PROCEDURES FOR PREVENTING FAILURES DURING DESIGN

Typical phases during design include: planning and requirements definition, concept design, detail design and test and validation. The iterative nature of design requires a con-tinuous analysis and redesign process as described in the previous chapter.

Regardless of the procedure used for developing the design, at the conclusion of the design process, the required quality of the product should be defined and then described in a technical document which should also include a definitive layout or a final design related to the manufacture of the product. Only the minimum quality needed for the product to perform the function intended should be specified. Overspecifying, including the addition of restrictive features in quality description, may lead to unnecessary delays and increased end-product costs [9].

The design process requires the development of operational definitions (R. Lo´pez, personal communication, 2000) to assist in communicating the intent of a design concept [10]. An example of an operational definition is a specific test of a material or judgment criteria. Without operational definitions, a specification is meaningless. For example, a casting specification containing the clause: ‘‘The casting shall be delivered reasonably clean’’ requires operational meaning of ‘‘reasonably clean’’ [10].

For this discussion, a design process is divided into PHASE I, which corresponds to the basic definition of the product (including concept and detail design), and PHASE II, which corresponds to the design review to prevent failures and minimize risks.

A. PHASE I: Basic Definition of Design

Figure 1 summarizes the scope of PHASE I which addresses the basic definition of the product. This phase begins with concept definition and ends with a basic description of design. During this phase, all the working loads shall be addressed. Loads, in general, refer to a combination of mechanical, thermal and chemical loads, although other load sources (electrical, magnetic, etc.) may also intervene and vary throughout the product life cycle.

As the first step, the type of material and its geometric configuration should be selected. The properties of the material, such as weldability, machinability, and formabil-ity must be suitable for the different manufacturing processes to be used. It is also impor-tant to consider the impact of these manufacturing processes and procedures on dimensional, microstructural and compositional variations of the material. The resulting product must be able to perform the required functions in the desired time. In this first phase concerning quality definition and preliminary description, legal regulations, design codes, safety and reliability aspects, maintenance and esthetical appeal requirements must be met while simultaneously meeting the technical requirements at the lowest cost.

As an example, consider a reactor typical of those used in a petrochemical plant which must operate in a certain environment, under certain pressure, at high temperatures during a specified design life. During this first phase of design, it can be established that the plate will be made of 2¹₄ Cr–1 Mo steel (ASTM SA-387 Grade 22) with calculated dimensions consistent for this steel with allowable design stresses and tolerances specified in the ASME code.

Figure 1 PHASE I, basic definition of design.

In this stage of basic definition, some potential failure modes have been taken into account. Design allowances for corrosion=oxidation resistance were incorporated by the particular corrosion and oxidation-resistant material selected. By calculating the corrosion allowance, considering the material properties exposed to the design temperature, and the design life, ductile failure associated with a plastic collapse of the component under service conditions is avoided. The brittle fracture mode, associated with the lowest temperatures of the metal that may be exposed to, may also be considered in the requirements set forth in the design code. This results in the basic specification of the product.

B. PHASE II: Design Review Oriented to Minimize Risks

Figure 2 shows the actions that are involved in PHASE II design review with a special reference on how to minimize risks. The purpose of this phase is to ensure that the basic design fulfills the requirements and review the design for potential failures. During PHASE II, the following questions are addressed:

Has product quality been properly defined to fulfill the user’s requirements?

Has product quality been properly described to order the product manu-facturing?

Has product quality been properly described to ensure the desired quality?

Has susceptibility to failures been minimized?

Which are the most critical aspects of design?

Is it possible to detect potential failures before the product is used?

What are the allowed risks?

Has the engineering design process been completed?

The following steps should be performed to review the design for failure prevention:

1. Perform a detailed review of the product design to ensure performance; at the time of loading, it will be subjected throughout its life cycle.

2. Determine design criteria, objectives and limitations used in the basic definition of design.

3. Verify the fulfillment of design criteria. These criteria involve combinations between the main properties of a product. The main criteria applied are presented in Sec. 5. During this step, basic types of failure modes have been properly controlled by design.

4. Determine failure modes during the product life cycle should be verified. Section 6 provides a list of potential failure modes and its consequent damage.

5. Determine the types of damages associated with each failure mode in order to implement methods for detection.

6. Determine the likelihood and severity of failure associated with each failure mode. The criteria used to assess susceptibility—described for each failure mode in Sec. 7—may be used as a guideline. Risk level is estimated in a qualitative manner.

7. Describe potential failure effects of the parts and their propagation over the system. Determine whether there are any failures that may affect the entire system, i.e., ‘‘single-point failures’’.

8. Prioritize risks by ranking critical points for the safety and reliability of the product.

9. Determine corrective actions to minimize risk for each failure mode making use of decision logic applied to the evaluation and reduction of risk, Fig. 3, and the criteria to minimize risk for each failure mode are presented in Sec. 7 of this chapter.

Figure 2 PHASE II, design review process used to minimize the occurrence of failures and its consequences.

Figure 3 Decision logic applied to the evaluation and reduction of risk.

10. Record the results and describe the required quality criterion in a detailed Technical Specification of the product being manufactured.

Returning to the example of the Cr–Mo reactor provided in Sec. 4, a design review may show that the operating temperature range may produce brittle fracture which may be associated with material degradation due to temper embrittlement. This type of embrittlement may reduce the useful service life and increased risk of a catastrophic col-lapse of the reactor, especially during a hydraulic test or during shutdown and start-up periods of equipment operation. The decision logic for corrective actions (see Fig. 3) shows that it is necessary to use a Cr–Mo type of steel because of its resistance to both creep and hydrogen attack in addition to cost. Therefore, these design criteria must be applied to minimize temper embrittlement risk.

Purchasing a material with a restricted chemical composition and using welding con-sumables capable of being exposed to operation temperatures and thermal treatments dur-ing the manufacturdur-ing process can also minimize risk. Step cooldur-ing tests which are intended to assess the material behavior over a long period of time at a high temperature by simulating the expected material degradation during operation. A similar analysis might identify the potential necessary for minimizing the hydrogen attack, or any other more specific failure mode.

The use of this second phase in design permits the verification of the occurrence of the various failure modes and the completion of the definition of quality which contri-butes to a proper specification for the product to minimize the different types of failure modes.

V. MATERIAL BEHAVIOR