Closed-loop process safety system

Figure 23-1 Closed-Loop Process Safety System.

External/Intemal Audit Reports ModKied Processes/New Facilities External Reviews Figure 23-1 Closed-Loop Process Safety System. 

V V methods include inspection, analysis, demonstration, and validation and verification. V V activities are determined by the perceived risks, safety, and criticality of the element under consideration. Use of a requirements management tool is essential once a design has been established and V V begins. A unique requirements identifier can be used for traceability to the V V plans, procedures, and reports to provide a closed-loop process from system capability, as proven through a V V process back to the source requirement. Basic V V activities are as follows ... 

Factors that affect the safety of a production system include (1) the scale of production (2) the quantity of hazardous chemicals involved (3) the hazardousness of the chemicals involved (4) batch versus continuous processing (5) the presence of pressure or temperature extremes (6) storage of intermediates versus closed loop processing and (7) multi-stream versus single-stream plants. These factors are discussed briefly below. 

As you may remember from Chapter 2, the system safety process is a closed-loop process. If there is no feedback in the system, it can never regulate itself nor fine-tune its processes. This closed-loop process is twofold tracking individual hazards in each system or subsystem and periodic review of the entire system safety process. Not only are you able to verify that safety controls are truly working, you can also optimize the safety process and make the entire system as cost-effective as possible. 

The cote system safety process can therefore be reduced to Hazard Identification -> Hazard Risk Assessment -> Hazard Risk Control -> Hazard Risk Verifica-tion-> Hazard Identification... (Ericson 2005). This is a closed-loop process where Hazards ate identified and tracked until acceptable closure action is implemented and verified. 

Final elements must provide the desired capacity with the required precision of flow throttling over the desired range, usually 10% to 95% of maximum flow. The valve characteristic should provide a linear closed-loop gain, except choose linear or quick-opening characteristics for valves that are normally closed but must open quickly. Select the valve failure position for safety. The valve body should satisfy such requirements as required flow at 0% stem position, plugging, pressure drop, or flashing. The nonideal final element behavior, such as friction and deadband, should be small, as required by each application. Control valves should have manual bypass and block valves to allow temporary valve maintenance when short process interruptions are not acceptable. However, the bypass should never compromise safety interlock systems. 

Periodic system review Probably one of the key points of the system safety process is that it is a closed-loop system. This means that the engineering and management organizations periodically review the safety program, engineering processes, management organizations, and product field use. Actually, this should be part of the SMS. The American automobile industry has lost billions of dollars in automobile recalls due to safety problems, some of which possibly could have been avoided by periodic review of product use. 

The other piece of the closed loop is periodic review of the system safety process. This is the fine-tuning of the system safety program and review at the macroscopic level and should be conducted by an independent safety group to avoid conflict of interest. This is precisely why the system safety office should report directly to the corporate office. The purpose of the review and audit is to verify that the system safety program is in place, performs its intended function, and adequately protects employees, consumers, and the eompany against accidents and other losses. It also serves as a documentation trail for OSHA inspectors, showing that your organization takes safety seriously. 

ABSTRACT In most cases, Model Based Safety Analysis (MBSA) of critical systems focuses only on the process and not on the control system of this process. For instance, to assess the dependability attributes of power plants, only a model (Fault Tree, Markov chain. ..) of the physical components of the plant (pumps, steam generator, turbine, alternator. ..) is used. In this paper, we claim that for repairable and/or phased-mission systems, not only the process but the whole closed-loop system Proc-ess/Control must be considered to perform a relevant MBSA. Indeed, a part of the control functions aims to handle the dynamical mechanisms that change the mission phase as well as manage repairs and redundancies in the process. Therefore, the achievement of these mechanisms depends on the functional/dysfunctional status of the control components, on which these functions are implemented. A qualitative or quantitative analysis method which considers both the process and the control provides consequently more realistic results by integrating the failures of the control components that may lead to the non-achievement of these mechanisms. This claim is exemplified on an industrial study case issued from a power plant. The system is modeled by a BDMP (Boolean logic Driven Markov Process), assuming first that the control components are faultless, i.e. only the faults in the process are considered, and afterwards that they may fail. The minimal cut sequences of the system are computed in both cases. The comparison of these two sets of minimal cut sequences shows the benefit of the second approach. 

In many passive safety systems natural circulation is an essential part of the system. The main aim is to use density differences in a pool or in a closed loop to transfer thermal energy from a source to a heat sink without using other energies than gravitational energy. This process can take place with or without phase changes. 

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