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

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2.2.6 CENTRIFUGAL PUMP MAGNETIC BEARINGS 2.293

FIGURE 15 Single-stage double-suction 10  26-size process pump [800 hp (0.6 MW)] with magnetic bearings.

(Reference 7)

TABLE 4 Design and experimental loads for the single-stage pump

Bearing Expected Load, lb, (kN) Design Load, lb (kN) Actual Load, lb (kN)

Radial 953 (4.2) 1,430 (6.4) 1,450 (6.4)

Axial 1,000 (4.4) 4,000 (17.8) 2,100 (9.3)

the axial thrust bearing. Greater design capacity would provide the same margin with

respect to radial loads.

The margins evident in Table 3 for the multistage pump may appear excessive, but

until more operating knowledge about such pumps is acquired, it would appear that this

degree of conservatism in designing magnetic bearings for pumps is merited.

COSTS _____________________________________________________________

Backed by the vision of complete magnetic suspension and the attendant benefits, mag-

netic bearing technology has been proven and demonstrated in pumping machinery. How-

ever, the major deterrent to further application of this technology is cost. Retrofitting the

magnetic bearings to the multistage pump described above cost at least twice the price of

the pump itself, and this included the analog controller, which accounted for about a third

of the retrofit cost. Digital controllers, a later development, are one third the size and cost

of analog controllers; this has significantly reduced the overall cost. Much of this overall

cost is in the engineering of the magnetic bearings, which includes matching this system

to the rotordynamic characteristics of the pump. Use of the same system in quantity pro-

duction would reduce the cost by up to 80 percent.

REFERENCES _______________________________________________________

1. Marks Standard Handbook for Mechanical Engineers. 9th ed. E. A. Avallone and

T. Baumeister, eds., McGraw-Hill, 1987, pp. 3

—

56.

2. McCloskey, T., and Jones, G. “Electric Utility Applications for Active Magnetic Bear-

ings.” Proceedings of MAG ‘92, Magnetic Bearings, Magnetic Drives and Dry Gas Seals

Conference and Exhibition, University of Virginia, July 1992, pp. 3

—

18.

2.294 CHAPTER 2

3. Cooper, P., McGinnis, G., Janik, G., Jones, G., and Shultz, R. “Application of Magnetic

Bearings in a Multistage Boiler Feed Pump.” Proceedings of the Second International

Symposium on Magnetic Bearings, July 1990.

4. McGinnis, G., Cooper, P., Janik, G., Jones, G., and Shultz, R. “A Boiler Feed Pump

Employing Active Magnetic Bearings.” Presented at International Joint Power Gen-

eration Conference, Boston, MA, ASME, October 1990, ASME.

5. Cooper, P., and Jones, G. “Operating Experience, Including Transient Response, of a

Magnetic-Bearing-Equipped Boiler Feed Pump.” Proceedings of MAG ‘92, Magnetic

Bearings, Magnetic Drives and Dry Gas Seals Conference and Exhibition, University

of Virginia, July 1992, pp. 19

—

28.

6. Jones, G., and Penfield, S. Jr. “Magnetic Bearing Boiler Feed Pump Demonstration”,

Research Report EP89-39, Empire State Electric Energy Research Corporation, Octo-

ber 1992.

7. Brown, E.,Thorp, J. M., Hawkins, L., and Sloteman, D.“Development and Application

of a Homopolar, Permanent-Magnet-Bias Magnetic Bearing System for an API 610

Pump.” Saudi Aramco Journal of Technology, Spring 1997, pp. 2

—

13.

8. Hanson, L., and Imlach, J. “Development of a Magnetic Bearing API Process Pump

with a Canned Motor.” Proceedings of the Ninth International Pump Users Sympo-

sium, Texas A&M University, March 1992, pp. 3

—

9. “Guidelines for the Use of Magnetic Bearings in Turbomachinery.” Prepared by Tech-

nology Insights, San Diego, California and published by the Electric Power Research

Institute, February 1996.

