condition monitoring of fd-fan using vibration analysis

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 1, January 2013)

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Condition Monitoring of FD-FAN Using Vibration Analysis N. Dileep

1, K. Anusha

2, C. Satyaprathik

3, B. Kartheek

4, K. Ravikumar

5 (ASST.PROFFESOR)

1, 2, 3, 4 B.Tech Graduate, Mechanical, V. R. Siddhartha Engineering College,Vijayawada, Krishna (Dt), Andhrapradesh, India.

Abstract-- Machines of some kind are used in nearly every

aspect of our daily lives; from the vacuum cleaner and

washing machine we use at home, to the industrial machinery

used to manufacture nearly every product we use in our daily

life. When a machine fails or break down, the consequences

can range from annoyance to the financial disaster or

personal injury and possible loose of life. For this reason early

detection, identification and correction of machinery

problems is paramount to anyone involved in the maintenance

of industrial machinery to insure continued, safe and

productive operation. In order to run the machines efficiently

and to know the onset of impending defects condition and

monitoring of machines is important. There are several

indicating phenomenon like Vibration, noise, heat, debris in

oil, sound beyond human abilities etc., which emanate from

these inefficiently running machines. Monitoring of these

indicators provide early warnings of impending failures.

This paper is primarily focused on the implementation of

vibration based maintenance on critical rotating machines

namely FORCED DRAFT FAN (FD FAN 6B) at DR.NTTPS

which is one of the boilers auxiliary. FD fans for boilers force

ambient air into the boiler, typically through a preheater to

increase overall boiler efficiency. The required vibration

readings were taken. The levels of vibration of the fan driving

end (hub1) are beyond the safe limits of desired velocity and

displacement limit values. These further may cause to failure

of the fan. This paper also explains about the Spike energy

readings which were calculated and explained. After reading

are taken it was noticed that fan driving end bearing was

failed due to long life time of bearing, which is one of the

cause for the increase of vibrations. This problem was

rectified by replacing the new bearing and again vibration

readings are noted found to be in safe limits of velocity.

Keywords-- MDE, MNDE, FDE (HUB 1), FNDE (HUB 2),

Spectrums, Spike energy, Displacement, Velocity, Condition

Monitoring

I. INTRODUCTION TO MONITORING

Monitoring is the systematic collection and analysis and

information as a project progresses. It is aimed at

improving the efficiency and effectiveness of a project or

organization. It is based on targets set and activities

planned during the planning faces of work. It helps to keep

the work on track and can let management know when

things are going wrong. If done properly, it is an invaluable

tool for good maintenance, and it provides a useful base for

evaluation.

It enables you to determine whether the resources you

have available are sufficient and are being well used,

whether the capacity you have is sufficient and appropriate

and whether you are doing what you planned to do.

1.1 Maintenance strategies are classified by three

developmental stages:

1. Break down maintenance

2. Preventive maintenance

3. Predictive maintenance

1.1.1 Break Down Maintenance:

This provides the replacement of defective part or

machine after the machine becomes incapable of further

operation. Break down maintenance is the easiest method

to follow and it avoids the initial costs and training

personnel and other related upfront costs.

1.1.2 Draw backs of break down maintenance are:

1. Failures are untimely

2. Since machine is allowed to run till to failure repair

is more expensive. Sometimes total replacement is

required

3. Failures may be catastrophic. Hence loss will be

more.

4. Production loss will be more, as it requires more time

to restore normalcy.

5. It reduces the life span of the equipment

1.2.1 Preventive maintenance:

In preventive maintenance, maintenance is scheduled on

calendar or hours to run and is performed irrespective of

machine condition.

1.2.2 Advantages:

1. Down time of machine is reduced by 50-80%

2. Lower expenses of over pay may same as much as

30%

3. Increases the equipment life expectancy.

4. Reduces the maintenance cost by reducing the capital

spending by 10-20%, labour cost by 10%,

material cost by 30%

5. Improve the employee’s safety

6. Preventive maintenance results in a catastrophic

failure and down time is required to complete all

scheduled maintenance costs

7. Damage to machine is less



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1.2.2 Disadvantages:

1. Periodically dismantling of each and every critical

machine is expensive and time consuming.

2. It may lead to unnecessary inspections even on

healthy machine also which may further lead to more

complications.

3. It is difficult to predict time interval between

inspections which ultimately may lead to break down

maintenance.

4. Preventive maintenance alone cannot eliminate break

down. The causes of equipment failure change with

the passage of time. Fig 1 shows the failure rate

curve which is also called as bath tub curve. Failure

rate is taken on ordinate and time is taken on abscissa

when the equipment is new there is high failure rate

due to design and manufacturing errors. Failure rate

increases once again since the equipment approaches

the end of its failure.

