ENGG ZC242-L4

BITS PilaniPilani Campus

Anil JindalDepartment of Mechanical Engineering

BITS Pilani

MAINTENANCE & SAFETY

BITS PilaniPilani Campus

Condition-Based Maintenance (CBM)Lecture 4

Chapter 3:Part-1

Introduction

Condition Monitoring means the use of advanced technologies in order to determine equipment condition,

and potentially predict failure.

CBM was introduced to try to maintain the correct equipment at the right time.

CBM is based on using real-time data to prioritize and optimize maintenance resources.

Observing the state of the system is known as condition monitoring.

Such a system will determine the equipment's health, and act only when maintenance is actually necessary.

Condition Based Maintenance

Developments in recent years have allowed extensiveinstrumentation of equipment, and together with better

tools for analyzing condition data.

Ideally condition-based maintenance will allow themaintenance personnel to do only the right things,

minimizing spare parts cost, system downtime and time

spent on maintenance.

It includes technologies such as: Vibration measurement and analysis Infrared thermograph Oil analysis and tribology Ultrasonic Motor current analysis Gas-leakage detection

Challenges

Despite its usefulness, there are several challenges to theuse of CBM. First and most important of all, the initial cost

of CBM is high.

It requires improved instrumentation of the equipment. Often the cost of sufficient instruments can be quite large,

especially on equipment that is already installed.

Therefore, it is important for the installer to decide theimportance of the investment before adding CBM to all

equipment.

Advantages

CBM has some advantages over planned

maintenance.

Improved system reliability

Decreased maintenance costs

Decreased number of maintenance operations causesdecreasing of human error influence

Advantaged

According to a 1988 multi-industry survey, the benefits of

CBM are:

Disadvantages

High installation costs, for minor equipmentitems more than value of equipment

Unpredictable maintenance periods are causing costs tobe divided unequally

Increased number of parts (CBM installation) that need maintenance and checking

Effective Improvements through

Condition Based Maintenance.

Improves the equipment reliability through effective prediction.

Minimizes downtime through the integrated planning and scheduling of repairs indicated by CBM techniques .

Maximizes component life by avoiding the conditions that reduce the equipment life.

Utilizes the condition monitoring technique to maximize the equipment performance and throughput.

Minimizes condition monitoring cost.

Effectiveness improvement through

condition monitoring

BITS Pilani, Pilani Campus

Visual

Smell

TemperatureVibration

Debris

Noise

Visual:


Crack, Leakage, Damage

Naked eye

Endoscope

Noise:

Abnormal noise, Cyclic spikes

Ear

Noise detectors

Vibration:

Bearing

Manual

Vibration sensors

Smell:

Burning

Manual

Temperature:

Running parts

Thermometer

Thermocouples

Pyrometer

Debris (in lube oil):

Wear particles

Microscope

11

Visual Temperature Vibration Lubricant monitoring Leakage monitoring Cracks monitoring Thickness monitoring Corrosion monitoring Noise / Sound monitoring Smell / Odour monitoring


Condition Monitoring Techniques

Vibration analysis

The most widely used technique in the present day condition monitoring is through vibration analysis.

Vibration analysis can forecast potential machinery problems and pinpoint their cause.

Provides periodic condition monitoring using instrumentation.

A database is developed to record performance, establish machine histories, assist maintenance diagnostics and

extend machinery reliability.

Problems detected include imbalance, misalignment, motor and electrical defects, gear train defects, and bearing

defects.

Vibration analysis is also used for verification of alignment between shafts.

Vibration analysis


Ultrasonic Evaluation

Periodic inspections of critical items of equipment should be carried out by using NDT (Non Destructive Testing).

It is essential to identify the critical areas where failures are likely to occur and select suitable NDE (Non Destructive

Evaluation) techniques for detection of such failures.

General applications of ultrasonic inspection are as follows: Hull plate thickness Piping Storage tanks Pressure vessels Heat exchangers

Motor condition monitor

Motor condition monitoring is a device capable of detecting impending mechanical and electrical failures in motors and

motor based systems at the early stage of fault

development.

