Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 39 pdf

Routine maintenance is the preventive action in maintenance that cares for the operational condition of equipment through inspection, adjustment, recording, monitoring, servicing and lub

Defect maintenance is the corrective action in maintenance through fixing or

repair-ing equipment after it has failed

Routine maintenance is the preventive action in maintenance that cares for the operational condition of equipment through inspection, adjustment, recording, monitoring, servicing and lubrication, to ensure that the equipment’s operational functions conform to the required limits of performance.

These routine maintenance activities can be grouped into the following categories

that can be scheduled on a fixed-time interval:

• Running checks:

This includes inspections and minor adjustments

• Monitoring checks:

This includes data log records and condition monitoring readings

• Service checks:

This includes replacement of lubricants and consumable parts

The concept of routine maintenance is based upon the type of preventive actions that can be routinely carried out or, by definition, performed according to a regular course of procedure on a fixed-time interval basis Evidently, this type of preventive

action can only be directed towards the operational condition of equipment Preventive maintenance is the preventive action in maintenance that strives to

reduce the likelihood of failure through the detection of identifiable potential

fail-ures in the equipment’s physical condition, and thus attempts to avoid functional failure occurrences This is done through scheduled checks and inspections of

phys-ical condition, fault diagnostics, measurement, scheduled shutdowns for opening and cleaning equipment, scheduled shutdowns for replacing worn components, and scheduled shutdowns for overhauling plant and equipment

These preventive maintenance activities can be grouped into the following

cate-gories that are scheduled on run-time intervals:

• Physical checks:

This includes scheduled checks of physical conditions, and fault diagnostics

• Measurement checks:

This includes measurement of physical conditions such as stress cracks, thickness tests, wear tolerances, etc., and scheduled shutdowns for opening and cleaning equipment

• Replacement shuts:

This includes scheduled shutdowns for replacement of worn components, and scheduled shutdowns for overhauling plant and equipment

Thus, the way in which corrective and preventive action in maintenance is

practi-cally implemented is through the different types of maintenance whereby:

• Defect maintenance is corrective action in restoring equipment to its operational state or repairing physical defects after it has failed.

• Routine maintenance is preventive action in caring for the operational condition

of the equipment before it has failed.

• Preventive maintenance is preventive action in caring for the physical condition

of the equipment before it has failed.

Condition monitoring and the concept of predictive maintenance One of the

routine maintenance activities included in the category of monitoring checks is con-dition monitoring Concon-dition monitoring is the assessment of the concon-dition of equip-ment whilst it is in operation.

Consequently, condition monitoring can be regarded as a routine maintenance

task that cares for the operational condition of equipment Thus, from an under-standing of the condition of equipment, condition monitoring can be properly

de-fined as “the assessment of the operational condition of equipment whilst it is in operation”.

There are two types of condition monitoring:

• Periodic monitoring

• Continuous monitoring.

Periodic monitoring is the monitoring of equipment operational condition

accord-ing to a regular course of procedure on a periodic fixed-time interval basis The simplest form of periodic condition monitoring is operational checks of equipment temperatures, vibration or noise by the operator or service technician The more sophisticated form of periodic condition monitoring is the use of specialised instru-mentation to monitor temperature (thermographics, infra-red scanning, etc.), vibra-tion (accellerometers, etc.), noise (ultrasonics) and contaminavibra-tion (lubricant and

de-bris analysis, etc.) Continuous monitoring is monitoring of equipment operational condition through the employment of electronic signal processing techniques to

de-termine certain equipment operational characteristics (such as vibration of rotating machinery, pump flow, etc.), with the aid of online sensors, onboard or mounted in-strumentation (geriometry), and computerisation (supervisory control and data ac-quisition, SCADA, systems)

The importance of condition monitoring, compared to walk-through inspections,

is the essential trending of accumulated monitoring data Through forecasting the

trend of an increasing divergence of the operational condition of equipment away from its standard limits of operational performance, predictions can be made con-cerning gradual degradation of the physical condition of the equipment This fore-casting of diverging trends of the operational performance of equipment and

pre-dicting failure is called predictive maintenance The term is not quite correct, as maintenance by definition implies action, and the forecasting of operational

con-ditions of equipment to predict failure is not a specific action or work carried out

on the equipment itself The only action that is carried out in condition monitoring

is the taking of readings of equipment operational condition—which is a routine maintenance activity The result of the prediction of failure can lead to the action of scheduled replacement or equipment overhaul—which is a preventive maintenance activity.

