Maintenance Strategy Selection

MAINTENANCE STRATEGY SELECTION

 

ABSTRACT:

Indian industries have transformed from the past & focus has been shifted from manual production system to large scale maschine assisted production system. Our industrial technicians & engineers manage huge asset base which have strategic importance for the organization. Safety & reliability of the assets / personnel / environment is of prime importance. Keeping the equipment’s production ready is main goal. This can be achieved only by adopting effective & efficient maintenance strategies. Certain factors like fault and failure mode analysis, asset base criticality ranking and adoption of correct maintenance strategy as per the nature of asset will not only increase the performance & availability of asset but also reduce the cost & maintenance time to a great extent. Using the wrong maintenance technique results into wastage of time, money and resources, and often has no effect on improving or maintaining availability.

The following topics are covered in this blog-

·         Introduction of different maintenance strategies.

·         Key features, advantages & disadvantage of different maintenance strategy.

·         Selection & Implementation of right maintenance strategy for different asset kind.

INTRODUCTION:

Our industries have various civil, mechnical, electrical & electronics assets. Efficient maintenance of the equipment’s improves the availability, life & reduces maintenance cost. Every year a budget is allocated for operation & maintenance. A portion of the budget is allocated for the maintenance of these assets on annual basis. The efficient operation & maintenance of these assets can be crucial for the upkeep of above equipment’s for critical operations. The requirement of effective & efficient maintenance strategies is not only critical in terms of money but to a great extent for the reliability of the system.  If we categorize the equipment’s / systems, they can be classified into three categories-

1.       Rotating assets like motors, pumps, blowers, compressors.

2.       Static Assets like control systems, associated electronics, electrical system etc.

3.       Civil Structures.

The basic driving force behind adoption of type of maintenance strategy is business objectives which in turn drives the technologies and solutions applied to meet desired goals. As discussed earlier, the top most business objective of the any industry is reliability / safety of the equipment’s & personnel, environment & the secondary objective is to reduce maintenance cost & time.

So in nutshell the effective strategy is to determine which maintenance activities will be performed on which assets, its frequency & its scope to meet the preset objectives? Out of all equipment’s listed above, maintenance of a rotating asset is most costly. It has different components like motors, turbines, DG sets, pumps, compressors, blowers etc. Most of the assets have been in long operation & it’s expected to have a proper maintenance and life extension plans till they are replaced with new superior assets. Adoption of suitable maintenance strategies results into deployment of adequate technology/solutions & overall an improved business performance.

The four main maintenance strategies adopted by the industry are:

·         Reactive Maintenance (RM)

·         Preventive Maintenance (PM)

·         Predictive Maintenance (PdM)

·         Proactive Centered Maintenance (PCM)

Reactive Maintenance: Reactive maintenance is a fire fighting approach where maintenance is performed after a failure of the asset.  In this strategy, machines are operated in a run-to-failure (RTF) mode and the maintenance is carried out only after the functional failure of the equipment.

Preventive Maintenance (PM): Preventive maintenance strategy is a template based strategy where periodic maintenance is carried out as per OEM recommendation / organizational recommendations. The schedule of the intervals is based on average statistical/anticipated lifetime to avoid failure. This includes inspection, service / replacement of the defective parts of the machine.

Predictive Maintenance (PdM): This maintenance based on the actual asset condition gauged by objective evidence of probable problem obtained from on site, non-invasive tests and operating and condition measurements.  This is also known as Condition Based Maintenance (CBM).

Proactive-Centred Maintenance (PCM):  This is program of continuous maintenance optimization based on feedback from Root Cause Failure Analysis repairs, quantitative preventive maintenance, predictive maintenance routines, Condition Monitoring Systems, and operational experience.

MAINTENANCE TECHNIQUES:

There are different techniques / solutions for achieving the ultimate business objectives-

Condition Monitoring (CM): The is process of recording measurements of different machine parameters that convey the condition of machine without disrupting operation (for example vibration, oil analysis, process variable analysis, acoustics electrical characteristics, and thermal imaging) and comparing each to the permissible limits for each asset.

Reliability Centred Maintenance (RCM): This is a systematic and disciplined process to ensure the safety and mission compliance that defines system boundaries and identifies system functions, functional failures, and likely failure modes for equipment and structures in a specific operating context. RCM develops a logical identification of the causes and effects (consequences) of system and functional failures to arrive at an efficient and effective asset management strategy to reduce the probability of failure.

