2. El triunfo de la ópera italiana (1833-1852)
2.3 Cantantes nacionales para un repertorio internacional (1836-1839)
Life cycle costs (LCC) are all costs expected during the life of an asset. The term refers to all costs associated with acquisition and owner-ship, specifically operations and maintenance, of the asset over its full life, including disposal. Figure 6.15 shows a typical asset life cycle chart.
The total cost during the life of an asset includes:
• Acquisition Cost
• Design and Development
• Demonstration and Validation (mostly applicable to one-of-a-kind, unique systems)
• Build and Installation (including commissioning)
• Operations and Maintenance (O&M)
• Operating Cost (including energy and supplies)
• Maintenance Cost
• PM
• CM
• Disposal
Based on several studies reported, the distribution of estimated LCC is as follows:
For a Typical
DoD System* Industrial
Design and Development 10 – 20 % 5 – 10 %
Production / Fabrication /
Installation 20 – 30 % 10 – 20%
Operations and Maintenance (O&M) 50 – 70 % 65 – 85%
Disposal < 5 % < 5 %
*Assurance Technologies Principles and Practices by Raheja and Allocco
A graph showing the typical cost commitment and expenditures for the life of an asset is shown in Figure 6.16, as reported by Paul Barringer, a leading reliability expert. It is clear from the figure that the O&M cost is on average about 80% of the total life cycle cost of the asset. It is obvi-ously important that we need to minimize operations and maintenance (O&M) costs. As shown in the chart, the major portion of the O&M cost becomes fixed during early design and development phase of the asset.
There are ample opportunities to reduce the LCC during the design, build-ing, and installation of the asset.
Figures 6.15 Asset Life Cycle
Figure 6.16 Cost Commitment and Expenditures during an Asset Life Cycle
Assets should be designed so that they can be operated and main-tained easily with minimum operations and maintenance needs. As dis-cussed earlier in this chapter, reliability and maintainability are design attributes; they should be designed in, rather than added later.
To have reliable and easy-to-maintain assets, we need to insure that asset owners, including operators, are involved in developing the require-ments as well as in reviewing the final design. In designing for reliability and maintainability, attention must focus on:
• Reliability requirements and specifications
• Designing for reliability and maintainability
• Proper component selection and configuration to guarantee required reliability and availability
• Review design for maintainability
• Logistics support — maintenance plan and documentation to reduce MTTR
• Reducing the operations and maintenance costs Reliability Requirements and Specifications
In order to develop a reliable asset, there must be good reliability requirements and specifications. These specifications should address most, if not all, of the conditions in which the asset has to operate, includ-ing mission time, usage limitations, and operatinclud-ing environment. In many
instances, developing these specifications will require a detailed description of how the asset is expected to perform from a reliability perspective. Use of a single metric, such as MTBF, as the sole reliability metric is inadequate.
Even worse is the specification that an asset will be “no worse” than the existing or earlier model. An ambiguous reliability specification leaves a great deal of room for error, resulting in poorly-understood design require-ments and an unreliable asset in the field.
Of course, there may be situations in which an organization lacks the reliability background or history to properly define specifications for asset reliability. In these instances, an analysis of existing data from previous or similar assets may be necessary. If enough information exists to character-ize the reliability performance of a similar asset, it should be a relatively simple matter to transform this historical reliability data into specifications for the desired reliability performance of the new asset.
Indeed, the financial concerns will have to be taken into account when formulating reliability specifications. What reliability can we afford? How many failures can we live with? Do we need to have zero failures? Zero fail-ures is a great goal, but can we justify the cost in achieving it? A proper bal-ance of financial constraints and realistic asset reliability performbal-ance expectations are necessary to develop a detailed and balanced reliability specification.
Key Elements of Reliability Specifications
• Probability of successful performance
• Function (mission) to be performed
• Usage time (mission time)
• Operating conditions
• Environment
• Skill of operators / maintainers
An example of reliability requirements for an automotive system* con-sists of an engine, a starter motor, and a battery.
There shall be a 90% probability (of success) that the cranking speed is more than 85 rpm after 10 seconds of cranking (mission) at
— 20oF of (environment) for a period of 10 years or 100,000 miles (time). The reliability shall be demonstrated at 95% confidence.
