A vent duct effectively “leads” an explosion to the outside of a building and creates back pressure, the force of which depends on the length of the duct and properties of the combustible mixture. An enclosure needs to have a sufficient Pred to withstand this backpressure.

Pred, the reduced enclosure strength, is typically determined by taking 70% of the PES or design strength of an enclosure. Over time, due to corrosion, fluctuating pressure and stress in general, as the enclosure ages, the PES and Pred reduce from the original as-built design. 

Pred can be increased by stiffening the enclosure in critical areas, like reworking all welded areas, re-enforcing the frame, and increasing the thickness of the sheet metal of the enclosure casing.

To be done correctly, a certified structural engineer would calculate and determine where/how the design strength has to be increased and by how much.

This may be a costly procedure. In many cases back pressure is avoided by simply installing an indoor flameless vent, that will, in the event of an explosion, safely vent an explosion inside without the need for ducting to the outside.

The design of a process rupture disc is such that it will release the overpressure from inside an enclosure based on the real pressure difference. If it is correctly designed, a rupture disc will release this overpressure whether it be under water or at high altitudes. The disc specifications will take into account all the atmospheric conditions, including location of application, temperature, etc.

Your concerns are well founded. The ideal installation in this situation is to have the panels located such that recoil forces will not have a damaging impact on the silo or other enclosure – in other words, across from each other. If, for any reason, the vent panels have to be installed on one side, then the enclosure must be sufficiently strong to withstand the potential recoil force.

PVC piping could be used so long as the strength (Pred) is high enough and it is sufficiently temperature resistant. The larger concern would be the 90 degree bends. Although NFPA standards do not definitively say you cannot have such bends, we don’t recommend it.

Generally–and I say this with some hesitation–if your fuel is gas of some kind, the minimum could be approximately 5%. If your fuel is dust of some kind, the minimum could be approximately 10-12%. For smoldering dust, it could be as low as 2%. Note that I say could be. These generalizations really don’t mean anything for a specific application. The answer always depends on the specific fuel and a LOC (Lowest Oxygen Concentration) must be determined for that specific fuel.

I would prefer to know more about your process to better understand why you are considering nitrogen blanketing for this application. Having said that, I recommend you have your dust tested even if you don’t find a Kst value (I didn’t locate one either!) Testing your dust will give you additional information from which you can make your decision.

Per NFPA Standard 654 Section 7.1.4.1, isolation is always required. “Where an explosion hazard exists, isolation devices shall be provided to prevent deflagration propagation between pieces of equipment connected by ductwork.”
NFPA standard 69 annex section A. 11.2. seems to provide an exception for 100 mm (4 in.) pipes, on the theory that, “Piping less than 100 mm (4in.) diameter is far less likely to provide a conduit for flame spread than larger diameters.” This same section starts by indicating that isolation is necessary unless:
1. A qualified risk analysis is performed
2. It is determined that the risk involved in not isolating is acceptable to the AHJ.

NFPA standard 654 lays out the requirement for isolation to prevent deflagration propagation in section 7.1.5. Reference is made to section 7.1.4.2, which lists several types of isolation devices as examples of the types that might be used, and NFPA standard 69 is your guide for further details about this list. Standard 654, section 7.1.5.3 makes clear the list is not exclusive.
NFPA standard 69 Chapter 11 provides a description of what is required with respect to any isolation device:
1. Interruption or mitigation of flame,
2. Interruption or mitigation of pressures, pressure piling,
3. Interruption or mitigation of flame-jet ignition between enclosures

11.1.2 says the isolation technique can be active or passive; 11.1.3 lists examples of devices that may be used, but again, makes clear the list is not exclusive:

1. Flame front extinguishing system
2. Fast-acting mechanical valve
3. Actuated float valves
4. Actuated pinch valve

The rest of Chapter 11 provides details about some isolation devices, but not all are described in the standard. Your combustible dust explosion protection equipment provider can best assist you with details about the various devices available. You should carefully evaluate your options to be certain you are not spending more or less than is necessary. It is highly recommended you consult your authority having jurisdiction related to your specific application to confirm the isolation technique you intend to use is acceptable to them.

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.