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Product category: Shutdown, SIS and safety systems
News Release from: Smith Flow Control | Subject: Safety interlocks
Edited by the Processingtalk Editorial Team on 16 May 2003

The case for safety interlocks

Mike Smith of Smith Flow Control, supplier of safety interlocking equipment for the oil, gas and chemical industries, describes the benefits of safety interlocks for the hydrocarbon engineering sector

The article concentrates on technology from Smith Flow Control, but also touches on safety equipment offered by two other Halma Group companies, Castell Safety International and Iso-Lok As a general principle, it can be said that activities in the processing industries which are safe when performed correctly can have catastrophic consequences when performed incorrectly

The hydrocarbon processing industries generally have a very disciplined approach to design and operating practice, governed largely by well-recognised international standards and enforced by certification authorities.

While good practice begins with good design, both are inevitably hostage to the 'human factor'.

An abundance of statistics and case examples exist to confirm this, and readers of this article will each have knowledge of high profile (and other less publicised) incidents where the 'human factor' contributed to, or directly caused, accidents.

Up to 70% of reported incidents in the oil and gas industry worldwide, for example, are attributable to human error, accounting for in excess of 90% of the financial loss to the industry.

The UK Health and Safety at Work Act (1974) places a responsibility on the people who design, manufacture or supply equipment for use at work to ensure, as far as is reasonably possible, that they are safe.

Throughout the developed world, and in particular within the European Community, the thrust of legislation is towards making all 'professionals' legally responsible for their actions.

As practitioners within the hydrocarbon processing industry, we should not only consider risk from the perspective of injury to persons or damage to plant, but also from the perspective of protecting the environment.

Indeed, the World Federation of Engineering Organisations (WFEO) has drafted an international code of Environmental Ethics for Engineers, which has been approved by European national engineering associations and UNESCO.

These points all gravitate towards the inevitable conclusion that 'responsible engineering' will become a by-word demanded by society and legislated for by the authorities.

Internationally, there are strong indications of a fundamental shift of emphasis in legislation from prescriptive regulations to the risk-management approach.

This approach places primary legal responsibility on 'owners' to adopt 'best available' technology and methods to ensure safety.

This legislative trend, set against the trend of 'downsizing' internationally by hydrocarbon processing (owner) operating companies for commercial imperatives, suggests a potential conflict of priorities.

Operating (owner) companies are increasingly reducing their own manning levels and contracting out the operation and/or maintenance of their assets to independent contractors.

These contracting arrangements may be characterised by a higher rotation of personnel at site because they are, typically, relatively short-term agreements.

The 'job-for-life' syndrome, which produced the '25-year' dedicated company man, is fast disappearing, being replaced by higher levels of process automation and increasing dependence on 'partnering' with contractors.

This 'casualisation' of on-site labour inevitably increases the risk of accidents through human error and demands higher levels of applied safety systems to mitigate this risk.

What are key interlocks? Many routine procedures are potentially dangerous if executed incorrectly, or in unsafe conditions, with the scope for injury and/or damage being significantly increased when high temperature, high pressure or toxic/flammable product is present.

Key interlocks are dual-keyed mechanical locking devices, which operate on a 'key transfer' principle to control the sequence in which process equipment may be operated.

Key interlock systems are gaining increasing recognition as an effective safety tool and are recommended in a number of internationally recognised standards for specific process applications, which can be listed as.

API RP 14E - Design and Installation of Offshore Production Platform Piping Systems (Para.5.8.b (2) - Relief Device Piping).

API RP 520 - Pressure Relieving Systems for Refinery Services (Part 2, Section 4 - Isolation Valve Requirements).

NFPA 12 - National Fire Protection Association (USA) - Carbon Dioxide Extinguishing Systems (1993 Edition).

BS 5306 - British Standard (Part 4, 1986 - Specification for Carbon Dioxide Systems).

BS 8010 - Code of Practice for Pipelines (Part 2, 1992 - Sect 2.8; Part 3, 1993 - Sect 6.6).

1996 No.825 - (UK) The Pipelines Safety Regulations (Section 6, Para 37 of Guidance on Regulations - published by UK Health and Safety Executive).

The hardware is relatively simple and is based on specialised mechanical locks designed as integral-fit attachments to the host equipment.

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 (eg hand-wheel or push-button) thereby enabling operation of the valve or switch.

Operating the unlocked equipment immediately traps the initial (ie inserted) key.

When the 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.

Mechanical key interlock systems are ideally suited for integration with Permit-to-Work (PtW) procedures; indeed, the Cullen Report on the Public Inquiry into the Piper Alpha Disaster (1990) strongly recommends the use of interlocking systems integrated with PtW procedures, especially where routine procedures cannot be accomplished in the time-scale of a single shift.

