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I am currently doing a study on wood shop dust collectors. The majority of the installations are shaker style with the tube sheet located on the bottom. ie the dirty air flows inside the filter bag. I am interested if any forum members have any experiance with retrofitting this type of dust collector with an explosion vent. The current installations would result in the explosion venting into the storage barrels. (I don't think this is a good idea) due to the pressure drop between the dirty air side and the clean air side access door which does have Brixon latches.

any suggetions would be appreciated.

John

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Hi John,

I consulted with John Dauber, our North American sales manager on this one. Here's what he said:

"Retro fitting an existing dust collector with an explosion vent can be tricky. First off, you need to know the strength of the vessel (dust collector) or maximum positive pressure it was designed for. If there is any structural damage or degradation of the dust collector like rusting or other damage, it makes it almost impossible to calculate the maximum pressure the collector can withstand. All explosion vents need to be vented outside to an unoccupied area. There are other options like explosion suppression that can cost more than the original collector."

By the way, John just did a live webinar on Dust Control last Thursday with two other panelists, plus Guy Colonna from the NFPA. This is posted for viewing on-demand at : http://video.webcasts.com/events/putm001/33721/. I think you would find this very informative toward your study.

Regards,

Sal
John,

One other thing. John Astad, the creator of this discussion group here on SafetyCommunity.com, also did a webinar on Monday for ISHN (Industrial Safety & Hygiene News) on the subject of combustible dust and explosions. It's been posted for viewing on-demand here. John was in good form, as usual!

Sal
John, the only way to protect a dust collector from an explosion occuring on the dirty side is by protecting the dirty side. This is true for explosion venting as well as for explosion suppression. Of course the explosion pressure will propagate to the clean side, but the presence of the filter bag means that you cannot vent quickly enough through the clean side access door.
What is likely to happen in this case is that the filter bag will be blown off the attachment to the tube sheet, which will then lead to an explosion in the "clean" side which needs to be vented as well. But by that time the explosion on the dirty side will have progressed too far, unless the dirty side is independently vented as well.
The (usually) inherent weakness in the attachment of the unsupported filter bag to the tube sheet makes this kind of dust collector more complicated to protect than the more normal duct collector where the dirty side is outside the filter bags which will be supported on some sort of cage.
Peter,
Thanks, That is exactly what I thought. It is too bad they have installed this type of dust collector, however the cost for total replacement of all the units is prohibitive.
An interesting feature of this style of dust collector is the inside volume of the bags are technically on the dirty side, ie I need to consider adding the volume to my explosion vent size calculation. My estimation is that there are 1,000's of this style of dust collector installed in North America, however their is litle evidence that they have exploded and or any injuries have occured. The only evidence I have found is Tucker vs Reeves, where an explosion caused significant injury and another 20 incidents of fire. Any ideas where I might find any fire or explosion data on this type of dust collector.
John
John,
If you look for proof that any particluar type of dust collector has exploded, you will always come up with small numbers, unless you talk to manufacturers and operators of that type (and they are really honest).
One big reason for that is that most incident descriptions are not too specific about the type of dust collector involved. For example, in this case a description may simply state that it was a unit with filter bags (as opposed to, for example, cartridges) without recording that they were the "upside down" bags. Whoever recorded the information may not know the details or not know the relevance of them.
Incidents that do not escalate into major plant destruction or serious personal injury tend to be underreported and any reports tend to be low on specifics.

There is another factor to consider, and that is that there are many fires for every explosion. As I always tell people "it is not easy to make a good dust explosion". After all, you need to get a good dust air mixture with a dust concentration around the optimum concentration, and the ignition must happen in a "good" spot for the maximum explosion effects. If the conditions are not right, you will get "just" a dust fire, or maybe a flash fire, which is an explosion except there are no significant pressure effects. In my 30-odd years as process safety consultant I have stood next to burning dust collectors where the air jet pulsing was kept running throughout the incident and still they had no explosion.

Most people see these incidents as reasonably harmless instead of as the failed explosion experiment or "near miss" that it actually was. If they realised that they were one step away from a devastating dust explosion, we would not have such a hard time convincing industry that it is really a good idea to protect dust collectors (among others) against explosions, and not just fires.

And now we are talking about good practices when it comes to explosion protection, why not include explosion isolation in the considerations? They are even more a hard sell, in spite of all the guidance available, such as in NFPA standards in the US. And in Europe this is even included in the top-level requirements for explosion safety under the ATEX legislation, and still virtually every recommendation to provide explosion isolation is challenged as over the top in several types of industry. Sorry, just had to get that off my chest.
Peter,
Your observations are consistant with what I have found. For the one explosion documented for this type of dust collector , I found 20 fires , any of which could have resulted in an explosion.

During my previous experiance in the Masonite industry, I witnessed and investigated many significant explosions with less than ideal conditions than what I have found in my current study.

It's just nice to converse with someone who is distributing the same safety message as I have been for the last 30 years.
Thanks
John
John, It is my understanding as Pieter mentions, that you would need to vent both the clean and dirty side of this type dust collector.

Sal is right about the concern of the condition of the dust collector when calculating venting.

And you make a very important point about the number of dust collector fires that could have resulted in a deflagration. Most people do not understand the probability of occurrence, or that the fires they have experienced are precursors to deflagrations. Many Stakeholders believe their process to be safe, when in fact they may just be lucky!

This is a very important question you ask, as many dust collectors are running without proper protection.

I appreciate the opportunity to interact with other professionals as passionate about property loss and life safety.

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