Failure Modes & Effects Analysis (FMEA): This is a methodology to identify the functions of an asset, the ways it can fail to perform intended functions, the causes of those failures, and the methods & techniques for detecting or mitigating those failures.

FMECA: Failure Modes, Effects, and Criticality Analyses is an integral part of RCM directed to determining type, probability, cause and consequences of potential failures.

Root Cause Failure Analysis (RCFA):  A methodology used to identify the fundamental cause(s) that, if corrected, will prevent recurrence of an event or adverse condition.

Computerized Maintenance Management System (CMMS):  CMMS is a computer maintenance management system for measuring, managing, and analyzing the maintenance process.  It includes MRO task planning and scheduling, inventory control and management, labor and material cost accounting, and asset historical data.

Taxonomy:  This is technique of classification and cataloging of items that shows the relationships between items. A proper taxonomy includes a consistent method for numbering, describing, naming and classifying items which simplifies management of the items by facilitating searching, understanding functional relationships, elimination of duplicate records, etc.

The P-F Interval – Mechanical Asset Example (Centrifugal Pump):

PF curve is a common curve that shows the behavior of equipment as it approaches failure. The curve shows that as a failure starts and progresses, the equipment deteriorates to the point at which it can possibly be detected and is denoted by point ‘P’.  If the failure is not detected at that point & and the fault is not mitigated, it continues until a complete failure occurs ‘F’. This is known as functional failure i.e. the system is no longer capable of performing the intended function.  For example, a compressor that is designed to produce 2000 cfm at 7 Kg/cm² is considered to have functionally failed if it can only produce 900 cfm at 7 Kg/cm². The time range between P and F, commonly called the P-F interval, is the window of opportunity during which an inspection can possibly detect the imminent failure and address it.

 

 Adapted from Moubray, John, Reliability-Centered Maintenance

Time can be measured in seconds, minutes, days, months or years. P1-Px indicates detectability intervals by various techniques or technologies. In this example point ‘P’ is start of potential failure point. As fault progresses, the point P1 is approached where change in vibration pattern starts appearing.  Further due to metallic part wear one can detect the worn out metal particles in the oil filter. Further on the variation in process data starts appearing. Later on the temperature rise can be detected by IR cameras & finally it can be felt by touch & audible noise can be heard. This is the point which is very near to failure & finally the machine reaches point ‘F’ where machine breaks down completely.

Key Features of the Four Fundamental Maintenance Strategies:

 

Reactive Maintenance (RM): Reactive maintenance strategy is just like living life at the bottom of the P-F curve. In this maintenance strategy, maintenance is performed after a failure of the asset, or after an obvious & unforeseen threat of immediate failure. Running machines in run-to-failure mode is an appropriate strategy for assets where the consequence of failure & also including the cost of replacement is so low that the expense of maintenance time by doing preventive maintenance or predictive maintenance cannot be justified.  Reactive maintenance in long run can be a very expensive type of maintenance strategy when applied indiscriminately to all assets in the plant. This can consume up to 80 percent of the total time and budget of companies adopting this mode invariably.

Preventive Maintenance (PM): Preventative Maintenance strategy was one of the very first strategies adopted by OEM & asset owners and it is still effective.  There are two kinds of PM –

·         Inspection and observation.

·         Intervention and replacement.

The first Preventative Maintenance form is the usual response used for equipment and parts that show signs of age and wear-out by adopting LLF (look, listen, feel) technique.  It involves inspection and noting down the condition of equipment and servicing it on regular intervals like changing oil lubricant, greasing, filter cleaning etc. If evidence of failure is found, the part is changed for new immediately or at the earliest convenient time before actual breakdown. This strategy is often based on OEM recommendations with PM template & maintenance is performed at time-based intervals. The maintenance intervals are generally based on the MTBF (Mean Time Between Failure) data compiled by the OEM & often very conservative. PM includes intrusive time-based inspections and requires shutdown of asset for recommended maintenance activity. Quite often as the asset is already opened for inspection, wearable parts may be replaced even though they do not show any sign of wear. As we know, asset failures can happen in between scheduled maintenance intervals, a time-based maintenance strategy may not be right for many assets. Intrusive PM employs time based maintenance as per OEM recommendation where parts would be replaced irrespective of the condition of the same where as non-intrusive PM for the same machine would comprise of the inspection, taking measurement of the part & decide not to replace the part. But non-intrusive maintenance is still time based & they cannot be performed if the machine is running. Thus PM strategy is better than RTF strategy. It improves production by reducing unplanned downtime & mitigates wear and tear on equipment by keeping it clean & lubricated. It reduces maintenance and repair costs by eliminating emergency repair services.

