mech7350 14 condition monitoring
TRANSCRIPT
MECH7350 Rotating Machinery 14. Condition Monitoring
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14. CONDITION MONITORING
14.1 Introduction to Condition Monitoring Techniques
Maintenance is the management, control, execution and quality of those activities which will
ensure that optimum levels of availability and overall performance of plant are achieved, in
order to meet business objectives - The British Department of Trade & Industry (DTI) (Rao,
B.K.N.).
Maintenance strategies can be characterised as a) general purpose, b) essential and c) critical
(Scheffer and Gridhar).
a) General Purpose
• Failure does not affect plant safety
• Not critical to plant production
• Machine has an installed spare or can operate on demand
• These machines require low to moderate expenditure, expertise and time to repair
• Secondary damage does not occur or is minimal
Fig. 14.1 Maintenance Strategies (from Scheffer and Gridhar).
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b) Essential Equipment
• Failure can affect plant safety
• Machine that are essential for plant operation and where shutdown will curtail a unit
operation or part of the process
• They may or may not have an installed spare available
• Start-up is possible but may affect production process
• High power and speed might not be running continuously
• Some machines that demand time-based maintenance
• These machines require moderate expenditure, expertise and time to repair
c) Critical Equipment
• If their failure can affect plant safety
• Machines that are essential for plant operation and where a shut-down will curtail the
production process
• Machines which do not have spare parts
• These machines have high capital cost, they are very expensive to repair or take a
long time to repair
14.1.1 Run-to-failure Maintenance
This applies to non essential equipment and machinery where shutdowns do not affect
production, materials and replacement are readily available. It allows the machinery to run to
failure and only repair or replace damaged components when the machine comes to a
complete stop.
Disadvantages:
Interrupt production
Large inventory of spare parts
Maintenance personnel have to work at odd time and interrupt normal activities and
tend to work overtime.
14.1.2 Preventive Maintenance
Preventive or time-based maintenance is to schedule maintenance at predetermined time
intervals, based on running hours of machines. In this case replacement of damaged
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equipment is carried out before problems occur. This allows the machine to run continuously
and where the personnel have enough skill, knowledge and time to perform the preventive
work.
Disadvantages:
Performing maintenance tasks either too early or too late
Replacement of components before the end of residual life
Reduced production due to unnecessary maintenance
Possibility of diminished performance due to incorrect repair methods
Possibility good parts being disassembled and discarded and improper fixing of
replaced parts can lead to other problems
14.1.3 Condition-based Maintenance
Condition-based or predictive maintenance periodic monitoring involves periodic monitoring
on the health of the machine and scheduling maintenance only when a functional failure is
detected. This allows trends of the machine component be constructed and time to failure be
estimated. Maintenance can be conveniently planned and allows lead-time for organisation of
parts and maintenance personnel and be scheduled. This leads to full utilisation of the
machine and possible increase in production capacity.
Disadvantages:
Incorrect assessment of the deterioration of machines
Inaccurate prediction of the lead-time
Requires specialised equipment to monitor the trend and highly skilled personnel.
14.1.4 Proactive Maintenance
Proactive or prevention maintenance involves tracing all failures to their root cause and to
ensure that failures are not repeated. It utilises predictive/preventive maintenance techniques
in conjunction with root cause failure analysis (RCFA). RCFA detects and identify the cause
of failure and ensures that proper installation and repair techniques are used. It also identifies
need for redesign of machine to avoid future occurrence of the same problems and improve
the reliability of the machine.
Disadvantages:
Needs highly skill personnel with a vast knowledge of all aspects of maintenance
May require outsourcing to private consultants and problems with confidentiality
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Requires specialised monitoring equipment and management support.
14.2 Introduction to Condition Monitoring
Condition monitoring and diagnostics of machines – according to ISO, Sub-committee
9ISO/TC/108/SC5. The scope of this Sub-committee – Standardisation of the procedures,
process and equipment requirement uniquely related to the technical activity of condition
monitoring and diagnosis of machines in which selected parameters associated with an
operating are periodically or continuously sensed, measured and recorded for interim purpose
of reducing, analysing, comparing and displaying the data and information so obtained and
for the ultimate purpose of using this results to support decisions related to the operation and
maintenance of the machine (Rao, B.K.N.).
