Pump Reliability

8/3/2019 Pump Reliability

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PUMP RELIABILITY

IMPROVING PUMP RELIABILITY


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3/63

Maintenance vs. Capital

What does a pump actually cost ?

Most plants regard the pump as a commodity...

purchased from the lowest bidder with little

consideration for:

The operation and maintenance cost of the pumpover its life cycle... which could be 20 - 30 years

Costs to be considered:

Spare parts (inventory costs)

Operation downtime (lost production) Labor to repair (maintenance costs)

Power consumption based on pump

efficiency

Environmental, disposal, and recycle costs


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TRUE PUMP COSTS

Repair costs can easily exceed the price of a

new pump (several times) over its life of 20 -

30 years

Documented Pump failures costRs.2.00.000/- or more per incident ( parts and

labor)

If MTBF was improved from 1 to 2 years for a

pump in a tough application Results in savings of Rs. 1,00,000/- per

year over the life of the pump


5/63

WHY PUMPS AND SEALS FAIL

MECHANICALAffects Bearings, Seals and Shafts

-EXTERNAL1. Operation off the BEP

2. Coupling Misalignment

3. Insufficient NPSH4. Poor Suction and Discharge

Piping Design

5. Pipe Strain / Thermal Expansion

6 Impeller Clearance

7. Foundation and Baseplate

-INTERNAL1. Pump Design and Manufacturing

Tolerances

2. Impeller Balance (Mechanical and

Hydraulic)

3. Mechanical Seal Design

ENVIRONMENTALAffects Wet End Components,

Bearings and seals

1.High Temperature

2. Poor Lubrication

/ Oil Contamination

3. Corrosion

4. Erosion

5. Abrasion


6/63

HOW ARE FAILURES INITIATED?

Installation Piping system & Pipe Strain

Alignment

Mechanical Seal installation

Foundation

Operational System: cavitation, dry running, shutoff

Product changes: viscosity, S.G., temp.

Seal controls: flush, coolingMisapplication

Pump, seal, metallurgy selection


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RADIAL LOAD

Operation of a pump away from the BEPresults in higher radial loads ...creating vibration and shaft deflection

H

E

A

D

FLOW

B.E.P


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Radial Forces

By design, uniform pressures exist around the

volute at the design capacity (BEP)

Resulting in low radial thrusts and minimal

deflection. Operation at capacities higher or lower than

the BEP

Pressure distribution is not uniform resulting in

radial thrust on the impeller

Magnitude and direction of radial thrust

changes with capacity (and pump specific

gravity)


9/63

SHAFT DEFLECTION

Most pumps do not operate at BEP: Due to improper pump selection (oversized)

Changing process requirements (throttling)

Piping changes

Addition of more pipe, elbows and valves

System head variations

Change in suction pressure, discharge headreqd

Buildup in pipes

Filter pluggedAutomatic control valve shuts off pump flow

Change in viscosity of fluid

Parallel operation problems (starving one pump


10/63

Impeller Radial Force

At Any Flow F(lbs.,Kg)

D

B

K= THRUST FACTOR

H = HEAD (ft, m)

S = SPECIFIC GRAVITY

D= IMPELLER DIAMETER (in.,cm)

B = IMPELLERWIDTH (in., cm)

FF == K x H x SK x H x S

2.312.31x D x Bx D x B


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KK

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100 120 140 160

500500(10)(10)

10001000(20)(20)

15001500(27)(27)

20002000 (40)(40)

35003500(71)(71)

PERCENT CAPACITY

SPECIFIC SPEED - K vs. CAPACITY

Ns (SI)