*Assurance Technologies Principles and Practices by Raheja and Allocco
Let us take another example of a manufacturing cell / system that needs to produce xyz product, at a rate of ## / hour or day, at a quality level of Qx. The reliability-related requirements can be developed using operational data and some assumptions. A suggested approach is:
• Define operating environment / duty cycle, i.e.,
• 20 hours/day and 250 days/year or 5000 hours/year
• Expected number of failures < 5/yr (This is an assumption — what can we afford? Can we live with fewer than 5
failures/year?)
• Reliability and Maintainability requirements (based on above data and assumptions)
• MTBF = 5000/5 = 1000 hours; FR = 1/1000 = 0.001 failures/hr
• Estimated repair time or MTTR can be calculated based on the following assumptions
• 3 failures @ < 2 hours = 6 hours
• 1 failure @ < 10 hours = 10 hours
• 1 failure @ < 24 hours = 24 hours Therefore, the required MTTR = 40/5 = 8 hours
• Reliability and Availability requirements:
• Reliability for 20 hours/day operation
• R
20 = e –(0.001x20) = 98%
• Availability = MTBF / (MTBF+MTTR) = 1000/1008 = 99%
• Required (desired) operating costs
• 2 man-hour / hour of operation (currently is 3 man-hour / hour)
• Energy plus other utility cost
• 20% less than current (current usage is 2 MW plus other)
• Maintenance cost (preventive and corrective)
• 2% or less of Replacement Asset Value (currently 2.7%
increasing by 0.2% per year)
Based on the calculations and data above, we can specify the following requirements for this new system we are procuring.
• MTBF of 1000 hours or FR = 0.001 failures/hr
• MTTR of 8 hours
Or we can ask reliability of 98% for 20 hours of operations/day and availability of 99%. Similarly, we can specify that total operating cost and maintenance costs may not exceed some number or percent of system replacement value. However, these numbers should be validated by sys-tem designers/builder by performing an FMEA.
In addition, the requirements and specifications should include:
• Display of asset performance data — such as early warnings
• Current, Temperature, Pressure, etc.
• Other operating / asset condition data
• Diagnostic display — pinpointing problem areas
• Use of modular and standard components
• Use of redundant parts / components to increase reliability
• Minimize special tools — parts
• Operations and maintenance training material
• FMEA / RCM-based maintenance plan
• Maximum use of CBM technologies
• Basis of spares recommendations
• Life Cycle cost analysis
• O&M cost estimates Reliability Approach in Design
It has been found that as much as 60% of failures and safety issues can be prevented by making changes in design. Assets must be:
• Designed for fault tolerance
• Designed to fail safely
• Designed with early warning of the failure to the user
• Designed with a built-in diagnostic system to identify fault location
• Designed to eliminate all or critical failure modes cost effectively, if possible
The following analyses are recommended to be performed during the design phase — from conceptual design to final design.
• Reliability Analysis
• Lowers asset and system failures over the long term
• System reliability depends on robustness of design, as well as quality and reliability of its components
• Maintainability Analysis
• Minimizes downtime — reduces repair time
• Reduces maintenance costs
• System Safety and Hazard Analysis
• Identifies, eliminates, or reduces safety-related risks through out its life cycle
• Human Factors Engineering Analysis
• Prevents human-induced errors or mishaps
• Mitigates risks to humans due to interface errors
• Logistics Analysis
• Reduces field support cost resulting from poor quality, relia-bility, maintainarelia-bility, and safety
• Insures availability of all documentation, including PM plan, spares, and training needs
The following checklist is recommended as a guide to review the design and make sure that it adequately addresses reliability, maintainabil-ity, and safety issues.
Design Reviews Checklist
• Are reliability, maintainability, availability, and safety analysis performed?
• Is Failure Modes and Effects Analysis (FMEA) performed dur-ing the design — at preliminary design reviews (PDR) and criti-cal design reviews (CDR)?
• Can fault-free analysis be used to improve the design?
• Is fault-tolerant design considered?
• Are components interchangeabilities analyzed?
• Is modular design considered?
• Are redundancies considered to achieve desired reliability?
• Has the design been critiqued for human errors?
• Are designers familiar with the human engineering guidelines?
• Is Reliability-Centered Maintenance (RCM) considered in design?
• Is a throwaway type of design considered instead of repair (e.g., light bulbs)?
• Has built-in testing and diagnostics been considered?
• Are self-monitoring and self-checking desirable?
• Are components and assets easily accessible for repair?
• Are corrosion-related failures analyzed?
• Do components need corrosion protection?
• Is zero-failure design economically feasible?
• Is damage detection design needed?