Condition monitoring attempts to detect symptoms of eminent failure and approximates time
of a functional failure. It utilises a combination of techniques to obtain the actual operating
condition of the machines based on collected data such as vibration analysis, oil and wear
debris analysis, ultrasound, temperature and performance evaluation. The specific techniques
used depend on the type and operation of the machines.
Examples condition monitoring techniques (Scheffer and Gridhar):
(a) Vibration monitoring – this is the most commonly used and effective technique to
detect internal defects in rotating machinery.
(b) Acoustic emission monitoring – this involves detection and location of cracks in
bearings, structures, pressure vessels and pipelines.
(c) Oil analysis – lubrication oil is analysed and the occurrence of certain
microscopic particles in it can be connected to the condition of bearings and gears.
(d) Particle analysis – worn machinery components, whether in reciprocating
machinery, gearboxes or hydraulic systems, release debris. Collection and
analysis of this debris provides vital information on the deterioration of these
components.
(e) Ultrasonic monitoring – this is used to measure thickness of corrosion or crack on
pipelines, offshore structures, pressure vessels.
(f) Thermography – this is used to detect thermal or mechanical defects in generators,
overhead lines, boilers, misaligned coupling and cell damage in carbon fibre
structures on aircrafts.
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(g) Performance monitoring – this is used to determine the performance problems in
equipment. The efficiency of machines provides a good inside on their internal
conditions.
14.3 Relevant Industrial Standards
a) ISO 18436-1 Condition monitoring and diagnostics of machines – Requirements for
training and certification of personnel – Part 1: Requirements for certifying bodies and
certification process. This part of ISO 18436 defines the requirements for bodies operating
certification systems in no-intrusive machine condition monitoring, diagnostics and
correction technologies. General requirements for certification body personnel are contained
in this part of ISO 18436. Specific requirements for personnel in condition monitoring and
diagnostics will be contained in subsequent parts of ISO 18436.
b) ISO 18436-2 Condition monitoring and diagnostics of machines – Requirements for
training and certification of personnel – Part 2: Vibration condition monitoring and
diagnostics. The part of ISO 18436 defines the requirements against which personnel in the
non-intrusive machine condition monitoring and diagnostics technologies associated with
vibration analysis are to be carried and the methods of testing such personnel. Conformity
assessment for certification in vibration analysis will be performed by a body accredited to
the requirements of ISO 18436-3.
c) ISO 17359:2003(E) Condition monitoring and diagnostics of machines – general
guidelines. This International Standard presents an overview of a generic procedure
recommendation to be used when implementing a condition monitoring programme and
provides further detail on the key steps to be followed. It introduces the concept of directing
condition monitoring activities towards root cause failure modes, and describes the generic
approach to setting alarm criteria, carrying out diagnosis and prognosis and improving the
confidence in diagnosis and prognosis, which are developed further in other International
Standards.
d) ISO 13379:2003(E) Condition monitoring and diagnostics of machines – General
guidelines on data interpretation and diagnostics techniques. This International Standard
contains general procedures that can be used to determine the condition of a machine relative
to a set of baseline parameters. Changes fro the baseline values and comparison to alarm
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criteria are used to indicate anomalous behaviour and to generate alarms: this is usually
designated as condition monitoring. Additionally, procedures that identify the cause(s) of the
anomalous behaviour are given in order to assist in the determination of the proper corrective
actions: this is usually designated as diagnostics.
e) ISO 13380:2002(E) Condition monitoring and diagnostics of machines – General
guidelines on using performance parameters. This International Standard provides guidelines
for condition monitoring and diagnostics of machines using parameters such as temperature,
flow rates, contamination, power and speed, typically associated with the performance,
condition, safety and quality criteria. The evaluation of machine function may be based on
performance, condition, product quality or safety.
f) ISO 13374-1:2003(E) Condition monitoring and diagnostics of machines – Data
processing, communication and presentation – Part 1: General guidelines. This part of ISO
13374 establishes general guidelines for software specifications related to data processing,
communication and presentation of machine condition monitoring and diagnostics
information.
