How Centrifugal Pumps Work

Stripped of all nonessential details, a centrifugal pump (Figure 1.4) consists of an impeller attached to and rotating with the shaft and a casing that encloses the impeller. In a centrifugal pump, liquid is forced into the inlet side of the pump casing by atmospheric pressure or some upstream pressure. As the impeller rotates, liquid moves toward the discharge side of the pump. This creates a void or reduced pressure area at the impeller inlet. The pressure at the pump casing inlet, which is higher than this reduced pressure at the impeller inlet, forces additional liquid into the impeller to fill the void.

If the pipeline leading to the pump inlet contains a noncondensable gas such as air, then the pressure reduction at the impeller inlet merely causes the gas to expand, and suction pressure does not force liquid into the impel-ler inlet. Consequently, no pumping action can occur unless this noncon-densable gas is first eliminated, a process known as priming the pump.

With the exception of a particular type of centrifugal pump called a self-priming centrifugal pump, centrifugal pumps are not inherently self-self-priming if they are physically located higher than the level of the liquid to be pumped.

That is, the suction piping and inlet side of centrifugal pumps that are not self-priming must be filled with noncompressible liquid and vented of air and other noncondensable gases before the pump can be started. Self-priming pumps are designed to first remove the air or other gas in the suc-tion line, and to then pump in a convensuc-tional manner.

If vapors of the liquid being pumped are present on the suction side of the pump, this results in cavitation, which can cause serious damage to the pump. Discussed in greater detail in Chapter 2, Section VI, cavitation may also cause the pump to lose prime.

Once it reaches the rotating impeller, the liquid entering the pump moves along the impeller vanes, increasing in velocity as it progresses. The vanes in a centrifugal pump are usually curved backward to the direction of rotation.

Some special types of pump impellers (Chapter 7, Section VII) have vanes that are straight rather than curved. The degree of curvature of the vanes and number of vanes, along with other factors, determines the shape and characteristics of the pump performance curve, which is described in Chapter 2, Section IV.

When the liquid leaves the impeller vane outlet tip, it is at its maximum velocity. Figure 1.5 illustrates typical velocity and pressure changes in a cen-trifugal pump as the liquid moves through the flow path of the pump. After the liquid leaves the impeller tip, it enters the casing, where an expansion of cross-sectional area occurs. This expanded area is offset in many pumps by the additional flow being directed into the casing by the rotating impel-ler vanes, so that there is an area along the flow path where velocity and

Typical pump section Section through impeller and volute along mean flow surface Flowline

Flowline

Volute

Impeller Point of entrance

to impeller vanes

FIGURE 1.4

Centrifugal pump with single volute casing. (From Karassik, I.J. et al., Pump Handbook, 4th Ed., McGraw-Hill, Inc., New York, 2008. With permission.)

pressure are neutral in this part of the casing. This is illustrated by the hori-zontal portions of the velocity and pressure lines in Figure 1.5.

Continuing on through the flow path in the remainder of the casing, the casing design ensures that the cross-sectional area of the flow passages increases as the liquid moves through the casing. Because the area is increas-ing as the liquid moves through the path of the casincreas-ing, a diffusion process occurs, causing the liquid’s velocity to decrease, as Figure 1.5 illustrates. By the Bernoulli equation (see Ref. [1] at the end of this book), the decreased kinetic energy is transformed into increased potential energy, causing the pressure of the liquid to increase as the velocity decreases. The increase of pressure while velocity is decreasing is also illustrated in Figure 1.5.

A centrifugal pump operating at a fixed speed and with a fixed impeller diameter produces a differential pressure, or differential head. Head is usually expressed in feet or meters, and abbreviated TH (total head). The amount of head produced varies with the flow rate, or capacity delivered by the pump, as illustrated by the characteristic head–capacity (H–Q) curve shown in Figure 1.6.

As the head developed by the pump decreases, the capacity increases.

Alternatively, as the pump head increases, the flow decreases. Pump capacity is usually expressed in gallons per minute (gpm) or, for larger pumps, in cubic feet per second (cfs). Metric equivalents, depending on the size of the pump, are cubic meters per second, liters per second, or cubic meters per hour.

The centrifugal pump casing is one of several types. A single volute casing is illustrated in Figure 1.4. Note that the single volute casing has a single

Suction

Velocity

Discharge Outlet tip of

impeller vane Inlet tip of

impeller vane

Flow path Velocity

(ft/sec) Pressure

(PSI) Pressure

FIGURE 1.5

Velocity and pressure levels vary as the fluid moves along the flow path in a centrifugal pump.

cutwater where the flow is separated. As the flow leaves the impeller and moves around the volute casing, the pressure increases. This increasing pres-sure as the liquid moves around the casing typically produces an increasing radial force at each point on the periphery of the impeller, due to the pres-sure acting on the projected area of the impeller. Summing all of these radial forces produces a net radial force that must be carried by the shaft and radial bearing system in the pump. The radial bearing must also support the load created by the weight of the shaft and impeller.