Condition monitoring, including forecasting trends in the deviation of opera-tional conditions and, thus, predicting the possibility of failure in the physical

con-dition of equipment, forms the link between routine maintenance and preventive

maintenance Condition monitoring is the ‘stepping stone’ from caring for equip-ment operational condition (routine maintenance), to caring for equipequip-ment physi-cal condition (preventive maintenance) Thus, condition monitoring is the routine

maintenance assessment of the operational condition of equipment that will give an indication for the need for preventive maintenance action

Condition measurement and the concept of fault diagnostics There are many

benefits that can be derived from the ability to anticipate the need for preventive maintenance The occurrence of failure that results from degradation of the physical condition of equipment takes place in a sequence or cascade of events with each event increasing the probability of a partial loss of function or total loss of function

of the equipment If the rate of deterioration of the physical condition of equipment can be measured before a total loss of function occurs, then preventive maintenance can be systematically planned to avoid such an occurrence of failure Such a mea-surement of the rate of deterioration of the physical condition of equipment is called

condition screening, and incorporates the use of condition measurement.

It is essential, in designing for maintainability, to have an understanding of the

patterns of functional failure of equipment with a physical condition that is

dete-riorating Only then can preventive maintenance be carried out Most engineered installations have scheduled shutdowns for either production or process changes, physical condition inspections, or for general overhauls, in which the opportunity arises for the physical condition of critical components to be examined and tested

by non-destructive test (NDT) methods of condition measurement.

Condition inspection is the most basic examination of an equipment’s physical

condition, and can be enhanced by the use of condition measurement methods for the detection and fault diagnostics of cracks, surface wear or defects, deformation, corrosion, thickness reduction, and stress marks due to aged equipment or excessive use Fault diagnostics is the analysis of the deterioration of the physical condition of equipment to determine the causes and effects of wear, cracks, defects, deformation, corrosion and stress in the equipment

b) Mathematical Model of Preventive Maintenance Physical Checks

Although condition inspection is the most basic examination of physical condition,

it is often disruptive to the continued operation of equipment However, it usually decreases downtime due to preventive maintenance because it results in fewer break-downs Typical mathematical models for calculating the optimum number of phys-ical condition inspections, with resulting minimum preventive maintenance down-time, are of the following format (Dhillon 1999b):

Tp m= ITi d+kTb d

where:

I = number of physical condition inspections

k = operational constant for a particular system

Ti d = downtime per physical condition inspection

Tb d = downtime due to equipment breakdown

Taking derivatives of Eq (4.97) with respect to I gives

dTp m

dI = Ti d+kTb d

Setting Eq (4.98) to zero for optimisation, and rearranging the variables:

¯= [kTb d/Ti d]1/2

where:

¯ = optimum number of physical condition inspections

Substituting Eq (4.98) into Eq (4.97) yields the optimum downtime due to physical condition inspections that contribute to the preventive maintenance downtime

c) Mathematical Model of Preventive Maintenance Replacement Shuts

Similar to the previous model, the objective of this model is to minimise preven-tive maintenance downtime as a result of scheduled shutdowns for replacement

of worn components The model represents a constant interval replacement pol-icy Such a constant interval replacement model implies the following (Elsayed 1996):

• Replacements are carried out at predetermined intervals, irrespective of the age

condition of the equipment’s components

• Replacements are made of failed equipment (i.e unit replacement and repair

cycle)

Preventive maintenance downtime, Tp m, can be expressed in the form of system

downtime (inclusive of the system’s equipment) over the length of the preventive maintenance cycle (Jardine 1973):

