Design, process safety tolerance

Note that the example SIL levels provided in this chapter are only examples. They are not to be assumed recommended levels of protection. The selection of an appropriate Safety Integrity Level (SIL) is site-specific and the analysis requires selecting criteria for tolerable risk, and evaluating process conditions, specific chemicals, equipment design-limits, control schemes, process conditions, and unique hazards. Experts in process engineering, instrumentation, operations, and process safety should imdertake SIL selection. 

The amount of process plant that can be defined accurately as automatic is relatively small, and manual intervention is often involved at some stage. The relevant design criteria are therefore often IM/12 or IM/18. In practice, fully automatic burner controllers tested and certified by British Gas are available that comply with the requirements of BS 5885. Although these have features which may not be applicable to non-automatic plant, it may be more appropriate to use such a controller, particularly as its safety is well proven. It may also be less expensive than buying and installing separate timers, relays, etc. For some processes (for example, those that do not need and cannot tolerate a long purge) such controllers may not be appropriate. 

In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 

A process or a facility satisfies the single fault tolerance principle when it has been designed or equipped in such a way that a single fault does not lead to the occurrence of the undesired event. This applies also to safety-related systems. 

When the patients who received a drug candidate have the disease manifestations completely eradicated and experience no other effects while patients treated with a placebo have a continuation of the disease process, the evaluation is not difficult. However, that is rarely, if ever, the case, and evaluation requires detailed statistical analysis of the collected data. An ICH guide-line covers statistical issues related to the scope of clinical trials, design techniques to minimize bias, types of clinical trial designs, conduct considerations, data analysis for efficacy, evaluation of safety and tolerance, and reporting. 

Because of their size, these molds are often stand-alone equipment packages and as such need to be sufficiendy robust to permit repeated daily process cycles with the minimum of maintenance downtime. They must offer the blade manufacturer the means to reliably produce blades with the minimum number of operators in the shortest possible time. Benefits gained with proper designed blades include reduced mold height to eliminate cumbersome platforms around the molds improved speed and safety of mold closing mechanisms accurate blade shape and blade edge tolerances and all electrical, vacuum, and other services within the structural framework. 

Remote actuated valves must be chosen to match process requirements. Design decisions include materials of construction, valve seat material, valve t q)e, actuator type and controls characteristics. The design must match utility supply levels (air pressure, hydraulic pressure, flow capacity) and tolerances with actuator design to provide the correct torque/thrust to the valve. The torque/thrust must be above breakaway requirements and must be below stress limits of the drive train. With some valves, the ratio of these values may limit the available operating safety factor. 

Gibson, S.B., 1976. The design of new chemical plant using hazard analysis. Process Industry Hazards, Symposium Series No. 47. 135 (IChemE. Rugby. UK). HSE, 1992, Tolerability of Risk from Nuclear Power Stations, revised edition. Pantony, M.F.. Scilly. N.F. and Barton. J.A.. 1989. Safety of exothermic reactions a UK strategy, Int Symp on Runaway Reactions. 504—524 (CCPS, AIChE. USA). Kauffman, D. and Chen, H-J.. 1990, Fault-dynamic modelling of a phthalic anhydride reactor, J Loss Prev Process hid. 3 386-394. 

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