The starting point of a new industrial plant concerning the levels of reliability required to keep the process under a defined tolerable risk is a challenge most contractors company face. During the embryonic phases, in the bidding process and for budget purposes, a pre-defined Safety Instrumented System (SIS) design must be provided to the contractor, sometimes even before the process conceptual design is well defined. The consequences of such situation, in which no risk analysis have been considered, not only disregards the Safety Lifecycle template suggested by the recent versions of the functional safety standards applied to the process industry, IEC 61511  and ANSI/ISA 84.01  but also implies in unpredictable outcomes.
By means of actual examples, where the customers names will be suppressed for confidentiality matters, this paper will present and briefly discuss the pros and cons of some actual applications, the achieved safety of the resulting design and the impact of investments during implementation and operation phases of the enterprise.
Risk is a condition in which there is a possibility of an adverse deviation from a desired outcome that is expected or hoped for . Because intrinsically in every enterprise there is a lack of knowledge of what will or will not happen in the future, risk management became an important tool to assess the possibility of loss, existence of risk and exposition, insurance negotiation, etc.
When it comes to the process industry, the functional safety standards state the need of an in depth study of the potential risks that can jeopardize people, environment, company image and financial health, among others. So, after identifying independent layers of protection that mitigate the consequences of an undesired event (dikes, relief valves, control system, alarm management, etc.), it may be necessary to develop a Safety Instrumented System that will reduce the residual risk to a tolerable level .
In real life, however, despite all requirements stated in the international standards concerning Safety in the Process industry, it is still a challenge for engineers to justify and estimate investments in Safety Instrumented Systems, mainly during the enterprise initial phases.
It’s a common practice these days for operator companies, regarding new facilities, to assign to a contractor a fixed price supply contract. So, in the bidding process and for budget purposes, a pre-defined SIS design must be provided to the front- end engineering and design (FEED) contractor, most of the times even before the process conceptual design is well defined. This pre-defined SIS design provides an equal basis for all the contractors participating in the bidding process, all of them looking for the contract. Questions are:
1. What if the pre-defined SIS design is below the real safety requirements?
2. What if the pre-defined SIS design is above the real safety requirements?
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