• Is software reliability specified and considered in design?
• Is fault-isolation capability needed?
• Do electronic circuits have adequate clearances between them?
• Are software logic concerns independently reviewed?
• Has software coding been thoroughly reviewed?
• Is self-healing design feasible or required?
• Are redundancies considered for software?
• Are the switches for backup devices reliable? Do they need maintenance?
• Are protective devices such as fuses, sprinklers, and relief valves reliable?
• Does the asset need to withstand earthquakes and unusual loads?
If yes, are design changes adequate?
• Can manufacturing/fabrication or maintenance personnel intro-duce any defects? Can they be prevented by design?
• Can the operator introduce wrong inputs — wrong switching or overloads, etc.? If so, can the asset be designed to switch to a fail-safe mode?
• Can a single component cause the failure of a critical function?
If yes, can it be redesigned?
• Are there unusual environments not already considered? If haz-ardous material is being used, how will it be contained or han-dled safely?
• Is crack growth and damage tolerance analysis required?
• Are safety margins adequate?
• Are inspection provisions made for detecting cracks, damage, and flaws?
• Are production tests planned and reviewed?
• How will reliability be verified and/or validated?
6.5 Summary
Improving reliability is essential to the success of any organization, particularly to its operation and maintenance. Understanding reliability and maintenance and how they’re interrelated are the basis for reducing the life cycle costs of assets and plant.
Reliability focuses on the ability of an asset to perform its intended function of supporting manufacturing a product or providing a service.
Reliability terminates with a failure — i.e., when unreliability occurs.
Unreliability results in high cost to the organization. The high cost of unreliability motivates an engineering solution to control and reduce costs.
Maintenance is an act of maintaining, or the work of keeping the asset in proper operational condition. It may consist of performing main-tenance inspection and repair to keep assets operating in a safe manner to produce or provide designed capabilities. Thus, maintenance keeps assets in an acceptable working condition, prevents them from failing, and, if they fail, brings them back to their operational level effectively and as quickly as needed.
Reliability should be designed in. It is a strategic task. In contrast, maintenance keeps the asset functioning and is a tactical task. The relia-bility and maintainarelia-bility attributes are usually designed into the product or asset. These attributes minimize maintenance needs by using reliable components, simpler replacements, and easier inspections. Reliability is measured by MTBF, which is the inverse of failure rate. Maintainability
— the ease of maintenance — is measured by MTTR.
It has been found that the Operations and Maintenance (O&M) costs are about 80% or more of the total life cycle cost of an asset. It is impor-tant to minimize O&M costs. The majority of the O&M costs to be
incurred in the future are set during the design and development phase of the asset. Therefore, we must adequately address reliability, maintainabil-ity, and safety aspects of the system in order to reduce the overall life cycle cost of the assets during the design and building of the assets.
6.6 Self Assessment Questions
Q6.1 Define reliability and maintainability.
Q6.2 What’s the difference between maintenance and maintainability?
Q6.3 If an asset is operating at 70% reliability, what do we need to do to get 90% reliability? Assume assets will be required to operate for 100 hours.
Q6.4 If an asset has a failure rate of 0.001failures/hour, what would be the reliability for 100 hours of operations?
Q6.5 What would be the availability of an asset if its failure rate is 0.0001failures/hour and average repair time is 10 hours?
Q6.6 What would be the availability of a plant system if it is up for 100 hours and down for 10 hours?
Q6.7 If an asset’s MTBF is 1000 hours and MTTR is 10 hours, what would be its availability and reliability for 100 hours of operations?
Q6.8 Define availability. What strategies can be used to improve it?
Q6.9 What is the impact of O&M cost on the total life cycle cost of an asset?
Q6.10 What approaches could we apply during the design phase of an asset to improve its reliability ?
6.7 References and Suggested Reading Paul Barringer. www.Barringer1.com
Blanchard, B.S., D. Verma, and E.L. Peterson. Maintainability: A Key to Effective Serviceability and Maintenance Management. John Wiley and Sons, 1995.
Ebeling, C.E. An Introduction to Reliability and Maintainability Engineering. McGraw Hill, 1996.
Narayan, V. Effective Maintenance Management. Industrial Press, 2004.
O’Conner, Patrick D.T. Practical Reliability Engineering. John Wiley and Sons, 1985.
Raheja, Dev and Michael Allocco. Assurance Technologies Principles and Practices, 2nd edition. Wiley-Interscience, 2006