Pump Failure Article

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WORLD PUMPS October 2012

Before embarking on a pump failure

reduction programme it is impor-

tant to know how well a particular

facilitys pumps perform when compared

to those installed at a competitors plants.

There probably is room for improvement

if a plant does not measure up to above-

average pump failure statistics.

MTBFs and failure analysis

An easy comparison among pump users

is feasible. It consists of adding up all

process pumps installed at a plant and

then dividing by the number of pump

repairs per year. For a well-managed and

reasonably reliability-focused US refinery

with 1,200 installed pumps and 156

repair incidents in one year, the mean

time between failures (MTBF) would be

(1,200/156) = 7.7 years.

The refinery would count as a repair

incident the replacement of parts any

maintenance and better pump monitoring

may have contributed to reduced failure

severity on the typical pump, although the

ultimate consequences of some pump fail-

ures are grave indeed.

The MTBFs (installed life before fa ilure)

shown in Table 1 have been estimated in2004. Published data and observations

made in the course of performing main-

tenance-effectiveness studies and relia-

bility audits in the late 1990s and early

2000s were used in these estimates.

Seal-life statistics were estimated in the early

2000s (Table 2). These led to an estimation

of reasonable goals (Table 3). Note that the

target is less than best actually achieved.

Many plants achieve the months of

installed lives indicated in Tables 2 and 3.

Note that the actual operating life of a

component would thus be about one-

half of its installed life. To reach these

pump life expectancies, the pump

components themselves must be oper-

ating at the highest levels of reliability.

An unsuitable seal with a lifetime of one

month or less would have a serious

negative effect on pump MTBF, as would

an under-performing coupling or bearing.

Calculating MTBF projectionsSimplified calculations2 will give an indi-

cation of the extent to which improving

one or two key pump components can

improve overall pump MTBF.

parts regardless of cost. In this case, a

drain plug worth two dollars or an alloy

impeller costing $5,000 would show up

in the same way on the MTBF statistics.

Only the replacement or change of lube

oil would not be counted as a repair.

A best-practices plant counts in itstotal pump repair cost all direct labour,

materials, indirect labour and overheads,

administration cost and the cost of labour

to procure parts. It assigns a value to

failure avoidance, even the pro-rated

value of avoiding pump-related fire inci-

dents. Likewise, it assigns a monetary

value to a workforce relieved of pump

repair burdens and assignable to proac-

tive asset failure avoidance tasks.

Typical published pump repair costs have

averaged $10,287 in 1984 and $12,000 in

2008. After inflation is factored into a repair,

an actual cost reduction trend is indicated

over this 24-year time span. Predictive

Widely accepted statistics point to around 7% of a plants process

pumps consuming c. 60% of the maintenance funds allocated to

its entire pump population. With repeat failures largely responsible,Heinz Bloch outlines successful failure avoidance actions that can

be employed to extend the mean time between failures.

Problem pumps:a thing of the past

Operating

0262 1762/12 2012 Elsevier Ltd. All rights reserved

Table 1. Pump mean times between failures1

Pump type and location MTBF (years)

ANSI pumps, average, USA 2.5

ANSI/ISO pumps average, Scandinavian pulp & paper plants 3.5

API pumps, average, USA 5.5

API pumps, average, Western Europe 6.1

API pumps, repair-ocused refnery, developing country 1.6

API pumps, Caribbean region 3.9

API pumps, best-o-class, US refnery, Caliornia 9.2

All pumps, best-o-class petrochemical plant, USA (Texas) 10.1

All pumps, major petrochemical company, USA (Texas) 7.5


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Say, for example, that theres agreement

that the mechanical seal is the pump

component with the shortest life,

followed by the bearings, coupling, shaft

and sometimes impeller, in that order.

The anticipated MTBF (operating MTBF)

of a complete pump assembly can beapproximated by summing the individual

MTBF rates of the individual components,

using the following expression:

1/MTBF = [(1/L1)2 + (1/L2)

2 + (1/L3)2

+ (1/L4)2]0.5 (1)

In a 1980s study, the problem of mechan-

ical seal life was investigated. An assess-

ment was made of probable failure

avoidance that would result if shaft

deflections could be reduced. It was

decided that limiting shaft deflection atthe seal face to a maximum of 0.001 in

(0.025 mm) probably would increase seal

life by 10%. It was thought that increasing

seal housing dimensions to accommodate

modern seal configurations would more

than double seal MTBF. All components

that could be upgraded were examined.

