Hazard control sequence

The prioritized list of controls for hazards may be called the safety precedence sequence, the hazard reduction precedence, the hazard control sequence, the system safety sequence, or any of several other terms. The only real difference between an operating hazard analysis (OHA) and an operating and support hazard analysis (O SHA) is the name. 

Without proper control of hazards, a sequence of events (scenario) occurs which results ia an accident. A hazard is defined as anything which could result ia an accident, ie, an unplaimed sequence of events which results ia iajury or loss of life, damage to the environment, loss of capital equipment, or loss of production or inventory. 

It is not, however, always called the safety precedence sequence. It is also known as the system safety precedence, the hazard reduction precedence sequence, the hazard control precedence sequence, fhe risk control sequence, and several other names. The fifth step (accept residual risks) is not always included. 

Hazard control involves applying the safety precedence sequence to identified hazards that present unacceptable risks until the risks are controlled to an acceptable level. Inherent in the analysis and control process is verification... 

System safety methods assume that accidents and mishaps result from multiple causal factors. System thinking views hazards and causal factors as moving in logical sequences to produce accident events. Traditional approaches to accident prevention simply classify causal factors as unsafe acts and unsafe conditions. Hazard control personnel should use root cause processes to discover, document, and... 

Events leading up to the hazard state are an addition to the hazard analysis format. The events can occur either sequentially or in parallel or both. In developing controls to prevent the development of the hazard state, ranember that you have the opportunity to interrupt the hazard event sequence at various points. It is critical that you spend some time in deciding where you wish to intervene. A poor decision can be very expensive. 

Barriers can be designed to be passive or active in nature. Passive safety barriers provide a natural fixed roadblock, such as a tank pit or a firewall. Active barriers provide a response to certain states or conditions they involve a sequence of detection, diagnosis, and action (also referred to as detect-diagnose-deliect). Both physical and nonphysical barriers are utilized and apphed in hazard control and risk mitigation. Anything used to control. 

Figure 23-1 shows the hazards identification and risk assessment procedure. The procedure begins with a complete description of the process. This includes detailed PFD and P I diagrams, complete specifications on all equipment, maintenance records, operating procedures, and so forth. A hazard identification procedure is then selected (see Haz-ard Analysis subsection) to identify the hazards and their nature. This is followed by identification of all potential event sequences and potential incidents (scenarios) that can result in loss of control of energy or material. Next is an evaluation of both the consequences and the probability. The consequences are estimated by using source models (to describe the... 

Physical or chemical processes involving chemical reactivity hazards require carefully determined, facility-specific operating limits, which may go well beyond temperature control. Limits may need to be specified for addition quantities, rates and sequences agitation pH conductivity concentration pressure and other variables that either keep an undesired chemical reaction from starting or control a desired chemical reaction. Determination of these limits is outside the scope of this publication references such as Barton and Rogers (1997), CCPS (1995a) and HSE (2000) can be consulted for further information. 

A complete understanding of the system under consideration and of the mechanisms that lead to all the hazardous outcomes is required. This may be in the form of a time sequence of instructions, control actions, or in the sequence of physical events that lead to hazardous consequences. 

The full range of process maloperations, including system failures that might lead to process runaway will first have to be considered by a systematic evaluation of the plant and process concerned141. These may, for examplel be due to human error, hardware failure, or due to failure of a computerised sequence controller. To assess the likely/ credible maloperations accurately, it is recommended that personnel who will be operating the plant are involved in the hazard assessment. 

The sintered product is cut into the required shape and the active material is infused using one of a number of techniques - e.g. for the negative electrode by impregnating the sinter with concentrated aqueous cadmium nitrate, followed by thermal decomposition, or by cathodic polarization in molten Cd(NC>3)2 baths, etc. The plates are washed and the impregnation cycle is repeated up to 5-10 times until the required loading is attained. Finally the plates are formed by a sequence of carefully controlled charge/discharge cycles. Safety precautions are very important in the manufacture of cadmium-based electrodes because of the health hazards associated with this material. 

The scope and limitations of the use of DSC as a means of investigating the reaction dynamics of potentially hazardous chemical reactions is discussed. For existing processes, examination of production samples taken at various stages of the overall process sequence can identify the stage at which maximum heat production is occurring, as the most critical phase for control requirements. The existence of an induction period can also be established, and use of the instrument in its more sensitive isothermal mode can yield information on consecutive reactions and catalytic effects... 

With attenuated viral vaccines the potential hazards are those associated with reversion of the virus during production to a degree of virulence capable of causing disease in recipients. To a large extent this possibility is controlled by very careful selection of a stable seed but, especially with live attenuated poliomyelitis vaccine, it is usual to compare the neurovirulence of the vaccine with that of a vaccine known to be safe in field use. The technique involves the intraspinal inoculation of monkeys with both the reference vaccine and the test vaccine followed by comparison of the neurological lesions and symptoms, if any, that are caused. If the vaccine causes abnormalities in excess of those caused by the reference it fails the test. A modification of this test which uses transgenic mice instead of monkeys is now available. An in vitro method (MAPREC test) which relies on detecting RNA sequences specific to virulent virus has also been developed. 

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