Protective measure

However, in order to maintain a viable process, significant hazards may still remain and further safety measures will be necessary. These can be grouped broadly as preventive or protective measures. 

Preventive measures either take automatic remedial action or allow for manual intervention to prevent the conditions for uncontrolled reaction being achieved. They include the use of sensors, trips, alarms, control systems, and other safety features. These measures require a.thorough understanding of the safe operating envelope within which the process must be maintained.. .  

Where systems incorporating manual intervention are used, they have the advantage that they can be flexible and allow operator judgement to be applied. However, their reliability may be open to question as they are dependent upon a number of factors, including complexity of the task, system design, time available for response and operator training. Further information on this is given in references 7 and 8. 

Protective measures mitigate the consequences of a runaway reaction. They are rarely used on their own, as some preventive measures are usually jxesent to reduce the demand on the protective system. The main options are  

A detailed knowledge of the runaway reaction, in particular the rate of runaway, is needed for the specification or design of protective systems. With the exception of (a), these measures are normally reliant upon control systems to operate, and the 

In the event of an alarm, an automatic sequence of actions should make the plant safe without the intervention of an operator. The sequence is likely to be simplest for continuous or semi-batch processes typically it may involve no more than stopping the reactant feeds, provided there are no problems with accumulation of reactants. Batch reactors are more difficult, particularly if they contain large amounts of unreacted material and are more likely to require the provision of protective measures such as emergency relief venting, or the provision of dump tanks with drown out facilities. 

Protective measures include emergency relief systems, quenching or inhibition and containment. 

In common with using prevention as a basis of safety, it is essential that a full evaluation of the hazards of the process is carried out, before the type of protective measure is chosen and designed. The identification and definition of the worst case scenario is particularly important as, in contrast to prevention, any protective measure has to be able to cope with this worst case runaway reaction. In addition, the course of the runaway reaction has to be fully characterized and evaluated using the techniques described in Chapters 3 and 4. 

The period of time during which the particles are exposed well exceeds the standard residence time in the equipment since such layers are not generally removed during the production run. Self-heating can occur. 

Both self-heating and self-ignition characteristics are commonly determined by adiabatic tests (self-heating) and hot-plate tests (self-ignition). These tests are limited in their applicability because they do not fully simulate the plant conditions. 

A mathematical model is described [138] in which the self-heating of material layers under industrial conditions is simulated. The model takes into account oxygen (or gas) diffusion and consumption, reactant conversion, heat conduction in, and heat transfer to and from the layer. Scale-up experiments were performed which showed the model can be successfully applied to predict the self-heating phenomenon in the layers. 

A general approach to evaluating decomposition and oxidation of product during drying follows  

Determine the lowest temperature at which exothermic decomposition results in heat accumulation. 

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Protective measures iavolve prompt detection and adequate ventilation. Continuous monitoring is recommended to signal an evacuation alarm if the workplace concentration exceeds 70 mg/m (50 ppm) and a warning alert if it is present at 15—70 mg/m (10—50 ppm). 

Like other proteins, enzymes are potential allergens. In addition, proteases may act as skin and eye irritants. However, during the production and handling of industrial enzymes, the occupational health risks entailed by these properties can be avoided by protective measures, and by the form in which... 

Explosion protection encompasses the measures implemented against explosion hazards in the handhng of combustible substances and the assessment of the effectiveness of protective measures for the avoidance or dependable reduc tion of these hazards. The explosion protection concept is vahd for all mixtures of combustible substances and distinguishes between ... 

From a safety standpoint, priority must be given to the measures in item 1. Group 2 cannot be used as a sole protective measure for flammable gas or solvent vapors in industrial practice with sufficient reliability, but can be applied as the sole protective measure when only combustible dusts are present if the minimum ignition energy of the dusts is high (>10 mj) and the operating areas concerned can easily be monitored. 

For combustible dusts, the explosibility limits do not have the same meaning as with flammable gases and flammable vapors, owing to the interaction between dust layers and suspended dust. This protective measure can, for example, be used when dust deposits are avoided in operating areas or in the air stream of clean air lines after filter installations WTiere in normal operation the lower explosibility limit is not reached. However, dust deposits must be anticipated with time. When these dust deposits are whirled up in the air, an explosion hazard can arise. Such a hazard can be avoided by regular cleaning. The dust can be extracted directly at its point of origin by suitable ventilation measures. 

The maximum allowable oxygen concentration (MAOC), which is, in general, 2 vol % below the LOC, has to include the following considerations fluc tuatiou in oxygen concentrations due to process and breakdown conditions per time and location, as well as the requirement for protective measures or emergency measures to become effective. In addition, a concentration level for an alarm has to be set below the MAOC. 

For industrial practice, the following principles have resulted for the protective measures to be implemented. Their application is selective and depends on the prevailing circumstances. 

Containment (Explosion-Pressure-Resistant Design for Maximum Explosion Overpressure) An explosion-resistant construction is understood to mean the possibihty of designing vessels and equipment for the full maximum explosion ove (pressure, which is generally of the order P = 9 bar. The explosion-resistant vessel can then be designed as explosion pressure resistant or explosion pressure shock resistant. This protective measure is generally employed when small vessel volumes need to be protected, such as small filter units, fluidized-bed dryers, cyclones, rotaiy valves, or mill housings. 

Mitigation Reducing the risk of an accident event sequence by taking protective measures to reduce the likelihood of occurrence of the event, and/or reduce the magnitude of the event and/or minimize the exposure of people or property to the event. 

Kaufmann, R.H. and Halberg, M.H., System over- voltages, causes and protective measures. 

Table 3-2 Survey of measuring instruments for corrosion protection measurements...

Corrosion susceptibility in aqueous media is assessed on the basis of the rating numbers [3, 14], which are different from those of soils. An increased likelihood of corrosion is in general found only in the splash zone. Particularly severe local corrosion can occur in tidal regions, due to the intensive cathodic action of rust components [23, 24]. Since cathodic protection cannot be effective in such areas, the only possibility for corrosion protection measures in the splash zone is increased thickness of protective coatings (see Chapter 16). In contrast to their behavior in soils, horizontal cells have practically no significance. 

Stray currents from foreign sources are to be regarded in the same way as galvanic currents. The explanations for Eq. (4-11) are relevant. Protective measures afed cribed in Chapters 9 and 15. 

Corrosion protection measures are divided into active and passive processes. Electrochemical corrosion protection plays an active part in the corrosion process by changing the potential. Coatings on the object to be protected keep the aggressive medium at a distance. Both protection measures are theoretically applicable on their own. However, a combination of both is requisite and beneficial for the following reasons ... 

Fig. 10-5 Protection measure by separation of electrical operational equipment that is connected to the cathodically protected object via the housing, with an FI protection circuit leakage current circuit breaker (see Ref. 14) Tj and isolating transformers (see Ref. 15).

Locate and eliminate contact. If not possible for structural reasons— depending on the individual case—take other protection measures, e.g. insert hydraulic sealing medium or organic materials into the annulus. 

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