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BITS PilaniPilani Campus

Anil JindalDepartment of Mechanical Engineering

BITS Pilani

MAINTENANCE & SAFETY

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Condition-BasedMaintenance(CBM)and

ReliabilityCentered Maintenance (RCM)

Lecture 5

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Effectiveness improvement through

condition monitoring

BITS Pilani, Pilani Campus

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Visual

Temperature

Vibration

Lubricant monitoring

Leakage monitoring Cracks monitoring

Thickness monitoring

Corrosion monitoring

Noise / Sound monitoring Smell / Odour monitoring


Condition Monitoring Techniques

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Alignment of shafts


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Alignment of shafts


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Alignment of shafts


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Evaluation of electric motors and other electrical equipment

is critical to a total plant predictive maintenance program.

To an extent, vibration data isolate some of the

mechanical and electrical problems that can develop in

critical drive motors. However, vibration cannot providethe comprehensive coverage required to achieve


optimum plant performance. Therefore, a total

predictive maintenance program must include

plant

data

acquisition and evaluation methods that are specificallyand otherdesigned to identify problems within motors

electrical equipment.

ELECTRIC MOTOR ANALYSIS

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1. Insulation Resistance

Normally these tests are conducted using (1) megger, (2)

Wheatstone bridge, (3) Kelvin double bridge, or (4) anumber of other instruments.

2. Other Electrical Testing

Total plant program should also include (1) dielectric lossanalysis, (2) gas-in-oil analysis, (3) stray field monitoring,

(4) high voltage, switchgear discharge testing, (5)

resistance measurements, (6) Rogowski coils, and (7)

rotor bar current harmonics.BITS Pilani, Pilani Campus

ELECTRIC MOTOR ANALYSIS

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Thickness Monitoring

Thickness monitoring is very effective and useful techniquefor assessing the thickness (and thus condition) of the

pipelines, pressure vessels, tanks, bottles, cylinders, radar

domes, aircraft wings and body panels etc.

Most thickness monitoring equipment work onultrasonic system.

A sound pulse, generated by a probe, travels through the

material, bounces-off the back surface of the material and

returns to the probe. By accurately measuring the time taken between the

transmission and reception of the pulse, the instrument

calculates the thickness using the velocity formula.

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Thickness Monitoring

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Crack Monitoring

Crack monitoring is more used for quality assurance and

metallographic analysis to assess the quality of metals and

quality of procedures during making, shaping and treating

of metals in industries.

Crack monitoring programmes measures not total crack

depth and width but change in crack width. This change in

crack width is called crack displacement. The crack

displacement measured by the sensors may be driven by

any combination of the factors listed below

Differential thermal expansion,

Structural and machine overloading,

Chemical changes in various components of machine,

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Crack Monitoring

Shrinkage arid twisting of different components

temperature and humidity changes etc.

Fatigue and aging of components, etc

Various techniques used for crack monitoring are-

Dye-penetrant Test,

Magnetic flux (Magna-flux),

Electric resistance, Eddy current,

Ultrasonic and Radiographic tests etc

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Corrosion Monitoring

The principles of corrosion monitoring equipment is based

on corrosion or chemical wear of the material. The use of

such techniques for condition monitoring of machines/

components is very limited and selective.

Few common corrosion monitoring techniques are

enumerated below

Weight Loss Method,

Electrical Resistance Method,

Linear Polarization Resistance (LPR) Method,

Galvanic or Zero Resistance Method,

Hydrogen Monitoring Method: etc.

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NOISE / SOUND (ACOUSTIC)MONITORING

Acoustic is a general word for noise and sound. Noise

and sound are basically the same except that noise is

considered as harsh, unpleasant and undesirable sound.

The human ear can detect frequencies between 20 Hz

and 20 kHz. This range is referred to as the audible, or

sonic, range. Frequencies above this range are referred to

as ultrasonic or ultrasound.

Noise monitoring is very important for controlling noise

pollution and environmental protection as noise affecthuman-being both ways, physically and psychologically

and prolonged exposure to high noise level can lead to

permanent hearing loss.

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NOISE / SOUND (ACOUSTIC)MONITORING

Noise monitoring can also be used, to some extent, to

monitor the health and condition of machines. For,

identifying the noise sources, following techniques may

be used

Subjective assessment,

Acoustic ducts (such as horn etc),

Surface intensity approach (using accelerometer on

vibrating surface and a microphone),

Acoustic intensity approach and sound-pressure

monitoring (using microphone devices),

Impulsive noise monitoring,

Infrasonic noise monitoring and microbarograph; etc.

