Reliability Centered Design

by Ramesh Gulati, CMRP

Designing for Reliability and Maintenance

Bruce Dean, CMRP
Manager, Design Engineering
Ramesh C. Gulati, PE, CMRP
Manager, Asset Mgmt. & Reliability Planning
AEDC / ATA
Arnold Air Force Base, TN 37389

 

Abstract

One of the key factors in asset/system performance is its reliability- “inherent reliability” or designed in reliability. Are we designing the system with reliability and maintenance in mind? The O&M cost, which is typically about 80% of the total life cycle cost of the system, becomes fixed –whether intentionally or not- during the early design phase. Are we specifying the reliability and maintenance needs in our requirements document before the design phase begins? The reliability & maintainability are design attributes that should be designed into the assets to minimize maintenance needs by using reliable components, simpler replacements and easier inspections.

Introduction

Having high reliability of assets /plants is essential to the success of any organization, particularly with respect to its overall operations and maintenance cost. Understanding reliability and maintenance and how they’re interrelated is important.

Reliability focuses on the ability of an asset to perform its intended function to support 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.

Maintenance is an act of maintaining, or the work of keeping the asset in proper –operational condition. Maintenance is a “field” action and may consist of performing maintenance inspection and repair to keep assets operating in a safe manner to produce or provide the designed capabilities. So maintenance is to keep assets in an acceptable working condition, to prevent them from failing, and if they fail, bring them back to their operational level effectively and as quickly as needed.

Reliability should be “designed in” and is a strategic task. Maintenance is to keep assets functioning and is a tactical task. The reliability & maintainability should be designed into the asset to minimize maintenance needs. It could be accomplished by using reliable components that are easier to inspect and replace.

It has been found that the Operations & Maintenance (O&M) costs are about 80 % or more of the total life cycle cost of an asset. It is obviously important that there is need to minimize O&M costs. Also, a majority of the O&M costs get fixed during the design and development phase of the asset. Therefore it is highly important that during the design and building of the assets, we adequately address reliability, maintainability, and safety issues to reduce overall life cycle cost of the assets.

Examples of Unreliability & Maintainability:

How somebody is going to perform maintenance on this
equipment?

How long it will take to replace a leaky hose
on this equipment?

It will be a challenge to replace pump or motor
on this unit.

Maintenance had to cut a hole to make an
opening to reach hidden (broken) component.

Asset Performance and Designed Reliability

The objective of maintenance & reliability in an organization is to ensure that the assets are available, when needed, in a cost effective manner. The performance of an asset is based on three factors,

Asset Performance

Figure 1 Asset Performance “A Three-legged stool”: Reliability, Operating Environment, & Maintenance

Usually assets are designed with a certain level of reliability built-in based on an effective use of reliable components and their configurations. This level of reliability is called inherent reliability. We can’t change or improve reliability of an asset without redesigning or replacing its components with improved components. The second factor is considering the operating environment to ensure that operating conditions are considered during the design and also in the development of maintenance plan. The operators need to be trained with appropriate skills to operate the assets effectively. Several studies have indicated that 40 percent or more of failures are the result of operational errors. The third factor, an effective maintenance plan, is needed to detect or find potential failures before they create unscheduled downtime. This also requires doing the necessary service type actions such as replacing filters or performing the necessary repairs when a failure occurs. This maintenance plan should sustain the reliability and help to improve asset availability.

Reliability, a design attribute, is a broad term that focuses on the ability of an asset to perform its intended function. Maintainability is another design attribute which goes hand in hand with reliability. It’s ease of maintenance. The objective of maintainability is to ensure that maintenance tasks can be performed easily and effectively.

There are a number of reasons why reliability & maintainability are important asset attributes, including:

A reliable plant means that the plant and its assets are available, as and when needed, to meet customer’s needs on schedule and at cost.

Asset Life Cycle Cost:

Life cycle costs (LCC) are all the costs expected during the life of an asset. It refers to all the costs associated with acquisition and the ownership, specifically operations and maintenance, of the asset over its full life, including disposal. Fig. 6.14 shows a typical asset life cycle chart. The total cost during the life of an asset includes:

Based on several studies reported, the distribution of estimated LCC is:

Figure 2 LCC- Commitment and Expenditure of an Assets Life Cycle

It has been found that the operations and maintenance (O&M) cost is about 80 percent of the total life cycle cost of the asset. It is obviously important that we need to minimize O&M costs. As shown in the chart, the major portion of the O&M cost are fixed during the early design and development phase of the asset. There are ample opportunities to reduce the LCC during the design, building and installation of the assets.

Reliability requirements / 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 the asset has to operate, including mission time, usage limitations, operating environment, etc. In many instances, this will require a detailed description of how the asset is expected to perform. Use of a single metric, such as MTBF, as the sole reliability metric is inadequate. Even less effective is the specification that an asset will be “no worse” than the existing or previous model. An ambiguous reliability specification leaves a great deal of room for error, and this can result in a poorly-understood and unreliable asset in the field.

Financial concerns will definitely 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? It’s a great goal but, can we justify in achieving zero failures. A proper balance of financial goals and realistic asset reliability performance expectations are necessary to develop a detailed and balanced reliability specification.

Key elements of reliability specifications:

Designing for Reliability:

It has been found that as much as 60 % of failures and safety issues that can be prevented by making changes in design. We need to ensure that assets are:

The following analyses are recommended to be performed during design phase, from conceptual design to final design.

The following design reviews check list is recommended to be used as guide to ensure design is adequately addressing reliability, maintainability, and safety issues;

Design Reviews Checklist:

Summary

Assets should be designed so that they can be operated and maintained easily with minimum operations & maintenance needs. Reliability and maintainability are the design attributes and should be designed in, rather than added later.

To have reliable and easy to maintain assets, we need to ensure that asset owners, including operators are involved in developing the requirements as well as in reviewing the design. In designing for reliability and maintainability, attention should be focused to:

Maintenance cannot improve the inherent reliability. It is dictated by the design. Maintenance at best can sustain it. Get involved early in the concept phase of the design to ensure reliability; maintainability and safety are being addressed. Develop a maintenance plan for the asset using FMEA/RCM to mitigate failure modes which cannot be eliminated through design

Design your assets / systems for RELIABILITY and MAINTENANCE in mind!