Metal dusts are very tricky when it comes to indoor venting systems such as the REMBE Q-Rohr-3. With your particular application, aluminum, the Kst is low but the immediate temperature increase in the event of an incident needs to be considered before determining that an indoor venting system will be effective for this situation. For this reason, generally, metal dusts are not included in ATEX approvals. You will need to have the dust tested by the manufacturer to verify that an indoor venting system is a viable option.

Under typical circumstances where you have complete information about your dust collector, such as the strength of the collector, retrofitting should be no problem. In that situation, in accordance with NFPA standards 654, 68, 69, and perhaps other standards that specifically address your industry, a dust collector must be vented/suppressed and isolated ( the inlet always needs to be isolated; the clean air side must be isolated if it is a return-air installation). If the dust collector is inside, the dust collector might be vented through a duct to the outside, an indoor flameless vent can be installed or chemical suppression might be used. If the dust collector is located outside, explosion panels or flameless vents can be used depending on the proximity to other structures and people. NFPA standard 68 provides the method by which to calculate the required vent areas.
If you are in a situation where you don’t know the strength of the dust collector and you have no way of finding out the strength, you will either need to have an engineering analysis done on the dust collector or replace the collector to be absolutely sure you are properly calculating the vent area. There is no way to calculate the vent area if you don’t know the strength of the dust collector, and effective vent area is the critical component to minimizing damage to people and structures should there be a combustible dust explosion in the dust collector. In that case, you may decide you are better served by replacing the dust collector but you will still need to equip the collector with the appropriate venting/suppression and isolation equipment as indicated above.
Note: Even if you have all the information about your dust collector, you may find that the strength of the dust collector is such that it is more cost effective to increase the strength of the collector to reduce the costs of equipping with explosion protection equipment or to buy a new collector and properly equip it to protect it.

Per NFPA regulations, each enclosure containing a combustible particulate has to be vented or suppressed, no matter how old the enclosure is. In the case of a dust collector, if the reduced pressure (Pred) for the existing dust collector can be determined either by the manufacturer’s specifications or through a structural analysis, the old dust collector can be upgraded and still be used. In some cases, the cost to determine the Pred could be quite high, and the dust collector may need too much “strengthening” or other redesign—retooling doors that will not withstand explosions, for example, so that purchasing a new dust collector could be the more economic solution. Whether you retrofit an old dust collector or install a brand new one: either one of them has to include explosion protection and explosion isolation.

NFPA Standard 68 gives you precise guidelines for how explosion vent panels should be installed. The goal of NFPA Standard 68, 8.7.1 is to prevent the bags from blocking the vent such that the bags might be “sucked through” the vent if there is an incident.
Explosion vent panels should be installed underneath the bottom of the filter bags as described in NFPA Standard 68, 8.7.1. If, for any reason, there is not enough space to install the panels underneath the filter bags, you may, per NFPA Standard 8.7.1 (2), install the panels along the dirty air wall if “… bags are either completely removed or shortened so that they do not extend below the top of the vent for a distance of one vent diameter from the vent. In addition, bags immediately adjacent to the vent shall be removed and the remaining bags shall be restrained from passing through the vent.” NFPA Standard 68, 8.7.1(2), 2007 edition.
Another acceptable explosion vent panel placement option, per NFPA Standard 8.7.1 (3): “…the bottom of the vent(s) is at or above the bottom of the bags,…and the row of bags closest to the vent are restrained from passing through the vent… For this case, the volume used to calculate the vent area shall be the entire volume (clean and dirty) below the tube sheet.”

Explosion venting panels on a dust collector should be sealed so that there is no water entering the dust collector or dust escaping. If there is any type of premature opening due to pressure pulses/low vacuum in the dust collector, then an incorrect panel was used for the application.
You can certainly retro-fit the proper explosion venting panels to protect your dust collector. The vent area would need to be calculated so that the panel would open only when the pressure inside the dust collector exceeded the Pstat pressure, that is, the pressure at which the panel is designed to open in the event of a developing explosion (usually .1 barg).

Any enclosure containing a combustible material has to be vented/suppressed and isolated if there is any risk that a spark could cause an explosion. Without having full knowledge of the iris valve design, I can tell you that for isolation purposes, the valve has to be certified as an explosion proof design. The need for an explosion proof design is to prevent the spread of any combustion incident that may occur in the hopper.
Separately, I am concerned about your mention of nylon material – what you refer to as a “nylon covered hole”. Using nylon or anything similar, coupled with the flow of bulk material, can create a favorable environment for static electricity, the discharge of which could create a “spark” event or, in other words, the ignition source for a combustible event.

The short answer is, “Yes there is a chance” and it is difficult to quantify. NFPA specifically mentions dryers, including vacuum tray dryers, “should be designed, constructed, installed and monitored so that the required conditions of safety for operation of the air heater, the dryer, and the ventilation equipment are maintained.” NFPA 654 A.7.18.
With respect to dryers, there is a risk of accumulated dust on the dryer or in the dryer and there is always the risk of a spark igniting a dust cloud. Safety measures to be taken are the standard: provide effective venting and isolation systems to minimize the impact of the explosion and prevent the explosion from traveling to other parts of the process.

Good question. Technically, you are absolutely correct in terms of the potential for harmful effects. Much of the standard was developed based on the best expertise and knowledge base of an earlier day, when chemical suppression was pretty much the only option for enclosures located inside a building and venting to the outside wasn’t practical. You really can’t use suppression in this case because of the pressure buildup in the enclosure, should the suppression agent be triggered. And if you were to build your 8 cu ft enclosure to withstand the pressure to accommodate suppression as a protective measure, you may as well spend the extra few dollars to build a pressure vessel to withstand a combustible dust explosion.
Today, with flameless venting systems designed for indoor or outdoor venting, an 8 cu ft enclosure can easily be protected. What might prevent this volume requirement from being changed is the need to recognize that compliance with the standards depends on reasonableness and competing priorities might require a balance to gain acceptance in the business community.

It really depends on how the rupture disc system was designed. The system you are using is a double bursting disc unit that provides double the protection for production equipment working under extreme conditions, such as highly corrosive environments or environmentally harmful or highly toxic substances. The response of the pressures of the two discs depends on the operating conditions and the burst pressures of the discs. If the burst pressure of the second disc (further from the vessel) was designed to be higher than the first disc, then it should remain intact. Also, you should have independent signaling units on each rupture disc so you would know if the second disc did rupture. Of course, if the first disc should rupture, to maintain the integrity of the system and its built-in safeguards, you should replace it immediately

Not safely. Any spark generated by cutting or drilling may start a chain of events that could lead to an explosion. To be absolutely certain you are proceeding safely, I would recommend the silo be emptied and cleaned so that in any case the dust concentration remains below the lower explosion limit (LEL).