Alarms, Safety Trips, and Interlocks

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 

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 or switch off a motor. 

A description of some of the equipment (hardware) used is given by Rasmussen (f975). 

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