Explosion Vents FAQ's

Q.Why use explosion venting?
Q.Where are they required?
Q.How do they work?
Q.Where can explosions happen?
Q.What is considered to be a fuel?
Q.How do I size my vents?
Q.When I calculate the vent area, it turns out that I don't have enough wall available. What can I do?
Q.What is the difference between FM and NFPA?
Q.Why is FM testing needed?
Q.Why is FM approval important?
Q.At what value should my vents be set to release?
Q.Bottom hinged vs. top C/S Explovent^® panels? Why would I specify one over the other?
Q.What about security? How easy are C/S's Explovent^® explosion vents to open?

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Q. Why use explosion venting?

A. The issue of venting violent overpressures is a critical one for owners of facilities where potentially explosive atmospheres, materials, and processes exist. Recent statistics indicate that the average damage due to explosions in industrial establishments is approximately $3.4 million, compared to $210,000 for an average fire. The possibility of an explosion occurring must be seen as a real concern considering only three common elements must be in place for an event to occur: ignition source, a fuel, and confinement. If something can burn, it can likely explode.

Q. Where are they required?

A. Any facility where potentially explosive atmospheres exist requires explosion venting. Facilities handling hazardous materials are required by numerous building codes to provide adequate safeguards against explosions. The following code and guideline authorities specify the requirements for explosion venting and set standards of protection that must be met.

• National Fire Protection Association Guide NFPA 68 (1998) Explosion Venting
• BOCA National Building Code (1999) Section 417.5.1
• SBCCI Standard Building Code (1999) Section 407.2.2
• SBCCI Standard Fire Code (1999) Section 611, 2203.1.16
• ICBO Uniform Building Code (1997) Section 307.10
• ICBO Uniform Fire Code (1997) Section 8004.2.2.4
• International Building Code (2000) Section 415.5.1
• International Fire Code (2000) Section 911
• National Fire Code (Canada) section 3.2.8.2, 4.2.9.6, 4.3.13.3, 4.8.3.1, 4.8.4.2, 5.3.1.6 and 5.6.1.5
• National Building Code (Canada) Section 6.2.2.5

Q. How do they work?

A. An explosion vent is designed to be the weakest part of the external structure. As the explosion vent experiences the pressure rise, it opens quickly allowing the rapidly expanding heated gases to be released to the outside. By doing so, the internal walls, floor, and ceiling are spared from the damaging overpressure experienced during a deflagration. To successfully limit damage to the vented area, vent design and the pressure resistant structure must be in keeping with guideline authority recommendations. As such, C/S Explovent^® explosion relief systems have been designed in accordance with the NFPA 68 Venting of Deflagrations guideline and Factory Mutual 1-44 Damage Limiting Construction document. Explovent^® has been tested, approved, and labeled by Factory Mutual and tested and approved by the Canadian Center for Mineral and Energy Technology for explosion relief applications. Types of industries where explosion venting is used and required

NFPA 68 Venting of Deflagrations
The National Fire Protection Association is recognized worldwide as a leading authority on explosive events. Its guideline document NFPA 68 Venting of Deflagrations, provides specific recommendations for the design and operation of explosion vents.

Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium constitutes as a deflagration.

NFPA - 2-1.4 "Deflagration venting is one means of controlling damage caused by deflagration explosions. By releasing expanding gases through an opening it is possible to reduce the pressure below the level that would cause damage."

NFPA - 3-1.1 "A deflagration vent is an opening in an enclosure through which burned and unburned material expands and flows. In many cases it is impractical and economically prohibitive to construct an enclosure that will withstand or contain such pressures."

Q. Where can explosions happen?

A. Explosions can happen in any building that contains hazardous materials. Any material capable of reacting with an oxidizing medium can be classified as fuel. Fuels with concentrations between their lower and upper flammable limits are considered combustible.

Q. What is considered to be a fuel?

A. NFPA 68 2-3.1 "Any material capable of reacting rapidly and exothermically with an oxidizing medium can be classified as a fuel. A fuel can exist in a gas, liquid, or solid state. Liquid fuels that are dispersed in air as fine mists, solid fuels that are dispersed in air as dusts, and hybrid mixtures pose similar deflagration risks as gaseous fuels."