14.4 Vibration Monitoring
Vibration generated from a machine contains vital information on the health of the machine
and can be used to identify developing problems. Regular vibration monitoring can detect
deterioration or defective bearings, mechanical looseness, worn or broken gears,
misalignment and unbalance of rotor.
Fig. 14.2 Simple harmonic motion (from Scheffer and Gridhar).
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All rotating machines produce vibrations that are a function of the machine operating
conditions and machine dynamics. The most classical example is that of a body with mass M
attached to a spring of stiffness K. Due to weight of mass M, the object will stabilised at an
equilibrium position at a distance xo. When the mass is displaced by a certain displacement
x and released, it moves up and down about the equilibrium position and reaches the top and
bottom limits. The motion can theoretically continue indefinitely if there is no damping and
is called periodic or harmonic motion. The relationship between the displacement of the mass
and time is expressed in the form of a sinusoidal equation:
X = X0 sin ωt (14.1)
Where X – displacement at any given time t; X0 - maximum displacement; ω = frequency
(rad/s).
Velocity can be obtained by taking the first derivative of the displacement equation.
V = X0 ω cos ωt (14.2)
Similarly, the acceleration can be obtained by taking the derivative of the velocity equation
or the second derivative of the displacement equation.
A = -X0 ω2 sin ωt (14.3)
Fig. 14.3 Waveform of displacement, velocity and acceleration of mass in SHM (from Scheffer and Gridhar).
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Table 14.1 Some useful vibration parameters
Displacement (m) Velocity (m/s) Acceleration (m/s2 )
Frequency (Hz) Bandwidth (Hz) Spike Energy (gSE)
Power Spectral Density Peak Value Root mean square (RMS)
Crest factor (CF) Arithmetic mean (AM) Geometric mean (GM)
Standard deviation (SD) Kurtosis (K) Skewness
Phase (deg)
Using Vibration to Machinery Fault Detection
A typical machine system is shown in Fig. 14.4. It consists of a driver, such as electric motor,
diesel engines, gas engines, steam turbines and gas turbines. The driven equipment could be
pumps, compressors, mixers, agitators, fans, blowers and others. The driven equipment is
connected to the prime mover via a gearbox, belt drive, coupling and other connectors.
Each of these rotating parts is further comprised of simple components such as:
• Stator (volutes, diaphragms, diffuser, stator poles, etc)
• Rotors (impellers, rotors, lobes, screws, vanes, fan blades, etc.)
• Seals
• Bearings
• Couplings
• Gears
• Belts and pulleys
Fig. 14.4 A typical machinery system (from Scheffer and Gridhar).
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All rotating and moving parts are prone to wear and failure after a period of service and when
mechanical defects occur, they generate high vibration levels. Some of the common faults are
listed in Table 14a.
Table 14.1a Common machine faults
Unbalance of rotating parts
Misalignment of couplings and bearings
Bend or bow shafts
Worn or damage gears and bearings
Bad drive belts and chains
Torque variations
Electromagnetic forces Aerodynamic forces Hydraulic forces
Looseness Rubbing Resonance
The causes of machinery vibration and resulting vibration characteristics can be classified in
terms of characteristics vibration frequencies and their harmonics. Table 14.2 shows the most
common causes of machinery vibration and the resulting characteristic frequencies. Table
14.3 shows possible causes of vibration from known characteristic frequencies. Some of the
common causes of bearing failure are shown in Table 14.4. It has to be pointed that these
faults are not easily identifiable and these tables are provided to be used as a reference guide.
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Table 14.2 A guide to causes of vibration (from Bruel & Kjaer 2).
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Table 14.3 Common faults from known vibration characteristic frequencies (from Rao, B.K.N.).
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Table 14.4 Troubleshooting rolling element bearing failures (from Rao, B.K.N.).