In addition to the standards referred to earlier, the Technical Guidance Notes supporting the UK 'Pipeline Regulations (1996) Act' also recommend interlocks as a suitable safety system in the operation of Pig Traps.

Key interlocks date back to the 1890's, where they were first used in the French railway system to control track-switching operations.

In the UK in 1928, James Harry Castell developed the original form of modern day key interlocks, which came quickly and prominently into common usage in electrical switchgear applications to prevent the paralleling of electrical supply in bus-bar systems (these will be touched on later).

Modern integral-fit key interlock systems for hydrocarbon processing and pipeline systems did not emerge until the early 1980's.

In the interval since then, industry acknowledgement of their effectiveness has led to their extensive use worldwide and an increasing degree of adoption within international standards and codes of practice.

Whether a process module is of simple design with basic functions controlled by manually-operated valves, or of a complex design controlled by sophisticated mainframe Distributed Logic Control (DLC) systems, key interlocks can provide a totally reliable mechanical assurance of safe operating practice in which the operator's scope for error is eliminated.

Indeed, within DLC-controlled systems, which invariably incorporate electrical interlocking ('trips'), these are limited to governing only the operation of motorised valves (MOVs); associated sundry services valves (eg for venting), may well be served by manually-operated valves, and correct operation of these valves is dependent on the operator following written operating procedures.

In such systems, key interlocks are not intended as the primary safety system but as a secondary total back-up system to the primary (DLC) system.

The inherent advantage with key interlocks is that by being mechanical, they are not power-dependent, and yet may be designed to attach to MOVs without compromising their operating or failsafe function.

Designs have been developed in recent years to provide key interlocking solutions that offer the only total form of interdependent control over the operation of MOVs and manually-operated valves in one fully-integrated system.

While microprocessor-based DLC systems are enormously efficient and are now invariably standard as a process systems management tool, they are not, however, 'scrutable'.

Software faults may not become apparent until a process malfunction actually occurs.

(Indeed, the American standard ISA-S84.01-1996 - 'Application of Safety Instrumented Systems for the Process Industries', recognises this characteristic, stating that software 'has a high probability of systematic failures').

Key interlock systems on the other hand - being mechanical devices - are entirely 'scrutable', and therefore reliable, as effective safety management tools.

The interfacing of both technologies offers enhanced operational efficiency with improved assured safety.

In process systems where the valving and/or control components are all manually-operated (i.e not DLC controlled), key interlocks become the primary safety system.

Whether adopted as a primary or secondary safety system, the key interlock installation can be customised to intelligent format by electronic tagging of the individual keys.

This is achieved by fitting each key with an ID chip, which is read by a tag reader in the control room key cabinet.

The key cabinet system incorporates a PC, which manages the system software, and this can be interfaced with the mainframe DLC system by a simple twin-wire connection.

Other uses for interlocks Castell Safety International, also part of the Halma Group, specialises in trapped key interlocking systems for a broad range of industrial applications beyond valve interlocking.

These include CO2 access control, electrical switchgear and electrostatic discharge safety procedures.

Automatic CO2 total flood systems are designed to rapidly reduce the oxygen content within a protected area to extinguish a fire.

The Castell EDIX system, for example, provides personnel protection by making sure that both automatic and manual trigger systems are isolated before access to the protected area is permitted.

An emergency crash-out feature is incorporated as an additional safety measure.

Switchgear interlocks, on the other hand, control the operation of disconnectors, isolators and earth switches, as well as access to capacitor banks, within electrical substations.

They ensure that disconnectors can never be operated whilst on load, that power is isolated before earth switches are closed and that maintenance operations are carried out safely.

Finally, the Castell Earth-Line system obliges operators to follow a pre-determined process to guarantee that chemical tankers, storage vessels and vehicles undergoing refuelling are correctly earthed before the handling of potentially explosive substances can proceed.

If the resistance to earth exceeds a pre-set limit, an alarm sequence is triggered and the loading/unloading operation is automatically shut down.

Iso-Lok, another Halma company, which is affiliated to Castell, manufactures specialised valve safety covers and lockout devices for ball and butterfly valves.

The Iso-Lok Ball Valve Lockout is a unique device that allows a valve to be locked in either an open or closed position; while its B-Safe Butterfly Valve Lockout is currently the only way to secure butterfly valves efficiently.

All these locks come in a variety of sizes and are securely locked with Iso-Lok padlocks or multi-clasps.

From the above discussion, I hope to have shown that key interlocks and associated mechanical interlocking systems provide a highly effective front-line safety management tool which can significantly mitigate against the risk of human error in the hydrocarbon engineering sector.

In most key interlocking procedures, there is only one key 'free' at any time.

This is the essence of a well-designed interlock system: it must always be operator-friendly.

An overly complex system will defeat the objective.

The message is, 'Keep it simple'. Request a free brochure from Smith Flow Control ...

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