Safety systems, react

The safety requirements must correctly reflect the critical properties of the environment in which the software is to operate. This is of importance since the software system may react differently in different environments and it is imperative to exercise the programs with representative inputs. In practice, general user requirements will change over a period of time and a necessary feature of the safety specification will provide for maintainabihty of the safety systems so that the achieved safety level is not compromised by future alterations. 

Training of operators to react in case of failures in safety systems... 

Training sessions are designed to train the operators in usage of both columns of the EOPs. Operators are trained to react in the case of a failure of the system designed to cope with the accident, i.e. they are trained to react in the case of failures in the safety systems. 

The proposed safety analysis method is based on simulation of failures. Component models that only reflect normal operation have to be enhanced accordingly. At system level, control or reconfiguration logics have to be implemented that define how a system reacts to the occurrence of failures. The pursued modelling approach is structure invariant, i.e. the DAE structure (equation 1) remains always the same. Failures are represented by model parameter (values in matrices A, B) changes. 

Software diversity has been advocated as a means of improving the reliability of safety related software and in particular safety systems that react to a demand, where a 1 out of 2 or a 2 out of 3 voting scheme can be used to ensure that some safety action is performed. This approach is used in industry (e.g. for railway interlocking), but development and maintenance is costlier than for a non-diverse system and it is not easy to predict in advance the likely safety improvement that can be achieved. 

Liquid metals, however, present several disadvantages. Their weights must be considered with regard to equipment design. Additionally, Hquid metals are difficult to contain and special pumps must be used for system safety. Alkali metals react violentiy with water and bum ia air. Liquid metals also may become radioactive whea used for cooling auclear reactors (qv). 

Humans require time to react to process alarms and control requirements. Reaction time must always be considered early in the plant design. It is inherently safer to decide early in process design what administrative controls the operator will be assigned to activate for safety control. Requiring periodic operator interface to the process system relieves boredom and heightens interest in knowing the current condition of the process. See Sections 6.4 and 6.5. 

Chemical kinetics involves the study of reaction rates and the variables tliat affect these rates. It is a topic that is critical for the analysis of reacting systems. The objective in tliis sub-section is to develop a working understanding of tliis subject that will penuit us to apply chemical kinetics principles in tlie tu ea of safety. The topic is treated from an engineering point of view, tliat is, in temis of physically measurable quantities. 

The molten salt electrolyte also contributes to the safety behavior of ZEBRA cells. The large amount of energy stored in a 700 g cell, which means about 30 kWh in a 300 kg battery, is not released suddenly as heat as be expected in a system with liquid electrodes such as the sodium sulfur cell. In the case of accidental destruction of ZEBRA cells, the sodium will react mainly with the molten salt, forming A1 sponge and NaCl. -The diffusion of the NaAICI ... 

It is possible to directly measure the instantaneous heat output of a nonexplosively reacting system due to chemical or physical processes as a function of the process time. This quantity shows directly whether and how quickly chemical conversions occur in the process phase under consideration. Such an approach can be useful, not only from a safety perspective but also for process design and optimization. 

A reactor has a volume of 2 m3. The worst case runaway reaction has been identified and the data from a suitable adiabatic, low thermal inertia test, with a thermal inertia ( ) of 1.05, is given in Figure 6.4. Under these conditions, the reactor would contain 793 kg of reactants. The reacting system is a vapour pressure system. It is desired to relieve the runaway via a safety valve, if possible, with a set pressure of 0.91 barg (relief pressure of 1.0 barg = 2.0 bara). Evaluate the required relief size for an overpressure of 30% of the absolute relief pressure, which gives a maximum pressure of 2.6 bara = 1.6 barg. 

The iodine value (IV) is used to determine the level of unsaturation in a fat/oil system. It is expressed as the number of grams of iodine that add to/react with 100 g of sample. The traditional iodine value method using the Wijs reagent requires carbon tetrachloride (CC14). For safety reasons, CC14 is no longer considered to be an acceptable chemical and it is not readily available for purchase, and if offered it is extremely expensive. Therefore the traditional method has been modified to a more human-friendly system which uses cyclohexane. 

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