Canned motoer pump

7/25/2019 Canned motoer pump

1/5

62 PLANT ENGINEERING JUNE 3,1993

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ontaining a fluid while moving it

under pressure has been a problem as

long as there have been pum ps. Early

pumps with packings leaked, but develop-

ments in mechanical face seals dramatical-

ly reduced this problem. However, a study

of

pump failures cited in Practical Ma

chinery Management fo r Process Plants by

H Bloch indicates the nu mber-one cause of

failures is m echanical seals (40%) and, in

second place, antifriction bearings

(7 ).

Sleeve bearings accounted for a small

amount of pum p failures (1 ).

Canned m ot o r and m agne t i c d r i ve

pumps have bypassed the seal problem by

eliminating it. Both pum ps have sealed pri-

mary enc losures, but thats where the sim i-

larity ends. Both canned m otor pu mp bear-

ings are sleeved. M agnetid drive p um ps

have two sleeved bearings and two to four

antifriction bearings.

Canned m otor pumps are one-piece, stat-

ically-sealed, primarily volute-types avail-

able for moving corrosive, volatile, envi-

ronmentally sensitive, and other hard-

to-handle fluids. The pump and motor ro-

tor are assembled on a com mon shaft in a

single unit (Fig. 1 . This design eliminates

alignment, does not re quire external lubri-

ca t ion , and of fe r s double conta in-

ment.

Retrofit canned mo tors are av ailable that

fit an existing pum p w ith little or

no

change

required in piping and hydraulics. Data

such s bearing loads and recirculation

flow are evaluated to en sure that motor and

pump performance do not change (Fig. 2).

The pumped fluid is circulated through

the rotor section to provide cooling for the

mo tor and bearings, lubricant fo r the sleeve

bearings, and thrust control. Fluid is isolat-

ed from motor stator windings by a corro-

sion-resistant nonmagnetic liner. This lin-

er, or can, leads to the term canned

pump.

Liner thickness ranges from 0.0

15

to

0.030 in. and contains pressure to 300 psig.

An outer shell, or secondary containment,

is thicker and capable of containing pres-

sures up to

450

psig, with special designs

rated at

5000

psig.

Pump s are available from 1 to 250 hp,

capacities to

2000

gpm, and fluid tempera-

tures from

-300

to 1000 F.

A common design has a dry stator.

No

dielectric oil or other ty pe of fluid is in th e

stator cavity. This a pproa ch eliminates the

need for an expansion chamber.

Some designs provid e additional cooling

capacity by filling the stator cavity with a

dielectric oil. Besides dissipating heat, th e

oil protects against condensation damage

resulting in expected motor life exceeding

NEMA standards. In this case, an expan-

sion cham ber is required.

An other design fills the stato r cavity with

an inert, high-thermal-conductivity com-

poun d th at helps dissipate heat. Stator vol-

ume is completely filled to prevent the ac-

cumulation of vapor. E xpansion chambers

are not required.

A canned motor pump offers a simple

design, which is inherently efficient. The

pump has two bearings, no coupling or

seals, primary and secondary leak contain-

ment, and low noise levels. In addition, it

doesnt require alignment or lubrication.

Th e overall size is sm all, about 50 of the

space requi rement of a convent iona l

pumping arrangement (Fig. 3 .

Conven tional pumps have four bearings,

seals and seal support system, coupling,

and rigid baseplate to absorb loads and

maintain alignment. There

is

noise from

the motor cooling fan and no secondary

containment.

Magnetic drive pumps have four to six

bearings, are close coupled, or have a cou-

pling and rigid baseplate to absorb loads

and maintain alignment. There is noise

from the motor cooling fan and normally

no

secondary containment.

Fig 1 Canned

motor

pumps

have the stator

chamber dry, oil-Blled, r

solid-fitled. Some designs

providefor dry running

(Courtesy Sundstrand Fluid

Handling,

Diu.

of Milton

Roy Co.).

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Canned

motor

pumlbs use sleeve bearings made of

carbon graphite or s i l b n curbide

Fig

2.

Remfitting canned

motors to existing pumps i s

done quickly without dis-

turbing piping. Bearing

flush systems and monitors

are available. (Courtesy

Hayward Tyler, Inc.).

Bearings

Typical sleeve bearing m aterials a re car-

bon graphite and silicon carbide. Carbon

graphite is widely used because of its low

cost. It is self-sacrificing an d un de r certain

conditions runs dry.

