Manufacturing flaws

There are numerous processes, respectively process steps which operate at pressures below atmospheric or where processes like unwanted condensation may produce underpressure. In general underpressure is a smaller problem than overpressure. Yet, there is the possibility of ingress of air and consequent reactions with the contents of the equipment. For example, ingress of air into a distillation column may cause an explosion [13]. Furthermore one must observe that the stability of vessels and such like is reduced by underpressure because manufacturing flaws (slight distortions) may be enhanced. With overpressure, on the other hand, they are compensated ( the vessel is blown up )-... 

Technical systems are made up of components such as valves, pumps, pipes, measuring devices, control loops etc. These have to fulfil certain tasks within the system. If they do not comply with these tasks because their technical properties have changed in such a way that they lie outside the permissible regions of tolerance of the system, they have failed. There are many reasons for component failures, e.g. manufacturing flaws, corrosion, overload, unfavourable environment or wear to name just a few. It is known that components fail after a certain period of time, but it is impossible to predict the point in time of failure. Yet, models describing the operating behaviour can be developed. They are based on the observation of large numbers of components and they are statistical in nature. 

This period with its constant failure rate represents random failures of such nature that future failures do not depend on past operation. These random failures are caused mainly by random fluctuations of operating and environmental conditions which cause loads exceeding design strength. In addition failures may occur due to unprofessional maintenance and hidden design and manufacturing flaws which have remained despite the measures mentioned above. 

So far it has been assumed that the primary events of a fault tree are independent of one another. However, this is not always true. Failures of components from the same production may occur due to a manufacturing flaw which affects all of them. A corrosive atmosphere may shorten the lifetimes of all components exposed to it. Errors in testing and maintenance may occur, for example an erroneous calibration of several redundant measuring devices. These examples belong to a class of failures called dependent failures . They are discussed in detail in [48]. Dependent failures are failures which occur simultaneously or within a short interval of time so that several components are not available simultaneously. This type of failures is especially grave if it affects redundant sub-systems or systems. An overview of the different types of failures is provided by Fig. 9.33. 

The fuel, coolant, and internal structural materials are chemically compatible such that clad/coolant chemical interaction is avoided with control of coolant chemistry and such that run beyond clad breach due to manufacturing flaws would not lead to autocatalytic degradation - even for the very long duration of refuelling operations. In situ monitoring of coolant and cover gas conditions would be used to confirm normality of conditions. 

The crack in the shaft, initiated by a manufacturing flaw, had apparently grown slowly but deliberately over the four million striking cycles since the clock was installed in 18S9—a classic fatigue failure. 

A CMF is the failure of multiple similar components in the same mode, for various reasons. A CMF is an event or failure that simultaneously affects a number of elements otherwise considered as being independent. For example, a set of identical resistors from the same manufacturer may all fail in the same mode (and exposure time) due to a common manufacturing flaw. The term CMF, which was used in the early safety literature and is still used by some practitioners, is more indicative of the most common symptom of the CCF, but it is not a precise term for describing all of the different dependency situations that can result in a CCF event. A CMF is a special case or a subset of a CCF. Typically, the term CCF is used to include both CCFs and CMFs. 

The disks are likely to have similar serial numbers thus, they may share any manufacturing flaws affecting production of the same batch. 

Lessons learned from prior incidents involving Clow Model GMZ check valves installed at Shell facilities were not adequately identified, shared, and implemented. This prevented recondition and correction of the valve s design and manufacturing flaws at OP-III prior to the accident. 

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