Trips, interlocks, permissives, and sequences

Trips, interlocks, permissives, and sequences

are fundamental concepts in instrumentation control systems such as ESD (Emergency Shutdown), DCS (Distributed Control System), and PLC (Programmable Logic Controller). Here's a summary of each concept:

Trip:

A trip is an action initiated by the control system that forces a device or devices to a predetermined state. Trips are often related to safety functions and are typically initiated by a Safety Instrument System (SIS) or a hardwired system. However, in some cases, PLCs or DCS may also initiate trips if they meet the necessary independence and safety integrity level (SIL) requirements. Examples of trip signals include closing a valve, opening a valve, stopping a motor, etc. Once a trip is activated, the devices remain in the specified state until manually reset by an operator. Trips are usually controlled procedures and can only be reset if certain conditions are met.

Interlock:

An interlock is similar to a trip but is considered a "self-resetting" trip. Interlocks are typically used for on/off control and are not usually classified as safety-related. They are often initiated by DCS or PLC systems. However, if an interlock has safety implications and meets the required SIL rating, it may be implemented in an SIS or hardwired system. Like trips, interlocks force devices to predetermined states (e.g., closing a valve, stopping a motor) but automatically remove the interlock when the initiating cause returns to a healthy condition. Interlocks can usually be overridden for operational reasons or defeated for maintenance purposes.

Permissive:

A permissive is a specific type of interlock used to prevent actions from taking place until certain criteria are met. Permissives are typically initiated by DCS or PLC systems, but if they have safety implications and meet the required SIL rating, they may be implemented in an SIS or hardwired system. Permissives ensure that predefined conditions are satisfied before allowing an action to occur. For example, a permissive may prevent a pump from starting until the suction valve is open. Once the permissive conditions are met and the action is executed, the permissive becomes inactive. Permissives can usually be overridden for operational reasons or defeated for maintenance purposes.

Sequence:

A sequence refers to a prearranged set of actions carried out by the control system. Sequences can be initiated by events or operator actions. They can be either "single pass" or "cyclic." A single-pass sequence involves a series of actions performed once, based on a specific trigger or operator command. For example, an agitated vessel reaching a predetermined level may trigger a sequence that stops the feed pump, closes the filling valve, stops the agitator, and then opens the discharge valve after a 30-second wait. A cyclic sequence, on the other hand, involves actions that repeat based on certain conditions.