Inherent safety measures

Although inherent safety measures are reasonable at all levels of the safety concept their important field of application is before the safety concept has been fixed. The following strategies are used  

However, it is conceivable that no inherent safety measures exist for a given process. In such a case only engineered safety measures inscribed in the categories of Table 4.2 (passive, active, organizational) can be applied. 

In the first place alternative processes may be considered, as in Example 4.1. It is important that bulfer volumes are kept small by synchronizing the different process steps. Contributions to minimization are to be expected as well from microreactors, which are, however, still in the pilot plant stage [12]. 

Example 4.1 Reduction of the process inventory in producing nitroglycol 

Data Inventory of the stirred tank reactor 100 kg of nitroglycol inventory of the injector reactor 3 kg of nitroglycol energies of explosion nitroglycol 6,743 kJ/kg, TNT 4,650 kJ/kg. 

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Although inherent safety and pollution prevention are similar in concept, there are practical differences between the two that have, so far, made the adoption of inherent safety measures less attractive to industry than pollution prevention/cleaner production. 

In financial analysis two final indices are available Conventional Safety Cost Index (CSCI) which is the ratio of conventional safety measures of the system over the probable loss cost, and Inherent Safety Cost Index (ISCI) which is the relative amount of the cost of inherent safety measures added to the system to the loss cost (Khan and Amyotte, 2004). Smaller ISCI in comparison to CSCI shows enviable impact of safety features on safety costs. In other words the smaller the ISCI/CSCI fraction the better the response. 

Storage and receiving are activities that can greatly contribute to a safe and economic operation. It is here that quality control can be achieved at minimal cost. Label verification and other quality assurance measures can increase the confidence level that the correct chemicals have arrived, thereby potentially circumventing the use of wrong chemicals. Wrongly shipped chemicals can be returned to the manufacturer with minimal or no cost to the batch operation owner. As with all processes and activities it is of great importance to apply the principles of inherent safety, in particular the minimization and attenuation principles (CCPS G- 41). 

Develop methods to measure the effectiveness of inherent safety efforts and to provide feedback to personnel to improve performance. 

Dow (Sheffler, 1996 Gowland, 1996a,b) describes the use of the Dow Fire and Explosion Index (Dow, 1994b) and the Dow Chemical Exposure Index (Dow, 1994a) as measures of inherent safety, along with the use of inherently safer design principles to reduce hazards. 

The worst hazard scenarios (excessive temperature and pressure rise accompanied by emission of toxic substances) must be worked out based upon calorimetric measurements (e.g. means to reduce hazards by using the inherent safety concept or Differential Scanning Calorimetry, DSC) and protection measures must be considered. If handling hazardous materials is considered too risky, procedures for generation of the hazardous reactants in situ in the reactor might be developed. Micro-reactor technology could also be an option. Completeness of the data on flammability, explosivity, (auto)ignition, static electricity, safe levels of exposure, environmental protection, transportation, etc. must be checked. Incompatibility of materials to be treated in a plant must be determined. 

Inherent safety can rarely be attained in an established process by simple measures. In cases of process modification, one of the following must be involved to make the process safer (Sharkey et ciL, 1992) (1) new chemistry, (2) new reaction solvent, (3) added recycle streams, and (4) major concentration changes (reactants, products, by-products, solvents). 

Preventive and protective measures. Technical safety measures must always be developed no matter how inherently safe we think the process is. The scope of preventive and protection measures is large for the whole plant. Below, some preventive measures to be taken to prevent thermal runaways when a dangerous situation is recognized are listed ... 

Thus, against this measure of inherent safety the fire hazard is 3.5 times larger for operation at 150°C compared to operation at 100°C. 

In most cases, however, the design will not be inherently safe so that measures and/or controls will have to be introduced into the design. Two types of safety measures are defined (1) preventive, and (2) defensive (protective or mitigating). 

Both have chosen temperature and pressure to illustrate inherent safety. This is because temperature is a direct measure of the heat energy available at release. Pressure is both a measure of the energy available at release and the energy available to cause a release. 

A synthesis problem on purification of butenes is discussed (Hurme and Heikkila, 1998). The aim is to synthesize the separation process by maximizing the inherent safety. The inherent safety is measured as the Inherent Safety Index. 

Hendershot, D. C. 1997. Measuring Inherent Safety, Health and Environmental Characteristics Early in Process Development. Process Safety Progress, Vol. 16, No. 2, pp. 78-79. 

A chemical engineer may have a choice of inherent safety variables, such as quantity stored or process temperatures and pressures, or process safety measures such as emergency isolation valves or containment systems, all of which may greatly reduce the vulnerabilities or the consequences of intentional loss. These are in addition to traditional security measures, which may include physical security, background checks, administrative controls, access controls, or other protective measures. For a more complete discussion of the options, refer to the AIChE Center for Chemical Process Safety Guidelines for Analyzing and Managing the Security Vulnerabilities of Fixed Chemical Sites and other references.f... 

Most petroleum and chemical facilities rely on inherent safety and control features of the process, inherent design arrangements of the facility, and process safety ESD features as the prime loss prevention measures. These features are immediately utilized at the time of an incident. Passive and active explosion and fire protection measures are applicable after the initiating event has occurred and an adverse affect to the operation has been realized. These features are used until their capability has been exhausted or the incident has been controlled. 

A hierarchical perspective can be applied. Preventive measures that increase inherent safety are generally considered the most effective approach. See examples for assessing the effectiveness of individual recommendations in Figure 10-3. 

There, are a number of means of achieving safe operation of chemical, reactors. Usually, the best option is to avoid the hazards completely, or at least minimise them, by inherently safer design. However, this may not be reasonably practicable and,.to maintain a viable process, other safety measures will. be required, either alorie or in combination.. , ... 

Measure the inherent safety or lack of inherent safety in processes... 

Measuring the Inherent Safety or Lack of Inherent Safety 45... 

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