FMEA Example

To further understand the use of the FMEA, the following example of an overhead bridge crane will be evaluated. It is again noted that this analysis, as with other sample analyses discussed in this text, is provided only in an effort to demonstrate the utility of a specific system safety analysis tool. It is therefore superficial in presentation and will examine only a select few of the many possible failure modes associated with the overhead bridge crane system described here. 

This example will develop a hardware FMEA for a proposed system that is well into the design phase of the product life cycle. For informational purposes, it is assumed that a preliminary hazard analysis was previously performed during the early stages of the design phase of this system. The information from the PHA will be used to assist in the development of the hardware FMEA. It should also be noted that the nature of a FMEA requires evaluation of subsystems, subassemblies, and/or components. For this reason, more detailed and specific descriptive information is provided here than that supplied for previous examples discussed in this text. 

Component Electric Hoist Motors and Controls. Both hoists have enclosed, nonventilated, 220/440-V, three-phase, 60-Hz, 1200-rpm (rev/min) motors. The control used on hoist motions is referred to as the static stepless magnetorque control on the manufacturer s drawings. With this control, drive motor torque is controlled by means of fixed resistors and saturable reactors in the 

Subsystem Motor-Driven Power Wheel. The motor driven power wheel is mounted on the bridge assembly and functions as a caretaker of the electric cable. It provides automatic operation as it rolls up, or releases, the cable as required by crane travel. It is powered by a dripproof, brake-equipped AC motor protected by a limit switch. 

Subsystem Trolley Drive Assembly. The trolley drive assembly moves the hoist assembly laterally across the 14 ft span between the runway rails. This is accomplished by applying torque through the trolley drive gear case to the trolley wheels. The assembly consists of a 5-horsepower, 220/440-V, three-phase, 60-Hz enclosed motor, driving through the trolley drive gear reducer assembly. The motor is reversible and therefore able to drive the trolley in either direction. An electrically released, spring-set motor brake provides frictional torque to brake and hold the trolley. Contact limit switches are provided for trolley travel in either direction. 

The 10 ton (main) hoist assembly contains two main hoist motor brakes, a 30 horse power main hoist AC motor, the main hoist magnetorque load brake, main hoist gear reduction assembly, and the wire rope drum assembly. The 1.5 ton (auxiliary) hoist assembly contains the same components as the 10 ton. However, the auxiliary hoist motor is rated at 18 horse power and a single auxiliary hoist brake is provided. 

Component Hoist Gear Reduction Assembly and Wire Rope Drum The 10 ton 

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Examples of analysis and results obtained by the FMEA Example 1 failure mode bi execution does not meet time limit ... 

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. 

Design teehniques (for example, FMEA or Fault Tree Analysis (FTA))... 

While 30 ppm may be acceptable as a maximum probability of occurrence for a failure of low severity, it is not acceptable as severity increases. An example table of FMEA Severity Ratings was shown in Figure 2.20. In the definite return to manufacturer (a warranty return) or violation of statutory requirement region (S = 5 or S = 6), the designer would seek ways to enhance the process capability or else utilize some inspection or test process. Reducing d will reduce occurrence, as indicated by equation 2.11, but inspection or test is of limited efficiency. 

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. 

For example, the characteristic dimension A on the cover support leg was critical to the success of the automated assembly process, the potential failure mode being a major disruption to the production line. An FMEA Severity Rating (S) = 8 is allocated. See a Process FMEA Severity Ratings table as provided in Chrysler Corporation et al. (1995) for guidance on process orientated failures. The component cost, Pc = 5.93 and the number planned to be produced per annum, N = 50000. 

The so-called Q7 tools and techniques, Cause and Effect Diagrams, Pareto Analysis, etc. (Bicheno, 1994 Dale and McQuater, 1998 Straker, 1995), are applicable to any stage of the product development process. Indeed they support the working of some of the techniques mentioned, for example using a Pareto chart for prioritizing the potential risks in terms of the RPN index for a design as determined in FMEA (see Appendix III). 

QFD should be applied at the start of produet development to help understand and quantify eustomer requirements and support the definition of produet requirements, giving an overall pieture of the requirements definition throughout the produet development proeess. Other tools and teehniques ean be used in eonjunetion with it, for example Pareto ehart, histogram, or data eoming out of the FMEA ean be fed baek at an early stage. 

The FMEA is a methodieal study of eomponent failures. This review starts with a diagram of the operations, and ineludes all eomponents that eould fail and eoneeivably affeet the safety of the operation. Typieal examples of eomponents that fail are instrument transmitters, eon-trollers, valves, pumps, and rotometers. These eomponents are listed on a data tabulation sheet and individually analyzed for the following ... 