Shortcomings of Preventive Maintenance:

Preventive maintenance is not developed based on FMEA analysis for the assets. No condition monitoring technique is used to develop the PM strategy. It is only a time-based maintenance practice. PM includes intrusive time-based inspections and requires taking the asset out of service and opening it to look for worn parts or incipient failures. It is a time based, OEM recommended practice to change parts etc. that is adopted. For quantitative PM the asset needs to be shut down. This may reduce the life cycle of the assets for which starting & stopping incurs greater wear than steady state operations. Often we land up doing over maintenance & high maintenance costs.

Predictive Maintenance (PdM): Predictive Maintenance (PdM) is a powerful maintenance strategy that involves monitoring for evidence of changed conditions within the equipment over a period of time.  The amount of change and the rate of change are tracked and used to predict the time of failure in future. Typically there is a start point, a gradual worsening, and eventually a point where the item cannot perform its duty & termed as functional failure. If it is possible to detect early onset of the failure then there is often time to manage the equipment carefully and continue operation until a replacement is actually needed. PdM techniques include thermography, oil debris analysis, vibration monitoring, process variable analysis, acoustics, and NDT testing.  These methods detect a change, and measure the rate of change & predictions are on the equipment’s continuing performance. Predictive Maintenance management strategy enables to detect the problems immediately and gives sufficient time to act before a failure occurs that shuts down the operation. It costs less than preventive maintenance or reactive maintenance. So, without a well-developed PdM Strategy, Condition Monitoring may not be effective. The only short coming of Condition Monitoring tasks is that it may not be based on failure analysis & may not be well integrated with other reliability tools like PM, RCFA, Lean Six Sigma etc. & it’s difficult to train all maintenance personnel on different technologies.

Proactive-Centred Maintenance (PCM): A one-size-fits-all approach utilizing Reactive Maintenance has already been discussed as the most expensive and least effective maintenance strategy when indiscriminately applied to all assets. However, the same can be said for both PM and PdM. Simply applying any particular strategy to all assets, independent of the asset’s criticality, is non-optimal. PCM recognizes this and emphasizes doing the right maintenance on the right assets at the right time.  In most cases, a PCM approach increases the use of PdM, while continuing to utilize PM. It also utilizes RM, but correctly limits this approach to assets with little or no consequences of failure. However, PCM’s purview encompasses more than just where to apply RM, PM, and PdM. It also concerns itself with procedures, operating parameters, processes, and designs in order to limit or prevent recurring failures, thus reducing the total number of asset failures and extending the mean time between asset failures. A PCM program is continually being optimized with feedback from Root Cause Failure Analysis (RCFA) repairs, Quantitative PM’s, PdM routines, CM systems, and operations. This feedback is used proactively to keep assets in their optimal operating condition. In the long run, PCM offers the lowest maintenance expenditure.

 Selecting the Right Strategy:

 The maintenance strategy is unique to each asset, and should address the specific root-cause failure modes of the asset. A mix of strategies can be applied to any one asset – for example, PdM may be used for detectable fault types, but PM or RM may apply for other failure modes. The purpose of this section is to introduce the main steps in selecting the right strategy:

1. Identify the Asset Base

2. Criticality Analysis

3. Develop the Strategy

4. Assign Technology & Tools

5. PM Task Optimization

 Identify the Asset Base

The Master Equipment List (MEL):

The equipment data library is a database of equipment and their attributes that is loaded into the Computerized Maintenance Management System (CMMS). This is built using logical conventions and taxonomy. The first step is to develop a complete and accurate equipment data library in the CMMS, along with corresponding ID tags on equipment in the field. The extra effort required to develop this equipment data library will pay for itself many times over.  For example, value from the equipment data library is delivered every time a repair work order is issued, a piece of equipment needs to be identified in the field, a reliability or cost history is needed, and when spare parts need to be identified and purchased. The equipment library starts with development of a logical equipment hierarchy, tagging convention, description, equipment class and subclass identification, and nameplate data.  This information is compiled in a database that is uploaded into the CMMS.  Corresponding field tags are also physically placed on the equipment.

