The Safety-Guided Design Process

One key to having a cost-effective safety effort is to embed it into a system engineering process from the very beginning and to design safety into the system as the design decisions are made. Once again, the process starts with the fundamental activities in chapter 7. After the hazards and system-level safety requirements and constraints have been identified the design process starts  

If any of the hazards cannot be eliminated, then identify the potential for their control at the system level. 

Safety-guided design entails tightly intertwining the design decisions and their analysis to support better decision making. 

Create a system control structure and assign responsibilities for enforcing safety constraints. Some guidance for this process is provided in the operations and management chapters. 

The next section provides an example of the process. The rest of the chapter discusses safe design principles for physical processes, automated controllers, and human controllers. 

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In the initial candidate architecture (control structure), a decision is made to introduce a human operator in order to supervise robot movement as so many of the hazards are related to movement. At the same time, it may be impractical for an operator to monitor aU the activities so the first version of the system architecture is to have the TTPS control system in charge of the non-movement activities and to have both the TTPS and the control room operator share control of movement. The safety-guided design process, including STPA, will identify the implications of this decision and wUl assist in analyzing the allocation of tasks to the various components to determine the safety tradeoffs involved. 

The safety-guided design process, including STPA, will identify the implications of the basic decisions in the candidate tasks and will assist in analyzing the allocation of tasks to the various components to determine the safety tradeoffs involved. 

The primary responsibility of the process controller is to produce conutiands to fulfill its control responsibilities. Again, the STPA hazard analysis and safety-guided design process will produce the application-specific behavioral safety requirements and constraints on controller behavior to ensure safety. But some general guidelines are also useful. 

Setting up a safety information system for a single project or product may be easier. The effort starts in the development process and then is passed on for use in operations. The information accumulated during the safety-driven design process provides the baseline for operations, as described in chapter 12. For example, the identification of critical items in the hazard analysis can be used as input to the maintenance process for prioritization. Another example is the use of the assumptions underlying the hazard analysis to guide the audit and performance assessment process. But first the information needs to be recorded and easily located and used by operations personnel. 

The process of safety-guided design and the use of STPA to support it is illustrated here with the design of an experimental Space Shuttle robotic Thermal Tile Processing System (TIPS) based on a design created for a research project at CMU [57]. 

Implement special training for developers and development managers in safety-guided design and other necessary skills. Update this training as events occur and more is learned from experience. Create feedback, assessment, and improvement processes for the training. 

The safety status of the process should be periodically reviewed against the guiding principles for the original design. Monitoring of add-ons can detect potentially dangerous modifications. Process hazards analysis or process safety audits are useful tools for this review. Documentation of inherently safer principles is critical to ensure that future changes don t nullify the positive features of the initial installation. 

The basic safety and fire protective measures that should be included in all chemical process designs are listed below. This list is based on that given in the Dow Guide, with some minor amendments. 

The Guide to Process Safety is designed to help with implementation of the Responsible Care Process Safety Code (SOCMA, 1999). The guide presents voluntary, proactive initiatives for the continuous improvement of process safety performance. 

The safety and loss prevention guide developed by the Dow Chemical Company provides a method for evaluating the potential hazards of a process and assessing the safety and loss-prevention measures needed. In this procedure, a numerical Fire and Explosion Index is calculated, based on the nature of the process and the properties of the materials. The index can be used two different ways. In the preliminary design, the Dow index will indicate whether alternative,... 

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