FIG 1: BATH TUB CURVE

1.3.1 Predictive Maintenance:

Trending and analyzing machinery parameters we can

detect the developing problems in early stages. Hence

repair works can be carried out before failure of a machine.

1.3.2 Advantages:

1. Shut down can be done at convenient times

2. Work schedule can be prepared for mobilizing men ,

tools and replacement parts before shut down

reducing machinery down time

3. Identifying problem, costly trial and error procedures

to solve a problem can be avoided.

4. Machine in good running condition can run

continuously as long as problem develops


1. Require skilled labour

2. It is costly affair.

For all machines common characteristic is vibrations

and hence vibrations become a powerful tool in

implementing predictive maintenance program

The vibration predictive maintenance program has

four steps:

i. Detection

ii. Analysis

iii. Correction

iv. Conformation

i. Detection:

First select all available critical machines in the plant.

Prepare a schedule for all these machines for data

collection identify bearing locations of the machine train

motor non drive end, MND, FNDE, FDE, PNDE, PDE,

etc…identify the directions where vibration data is

collected like H, V, A etc. define which vibration

parameters are to be collected via displacement, velocity,

acceleration etc. after doing all these start collecting

vibrating data and related data and record them. Collect the

data for every fortnight or monthly or so by trending and

interpreting the data, identify source of vibration.

ii. Analysis:

After identifying the source of vibrations analyze to pin

point the root cause for vibrations. This can be achieved by

eliminating process. Follow confirmative procedures in

support of analysis.

iii. Correction:

Open and inspect he machine at a convenient time and

make necessary corrections

iv. Confirmation:

After corrections put the machine in service and again

collect vibration data and look for elimination of source.

II. CONDITION MONITORING

Condition monitoring is the process of monitoring a

parameter of condition in machinery, such that a significant

change is indicative of a developing failure. The most

efficient way of doing predictive maintenance is by

condition monitoring technique. Predictive maintenance by

condition monitoring technique will boost up the

availability of the equipment; will increase the efficiency

and industrial safety.

The various steps involved in condition monitoring

program are:

1. Plant survey feasibility report.

2. Machine selection strategic and economic importance.



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3. Select optimum monitoring techniques there is a large

number of parameters that can be collected and analyzed in

order to determine machine condition. No single parameter

has given consistent results.

4. Establish a predictive maintenance programmed-

inspection schedule, data handling, administration and

training.

5. Set acceptable condition, data and lists based on machine

severity charts, manufacturer’s specializations and

experience.

6. Machine base line measurements are taken after many

corrective actions.

7-10. Routine monitoring programmed – the object of this

is to detect significant deterioration in machine condition

through trend analyzed of the measured data.

11. Condition analysis – is in the death analysis of machine

condition often involving the joint application of number of

techniques. The object of this is to confirm the existing

fault, location and the corrective action required.

12. Fault correction – having diagnosed the fault it is

required to schedule the corrective action. The details of

the identified faults should feedback to the diagnosis and

improve the diagnostics capabilities of the program.

CONDITION MONITORING TECHNIQUES:

Vibration monitoring

Debris analysis lubes oil analysis

Corrosion monitoring

Thermography

Visual monitoring

Contaminant monitoring

Performance and behavior monitoring

This paper is mainly focused on vibration monitoring

which is the most commonly used method for rotating

machines

2.1. Vibration Monitoring:

Vibration monitoring is well established method for

determining the physical movement of the machine or

structure. Vibration is the best indicator of overall

mechanical condition and the earliest indicator of the

developing defects. There are other indicators like

temperature, pressure and flow and oil analysis. If only one

indicator is to be used to monitor machine health then

vibration is usually the best choice.

All rotating and reciprocating machines vibrate either to

a smaller or to a greater extent. Machines vibrate because

of defects or in accuracies in the system. When the

inaccuracies are more it results in increased vibration.

Each kind of defect produces vibration, characterized in

a unique way. Therefore, recording vibration level of a

machine indicates the condition of the machine.

2.1.1 Advantages of condition monitoring over planned

maintenance:

Improved system reliability

Decreased maintenance cost

Decreased number of maintenance operations causes

decreasing of human error influence


High installation costs, for minor equipment items

more than value of equipment

Unpredictable maintenance periods are causing

costs to be divide un equally

Increased number of parts(CBM installation ) that

need maintenance and checking

III. VIBRATION ANALYSIS

Vibration analysis is a non-destructive technique

which helps early detection of machine problems by

measuring vibration.

Vibration analysis has been proven to be the most

successful predictive tool when used on rotating

equipment, both in increasing equipment availability and

reliability. In order to maximize the finite life

associated with rolling element bearings and optimize

equipment production life, excessive wear caused by

misalignment, unbalance, and resonance must be

minimized. The presence of trained vibration specialists

with equipment to conduct analysis will form the basis of a

strong vibration program.

