TABLE OF CONTENTS Many devices have been used as final elements in safety instrumented functions. These include annunciation devices like horns, flashing lights or sirens in functions designed for consequence reduction. Some safety instrumented functions need only simple devices like relays, motor controllers and solenoid valves as the final element. In machine control applications the final element may be a motor controller or a device as complex as a clutch-brake assembly which rapidly removes kinetic energy.
In the process industries the most common final element is a remote actuated valve. This assembly usually consists of a pneumatic/hydraulic control assembly, an actuator and a valve (Figure 11-1). The control assembly may be as simple as a three-way solenoid, a smart partial valve stroke box or a complex electro-pneumatic assembly with solenoids, test switches, pneumatic booster relays and quick exhaust valves.
There are many different types of actuators. They are defined by their power source and range of motion. Actuators typically use electric, hydraulic or pneumatic power sources. The range of motion is typically linear, partial turn or multi-turn.
There are many different types of valves used in safety instrumented functions. Typical valve types include ball, butterfly, offset butterfly, gate, globe and other special designs that are not readily categorized into the standard design groups.
All of these different designs have different performance characteristics, different failure rates and different failure modes.
Remote actuated valves must be chosen to match process requirements.
