Failure modes requirements

The purpose of the criticaUty rating is to provide guidance as to which failure modes require resolution. However, critical modes of failure resulting in unsafe operation should be given special attention, and design/verification actions should be taken to ensure that they never occur. 

The three scenarios are different with respect to the associated vapor cloud explosion risk. Investigations by Giesbrecht et al. have shown that highly destructive explosion pressures may occur in the first scenario. Because of the exceptional failure mode required and because of special conditions to be fulfilled for optimal ignition, the probability of this event is low. 

Failure Mode. The failure mode identifies how the component/subsystem can fail to perform each required function. A function may have more than one failure mode. 

Process Hazards Analysis. Analysis of processes for unrecogni2ed or inadequately controUed ha2ards (see Hazard analysis and risk assessment) is required by OSHA (36). The principal methods of analysis, in an approximate ascending order of intensity, are what-if checklist failure modes and effects ha2ard and operabiHty (HAZOP) and fault-tree analysis. Other complementary methods include human error prediction and cost/benefit analysis. The HAZOP method is the most popular as of 1995 because it can be used to identify ha2ards, pinpoint their causes and consequences, and disclose the need for protective systems. Fault-tree analysis is the method to be used if a quantitative evaluation of operational safety is needed to justify the implementation of process improvements. 

Eault tree analysis (ETA) is a widely used computer-aided tool for plant and process safety analysis (69). One of the primary strengths of the method is the systematic, logical development of the many contributing factors that might result ia an accident. This type of analysis requires that the analyst have a complete understanding of the system and plant operations and the various equipment failure modes. 

The cost of performing the hazard identification step depends on the size of the problem and the specific techniques used. Techniques such as brainstorming, what-if analyses, or checklists tend to be less expensive than other more structured methods. Hazard and operability (HAZOP) analyses and failure modes and effects analyses (FMEAs) involve many people and tend to be more expensive. But, you can have greater confidence in the exhaustiveness of HAZOP and FMEA techniques—their rigorous approach helps ensure completeness. However, no technique can guarantee that all hazards or potential accidents have been identified. Figure 8 is an example of the hazards identified in a HAZOP study. Hazard identification can require from 10% to 25% of the total effort in a QRA study. 

The Conformability Matrix (see later for an example) primarily drives assessment of the variability effeets. The Conformability Matrix requires the deelaration of FMEA Severity Ratings and deseriptions of the likely failure mode(s). It is helpful in this respeet to have the results from a design FMEA for the produet. 

The link with FMEA brings into play the additional dimension of potential variability into the assessment of the failure modes and the effects on the customer. The Conformability Matrix also highlights those bought-in components and/or assemblies that have been analysed and found to have conformance problems and require further communication with the supplier. This will ultimately improve the supplier development process by highlighting problems up front. 

Before setting about the task of developing such a model, the product development process requires definition along with an indication of its key stages, this is so the appropriate tools and techniques can be applied (Booker et al., 1997). In the approach presented here in Figure 5.11, the product development phases are activities generally defined in the automotive industry (Clark and Fujimoto, 1991). QFD Phase 1 is used to understand and quantify the importance of customer needs and requirements, and to support the definition of product and process requirements. The FMEA process is used to explore any potential failure modes, their likely Occurrence, Severity and Detectability. DFA/DFM techniques are used to minimize part count, facilitate ease of assembly and project component manufacturing and assembly costs, and are primarily aimed at cost reduction. 

Proteetion of the load and of the power supply from failures in the load should be an important eonsideration within all switehing power supply designs. It is important to know what failures are likely to oeeur within the switehing power supply and the load. An exereise that is frequently required in military designs is what is ealled a FMEA (failure modes and effeets analysis), where eaeh eom-ponent is hypothetieally assumed to fail open-eireuited and then short-eireuited. With sueh failures, how does eaeh failure affeet the other seetions of the eireuit This antieipation of failures ean make a power supply design robust. It is the responsibility of the power supply designer to provide proteetion to the load eireuitry from anomalies eneountered from the input line and failures within the supply and the load eireuitry. Often, proteetion sehemes ean be easeaded to provide redundant proteetion in the event of a failure in a proteetion eireuit. A fuse or a eireuit-breaker usually provides sueh a baek-up funetion. 

Fig. 1, Schematic of commonly u.sed methods for testing the strength of adhesive joints, (a) Peel test. Note that the peel angle can be changed depending on the test requirements, (b) Double overlap shear test. In this test, the failure is predominantly mode II. (c) Single overlap shear test. In this test the failure mode is mixture of mode I and mode II. (d) Blister test.

An important application of a rupture disc device is at the inlet of a pressure relief valve. The sizing of the pressure relief valve or rupture disc device combination requires that the pressure relief valve first be sized to meet the required relieving capacity. The normal size of the rupture disc device installed at the inlet of the pressure relief valve must be equal to or greater than the nominal size of the inlet connection of the valve to permit sufficient flow capacity and valve performance. The failure modes of rupture discs are [40] ... 

Clearly and uniquely specify the top event to the precise requirements without ( /er-specification which might exclude important failure modes. Even if constructed by difh ent analysts, the top event specification should produce trees that have the same Boolean equatio. ... 

A risk assessment analyses systems at two levels. The first level defines the functions the system must perform to respond successfully to an accident. The second level identifies the hardware for the systems use. The hardware identification (in the top event statement) describes minimum system operability and system boundaries (interfaces). Experience shows that the interfaces between a frontline system and its support systems are important to the system cs aluaiion and require a formal search to document the interactions. Such is facilitated by a failure modes and effect analysis (FMEA). Table S.4.4-2 is an example of an interaction FMEA for the interlace and support requirements for system operation. 

A failure modes and effects analysis delineates components, their interaction.s ith each other, and the effects of their failures on their system. A key element of fault tree analysis is the identification of related fault events that can contribute to the top event. For a quantitative evaluation, the failure modes must be clearly defined and related to a numerical database. Component failure modes should be realistically and consistently postulated within the context of system operational requirements and environmental factors. 

Initially, a system s hierarchy is identified for subsystems, sub-subsystems and so on to the components for which data must be found. The top event specifies system failure subsystems required for operation of the system in the mode specified are input to the top event s OR gate. Redundancy is represented by the redundant systems inputting an AND gate. This process of grouping subsystems under OR gates, if they can individually fail a function, or under AND gates if concurrent failures are necessary, is continued to the component or support system level until the tree is completed. This process grades the hierarchy from top to bottom, down the fault tree. 

Failure modes analysis Statistical process control Measurement systems analysis Employee motivation On-the-job training Efficiency will increase through common application of requirements for Continuous improvement in cost Continuous improvement in productivity Employee motivation On-the-job training... 

One of the procedures used to determine which sensors are needed to sense process conditions and protect the process is called a Failure Mode Effect Analysis—FMEA. Every device in the process is checked for its various modes of failure. A search is then made to assure that there is a redundancy that keeps an identified source or condition from developing for each potential failure mode. The degree of required redundancy depends on the severity of the source as previously described. Table 14-2 lists failure modes for various devices commonly used in production facilities. 

Two failure modes were considered in the study. The first is failure to start in addition to unplanned demands this includes both fast start tests and slow start tests. The failure to run mode includes all failures occurring from the time when load was applied to the DG until the diesel is no longer needed or until the end of the running duration required by technical specifications. 

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