The life estimates were collected and then

used in MTBF calculations.

In Equation 1, L1, L2, L3 and L4 represent

the life, in years, of the component

subject to failure. Using applicable datacollected by a large petrochemical

company in the 1980s, MTBFs and esti-

mated values for a reliability-upgraded

pump were calculated. The results are

presented in Table 4. As an example, a

standard construction ANSI B73.1 pump

with a mechanical seal MTBF of 1.2,

bearing MTBF of 3.0, coupling MTBF of 4.0

and shaft MTBF value of 15.0, resulted in

a total pump MTBF of 1.07 years (actual

operating hours). By upgrading the seal

and bearings, the estimated achievable

pump MTBF (operating hours) can be

improved by 80%, to 1.93 years.

Table 4 shows the influence of selectively

upgrading either bearings or seals or both

on the overall pump MTBF of relatively

small, inexpensive pumps. Choosing a

2.4-year MTBF seal and a six-year MTBF

bearing (considered achievable by

preventing lube oil contamination via

superior bearing housing protector seals)

had a major impact on increasing the

pump MTBF. Assuming the upgrade cost is

reasonable, better seals are the best choice.

Based on year 2002 reports, a typical ANSI

pump repair costs $5,000. This average

cost includes material, parts, labour and

overheads. Assume that the MTBF for a

particular pump is 12 months and that it

could be extended to 18 months. Thiswould result in a cost avoidance of

$2,500/year which is greater than the

premium one would pay for the relia-

bility-upgraded centrifugal pump.

Reduced power demand would, in many

cases, further improve the payback.

Selecting advantageous pump hydraulics

benefits both pump life and operating effi -

ciency. Audits of two large US plants iden-

tified seemingly small pump and pumping

system effi ciency gains that resulted in

power-cost savings of many hundreds of

thousands of dollars per year. Thus, the

primary advantages of reliability-upgraded

process pumps are extended operating life,

higher operating effi ciency and lower

operating and maintenance costs.

Table 4 provides a quick means of

approximating the annual pump repair

frequency based on the total (installed

life) MTBF. Equation 1 and Table 4 also

can be used to determine potential

savings from upgrades and should shape

the pump users strategies.

An experience-based observation assumes

that every missed upgrade item reduces

pump life by 10% to a new life factor of

only 0.9 years. If we miss six such upgrade

items we will have reduced the antici-

pated life or MTBF to (0.9)6 = 54% ofwhat it might otherwise have been.

Older pumps versus newer pumps

After 50 or 60 years of service and many

maintenance actions, a large number of

standard ANSI and ISO-compliant pumps

are still operating. When they were

designed in the 1950s and 1960s time

frame, frequent repairs were accepted.

Also, plant maintenance departments

were staffed with more personnel than in

later decades.

Unless selectively upgraded, a decades-

old standard pump population will not

allow 21st Century facilities to reach

their true reliability and profitability

potentials. An older pump will generally

fail more often than a newer pump. Like-

wise, a standard process pump will fail

more often than an upgraded process

pump. It should also be realized that

even the latest industry standards tend

to list minimum component require-

ments only. So, consider upgrading

beyond the standard.

Buried in Table 1 is a plant with more

than 2,000 installed pumps; their average

Table 2. Suggested refnery seal target MTBFs1

Target or seal MTBF in oil refneries

Excellent >90 months

Very good 70/90 months

Average 70 months

Fair 62/70 months

Poor 60 months

Bearings All plants

Continuous operation 60 months

Spared operation 120 months

Pumps General types

Based on series system

calculation

48 months


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Time

Temperature.

It is extremely important to accept the

basic premise that components will only

fail due to one, or perhaps a combination

of several, of these four failure agents. We

use the acronym FRETT to recall thesefour agents.

