The subject of Grain facilities is not easily answered by venting or suppression.
For instance if the protected area is a grain elevator venting can only mitigate the damage. Venting will allow the deflagration to spread to other areas. NFPA 68 indicates that venting in ducts will not stop the explosions spread only lower pressure drop.
In other words with venting the explosion can spread from the boot to the head.
In other areas venting is the most cost effective method.
In all cases in a grain facility the first area of concern should be to stop the propagation of the deflagration. Then good vessel prevention and protection methods will work.
Old suppression systems that use point detection and static detection were subject to false product activations. New analog sensors are a better choice. Proper explosion prevention and protection is not the application of one method for all areas but the right choice for each perticular hazzard.
P.S. Nothing beats good houskeeping and operational methods.
We are a Company manufacturing prevention systems for grain silo, based upon oxygen continuous analysis and inert with nitrogen.
We are interested in the possibility of cooperating with Companies with prevention problems in silos.
We had some interesting experiences in France and Italy, with good results.
14th April 2003, 19:54
We have only two methods here: explosion PREVENTION, or explosion PROTECTION. Both are acceptable. Follow NFPA 69 if you prefer PREVENTION, and NFPA 68 if you prefer explosion PROTECTION.
NFPA 68 does not prohibit use of venting ducts, it gives design criteria that must be met if ducts are used.
In my view PREVENTION by the use of inert gases such as N2 is nomally not used in the grain industry because of its cost.
Pneumatic Conveying Consultants
27th April 2003, 14:36
When handling an explosive dust look for handling equipment that does not contain sources of ignition so preventing an explosion.
PREVENTION should always be the preferred option "if practicable".
PROTECTION comes after.
It makes sense NOT to have an explosion in the first place, rather than attempt to cope with an explosion AFTER it has taken place.
30th April 2003, 11:36
Constructive Explosion Protection
Each method has its pros and cons but regarding suppression systems, since there are appropriate passive venting systems for in- and outdoor applications, the use of these maintenance-intensive, toxic powder blowing devices is no longer the state-of-the-technique!
What answer do the suppression system manufacturers give to the question: How do you guarantee that the system also suppresses a secondary explosion - a follow up ignition from somewhere else? With venting devices you already vented the explosion and your equipment is "released from the pressure" - even if there is a follow-up event pressure can no longer rise.
When it comes to protect Elevators it is necessary to avoid the flame propagating from the bottom to top. Vent the foot and head and use in between a water-mist-extinghuishing system -
this is not toxic and easy to reset!
Generally, prevent an explosion and try to minimize possible ignition sources: One mean is to eliminate static accumulation
by earthing, grounding and monitoring of these measures!
More info: www.electro-static.de
More explosion protection info concerning passive
venting systems: www.rembe.com
Stefan Penno, MBP
30th April 2003, 21:24
On the subject of bucket elevators, does anyone have any views on difference between BELT and bucket elevators and CHAIN and bucket elevators regarding their susceptability to explosions?
belt slips on drum, belt heats up, fire, explosion
belt tracks off rubs casing, belt heats up, fire, explosion
positive sprocket drive, NO SLIP, NO HEATING, NO FIRE, NO EXPLOSION
positive chain location on sprocket, NO TRACKING OFF, NO HEATING,NO FIRE, NO EXPLOSION
Bit of discussion on this?
26th August 2003, 22:33
Chain versus belts for bucket elevators
This is a difficult question to answer because there is not a statistically significant set of losses framed around this issue and it is doubtful if this information is being tracked. Generally it is believed that belts are safer for if they break there is no metal chain hitting the housing to cause sparks. Chains do break of course. Stainless steel chain would not spark and could be considered safer, but it is not widely used presumably for cost reasons. Chain also is subject to high levels of wear in dusty environments and stainless chain is more susceptible to galling.
Naturally the type of conveyor and the materials being handled would play a big part in the decision about the best materials for any given application. My sense of it is that non-metallic belts and buckets offer practical safety advantages where they are compatible with the process and products being conveyed. There could be specific applications, however, where a chain drive, particularly a non-sparking one, might offer advantages that outweigh their shortcomings. So in the end there is no single right answer to this question.
28th August 2003, 20:00
Explosion protection or prevention? Always a choice to make. Prevention IS the first step under all conditions: preventing an explosion from happenig is best but the reality of the installation, process conditions and operator interference with process or preventive measures may result in a need to add explosion protection (venting, suppression, pressure resistent design or decoupling) to the solution. Insurers may also have a say....
Best approach is to get someone knowledgable in explosion matters to evaluate the process and installation, get a good understanding of the explosability of the product, identify highest risk proces equipment and draft a "strategy".
Elevators offer quite a challenge. NFPA 651 as an example provides some guidance specific to these high frequency explosion sources. Prevention through belt speed monitors, alignment detectors and slippage detectors, ect are recommended. Explosion venting may be an option, especially for those parts of the elevators being outside. Footers which may be located inside or underground may require other solutions flameless venting or suppression - check with www.fike.com)
Decoupling the elevator inlet and outlet to avoid run-through of explosions to connected equipment may need to be considered.
29th August 2003, 12:25
I believe that there is a real question as to if explosions in bucket elevators are actually caused by steel buckets hitting the casing.
There seems to be information from a study into the causes of elevator explosions in the USA that did not identify sparks from metal buckets as a possible cause, and another study from the USA indicated that single sparks from mild steel did not generate an explosion.
Also it is reported that in the UK during explosion trials on bucket elevator venting it was not found possible to ignite grain dust by buckets hitting other steel parts. It was necessary to introduce a specific ignition source to cause an explosion.
I think there is still much practical work to be done on dust explosions in materials handling equipment.
29th August 2003, 14:51
Thank you for your thoughts. I would be very interested in taking a look at the studies you cite on bucket elevator explosions. Would you post the information so I can obtain copies please? I have also investigated explosions involving bucket elevators that had nothing to do with metal buckets or chains hitting the casing. The thing to keep in mind is that dust explosions, in bucket elevators or any other equipment, can be caused by a wide variety of things. It has been my experience that the the events leading up to an explosion are always a very unusual sequence and that if any one of them had not occured, there would not have been an explosion. For this reason it is not likely to be possible to come up with a definitive study that puts to rest all of the issues. This would be true for bucket elevators perhaps moreso, but for any type of system.
Finally, Dr. Rolf Eckhoff did some research on sparks that supports your idea that single sparks of the type that would be generated by metal striking metal are not likely to be a viable source for ignition of pure dusts. Dr. Lawrence Britten has also done research in this area that strongly reinforces this information. So you are on firm ground on this one.
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