3.1 Causes for vibrations:

1. Change in direction with time, such as a force

generated by a rotating unbalance.

2. Change in amplitude with time, such as unbalanced

magnetic forces generated in an induction motor due

to unequal air gap between the motor armature and

stator.

3. Result in friction between rotating and stationary

machine components is much.

4. Cause impacts, such as gear tooth contact or the

impacts generated by the rolling elements of a bearing

passing over a flaw in the bearing raceways.

5. Cause randomly generated forces such as flow

turbulence in the fluid handling Devices such as fans,

blowers and pumps; or combustion turbulence in gas

turbines or boilers.



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3.2 Machine vibration:

A vibrating object moves to and fro, back and forth

motion. We experience many

An example in our daily life like vehicles driven on

rough terrain vibrates.

There are various ways we can tell that something is

vibrating. We can touch a vibrating object and feel the

vibration. We may also see the back and forth

movement of a vibrating objects.

Fig 2: Machine Vibrations

3.3 Sources of vibration:

1. Misalignment of couplings, bearings and gears.

2. Unbalance of rotating components.

3. Looseness

4. Deterioration of rolling element bearings

5. Gear wear

6. Eccentricity of rotating components such as "v "belt

pulleys or gears.

3.4. Detection by Vibration Analysis:

1 Unbalance(Static, Couple, Quasi-Static),

2 Misalignment(Angular, Parallel, Combination)

3 Eccentric Rotor, Bent Shaft

4 Mechanical Looseness, Structural Weakness, Soft

Foot

5 Resonance, Beat Vibration

6 Mechanical Rubbing

7 Problems Of Belt Driven Machines

8 Journal Bearing Defects

9 Antifriction Bearing Defects (Inner race, Outer race,

Cage, Rolling Elements)

10 Problems of Hydrodynamic & Aerodynamic

Machines (Blade or Vane, Flow turbulence,

Cavitations)

11 Gear Problems (Tooth wear, Tooth load, Gear

eccentricity, Backlash, Gear misalignment, Cracked

or Broken Tooth)

12 Electrical Problems of AC & DC Motor (Variable

Air Gap, Rotor bar Defect, Problems of SCRs)

3.5 Methods to detect causes of vibration:

There are literally hundreds of specific mechanical and

operational problems that can result in excessive machinery

vibration. However, since each type of problem generates

vibration in a unique way, a thorough study of the resultant

vibration characteristics can go a long way in reducing the

number of possibilities hopefully to a single cause. A

simple, logical and systematic approach that has been

proven successfully in pinpointing the vast majority of the

most common day-to-day machinery problems.

Interpreting the Data:

Obtain horizontal, vertical and axial spectrums at each

bearing of the machine train in order to take the readings.

Once horizontal, vertical and axial FFTs have been

obtained for each bearing of the machine train, the obvious

next question is: "What is this data telling me?" Essentially,

amplitude-versus-frequency spectrums serve two very

important purposes in vibration analysis:

1. Identify the machine component (motor, pump, gear

box, etc.) of the machine train that has the problem And

2. Reduce the number of possible problems from several

hundred to only a limited few.

Identifying the Problem Component Based On Frequency:

Figure3 shows a fan operating at 2200 RPM, belt driven

by an 1800 RPM motor. The rotating speed of the belts is

500 RPM. Assume that a vibration analysis was performed

on this machine and the only significant vibration detected

had a frequency of 2200 CPM or 1 x RPM of the fan. Since

the vibration frequency is exactly related to fan speed, this

clearly indicates that the fan is the component with the

problem. This simple fact eliminates the drive motor, belts

and possible background sources as possible causes.

Most problems generate vibration with frequencies that

are exactly related to the rotating speed of trip in trouble.

These frequencies may be exactly 1 x RPM or multiples

(harmonics) of 1 x RPM such as 2x, 3x, 4x, etc. In addition,

some problem's may cause vibration frequencies that are

exact sub harmonics of 1 x RPM such as 1/2x, 1/3x or 1/4 x

RPM. In any event, the FFT analysis data can identify the

machine component with the problem based on the direct

relationship between the measured vibration frequency and

the rotating speed of the various machine elements.



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Fig 3: Components generate different vibration frequencies

Identifying the Problem Component Based On Amplitude:

Identifying the fan as the source of vibration based on

vibration frequency was quite easy in the above example

because of the notable differences in the rotating speeds of

the various machine components. The obvious question, of

course is: What about direct-coupled machines that is

operating at exactly the same speed?" In this case, the

component with the problem is normally identified as the

one with the highest amplitude. For example, consider a

motor direct coupled to a pump. Examining the analysis

data, it is noted that the highest vibration amplitude on

the motor is 1.0 in/sec compared to 0.12 in/sec on the

pump. In this case, the motor is clearly the problem

component since its vibration amplitude is nearly 8

times higher than that measured on the pump.

