61. Cooper, P. “Hydraulics and Cavitation.” Symposium Proceedings: Power Plant Pumps.

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tled NASA Space Vehicle Design Criteria (Chemical Propulsion). NASA, 1975.

65. Makay, E., Cooper, P., Sloteman, D. P., and Gibson, R. “Investigation of Pressure Pul-

sations Arising from Impeller/Diffuser Interaction in a Large Centrifugal Pump.” Pro-

ceedings: Rotating Machinery Conference and Exposition, ASME, Vol. 1, 1993.

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Pump.” Transactions of the ASME. Vol. 54, 1932, pp. 143

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67. Fraser, W. H. “Recirculation in Centrifugal Pumps.” Materials of Construction of

Fluid Machinery and Their Relationship to Design and Performance. ASME, Nov.

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68. Iino,T., Sato, H., and Miyashiro, H.“Hydraulic Axial Thrust in Multistage Centrifugal

Pumps.” Transactions of the ASME, Journal of Fluids Engineering. Vol. 102, Mar.

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69. Makay, E., and Barrett, J. A. “Changes in Hydraulic Component Geometries Greatly

Increased Power Plant Availability and Reduced Maintenance Cost: Case Histories.”

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lege Station Texas, May 1984, pp. 85

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71. Heald, C. C., and Palgrave, R. “Backflow Control Improves Pump Performance.” Oil

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74. Sloteman, D. P., Cooper, P., and Dussourd, J. L. “Control of Backflow at the Inlets of

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75. Cooper, P., Sloteman, D. P., Graf, E., and Vlaming, D. J. “Elimination of Cavitation

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Related Instabilities and Damage in High

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Energy Pump Impellers.” Proceedings of the

Eighth International Pump Users Symposium. Texas A&M University, 1991, pp. 3

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Damage.” Transactions of the ASME, Oct. 1955, pp. 1045

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CLASSIFICATION AND NOMENCLATURE ________________________________

A centrifugal pump consists of a set of rotating vanes enclosed within a housing or casing

that is used to impart energy to a fluid through centrifugal force. Thus, stripped of all

refinements, a centrifugal pump has two main parts: (1) a rotating element, including an

impeller and a shaft, and (2) a stationary element made up of a casing, casing cover, and

bearings.

In a centrifugal pump, the liquid is forced by atmospheric or other pressure into a set

of rotating vanes. These vanes constitute an impeller that discharges the liquid at its

periphery at a higher velocity. This velocity is converted to pressure energy by means of a

volute (see Figure 1) or by a set of stationary diffusion vanes (see Figure 2) surrounding

the impeller periphery. Pumps with volute casings are generally called volute pumps,

while those with diffusion vanes are called diffuser pumps. Diffuser pumps were once

quite commonly called turbine pumps, but this term has become more selectively applied

to the vertical deep-well centrifugal diffuser pumps usually referred to as vertical turbine

pumps. Figure 1 shows the path of the liquid passing through an end-suction volute pump

operating at rated capacity (the capacity at which best efficiency is obtained).

Impellers are classified according to the major direction of flow in reference to the axis

of rotation. Thus, centrifugal pumps may have the following:

• Radial-flow impellers (see Figures 25, 34, 35, 36, and 37)

• Axial-flow impellers (see Figure 29)

• Mixed-flow impellers, which combine radial- and axial-flow principles (see Figures 27

and 28)

2.2.1

CENTRIFUGAL PUMPS: MAJOR

COMPONENTS

IGOR J. KARASSIK

C. C. HEALD

2.97

SECTION 2.2

CENTRIFUGAL PUMP

CONSTRUCTION

2.98 CHAPTER TWO

FIGURE 1 A typical single-stage, end-suction volute pump (Flowserve Corporation)

FIGURE 2 A typical diffuser-type pump

Impellers are further classified in one of two categories:

• Single-suction, with a single inlet on one side (see Figures 25, 33, and 37)

• Double-suction, with liquid flowing to the impeller symmetrically from both sides (see

Figures 26, 27, and 38)

The mechanical construction of the impellers gives a still further subdivision into

• Enclosed, with shrouds or side walls enclosing the waterways (see Figures 25, 26, and 27)

• Open, with no shrouds (see Figures 29, 33, and 34)

• Semiopen or semi-enclosed (see Figure 36)

2.2.1 CENTRIFUGAL PUMP: MAJOR COMPONENTS 2.99

FIGURE 3 Horizontal single-stage double-suction volute pump (the numbers refer to parts listed in Table 1)

(Flowserve Corporation)

A pump in which the head is developed by a single impeller is called a single-stage

pump. Often the total head to be developed requires the use of two or more impellers oper-

ating in a series, each taking its suction from the discharge of the preceding impeller. For

this purpose, two or more single-stage pumps can be connected in a series or all the

impellers can be incorporated in a single casing.The unit is then called a multistage pump.

