Can an explosion vent discharge inside a building?

Not with an ordinary open vent. A conventional explosion vent releases a fireball of burning dust and a pressure wave, and inside a building that endangers people, starts fires and can raise settled dust into a secondary explosion. Indoor equipment can still be vented, either through a duct that leads the explosion outdoors or through a flameless venting device certified for the dust and the vessel, or it can be protected by explosion suppression or containment instead.

Short answer

• HSE guidance says vents discharging inside a building give people inside little protection and should not relieve into regularly occupied areas.
• NFPA 652 (2019 edition), since replaced by NFPA 660 (2025), stated that pressure relief devices must not vent to an area where a dust explosion or dust flash-fire hazard exists.
• Option 1: a vent duct to a safe place outdoors. It must be at least the size of the vent, short and straight; it reduces venting efficiency, so the vent area or vessel strength must be increased.
• Option 2: a flameless venting device, which quenches the flame and retains most of the dust. In Europe it is tested to EN 16009; NFPA 68 has a section on venting inside buildings with such devices.
• A flameless vent is only acceptable within its certificate: dust types, Kst and Pmax, vessel volume and pressures. Hot gases and pressure still leave the device, so clearances must be kept.
• Option 3: no vent at all, using explosion suppression or an explosion-pressure-resistant design.

Why an open vent cannot discharge indoors

Venting does not remove an explosion; it moves it out of the vessel. HSE guidance warns that a fireball and a shock wave emerge when a vent opens, that the fireball is usually much larger than the vessel it came from, and that it can throw burning particles some distance. Indoors this has three consequences:

• People. Anyone near the vent is inside the flame and pressure zone.
• Fire. Burning dust lands on equipment and stored materials and can start further fires.
• Secondary explosion. The blast raises dust lying on floors, beams and cable trays, and the fireball ignites it. The result can be far worse than the original event; see Why is a secondary dust explosion usually worse than the first?

US rules reflect the same principle. The OSHA grain handling standard, for example, allows filter collectors installed after 30 March 1988 inside a facility only in an area protected by explosion suppression, or in an area with at least one hour fire-resistant separation, against an outside wall and vented to the outside through a vent and ductwork designed not to rupture.

Option 1: duct the vent outdoors

HSE describes a duct leading the explosion products to a safe place in the open air as the usual solution. The duct must have at least the cross-sectional area of the vent panel, should be short and straight, and must direct burning material away from anywhere people regularly go. The cost is back pressure: the explosion has to push through the duct, so the reduced explosion pressure Pred rises. The vent area or the strength of the vessel must be increased to compensate, using the duct corrections of EN 14491 or NFPA 68. Long ducts, bends and large volumes need careful design, and the duct itself must withstand the explosion.

This option suits equipment close to an outside wall or roof. For a vessel in the middle of a large building, the duct often becomes too long.

Option 2: flameless venting

A flameless venting device combines a bursting panel with a flame-quenching element, typically a metal mesh. The burning cloud is forced through the element, which cools it below ignition and retains most of the dust, so no flame leaves the device. EN 16009 sets the requirements for design, testing, marking and documentation of these devices, which are protective systems under Directive 2014/34/EU.

The limits that matter for an indoor installation:

Research supports caution in applying test results. An AIChE paper on evaluating flameless venting devices notes that the mesh adds flow resistance that can raise the internal overpressure, and a study based on the testing experience of INERIS, the French national institute for industrial risks, points out that because the flame stays inside the vessel, combustion lasts longer than with a classical vent, which questions treating these devices simply as an open vent with an efficiency factor. Use certified data for the specific device, dust and volume.

Option 3: protect without venting

Where neither ducting nor flameless venting fits, explosion suppression extinguishes the explosion inside the vessel within milliseconds, and containment designs the vessel to withstand the full explosion pressure. Both keep the event inside the equipment. Whichever option is chosen, connected ducts still need isolation, otherwise the explosion can travel back into the building; see Dust explosion protection.

Common mistakes

• Pointing a vent at a wall or ceiling inside the building. The fireball is still indoors and raises dust everywhere it reaches.
• Using a flameless vent outside its certificate. A different dust, a higher Kst or a larger vessel needs new evidence.
• Forgetting the duct penalty. A vent sized for free discharge is too small once a duct is added.
• Letting dust settle around a flameless vent. The discharge can raise it and create a local cloud.

Related on excodex

• Dust explosion protection: venting, suppression, isolation and containment
• When is an explosion vent required on a dust collector?
• Should a dust collector be located inside or outside a building?
• How often must explosion vents and suppression systems be inspected?

Standards and further reading

• EN 16009:2011, flameless explosion venting devices: standards.ie
• NFPA 68 (2023 edition), explosion protection by deflagration venting, including a section on venting internal to a building with flame-arresting and particulate retention devices: standards.ie
• EN 14491:2012, dust explosion venting protective systems (vent ducts and effects outside the vent).
• HSE, HSG103 Safe handling of combustible dusts (vents discharging indoors, vent ducts, flameless venting): hse.gov.uk (PDF)
• NFPA 652, first revision record for the 2019 edition (pressure relief devices not to vent into hazard areas); NFPA 652 has been replaced by NFPA 660 (2025): nfpa.org (PDF)
• Dorofeev, A methodology to evaluate flameless venting devices for dust explosions, AIChE Global Congress on Process Safety 2013: aiche.org
• Flameless venting: achievements and difficulties, based on INERIS testing experience (abstract): core.ac.uk
• OSHA 29 CFR 1910.272, grain handling facilities, paragraph (l) on filter collector location: osha.gov

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.