Was Port Wentworth an accident waiting to happen?
01 November 2008
The catastrophic explosion at the Imperial Sugar Refinery at Port Wentworth in Georgia, US last February demonstrates that there are important lessons to be learned with regard to dust explosion hazards. David Brearley asks whether a major dust explosion was inevitable and looks at the lessons to be learned from the disastrous sugar refinery explosion in Georgia earlier this year
The Imperial Sugar Refinery accident claimed 13 lives
Thirteen people were killed and many more injured. And it could have been even worse had the explosion occurred a few hours earlier during the day shift, when there were three times as many staff on site.
The handling and processing of potentially flammable dusts and powders is an everyday part of many industries, but it can be a particular problem for food, fertiliser and pharmaceutical manufacturers. Anyone working in these industries should be well aware of the potential problems they face and have taken the appropriate steps to prevent any chance of explosions.
When faced with any potential explosion risk, the preferred solution is to try to remove the fuel source, or reduce its volume, and therefore limit the extent of the hazardous zone. For those faced with gases and vapours as a fuel source this can often be achieved, but when handling powders and dusts it is very difficult, because the powder being processed cannot be removed.
When processing combustible powders, it is often difficult to completely remove the risk of primary explosions, which might occur within the processing equipment. However, with well designed venting or containment those primary explosions on their own do not usually result in the devastating destruction and loss of life seen at the Port Wentworth Refinery.
Explosions of this magnitude are almost always the result of secondary explosions of combustible powder, which has accumulated on surfaces throughout the plant. The primary explosion can disperse the powder, especially on elevated surfaces, resulting in a large flammable cloud ignited by the primary explosion. It is vital therefore in any powder handling facility to observe a very strict housekeeping regime to ensure that the build-up of powder on any surface is kept to an absolute minimum.
This would appear to be one of the major failings at the Imperial Sugar refinery. Photographs taken two years before the incident show large drifts of sugar on the floor, up to several feet in places, and also as much as a few inches on elevated surfaces. Company records indicate the need to clean up about 15,000lbs per week from the mill room, and losses of 34,000lbs in one month from an elevator.
To put these figures into perspective, National Fire Protection Association (NFPA) guidance indicates that accumulations of combustible dust of just 1/32 of an inch covering as little of 5 per cent of the available surface area must be considered hazardous.
The source of ignition for the primary explosion at Port Wentworth has not yet been determined, but one likely location is underneath the sugar silos. An enclosed conveyor runs beneath the silos and some of the steel panels forming the enclosure were blown out.
If this was not the source then there are plenty of other potential candidates. This really was an accident waiting to happen. A lot of the electrical equipment on site was not adequately protected against dust ingress and therefore should not be have been installed where there was a possibility of flammable dust clouds forming.
Ongoing investigations and enquiries into this incident have highlighted a number of issues, which might have contributed to this disaster, including the lack of appropriate standards in this area in the USA. This would therefore be a good opportunity to review the current status of the European standards that would apply had the refinery been located within the European Union.
The first principle as always would be to remove or reduce the flammable material as far as reasonably practicable. The whole plant would then be subjected an inspection in accordance with EN 61241-14 to determine the nature and extent of the hazardous zones present. The zones for flammable dusts are defined in table 1.
The ATEX User Directive, 1999/92/EC, allocates equipment, categorised in accordance with 94/9/EC, to the zones in which it may be used. Category 1 equipment is for use in Zone 20, Category 2 for Zone 21 and Category 3 for use in Zone 22.
Having zoned their premises, users are then in a position to specify the category of equipment they need to purchase for each zone and approach the manufacturer with this specification. For existing plant, the owner should have already undertaken a risk assessment on all existing equipment located in hazardous zones to make sure it is appropriate for use in that zone.
The standards applicable to electrical equipment for use in hazardous dust areas are changing rapidly. Electrical equipment for hazardous dust atmospheres has required certification for many years but until recently, manufacturers have been limited to a single approach when certifying their equipment for use in these areas.
Since 1998, a single standard, EN50281-1-1, has been applied to all equipment regardless of type or the zone in which it is intended for use. The same standard applied to a micro-switch for use in Zone 20 and a 500kW motor for use in Zone 22. EN50281-1-1 was based on the very simple requirement that all equipment should have an enclosure, which provided adequate ingress protection (IP) depending on the likelihood of encountering a dust atmosphere.
