An important metric used to determine the safety integrity level (SIL) of a safety system is the probability of failure on demand (PFD) of the system. PFD is nothing more than the likelihood that a safety function will work when called upon, and it depends on the reliability of the components from which is it is constructed. Without getting too deep into the details of all the specific terms and methods pertinent to the application of these standards, it should be easy to make the connection between the need for reliability of function during an emergency and the benefit of simple, consistent and effective means for the software to carry out the required emergency action.
Many processes are not so dangerous as to require a completely separate system for implementing safety functions. Instead, safety countermeasures are integrated directly into the basic control functionality. The best way to accomplish this is through simple, uniform mechanisms to override the current state or action of an object, along with verification of the desired result. Each object, therefore, should provide a configurable override capability that both standardizes and fully supports the means to deploy whatever safety countermeasures result from the application of a safety standard. In this way, safety functions can be realized with little effort during initial programming and readily accommodated later if safety requirements change.
Support S88-based recipe execution. This optional feature is sometimes relevant when the process being controlled is a batch process using multiple recipes. That in itself does not require the sophisticated recipe execution capabilities ascribed to S88, but certainly warrants their consideration. When it is determined that the process can benefit from this functionality, then some additional logic elements are needed. First and foremost is a routine which sequences the process operations contained in the recipe and translates them into specific actions. This routine also works with the recipe manager to collect associated parameters from the recipe and pass them along with the action information.
Another important feature is intelligent command and parameter processing. This capability provides filtering and re-direction of commands and parameters that result from the actions being executed by the current recipe operation(s). Since multiple units may be capable of performing the necessary action, the pertinent data must be routed to the resource which has been selected by the recipe execution manager (after being filtered by mode), and subsequent status information must be routed back.
Mentioned previously, resource allocation is another important capability that permits a recipe operation to temporarily take exclusive ownership of the object(s) which are needed to carry out its associated actions. This can be accomplished through object-level functionality or by means of a system-level utility, but is nonetheless a required component of a well-designed S88 batch execution system.
Recipe execution management is an operator-driven function which provides storage and retrieval of recipes, the means to define the logic and data associated with a recipe, and also provides capability to execute a recipe. This functionality can be custom-built, but many turn-key products already exist and would most likely provide a higher level of functionality at a lower cost.
Standards Are the Key Which Unlocks the Treasure of Sustainability
For all of the reasons described above, the application of standard methodologies is a wise choice for the implementation of a process control system. Given the time and effort committed to these standards, it's very unlikely that many needs, details or aspects of integration have been overlooked. Quite the contrary, published standards are a storehouse of useful and holistic information about the numerous details involved in developing a quality process control solution. They contain a thorough explanation and breakdown of techniques which are considered the best practices in the industry.
Often considered an over-used buzzword, sustainability simply refers to the capacity of something to endure. Many things affect this capacity, like continued relevance, adaptability to change, ease of management and simplicity of use. When standards form the basis of the approach used for the design and implementation of a control system, sustainability is a logical and expected outcome. It may stretch the willingness of project participants to consider new ideas, and may even impose an uncomfortable and unpopular learning curve on the time and budget constraints of a project, but pushing through these hurdles will pay dividends for years to come – and if done well, perhaps decades.
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