fans - maintenance strategy

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Fans - MaintenanceStrategyA Best Practice Approach

Summary

Maximizing the efficiency of a plant and its machine assetsrequires a shift from discrete maintenance activities to a strategicprocess that is embraced company-wide. A well-foundedmaintenance strategy will have a sound financial and technicaljustification. This is derived from a proper understanding of allof the functions that the machine is required to perform, and theirimpact upon the business in economic terms, and health / safety /environment (HSE) consequences.

AoM04002_Fan_SMel Barratt14 pagesJanuary 2005

SKF Reliability Systems@ptitudeXchange

5271 Viewridge CourtSan Diego, CA 92123United Statestel. +1 858 496 3554fax +1 858 496 3555email: [email protected]: www.aptitudexchange.com

Use of this document is governed by the termsand conditions contained in @ptitudeXchange.Copying or distribution of this document is prohibited.
mailto:[email protected]://www.aptitudexchange.com/http://www.aptitudexchange.com/mailto:[email protected]


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AoM04002_Fan_S - Fans - Maintenance Strategy

Introduction......................................................................................................................................3

Formulation of Maintenance Strategy .............................................................................................3

Identify System.........................................................................................................................4

Identify Functions.....................................................................................................................4

Failure Modes & Effects Analysis (FMEA).............................................................................5

Criticality Analysis ...................................................................................................................7

Maintenance Strategy Methods........................................................................................................7

Maintenance Strategy Templates.....................................................................................................9

The Role of Maintenance templates .........................................................................................9

Generic Templates..................................................................................................................13

Output from the Strategy Process ..................................................................................................13

Suggested Further Reading............................................................................................................14

2005 SKF Reliability Systems All Rights Reserved 2


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Introduction

A key aspect of any world-class assetmanagement program is a proactive,efficient work management process,designed to ensure the effective performanceof maintenance on critical assets. To achievemaximum return on investment andmaintain the greatest degree of productivity,it is pivotal that organizations have a processthat effectively translates asset informationto knowledge, and ultimately gain valuefrom that knowledge.

To help organizations achieve these goals,SKF offers Asset Efficiency Optimization

(AEO), a management process designed toachieve maximum efficiency andeffectiveness from work managementactivities focused on business goals.

The AEO process encompasses four keyelements [1]:

Strategy

Identification

Control

Execution

Maintenance Strategyinvolves theevaluation of work activities in relationshipto a facilitys business objectives, aprocedure that creates the documented basisfor the maintenance program.

Work Identification is where work isidentified from the evaluation of a

comprehensive flow of data in conjunctionwith an integrated decision-making process.Key to the success of Identification is acomprehensive computerized maintenancemanagement system.

Work Control involves establishingprocedures for planning and scheduling thework identified by the CMMS. Tasks are

organized based on several parameters,including time and condition; job plans orprocedures; man-hours required; datafeedback; special requirements; and many

other factors.

Work Execution is where identified,planned and scheduled work is performed.Once work is completed, feedback from thefield plays a key role in measuring theoverall effectiveness of the AEO processand making refinements for even greaterefficiency in the future.

The AEO process transforms conceptual

asset management to tangible competitiveedge. Each of the elements listed abovecontributes to the success of the AEOprocess. No matter how well defined anddocumented the maintenance strategy maybe, it can not achieve optimum effectivenessif badly implemented. Similarly goodcontrol and implementation practices arewasted if they are expended in performingthe wrong tasks. This article focuses on the"strategy" element of the process.

Figure 1. Maintenance Strategy Process - Principal

Inputs and Outputs.

Formulation of Maintenance

StrategyMaximizing the efficiency of a plant and itsmachine assets requires a shift from discretemaintenance activities to a strategic processthat is embraced company-wide [2]. A well-founded maintenance strategy will have asound financial and technical justification.This is derived from a proper understandingof all of the functions that the machine is



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required to perform, and their impact uponthe business in economic terms, and health /safety / environment (HSE) consequences.