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Radiography

Deep penetration for several inches and thickness of steel

and other metals.

According to strength of radiation, defects detected to quite

a depth.

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Ultrasonic Testing

Based on the strength of ultrasonic sound waves getting

reflected to the source.

Complementary to radiography or X-Ray.

Can investigate several inches depth in metals.

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Magnetic Particle Inspection

Used for shallow sub surface defects.

Used for materials which are magnetic, particularly for steel

welds.

Portable and cheaper technique.

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Hydrostatic Technique

Pressure testing of a system having boundaries prior to

operation.

Usually carried out with water as medium for a specified

period of time for testing leakage.

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Electro magnetic Induction

Coil surround the component and are magnetize which

induce current .

Used in testing thickness of sheets especially in tube

thickness.

Portable and can be used for detecting defects on thesurface and sub surface.

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Acoustic Emission Technique

Similar to ultrasonic testing, but basically used for detection of

crack growth through piezoelectric crystal placed on the

member to be inspected.

The electric current from the transducer is proportional to the

energy disseminated by crack development.

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Dye Penetration Inspection

It also called liquid penetrant inspection (LPI) or penetrant

testing (PT), is a widely applied and low-cost inspection

method used to locate surface-breaking defects in all non-

porous materials (metals, plastics, or ceramics).

The penetrant may be applied to all non-ferrous materialsand ferrous materials, but for inspection of ferrous

components magnetic-particle inspection is also preferred

for its subsurface detection capability.

LPI is used to detect casting, forging and welding surfacedefects such as cracks, suface porosities, and leaks in new

products, and fatigue cracks on in-service components.

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Dye Penetration Inspection

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Typical case study


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Reliability

Centered Maintenance(RCM)

Chapter 4:Part-1

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Reliability

Reliability provides the means to estimate the likelihood that asystem will achieve its mission in a given duration and operating

conditions.

Reliability refers to the consistency of a measure. A test is

considered reliable if we get the same result repeatedly. For example, if a test is designed to measure a trait, then each

time the test is administered to a subject, the results should be

approximately the same.

Unfortunately, it is impossible to calculate reliability exactly, but it

can be estimated in a number of different ways.

the probability that no (system) failure will occur in a given time

interval

A reliable system is one that meets the specifications.

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Various aspects ofreliability centered maintenance

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Practical steps towards achievingreliabilitycentered maintenance

Step 1: Educate from Top to Bottom on Reliability-Centered Maintenance

Shatter the old myths

Presentation to the staff the better way

Use multiple formats Make the employees understand the importance of

their benefit in following the new techniques

Planting lots of small seeds

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Step2: Benchmarking the Present position

Companies will realize that once they are bench-marked,

they will realize that how far they are behind. The realities

will provide the necessary attitude adjustment.

For safety, the International Standards Organization hasdefined to calculate lost time incident rate (LTIR) and

recordable incident rate (RIR).

It is understood that an RIR of 0.5 and LTIR of 0.05 are

considered to be high. Similar norms are not available forreliability centred maintenance

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Step 3: Establishing Long Term VisionOnce the benchmark is established, the next challenge is to define

where to proceed further.

The key to establish a vision is to begin with a goal in mind.

The metrics from benchmark are used to set specific, measurable

targets for the performance outcomes with in 3-5 years in future.

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Step4: Building up of a Business CaseThe reason to carry out RCM is to improve the net profit by

reducing the maintenance costs.

Some examples of improvement are given below:

A 5 percent increase in availability = 5 percent increase in revenue

for a continuous process plant that can sell all that it makes. For

example, a plant that produces Rs. 1,000 crore per year generates

another Rs. 50 crore in revenue.

Reducing overtime from 20 to 10 percent moves 10 percent of

labour from overtime rates to straight time rates. If the overtimemultiplier is 1.5 and a plant has a Rs. 10 crore labour budget

towards overtime Rs. 1 crore is saved.

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Step5: Conducting a Pilot ProgramIt may be necessary to conduct a pilot programme so as

to get the real feel of the benefits of following an

organized maintenance scheme.

The pilot serves the following critical functions:

Reduce initial investment.

Business case need approval

Test Lab.

Selecting a pilot project is critical, i.e. an operationallyimportant yet small-sized project which can be fulfilled

in 3-6 months.