Consequences of Explosions NFPA 68 - 3-2.1 "Damage can result should a deflagration occur in any enclosure that is too weak to withstand the pressure from a deflagration. The area of the vent must be large enough to limit the deflagration pressure to some predetermined safe level. In addition to the deflagration pressure, there is a thermal hazard associated with the flame. This thermal hazard exists both within the enclosure and in the path of the vented flame."

NFPA 68 - 3-2.4 "The effect of a deflagration depends on the maximum pressure attained, the maximum rate of pressure rise, and the duration of the peak pressure. The total impulse imparted to the enclosure is reduced as the vent area increases. However, total impulse is not a useful design basis. The stress developed on the enclosure is calculated on the basis of the equivalent static load."

NFPA 68 - 3-2.6 "When a gas or dust deflagration is vented, a tongue of flame of brief duration issues from the vent. Unburned dust will be ignited as it flows out the vent and can produce a large fireball that can extend not only outward and upward, but also downward from the vent. This has been shown in numerous tests conducted with full-scale equipment."

Q. How do I size my vents?

A. Either the NFPA Guideline 68 for Venting Deflagrations or the Factory Mutual 1-44 Damage Limiting Construction document may be used. They offer formula approaches to determine the appropriate vent area for low strength enclosures.

Q. When I calculate the vent area, it turns out that I don't have enough wall available. What can I do?

A.

1. Consider reorienting your room so that the long wall is an exterior wall.
2. Relocate the room to an outside corner so that two exterior walls are available.
3. Locate the room as an extension off an exterior wall to allow for three exterior walls.
4. Consider venting through the roof as well as the wall(s).
5. As room surface area plays a key role in the vent area calculation, consider reducing room size or height.
6. Similarly, within the vent area formula, if the strength of the pressure resistant structure is increased the required vent area is reduced.

Q. What is the difference between FM and NFPA?

A. The Factory Mutual Research Corporation is an organization financially supported by three Industrial Risk Insurance Companies. FM's research, engineering, education, testing, and approval are aimed at the risk reduction and property protection of its policyholders through the setting and implementation of construction and operation practices.

The National Fire Protection Association is an independent organization whose interest is the development of codes, standards, recommended practices, and guides for the education and furthering of fire safety practices in North America and around the world. Members include individuals, corporations, trade, or professional associations, institutes, fire departments, fire brigades, and any other private or public agencies desiring to advance the purposes of the association.

Q. Why is FM testing needed?

A. Insurance companies offer premiums to companies who provide tested and approved products on their buildings. FM offers the only test facility in the world with the ability to duplicate warehouse-sized protection schemes and loss scenarios.

Q. Why is FM approval important?

A. Third party independent approval is your assurance that the product or system in question has been tested and approved to be compliant with specified standards for both its manufacture and performance. This is further enforced through random manufacturing audits.

Q. At what value should my vents be set to release?

A. The vents should be set as low as practical, yet remain in place during wind storm conditions. In general 20 lb/ft2 is accepted as that practical value. The NFPA suggests a maximum of 30 lb/ft2 in areas subject to severe wind conditions whereas FM recommends a maximum of 40 lb/ft2.

Q. Bottom hinged vs. top C/S Explovent^® panels? Why would I specify one over the other?

A. Top hinged panels are the most commonly specified system. If they open, they will come to a near closed position once the internal and external pressure has equalized. In this near closed position, the canopy effect offers the facility protection from wind, rain, and snow. Whereas a bottom hinged panel will remain open at approximately 60 degrees from vertical, thus allowing easy entry of the elements.

Bottom hinged vents would most commonly be specified when panels are located at grade or at roof level where snow drifting might be a concern. It is much easier for a bottom-hinged panel to open downward on top of the drift than have a top hinged unit power through the snow.

Q. What about security? How easy are C/S's Explovent^® explosion vents to open?

A. The latch of a 4' x 8' panel specified to release at 20 lb/ft2 would be calibrated to release when experiencing a 320 lb force. In order to open the panel from the outside, a point load pull of 320 lb right at the latch position would be required. This is fairly significant and difficult when you consider the smooth exterior finish of the system. Also, if you specify a bottom hinged panel in these security areas, you raise the latch point to a minimum of 8' above grade or roof level making the application of this point load even more difficult.

Consideration may also be given to specifying factory applied burglar bars or other electronic measures for detection.

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