10. Meeks, C. R., DiRusso, E., and Brown, G. V. “Development of a Compact, Lightweight

Magnetic Bearing.” Proceedings of the 26th Joint Propulsion Conference. AIAA/SAE/

ASME/ASEE, Orlando, FL, 1990.

Sealless pumps are developed to eliminate the liquid leakage to the atmosphere that

occurs from pumps that employ packing or mechanical seals. This leakage is usually toxic

or dangerous to the environment. Sometimes the leakage is valuable. Eighty percent of the

applications are for pressures below 200 lb/in

(13.8 bar) and below 250°F (120°C).

Sealless pumps are divided into two categories: magnetic drive pumps and canned

motor pumps. The two categories compete against themselves in certain applications, but

for the most part they each have their own market niche into which they are applied. Both

have encapsulated inner driven mechanisms. On a magnetic drive pump (see Figure 5 in

Section 2.2.7.1), the impeller is mounted to an inner magnet carrier. The inner and outer

magnetic carriers are sealed by what is called a shell, which contains pump internal pres-

sure. On a canned motor pump (see Figure 1 in Section 2.2.7.2), the impeller is mounted

directly to the motor rotor. The atmospheric sealing element between the motor stator and

rotor is called a liner or “can.” Both magnetic drive and canned motor pumps use product

lubricated bearings of compatible design and materials.These bearings are usually cooled

and lubricated by the pump liquid.

The following sections describe both magnetic drive and canned motor pumps.

SECTION 2.2.7

SEALLESS PUMPS

FREDERIC W. BUSE

STEPHEN A. JASKIEWICZ

2.296

The principle of a magnetic drive pump is the elimination of the seal or packing in an

overhung pump (Figure 1) and cutting the shaft in two at the seal location (Figure 2a).

On the inner or wet end half of the shaft (Figure 2b), an inner magnet assembly (a) is

placed on a shaft (b) supported by product lubricated bearings (c). On the other cut por-

tion of the shaft, an outer magnet assembly (d) is placed on the power shaft (e). Between

the inner and outer magnet assemblies, a static seal (f) isolates the shell (g) or

diaphragm and the pumped liquid from the atmosphere. The magnetic flux from the

outer magnet assembly drives the inner magnet assembly and impeller(s). The outer

assembly is mounted directly to either its own bearing assembly in a frame housing (Fig-

ure 3) or motor shaft (Figure 4). Figure 5 shows a cross-section of a magnetic drive frame

mounted pump.

Magnetics A magnetic circuit usually consists of two sets of permanent magnets and

inner and outer conducting rings (Figure 6). The conducting rings can be cast iron, duc-

tile iron, or a 400 series stainless steel.

Magnet Materials The first permanent magnets, developed in the 1940s, were made

of aluminum-nickel-cobalt (AlNiCo) and used in small chemical pumps. Development of

rare earth magnets in the 1980s made it possible to have a small power package that

could drive larger pumps. The two types of rare earth materials commonly used are

neodymium iron boron (NdFeB) and samarium cobalt (SmCo). The SmCo is four times

stronger than AlNiCo. NdFeB, at 70°F (21°C), is 20% stronger than SmCo. The advan-

tage of SmCo is the maximum service temperature of 550°F (288°C),almost twice that of

NdFeB, which is 300°F (149°C). Figure 7 shows the strength versus temperature char-

acteristics of the two materials.The cost of SmCo, however, is about twice that of NdFeB.

SECTION 2.2.7.1

MAGNETIC DRIVE PUMPS

FREDERIC W. BUSE

2.297

2.298 CHAPTER TWO

FIGURE 1 Pump with conventional bearing housing and mechanical seal (Flowserve Corporation)

The magnets have a maximum temperature at which they lose all magnetism in an

irreversible process; the magnets do not regain magnetism as they cool. This temperature

is the Curie temperature and is shown for each magnet material in Table 1.

Conduction Ring The amount of transmittal torque depends on the overall gap (Fig-

ure 8) between the poles of the magnets and the thickness of the conducting ring (Figure

9). If the thickness of the conducting ring is too small, it will become saturated with flux

and the torque capability will be reduced.