Properly structured and implemented, a wellthought-out maintenance strategy transformsmaintenance from a cost to a profit center. Acomprehensive maintenance strategy islikely to employ a number of techniquesworking to support each other. Choosing theright technology tool, or combination oftools, requires a sound understanding of theplant and the relative data that may or maynot be available. A well definedmaintenance strategy process will:

Identify what is truly needed (i.e.justified commercially and technically)

Create a maintenance strategy that mosteffectively combines the righttechnologies to meet those needs.

In deriving maintenance strategy for amachine a number of steps should befollowed.

Identify SystemFirst stage is the unambiguous definition ofthe system under study. In some cases the"system" may be a process line or plant,comprising a number of individual plantitems. In other cases the study may berestricted to an individual plant item, or to aspecified group of items.

It is important to clearly define the boundaryof the item under study.

For example some might decide that a fanshould be considered to comprise the fanitself, plus its coupling (or belt drive) anddrive motor. Others may decide to considerthe drive motor as a separate item. Cleardefinitions are important to avoidduplications of effort, or more importantlythe possibility of important componentsbeing overlooked. In the case of a fan being

studied independently of its drive motor, forexample, is the coupling (or belt drive)considered to be part of the fan, or part ofthe motor / drive system?

The manner in which such boundaries aredefined should be consistent with thestructure of the asset register that willtypically identify individual plant items andthe various sub-components that theycomprise [3].

Identify Functions

The second step is to properly understand allof the functions that the subject machine is

intended to fulfill.

Primary Functions

In the case of a fan these will typicallyinclude:

Ventilation or pressurization of enclosedmodules.

Air circulation / dilution in open processareas.

Secondary Functions

It is important that secondary functionsserved by a machine (sometimes calledhidden or tacit functions) are not ignoredwhen devising the maintenance regime.

In the case of a fan these might include (forexample) forming an integral part of thecontainment boundary for a corrosive ortoxic gas or vapor. In such a case adeveloping fault (leak) on the fan casingmight have little impact upon the primaryfunction, but the failure to perform thesecondary function might well haveprofound consequences.



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Figure 2 A typical maintenance strategy process

Failure Modes & Effects Analysis(FMEA)

The FMEA process identifies failures,effects, and risks within a product orprocess, and then eliminates or reduces them

[4]. The process begins by investigating theways a product or process can fail, or aproduct or processfailure modes. Eachfailure mode has a potential effect, and someare more likely to occur than others.



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The relative risk of a failure and its effectsare determined by three factors:

1. Severity: The consequence of the

failure, should it occur.2. Occurrence: The probability or

frequency of the failure.

3. Detection: The probability of detecting afailure before its impact is realized.

Each potential failure mode and effect israted on a scale of 1 to 10 (low to high). Bymultiplying the rating for the three factors, aRisk Priority NumberorRPNis determined.

The RPN can range between 1 and 1000,and is used to rank the need for corrective orpreventive action. Once actions are taken, anew RPN can be determined by re-evaluating the severity, occurrence, anddetection ratings.

The FMEA process consists of these 10steps:

1. Define the system and sub-systems.

2. Define/Analyze all system functions

3. List potential failure modes.

4. List potential effects for each failuremode.

5. List the failure causes

6. List the current controls

7. Express Occurrence, Severity, andDetection of failure modes.

8. Calculate the risk priority number.

9. Take action to eliminate or reduce high-risk priority number failure modes.

Implement and review the results

FMEA Worksheet

A FMEA worksheet starts with generalinformation regarding:

Product (product FMEA), process(process FMEA), or manufacturing

equipment / component (maintenanceFMEA)

FMEA team and team leader

FMEA number

FMEA date (original and revised)

Page number(s)

The main part of the FMEA worksheet isdepicted in Figure 3 below. The item

(product, process, or equipment), failuremodes, effects, and causes are investigatedfirst. The FMEA worksheet table is oftenextended with action results: Actions taken,Severity, Occurrence, Detection, and RPNcalculation. Then, current actions andcontrols are listed. Severity, occurrence, anddetection ratings are assigned, whichdetermine the RPN.

An example of the failure modes listing for atypical fan is given at Figure 4 below.

Item orfunction

FailureMode

FailureEffect

S Potential FailureCause /Mechanism

O Currentcontrols orpredictivemethods

D RPN(=SxOxD)

Recommendedactions

Responsibleperson+ date

Figure 3 Typical FMEA Worksheet headers.