The radial bearing loads generated by a pump also vary as the pump oper-ates at different points on the pump performance curve, with the minimum radial force being developed at the best efficiency point (BEP) of the pump (Figure 1.7). See Chapter 2, Section V, for a discussion of BEP. Operation at points on the pump curve to the right or to the left of the BEP produces higher radial loads than are produced when operating at the BEP. This is especially true of single volute casing pumps, as Figure 1.7 illustrates.

Symptoms of excessive radial loads include excessive shaft deflection and premature mechanical seal and bearing failure. Continuous operation of the pump at too low a minimum flow is one of the most common causes of this type of failure. Because for rolling element bearings, bearing life is inversely proportional to the cube of the bearing load, operating well away from the pump BEP can cause a reduction in bearing life by several orders of magnitude.

A diffuser (shown in orange in Figure 1.8) is a more complex casing arrange-ment, consisting of multiple flow paths around the periphery of the impel-ler. The liquid that leaves the impeller vanes, rather than having to move completely around the casing periphery as it does with the single volute

Flow rate

Total head or total differential pressure

Slip

Centrifugal

Positive displacement

FIGURE 1.6

Typical head–capacity relationship for centrifugal and positive displacement pumps.

casing, merely enters the nearest flow channel in the diffuser casing. The diffuser casing has multiple cutwaters, evenly spaced around the impeller, as opposed to the one cutwater found in a single volute casing. The main advantage of the diffuser casing design is that this results in a near balancing of radial forces, thus reducing shaft deflection and eliminating the need for a heavy-duty radial bearing system. The dead weight of the rotating element

Impeller Casing

Diffuser

FIGURE 1.8

Diffuser casing minimizes radial loads in a centrifugal pump.

Q (gpm) (ft)H

Radial load (#)

Single volute

Diffuser

Best efficiency point Double volute

FIGURE 1.7

Typical radial loads produced by single volute, double volute, and diffuser casings.

must still be carried by the radial bearing, but overall the diffuser design minimizes radial bearing loads compared with other casing types.

Because the diffuser design produces minimal radial bearing loads, one might wonder why all pumps do not have diffusers rather than volute type casings. The reason is partially due to economics, as a pump with a diffuser casing generally has more parts or more complex parts to manufacture than a pump with a volute casing. Depending on pump size and materials of con-struction, economics often do not justify the use of diffuser casings except where significant savings can be achieved in the size of shaft or radial bear-ing that is used in the pump. This is usually only found to be the case in multistage, high-pressure pumps. However, with multistage pumps there are other considerations as well. Volute designs in some multistage pumps allow, by the use of a cross-over, some of the impellers to be oriented in the opposite direction, providing balancing of axial thrust loads. (Refer to Chapter 4, Section II.D, and Section VII.) The leading manufacturers of mul-tistage pumps are themselves not in agreement on this subject.

Vertical turbine pumps (Chapter 4, Section XI) usually have diffuser casings.

Because the bearings for these vertical pumps are submerged in the liquid being pumped, it is not practical to have a ball- or roller-type radial bearing for this type of pump. Rather, the radial bearing loads must be accommo-dated by a wetted sleeve type bearing, which is not an ideal bearing system in this type of arrangement. Therefore, to minimize radial bearing loads, diffuser type casings are used in this type of pump.

A hybrid between a single volute casing and a diffuser casing is a double volute casing (Figure 1.9). With this casing design, the volute is divided, which creates a second cutwater, located 180° from the first cutwater. This design results in much lower radial loads than are present with single volute designs (Figure 1.7). Double volute casings are usually used by pump designers for larger, higher flow pumps (usually for flows greater than about 1500 gpm) to allow the use of smaller shafts and radial bearings.

FIGURE 1.9

Double volute casings are used in larger centrifugal pumps to reduce radial loads. (Courtesy of Goulds Pumps, Inc., a subsidiary of ITT Corporation.)

VI. PD Pumps A. General

This book is primarily about centrifugal pumps. However, as Figure 1.3 illus-trates, there is an entire other class of pumps known as Positive Displacement (PD) pumps that deserves some attention. One of the earliest decisions that must be made in designing a system and applying a pump is the selection of the type of pump to be used. The first issue is the general decision whether the pump should be of the centrifugal or the PD type. Surveys of equip-ment engineers and pump users indicate that the majority of them have a strong preference for centrifugal pumps over PD pumps (if the hydraulic conditions are such that either type can be considered). Many reasons are given for this preference for centrifugals, but most are related to the belief that centrifugal pumps are more reliable and result in lower maintenance expense. Centrifugal pumps usually have fewer moving parts, have no check valves associated with the pumps (as reciprocating PD pumps do), produce minimal pressure pulsations, do not have rubbing contact with the pump rotor, and are not subject to the fatigue loading of bearings and seals that the periodic aspect of many PD pumps produce. Centrifugals should be considered first when applying a pump, but they are not always suited to the application.