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System ReliabilityReliability of the product (made up of a number of components) is

determined by the reliability of each component and also by the

configuration of the system consisting of these components

Product design, manufacture, maintenance influence reliability, but

design has a major role

One common approach for increasing the reliability of the system isthrough redundancy in design, which is usually achieved by placing

components in parallel.

As long as one component operates, the system operates

Systems with components in series

For the system to operate, each component must operateIt is assumed that the components operate independently of each

other (Failure

of one component has no influence on the failure of any other

component)A CB

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Systems with components in

series contd..

If there are n components in series, then system reliability is givenby Rs = R1 x R2 x - - - - - - Rn

System reliability decreases as the number of components in series

increases

Manufacturing capability and resource limitations restrict the

maximum reliability of any given component

Product redesign that reduces the no. of components in series is the

viable alternative

Use of the Exponential Model

If the system is in chance failure phase, a constant failure rate

could be justified based on which we can calculate failure rate,

mean time to failure and system reliability

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series contd..

The system reliability is given by

Thus if each component that fails is replaced immediately with

another that has the same failure rate, the mean time tofailure for the system is given by

Use of the Exponential Model Suppose the system has n components in series

Each component has exponentially distributed time-to-failurewith failure rates given by 1, 2 n

sR e 1tX e2tX e3tX en t

n

i e i 1

t

MTTF

i1

When all components have same failure rate, If

1n

i1

then nMTTF

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Fs 1R11R21Rn (1Ri)

System reliability can be improved by placing components in parallel as system will operate as

long as at least one of the components operates.

The only time the system fails is when all the parallel components fail

All components are assumed to operate simultaneously.

A system having n components in parallel, with the reliability of the ith

component denoted byRi, i=1, 2, ----- n.

Also assume that the components operate randomly and independently of each other.

The probability of failure of each component is given by Fi = 1-Ri.

System fails only if all the components fail and hence the probability of system failure is

System with components in parallel

i1

n

S i h i

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parallel contd..

Mean time to failure for a system of n components in parallel is given by

11 1

MTTF 1/1

If the time to failure of each component can be modelled by the exponential

distribution, each with a constant failure rate i, then the system reliability, assuming

independence of component operation is

Time to failure of the system is not exponentially distributed

In the special case, where all the components have the same failure rate

the system reliability is Rs = 1- (1-e- t)n

2 3 n

Reliability of the system is the complement of Fs andis given by Rs = 1-Fs

Use of Exponential model

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Reliability block diagrams

Once the reliability of the subsystems is determined, the overall system canbe effectively modeled from the reliability perspective. Once modeled, the

weak links usually become evident and can be addressed with reliability

growth measures to eliminate the deficiencies. Figure illustrates block-

diagrammed examples of simple serial, parallel and combination systems.

R li bilit hil ti d

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Reliability while active andstandby

R li bilit hil ti d

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R li bilit hil ti d

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R li bilit t d

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RCM is a process to optimize reliability and associatedmaintenance tactics with respect to operational

requirement.

Economic optimization of machine reliability with

organizational goal is the primary objective of RCM. RCM guides the reliability investment with improvement

measures and techniques including lubrication

management and analysis such that the economic

optimization is realized.

Reliability centeredmaintenance

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Reliability centered maintenance

Law of diminishing marginal

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The implementation of RCM follows the law of diminishingmarginal returns.

The money invested in reliability improvement tends to

yield a higher return on investment than any money

subsequently invested. The objective is to reach the point of optimization at which

the benefits of reliability expresses as total operating cost,

are maximized through cost reduction.

Law of diminishing marginalreturns

Law of diminishing marginal

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Law of diminishing marginalreturns

Plot between time & condition

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The warning in advance of a functional failure that amonitoring technique provides is called the P-F interval.

P refers to the time at which the potential failure occurs.

F refers to the time at which actual failure occurs.

Longer the P-F interval, more time one has to make agood decision and plan action.

Plot between time & condition

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An amplifier has an exponential time to failure distributionwith a failure rate of 8% per 1000h. What is the reliability

of the amplifier at 5000 h? Find the mean time to failure.

Example

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Solution:

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What is the highest failure rate for a product if it is to have aprobability of survival (that is, successful operation) of

95% at 4000h? Assume that the time to failure follows an

exponential distribution.

Example

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Solution:

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A module of a satellite monitoring system has 500components in series. The reliability of each component

IS 0.999. Find the reliability of the module. If the number

of components in series is reduced to 200. What is the

reliability of the module?

Example

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Solution:

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