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PUMP SPECIFIC SPEED

CLASSIFIES IMPELLERS ON THE BASIS OF

PERFORMANCE AND PROPORTIONS REGARDLESS

OF SIZE OR SPEED

FUNCTION OF IMPELLER PROPORTIONS

SPEED IN RPM AT WHICH AN IMPELLER WOULDOPERATE IF REDUCED PROPORTIONALLY IN SIZE

TO DELIVER 1 GPM AND TOTAL HEAD OF 1 FOOT

DESIGNATED BY SYMBOL Ns

Ns = RPM(GPM)1/2

H3/4

RPM = SPEED IN REVOLUTIONS / MINUTE

GPM = GALLONS /MINUTE AT BEST EFF. POINT

H = HEAD IN FEET AT BEST EFF. POINT


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PUMP SPECIFIC SPEED (Metric)

CLASSIFIES IMPELLERS ON THE BASIS OFPERFORMANCE AND PROPORTIONSREGARDLESS OF SIZE OR SPEED

FUNCTION OF IMPELLER PROPORTIONS SPEED IN RPM AT WHICH AN IMPELLER WOULD

OPERATE IF REDUCED PROPORTIONALLY INSIZE TO DELIVER 1 M3/h AND TOTAL HEAD OF 1M

DESIGNATED BY SYMBOL NsNs = RPM(m3/h )1/2

H3/4

RPM = SPEED IN REVOLUTIONS / MINUTEm3/h = CUBIC METERS / HOUR AT BEPH = HEAD IN METERS AT BEST EFF. POINT


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PUMP TYPE VS. SPECIFIC SPEED

SPECIFIC SPEED, ns (Single Suction)

CENTRIFUGALCAPACITY

HEAD,POW

ER

EFFICIENCY

CAPACITY

HEAD,POW

ER

EFFICIENCY

AXIAL FLOW

CAPACITY

HEAD,POW

ER

EFFICIENCY

VERTICAL TURBINE

HEAD

EFFICIENCY

POWER

10 20 40 60 120 200 300

500 1,000 2,000 3,000 6,000 10,000 15,000

SI

US

RADIAL-VANE FRANCIS-VANE MIXED FLOW AXIAL FLOW


15/63

CUTWATER

SHUTOFF 0%Length of Line = Force

50%

BEP 100%

%CAPACITY of

BEP

125%

150%

FLOWR

ADIALLOAD

BEP

RADIAL FORCES ON IMPELLER


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THE IMPORTANCE OF ALIGNMENT

Any degree of misalignment between the

motor and the pump shaft will cause

vibration in the pump.

Every revolution of the coupling places a

load on the pump shaft and thrust bearing

At 2900 RPM, there will be 2900 pulses per

minute applied to the shaft and bearing


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MISALIGNMENT

Pipe strain

Thermal growth

Poor foundation / base plate

Improper initial alignment.

System vibration / cavitation

Soft foot on motor


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NET POSITIVE SUCTION HEAD (NPSH)

NPSH (Net Positive Suction Head)

Pressure in terms of head above vapourpressure at the inlet / eye of the impeller isknown as NPSH (Net Positive Suction Head)

NPSH available

Pressure in terms of head above vapourpressure available at the inlet / eye of the

impeller is known as NPSHA (Net PositiveSuction Head Available)


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NPSH cont.

NPSH required

Pressure in terms of head above vapour

pressure required at the inlet / eye of the

impeller to avoid cavitation is known asNPSHR (Net Positive Suction Head Required)

NPSHavailable must always be > NPSH

required by a minimum of 3-5 feet (1-1.5m)

margin


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CAVITATION

Results if the NPSH available is less than theNPSH required

Occurs when the pressure at any point inside thepump drops below the vapor pressure

corresponding to the temperature of the liquid The liquid vaporizes and forms cavities of vapor

Bubbles are carried along in a stream until a regionof higher pressure is reached where they collapseor implode with tremendous shock on the adjacentwall

Sudden rush of liquid into the cavity created by thecollapsed vapor bubbles causes mechanicaldestruction (cavitation erosion or pitting)


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CAVITATION cont.