In general, the machine component that has the

problem is usually the one with the highest amplitude

of vibration. The forces that cause vibration tend to

dissipate in strength at increased distances from the

source.

However, there are exceptions to this rule such as the

example given earlier where a vertical pump was vibrating

excessively due to a resonance problem with the discharge

piping. In this case, the exciting force was actually

generated by the motor/pump but was being amplified by

the resonant condition of the piping.

Another exception to this rule involves misalignment of

direct coupled machines. Sir Isaac Newton's third law of

physics slates that "whenever one body exerts a force on

another, the second always exerts on the first a force which

is equal in magnitude but oppositely directed." In other

words, "for every action, there is an equal but opposite

reaction." In the case of coupling misalignment, the

vibratory force (action) is generated at the coupling

between the driver a driven components. As a result, the

"reaction" forces on the driver and driven unit; will be

essentially equal, resulting in reasonably comparable

vibration amplitudes. The only reason one component may

have a slightly higher or lower amplitude than the other is

because of differences in the mass and stiffness

characteristics of the two components.

The following chart lists the most common vibration

frequencies is they relate to machine rotating speed (RPM),

along with the common causes for each frequency. To

illustrate how to use the chart, assume that the belt-driven

fan pictured in Figure3 has excessive vibration at 2200

CPM which is 1 x RPM of the fan. Of course, this clearly

indicates that the fan is the component with the problem

and not the drive motor or belts. In addition, since the

vibration frequency is 1 x RPM of the fan, the possible

causes listed on the chart are:



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TABLE 1

Vibration Frequencies and the Likely Causes

Comparing Horizontal and Vertical Readings:

When comparing the horizontal and vertical data, it is

important to take note of how and where the machine is

mounted and also, how the bearings are mounted to the

machine. Basically, the vibration analyst needs to develop a

"feel" for the relative stiffness between the horizontal and

vertical directions in order to see whether the comparative

horizontal and vertical readings indicate a normal or

abnormal situation. Machines mounted on a solid or rigid

base may be evaluated differently than machines mounted

on elevated structures or resilient vibration isolators such as

rubber pads or springs.

Comparing Radial (Horizontal & Vertical) Data to Axial

Data:

The second important comparison that needs to be made

to tri-axial analysis data is how the radial (horizontal and

vertical) readings compare to the axial readings.

Relatively high amplitudes of axial vibration are

normally the result of

1. Misalignment of couplings

2. Misalignment of bearings

3. Misalignment of pulleys or sheaves on belt drives

4. Bent shafts

Unbalance of "overhung" rotors.

IV. SPIKE ENERGY

When flaws or defects appear in a bearing, the resulting

vibration will appear as a series of short duration spikes or

pulses such .The duration or "period" of each pulse

generated by an impact depends on the physical size of the

flaw; the smaller the flaw, the shorter the pulse period will

be. As the size of the defect increases, the period of the

pulse becomes longer.

Frequency

in Terms Of RPM

Most Likely causes

Other possible causes & Remarks

lx RPM

Unbalance

I) Eccentric journals, gears or pulleys

2) Misalignment or bent shaft if high axial

vibration

2 x RPM

Mechanical looseness

1) Misalignment if high axial vibration

2) Reciprocating force

3 x RPM

Misalignment

Usually a combination of misalignment and

excessive

Axial clearance (looseness).

Less than

lx RPM

Oil Whirl(Less than1/2 x RPM)

I) Bad drive belts

2) Background vibration

3) Sub-harmonic resonance

4) "Seat" Vibration

Synchronous (A.0 line frequency)

Electrical Problems

Common electrical problems include broken

rotor bars,

eccentric rotor, and unbalanced phases in poly-

phase

Systems, unequal air gap.

2xSynch.Frequency Torque Pulses Rare as a problem unless resonance is excited

Many Times RPM

(Harmonically

Related Freq.)

Bad Gears, Aerodynamic Forces,

Hydraulic forces, Mechanical Looseness,

Reciprocating Forces

Gear teeth times RPM of bad gear

Number of fan blade times RPM

Number of impeller vane times RPM

May occur at 2, 3, 4 and sometimes higher

harmonics

High Frequency

(Not Harmonically

Related)

Bad

Anti-Friction bearing

1) Capitation, recirculation and flow turbulence

causes

random high frequency vibration

2)Improper lubrication of journal bearings

3)rubbing



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A short-term (40 millisecond sec) time waveform that

was taken on a ball bearing with a small nick purposefully

ground on the bearing inner race way. It can be seen that

the pulse period lasts only a few microseconds (1

microsecond = 1 millionth of a second). Of course, if the

period of a vibration signals is-known, the frequency of the

vibration can be found by simply taking the inverse of the

period. For example, if it takes 1/3600 minute to complete

one cycle of a vibration, then the vibration frequency is

3600 cycles per minute (CPM) or the inverse of the period.