Because there are no failure agents

beyond these four, the troubleshooter

must remain fully focused on these four

agents. To re-emphasize by an example, a

bearing can only fail if it has been

subjected to a deviation (or deviations) in

allowable force (F), or has been exposed

to a reactive environment (RE), has been

in service beyond its design life (T), or

was subjected to temperatures outside

the permissible range (T).

The need for knowledge must not be

overlooked. For instance, bearings can fail

(overheat) when they are too lightly

loaded. The rolling elements will then

skid. But there we go again: skidding is

traceable to an inadequate force (F) and

will manifest itself as a temperature excur-

sion (T). Two of the four FRETT agents

are at work.

Each failure, and indeed each problem

incident, is the effect of a causal event. In

other words, for every effect there is a

cause; or, there is a reason for every

failure. Heres an example:

[Man injured] because man fell

[Man fell] because man slipped

[Man slipped] because there was

oil on the floor

[Oil on floor] because a gasket

leaked.

By arriving at the word gasket, the cause-

and-effect chain is focused at the compo-

nent level. Once we have narrowed issues

down to the component level, we know

that one or sometimes two troublesome

or unexpected or overlooked FRETT

Maintenance deficiencies, including

neglect/procedures

Improper operation.

Searching for additional cause categories

will not add value because anything

uncovered will, at best, be a subset of

these seven. However, if one systemati-cally concentrates on eliminating five or

six of the seven categories in succession,

one will arrive at the category where a

deviation exists. That will make it possible

to concentrate on understanding what

led to the deviation.

The pump failure analyst must pay close

attention to the under-appreciated,

generally non-glamorous basics and do

so before opting for the often costly

and sometimes unnecessary high-tech

solution. Pumps obey the laws ofphysics and there is always a cause-

and-effect relationship. It follows that

even seemingly elusive and generally

costly repeat problems can usually be

eliminated without spending much

money.

An integrated, comprehensive approach

to failure analysis starts out by either

describing the deviation, or by stating the

problem. Next, such an approach encour-

ages, or even mandates, careful observa-

tion and definition of failure modes. The

approach should employ pre-existing or

developed-as-you-go checklists and trou-

bleshooting tables1,3. Already-existing

checklists are supplied by pump manufac-

turers and can also be found in a very

large body of literature.

The FRETT approach

From observation and examination of a

failed part one identifies failure agent(s),

realizing that there are only four

possibilities3:

Force

Reactive environment

size is close to 30 hp. In 2010 this pump

population had an MTBF of slightly more

than nine years. Its owner-operators

prided themselves in cultivating effective

interaction between the mechanical and

process-technical workforce members.

The reliability professionals at this plant

fully understood that pumps are part of

a system and that the system must becorrectly designed, installed and oper-

ated if high reliability is to be achieved

with consistency. It should also be

pointed out that this plant (and others in

its peer group) conducted periodic

pump reliability reviews.

Structured analysis solves problems

Repeat pump failures are an indication

that the root cause of a problem has not

been found. Alternatively, and if the

problem cause is known, someone musthave decided not to do anything about it.

Pursuing a structured failure analysis

approach is necessary to solve problems.

Guessing or going by feel will never do.

Structured analysis means a repeatable

approach that can be learned and

employed by more than one person3.

Once an accurate analysis is documented,

remedial steps can be agreed upon and

can be implemented. Also, whenever it can

be established that a pump at location A

suffers more failures than an identical

pump at location B, we can be sure that

an explanation exists. The explanation is

found in deviations from best practices in

one or more of the following seven cause

categories:

Faulty design

Material defects

Fabrication and/or processing

(machining) errors

Assembly or installation defects

Off-design or unintended service

conditions

Table 4. How selective component upgrading infuences MTBF2

ANSI pump upgrade

measure

Seal MTBF

(years)

Bearing

MTBF (years)

Coupling

MTBF (years)

Shat MTBF

(years)

Composite

pump MTBF

(years)

None, i.e. standard 1.2 3.0 4.0 15.0 1.07

Seal and bearings 2.4 6.0 4.0 15.0 1.93

Seal housing only 2.4 3.0 4.0 15.0 1.69

Bearing environment 1.2 6.0 4.0 15.0 1.13


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Head

% Flow110%80%

Reliability Curve

Best Efficiency Point

Hightemperature

rise

Low flowcavitation

Cavitation

Lower bearing &seal life

Lower impeller life

Lower bearing &low seal life

Dischargerecirculation

Suctionrecirculation

Reactive environment: none found;

normal chemical plant location and

ambient environment.

