Two classes of technology apply to protecting against combustible dust explosions – active and passive. Tulsa-based BS&B Pressure Safety Management specializes in both, said Geof Brazier, the company’s director of product and market development.
The classic mistake is to treat passive technology such as explosion vents as a foolproof, no-maintenance alternative to active technology solutions triggered by electronic means, Brazier said.
“It used to be thought that passive technology was the best way to go simply because it was zero maintenance, but that is not true,” he said. “Since the 2007 edition of NFPA 68, explosion vents have a requirement for periodic inspection to insure that they remain in operable condition.”
Brazier serves on the committee for NFPA 68 – Standard on Explosion Protection by Deflagration Venting and NFPA 69 – Standard of Explosion Prevention Systems.
A physicist with more than 30 years experience in the industrial dust explosion field, Brazier holds more than 20 U.S. patents in the areas of explosion prevention and protection, pressure relief and industrial wireless devices.
“Explosion venting grew out of the rupture disk industry, which is perhaps what BS&B is most famous for,” Brazier said. “The difference between rupture disks and explosion vents is very subtle. The technology is basically the same.”
BS&B traces its roots in explosion venting back to 1931. Beginning in the 1980s, the company began pursuing combustible dust risk management as a business activity separate from its other fields of interest.
“Since 1990, we have made a number of strategic acquisitions to gain the supply of suppression equipment, spark protection equipment and flameless venting equipment to provide all the platforms of technology that might be used for combustible dust risk management,” Brazier said.
Explosion vents qualify as passive technology, he said.
“Passive technology is where the design of the equipment or building involved automatically kicks the safety solution into action without intervention of electrical sensors or actuators,” Brazier said.
Explosion venting typically allows the full evolution of the combustion event. Venting can be applied to individual pieces of equipment or entire buildings.
“The objective is to select a vent big enough to enable most of the combustion process to take place outside what is being protected so that the energy is rapidly dissipated into the atmosphere rather than inside where the dramatic pressure rise would be destructive,” Brazier said.
Typically, materials handling equipment such as a dust collector has a design stress of between two or three pounds per square inch. The maximum pressure generated by a fully contained explosion depends on the type of combustible dust involved.
“For fine corn starch, that number could be as high as 150 psi,” Brazier said. “Even for wood type products and agricultural product dusts, that number is easily over 100 psi. Obviously, there is a problem if the venting scenario – the explosion – is not pushed out of the equipment as rapidly as possible.”
Ideally, the desired set pressure for explosion vents should be half or less than the strength of the equipment.
“Even if the explosion vent opens at one psi, which is a very typical value, the pressure inside the equipment easily will continue to rise for a few milliseconds and reach three or four psi in less than 100 milliseconds,” Brazier said. “Explosion vents deal with the issue on a millisecond time frame.”
To be 100 percent efficient, venting devices require adequate design work.
“An explosion device is made of relatively lightweight material, which typically is thin gauged such as 0.030 to 0.060 of an inch stainless steel,” Brazier said. “As long as you are in that range, explosion venting devices are usually 100 percent efficient.”
Explosion venting requires careful design work. A typical vent measures three feet by two feet.
“If it is 100 percent efficient, you are getting the benefit of all six square feet,” Brazier said.
Other technologies, such as explosion doors, tend to be heavier in construction, made from multiple layers of material. These technologies may also use springs to assist in their operation.
“As a consequence, an explosion door might have only an 80 percent efficiency factor,” Brazier said. “So instead of getting the benefit of six square feet out of a two-foot by three-foot vent, you’re only getting 80 percent of that number. You’re only getting 4.8 square feet of effective vent area.”
A better understanding of the importance of efficiency in venting devices has resulted in a trend toward lightweight, thin metal construction of explosion vents and away from explosion door type devices of a much heavier construction.
“Until fairly recently, this characteristic was controlled in the NFPA standards by just saying that any vent weighing 2½ pounds per square foot or less in mass qualifies as 100 percent efficient,” Brazier said. “If it was heavier than that then you had to deal with an inertia change factor.”
Vent efficiency, covered in the current NFPA 68 standard, is a fairly new parameter in North America and still not well understood, he said.
“Sometimes it is just unavoidable to have a certain amount of household related ambient dust lying around,” Brazier said. “There are some facilities that take specific action in the construction of their building to provide a preferred venting patch.”
Multi-story buildings are particularly problematic when it comes to building venting, he said.
“If you were to have a combustible dust explosion on level five of a 10-story building, the structural weakness risks having the floors above collapse down on top of the building,” Brazier said. “Multi-story construction is an area of great sensitivity if perfect housekeeping cannot be lived up to.”
Building venting presents two specific building challenges. The first is that buildings are even weaker than industrial equipment such as dust collectors and bucket elevators.
“The strength or the peak pressure in an explosion that even a concrete building could withstand is only in the range of one psi,” Brazier said. “Weaker constructions that are not reinforced concrete could be half that value.”
As a result, passive venting technology must use extremely low release set pressures.
“In the last five years there has been new technology that gives designers choices of explosion vents that have set pressures as low as 10 inches of water column, which is far below the limits of design for traditional metallic vent constructions,” Brazier said. “BS&B has been in the forefront of these developments.”
Modern building vents use extruded plastic materials such as polycarbonate, which has the advantage of being many times stronger than glass or other typical building materials. With enough of this extruded polycarbonate sheeting mounted into framework as explosion vents, the building can be structurally protected from damage.
