Failure types thermal

Since an understanding of the importance of any one process contributing to the failure in thermoplastics and the control over these processes is only partly attainable, a knowledge and understanding of the nature of endurance Hmits is of extreme importance for successful use of plastics, in particular engineered thermoplastics [27]. In terms of the failure type, polymer fracture may occur as a rapid extension of an initial defect, plastic flow of the matter and the thermally activated flow of the macromolecules. In all these cases, however, fracture is a localized phenomenon characterized by a large inhomogeneity of deformations. 

Dielectric Strength. Dielectric failure may be thermal or dismptive. In thermal breakdown, appHed voltage heats the sample and thus lowers its electrical resistance. The lower resistance causes still greater heating and a vicious circle, leading to dielectric failure, occurs. However, if appHed voltage is below a critical value, a stabilized condition may exist where heat iaput rate equals heat loss rate. In dismptive dielectric failure, the sample temperature does not iacrease. This type of failure is usually associated with voids and defects ia the materials. 

Salt accumulations and other deposits on the firetube can sometimes be detected by smelling the vapors from the still vent. A burned" odor emitted from these vapors usually indicates this type of thermal degradation. Another detection method is to observe the glycol color. It will darken quickly if the glycol degrades. These detection methods may prevent a firetube failure. 

In addition to the many different forms of boiler section ferrous corrosion already described, several other less common types occasionally develop. In particular, corrosion processes may evolve that are interrelated with stress, deposition, and/or high temperatures (thermal effect corrosion), and together these may lead to metal fatigue (metal fatigue corrosion), metal failure, and even more serious problems such as the risk of a boiler explosion. 

Thermal reactions of the second type, which do not involve adding ligands toward formation of metal organic product compounds, are less widely recognized as chemical phenomena. There may be here a simple failure of reaction, as in Eq. (16), because (99) of the diffusion of CO from the reaction site there may be stabilization of an intermediate stage by scavenger action (26, 27, 56) ... 

While redundancy—backing up one system with another just like it—is a logical way to protect against instrument failure, diversity may be a better choice in many cases. For example, dual thermocouples in the same well could corrode out at the same time. It might be better to have two different types of measurement, one a thermocouple in a thermowell and the other possibly a resistance thermal detector. 

Catalytic reduction of aromatic nitro compounds to the amines is highly exothermic (AH = —548 12 kJ/mol) and has high potential for hazard in the event of cooling- or other process-failure. The total reaction proceeds via nitroso and hydroxylamino intermediates, both of which are reactive and may undergo undesired condensation or disproportionation reactions, and the thermochemistry of all these possibilities was investigated. The reduction or disproportionation of the hydroxylamino intermediate (which is of low thermal stability) is identified as the fastest and most exothermic step (despite which it can frequently be concentrated or trapped) implications for process safety are considered in detail and verified by experiment with typical compounds and intermediates [1]. A calorimetric study of the hazards inherent in hydrogenation of nitroaromatics was made, using nitrobenzene as model compound [2]. Individual incidents of this type are ... 

---