In the case of the pulses generated by the bearing

defects, since the pulse periods are so short, the period

inverses (frequencies) are typically very high. To illustrate,

a MICRO-FLAW is generally defined as a defect that is so

small that it is essentially invisible to the naked eye. The

pulses generated by a micro-flaw are typically less than 10

micro-seconds (i.e. 10 millionths of a second). By taking

the inverse of a 10 micro-second pulse, the fundamental

frequency becomes 100,000 Hz (TOOK Hz) or 6,000,000

CPM. As bearing deterioration progresses, the flaw gets

larger. The next stage is a MACRO-FLAW or one that is

detectable with the naked eye. Since the macro-flaw is

larger, the duration or period of the pulse generated is

longer and, thus, the fundamental pulse frequency is lower.

Typically, a macro-flaw will generate a pulse with a period

exceeding 20 microseconds, resulting in a fundamental

pulse frequency of 50K Hz (3,000,000 CPM) or less. Of

course, as the bearing defects continue to increase in size,

the resultant pulse periods become even longer resulting in

a decrease in fundamental pulse frequency.

Experimentation has revealed that by the time the

fundamental pulse frequency has reduced to approximately

5k Hz (300,000 CPM), bearing deterioration has generally

reached severe levels.

With the above facts in mind, the following outlines the

basic features of the SPIKE ENERGY (abbreviated gSE).

Since the frequencies of bearing vibration are very

high, utilize a vibration acceleration signal from an

accelerometer transducer.

Incorporate a "band-pass" frequency filter that will

electronically filter out frequencies above 50K Hz

(3,000,000-GPM) -and below 5K Hz (300,000 CPM).

By eliminating frequencies above 50K HI, micro-

flaws, defects that are undetectable with the naked eye,

will not affect the measurement. In other words, when

the SPIKE ENERGY (gSE) measurements reveal a

significant increase, a visual inspection of the bearing

should provide confirmation with a visible flaw.

Since the spike-pulse signals generated by bearing

defects have very low RMS values, incorporate a true

peak-to-peak detecting circuit instead of an RMS

detecting circuit.

V. DATA PAC 1500

Instrument: Data Pac 1500

Company: ENTEK IRD

Feature: portable data collector / analyzer in a small

lightweight package.

Data PAC 1500 is part of Entek's complete range of

monitoring products and services to all industry segments

worldwide. The data PAC 1500 is a fully featured portable

data collector.

Figure 4: DATA PAC 1500

Supported Measurements:

Acceleration

Velocity

Displacement

G Spike Energy (GSE)

Temperature

Thrust or axial position

DC voltage

VI. BEARING FREQUENCIES

The bearing frequencies can be calculated based on 4

typical components of bearings.

1. BALL PASS FERQUENCY OUTER RACE

(BPFO)

2. BALL PASS FREQUENCY INNER RACE(BPFI)

3. BALL SPINNING FREQUENCY(BSF)

4. FUNDEMENTAL TRAIN FREQUENCY (FTF)



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6.1 FORMULAS FOR BEARING FREQUENCIES:

BPFO: (nb/2 - 1.2) rpm

BPFI: (nb/2 + 1.2) rpm

BSF: 1/2(nb/2 -1.2/nb) rpm

FTF: (1/2 – 1.2/nb) rpm

Where nb = no. of balls

Fig5 Rolling bearing element components.

Understanding Bearing Vibration

Example 1:

If you have the defect multipliers at your disposal, the

process of calculating the defect frequency is as follows:

1. Look up the bearing number that is exhibiting the

suspect vibration frequency on a table like the one below:

BEARING

ID

NO.OF

ROLLING

ELEMENTS

FTF

BSF

BPFO

BPFI

9436 19 0.434 3.648 8.247 10.753

9437 19 0.434 3.648 8.247 10.753

9442 22 0.443 4.191 9.740 12.260

2. Multiply this number by the shaft speed mated with

this bearing and you have the defect frequency that would

be generated by a defect on the element in question. See

Figure 7.4 9.740 x 351 rpm shaft speed = 3419 cpm

Outer race bearing defect

If the frequency and harmonics (multiples) of it are

present on the vibration spectra, you most probably have an

outer race bearing defect. It could be a spall on the

raceway, electrical fluting, false brinelling acquired during

bearing storage or equipment transport, etc.

Be advised that there will be occasions when the

calculated defect frequencies don’t exactly match the

bearing defect frequencies that appear in the vibration

spectra. Typically this is due to higher than normal thrust

loads which cause the bearing to run at a different contact

angle. These abnormal thrust loads can be caused by

sources such as mis-alignment.

Also, not all bearing manufacturers use the same number

of rolling elements in a particular bearing size.

The most common bearing problem is the outer race defect

in the load zone; inner race faults are the next most

common. It is very rare to see a fault at the bearings ball

spin frequency or BsF.