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Figure 4 Failure Modes for a typical fan [From the

Asset Knowledge Science (AKS) in SKF @ptitude

View Decision Support System.

Criticality Analysis

This part of the exercise concerns itself withthe consequencesof loss of function. Thevarious ways in which function may be lostare referred to as "failure modes".

Criticality assessment may take a number offorms ranging from a quantitativeassessment of risks in financial terms, to asimpler quantitative assessment such as that

depicted at Figure 5 in appendix A of thisdocument [5]. Examples of Critical Criteriainclude:

Personnel injury

Reduction in load/output > X units

Delay in startup > X hours

Unit trip

Violation of regulatory requirements

Equipment damage > $X

Another commonly used method to evaluatevarious criteria is construction of a matrixsuch as that shown in figure 4 below. Eachfailure mode is evaluated against the three"impact" columns and the "worst case"determines the criticality level.

Maintenance Strategy Methods

A number of tools are available thatincorporate the various steps outlined above

into a formal, documented process [6].These include:

RCM Reliability-Centered Maintenance

This is often recommended for equipmentwhere criticality is high and confidence inexisting maintenance is low [7]. The role ofRCM in most cases is to provide adefensible base for specific equipmentwhere there is limited knowledge andhistory. While RCM can be quite effectivefor reliability improvements in specificareas, in general it is inappropriate for therest of the plant. This aspect of RCM has ledto the development of other similarmaintenance techniques, which are moreappropriate on a plant-wide basis.


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IMPACTCRITICALITY

Health / Safety Financial Environmental

Extremely High S-1 Fatalities

Long-term healthimpact on public

F-1Greater than 1M$

E-1

Major plant-wideemergency response

Major clean-up for months /years

Potential widespread, long-term significant adverseeffects on the environment(including soil andgroundwater) or a smallcommunity.

High S-2 Disabling Injury

Lost-time injury

F-20.5M$ to 1M$

E-2

Unit emergency response

Significant clean-up forweeks / months

Potential localized,medium-term significantadverse effects on theenvironment (including soiland groundwater) or a smallcommunity.

Medium S-3 Medical aid

injury

Restricted work

F-3100K$ to 0.5M$

E-3

Local emergency response

Cleanup for days / weeks

Potential short-term minoradverse effects on theenvironment (including soiland groundwater) or a fewmembers of the public

Low S-4 First aid

F-4Less than 100K$

E-4

Confined to close proximity

Inconsequential or noadverse effects

Figure 5 Example of a criticality matrix

SRCM Streamlined Reliability-Centered Maintenance

This enhanced version of RCM focuseson the dominant failure modes ofequipment and the significant effects ofthose failures such as production losses,personnel safety, environmental releases,etc. A key benefit of an SRCM programis developing the understanding of plantpersonnel on the value of a modern-

based maintenance strategy. As such,SRCM is particularly appropriate wherea major change in the maintenanceculture within the enterprise needs to be

made, which also makes it a good optionfor new builds [8].

RBM Risk-Based Maintenance

This is a financially based analysistechnique, which focuses on establishingthe relative worth of maintenance. It wasoriginally developed as a means of


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AoM04002_Fan_S


Function Failure Mode Failure Effect SPotential failure Cause /Mechanism O

CurrentControls D

Maintain pressure / flow withinprescribed limits Fails to Run Electrical:

Power switched off

Fuses Blown

Drive System

Broken Belts / Pulleys

Coupling Failure

Fan

Impeller fouled onhousing

Total In-service Failure Electrical:

Overload protectortripped

Fuses Blown

Drive System

Broken Belts / Pulleys

Coupling Failure

Fan

Shaft failure


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AoM04002_Fan_S


CurrentControls D

Maintain pressure / flow withinprescribed limits (Ctd)

Total In-service Failure(Ctd) Bearing failure

Lubrication fault

Misalignment

Unbalance

Looseness

Reduced flow / Pressure Ducting

Closed dampers

Duct blockage

Leaks

Filters

Dirty / clogged

Incorrect filters fitted



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AoM04002_Fan_S


CurrentControls D

Maintain pressure / flow withinprescribed limits (Ctd) Reduced flow / Pressure Fan