Time: ascertained that run length was

not excessive; the hub failed after just a

few weeks of operation.

Temperature: suppose the coupling was

heated to facilitate installation. How

was the heat applied? What tells us

that the temperature was within limits?

The temperature could have been too

high (causing overstretch) or too low

(not allowing dilation to result in suffi -

cient axial advance).

In both of these examples, the pump

failure analyst has to determine in which

cause category there is a deviation fromthe norm, which item needs to be modi-

fied and how this modification must be

implemented so as to prevent a repeat

failure. Data will be required to support

any conclusions. With data one can

define the root causes of a problem.

Without data one can, at best, determine

a probable cause.

Change analysis

Change analysis parallels and supple-

ments the structured, comprehensiveapproach. It seeks to identify what is

different in the defective item as

compared to an identical but unaffected

item. The analyst probes into when,

where and why the change occurred.

The analyst then outlines a number of

contributors must now be found. In this

case, a gasket leaked. A gasket is clearly a

component. So:

[Gasket leaked] must be due to: force?

reactive environment? time? temperature?

We must check it out on the basis of data.Without data we would be guessing, and

guessing does not lead to repeatable results.

Force: too much why do we rule it in

or rule it out? Not enough why do

we rule it in or rule it out?

Reactive environment: wrong material

selected for the medium transported in

the pipe? Why do we rule it in or rule

it out?

Time: was the same gasket left in placefor many years? Why do we rule it in or

rule it out?

Temperature: too high? too low? Which

one of these two (or perhaps both)

might be ruled in or can confidently

be ruled out in a particular instance?

The pump failure analyst must take a very

similar approach with pumps and other

machinery. For every effect there is a

cause; there is a reason for every failure

and we have to find it:

[Pump is down] because the shaft broke

[Shaft broke] our failure mode inven-

tory was consulted; lets assume we

found the surface has fretting damage.

That is a deviation from the norm.

[Surface damaged] because the

coupling hub was loose. That would

explain the fretting damage.

An analyst can now try to get to the root

cause by remembering that all pump

failure events fit into one or more of the

seven cause categories listed above. If the

coupling hub was found to be loose, what

cause categories are likely and which ones

can we eliminate?

Design error? Unlikely, since other

couplings are designed the same way

and we have verified that they are

holding quite well.

Material defects? No, since a thorough

metallurgical exam checks OK.

Fabrication error? No, because the

hardness checked OK; dimensional

correctness was verified and had been

recorded upon installation three years

ago.

Assembly/installation defect? Suppose

we have no data and defer it for

possible consideration later.

Off-design or unintended service condi-

tions? No; we rule it out.

Maintenance deficiencies (neglect/

procedures)? No, since no preventative

maintenance (PM) is required on a

coupling hub.

Improper operation? No, because we

have ascertained operator activities

were in accordance with our estab-

lished standards.

At this stage the analyst would get

back to what needs to be investigated

further or requires follow-up examination.

This might be a good time to start

compiling:

(a) A checklist of possible assembly errors.

From discussions with maintenance

personnel we might conclude that

none apply in this instance.

(b) A checklist of possible installationerrors:

Force: could have overstretched hub,

or could have had insuffi cient axial

advance on taper (insuffi cient inter fer-

ence fit).

Figure 1. Staying near the centre of this reliability curve also known as a BarringerNelson curve is a wise

course of action4. (Source: Paul Barringer, www.barringer1.com, by permission.)


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remedial action steps and will have to

choose the steps that best meet defined

objectives. These objectives must achieve

highest safety and the analyst may pick

from a list that includes lowest life-cycle

cost (LCC), present value, highest initial

quality, meeting a certain industrystandard, a deadline, etc.