The experimental modelling technology used in MCM can automate fault detection and prevent unplanned machine

failure.

MCM utilizes model based fault detection technique which compares the dynamic behavior of the motor with a

mathematical model.



There are three operational modes of MCM:

1. Check mode: In this mode the MCM measures the current

drawn by the motor and the applied voltage to the motor. It

computes voltage and current imbalances, hence determines

whether the measured and computed quantities are below

user specified thresholds. It also determines the ordering of

voltage and current channels and can give information about

the frequency content of voltage or current signals.

Motor Condition Monitoring

2. Learning mode: In this mode the MCM determines the

model of the motor. It repeatedly takes data while the motor is

running through its typical load range and processes this set

of data to obtain the model.

3. Testing mode: In this mode the MCM tests if the motor

behaves as per the model obtained in the learning mode.

Towards this end, the MCM intermittently takes data from the

motor, models the taken data and compares this model with

the model obtained in the learning mode. Furthermore, it also

compares the output data with the outputs predicted by the

model obtained in the learning mode.

Thermography-Principle

According to Planks law of blackbody radiation, the radiation energy is proportional to the absolute

Temperature of the body.

Hence by measuring the radiation energy, the temperature of the body can be known.

Electromagnetic spectrum

Thermography

Thermography was first utilized in the inspection of electrical equipment to locate high resistance connections

or faulty components.

It has also been used to areas of reduced insulation in buildings and leaks in building roofs.

It has been found to be effective in identifying faults in bearings, coupling and shorts in motors.

Infrared thermography is an effective means to identify excessive heat loss in system or materials.

Advantages of Thermography

Advantages of thermography are as follows:

A noncontact, nondestructive means of testing. Reduce downtime. Virtually eliminates unnecessary work. Reduce unnecessary material expenses. Avoid catastrophic failures. Direct maintenance to the root of the problem

Temperature Monitoring Methods

Temperature crayons and tapes

Thermometers

Optical pyrometers

Thermocouples

Fusible plugs

Thermography (Infrared radiation scanners)

Infrared Thermography

Every object radiates electromagnetic energy invisible to human eye but visible to specialized equipment.

Thermal imaging radiometers can detect and measureinfrared radiation, and produce a map of colors or varying

shades of gray that show the surface temperatures of the

object.

The lighter colors or shades represent the warmesttemperatures, and the darker colors or shades the

coolest.

Infrared Thermography

Unlike other infrared techniques, thermal or infraredimaging provides the means to scan the infrared emissions

of complete machines, process, or equipment in a very

short time. Most of the imaging systems function much like

a video camera.

The user can view the thermal emission profile of a widearea by simply looking through the instrument's optics.

Type of cameras in thermography

Cooled Type

Detectors are cryogenically cooled. Their sensitivity is much higher as their own temperature is

very low ( 4 to 110K) compared to object being measured.

But it is expensive to both produce and run.


Type of cameras in thermography


Un-cooled Type

It works on ambient temperature. So need of expensive and bulky cryogenic coolers.

This makes it small and less costly. Works by change in voltage or current when heated by

infrared radiations.

Thermography


Oil and lubricant analysis in CBM

Used oil analysis is comparable to a medical analysis with a blood test.

Like blood, lubricating oil contains a good deal of information about the envelope in which it circulates.

Wear of metallic parts, for example, produces a lot of minute particles, which are carried by the lubricant.

These small metal particles can give information about the machine elements that are wearing, and can be detected

by various methods.

Various types of wear are Adhesive wear Abrasive wear Fatigue Cavitation of the lubricant/fluids Corrosion Erosion Electrical sparks


Adhesive wear, Abrasive wear, Fatigue

Adhesive wear: Caused due to welding between soft and hard materials due to lack of lubricant. Subsequently, the

welded material breaks away causing wear and wear

particles.

Abrasive wear: Due to pressure and relative motion between two surfaces caused by inadequate lubrication/or

use of lubricant which is long overdue for change

Fatigue: Caused due to severe conditions of vibration, pressure and temperature variations. The severity should

be minimized to eliminate this problem.


