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Future trends for instrumentation in hazardous areas

11 March 2009

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 to evolve. The change will be gradual and much slower than is usually predicted because of the conservative approach to safety related matters.

MTL
MTL

There are two fairly safe predictions: one is that any relaxing of the current level of safety will not be permitted because it is politically unacceptable; the second is that the need to document the risk will increase and the so will the consequent bureaucracy. This article aims to discuss some of the factors that will affect the decisions and reasoning behind the selection of equipment in the future.

There are always challenges when selecting equipment; for example when hazardous materials are at extreme temperatures and pressures that are outside the limits of ‘normal ambient conditions.’

Oxygen enrichment presents a similar problem. The IEC and most national standards currently do not address this problem and, in European applications, the ATEX directives are not applicable. The usual instrumentation solution is to choose ‘ia IIC T4’ apparatus, preferably with an increased factor of safety. None of the other methods of protection are acceptable in these circumstances.

INTERNATIONALLY ACCEPTABLE SOLUTIONS

There is an increasing demand to design plants so they can be constructed without significant modification in any part of the world. There is some merit in using a recognised practice to achieve a common acceptable level of safety. Fortunately, the IEC Ex certification scheme has made significant progress and although the dream of ‘one certificate acceptable everywhere’ is still some way off, the plan goes from strength to strength.

Intrinsic safety has a considerable advantage over other explosion proof techniques in that it is the only technique which is universally accepted, and can be used in all three zones. The introduction of ‘ic’ to replace the non-incendive technique of type ’n’ extends its applicability to Zone 2. The preferred route for the international use of instrumentation in all hazardous areas has now become an IEC Ex intrinsically safe certification, followed by local ratification of the IEC test report to overcome local legislative difficulties.

How far the ‘ic’ concept will become the established approach to Zone 2 instrumentation, has yet to be determined. The current Zone 2 documents are predominantly written with power systems in mind, and are not conveniently applicable to instrument systems.

The principal advantage of ‘ic’ is that it creates a well-defined approach to Zone 2 instrumentation, allowing existing apparatus certified as ‘ia’, ‘ib’ and ‘simple apparatus’ to be used in ‘ic’ systems. This enables all categories of intrinsically safe systems to be combined in cables and junction boxes. It might be that some end users will prefer to maintain their freedom, to make up their own rules for Zone 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 they are available and provide an economic solution; for example, ‘ia IIC’ certified switch isolators are frequently used in Zone 2 installations. The large factor of safety, versatility, and low cost of these isolators, including their permitted use with any type of switch, makes them an instant universal solution to all switch transfer problems. Their ‘fail-safe’ characteristics and well-established reliability frequently lead to them to be considered, and used, in non-hazardous locations.

RISK ANALYSIS

A side effect of risk analysis is the need to demonstrate that it has been carried out and the inevitable need for detailed documentation. Third party certification of equipment will become the ‘norm’ and installation instructions will need to be improved to provide more specific details.

The use of the IEC Ex and manufacturers’ websites will make the distribution of this information easier. Removing the need to store files of information, which unavoidably become out of date, will have both safety and economic advantages. However, the sole use of websites is not currently acceptable to all regulatory authorities.

FUTURE TRENDS IN INTRINSIC SAFETY

The information derived from intelligent systems such as fieldbus and the HART system could replace the inspection procedures required by the IEC standards. It would be of considerable advantage if electronic interrogation 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 checks can be completed frequently, say weekly, then the approach to fault tolerance can be modified. The result will be safer installations at a lower lifetime cost.

The increased diagnostic capabilities of “intelligent” systems enable some operational faults to be detected from the safe area. This reduces the need for complex test apparatus in the hazardous area. However, many failures - such as wiring faults, or the failure of simple detectors such as switches and thermocouples - involve final testing in the hazardous area and, of course, in these circumstances, ‘live working’ represents one of the most significant advantages of the intrinsic safety technique.

As another example, fieldbus networks also require ‘live working’ because removing power from the whole network in order to replace a single faulty transmitter is not a practical proposition. Fortunately, equipment failure rates are decreasing. Modern manufacturing techniques and the inherent reliability of modern electronics used in “smart” instruments is winning out over the disadvantage normally associated with greater complexity.

In practice any maintenance work on this type of instrumentation requires ‘work permits’ but any technique, which reduces the need for ‘gas clearance’ certificates, is desirable.

There is considerable evidence that plants are at greatest risk during maintenance operations. The ability to achieve ‘live maintenance’ simplifies the procedures and reduces the probability of mistakes, thus creating a positive contribution to safety.

The increasing use of wireless technology will begin to change the pattern of intrinsic safety and the safety level of batteries and the limiting of transmitted power will become increasingly relevant factors. It seems probable that a level of safety ‘ia IIC T4’ will emerge, giving the maximum flexibility in use and choice of location.

Fibre optic transmission is another useful solution to isolation problems and the transmission of high-speed digital information. Its safety requirements are now quite well documented and as a consequence this technique is likely to be increasingly adopted.

There is also an increasing use of battery operated portable apparatus such as personal radios, gas detectors and test equipment. With these products there is no alternative to intrinsic safety. It is often quite difficult to control where this equipment is used and so a high a level of protection is required; a minimum requirement of ‘ib’ is not unusual.

CONCLUSION

The natural synergy of intrinsic safety with the low power requirements of instrumentation when combined with the high level of safety achieved ensure that this technique will continue to be the preferred solution for instrumentation in hazardous areas for the foreseeable future. The ability to carry out live maintenance, replacement, relaxed wiring and electrical protection requirements, give this technique considerable advantages over all other methods of protection. The two areas that are difficult to predict are: to what extent will ‘ic’ become the preferred technique for Zone 2 and will intrinsic safety be widely used where dust is the hazard?

It must be recognised that trends in instrumentation are not entirely based on technical and economic factors. If this were the case, the use of “explosion proof” and “IEC flameproof” techniques in instrumentation would have almost disappeared by now. Many engineers continue to use the techniques that have served them well for many years and, it is true that, the argument “we have always done it that way, and we have never had any trouble” is a powerful one when applied to safety considerations.

It seems likely that in the future, as in the present, intrinsic safety will play a major part in the pool of techniques that are used to solve problems. It will inevitably become a solution worthy of serious consideration and perhaps the preferred choice of some people and organisations. This, with time, will lead to even greater acceptance.

L.C. Towle, MTL Instruments




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