The objective of aiming for lowest LCC

usually makes considerable sense. Calcu-

lating this parameter would include the

cost of staffi ng a pump selection or reli-

ability review with dedicated, knowl-

edgeable individuals. LCC analyses must

also include the value of downtime

avoidance and MTBF extensions, as well

as the value of avoided fire and safety

incidents.

Recall that fewer pump failures translate

into fewer fires and decreased insurancepremiums. Failure avoidance creates

goodwill and enhances a companys repu-

tation. Also, having to cope with fewer

failures encourages a safety culture and

frees up personnel whose proactive activi-

ties avoid other failures, etc.

Over the decades, we have come to

realize that pump failure statistics are

rarely very scientific. Still, they are experi-

ence-based and should not be disre-

garded. If your MTBF hovers around

average, identify the repeat offenders and

subject them to an uncompromising

improvement programme. In the hydro-

carbon processing industry, about 7% of

the pump population consumes 60% of

the money spent on pump maintenance

and repair. Getting at the root causes of

failures on these 7% will save a lot of

money.

A strategy that involves rational thinking

is solidly supported by a minutes worth

of looking up vendor documentation. A

sound strategy also mandates respect for

the simple laws of physics. Its a strategy

that results in failure cause identification;

it will lead to future failure avoidance and

will extend pump MTBF.

ContactHeinz P. Bloch

Process Machinery Consulting

3163 W. 111th Drive

Westminster, CO 80031, USA

Tel: +1 515 225 0 668

E-mail: [email protected]

www.heinzbloch.com

It is fitting, then, to conclude or recap this

article by pointing to a very simple illus-

tration (Figure 1). This illustration tries to

convey that many parameters interact to

cause repeat failures in pumps. Many of

these are classified as hydraulic issues and

much work has been done to improvepump hydraulics. However, the majority of

what we choose to call elusive failure

causes are linked to mechanical issues.

We have become accustomed to mainte-

nance routines that rarely question the

adequacy of a vendors design. Failure

causes have become elusive because we

overlook or forget (and even disregard)

the laws of physics.

It should also be pointed out that process

pump vendors often merely furnish a

barely adequate design5. Vendors are left

with the impression that users are unwilling

to pay for a superior design. Moreover,vendors and pump manufacturers benefit

from the sale of replacement parts and are

in business to generate income.

We must also not forget that pump

manufacturers have right-sized, down-

sized and economized the way they do

business. Few (if any) of these organiza-

tional realignments benefit the user and a

preponderance of repeat failures attests to

this lack of benefit. Some vendors and

manufacturers no longer employ process

pump experts and diligent craftsmen. The

user-purchaser may belatedly come to

realize that he has become the manufac-

turers quality control inspector. Many

must experience failures before they

accept this fact. When they learn the hard

way, they must allocate money to ward

off this eventuality by suitable pre-

delivery inspections.

Timely and competent up-front action by

the owner-purchaser is one of the keys to

failure avoidance. This up-front action

includes development of detailed specifi-

cations for process pumps and some of

the key components that go into good

process pumps. Once a process pump

arrives in the field, it must be properly

installed and maintained. To be effective,

the facility must adopt work processes

and procedures that harmonize with best-

of-class thinking5.

To avoid repeat failures, pump owner-oper-

ators must deliberately push certain routinemaintenance actions into the superior

maintenance category. Superior mainte-

nance efforts will lead to (or are synony-

mous with) pump reliability upgrading.

In essence, the course of wisdom

demands that we move away from

business as usual. Before one can apply

practical wisdom5, one must acquire

knowledge and understanding. I hope

that this article has helped the reader

in this regard.

References

[1] H.P. Bloch and A.R. Budris, Pump Users

Handbook, 2nd Edition, Fairmont Press,

Lilburn, GA, USA, ISBN 0-88173-517-5,

(2006).

[2] H.P. Bloch and D. Johnson, Downtime

Prompts Upgrading of Centrifugal Pumps,

Chemical Engineering, November 25, (1985).