What-if is a creative, brainstorming examination of a process or operation conducted by knowledgeable individuals asking questions. It is not as structured as, for example, RA/OP or FMEA. It requires the analysts to adapt the basic concept to the specific application. 

An FMEA is a qualitative, systematic table of equipment, failure modes, and their effects. For each item of equipment, the failure modes and root causes for that failure are identified along with a worst-case estimate of the consequences, the method of detecting the failure and mi "ation ofits effects. Tables 3.3.5-2 and 3.3.5-3 present partial examples ofFMEAs addressing the Cuoling Tower Chlorination System, and the Dock 8 HF Supply System. 

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. 

Table 3.4.4-2 Example Format for an Auxiliary Feedwater System Interaction FMEA ...

Existing AIAG, ANFIA, FIEV, and VDA manuals are listed in a bibliography to ISQ/TS 16949 and form part of the requirements to the extent specified in specific clauses. For example, suppliers to Ford, Ghrysler, and General Motors will be required to apply the APQP Manual, FMEA Manual, etc. 

In applying FMEA, a mechanical flow diagram must first be developed.. As an example, consider the check valve on a liquid dump line. It can fail... 

Assemble the components into the process system and apply FMEA techniques to determine if protection devices on some components provide redundant protection to other components. For example, if there are two separators in series, and they are both designed for the same pressure, the devices protecting one from overpressure will also protect the other. Therefore, there may be no need for two sets of high pressure sensors. 

Once the FMEA is completed, the specific system is analyzed to determine if all the devices are indeed needed. For example, if it is not possible for the process to overpressure the vessel, these devices are not required. If it is impossible to heat the vessel to a high enough level to effect its maximum working pressure, the TSH can be eliminated. 

For example, a traditional checklist is, by definition, based on the process experience the author accumulates from various sources. The checklist is likely to provide incomplete insights into the design, procedural, and operating features necessary for a safe process. The what-if part of the analysis uses a team s creativity and experience to brainstorm potential accident scenarios. However, because the what-if analysis method is usually not as detailed, systematic, or thorough as some of the more regimented approaches (e.g., HAZOP study, FMEA), use of a checklist permits the PrHA team to fill in any gaps in their thought process. 

Partial FMEAs for the example processes described in Section 4.0 are shown in Tables 4.22 and 4.23. 

The PHA procedure can be conducted using various methodologies. For example, the checklist analysis discussed earlier is an effective methodology. In addition, Pareto analysis, relative ranking, pre-removal risk assessment (PRRA), change analysis, failure mode and effects analysis (FMEA), fault tree analysis, event tree analysis, event and CF charting, PrHA, what-if analysis, and HAZOP can be used in conducting the PHA. 

Other examples of inductive tools that have limited application in incident investigation include failure mode and effects analysis (FMEA), hazard and operability study (HAZOP), and event tree analysis (ETA). These are detailed in the CCPS book, Guidelines for Hazard Evaluation Procedures... 

In this first case, system security is associated with preventing the accidental or intentional alteration and corruption of the data to be displayed on the screen, or be used to make a decision to control the operation. To avoid accidental or intentional loss of data, the data collected must be defined, along with the procedures used to collect it, and the means to verily its integrity, accuracy, reliability, and consistency. A failure modes-and-effects analysis (FMEA) is one of many methods used to uncover and solve these factors. For example, to avoid data corruption, an ongoing verification program (Chapter 18) should be implemented. 

If the verification confirms that the analyzed failure mode has no negative effects on system safety, the failure mode can be accepted. In the opposite case however we know that the failure mode, if actually occurs, can affect the system safety properties. In such case FDR can provide example event scenarios that lead to a contradiction of safety. Those scenarios can then be very helpful while considering possible redesign of the component objects. The results of the failure mode injection campaign are collected in the OF-FMEA tables (see Table 1). 

Determine the critical control points (base investigation on FMEA or other hazard analysis technique). Examples would be ... 

FMEA is best applied by a multidisciphnary team reviewing the design of a computer system. As with CHAZOPs, team members are selected for their particular knowledge of the production process, the computer system, and the software. The team steps through various computer system failures, considering their effect, risk, and how they might be controlled. The outcome of a FMEA is then documented in a template such as the example given in Table 8.3. 

There are many methods of control to eliminate or mitigate identified hazards. The selected controls for a hazard should be recorded on the FMEA template. Examples of hazard controls include change in design specification, implementing alarms/wamings/error messages, and instituting a manual process. The most cost-elfective hazard coutrol should be selected and described in the FMEiA template. 

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