Speed too low

Inlet vanes set wrong

Blocked inlet

Dirty / distorted impeller

Improper runningclearance

Running backwards

Excessive flow /Pressure Ducting

Dampers set wrong

Grills not installed

Access door open

Filters

Filters not in place

Fan

Speed too high

Inlet vanes set wrong



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Maintenance strategy templates used byconsultants may include figures for:

The various failure mechanisms thatrelate to the failure mode are thenconsidered. Against each mechanismis listed the likelihood of occurrence

(denoted by the "O" number) andany existing controls or practices thatare in place to minimize the risk,either in terms of reducing itsprobability or its effect). The numberin the "D" column represents thedegree of mitigation afforded by thecontrol.

Mean Time Between Failure

(MTBF)

Mean Time to Prepare (MTTP)

Mean Time to Repair (MTTR)

This information is of value in accurateassessment of failure effects andprobabilities. However, such data isusually reserved by them for use inclient studies, and is not typically madeavailable for general publication.

Maintenance strategy templates willtypically identify the various failuremodes to which a machine may besubject, and also suggest appropriatepreventive / predictive measures foreach, based on the consultanciesexperience in adoption of "bestpractices" across a range of clientorganizations.

The Risk Priority Number (RPN) isthen calculated as the product of thethree numbers, S x O x D. Thisnumber gives an indication of therelative risk posed by each of thepotential failure mechanisms, andthereby prioritizes the need forappropriate maintenance response.

The "Recommended controls"column represents the output fromthe study. It lists the appropriatemaintenance activities for themachine, which have thus been

identified as being technically andfinancially justified.

Generic templates may of course bemodified to suit more specific machineconfigurations, derivatives andapplications The revised data sodeveloped may then be itself used as atemplate for other similar machines /applications.

Generic Templates

Some maintenance consultancies havedeveloped, through long and wideexperience of dealing with many clientorganizations, their own libraries oftemplates which include not only suchgeneric information, but also morespecific templates relating to (for

example) specific machine types, fittedwith individual ancillary equipment, andconfigured for a particular application.They thus provide a way for theconsultant to record and re-use hisexperience and successes.

Output from the StrategyProcess

A "strategy" is a plan to achieve a goal,and the term can also be applied to the

process by means of which that plan isderived. So thestrategyis the outputfrom the strategyprocess.

This strategy (i.e. output) is essentially alist of maintenance tasks that can beshown to have a sound technical andfinancial justification.



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As such it provides a primary input tothe "Work Identification" element of theAEO process. This output can beconsidered to comprise two streams.

Some of the tasks will likely bepreventive in nature, and some portionof these will inherently be high priorityactivities (e.g. Regulatory compliance orSafety critical tasks). These "pre-defined" priorities feed directly to thework order generation system in thework control process.

The remaining PM tasks comprise the

main input stream to the workidentification process, along with othertasks (typically those relating to PdM orODR activities) that entail a degree ofdata collection and analysis which mustfirst be undertaken in order to identifyspecific work order requests.

Suggested Further Reading

[1] Toomey G, "Asset EfficiencyOptimization Work Management

Process" Document GS02010http://www.aptitudexchange.com

[2] Reed W, " Introduction to AssetEfficiency Optimization" DocumentMB04003http://www.aptitudexchange.com

[3] Barratt M "Asset Register"Document MB03011http://www.aptitudexchange.com

[4] Schram G, "FMEA, An Introductionto Failure Mode and Effects Analysis"Document GS02002http://www.aptitudexchange.com

[5] Barratt M & Schram G, "CriticalityAnalysis In Perspective" Document

MB02029http://www.aptitudexchange.com

[6] Barratt M, " Maintenance Strategy

Review (MSR) Methods" DocumentMB03006http://www.aptitudexchange.com

[7] Barratt M, " Reliability CenteredMaintenance" Document MB02008http://www.aptitudexchange.com

[8] Creecy Michael E, "SRCMR

Methodology" Document MB03017http://www.aptitudexchange.com

[9] Barratt M, "Risk BasedMaintenance", Document MB02017http://www.aptitudexchange.com
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