[3] H.P. Bloch and F.K. Geitner, Machinery

Failure Analysis and Troubleshooting, 4thEdition, Butterworth-Heinemann

Publishing, Stoneham, MA, USA, ISBN

978-0123-860453, (2012).

[4] P. Barringer, API Pump Curve Practices

and Effects on Pump Life from Variability

About BEP, Weibull Analysis Course [avail-

able from: www.barringer1.com] (see also

Ref. 2, p. 621).

[5] H.P. Bloch, Pump Wisdom, John Wiley &

Sons, Hoboken, NJ, USA, ISBN 978-1-118-

04123-9, (2011).

"Repeat pump failures are an indicationthat the root cause of a problem has not

been found..."

This article is excerpted, by permission,

from Heinz P. Blochs textbook Pump

Wisdom, published by John Wiley &

Sons, ISBN 978-1-118-04123-9 (2011).


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Any defect in the rotating assembly

of a pump (particularly when a

component becomes loose or

disconnected) could result in an excessive

unbalance and high vibration levels, which

in turn can lead to an emergency trip. In

case of any serious malfunction, the expec-

tation is that the pump will come to a safe

stop (through an emergency trip) with the

minimum possible damage.

Pump failure mechanisms

Rotating pump components (such asimpellers or blade-rows) may exhibit reso-

nance with any excitations generated by

the pump package. Resonances with the

first and second natural frequencies can

be particularly dangerous. Generally, there

could be numerous cases of resonance in

a given pump system; some of them

could even be unexpected. For example,

in one pump, the second natural

frequency of a rotating assembly proved

to be almost exactly an integer multiple

of the first natural frequency, which

resulted in excitation and some damage.

Liquid-induced vibration, oscillatory

changes in liquid pressure and turbulent

flow can be causes of high vibration levels

or even a failure in some pumps. The stress

amplitudes should be analysed. The

stresses should clearly be within accept-

able limits, for instance, fatigue limits such

as a high cycle fatigue (HCF) failure or a

low cycle fatigue (LCF) failure. A form of

fatigue is usually involved in most rotating

part failures and such failures can only take

place after a number of operating hours. In

other words, a few hours of a pump shop

performance test (in the manufacturers

shop) usually cannot show signs of a

future fatigue failure.

practice, some pump rotor assembly

designs are unbalance-insensitive while

other designs are sensitive to any unbal-

ance. Generally, an accurate rotordynamics

study can identify the sensitivity of the

rotating assembly to unbalance. In the

The sudden failure of a pump component can have serious and

costly consequences. Amin Almasi considers the failure modes in

a pumps rotating assembly, including resonance issues, fatiguefailures, vibration, the chain of events and secondary damage to

critical auxiliaries. The discussion is illustrated with case studies.

Practical notes oncomponent failure

Operating

0262 1762/12 2012 Elsevier Ltd. All rights reserved

Figure 1. An example of a damaged pump drive shaft.


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As a result of high vibration levels or

extreme dynamic forces, some auxiliary

piping may be damaged. The most critical

such element is the lubrication oil piping

system. Any serious damage to the piping

feeding the lubrication oil to the bearings

(particularly to the hydrodynamic bearings

in large pumps) could lead to loss of the

oil flow and destruction of the Babbitt

metal in the journal bearing liner in avery short time (say around 13 seconds).

A short time of operation (sometimes as

little as 38 seconds) without any lubrica-

tion oil for some bearings can result in

their total destruction and other serious

damage (even explosion or fire). Failure of

a rotating part inside a pumps electric

motor driver could result in a serious

short-circuit incident and probably an

explosion.

In the case of an explosion or a serious

incident, the pump (or motor) casing

should fulfil the function of containment.

Structural damage to auxiliaries (particu-

larly critical elements such as the lubrica-

tion oil system) is also of concern, and

the design should be such as to ensure

that the auxiliary piping, flammable liquid/

hydrocarbon systems and electrical facili-

ties will safely sustain any fault conditions.

Pump shaft failure

The major reasons for pump shaft failure

can be broken down into the following

categories:

Mechanical: such as overhung load,1.

bending load, torsional load and axial

load.

Dynamic: vibration, cyclic, shock, etc.2.

