Alarms and Interlocks

Alarm systems have a very close relationship to safety shutdown systems (SIS) but they do not the same function as a SIS. Alarm systems are essential to maintaining integrity of processing units. Alarm systems alert operators to plant conditions, such as deviation from normal operating limits and to abnormal events, which require timely action or assessment.

Alarm systems are not normally safety related, but do have a role in enabling operators to reduce the demand on the safety related systems, thus improving overall plant safety.

There are many types of alarm systems:

Traditionally, alarms are typically implemented using “light box” style annunciators. These devices use engraved plastic nameplates back lit with lamps. Upon activation, a horn will sound and the appropriate lamp will flash until the operator acknowledged the alarm. An important characteristic of such form of implementation is that there was limited or no integration with control functions.

Alarms implemented within the PCS are designed to emulate the traditional annunciators. When a variable goes into alarm the indication of the variable will flash and change colors and a horn or other audible will sound. The acknowledge sequence will usually be the same as that used in dedicated annunciators: pressing a key will silence the audible indication and cause the flashing to stop.

Interlocks provide logical constraints within control systems and often provide a safety-related function. Their role in providing safety-related function must be evaluated in each particular application to decide if they are part of the process control system, an independent safety device or part of the SIS.