What is a valve interlock?

A valve interlock is a trapped key lock assembly which locks the valve in one or two positions - open and/or closed - with one key trapped within the lock assembly and one key free. It's about controlling the sequence of events, conducting different valve process activities. 

To change the valve position (open-to-close or close-to-open), two keys need to be inserted into the lock assembly. The free key can only be released when the valve is in the open or closed position. Alternatively a 'single keyed interlock' only allows removal of the locked open or closed key when the valve is in the correct locking position.

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Objective of the valve interlock system

Valves play an important role in safety of operation in many process industries, especially in chemical and petroleum industry. The opening and closing of valves, especially block valves on PSVs for example, in the wrong sequence can lead to serious consequences such as human fatal injuries, loss of production and damage of equipment.

Therefore valves critical for safety of the operations can be equipped with an interlock system to ensure that valves are operated in a safe sequence and maintained in a safe position. The valve interlocks not only control the opening and closing sequence of process valves but also prevent their unauthorized operation.

What is a key interlock?

The types of interlock systems are based on the principle of key exchange or a key transfer principle. Valves can only be operated in a predetermined sequence which is designed to maintain production, availability and the safety of systems.

Integral-fit mechanical locking devices

Key interlocks are integral-fit mechanical locking devices, attached to the host equipment. Typical interlock systems are applied to valves, closure doors, rotary switches or any form of equipment operated by human intervention. In addition, keys can be customized to intelligent format: electronic tagging of individual keys and managed by system software.

The lock mechanism is designed to ensure that valves are always either fully open or fully closed. Generally, the system is based on the operating principle of one key free when the valve is locked closed and the other key free when the valve is locked open.

Control of the sequence of operations

Popular Permit to Work (PtW) or Lockout/Tagout procedures provide a lock off capability. But they don't provide any control of the sequence of operations or the status of equipment. This can generate dangerous conditions through negligence. Interlocks provide an effective front-line safety tool that mitigates risks of human error. 

Visualizing interlock explanation

In order to visualize all information above we made an attractive inforgraphic for you.
You can download this infographic below. 

Quarter-turn interlocks on Rotork 242P quarter-turn worm gearbox Gear operated valve interlock Rotork handwheel valve Interlocks for Rotork IQ3 Actuators

Valve interlock system sample

The type of interlock system below is designed to mechanically interlock two pressure safety valves having two upstream block valves and two downstream block valves to prevent isolation of both the relief valves at the same time and allow safe isolation of the safety valve for maintenance.

Did you know we made a video about this valve interlock system sample?

Operating condition of the valve interlock

  • One PSV (PSV-A) is operational and other PSV (PSV-B) is standby
  • Key B is taken from the control room

To put PSV-B into maintenance:

  • Insert key B into valve #4 and close by releasing key A
  • Key A can be taken to the control room to keep PSV-B isolated for maintenance

To change-over putting PSV-A into operation and PSV-B into operation:

  • Insert key B into valve #1 and open by releasing key C (both safeties are online)
  • Insert key C into valve #2 and close by releasing key D, which can be taken to the control room

PSV-B is now in operation and PSV-A is in standby. 

To further isolate PSV-A for maintenance:

  • Insert key D into valve #3 and close releasing key E
  • Key E can be taken to the control room to keep PSV-A isolated for maintenance

Both PSVs will never be closed simultaneously, ensuring that at least one PSV is always available for equipment protection.

Example sequence PSV


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