Tp m=SDT

where:

where:

Tp r = downtime due to equipment replacement

T C = uptime time interval between replacements

Preventive maintenance downtime, Tp m, over the length of the preventive mainte-nance cycle can thus be expressed as the comparison of downtime due to equipment replacement plus the downtime due to system equipment breakdowns, to the down-time due to equipment replacement plus the updown-time interval between replacements (i.e the preventive maintenance cycle)

For several replacement tasks over the length of the preventive maintenance cy-cle, CL, the variables can be expressed as the following

Tp r=∑k

i=1(T pti )(F pti) (4.103)

Tb d =∑m

i=1

where:

T pti = the estimated lapse time for preventive maintenance replacement task i

for i = 1,2,3, ,k

F pti = the estimated frequency of preventive maintenance replacement task i

for i = 1,2,3, ,k

λi = the constant failure rate of item i = 1,2,3, ,m

T i = the corrective maintenance time needed to replace item i = 1,2,3, ,m.

Inserting Eqs (4.101) to (4.104) into Eq (4.100) yields an expression for Tp m that

can then be optimised in terms of the uptime interval between replacements, T C, by

taking derivatives of Eq (4.105) with respect to T Cand setting it to zero

= ∑k i=1(T pti )(F pti) +∑m

i=1λi T i

∑k

i=1(T pti )(F pti ) + T C

d) Maintenance Strategy

The term strategy is defined as “an overall plan with a choice of activities to be effectively carried-out” Maintenance strategy is closely related to the definition of maintenance as well as to the concept of effective maintenance.

To be able to understand the concept of effective maintenance, it is necessary

to first examine the principles underlying the goal of maintenance The goal of maintenance is defined as “that maintenance action necessary to achieve the cor-rect balance between the costs of input resources and the benefits derived from the performance of effective maintenance action”.

Two principles can be discerned from this definition of the goal of maintenance

The first principle is the correct balance between the costs of maintenance resources

and the benefits of maintenance This balance can be represented in the form of

a ratio:

Balance=Benefits of maintenance

Costs of maintenance This ratio can also be rewritten as:

Balance=Output of maintenance

Input of maintenance =Output

Input

This ratio is known as the productivity ratio, or the cost efficiency ratio It is the ratio

of the amount of maintenance work performed (output) to the total cost expended (input)

Maintenance action is often measured in terms of manpower utilisation and

re-source costs This is a measure of efficient maintenance However, the definition of

the goal of maintenance describes the correct balance of input to output, derived

from the performance of effective maintenance action.

The question to be asked then is ‘what is effective maintenance, and what is the

difference between efficient maintenance and effective maintenance?’

Efficient maintenance in simple terms can be described as ‘doing the job right’, and effective maintenance in simple terms can be described as ‘doing the right job’.

The second principle in properly understanding the goal of maintenance is that

it is not so much a determination of the amount of work that is to be carried out

that is crucial but, rather, whether the maintenance work that needs to be done is

the right type of maintenance that will be done at the right time This is effective

maintenance

The definition of maintenance strategy From an understanding of the definitions

of maintenance and the goal of maintenance, equipment maintenance strategy can

be defined as “the continuous corrective or preventive action for the care of equip-ment operational and physical condition on which the equipequip-ment’s functions depend

to achieve the necessary technical benefits through the application of defect main-tenance, routine mainmain-tenance, and preventive mainmain-tenance, in an overall plan” In

other words, a maintenance strategy is carrying out the right types of maintenance (scope of work) at the right time (overall plan) A maintenance strategy implies effective maintenance

An overall maintenance plan, with a choice of the essential types of maintenance

activities to be carried out, takes into account the following design criteria:

• The operation of the system and its output demand.

• The functions and criticality of the equipment.

• The required level of maintenance service.