Efficiency will be reduced as energy is

consumed in the formation of bubbles

Water @ 70oF (20oC)will increase in volume

about 54,000 times when vaporized Erosion and wear do not occur at the point of

lowest pressure where the gas pockets are

formed, but farther upstream at the point

where the implosion occurs Pressures up to 150,000 psi have been

estimated at the implosion (1,000,000 Kpa)


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E

A B CD

TURBULENCE,

FRICTION,ENTRANCE

LOSS

AT VANE TIPS

INCREASINGPRESSURE

DUE TO

IMPELLER

A B C D E

ENTRANCE

LOSS

FRICTION

INCREASING

PRESSURE

POINTOFLOWEST

PRESSUREWHERE

VAPORIZATIONSTARTS

POINTS ALONG LIQUID PATH

RELATIVE PRESSURES IN THE PUMP SUCTION


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(friction in suction

pipe)

Hf

Z

PAtmospheric

NPSHAvailable = P Atm. - Pvap. pressure - Z - Hf

Correct for specific gravity

All terms in feet (meters) absolute

NET POSITIVE SUCTION HEAD

AVAILABLE


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Results of Operating Off BEP

High Temp. Rise

Head

Head

FlowFlow

BEPBEP

Low Flow Cavitation

Discharge Recirculation

Reduced ImpellerLife

Suction Recirculation

Low Brg. & SealLife

Cavitation

Low Brg. & SealLife


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TEMPERATURE RISE

Overheating of the liquid in the casing can cause:Rubbing or seizure from thermal expansion

Vaporization of the liquid and excessive vibration

Accelerated corrosive attack by certain chemicals

Temperature rise per minute at shutoff is:

(T oF (oC) / min.= HP (KW)so x K

Gal (m3) x S.G. x S.H.

HPso = HP (KW) @ shutoff from curve

Gal. (m3) = Liquid in casing

S.G. = Specific gravity of fluidS.H. = Specific heat of fluid

Ex.: Pump takes100HP (75KW) @s.o. , 6.8 gal casing (.03m3)

water (at 16 deg C) would reach boiling in 2 min.

A recirculation line is a possible solution to the low flow or

shut off operation problems....


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CASING GROWTH

DUE TO HIGH TEMPERATURE

T F T C INCHES MILLIMETERSEXPANSION

100 F 55 C 0.0097 IN 0.245 MM

200 F 110 C 0.0190 IN 0.490 MM300 F 165 C 0.0291 IN 0.735 MM

400 F 220 C 0.0388 IN 0.900 MM

500 F 275 C 0.0485 IN 1.230 MM

600 F 330 C 0.0582 IN 1.470 MM

10 inches

250 mm

ROTATION

COEFFICIENT OF THERMAL EXPANSION FOR 316 S/S

IS 9.7X10-6

IN/IN/F OR 17.5 X10-6

MM/MM/CCALCULATION IS T x 9.7 X10-6 X LENGTH IN INCHES

T x 17.5X10-6 X LENGTH IN MILLIMETERS


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IMPELLER CLEARANCE

Critical for open impellers Normal setting .015 (.38mm) off front cover

High temperature requires more clearance

- Potential rubbing problem causes vibration

and high bearing loads

- Set impeller .002 (.05mm) addl clearance

for every 500 F (280C) over ambient temp.

Important for maximum efficiency


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IMPELLER BALANCE

MECHANICAL

- Weight offset from center of impeller

- Balance by metal removal from vane

HYDRAULIC- Vane in eye offset from impeller C/L

- Variation in vane thickness

- Results in uneven flow paths thru

impeller- Investment cast impeller eliminates

problem

- Careful machining setup can help


29/63

TYPICAL ANSI (or DIN) PROCESS PUMP

Small dia. shaft with excessive overhang

Stuffing box designed for packing

Shaft sleeve

Light to medium duty bearings Rubber lip seals protecting the bearings

Snap ring retains thrust bearing in housing

Shaft adjustment requires dial indicator

Double row thrust bearing

Cast jacket on bearing frame for cooling

Small oil reservoir


30/63

ANSI (ISO/DIN) STANDARD PUMPS

Industry standards for dimensions based on

requirements for packed pumps

Shaft overhang a function of no. of packing rings

and space for gland and repack accessibility Clearance between shaft and box bore based

on packing cross-section

If most pumps today use mechanical seals -

why do we continue to use inferior designsmade for packing ??