If applied properly u nder specific condi-

tion s, silicon carbide p rovides longer bear-

ing life than carbon graphite, particularly

when particles are present. Dry running

and thermal shock are two serious prob-

lems for silicon carbide bearings, because

of their tendency to crack. They ar e consid-

ered an expensive alternative to carbon

graphite.

Since sleeve bearing cleara nces ar e typi-

cally

0.002

to

0.006

in. in canned motor as

well as magnetic drive pumps, large parti-

cles in the fluid stream must be removed

or

ground up by the bearings. Particulate in a

pumping system can consist of weld slag,

pipe scale, sand , ferrous particles, and car-

bonized fluid. Slurries up to

6

by weight

and up t o 0.03-in. in size are removed from

the fluid stream

by

various devices, includ-

ing centrifugal separators, filters, and con-

trolled clearances.

When slurries cannot be removed by

these methods, they are excluded from the

bearing-rotor area by a flush fluid. The

flush fluid is constantly circulated to cool

the m otor, lubricate the bearings, and even-

tually mix with the process fluid. Flush flu-

id

loss

t o the process stream is minimized

by

a

close clearance bushing between the

motor and pum p or a lip seal.

Many types

of

thru st control devices are

available in canned motor pumps. Some

utilize double-acting thrust bearings, com-

bination

of

hydraulic systems,

or

variable

orifices.

A canned motor pump adds up to 100

deg

F

temperature rise (based o n water) to

the fluid circulating through the m otor.

Ap-

proximately 2 to

10 of

total pump flow

is circulated through the motor, depending

on design.

A variety of design configurations pre-

vent flashing

of

volatile fluids. One design

uses a connection from th e pu mp discharge

to inject process fluid into the rear of the

motor, where the process fluid is directed

into an auxiliary impeller which pressur-

izes the fluid above discharge pressure. T he

fluid circulates through t he m otor, removes

heat, and returns to the pump discharge.

This design allows the addition of heat to

process f lu ids bu t main ta ins p ressure

above the flash point.

Another design uses reverse circulation

through the motor section of the pump.

Process fluid is returned to the vapor zone

of the supply vessel through a connection a t

the rear of the motor. The return line is

throttled to maintain high liquid pressure

within the m otor an d serves as a vent line.

Shaft Deflection

The L3/D4 ratio, sometim es called the

shaft flexibility factor (SFF), is a measure

of shaft deflection. The higher the ratio,

or

more shaft deflection, the greater the main-

tenance cost on overhung cen t r i fugal

pumps. Increased deflection causes more

wear on bearings, shaft sleeves, wear rings,

and other close-tolerance pump parts.

For

this reason, many users making an econom-

ic analysis assess a maintenance penalty

against pum ps with high ratios.

Canned motor pum ps have low

L3/D4

a-

tios elow 30. Stand ard API pump ra-

tios range from 35 to over 200.

ccessories

Several accessories are available to up-

grade canned motor pump installations.

Bearing

moni to rs

The primary failure

items in canned motor pumps ar e bearings.

Many times they are cooled an d lubricated

by process fluids with very low viscosities

and poor lubricating qualities. T he solution

is to monitor bearing wear in order to re-

place them before failure occurs.

Bearing monitors are available in several

designs and on most ca nned motor pumps.

They are up to 98 accurate in preventing

catastrophic failure and indicate when a

bearing change needs to be m ade. Th is sig-

nal eliminates dismantling a canned motor

pump according to a fixed schedule to

check

for

bearing wear.

Bearing wear causes material in a sensor

64 PLANT

ENGINEERING* JUNE 3,1993

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to be removed, creatin g a signal that warns

of the amo unt of bearing life remain ing be-

fore failure. One design has a tube with a

mushroom -shaped, gas-pressurized contact

tip. Wear at the monitor tip is caused by

either or both radial and axial shaft move-

ment corresponding to sleeve or thrust

washer wear. At two-thirds of maxim um al-

lowable bearing wear, the tip and end nut

contact, rupturing the tip. The resulting

change in pressure in the tube displays on

an external gauge or actuates a pressure

switch. In ad ditio n, this design is useful in

detecting corrosion of th e rotor sleeve, be-

cause the con tact tip is supp lied in a metal-

lurgy similar to, bu t one -half the thick ness

of, the sleeve.

Another design consis ts of a,Teflon-coat-

ed ring affixed to the rear bearing assembly

and wired to a detector module. When

maximum allowable bearing wear occurs,

the rotor shaft rubs through the Teflon

coating, makes contact with a metal inner

ring, and co mp letes an electrical circuit. A

signal is generated indicating the bearings

shou ld be replaced.

