Alarms are used to alert operators of serious, and potentially hazardous, deviations in process conditions.
Key instruments are fitted with switches and relays to operate audible and visual alarms on the control panels and annunciator panels. Where delay, or lack of response, by the operator is likely to lead to the rapid development of a hazardous situation, the instrument would be fitted with a trip system to take action automatically to avert the hazard; such as shutting down pumps, closing valves, operating emergency systems.
The basic components of an automatic trip system are:
A sensor to monitor the control variable and provide an output signal when a preset value is exceeded (the instrument).
A link to transfer the signal to the actuator, usually consisting of a system of pneumatic or electric relays.
An actuator to carry out the required action; close or open a valve, switch off a motor.