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. ■

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.

Regards,

Amrit Agarwal

Pneumatic Conveying Consultants

powderandbulk/pcc.com ■

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. ■

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

Germany ■

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

belt slips on drum, belt heats up, fire, explosion

belt tracks off rubs casing, belt heats up, fire, explosion

CHAIN

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? ■

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.

Regards,

Bill ■

Peter,

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.

Regards,

Bill ■

Roger,

You obviously work for Fike. In the interests of fairness there are other sources of information and help that should be cited. To name a few: Fenwal, Rembe, Cv Technology, Firefly, Chilworth, FM Global, IPD, BS&B, Oseco, Continental to name just a few.

FWIW, I work for one of these and, as occassion demands, cooperate with all of them. I think it would be better if we could keep this forum non-commercial to the maximum extent possible and to that end have offered these alternative resources for other readers of the thread.

Regards,

Bill ■

Another point worth considering is the density of dust clouds that actually occur inside enclosed machines (bucket elevators, chain conveyors etc) and at transfer points (belt conveyor loading and discharges, free fall into silos). In addition to a source of ignition and a combustible dust, the dust cloud must have a minimum concentration to explode. These are actually quite high. Has anyone ever measured dust concentration levels at the points listed above? ■

Designer,

You are quite correct that there is a minimum explosible concentration (MEC) that would be necessary for a dust explosion. While the actual value would vary for each dust, values in the range of 25 to 60-grams per cubic meter would cover most. Normally the precise value is of greatest significance when evaluating the risk associated with a process that might be below MEC. A common example would be a spray dryer. By the way, the term MEC is slowly being replaced with Lower Explosive Limit (LEL).

As for your question concerning testing the actual dust concentration in some of the equipment under discussion, I am not aware of any. Developing such a test would be expensive and difficult, and would lead to not much useful information. We already know that the MEC/LEL is being exceeded in these vessels whenever there is a dust explosion in one.

Regards,

Bill ■

keeping a forum non comercial , sounds good , but some forum users are actually looking for commercial alternatives.

Academics are in search of knoledge.

engineers are looking for ways to implement , standards , costs.

Purchace depatraments are looking for alternatives.

If I were to vote, I would vote to keep it in the encineering scope, but I realize that much of the richness of a forum is precisely the input of all sorts of people.

Therefore I would suggest not to limit the forums to one point of view even if they work or not for a supplier or not .

Even if they are partial to their solution or product .

It is a fact of life , salesmen will push their products openly or not .

Forums like this give access to millions of users or potential customers .

I would say keep them free , open to discussion .

That is what makes it valuable.

Marco ■

keeping a forum non comercial , sounds good , but some forum users are actually looking for commercial alternatives.

Academics are in search of knoledge.

engineers are looking for ways to implement , standards , costs.

Purchace depatraments are looking for alternatives.

If I were to vote, I would vote to keep it in the encineering scope, but I realize that much of the richness of a forum is precisely the input of all sorts of people.

Therefore I would suggest not to limit the forums to one point of view even if they work or not for a supplier or not .

Even if they are partial to their solution or product .

It is a fact of life , salesmen will push their products openly or not .

Forums like this give access to millions of users or potential customers .

I would say keep them free , open to discussion .

That is what makes it valuable.

Marco ■

Installing and maintaining explosion protection is vital to the well being of workers. However, PREVENTION, is also needed to protect the workers, and prevent damage to the facility.

In most cases, central dust collection systems are the best at controlling dust dispersion, and accumulation through a facility. As with everything else however, proper maintenance, and employee training are needed for the system to work properly.

You can read more about the prevention systems here in this article on QFilter.com

http://www.qfilter.com/res/Dust-Expl...hem/detail/136 ■

The explosion should be protected by different technologies which are specially designed for protection

such as explosion doors, vents, inerting, explosion suppression, isolation and their combination can

also be used. ■