A complete and accurate equipment library is required for effective:

·         PM/PdM work planning and execution

·         Spare parts inventory management

·         Cost accounting

·         Reliability management

·         Lifecycle cost management

Asset Criticality Ranking:

After the equipment data library has been developed, the relative criticality of all equipment must be ranked in terms of safety, the environment, production, maintenance costs, and product quality.  Weightings for each ranking criteria are determined based on previous experience of team members from facility, and then the ranking of each asset is determined using consistent and balanced ranking criteria.  The criticality ranking data is then uploaded to the CMMS so that each equipment record in the CMMS has a corresponding criticality value. A fine Criticality Ranking of the equipment focuses more attention on the asset that has the largest financial impact on your overall operation. Criticality is used by maintenance planners to prioritize the work based on the risk the asset poses to the business. It is also used to determine the optimum level of spare parts inventory. A cross-functional team is usually assembled to agree on criteria weighting and asset evaluation.

The Primary criticality ranking criteria are:

·         Safety

·         Environment & Regulatory Compliance

·         Production

·         Maintenance & Operating Costs

·         Product quality

·         Other relevant criteria

Develop the Maintenance Strategy:

Point out that as critical equipment is put on PdM and results are seen, decisions can be made on a Cost-Benefit-Analysis basis whether to expand the amount of equipment monitored. In a typical implementation, 10% of equipment will receive RCM 35% are candidates for Failure Mode Effects Analysis, and about 50% will benefit from well written PM’s. Possibly 5% of equipment may be left off a maintenance program altogether and simply repaired or replaced when it fails. This would only be if it meets certain criteria. A granular Criticality Ranking of your equipment allows you to focus attention on equipment that has the largest financial impact on your overall operation. Critically Ranking Equipment allows ‘Work Order Prioritization’ supporting both planning and scheduling, as well as Emergency Work Order generation.

The three alternative tools that can be applied for maintenance Strategy Development are:

·         RCM – carried by teams on only the most critical systems. An individual RCM analysis may take 8 to 10 person-weeks to complete.

·         FMEA – developed by single analyst on critical systems and equipment. The use of FMEA libraries helps fastrack this process.

·         Templates – consist of standard maintenance strategies for low criticality assets.

Technology and Tools:

It is important to understand the asset and its criticality ranking to select the technology and application frequency to support the Maintenance Strategy. The criticality ranking of equipment in the system will influence the Maintenance Strategy and the level of Condition Monitoring that will be applied.

 

Adapted from Asset Management 101, Maintenance Strategy Overview

PM Optimization:

The most tangible and immediate benefits of implementing a condition monitoring and PdM program is that present the existing time-based PM program can be optimized.  Many PM programs are based on OEM recommendations or generic templates with little consideration of available predictive technologies or the operating context of the asset.  PM Optimization involves a systematic screening of existing time-based preventative maintenance tasks with respect to asset criticality, common failure modes, and available predictive technologies. Typical benefits are significant reduction in the number of PM tasks through replacement with non-intrusive tasks, condition-based maintenance tasks, and deletion of ‘no value’ tasks.  Many PM’s are also strengthened through revision of PM intervals and improved work packages that detail procedures, parts, and tools.

This is a systematic review of routine Preventive Maintenance (PM) tasks with reference to the ideal maintenance strategy. Work plans are strengthened with improved procedures, fully developed Bill-of-Materials, tools and parts kits. By optimizing existing PM tasks, the cost of the maintenance program is reduced keeping the reliability high. Savings from the PM Optimization process can fully fund the introduction of a Predictive Maintenance program. Typical results from PM Optimization:

·         Approx. 30% of PM tasks can be replaced by more cost effective Predictive Maintenance tasks.

·         Approx. 30% of PM tasks can be deleted (no value).

·         Approx. 30% of PM tasks need strengthening.

CONCLUSION:

 The adoption of any maintenance strategy is based on kind of asset chosen for maintenance. There is not a single solution for complete asset base. So, the ideal situation is to classify different naval assets into different categories based on their criticality. Accordingly, right maintenance strategy should be selected. This will not only result into improved performance & availability of assets but also reduce the maintenance cost & time. The resource saved can be utilized for other critical jobs.

References:

 

·         Optimizing Your Maintenance Strategy- Michael Hanifan- GE

·         Moubray, John, Reliability-Centered Maintenance, Industrial Press, Inc., New York City, NY, 1997.

·         LICENTIATE THESIS PROPOSAL- CONDITION BASED MAINTENANCE IN TECHNICAL SYSTEMS- Marcus Bengtsson- Department of Innovation, Design and Product Development

·         Innovation and Design, Product- and Process Development, Mälardalen University Eskilstuna, Sweden