Mode Failures

The failure modes of lead-acid batteries depend on the battery applications and battery construction and design. Lead-acid batteries usually fail from positive plate degradation, which is caused by grid corrosion or paste shedding. Table 1.15 shows the common failure modes. 

Positive grid corrosion can be caused by the grid alloy, grid casting conditions, and active material composition. Shedding of positive active material can be caused by battery construction, active material structure, battery cycles, DOD, and charge 

Several years Water loss, grid corrosion, positive material 

350-600 cycles Positive material shedding, grid corrosion, 

Designing in such a way that failures result in known failure modes is one method of accommodating expected failures of systans or components. Failures should produce not only predictable failure modes but also failure modes that place the system in a safe state. The Requirements for Design require that the principle of fail-safe design be considered and incorporated as appropriate into the design of plant systems and components important to safety (Ref. [1], para. 5.40). 

Access to equipment in systems important to safety should be appropriately limited, in view of the need to prevent both unauthorized access and the possibiUty of error by authorized personnel. Effective methods include appropriate combinations of physical security (locked enclosures, locked rooms, alarms on panel doors) and administrative measures according to the degree of supervision in the area where the equipment is located. 

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The failure mode of an equipment item describes the reason for the failure, and is often determined by analysing what causes historic failures in the particular item. This is another good reason for keeping records of the performance of equipment. For example, if it is recognised that a pump typically fails due to worn bearings after 8,000 hours in operation, a maintenance strategy may be adopted which replaces the bearings after 7,000 hours if that pump is a critical item. If a spare pump is available as a back-up, then the policy may be to allow the pump to run to failure, but keep a stock of spare parts to allow a quick repair. 

A suitable maintenance strategy should be developed for equipment by considering the criticality and failure mode, and then applying a mixture of the forms of maintenance described above. In particular, the long-term cost of maintenance of an item of equipment should be estimated over the whole life of the project and combined with its capital cost to select both the type of equipment and form of maintenance which gives the best full lifecycle cost on a discounted basis), while of course meeting the technical, safety and environmental specifications. 

It has been observed [4], that the effect of mean stress on the damage rate is much smaller than that of the stress range when delamination is the dominant failure mode. 

Methods for performing hazard analysis and risk assessment include safety review, checkhsts, Dow Fire and Explosion Index, what-if analysis, hazard and operabihty analysis (HAZOP), failure modes and effects analysis (FMEA), fault tree analysis, and event tree analysis. Other methods are also available, but those given are used most often. 

Fig. 6. A fault tree for the pumped storage example of Figure 5. For a real system the tank and pump failures would be more precisely defined, or set as intermediate events having further definition by subsequent basic events and more detailed failure modes.

The rehabihty level of a product also depends on the operating or environmental conditions, which may produce a variety of failure modes. Rehabihty can only be assessed relative to a defined environment. Unless these points are estabhshed clearly, confusion surrounds any quoted rehabihty number for a product. 

Failure Mode and Effects Analysis. The system design activity usually emphasizes the attainment of performance objectives in a timely and cost-efficient fashion. The failure mode and effects analysis (FMEA) procedure considers the system from a failure point of view to determine how the product might fail. The terms design failure mode and effects analysis (DFMEA) and failure mode effects and criticaUty analysis (EMECA) also are used. This EMEA technique is used to identify and eliminate potential failure modes early in the design cycle, and its success is well documented (3,4). 

The EMEA begins with the selection of a subsystem or component and then documents all potential failure modes. Their effect is traced up to the system level. A documented worksheet similar to Eigure 4 is used on which the following elements are recorded. 

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. 

Failure Cause. The failure cause is the physical, chemical, electrical, thermal, or other design deficiency which caused the failure. The agent, physical process, or hardware deficiency causing the failure mode must be identified, ie, what caused the failure for each failure mode. There may be more than one cause. Failure Fffect. The failure effect is the local effect on the immediate component/subsystem and the global effect on system performance/operation. In commercial products, the effect on the customer, ie, the global effect, must be addressed. 

Criticality Mnalysis. The criticaUty assessment provides a figure-of-merit for each failure mode. This figure of merit is based on the likelihood of occurrence of the failure mode (Occ), the criticaUty (severity) of the failure mode on system performance (Sev), and the detectabiUty of the failure mode by the user prior to occurrence (Det). 

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 most popular scheme among commercial companies is the assignment of a risk priority number (RPN) based on probabiUty of occurrence, detectabihty, and severity of a particular failure mode. The factors (Occ, Sev, and Det) are each rated on a 1 to 10 scale and then an RPN is based on the product of the three rating values. 

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. 

Explosibility and Fire Control. As in the case of many other reactive chemicals, the fire and explosion hazards of ethylene oxide are system-dependent. Each system should be evaluated for its particular hazards including start-up, shut-down, and failure modes. Storage of more than a threshold quantity of 5000 lb (- 2300 kg) of the material makes ethylene oxide subject to the provisions of OSHA 29 CER 1910 for "Highly Hazardous Chemicals." Table 15 summarizes relevant fire and explosion data for ethylene oxide, which are at standard temperature and pressure (STP) conditions except where otherwise noted. 

Failure Mode and Ejfect Analysis (FMEA) This is a systematic study of the causes of failures and their effects. All causes or modes of failure are considered for each element of a system, and then all possible outcomes or effects are recorded. This method is usually used in combination with fault tree analysis, a quantitative technique. FMEA is a comphcated procedure, usually carried out by experienced risk analysts. 

The Baum correlations for several vessel failure modes are given in Eqs.(26-7) to (26-16). 

In.strument air failure. The consequences of the loss of instrument air should be evaluated in coujuuc tiou with the failure mode of the control valve ac tuators. It should not be assumed that the correct air failure response will occur on these control valves, as some valves may stick in their last operating position. 

Introduction to Failure Modes Involving Mechanical Damage... 

Erosion-corrosion is a fairly complex failure mode influenced by both environmental factors and metal characteristics. Perhaps the most important environmental factor is velocity. A threshold velocity is often observed below which metal loss is negligible and above which metal loss increases as velocity increases. The threshold velocity varies with metal and environment combinations and other factors. 

Flaw. A flaw can be defined as an imperfection in a material that does not affect its usefulness or serviceability. A component may have imperfections and still retain its usefulness. This fact is recognized by most material codes that permit, but limit, the size and extent of imperfections. This is particularly true of welds, which commonly contain harmless imperfections. It is not uncommon for failures to occur in the vicinity of flaws that have contributed nothing to the failure mode. 

Surface defects, if sufficiently severe, may result in failure by themselves. More commonly, they act as triggering mechanisms for other failure modes. For example, open laps or seams may lead to crevice corrosion or to concentration sites for ions that may induce stress-corrosion cracking. 

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