The mechanical design of the casing provides the added pump classification of axially

split or radially split, and the axis of rotation determines whether the pump is a horizon-

tal or vertical unit.

Horizontal-shaft centrifugal pumps are classified still further according to the location

of the suction nozzle:

• End-suction (see Figures 1, 8, 11, and 13)

• Side-suction (see Figures 7 and 10)

• Bottom-suction (see Figure 15)

• Top-suction (see Figure 22)

Some pumps operate in air with the liquid coming to and being conducted away from

the pumps by piping. Other pumps, most often vertical types, are submerged in their suc-

tion supply. Vertical-shaft pumps are therefore called either dry-pit or wet-pit types. If the

wet-pit pumps are axial-flow, mixed-flow, or vertical-turbine types, the liquid is discharged

up through the supporting drop or column pipe to a discharge point above or below the

supporting floor. These pumps are consequently designated as aboveground discharge or

belowground discharge units.

Figures 3, 4, and 8 show typical constructions of a horizontal double-suction volute

pump, the bowl section of a single-stage axial-flow propeller pump, and a vertical dry-pit

single-suction volute pump, respectively. The names recommended by the Hydraulic Insti-

tute for various pump parts are given in Table 1.

CASINGS AND DIFFUSERS ____________________________________________

The Volute Casing Pump

This pump (refer to Figure 1) derives its name from the spi-

ral-shaped casing surrounding the impeller. This casing section collects the liquid dis-

charged by the impeller and converts velocity energy to pressure energy.

A centrifugal pump volute increases in area from its initial point until it encompasses

the full 360° around the impeller and then flares out to the final discharge opening. The

2.100 CHAPTER TWO

TABLE 1 Recommended names of centrifugal pump parts

Item no. Name of part Item no. Name of part

1 Gasing 33 Bearing housing (outboard)

1A Gasing (lower half) 35 Bearing cover (inboard)

1B Gasing (upper half) 36 Propeller key

2 Impeller 37 Bearing cover (outboard)

4 Propeller 39 Bearing bushing

6 Pump shaft 40 Deflector

7 Gasing ring 42 Coupling (driver half)

8 Impeller ring 44 Coupling (pump half)

9 Suction cover 46 Coupling key

11 Stuffing box cover 48 Coupling bushing

13 Packing 50 Coupling lock nut

14 Shaft sleeve 52 Coupling pin

15 Discharge bowl 59 Handhole cover

16 Bearing (inboard) 68 Shaft collar

17 Gland 72 Thrust collar

18 Bearing (outboard) 78 Bearing spacer

19 Frame 85 Shaft enclosing tube

20 Shaft sleeve nut 89 Seal

22 Bearing lock nut 91 Suction bowl

24 Impeller nut 101 Column pipe

25 Suction head ring 103 Connector bearing

27 Stuffing box cover ring 123 Bearing end cover

29 Lantern ring (seal cage) 125 Grease (oil) cup

31 Bearing housing (inboard) 127 Seal pipe (tubing)

32 Impeller key

These 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 dif-

fuser 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, dif-

fusion 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 effi-

ciently 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 single-

stage, low-head propeller pumps (see Figure 4).

Radial Thrust In a single-volute pump casing design (see Figure 5), uniform or near uni-

form pressures act on the impeller when the pump operates at design capacity (which coin-

cides 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 discus-

sion 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 diame-

ter will have a much greater radial reaction force at partial capacities than a low-head

2.2.1 CENTRIFUGAL PUMP: MAJOR COMPONENTS 2.101

FIGURE 4 Vertical wet-pit diffuser pump bowl (the

number refers to parts listed in Table 1) (Flowserve

Corporation)

FIGURE 5 Uniform casing pressures exist at

design capacity, resulting in zero radial reaction.

pump with a small impeller diameter. A zero radial reaction is not often realized; the min-

imum reaction occurs at design capacity.

Although the same tendency for unbalance exists in the diffuser-type pump, the reac-

tion 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 manufac-

turer, if properly advised, would supply a heavier shaft, usually at a much higher cost. Sus-

tained 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 capaci-

ties, 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

Pump Handbook by Igor J. Karassik, Joseph P. Messina, Paul Cooper, Charles C. Heald - 3rd edition

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