TABLE OF CONTENTS Plants that are implementing alarm and trip systems must follow the legislative and regulatory requirements in effect at the site. ANSI/ISA-84.01 1996, Application of Safety Instrumented Systems for the Process Industries provides detailed information on implementing critical trips for process applications, in particular where programmable electronic systems (PESs) are used as the logic function.
The purpose of a plant alarm system is to bring a malfunction to the attention of the operator(s), whereas a trip system takes protective or corrective action when a fault condition occurs. A plant trip system could shut down the process in an orderly fashion, or it could switch over from some defective unit (such as a pump) to a standby unit. In most cases, a trip system remains dormant and quite frequently unused until there is a demand on the system (or if it is being tested). Alarm and trip systems protect only when they work.
The reliability of alarm and trip systems is achieved through the following:
Their fundamental design.
The conditions under which they operate.
The capabilities of properly trained plant personnel.
The frequency at which they are tested.
Processes are generally provided with two trip systems. The first is a trip system for normal operation (commonly part of the control system) and typically related to production, quality, and financial issues. The second is a safety instrumented system (SIS) for handling critical trips. Additional categories can be generated to account for plant/process-related requirements. Critical trips protect the safety and health of people, and...
