Failure of service

The low temperature which, if the materials come into contact with the body, can cause severe tissue burns. Flesh may stick fast to cold uninsulated pipes or vessels and tear on attempting to withdraw it. The low temperatures may also cause failure of service materials due to embrittlement metals can become sensitive to fracture by shock. 

The failure of a concrete structure is of course not confined to catastrophic collapse. A concrete structure has failed or reached the end of its serviceability life when it is no longer capable of fulfilling its design functions, e.g. leak-tightness or as a barrier against deleterious elements which may cause corrosion. 

Careflil material selection is required to prevent brittle failure of tanks at low temperatures. In addition, for tanks where the service temperatures are reduced, it is essential that an engineering analysis be performed to ensure that the tanks are not subject to brittle failure at the house temperature. The tank and vessel codes usually specify allowable materials based on design temperature. Further information about selection of metals for low temperature is available (8). 

Heat for soldering is usually obtained from torches. The high conductivity of copper makes it necessary to use large flames for the larger sizes, and for this reason the location in which the joint will be made must be carefully considered. Soldered joints are most widely used in sizes 2 in and smaller for which heat requirements are less burdensome. Soldered joints should not be used in areas where plant fires are hkely because exposure to fires resiilts in rapid and complete failure of the joints. Properly made, the joints are completely impervious. The code permits the use of soldered joints only for Category D fluid service and then only if the system is not subject to severe cychc condions. 

The confinement of the cracks to a specific area of the cooler suggests that condensate from atmospheric moisture initially formed in this area and dissolved a corrodent from the atmosphere such as ammonia, sulfur dioxide, or oxides of nitrogen. Since the previous cooler had been in service for 20 years, it is conjectured that the rapid failure of this exchanger was due principally to very high bending stresses, which may have been induced during construction of the cooler. 

Figure 10.5 shows the appearance of numerous failures that had occurred in a short time in the water box inlet end. Cracking of this type was a recurrent problem in this condenser. Approximately 9% of the tubes in the condenser had been plugged. The condenser was in cyclic service, although the failures had occurred while the boiler itself was out of service. 

It is worrying that a vessel which is safe when it enters service may become unsafe by slow crack growth - either by fatigue or by stress corrosion. If the consequences of catastrophic failure are very serious, then additional safety can be gained by designing the vessel so that it will leak before it breaks (like the partly inflated balloon of Chapter 13). Leaks are easy to detect, and a leaking vessel can be taken out of service and repaired. How do we formulate this leak-before-break condition ... 

It has always seemed strange to us that the mechanics, or the manufacturer, are blamed when a mechanical seal fails after 3 months of service. If the seal fails on start-up, maybe you could point to the mechanic or the seal, but not after 3 weeks or 4 months of operation. This would most likely be an operational failure (a failure in operations), or a design failure (a failure in the system s design). And what is really amazing is that this statement and these words have never been recognized or said before.)... 

The result can be a wheel that fails prematurely. Often, a user will experience several wheel failures before deciding on a redesign (Figure 5-10). Yet, properly designed one-piece wheels have withstood decades of service without failures and, therefore, represent a desirable design in many cryogenic expanders. 

The aim of a risk assessment is to develop a product which is safe for the proposed market. A safe product is any product which, under normal or reasonably foreseeable conditions of use, including duration, presents no risk or only the minimum risk compatible with the product s use and which is consistent with a high level of protection for consumers (DTI, 1994). In attempting to protect products against failure in service and, therefore, the user or environment, difficulty exists in ascertaining the... 

From an FMEA of the system design, a Severity Rating S) = 1 was allocated, relating to a safety critical failure in service. It is required to find the optimum unequal angle section size from the standard sizes available. It is assumed that the load is carried at the section s centre of gravity, G, and only stresses due to bending of the section are considered, that is, the torsional effects are minimal. The combined weight of the beam and tie rod are not to be taken into account. 

The effect of ozone is complicated in so far as its effect is largely at or near the surface and is of greatest consequence in lightly stressed rubbers. Cracks are formed with an axis perpendicular to the applied stress and the number of cracks increases with the extent of stress. The greatest effect occurs when there are only a few cracks which grow in size without the interference of neighbouring cracks and this may lead to catastrophic failure. Under static conditions of service the use of hydrocarbon waxes which bloom to the surface because of their crystalline nature give some protection but where dynamic conditions are encountered the saturated hydrocarbon waxes are usually used in conjunction with an antiozonant. To date the most effective of these are secondary alkyl-aryl-p-phenylenediamines such as /V-isopropyl-jV-phenyl-p-phenylenediamine (IPPD). 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

A distinction must be made regarding the length of service of the pressure reducing systems. Fatigue failure of any mechanical system depends on time, i.e., the number of cycles to failure. Therefore, the treatment required for a continuous service may not be justified for a short term service. A System in short term service is defined as one which operates a total of 12 hours or less during the life of the plant. Pressure relief valves typically meet this limit. Systems in short term service exceeding the screening criteria indicated above should be evaluated. 

IEEE Std 500-1984 (IEEE, 1984) contains failure rate and out of service, repair and restoration times for electrical electronic, and sensing component, and mechanical equipment. It is a considerable improvement over IEEE STD 500-1977. The reported values are the consensus of over 200 experts. Each expert submitted a low, recommended, and a high value for the failure rate... 

External events are accident initiators that do not fit well into the central PSA structure used for "internal events." Some "external events" such as fire due to ignition of electrical wires, or flood from a ruptured service water pipe occur inside the plant. Others, such as earthquakes and tornados, occur outside of the plant. Either may cause failures in a plant like internal events. External initiators may cause multiple failures of independent equipment thereby preventing action of presumably redundant protection systems. For example, severe offsite flooding may fli 1 the pump room and disable cooling systems. An earthquake may impede evacuation of the nearby populace. These multiple effects must be considered in the analysis of the effects of external events. 

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