The operation of the system and its output demand are variables that relate to pro-cess efficiency, utilisation and productivity, all of which represent the functional characteristics of the process The functions and criticality of the equipment are de-termined from FMEA and FMECA The level of maintenance service is based on the required operational and physical conditions of the equipment, as well as on the amount of planning that is required for each type of maintenance to achieve these conditions

A maintenance strategy, not only in designing for maintainability but in the gen-eral context of process engineering design, outlines the best way to develop the most suitable scope of maintenance work or service to be conducted on the proposed engi-neered installation, within an overall maintenance plan This is established through taking cognisance of the following:

• What type of maintenance must be done.

• Why each type of maintenance must be done.

• Where each type of maintenance must be done.

• How each type of maintenance must be done.

• When each type of maintenance must be done.

• What technical expertise is required for the work.

• How frequently each type of maintenance must be done.

This maintenance service is developed according to a strategy that includes all or some of the following concepts that need to be adopted for each item of designed equipment The selection of and/or combination of these concepts will inevitably impact upon the necessary decisions in designing for maintainability:

• Run-to-failure (defect maintenance).

• Fixed-time-interval (routine maintenance).

• Run-time-interval (preventive maintenance).

In simple terms, then, a maintenance strategy is concerned with matching the best combination of the various types of maintenance to particular equipment according

to the following criteria:

• The operation of the plant and output demand.

• The functions and criticality of the equipment.

• The required operational and physical conditions of the equipment.

• The amount of planning required for each type of maintenance.

• The frequency of each type of maintenance.

• The necessary technical benefits to be achieved.

It is thus the balanced combination of the application of the different types of main-tenance that constitutes a mainmain-tenance strategy However, a question that can

justifi-ably be asked at this point is ‘why is it necessary to have a maintenance strategy?’—

it is essential to develop a maintenance strategy for process equipment, particularly

during the engineering design stage, so that the necessary technical benefits can be achieved according to the designed measures of performance.

Measures of performance Returning to the definition of the goal of maintenance

as the maintenance action necessary to achieve the correct balance between the costs

of input resources and the benefits derived from the performance of an effective

maintenance action, it is evident that there are specific benefits to be achieved from

effective maintenance As indicated, these benefits are predominantly technical ben-efits and can be achieved through developing a maintenance strategy, particularly

during the engineering design stage However, not all technical benefits are derived from the performance of an effective maintenance action, as the design criteria of

maintainability refers to measures taken during the design stage that strive to re-duce the required maintenance action, repair skill levels, logistic costs or support

facilities

The technical benefits relating to the engineering design that can be derived from

the performance of effective maintenance are the following:

• Properly maintained operational conditions.

• Properly maintained physical conditions.

• Corrective action being carried out on time.

• Preventive action being carried out on time.

• Achieving the designed equipment reliability.

• Achieving the designed equipment availability.

• Achieving the designed equipment maintainability.

• Achieving the required operational safety.

An important question at this point is ‘how would one know whether a developed maintenance strategy for a particular engineering design will, in fact, achieve the necessary technical benefits?’ The effectiveness of a maintenance strategy devel-oped during the engineering design stage can be determined only through the mea-sures of performance of the benefits that are achieved in the completed engineered installation These measures of performance are the measures of operational equip-ment reliability, availability, maintainability and safety (i.e operational integrity) that need to be compared to the original design benchmark measures It is evident that the only means of determining whether a maintenance strategy is effective is

to establish measures of design integrity as a benchmark against which measures of operational integrity can be compared It thus becomes a comparison of

engineer-ing design intention against engineerengineer-ing design application durengineer-ing the equipment life cycle, from design through to restoration, rather than single points of measure

in the equipment’s life

Maintenance strategy can now be defined as “the continuous action of caring for equipment condition through a balanced application of preventive maintenance, routine maintenance and defect maintenance, to achieve benefits of reliability, avail-ability, maintainability and safety”.

