Mechanical trapped key safety interlocks offer a way round human error in production

The UK HSE (Health and Safety Executive) at Work Act 1974 places responsibilities on people who design, manufacture or supply equipment for use at work. PUWER (Provision and Use of Work Equipment Regulations, 1998)  places duties on people and companies who own, operate or have control over work equipment, and on businesses and organisations whose employees use work equipment, whether owned by them or not.

Throughout the world, the thrust of legislation is towards making professionals legally accountable for their actions, a responsibility framed in civil law, extending through to criminal law.

Aas engineering practitioners throughout the industries, we should not only consider risk from the perspective of injury to persons or damage to facilities and assets, but also from the perspective of the environment.

Whether operating procedures with basic functions are controlled by manual operation or complex designs controlled by sophisticated main frame DCS (Distributed Control Systems) Mechanical Trapped Key Safety Interlocks provide a reliable mechanical assurance of safe operating practice in which scope for error is eliminated.

Within DCS systems, which invariably incorporate electrical interlocking ‘trips’, these are limited to governing only the operation of motorised valves (MOV’s). Associated service valves i.e. manually operated valves and their correct operation, are dependent on the operator following a pre-determined sequential operating procedure.

DCS Mechanical Trapped Key Safety Interlocks are intended as a secondary total back-up system. Being mechanical, they are not power dependent and yet may be designed to attach to MOV’s without compromising their operating of fail-safe principal function.

Designs have been developed in recent years to provide Trapped Key Interlocking solutions that offer the only total form or interdependent control over the operation of manual valves - utilised in PSV (Pressure Safety Valve) systems and MOVs, plus manual valves and closure doors – utilised in Pipeline Pig Trap Systems.

Many routine procedures are potentially dangerous if executed incorrectly or in unsafe conditions and the scope of injury and/or damage to assets and the environment is significantly increased when high temperature, high pressure or toxic / flammable product is present, as is frequently the case in most industrial applications.

Trapped Key Safety Interlocks operate on a key-transfer principle to control the sequence in which process equipment may be operated. The hardware equipment is relatively simple but the product lock portions, uniquely coded operating keys and principle product design has to withstand any form of unauthorised tampering, overriding or ‘operated-in-error’ actions.

Products should meet all the latest ISO manufacturing standards for trapped key safety interlocks. They should be designed as an integral-fit attachment to the host equipment and be applied to valves, closures and switches or any form of equipment which is operated by human intervention.

In the best products, the open/closed status of an interlocked valve or on/off status of an interlocked switch can only be changed by inserting and rotating a uniquely coded key. Operating the unlocked equipment immediately traps the initially inserted key. When the operation is complete, a secondary (previously trapped) key will be released, thereby locking the host equipment in the new position. The secondary key will be coded in common with the next Interlock (equipment) in the sequence: this simple coded rotary key transfer principle creates a ’mechanical logic’ system which removes potential for operator error.

Mechanical Trapped Key Safety Interlock systems are ideally suited for integration with PtW (Permit-to-Work) procedures. The Cullen Report following the Public Inquiry into the Piper Alpha disaster strongly recommended the use of locking systems integrated with PtW procedures.

Wayne Hodgson is MD at ENEX Group