31/63

BEARING OIL SEALS

Rubber Lip Seals Provided To Protect Bearings in

standard ANSI pumps

Have life of less than four months

Groove shaft in first 30 days of operation

External contamination causes bearing failure


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LIP SEAL LIFE

AUTOMOBILE

100,000 Miles @ 40 Miles /hr. = 2500 hrs.

of operation

PUMP

24 hrs./day x 365 days / year = 8760 hours

60% of lip seals fail in under 2000 hours

Lip seals may be fine for automobiles, butnot for pumps


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THRUST BEARING SNAP RING

Thrust bearings in standard ANSI pumps areheld in place with a snap ring

Snap ring material harder than bearing housing

Wear in bearing housing results in potential

bearing movement Difficult to remove and install

If installed backwards - potential loose bearing


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SIMULTANEOUS DYNAMIC LOADS

ON PUMP SHAFT

Impeller Axial

Thrust

Impeller Radial Thrust

HydraulicallyInducedForces due to

Recirculation

& Cavitation

Hydraulic

Imbalance

Seal

Radial Thrustdue to Impeller

and Misalignment

Axial Load

from Misalignmentand Impeller

Radial Thrust

due to Impeller

and Misalignment

Coupling

Motor


35/63

SHAFT DYNAMICS

Radial movement of the shaft occurs in 3 forms:

Deflection - under constant radial load in one direction

Whip - Cone shaped motion caused by unbalance

Runout - Shaft bent or eccentricity between shaft sleeve

and shaftIt is possible to have all 3 events occurring simultaneously

ANSI B73.1 and API 610

Limit radial deflection and runout of the shaft to 0.002

T.I.R. at the stuffing box face(0.05mm) Solid shafts are critical for pump reliability

Eliminate sleeve runout

Improved stiffness


36/63

SHAFT DEFLECTION

Shaft deflects because of unbalanced radial loads

on the impeller

Shaft revolves on own centerline even when

deflected

load is constant in direction and magnitude Shaft stays bent as long as operating conditions

remain the same


37/63

Shaft Whip

Shaft changes 180o from its centerline every

revolution

Usually caused by unbalanced impeller

Heavy side of impeller on same side of shaft

Whip and deflection can occur at same time Moved to one side by the amount the shaft

deflects


38/63

OPTIMUM PUMP DESIGN

OBJECT:

Create a better environment andgreater stability for the dynamic

pump components (seals and

bearings) .to withstand thedamaging forces inflicted upon them


39/63

SHAFT STIFFNESS

500 Lbs.(225Kg)

500 Lbs.(225Kg)


40/63

P = Load

E = Modulus of Elasticity

L = Length of Overhang

= PL3 I= 4 D4

3EI 64

= PL3 = L3

3E P D4 D4

64

Derivation of Stiffness Ratio

= Deflection of shaft

I = Moment of Inertia

cancel all common factors

LP

D


41/63

Stiffness Ratio Examples

LL

DD

D L3 4 3 4

1.50" 8" L /D = 8 /(1.50) = 512/5.06 = 101

1.62" 8" L3/D4 = 83/(1.62)4 = 512/6.89 = 74

1.75" 8" L3/D4 = 83/(1.75)4 = 512/9.38 = 55

1.87" 8" L3/D

4= 8

3/(1.87)

4= 512/12.23 = 42


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D L

LL

DD

1.87" 8" L3/D

4= 8

3/(1.87)