Residual: manufacturing processes,3.

repair processes, etc.

Thermal: temperature gradients, rotor4.

bowing, etc.

Environmental: corrosion, moisture,5.

erosion, wear, cavitation and similar.

Before the cause of a shaft failure can be

accurately determined, it is necessary to

understand clearly the loading and stress

acting on the shaft. The ability to properly

characterize the microstructure and the

surface topology of a failed shaft are also

critical steps in analysing a failure. The most

common tools available to do this are:

visual inspection; optical microscopy; scan-ning electron microscopy; transmission elec-

tron microscopy; and metallurgical analysis.

With experience and a fundamental knowl-

edge of shaft failure causes, a significant

number of failures can be diagnosed by

visual inspection. Confirmation may then be

sought from a metallurgical laboratory.

Table 1 gives an approximate breakdown

(rule of thumb) of the main causes of

pump shaft failures. Cavitation could be

considered as an impeller/casing failure

mechanism. There are other studies thatsuggest that fatigue-related failures are

more important for a pump shaft (>40%).

The failure mode is also dependent on the

pump type and the pump service/

application.

Table 1. Main causes of pump shaft failure

Cause of shaft failure Percentage (%)

Corrosion (various) 36

Fatigue (various) 31

Brittle fracture 16

Overload 11

Creep, wear, abrasion, erosion, etc. 6

"Before the cause of a shaft failure can beaccurately determined, it is necessary tounderstand clearly the loading and stressacting on the shaft."


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Residual stresses and fretting

Residual stresses or initial deflections are

usually independent of external loading

on the shaft. A wide variety of manufac-

turing or repair processes can affect the

amount of residual stress or initial deflec-tion including: drawing, bending,

straightening, machining, grinding,

surface rolling, shot blasting and

polishing. All of these operations can

produce residual stresses and initial

deflection by plastic deformation. In

addition, thermal processes that can

introduce residual stress and deflection

include: hot rolling, welding, torch

cutting and heat treatment.

Shaft fretting can cause serious damage

to the shaft and mating part. Typicallocations are points on the shaft where a

press or slip fit exists. The presence of

reddish-brown ferric oxide (rust) between

the mating surfaces is a strong indication

that fretting has occurred. The cause of

this condition is some degree of move-

ment between the two mating parts and

the presence of oxygen. Once fretting

occurs, the shaft may be very sensitive

to fatigue cracking (which could result in

a fatigue failure). Shaft vibration can

worsen this situation.

Case studies

Axial pump blade

This first case study concerns damage to

one of the blades of a large axial pump,

which resulted in extremely high vibra-

tions, damage to the lubrication feed line

piping and serious damage to the bearings

(extensive and costly consequences). The

blade in question failed because of reso-

nance and LCF failure, additionally

damaging neighbouring blades. The blade

loss incident led to an unusually large

unbalance of the pump, causing it to trip

as a result (a high-vibration emergency

trip). The pump rotor came to a stop, and

the pump casing fulfilled its containment

function perfectly. Because of the high

vibration level, the lubrication feed line to

the bearings was also damaged, which led

to the lubrication oil being cut off to the

sensitive hydrodynamic bearings during

the emergency shutdown period. All bear-

ings and many associated systems/compo-

nents were extensively damaged.

The lesson learned here was that the lubri-

cation oil piping (and any critical auxiliary

piping) should be designed to be robust

it is located in the area where the highest

shaft loading occurs. Fatigue cracks

usually start in the fillets or roots of the

keyway. A keyway that ends with sharp

step(s) has a higher level of stress concen-

tration than a keyway that uses a sled-runner type design. In the case of heavy

shaft loading, cracks frequently emanate

from this sharp step. A connection using

any form of key should be avoided to the

maximum extent possible. In special

cases, when other non-key solutions

cannot be used, it is important to obtain

an adequate radius on the edges of the

keyway.

Fatigue failures usually follow the weak-

link theory. The shaft fatigue failure

process usually consists of the following:

The fatigue leads to an initial crack on

the surface of the shaft.

The crack or cracks propagate until the

remaining shaft cross-section is too

weak to carry the load.

