MECH7350 Rotating Machinery 14. Condition Monitoring

14-1

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).

MECH7350 Rotating Machinery 14. Condition Monitoring

14-2

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

MECH7350 Rotating Machinery 14. Condition Monitoring

14-3

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

MECH7350 Rotating Machinery 14. Condition Monitoring

14-4

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.

MECH7350 Rotating Machinery 14. Condition Monitoring

14-5

(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

MECH7350 Rotating Machinery 14. Condition Monitoring

14-6

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).

MECH7350 Rotating Machinery 14. Condition Monitoring

14-7

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).

MECH7350 Rotating Machinery 14. Condition Monitoring

14-8

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).

MECH7350 Rotating Machinery 14. Condition Monitoring

14-9

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.

MECH7350 Rotating Machinery 14. Condition Monitoring

14-10

Table 14.2 A guide to causes of vibration (from Bruel & Kjaer 2).

MECH7350 Rotating Machinery 14. Condition Monitoring

14-11

Table 14.3 Common faults from known vibration characteristic frequencies (from Rao, B.K.N.).

MECH7350 Rotating Machinery 14. Condition Monitoring

14-12

Table 14.4 Troubleshooting rolling element bearing failures (from Rao, B.K.N.).