What are key interlocks?

Mechanical key interlocks work by controlling the sequence of events in which valve process activities are conducted.

Key interlocks are dual-keyed mechanical locking devices designed as integral-fit attachments to the host equipment and operate on a ‘key transfer’ principle. This limits actions to only those that produce a safe and desired outcome i.e. preventing a tanker from departing a loading/discharge station until the cargo hoses have been disconnected.

Typically key interlock systems are applied to valves, closures, switches or any form of equipment which is operated by human intervention. The ‘OPEN’ or ‘CLOSED’ status of an interlocked valve, or the ‘ON’ or ‘OFF’ status of an interlocked switch, can only be changed by inserting a unique coded key. Inserting the key unlocks the operating mechanism (e.g. hand wheel or push-button) enabling operation of the valve or switch. Operating the unlocked equipment immediately traps the initial (i.e. inserted) key.

When this operation is complete, a secondary (previously trapped) key may be released, thereby locking the equipment in the new position. This secondary key will be coded in common with the next lock (item of equipment) in the sequence.

By this simple coded-key transfer principle, a ‘mechanical logic’ system is created which denies the scope for operator error.

In addition, keys may be customized to intelligent format by electronic tagging of individual keys and managed by system software that interfaces with the mainframe DCS system.

While popular Permit to Work (PtW), Lockout/Tagout procedures provide a ‘lock-off’ capability, they do not provide any control of the sequence of operations, nor do they assure or confirm the status of the equipment to which they are fixed which can generate dangerous conditions through error or negligence. Interlocks are ideally suited for integration with PtW procedures and provide an effective front-line safety management tool that mitigates the risk of human error.

Published: 25th September 2009