Finally, a sudden fracture of the

remaining area occurs.

Crack formation and fatigue

The origin of cracks caused by fatigue is

usually the presence of surface disconti-

nuities, which are commonly referred to

as stress raisers. Examples of this on

pump shafts are keyways, steps, shoulders,collars, threads, holes, snap ring grooves,

shaft damage or flaws that would

produce a stress raiser (wherever there is

a surface discontinuity a stress raiser will

exist; Figure 3). Corrosion can also create

stress raisers. For typical pumps, the two

most common problematic areas are at

the shoulder on the bearing or in the

coupling keyway region.

In the case of fatigue caused by axial

loads, the thrust bearing carrying the axial

load would most often show fatigue

(often contact fatigue) before the shaft.

However, there are numerous examples

where the shaft is damaged before thepump is stopped.

Keyways are commonly used to secure

rotating components, rotor cores and

couplings to the shaft. The keyway on the

take-off end or drive/driven end of the

shaft is the one of most concern because

Figure 2. A degraded pump after some years of operation.

"[For custom-engineered equipment]accurate modelling techniques and proper

analysis methods should be employed forall parts and components."


9/9

Feature 43

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phenomenon responsible for this fracture

was fatigue.

The clearance between the impeller and the

static part was relatively tight, which is often

seen in non-API small pumps. As a result of

operational forces (unbalance, liquid, thermaland dynamic forces), rotor rubbing occurred,

which caused extremely high bending

moments and stresses on the shaft. Proper

condition monitoring was not provided for

this small pump and the rubbing was unde-

tected. The main cause of fracture was the

rubbing of the rotor assembly. This then led

to an excessive fatigue cycle, resulting in

fatigue failure of the shaft.

enough to be able to feed vital lubrication

oil to the bearings at all times of emer-

gency (such as extreme conditions of high

vibration, extraordinary forces, etc.) to bring

the pump to a safe stop.

Motor fan

The second study relates to a catastrophic

failure in the electric motor driver of a

pump. An axial fan with 13 blades was

used as part of the electric motor cooling

system to dissipate the generated heat.

Fracture of the cooling fan blades occurred

just 260 hours after start-up. The fracture

caused a serious short-circuit incident

between the rotor and stator and conse-

quently an internal explosion in the motor

with extensive damage. An accurate finite

element analysis (FEA) modelling and thor-ough investigations showed that the first

natural frequency of the fan blade at ~659

Hz (accurately calculated after the failure)

was very close (almost within a 1.5%

margin) to the frequency of the exciting

force caused by the shaft rotation (13

blades 50 Hz = 650 Hz). This resonance

was the initial cause of the failure.

Further investigations indicated that when

the fan blades were excited by the 650 Hz

frequency, the blades vibrated intensely

and the stresses exceeded 300 MPa, whichcould be suffi cient for a fatigue failure. This

resonance and associated strong vibration

led to crack initiation/propagation, blade

failure, the short-circuit incident and the

electric motor internal explosion. The

explosion was contained, but the damage

to the electric motor was extensive and

costly. The lesson learned: a relatively large

(or medium-size) electric motor driver for a

pump will most probably be custom-engi-

neered equipment, and accurate modelling

techniques and proper analysis methodsshould be employed for all parts and

components.

Rotor failure

In this third case study, a rotor failure

occurred in a low-speed, small, manufac-

turer-standard pump after only approxi-

mately 2,000 hours of operation due to a

shaft fracture. The fracture was exactly at

a diameter-change section (a step) of the

shaft. The fracture surface was smooth

and perpendicular to the shaft axis.When the crack propagated to around

60% of the shaft diameter, the remaining

shaft section could not tolerate the

applied stress and a fast fracture

occurred. Experiments showed that the

Contact

Amin AlmasiLead rotating equipment engineer

WorleyParsons Services Pty Ltd

Level 10, 151 Roma Street (East Tower)

Brisbane, QLD 4000, Australia

Tel: +61 7 3319 3902

Email: [email protected]

Figure 3. An example of a pump shaft with obvious surface discontinuities (stress raisers).

Documents

Pump Failure Article