Technology choices and the conceptual design stage

This chapter provides an outline of the issues to be faced by the designer at the conceptual design stage. Conceptual design is the stage where the control engineer prepares the whole safety instrumented system (SIS) scheme from sensors through logic solver to the final element, control valve, or motor trip. This means getting the concepts right for the specific application. It also means choosing the right type of equipment for the job—not the particular vendor but at least the right architecture for the logic solver system and the right arrangement of sensors and actuators to give the quality of system required by the requirements specification. Some typical issues decided at this stage include decisions on the type of sensor system to be used and measures required to avoid spurious trips. All basic design decisions are taken at this stage for the SIS, which are, however, subject to evaluation, review, and finally verification before the design is finalized and the detail engineering proceeds. The chapter reviews the essential features of various logic solver technologies in the context of conceptual design. Pneumatic devices have been used extensively in the offshore oil industry and continue to be used in freestanding installations in petrochemicals where the great advantage is that they are inherently safe for hazardous atmospheres. The most common application is to use a field-mounted pneumatic controller to provide wellhead pressure protection. Relay systems are simple to design with a good safe failure proportion by using relays with normally open contacts.