An SIS is intended to perform specific control functions to prevent unsafe process operations when unacceptable or dangerous conditions occur. Because of its criticality, safety instrumented systems must be independent from all other
control systems that control the same equipment, in order to ensure SIS functionality is not compromised. A SIS is composed of the same types of control elements (including
sensors,
logic solvers,
actuators and other control equipment) as a Basic
Process Control System (BPCS). However, all of the control elements in an SIS are dedicated solely to the proper functioning of the SIS. The essential characteristic of an SIS is that it must include instruments, which detect the process variables (flow, temperature, pressure etc. in the case of a processing facility) are exceeding preset limits (
sensors), a logic solver which processes this information and makes appropriate decisions based on the nature of the signal(s), and final elements which receive the output of the logic solver and take necessary action on the process to achieve a safe state. All these components must function properly for the SIS to perform its SIF. The logic solver may use electrical, electronic or programmable electronic equipment, such as
relays, trip
amplifiers, or
programmable logic controllers. Support systems, such as power, instrument air, and communications, are generally required for SIS operation. The support systems should be designed to provide the required
integrity and
reliability. One example of
SIF is a
temperature sensor that provides a signal to a controller, which compares the sensed process temperature to the desired temperature setpoint and sends a signal to an emergency on-off valve actuator which stops the flow of heating fluid to the process if the process temperature is exceeded by an unsafe margin. SIFs are implemented as part of an overall
risk reduction strategy which is intended to minimize the likelihood of a previously identified accident that could range from minor equipment damage up to the uncontrolled catastrophic release of energy or materials. The safe state must be achieved in a sufficiently short amount of time (known as
process safety time) to prevent the accident. == International standards ==