The optimal time to make decisions and take actions to minimize potential failures of equipment, facilities and components is during their design. This two-part blog post provides an overview of the types of issues involved and specific actions that can be useful to the design engineer. Archived blog posts (at www.dmme-engineering.com) that discuss specifics are provided parenthetically.
The choices made during design work directly affect the later reliability and life-cycle costs that will result. Once the design is set and service life commences often there are constraints, e.g., physical, economic and political, that prevent the engineer or organization from implementing the best improvements. This summary seeks to minimize some lost opportunities.
An important "thinking tool" to beneficially use during the initial design (or significant modification) of equipment or facilities is the Failure Mode & Effects Analysis (FMEA) procedure (July, 2013). In this procedure the engineer first attempts to identify all the ways that her design might fail, due to the most likely failure mechanisms, based on service parameters in the given application. He or she then assigns approximate probabilities to each method of failure AND assigns a relative level of consequence of each failure mode in the overall operation of the system or manufacturing process. A simple product of these estimates defines a rank ordering number for the different failure mechanisms by their importance to overall success. Design time and costs of control measures can then be most efficiently expended - usually via iterations of the process. The rigor of the thought process used during FMEA determines its ultimate value but when done well it can be very useful.
Clearly there is a wide range of failure mechanisms that engineers need to evaluate for the varieties of equipment, manufacturing processes and materials plus the specific service conditions encountered with each. This discussion will be confined to metallic material failures due to one of the several forms of corrosion and common mechanical failure modes of metals. Further, these ideas apply primarily to continuously operating equipment in plants that require 24/7 reliability for economic viability. Examples include equipment used in chemical processing, petrochemical processing, electric power generation, pulp & paper processing and automated manufacturing plants. Specific recommendations follow:
- Fabrication and assembly details must be designed so as to provide for complete drainage of contained process liquids to resist all forms of corrosion. Too often residual liquid left in equipment during maintenance shutdowns are locations where corrosion begins.
- Analyze the given application to define potential process upset conditions - not just normal conditions - and make provisions for these to resist both corrosion and mechanical failures.
- Define the effects of combined mechanical stresses, i.e., tensile, compressive, shear and static versus cyclic stress, and not just one type to resist SCC and mechanical failures. Eliminate or minimize stress concentration points.
- Determine if harmful residual stresses have been imparted to the finished materials and make provisions to remove or lessen them to minimize possible in-service failures by SCC, hydrogen embrittlement plus mechanical or corrosion fatigue (July, 2011).
- If use of contacting dissimilar metals is unavoidable, minimize galvanic corrosion by selecting metals that are close together in a "standard" galvanic series but realize the shortcomings of that approach (January,2014). Also assess the effect of the ratio of wetted areas of the two dissimilar metals in your application.
- Recognize the several forms of crevices that can be generated by certain design details and take actions to modify these details so as to avoid crevice corrosion (August, 2011).
- Acknowledge the susceptibility to SCC of specific alloys and corrosive media combinations in conjunction with tensile stresses (March, 2012). Identify dangerous combinations and use non-susceptible alloys for the media in your application.
- Cite the specific, desired standard specification, e.g., from ASTM, ASME, NACE, AWS, etc., in written specifications, on drawings and purchase orders. Never use vague descriptions such as "stainless steel" or "welded" in vital documents.
- Consider if a less corrosion resistant and cheaper alloy can be used along with a classic corrosion control method such as some type of organic or metallic coating or cathodic protection or chemical corrosion inhibitors instead of an expensive, corrosion resistant alloy used alone (August, 2012).
- Determine if a non-metallic solid or liner material can be used in the given application to avoid electrochemical corrosion of metals.
- If a form of mechanical wear or erosion-corrosion is likely in the given application be knowledgeable about best, resistant material selections and other design actions to combat these failure processes (January, 2012).
- Recommend using an independent engineer, either in-house from another group or an outside consultant, to review and critique the design for effective and economic resistance to anticipated failure modes in critical applications. This provides a practical check on the design by an objective, fresh mind and new set of eyes.
- Often it is justifiable to complete discounted, life-cycle cost analyses of competitive design approaches to preventing failures. The results can be most useful in convincing management that minimum initial cost is usually not the best way to assure best economic results for the typical multi-year engineering project (February, 2013).
Part 2 will provide more tactics the design engineer can evaluate to minimize in-service failures for his or her specific application.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
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