Action step: The presence of any of these four fundamental

fault frequencies should result in the repair technician

replacing the bearing and ensuring the housing fits and

shaft journals are within tolerance. Finally, it’s worth

discussing the presence of mechanical looseness, which

manifests itself as harmonics of 1x running speed, on a new

or re- built bearing housing or journal. This indication of

looseness could be coming from poor base mounting or one

of the following:

1. Loose housing-to-outer race fits

2. Loose journal-to-inner race fits

3. Excessive internal bearing clearance

Sleeve Bearing Defects:

Sleeve bearings do not make use of rolling elements;

rather, the shaft rides on a layer of lubricating oil inside the

bearing bore. The lubricant is either sealed inside the

bearing, gravity fed to the bearing or pumped in (pressure

fed). Sleeve bearings which have excessive wear/ clearance

exhibit a vibration spectrum similar to the one in Figure

7.5. Notice the series of running speed harmonics (up to 10

or 20). Wiped sleeve bearings often show much higher

vertical amplitudes than horizontal. A higher axial reading

on one end than the other provides further indication, with

the higher vibration level on the end with the damaged

bearing.



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Figure6: Bad bearing fits between housing &outer race.

Looseness from wear nd clearence problem

Clearance Problems In contrast, mechanical looseness

caused by loose mounting bolts or cracks in the frame

structure or bearing pedestal typically look like the

spectrum in Figure 7

Fig7: Loose mounting (Structural Looseness)

Excessive looseness can also cause sub harmonic

multiples at exactly 1/2 or 1/3 x rpm (.5x, 1.5x, 2.5x, etc.).

During bearing failure the harmonics are matched with

taking higher resolution of the spectrum, if it is found to

match with the spectrum it is said to be failure of bearing.

If the bearing failure occurs there will be little change in

the velocity values, no changes may occur in displacement

values but the spike energy value will be increasing.

During normal cases the spike energy value should be less

than one (gSE <1)

If the frequency (cpm) values are less than 600

displacement values are taken into criteria

If the frequency (cpm) values are in between 600-60000

velocity values are taken into criteria

If the frequency (cpm) values are >60000 spike energy

values come into criteria.

VII. SPECIFICATION OF FD FAN

FD-MOTOR:

SPEED: 993 RPM

VOLTS: 3300V

CURRENT: 229amps

TYPE: induction motor.

FREQUENCY: 50HZ

PHASE: 3PHASE.

FD-FAN:

TYPE: axial reaction

MEDIUM HANDLED: fresh air

SPPED: 1480 rpm

NO.OF BLADES: 23

BEARING NO: NU226E

HUB DIA: 1100mm

OUTERDIA: 1800mm

Displacement readings taken on 17 oct 2012.

Table 2

vibration data sheet of FD fan 6B before rectification

S.No

Position

Velocity

(mm/s)

Displacement

(µm)

H V A H V A

1 MNDE 8.71 0.751 1.75 108 363 14.8

2 MDE 9.93 1.35 1.66 111 8.34 14.6

3 HUB1 8.83 1.8 5.14 108 20.5 14.5

4 HUB2 9.24 1.76 5.41 114 20.8 16.2

Where,

MNDE: motor non drive end

MDE: motor drive end.

Hub1: fan drive end

Hub 2: fan non drive end

H: horizontal

V: vertical

A: axial

Limits as per ISO 10816 part2 (mm/sec peak)

Good : 0 to 6.0 mm/sec

Satisfactory: 6.0to 12.3mm/sec

Alarm : 12.3 to 16.0 mm/sec

Not Permitted : >16.0mm/sec

VIII. THE FOLLOWING ARE THE SPECTRUMS TAKEN TO

OBSERVE THE CAUSE OF VIBRATIONS

8.1 Spectrums

Velocity spectrum MNDE horizontal direction



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Velocity spectrum MNDE vertical direction

Velocity spectrum MNDE axial direction

Velocity spectrum MDE horizontal direction

Velocity spectrum MDE vertical direction

Velocity spectrum MDE axial direction

Velocity spectrum FDE (HUB 1) horizontal direction

Velocity spectrum FDE (HUB 1) vertical direction

Velocity spectrum FDE (HUB 1) axial direction

Velocity spectrum FNDE (HUB 2) horizontal direction

Velocity spectrum FNDE (HUB 2) vertical direction



180

Velocity spectrum FNDE (HUB 2) axial direction

8.2 Observations from the spectrums

1) Overall vibrations of all the bearings are in in

satisfactory zone as per ISO 10816.

2) Horizontal direction vibration at MNDE, MDE.HUB1,

and HUB2 are high side.