TABLE 1 Operating and Curie temperatures for magnet materials

Type of Magnet

Material Operating Temperature (°F/°C) Curie Temperature (°F/°C)

AlNiCo 660/305 800/425

NdFeB 250

—

300/120

—

137

(Depending on grade) 600/310

SmCo 500

—

660/260

—

350

(Depending on grade) 1300/700

2.2.7.1 MAGNETIC DRIVE PUMPS 2.299

FIGURE 3 Frame-mounted magnet drive arrangement

FIGURE 2 Comparison of typical mechanical seal arrangement and sealless pump drive configuration: a)

arrangement for typical mechanical through casing cover; b) sealless drive configuration

Gap The “overall gap” (Figure 10) is made up of the air gap, containment shell thick-

ness, liquid gap, and encapsulation. Gap dimensions are based on the pressure require-

ment for the shell, the number of magnets (single or dual), and the material of the shell

(metallic or nonmetallic) in the gap.

2.300 CHAPTER TWO

FIGURE 5 Cross-section of a frame-mounted magnet-driven pump (Flowserve Corporation)

FIGURE 4 Close-coupled magnet drive arrangement

2.2.7.1 MAGNETIC DRIVE PUMPS 2.301

FIGURE 6 Magnetic circuit

FIGURE 7 Magnet strength versus temperature

FIGURE 8 Magnet configuration

2.302 CHAPTER TWO

FIGURE 9 Rows of magnets

FIGURE 10A and B Encapsulation of magnets using nonmetallic composition and metallic clad

2.2.7.1 MAGNETIC DRIVE PUMPS 2.303

Mean diameter (on radius dimensions) 4.6 (117) 6.0 (152.4)

Air gap minimum .030 (0.762) .045 (1.143)

Containment shell thickness: Hastelloy C .040 (1.016) .060 (1.524)

Polymer .120 (3.048) .150 (3.810)

Liquid gap .035 (0.889) .035 (0.889)

Encapsulation .030 (0.762) .030 (0.762)

Overall gap (basis Hastelloy C) .135 (3.429) .170 (4.318)

Overall gap (basis polymer) .215 (5.461) .260 (6.604)

EXAMPLE Dimensions shown as in (mm)

In the previous example, the same overall gap was maintained for polymer and Hastel-

loy C shells. This allows for interchangeability of the magnetic assemblies independent of

shell material.

Transmittal Torque The torque transmitted by the magnets depends on the following:

• Flux density of the magnets, B

• Operating temperature of the magnets (which will change B

)

• Length of the magnet poles, L

• Number of magnets per ring, M

• Mean ratios between the ID of the outer assembly magnets and OD of the inner assem-

bly magnets, r

• Overall gap between the ID and OD of the magnets in the assembly, g

•  constant, K, which changes as a function of the specific design

Torque is determined from the following relationship, with appropriate units:

For a given design type and configuration, the torque varies inversely as the square of

the overall gap. Depending on costs and specific design construction, an assembly ring

of magnets can have either one continuous length of magnets of a series of 1, 2, 3 or

more rows of individual magnets.

TORQUE CAPABILITY_________________________________________________

The ultimate torque is the static “breakaway torque.” To determine this, the magnet car-

riers are assembled so the inner carrier is locked in position. Then a torque is applied by

bar and weights or by a torque wrench to the outer carrier. The torque value at which the

two carrier assemblies break loose from each other radially

—

or “decouple”

—

is called the

“breakaway torque.” The designer has to account for the driver start-up acceleration time,

start-up torque, and an appropriate safety factor to apply to the breakaway torque to

determine the allowable applied torque.

Table 2 gives examples of the torque capability of assemblies composed of NdFeB

blocks of magnet 0.75 in wide  0.38 in high  1.125 in long (19 mm  9.65 mm 

28.6 mm) with a 0.180 in (4.6 mm) thick conducting ring.

Basic Dimensioning of Magnet Blocks The magnet blocks can be made too long or

too short, resulting in handling, magnet molding, or flux density problems. The ratio of

the dimensions for magnet blocks for successful designs is as follows:

T 

K  B

 L  M  r

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

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