4= 512/12.23 = 42

1.87" 6" L3/D4 = 63/(1.87)4= 216/12.23 = 17


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LL

DD

D L

38mm 200mm L3/D

4= 200

3/ 38

4= 8000000/2085136 = 3.84

40mm 200mm L3/D

4= 200 3/ 40

4= 8000000/2560000 = 3.13

45mm 200mm L3/D4 = 200 3/ 454 = 8000000/4100625 = 1.95

48mm 200mm L3/D

4= 200

3/ 48

4= 8000000/5308416 = 1.51

L/D


44/63


LL

DD

D L

48mm 200mm L3/D

4= 200

3/ 48

4= 8000000/5308416 = 1.51

48mm 150mm L

3

/D

4

= 150

3

/ 48

4

= 3375000/5308416 = .64

L/D < 2.4 Considered Adequate


45/63

MAXIMUM STIFFNESS RATIO

L3 / D4 RATIO

Less than 60 (Inch)

Less than 2.4 (Metric)


46/63

FLOFLO

ZONE L3/D4ZONE L3/D4

INCHINCHA > 80A > 80

B 60 > 80B 60 > 80C 26 > 60C 26 > 60

D < 26D < 26

> 3.2> 3.2

B 2.4 to 3.2B 2.4 to 3.2

C 1.0 to 2.4C 1.0 to 2.4

D < 1.0D < 1.0

METRICMETRICAA

1515 25251010 2020

HEAD

HEAD

PUMP CURVEPUMP CURVEBEPBEP

A

B

C

D

001010202040408080PERCENT OF BEPPERCENT OF BEP

EFFECTIVE PUMP OPERATIONAL ZONES


47/63

ALIGNMENT

EVERY TIME A PUMP IS TORN DOWN,THE MOTOR SHAFT AND PUMP SHAFTMUST BE REALIGNED

UNPROFESSIONAL OPTION TO RE-ALIGNUSE A STRAIGHT EDGE

PROFESSIONAL OPTION IS TO USE DIALINDICATORSTO MINIMIZE TOTAL RUNOUT

MODERN METHOD IS LASER ALIGNMENTWHICH IS VERY ACCURATE


48/63

Drawbacks of Present Alignment Methods

All provide precision initial alignment Degree of accuracy varies

Cost of system, training, and time involved in theiruse is dramatic

Time consuming (possibly 2 workers, 4-8 hrs.)

Difficult to compensate for high temperatureapplications

Requires worker skill, dexterity, and training toachieve accurate results

After pump startup, cannot insure continuedalignment due to temperature, pipe strain, cavitation,

water hammer, and vibration


49/63

SEAL CHAMBERS

Designed specifically for seals

20 Times greater fluid volume Provides superior cooling,cleaning,

and lubrication for the seal

Solids centrifuged away from seal

Eliminate seal rub problems

Designed for packing

Small radial clearances-Seal contacting bore

Limited fluid capacity

-Poor heat removal

Easy to clog with solids

OLD STYLELARGE BORE


50/63

ELIMINATING SHAFT SLEEVES

REDUCES SEAL SIZE Sleeves are necessary for packed pumps, but with

todays new seals they serve no purpose

Add no stiffness to shaft

Run out tolerance between shaft and sleeve compoundsmotion of seal faces in addition to deflection and shaftrun out already present

Deflection must be a maximum of .002 at the sealfaces, yet faces are lapped within 2 helium light bands

Deflection or motion at seal faces is 1000 times greaterthan the face flatness


51/63

BEARING OIL SEALS

Three basic types:

Lip seal

Inexpensive, simple to install, very effective

when new Elastomeric construction

Contact shaft and contributes to friction

drag and temp. rise in bearing area

After 2000-3000 hours, no longer provide

effective barrier against contamination

Will groove shaft


52/63

BEARING OIL SEALS

Labyrinth seals

Required by API 610

Non-contacting and non-wearing

Unlimited life Effective for most types of contaminants

Do not keep heavy moisture or corrosive

vapors from entering the bearing frame(especially in static state)


53/63

BEARING OIL SEALS

Face seals and magnetic seals

Protect bearings from possible immersion

Good for moisture laden environment

Expansion chamber should be used toaccommodate changes in internal pressure

and vapor volume

completely enclosed system (can be

submerged) Generate heat

Limited life


54/63

BEARING LIFE

Bearing life calculations assume proper lubricationand an environment that protects the bearing fromcontamination

The basic dynamic load rating C is the bearing

load that will give a rating life of 1 million revolutions L10 Basic Rating Life is life that 90% of group of

brgs. will exceed ( millions of revs or hrs. operation)

Rating Life varies inversely as the cube of the

applied load Reduction of impeller dia. from maximum improves

life calculation by the inverse ratio of the impellerdiameters to the 6th power


55/63

BEARING LIFE cont.

