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BASICS OF TRIPS, INTERLOCKS, PERMISSIVES & SEQUENCES

This post provides information about basics of Trip, Interlock, Permissive and Sequences which are regularly used in instrumentation control systems like ESD, DCS, PLC etc.

Trip:

The term Trip refers to an action that is initiated by the control system and which forces a device or devices to a pre-determined state.
Example of Trip Signals: Close Valve, Open Valve, Stop motor, etc.
The Safety Instrument System (SIS) or a Hardwired systems normally initiate trips, however the PLCs or DCS may also initiate trips provided the necessary independence and SIL ratings are met.Once a device or devices have been forced to a pre-determined state by the action of a Trip they will remain in that state until the Trip is manually reset by a conscious operator action.
For example:
  • High level in a vessel initiates a trip system which stops the pump feeding that vessel, the pump will remain stopped even if the level in the vessel falls to a safe level.
  • The Trip must be ‘reset’ by the operator before the pump can be re-started.
  • The Trip can only be ‘reset’ if the level in the tank has fallen to a safe level.
  • Resetting the Trip will not cause the pump to automatically re-start, however it may be re-started by an operator action or a control system command e.g. part of a sequence.
The resetting of Trips is a controlled procedure which will only be possible if the operator is logged in and has the necessary access rights.Under normal circumstances it shall not be possible to ‘override’ or `defeat’ Trips.

Interlock:

An Interlock is in essence a ‘self resetting’ Trip. Interlocks are not deemed safety related and can be used for on/off control.Interlocks are normally initiated by the DCS or PLCs, however if an Interlock is deemed to be safety related it may, depending upon SIL rating, be implemented in the SIS or a Hardwired system.
An interlock will force a device or devices to a pre-determined state e.g. Close valve, stop motor, etc.Once a device or devices have been forced to a pre-determined state by the action of an Interlock they will remain in that state until the initiating cause returns to a ‘healthy’ condition, the Interlock will then be automatically removed.
Under normal circumstances it shall be possible to ‘override’ Interlocks for operational reasons or ‘defeat’ them for maintenance or other reasons.

Permissive:

A Permissive is a patricular type of Interlock used to prevent actions taking place until pre-defined criteria have been satisfied, for example prevents a pump starting until the suction valve is open.
Permissives are normally initiated by the DCS or PLCs, however if a Permissive is deemed to be safety related it may, depending upon SIL rating, be implemented in the SIS or a Hardwired system.
Once a Permissive has been satisfied and the resulting action implemented it becomes inactive, for example once the suction valve has been opened and the pump started the Permissive takes no further action, even if the suction valve is closed while the pump is running.
Under normal circumstances it shall be possible to ‘override’ Permissives for operational reasons or ‘defeat’ them for maintenance or other reasons.

Sequence:

A Sequence is defined as a pre arranged action or number of actions which are carried out by the control system. Sequences may be initiated by an event or operator actions.
Sequences may be ‘single pass’ or ‘cyclic’.
The following is an example of a ‘single pass’ sequence:
An agitated vessel reaches a pre-determined level. The operator initiates a sequence that carries out the following actions:
  • Stop the feed pump
  • Close the filling valve
  • Stop the agitator.
  • Wait 30 seconds.
  • Open the discharge valve.
The following is an example of a ‘cyclic’ sequence:
  • Low level in a vessel opens the filling valve.
  • The valve remains open until high level is detected.
  • On high level the valve closes.
  • The valve remains closed until low level is detected.
  • On low level the valve opens and the sequence it repeated.

Combined Functions:

It is common for Trip, Interlock, Permissive and Sequences to fulfill combined functions, for example the following pump protection system illustrates how the same system can perform various functions.
Permissive
Prevent pump starting until suction valve is open.
Interlock
Pump running – suction valve closed-pump stops.
Sequence
High level in vessel-Pump stops
Low level in vessel-Pump starts.
Pump running – suction valve closed – pump stops,
Suction valve re-opened – pump remains stopped.
Operator resets trip.
Pump available for re-start.

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