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IS2009-2 LCT 25/03/0815/04/08

Future trends for instrumentation in hazardous areas

Introduction

There has been continuous change in the detail of the methods of explosion protection applied to

instrumentation over the last fifty years and the probability is that the techniques will continue toevolve. The change will be gradual and much slower than is usually predicted because of the inevitablyconservative approach to safety related matters. There are two fairly safe predictions; one is that anyrelaxing of the current level of safety will not be permitted because it is politically unacceptable. Thesecond is that the need to document risk and the consequent bureaucracy will increase. Some factors,which will affect the choice of equipment in the future, are discussed in the following sections.

Area classification

The most significant factor to cause change in area classification is that the requirements imposed bylegislation and environmental concerns to minimise leakage of toxic and flammable materials means

that large Zone 1/Division 1 will become unacceptable. Open-air installations will becomepredominantly Zone 2/Division 2 to an even greater extent. Enclosed locations will also be affected butpossibly to a lesser extent. There will remain some small Zone 1 locations and inevitably some

instrumentation will be concentrated in these areas. In some circumstances, for example whereinstrumentation is in close contact with the process fluid, instruments create their own Zone 1locations.

Zone 0 areas are normally confined to spaces with very restricted ventilation such as the vapour spaceof storage tanks and within process vessels. It can be argued that many of such locations are above the

upper flammable limit for most of the time, but persuading an end user not to allocate a Zone 0 is notlikely to succeed. Area classification is difficult and consequently where there is a well-establishedacceptable standard practice this will be used and is almost impossible to change. Electricalinstallations in Zone 0 are always kept to a minimum because they are difficult to install and maintain.

However essential Zone 0 instrumentation such as the measurement of temperature and level areadequately covered by a wide range of intrinsically safe ia apparatus, and this will remain thestandard solution for the foreseeable future. There is an IEC standard IEC 60079-26 which suggeststhat certain combinations of explosion proof techniques are acceptable for use in conjunction with

Zones 0 but this approach is a second best compromise which solves a few difficult problems and isbest avoided if possible.

There is always the difficult problem of selecting equipment when the hazardous materials are attemperatures and pressures outside the limits of normal ambient conditions. Oxygen enrichmentpresents a similar problem. The IEC and most national standards do not address this problem and

fortunately in European applications the ATEX directives are not applicable. The usual instrumentationsolution is to choose ia IIC T4 apparatus, preferably with an increased factor of safety and offer aprayer to an appropriate god. None of the other methods of protection are acceptable in these

circumstances.

Internationally acceptable solutions

There is an increasing demand to design plants so that they can be constructed in any part of the worldwithout significant modification and there is some merit in using a uniform recognisable practice toachieve a common acceptable level of safety. Fortunately the IEC Ex certification scheme has madesignificant progress and although the dream of one certificate acceptable everywhere is some way offthe scheme goes from strength to strength. A major delaying factor is that the U.S., although permittingthe use of Zones, still predominantly uses the two Division approach to area classification with itsconsequent allegiance to U.S. explosion-proof. Additionally the requirement for local certification in

the U.S. is a hindrance, which is not likely to be removed in the near future since there are considerablevested interests in its retention. A similar situation exists in Europe where the ATEX apparatusdirective represents a similar barrier to trade. There are laudable attempts to reduce these problems butchange will require persistence and time.

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Intrinsic safety has a considerable advantage over other explosion proof techniques in that it is the onlytechnique, which is universally accepted and can be used in all three zones. The introduction of ic toreplace the non-incendive technique of typen extends its applicability to Zone 2. The preferred route

for the international use of instrumentation in all hazardous areas has now become IEC Ex intrinsicallysafe certification followed by local ratification of the IEC test report to overcome local legislative

difficulties. However how far the ic concept will become the established approach to Zone 2instrumentation has yet to be determined. The current Zone2/Division 2 documents are predominantlywritten with power systems in mind and are not conveniently applicable to instrument systems. Theprincipal advantages of ic are that it does create a well-defined approach to Zone 2 instrumentation, it

does allow existing apparatus certified as ia or ib and simple apparatus to be used in ic systemsand enables all categories of intrinsically safe systems to be combined in cables and junction boxes. Itmay be that some end users will prefer to maintain their freedom to make up their own rules for Zone2/Division 2 instrumentation as appears to be the current practice in some large organisations.

In practice many ic systems will use ia or ib certified apparatus because it is available andprovides an economic solution. For example ia IIC certified switch isolators are frequently used inZone 2 installations. The versatility, low cost and large factor of safety of these isolators plus their

permitted use with any type of switch make them an instant universal solution to all switch transfer

problems. Their fail-safe characteristics and well-established reliability frequently lead to them beingused in non-hazardous locations.

