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

4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 363 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 groupedinto the followingcategories that can be scheduled on a fixed-time interval: • Running checks: This includes inspections and minor adjustments. • Monitoring checks: This in cludes data log records and condition monitoring readings. • Service checks: This includes replacement of lubricants and consumable parts. The concept o f 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 a ction in maintenan ce 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 measurementof physical conditionssuch 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. 364 4 Availability and Maintainability in Engineering Design • Preventive main tenance 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 mainten ance activities included in the category of monitoring checks is con- dition monitoring. Condition monitoring is the assessment of the condition 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 o f 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 ty pes of conditio n 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 contamination (lubricant and de- bris analy sis, etc.). Continuous monitoring is monitoring of equipment operational condition through the employment of electronic signal processing techniques to de- termine certain eq uipment operational characteristics (such as vibration of ro tating 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, co mpared to walk-through inspections, is the essential trending of accumulated monitoring data. Through forecasting the trend of an increasing d ivergence 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 equipmen t 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.Theresult of the prediction of failure can lead to the action of scheduled replacement or equipment overhaul—which is a preventive maintenance activity. Condition monitoring, includin g forecasting trends in the deviation of opera- tional conditions and, thus, predicting the possibility of failure in the physical con- dition of equipment, fo rms the link between routine maintenance and preventive 4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 365 maintenance. Condition monitoring is the ‘stepping stone’ from caring for equip- ment operational condition (routine maintenance), to caring for equipment physi- cal condition (preventive mainten ance). Thu s, 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 th a t 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 screen ing, 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 o f critical components to be examined and tested by non-destructive test (NDT) methods of condition measurement. Condition inspection is the most b asic 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 downtimedue to preventivemaintenancebecause it results in fewer break- downs. Typical mathematical models for calculating the optimum number of phys- ical condition inspections, with resulting minimum pr eventive maintenance down- time, are of the following format (Dhillon 1999b): T pm = IT id + kT bd I (4.97) where: T pm = preventive maintenance downtime 366 4 Availability and Maintainability in Engineering Design I = number of physical condition inspections k = operational constant for a particular system T id = downtime per physical condition inspection T bd = downtime due to equipment breakdown. Taking derivatives of Eq. (4.97) with respect to I gives dT pm dI = T id + kT bd I 2 . (4.98) Setting Eq. (4.98) to zero for op timisation, and rearranging the variables: ¯ I =[kT bd /T id ] 1/2 where: ¯ I = 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 T pm = 2[kT bd T id ] 1/2 (4.99) 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, T pm , 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): T pm = SDT CL (4.100) where: T pm = preventive maintenance downtime SDT = system and system’s equipment d owntime CL = length of the preventive maintenance cycle 4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 367 and SDT = T pr + T bd (4.101) CL = T pr + T C (4.102) where: T pr = downtime due to equipment replacement T bd = downtime due to system equipment breakdown T C = uptime time interval between replacements. Preventive maintenance d owntime, T pm , 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 uptime 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 T pr = k ∑ i=1 (T pti )(F pti ) (4.103) T bd = m ∑ i=1 λ i T i (4.104) 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 p reventive maintenance replacement task i for i = 1,2,3, ,k λ i = 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 T pm 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 C and setting it to zero T pm =(T pr + T bd )/(T pr + T C ) (4.105) = ∑ k i=1 (T pti )(F pti )+ ∑ m i=1 λ i 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. 368 4 Availability and Maintainability in Engineering Design 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 b e discerned from th is 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 o f maintenance This ratio can also b e rewritten as: Balance = Output of maintenance Input of maintenance = Output Input This ratio is known as the productivity ratio,orthecost 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.Thisiseffective 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 equipment’s functions depend to achieve the necessary technical benefits through the application of defect main- tenance, routine maintenance, and preventive maintenance, in an overall plan”. In other words, a maintenance strategy is carrying out the right types of m aintenance (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: 4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 369 • The operation of the system and its output demand. • The function s 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 main tainability but in the gen- eral context of process engineering design, outlines the best way to develop th e most suitable scope of maintenance workor service to be conductedon 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 co mbination of these concepts will inevitably impact upon the necessary decisions in designing for m aintainability: • 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 fo llowing criteria: • The operation of the plant and output demand. • The function s 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 maintenance 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 370 4 Availability and Maintainability in Engineering Design 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 predominantlytechnical 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 d esign criteria o f 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 m eans 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 engineering design application during the equipment life cycle, from d esign through to restoration, rather than single points of measure in the equipment’s life. Maintenance strategy can now b e defined as “the continuous action of caring for equipment condition through a balanced application of preventive maintenance, routine maintenanceand 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— 4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 371 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 Main tenance Strategy Reliability of equipmen t has been de fined as “the p robability that equipment will perform a required function, under specific conditions, for a required period of time”. Operational reliability is the probability that equipm ent 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 b etween failures). What is significant in the concept of equipment reliability with in a maintenan ce 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 m ost useful performance measure 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 equ ipment’s r eliability. The performance measure 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 b y its quality of ma nufacture. 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 co ndition”. Availability is the equipment’s capability of being used. The measure of operation al availability is the relation ship 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 a nd 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 eq uipment availability within a maintenance strategy framework is that the operational condition of equip- ment is determined by its potential usage, which is a measure o f its availability. Availability is thus the most useful performance measure for determining the result 372 4 Availability and Maintainability in Engineering Design 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 effectivenessof the routine maintenance being carried out to care for its operatio nal 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”. Operation al 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 re pair 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 m aintainability. Maintainability is therefore the most useful p erformance 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 equipment is restored through defect mainte- nance, and its effect is determined by MTTR, which is the performance measure of the equipment’s maintaina bility. The performance measure of maintainability deter- mines the repairable cond ition 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 tim e. Maintainability is primarily a design parameter, and designing for maintainability defines how long equipment is expected to be d own 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 perform ance 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 maintainab ility of equipment, which is the performance measure of the repairable condition of equipment. . Overview of Availability and Maintainability in Engineering Design 363 Defect maintenance is the corrective action in maintenance through fixing or repair- ing equipment after it has failed. Routine. of equipment, fo rms the link between routine maintenance and preventive 4.2 Theoretical Overview of Availability and Maintainability in Engineering Design 365 maintenance. Condition monitoring. 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 engineering design