3) 1X Frequency is dominant in spectrums in all positions

4) Observed ½ Blade pass frequency and 1XBlade pass

frequency and its side harmonics are present

in HUB1 and HUB2 in axial direction

5) High vibrations in Horizontal direction at MNDE, MDE,

HUB1 and HUB2 due to any flow obstruction in fan blades

or may be unbalance in fan rotor

Remarks:-

1) Check for partial close/open position of dampers in connected duct.

2) check for impeller

Suggestion:

1) Check for bearing vibration and temperatures regularly until

correction.

IX. TABLE

Table3

Spike energy readings before rectification:

S.NO Date Position Direction Reading

1

17/10/12

MNDE HORIZONTAL 5.21

2 MDE HORIZONTAL 0.0339

3 HUB-1 HORIZANTAL 0.067

4 HUB-2 HORIZANTAL 0.033 RPM: 1487

Normal value: <0.8 at 1500 rpm

SAFE LIMITS OF spike energy (gSE):

3600 rpm: 1.4 gSE

1900 rpm: 0.70 gSE

1200 rpm: 0.50 gSE

900 rpm: 0.35 gSE

600 rpm: 0.25 gSE

X. ACTION TAKEN BASED ON REPORT

1. Since each unit runs with 2 FD fans, FD fan-6B is

stopped for one day and the failure bearing was

removed.

2. Maintenance was taken up and the FD fan was

replaced with new bearings.

3. After replacement again the vibrations data was

collected.

XI. VIBRATION READINGS AFTER RECTIFICATION

STAGE VI Load: 210MW Freq: 49.98Hz

11.1 vibration reading after changing of bearings:

Table 4

vibration data sheet of FD fan 6B after rectification

S. No Date Position Velocity (mm/sec) Displacement(µm)

19/10/2012

H V A H V A

1 MNDE 1.1 0.404 0.421 11.5 1.8 2.29

2 MDE 1.18 0.445 0.411 12.5 1.36 2.29

3 FDE(HUB1) 1.36 0.205 0.552 17.1 2.16 2.35

4 FNDE(HUB2) 1.37 0.228 1.19 17.6 2.67 2.94

11.2 Observations:

1. The values of velocity were reduced from the

satisfactory zone to good zone.

2. The high side displacement values are reduced into

desired limits.



Satisfactory : 6.0to 12.3mm/sec





181

11.3 spectrums after rectification:

Velocity spectrum of MNDE in horizontal direction

Velocity spectrum of MNDE in vertical direction

Velocity spectrum of MNDE in axial direction

Velocity spectrum of MDE in horizontal direction

Velocity spectrum of MDE in vertical direction

Velocity spectrum of MDE in axial direction

Velocity spectrum of MNDE in vertical direction

Velocity spectrum of FDE in horizontal direction

Velocity spectrum of FDE in vertical direction



182

Velocity spectrum of FDE in axial direction

Velocity spectrum of FNDE in horizontal direction

Velocity spectrum of FNDE in vertical direction

Velocity spectrum of FNDE (HUB 2) in axial direction

11.4 Spike energy readings:

RPM: 1487

Table 5

spike energy readings after rectification.(19/10/12)

S.NO Position Direction Reading

1 MNDE HORIZONTAL 0.032




Normal value: <0.8 at 1500 rpm


3600 rpm: 1.4 gSE

1900 rpm: 0.70 gSE

1200 rpm: 0.50 gSE

900 rpm: 0.35 gSE

600 rpm: 0.25 gSE

11.5 Observations after rectification:

1. Vibration frequencies are in normal ranges.

2. Even after the rectification the peaks are obtained at

MNDE, MDE, FDE, and FNDE in axial direction

which indicate high axial vibrations are existed.

3. The vibrations caused in axial direction indicate bent

shaft or angular misalignment

4. The rectification of bent shaft or angular misalignment

can be done only during overall shut down period of

the unit.

5. Hence the bearing replacement is carried out which

lead to reduction of vibrations and allowing smooth

running of FD fan.



183

XII. RESULTS

The results that were obtained after rectification of FD fan are as follows:

12.1 Velocity displacement data

Data before Rectification:

S.

No

Date Position Velocity (mm/sec) Displacement(µm)

17/10/2012

H V A H V A

1 MNDE 8.71 0.751 1.75 108 3.63 14.8

2 MDE 8.93 1.35 1.66 111 8.34 14.6

3 FDE(HUB1) 8.83 1.8 5.14 108 20.5 14.5

4 FNDE(HUB2) 9.24 1.76 5.41 114 20.8 16.2

Data After Rectification

S.

No

Date Position Velocity (mm/sec) Displacement(µm)

19/10/2012

H V A H V A

1 MNDE 1.1 0.404 0.421 11.5 1.8 2.29

2 MDE 1.18 0.445 0.411 12.5 1.36 2.29

3 FDE(HUB1) 1.36 0.205 0.552 17.1 2.16 2.35

4 FNDE(HUB2) 1.37 0.228 1.19 17.6 2.67 2.94



Satisfactory : 6.0to 12.3mm/sec



By observing the above tabular forms higher values of velocity and displacement are obtained indicating a flaw in the

machine, which were rectified based on several considerations which were further discussed.