Other designs indicate worn bearings

and eccentric shaft rotation by rubbed-off

dielectric coatings or interrupted fiber op-

tic paths.

Temperature sensors Pumped product

temperature is measured in the rotor area

near the bearings. A thermocouple used

with a temperature indicating device shuts

down the pump w hen abnormal rotor cavi-

t y temp eratures occur. Otherwise, flashing,

polymerization, or crystallization could oc-

cur and cause rapid bearing failure.

Over/under current relays These relays

detect dry running.

Direction

of

rotation indicators Because

rotating pa rts a re no t exp osed, it is difficult

to

determine whether the pump is rotating

in th e right d irection. An electrical*device

senses if the electrical phase sequence is

compatible with the motor windings. A

light indicates if the m otor is running in the

right direction. The usual way to determine

if the pump is rotating properly is to use

two gauges, one in the su ction end and one

in the discharge end. Normally the dis-

charge-end gauge should have the higher

reading.

Thermostats Thermostats are installed

in motor windings to protect insulation

from premature failure. In the event the

moto r windings reach a preset temp erature

limit, the pum p automatically shuts down

before permanent damage occurs.

Selection

Process and fluid information are re-

quired to select a canned m otor pump, be-

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4010

cause each application is fluid specific. D e-

signs are available to handle blended fluids

with different properties, temperatures,

pressures, and particles. Typical selection

factors include:

Substance pumped

Operating temperature and pressure

ranges

I Fluid properties (at startup and pump-

ing temperature)

Viscosity

Specific gravity

r Vapor pressure

Specific h eat

Solids in suspension

Largest solids particle size

c Average solids particle size

I Quantity of solids

Abrasive qualities

Fig.

3.

Spwe

requirements

for canned motor pumps

are approximately 50% of

those needed

by

conventfon-

al and magnetfc drives.

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Maintenance a n be done ' in-house

s

part of

re g maintenance with avaiGabGe kiis

V Operating conditions

V Flow

V Discharge head

I Suction head

V

NPSH available.

This info rmatio n is necessary to properly

size a pump, select bearing material, pro-

vide sufficient cooling flow to the motor,

lubricate bearings, ensure any axial thrust

is balanced, keep pu mp ed fluid from flash-

ing

or

overheating, and provide bearing

protection from particles. Pum p m anufac-

turers request specific information before

recommending a pump , and provide appli-

cation engineering for unusual or difficult

conditions.

Maintenance

Many canned motor pumps are field re-

pairable, which helps reduce costs when

performing routine maintenance, such as

changing bearings. Some liners in canned

motor pumps are welded and must be re-

turned to the factory for repair.

When repairs are done in the field, they

usually consist of bearing assembly (includ-

ing thrust washers and shaft sleeves),

0

ring seal, and bearing monitor replace-

men t. In many cases these are don e in a few

hours. Prepackaged kits are available and

include par t s , tools , and i l lus t ra te(

instructions.

PLANT ENGIN EERIN Gagazine would like to thank AB

Pumps Lawrence Pump Engine Div.; Carbone

USA

Corp

Chempump Div. Crane Co ; Cooper C ommunications: Gi

lette Brown Assoc.; Gou lds Pumps Inc.; Hayward Tyle

Inc.; Hydraulic Institute; KSB Inc.; Laing Therm otec. Inc

David Moore Assoc iates Inc.; Sundstrand Fluid Har

dling Div. of Milton Roy Co : and Teikoku USA

Inc.; fc

their special contribution s to the development of this articlc

For more information..

Sealless Centrifugal

Pump

Standards HI 5.1

5.6 1992 published by the Hydraulic Institute

is an excellent source of information on the

design use and maintenance of canned and

magnetic drive pumps. Contact the Institute at

9 Sylvan Way Parsippany

NJ

07054-3802

201 267-9700 for information.

API Standard 685 Sealless Centrifugal

Pumps

for Refinery Services is in the process

of being written and reviewed. For additional

information contact the American Petroleum

Institute at 1220

L

St. NW Washington DC

. Joseph

L. Foszcz

Senior Editor 708-390-

2699

20005; 202-682-8000.

For information on how t o order copies of

this

article

circle 10 on post card

For additional nformation on canned motor pumps,

circle the number on the Reader Service Card in this issue.

RS Company

i6 PLANT ENGINEERING JUNE 3,1993

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Canned motoer pump