Abrasive wear


Cavitations of the lubricant/fluids

It is caused due to the presence of air or gases in the

lubricants.


Corrosion

This is caused by galvanic or chemical reaction between the mating surfaces.

The effect can be minimized by the use of proper materials and additives in lubricants, which will inhibit the chemical

reaction.


Erosion, Electrical sparks

Erosion: Erosion occurs due to impingement of high velocity particulates upon the surface of the equipment. For

example, erosion of automobile silencer takes place due to

carbon particles of the exhaust gas.

Electrical sparks: This is caused in electric motors. Proper insulation and overload protection are some of the

solutions for this problem.


Erosion


1. Spectrometry

2. Viscosity

3. Dilution

4. Water detection

5. Acid number

6. Total base number

7. Particle counting

8. Microscopic examination in a laboratory


Methods of analysis of oil andlubricants

Spectrometry, Viscosity

A spectrometer is an instrument with which one can measure the quantities and types of metallic elements in a

sample of oil.

There are two types of viscosity: kinematic and dynamic (or absolute).

Oil analysis concerns itself almost exclusively with the former.

Kinematic viscosity is measured in centistokes (cSt) and is a measure of a fluid's resistance to flow or, more simply, its

thickness.

It must always be quoted at a stated temperature because a fluid's viscosity will change with temperature. At 40C, a 200 cSt oil is thicker than one, which has 100 cSt


Kinematic Viscosity


Dynamic Viscosity


Dilution, water detection

Dilution of used engine oil can be measured precisely by gas chromatography (GC) or by Fourier transform infrared

spectroscopy (FTIR).

Water is one of the more common contaminants. If it can be introduced via internal coolant leaks, high-pressure

hose cleaning procedures or condensation.

Water has several negative effects on the performance of oil. They are: Formation of rust, which in turn contaminates the oil. Increased wear rate from decreased lost load-bearing capacity. Creation of weak and strong acids from chemical reactions between additives

and base oils. Biological formation and growth in low-temperature applications. Loss of critical additives and additive function.


Acid number

The acidity of the oil is measured by titration through a base.

The Figure shows this graphically, depicting the evolution of

Total Acidity Number(TAN) as a function of time.


Total base number, Particle counting,

Microscopic examination in a laboratory

The alkalinity of oil is measured by titration through an acid, and expressed in mg KOH/g.

If the oil is in service for too long, then the TBN will drop significantly.

Particle Counting

This is an especially useful test for a hydraulic system with high sensitivity (e.g. servo valves).

In such a test certain quantity of hydraulic oil flows through a sensor, where all the insoluble material in the oil is

detected and counted using the principle of light

absorption.

The particles counted are classified cumulatively


Particle Counting

There is yet another method known as Particle Quantification Index (PQ or PQI).

In this test, each sample is passed over a sensor which measures the bulk magnetic content of the oil.

Since iron is the major wear element in virtually all components, the PQI is really a measure of how much iron

is present (ferrous density) in the sample, the amounts of

other magnetic elements being negligible.

The PQI does not mention size the bigger the number, the more iron. PQI will be able to specify grams of iron per

litre of oil.

Microscopic Examination in a Laboratory Microscopes of 1:500 magnification can detect metallic and non-metallic

particles as well as amount of water ingress.

Degradation of grease

Study of degradation of grease is also important in condition monitoring particularly for automatic lubricating

system with synthetic oil being used as base oil for the

grease in use.

The degradation of grease is nothing but oxidation of antioxidants.


Measurement Of Degradation

of Grease

Ruler method: In this method a variable voltage is applied to

the sample. While measuring the current flow, the presence

and concentration of various antioxidant additives can be

determined based on their unique electrochemical oxidation

potential and the magnitude of the induced current.

Direct scanning calorimetry (ASTM D5483): In this method

the grease is placed in a sealed vessel and subjected to heat

in an oxygen-rich environment.

Degradation of grease


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ENGG ZC242-L4