90% of all bearings will fail prematurely andnot reach their rated L10 life

- Calculated life by design over 20 years

- Actual life maybe 3 years

Failures:

-Fatigue due to excessive loads (20-50%

of failure)

-Lube failure - excessive temperatures &

contaminants

-Poor installation


56/63

BEARING LUBRICATION FAILURE

OXIDATION

Chemical reaction between oxygen & oil

New compounds produced which deteriorate the

life of oil and bearings

Reaction rate increases with the presence of water

and increases exponentially with temperature

CONTAMINATION

Water breaks down lube directly reducing brg.

life - .003% water in oil reduces life of oil 50%

Oil life decreases by 50% for every 20oF (11oC)

rise in temp. above 140oF (60oC)


57/63

SYNTHETIC

OILS Lower change in viscosity with temp. change-One synthetic can take place of several oils

Provides good lube at high temps. 300oF (160oC)

-Does not oxidize (breakdown)

At low temps.- good fluidity boosts efficiency and

reduces component wear during cold weather

Achieves full lubrication quickly

Offers longer life - less consumptionLasts 1.5-2 times longer than conventional oils

Maintains lube properties with water

contamination better than mineral oils


58/63

ANGULAR CONTACT BEARINGS

Used as thrust bearing in pairs (also carry radialload)

Mounted back to back (letters to letters)

Provides maximum stiffness to shaft

Avoid ball skidding under light loads

Small preload eliminates potential

Line to line design clearances

Shaft fit provides preload Eliminates shaft end play

Greater thrust capacity

Required by API 610 Specification


59/63

BEARING PRELOAD

Pump radial bearings have positive internalclearance

Thrust bearings can be either positive ornegative clearance.

Preload occurs when there is a negativeclearance in the bearing

Desirable to increase running accuracy

Enhances stiffness Reduces running noise

Provides a longer service life under properapplications

BEARING CLEARANCES /


60/63

BEARING CLEARANCES /

PRELOAD

LIFE

ClearancePreload


61/63

Goal: improved pump and Mechanical seal reliability

Eliminate or reduce mechanical and

environmental influences that cause pump and

seal problems.

Specify proper pump design criteria to minimize

the impact of mechanical and environmental

influences.

Specify proper mechanical seal and

environmental controls to maximize seal life


62/63

Optimum pump design summary

Low L3D4 ratio as possible Solid shaft ( no sleeves)

Large bore seal chamber

Large oil capacity bearing housing

Angular contact thrust bearings Retainer cover to hold thrust bearing (no snap rings)

Fin tube cooling for bearing housing

Labyrinth seals

Positive / precision shaft adjustment method Investment cast impellers

Magnetic drain plugs in oil sump

Centerline support for hot applications

R i t f i i


63/63

Requirements for proper emission

control and maximum seal life

Shaft runout at impeller within .001 T.I.R. (.03mm) Coupling alignment within .005 T.I.R. on rim & face

(.13mm)

Operation of the pump at or close to best efficiency point(definition dependent upon pump size, speed, and LD ratio)

NPSH available to be at least 5 feet (1.5m) greater thanNPSH required

Proper foundation and baseplate arrangement

Absolute minimum pipe strain on suction and dischargeflanges

All impellers dynamically balanced to ISO G 6.3 spec. Face of seal chamber square to shaft within .002 T.I.R.

(.05mm)

Seal chamber register concentric to shaft within .003 T.I.R.(.08mm)

Date post:	06-Apr-2018
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Pump Reliability

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