“One of the neat tricks we play with these extruded polycarbonates is that they are available in a translucent form,” Brazier said. “When a building owner is looking for a retrofit, there may not be many convenient places to put a large amount of vent area.”
Using these materials, windows can be replaced with explosion vents and still admit nearly 85 percent of the daylight permitted by glass.
“That’s a very attractive option because the windows are much more easily replaced than other parts of the building construction,” Brazier said. “Often the window area is pretty significant in existing buildings.”
Another explosion protection technique that qualifies as active rather than passive is explosion suppression. It involves detecting the earliest stage of the combustion event as a trigger for the rapid release of an extinguishing agent into the involved equipment. That injection snuffs out the developing fireball before the full combustion event can erupt.
“Suppression systems actually move faster than the original dust explosion,” Brazier said. “The detector devices, which are usually pressure based, look for a pressure wave. At the beginning of a dust explosion, the pressure wave is moving faster than the flame front.”
That difference in speed gives the suppression system a few precious milliseconds head start to respond before disaster.
“Typically, you can catch a combustion event when the volume of the flame ball is about the size of a basketball,” Brazier said. “So you really minimize the amount of combustion and, hence, the amount of clean up as well.”
Passive or active, every NFPA standard for explosion protection is under revision at this time, he said. NFPA 68 is early in the review process so the new venting standard is not likely to emerge with revisions until 2013.
“There will not be substantial changes for most combustible dust applications, but there will be increased information with regard to using the standard in a practical manner,” Brazier said. “A lot of the committee’s focus will be to ensure that the standard is user friendly for implementation. There are significant changes coming with respect to vent area determination for combustible gases.”
NFPA 69, which covers all other protection methods such as suppression and chemical isolation, is about to start its revision process. Revisions may not be issued until 2014, Brazier said.
“Again, there will not be substantial changes coming, just more clarification and focus on the proper use of the available and emerging technology,” he said.
Revisions for NFPA 654, the standard for prevention of fire and dust explosions in industrial settings, are due in 2012 though likely identified as a 2013 NFPA document.
“Even though these committees involve different people and different time cycles for development, there is a behind-the-scenes effort to harmonize the language and requirements of NFPA 68, 69 and 654, as well as some of the other industry specific standards such as NFPA 61, which covers agricultural products,” Brazier said. “The NFPA Standards development process looks for public input. Now is the time to provide that input. Interested parties can connect through the NFPA web site at www.nfpa.org.”
Meanwhile, OSHA has been conducting open meetings the past two years to assist in the process of writing a federal OSHA standard covering combustible dust.
“I met with the primary writer about a month ago,” Brazier said. “He said the draft is done and the next step in the regulatory process is an assessment of the impact on small businesses which should be occurring in December 2012.”
A 2006 study by the Chemical Safety Board reported at least 281 dust explosions in the U.S. between 1980 and 2005 that killed 119 workers and injured 718. The following year the CSB recommended that OSHA create workplace rules to control dust and cut down on explosions. OSHA began that lengthy process in 2009.
“Unfortunately there were a large number of combustible dust fatal accidents in the 2000 to 2003 time period,” Brazier said. “These were very publicly examined and reported by the CSB. Not only do they report on the root causes, but they have created very high quality educational animations that show what happened in these accidents, from metal dust to resins to rubber material.”
An important revision in the NFPA 68 standard makes it clear that passive protection, such as explosion vents, requires periodic inspection to ensure that it remains operable.
“There are no fit-and-forget explosion protection solutions,” Brazier said. “Twenty years ago you’d buy an explosion vent, install it and never look at it again. Today, NFPA 68 has a clear requirement that the inspection interval shall not exceed two years. The objective is to make sure the vent is not damaged, corroded or mechanically harmed to prevent it doing a good job.”
Most important is to check that process material has not built up on the vent so substantially that its weight is increased.
“A vent that is heavier than expected cannot open as fast as it should and provide the protection that is expected,” Brazier said. “This is particularly important with older vents built with a multi layered composite construction. There is a risk of accumulating process material internally between the layers. That can really increase the mass.”
With humidity, the material can solidify in the structure of the vent and render it inoperable, he said.
Another reason to require inspections is to make sure the vent’s purpose has not changed, Brazier said.
“It is very important that if an industrial process changes, the user has to look afresh at the protective mechanism,” he said. “Let’s say the process had changed to a more reactive dust. That means a bigger vent area is required to do the job of protection.”
Not having enough vent area is the same as not having vent area at all, Brazier said. Partial vent area results in significant destruction if the design is not rechecked.
Today, a greater awareness about the potential for dust explosions is apparent in industry, Brazier said. For example, periodic maintenance of process equipment now includes grounding and bonding to prevent any static electric charge that could be an ignition source.
“Ten years ago, it was not unusual to find a broken grounding wire on a piece of equipment and nobody paying attention,” Brazier said. “Today, it is much less likely that something like that would be missed in periodic maintenance.”
Industry has progressively improved at protecting itself from primary risks such as dust explosions, he said.
The quantity of explosion vent protection that BS&B produces is a good barometer of how things have changed, Brazier said. In 1980, the company made less than a thousand explosion vents per year. In 2011, BS&B made more than 15,000 explosion vents.
“That shows a huge increase in the conscientiousness of applying primary protection to dust explosions.”