Category 3 equipment located in Zone 22 only required an IP5x-rated enclosure because the probability of a flammable dust cloud in Zone 22 is very low. Category 1 and 2 equipment required a higher IP6x-rated enclosure because of the increased probability of encountering a flammable dust. The only exception was that if the potential dust cloud was electrically conductive then all enclosures had to be at least IP6x - regardless of category and zone.
The safety of equipment certified to EN50281-1-1 is not in question, but clearly not all equipment needs to be placed in an IP5x or IP6x enclosure to prevent it igniting a dust cloud. Dust clouds are much more difficult to ignite than gases, typically requiring mJ rather than µJ of energy so equipment which is certified Ex i or Ex m as safe for use in gas should also be safe for use in dust atmospheres without requiring an additional enclosure.
Consequently, there now exists a set of standards, the EN 61241 series, which offer a more flexible approach to dust certification (see table 2). The format of the standards closely follows the EN 60079 series of standards (electrical apparatus for gas atmospheres) with many concepts now common to both gas and dust.
EN 61241-1 is the exception. This, together with EN61241-0, is the replacement for the previous standard, EN 50281-1-1, relying mainly on the ingress protection of the enclosure to protect the equipment. The remaining standards, parts 11, and 18 are similar to their gas equivalents, meaning it is now simpler to do combined gas and dust certification for intrinsically safe and encapsulated equipment.
Work is already underway to combine the gas and dust standards. The next editions of EN 60079-0, EN 60079-11 and EN 60079-18 will include both gas and dust requirements. Baseefa’s presence on these standards committees means that we are able to advise you in advance of technical changes which affect your product enabling you to make any design changes in advance of the standards publication.
Historically there were no standards in place for non-electrical equipment for use in hazardous areas, but that changed in 2001 with the introduction of the EN 13463 series of standards. These would be particularly relevant to the Imperial Sugar Refinery explosion if the conveyor system was found to be the source of the primary explosion. The standards for non-electrical equipment are described in table 3.
Non-electrical certification takes a very different approach to the long established standards for electrical certification. Non-electrical certification is based on a risk assessment approach. The level of risk assessment depends on the hazardous zone and therefore the category of the equipment.
Category 3 equipment must be safe for use in normal operation. Category 2 equipment must be safe for use in normal operation and expected malfunction and Category 1 equipment must be safe for use in normal operation, expected and rare malfunction. Potential ignition sources identified in the risk assessment are made safe by applying one or more of the concepts described above.
Future standards development
At the moment the electrical standards, EN 60079 and EN 61241, begin life as IEC standards created by IEC working groups and are then adopted as European standards. The non-electrical standards, EN 13463, are just European standards and there are no IEC equivalents. IEC has begun work though to develop a set of non-electrical standards based on the EN 13463 approach that will result in technically identical standards between IEC and Europe for all equipment used in hazardous areas.
Could the Imperial Sugar Refinery explosion have happened in Europe? The simple answer is Yes, but provided that the European standards already in place had been applied properly such a catastrophic event is less likely. This does not give us any reason to be complacent. Processing, transporting, storing of flammable powders and dusts is always potentially dangerous. It is important that anyone working in the industry from top management down is fully aware of the dangers involved, and every practicable measure is in place to reduce the amount flammable dust in the workplace. Primary explosions will occur but if these do not initiate a secondary explosion the chance of massive destruction and loss of life are significantly reduced.
Table 1: The zones for flammable dust
Zone 20 – present continuously or for prolonged periods during normal operation
Zone 21 – present during normal operation or expected malfunction occasionally or for short periods
Zone 22 – only present as a result of a rare malfunction and then only for short periods
Table 2: The flexible approach to dust certification
EN 61241-0:2006 General requirements
EN 61241-1:2004 Protection by enclosure ‘tD’
EN 61241-11:2006 Intrinsically safe apparatus ‘iD’
EN 61241-18:2004 Protection by encapsulation ‘mD’
Table 3: Standards for non-electrical equipment
EN 13463-1 General requirements
EN13463-2 ‘fr’ Flow restricting enclosure
EN 13463-3 ‘d’ Flameproof enclosure
EN 13463-5 ‘c’ Constructional safety
EN 13463-6 ‘b’ Control of ignition sources
EN 13463-7 ‘p’ Pressurisation
EN 13463-8 ‘k’ Liquid immersion
David Brearley is a Certification Engineer with Baseefa
A Baseefa data sheet, available from http://www.baseefa.com/page166.asp, highlights the technical differences between EN 50281-1-1 and the newer standards.