Risk analysis

This approach to explosion prevention is the current flavour of the month. European legislation in theform of the ATEX installation directive [1999/92/EC] requires a risk analysis of all installations. TheIEC standards by introducing categories of apparatus are encouraging this approach. The fact thatnowhere is the acceptable risk defined is a slight problem but this is apparently not a deterrent tolegislation. The major implication of this approach is that the consequences of an explosion have to betaken into account. Area classification may be used as part of the risk analysis but alternative solutions

are permitted.

The consequences to be considered include an assessment of the number of people killed or injured bythe explosion and environmental damage caused directly or indirectly by the explosion. How fareconomic consequences such as loss of production, destruction of the installation and damage toadjacent property have to be considered is possibly not a safety concern but may influence decisions onthe precautions to be taken. A quantitative assessment of all the risks is not possible so anacknowledgement of their existence and an approximate estimate of risk has to suffice. Where the risk

is substantial then a high level of protection might be required of the instrumentation. For example afuel store within a densely populated area might require ia IIC T4 throughout, since this would be thesafest economic solution available. It is theoretically possible that an unmanned site in the middle ofthe dessert could use normal instrumentation, but this is a relaxation of the requirements and istherefore unlikely to be allowed.

A side effect of risk analysis is the need to demonstrate that it has been done and the inevitable need for

detailed documentation. Third party certification of equipment will become the norm and installationinstructions will need to be improved and possibly made more specific. The use of the IEC Ex andmanufacturers web-sites will make the distribution of this information easier. Removing the need tostore files of information, which inevitably become out of date will have both safety and economicadvantages. However the sole use of web-sites is not currently acceptable to all regulatory authorities.

Presumably the preservation of trees is not part of their green agenda.

Longer term it will be interesting to see how many engineers consider themselves competent to preparethe necessary risk analysis. It may be that the prudent engineer will minimise his personal risk byinsisting on well-documented certified apparatus of the highest category. Signing one of thesedocuments could adversely affect ones personal and corporate liability. The adage that nobody gotfired or caused an explosion by using ia IIC T4 apparatus may become engineering folklore.

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Dust

There is increased awareness of the hazard associated with combustible dust and some of theinstrumentation needs cannot be solved by any technique other than intrinsic safety. For example the

measurement of the temperature of a pile of grain is best done by an 'ia IIB system with a fault powerlevel less than 250 mW. The IEC standards now cover the use of intrinsic safety where dust is the

hazard. However the use of this technique in dust applications is at present limited, but is expected togrow as the appreciation of the risk of dust explosions increases and the pressure from legislationgrows.

There are some locations where there is a combined dust and gas hazard. The safety requirements ofapparatus for use in these environments are being considered by an IEC working group, but this work

is at a very early stage. Meanwhile the only acceptable solution for instrumentation is to useintrinsically safe apparatus taking into account both risks. In this situation ia IIC apparatus with amatched power less than 750 mW is frequently the solution.

Future trends in intrinsic safety

Changes in this technique are usually governed by IEC standards, which are in turn driven by changes

in technology. The current five-year cycle for these standards is extremely slow but it is difficult to seehow this can be improved. The present system ensures that the standard is a consensus document,which has been given considerable thought by many people from different backgrounds. It is not aperfect system but it is the only one we have got. That it can function reasonably well is illustrated bythe FISCO standards for Fieldbus systems, which have been produced at a reasonable speed.

Ideally future work ought to aim to make the application of intrinsically safe apparatus simpler. Forexample, the simpler interconnecting cable requirements used in the FISCO standard could be extendedto other systems.

Possibly the information derived from intelligent systems such as the HART system could replace the

inspection procedures required by the IEC standards. It would be a considerable advantage if electronicinterrogation removed the need to read largely unintelligible labels in the field. It should be possible to

modify the operational integrity checks to include safety checks. If this can be done and the checkscould be done frequently, say weekly, then the approach to fault tolerance could be modified. Theresult might be safer installations at a lower lifetime cost.

Conclusion

The natural synergy of intrinsic safety with the low power requirements of instrumentation togetherwith the high level of safety achieved ensure that this technique will continue to be the preferredsolution for instrumentation in hazardous areas for the foreseeable future. The ability to do livemaintenance and replacement, the relaxed wiring and electrical protection requirements, the simpleapparatus rules and the comprehensive code of practice give this technique considerable advantagesover all other methods of protection. The two areas, which are difficult to predict, are how far ic will

become the technique of Zone2 and whether intrinsic safety will be widely used where dust is the

hazard.

It must however be recognised that trends in instrumentation are not entirely based on technical andeconomic factors. If this were the case the use of U.S. explosion proof and IEC flameproof techniquesin instrumentation would have almost disappeared by this time. Inevitably many engineers continue to

use techniques, which have served them well for many years, and they believe they understand. Anelement of not invented here is also fairly widespread. It is also true that the argument that we havealways done it that way, and we have never had any trouble is a powerful one when applied to safetyconsiderations.

It seems probable that in the future, as in the present, intrinsic safety will be part of the pool oftechniques to solve problems. It will be the preferred solution of some people and some organisationsand its merits will lead to even greater acceptance