Up till now, the terms reliability, availability and maintainability have been used

as measures of design integrity and operational integrity It is, however, neces-sary to define these terms in the context of the basic principles of maintenance—

particularly as performance measures of the results that can be achieved from the application of the different types of maintenance in a maintenance strategy, and to understand which results are achieved from the application of which type of main-tenance

e) Concept of Equipment Reliability in Maintenance Strategy

Reliability of equipment has been defined as “the probability that equipment will perform a required function, under specific conditions, for a required period of time” Operational reliability is the probability that equipment will not fail in a given

period of operation

The fundamental indicator of reliability was previously given as the probability that the equipment has operated over a specific period of time, the average of which

is the measure of MTBF (mean time between failures) What is significant in the concept of equipment reliability within a maintenance strategy framework is that

the physical condition of equipment is determined by the MTBF, which is a

mea-sure of its reliability Reliability is thus the most useful performance meamea-sure for determining the result of the physical condition of equipment Furthermore, it was

previously stated that preventive maintenance is that type of maintenance that cares for the physical condition of equipment.

Thus, the physical condition of equipment is maintained through preventive maintenance, and its effect is determined by MTBF, which is the performance mea-sure of the equipment’s reliability The performance meamea-sure of reliability deter-mines the physical condition of equipment and the effectiveness of the preventive maintenance being carried out to care for its physical condition The inherent relia-bility of equipment is initially established by its physical design and by its quality

of manufacture Design for reliability thus plays an important role in the initial re-liability of equipment, the lack of which is often the cause of failures resulting in downtime stoppages

f) Concept of Equipment Availability in Maintenance Strategy

The availability of equipment has been defined as “that period of time in which the equipment is in a usable condition” Availability is the equipment’s capability of

being used The measure of operational availability is the relationship of the

equip-ment’s potential usage over a period of time, where usage is defined as “the period

of time that equipment is being utilized” Potential usage of equipment is the sum of

its actual utilisation and the period of time that the equipment was capable of being used but was not

The effect of potential usage is determined by the performance measure of the equipment’s availability What is significant in the concept of equipment availability

within a maintenance strategy framework is that the operational condition of

equip-ment is determined by its potential usage, which is a measure of its availability Availability is thus the most useful performance measure for determining the result

of the operational condition of equipment Furthermore, it was previously stated

that routine maintenance is that type of maintenance that cares for the operational condition of equipment Thus, the operational condition of equipment is maintained

through routine maintenance, and its effect is determined by the equipment’s poten-tial usage over a period of time, which is the performance measure of the equip-ment’s availability The performance measure of availability determines the opera-tional condition of equipment and the effectiveness of the routine maintenance being carried out to care for its operational condition In designing for availability, the in-herent availability of equipment is its potential usage in design operational time

g) Concept of Equipment Maintainability in Maintenance Strategy

The maintainability of equipment has been defined as “the probability that equip-ment which has failed can be restored to its required condition within a given period

of time” Operational maintainability is the probability that failed equipment is

re-paired within a given period of time The fundamental indicator of maintainability was previously given as the probability of repair within a given period of time, the average of which is MTTR (mean time to repair) What is significant in the concept

of equipment maintainability within a maintenance strategy framework is that the

ability to repair failed equipment within a given period of time is determined by

the MTTR, which is a measure of its maintainability Maintainability is therefore

the most useful performance measure for determining the repairable condition of equipment Defect maintenance is that maintenance work that fixes or repairs

equip-ment after it has failed Thus, failed equipequip-ment is restored through defect mainte-nance, and its effect is determined by MTTR, which is the performance measure of the equipment’s maintainability The performance measure of maintainability deter-mines the repairable condition of equipment and the effectiveness of defect mainte-nance being carried out to restore the equipment to its repaired state within a given period of time Maintainability is primarily a design parameter, and designing for maintainability defines how long equipment is expected to be down after failure

h) The Three Principles of a Maintenance Strategy

There are three fundamental principles of a maintenance strategy:

• The effectiveness of preventive maintenance is determined by the technical

ben-efit of reliability, which is the performance measure of the physical condition of equipment

• The effectiveness of routine maintenance is determined by the technical benefit

of availability, which is the performance measure of the operational condition of equipment

• The effectiveness of defect maintenance is determined by the maintainability

of equipment, which is the performance measure of the repairable condition of equipment