12. 2 Spike energy data

Spike energy data collected before rectification:

S.N

O

Date Position Direction Reading

1

17/10/12




4 HUB-2 HORIZANTL 0.033

Spike energy data collected after rectification:

S.NO Date Position Direction Reading

1

19/10/12





RPM 1487.


3600 rpm: 1.4 gSE

1900 rpm: 0.70 gSE

1200 rpm: 0.50 gSE

900 rpm: 0.35 gSE

600 rpm: 0.25 gSE

The spike energy data is taken into criteria because of

frequency obtaining >600000 rpm. The equipment used in

V.T.P.S is set only to horizontal direction, where the

readings are obtained.

From the above data flaw is found out at MNDE, and

still the spectrum criteria is taken into considerations in

order to find out the existing defects.



184

12.3 Spectrum data

In Vertical Direction at MNDE

Before Rectification

After Rectification

In vertical direction at MDE:


After Rectification

Even after the replacement of the failed bearing with

new bearing there are still some irregular harmonics

observed in axial direction at MDE, FDE (HUB 1) and

FNDE (HUB 2). The irregular harmonics in axial

directions are shown in the following spectrums.

In axial direction at MDE:


After Rectification

In axial direction at FDE (HUB-1):


After Rectification



185

In axial direction at FNDE (HUB-2)


After Rectification

Harmonics Observed In Axial Direction Indicates –

Bending Of Shaft

XIII. CONCLUSION

From the above observed graphs the irregular harmonics

are observed in the axial direction, which indicates that the

bending of the shaft caused due to bearing failure. After

replacement of bearing, the previously occurred irregular

harmonics were vanished in horizontal and vertical

direction.

The replacement of bearing requires a temporary pause

of FD-FAN (about a day). But the replacement of the shaft

requires the overall shut down of the unit. So instant

replacement of shaft is not possible as of bearing. So due

this factor the fan is allowed to run with irregular

harmonics in axial direction which doesn’t affect majorly

for a period of time, and can be rectified during overall

maintenance.

REFERENCES

[1 ] ISSN:0975-5462 International Journal of Engineering Science and

Technology

[2 ] http://electromotores.com/PDF/InfoT%C3%A9cnica/EASA/Understanding%20Bearing%20Vibration%20Frequencies.pdf

[3 ] http://www.bsahome.org/Archive/html/escreports/VibrationAnalysis.pdf

[4 ] R.bandal,”state of art in monitoring and rotating

machines”ISMA2002, International conference on noise & vibration engineering.

[5 ] R.k Biswas , December 2006 “vibration based condition monitoring on rotating machines”

[6 ] A.v Barkov, N.A.Barkov & Yu.Azovtsev, in 1997, “vibration based condition monitoring on rotating machines.

[7 ] DR.NTTPS vibration analysis material

[8 ] http://www.vibrotek.com/ref.htm

[9 ] Mechanical vibrations-G.K.Groover

[10 ] Tanver(ref8), P.J.School of engineering, Durham university, gave

areview on CMBS on rotating electrical machines

[11 ] Jones.N.B Yu-HuaLI (ref 1). Department of engineering, university of Leicester, Leisister, UK had presented a paper on reciprocating

compressor condition monitoring.

[12 ] http://en.wikipedia.org/wiki/Condition_monitoring

[13 ] http://en.wikipedia.org/wiki/Rolling-element_bearing



186

AUTHOR BIOGRAPHY:

N. DILEEP is pursuing 4/4 B.Tech in

Department of Mechanical Engineering at

V.R.Siddhartha Engineering College,

vijayawada , India. Participated in various

technical paper presentations and secured 2nd

prize for “Green Buildings” paper

presentation.

K. ANUSHA is pursuing 4/4 B.Tech in



vijayawada , India. Participated in various

technical paper presentations and secured 1st

prize for “OTEC” paper presentation.

C.SATYA PRATHIK is pursuing 4/4 B.Tech

in Department of Mechanical Engineering at

V.R.Siddhartha Engineering College, vijayawada , India.

B.KARTHEEK is pursuing 4/4 B.Tech in



vijayawada, India.

Sri K. RAVI KUMAR received B.Tech from

V.R. Siddhartha Engineering College,

Kanuru, Vijayawada. He has received

M.Tech from JNTU-A, ANANTPUR,India.

He is currently working as an Asst. Professor

in the Dept of MECHANICAL, V.R.

Siddhartha Engineering College, Kanuru, Vijayawada, India. He

has 5 years of teaching experience.

condition monitoring of fd-fan using vibration analysis

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