May 2021

Protection and Approvals for Process Exhaust Ductwork

The following is an update to our May 2019 technical web article Protection and Approvals for Combustible Plastic Ductwork.”

The removal of dust, smoke, and fumes is a critical part of many industrial and commercial operations. Any damage occurring to critical process ductwork can halt operations and result in property damage as well as process disruption, making it a crucial focus of property loss prevention. Aside from being a potential operations bottleneck, ductwork has the potential to introduce additional property risk into a facility in several ways:

  1. Can be of combustible construction (plastic materials)
  2. Can carry combustible contents (dusts and fumes)
  3. Can spread smoke and fire quickly to multiple areas of a facility

Years ago, ductwork was generally constructed of noncombustible materials, such as metal; however, combustible (i.e., plastic) duct construction has become increasingly common due to low weight and cost as well as its corrosion resistance when handling corrosive fumes. Ductwork made of combustible construction or carrying combustible contents presents the largest hazard, as a fire beginning within the ductwork or in an area adjacent to ductwork can be easily conveyed both horizontally and vertically, often leading to fire and/or smoke damage in multiple key production areas and lengthy process disruption. The loss history associated with combustible ductwork fires is significant. The key functional purpose of ductwork, to move air, dust, fumes, etc. quickly and efficiently throughout a facility, can be a detriment if fire and smoke are involved; active protection and housekeeping procedures can help to reduce the hazard associated with process ductwork.

Where combustible materials are being conveyed, it is best practice to establish a routine housekeeping procedure for the interiors of ductwork to ensure that combustible deposits are kept to a minimum; however, frequent cleaning will only reduce the likelihood of a fire, not the severity should one occur within the ductwork. In order to reduce the severity of a fire occurring within the ductwork, fixed automatic protection is recommended under the following circumstances:

  • Any portion of a system utilizes combustible components (i.e. plastic ducts, plastic-lined ducts, etc.)


  • The potential exists for the buildup of combustible materials inside the ductwork

Ductwork must be large enough to support sprinklers, as outlined in NFPA Standards and FM Data Sheets:

NFPA 13, Standard for the Installation of Sprinkler Systems (2019)FM Data Sheet 7-78, Industrial Exhaust Systems (2017)
Cross sectional area equal to or larger than 75 sq. in. (9.77 in. diameter)Cross-sectional area equal to or larger than 80 sq. in., or duct diameter equal to or larger than 10 in.

While the installation of sprinklers inside combustible ductwork is necessary to prevent fire spread, it is also problematic. A duct drainage system should be provided to prevent possible ductwork collapse due to the weight of discharged sprinkler water. In addition, a method to be able to remove the sprinkler head from the ductwork for periodic inspection is needed. Finally, if corrosive fumes are being exhausted, corrosion protection should be provided for the sprinkler heads located inside the ducts.

In order to avoid the installation of sprinklers in ductwork not subject to the buildup of combustible materials, the ductwork should meet the criteria set forth in FM Approval Standard 4922 “Approval Standard for Fume Exhaust Ducts or Fume and Smoke Exhaust Ducts,” 2016 Edition.

Ducts which are FM 4922 approved limit fire spread, inside and out, but also maintain their structural integrity which allows smoke and fumes to continue to be exhausted, It is important to note that this Approval Standard utilizes 12 in. diameter ductwork in the testing procedures. The 12 in. diameter is usually the smallest size approved and 60 in. is the current maximum size as found in the FM Global Approval Guide ( Manufacturers may request that smaller diameter ductwork is tested, and there are several manufacturers with FM Approved ductwork of 10 in., and some that have tested ductwork as small as 4 in. diameter. If FM Approved ductwork is desired, ensure the duct is FM Approved for the proper diameter and application.

There are three categories of FM 4922 approved ductwork as follows:

Fume Exhaust Duct SystemsNo fire spread but duct will lose its cross-sectional area (i.e., structural integrity) and collapse when exposed to heat and flame thus losing its ability to exhaust smoke and fumes.This category of FM 4922 Approved duct is not recommended for smoke sensitive occupancies such as cleanrooms
Fume and Smoke Exhaust Duct SystemsThis duct will maintain its cross-sectional area when exposed to heat and flameThe only shortcoming of this category is that the exterior surface of the duct when exposed to fire may produce a significant amount of smoke which could cause damage to a smoke sensitive occupancy.
Fume and Smoke Exhaust Duct Systems for Use in CleanroomsIn addition to no fire spread and maintaining its cross-sectional area, the exterior surface of the duct is tested to FM 4910 cleanroom materials standard. Very limited amount of smoke would be released when the exterior surface is exposed to fire.Recommended for use in smoke sensitive occupancies such cleanrooms

For new construction, it is recommended that all ductwork (of any size) is noncombustible or FM 4922 Approved in order to most effectively reduce the risk. The option to select FM Approved ducts having smaller diameter (less than 12 in.) may not be available or be more limited due to the testing procedures and default criteria outlined in FM Approval Standard 4922 above. The installation of fixed automatic protection in existing ductwork ultimately depends on an assessment of the criteria above as well as individual risk evaluation on site.

Risk Logic advises on process hazard reviews. Contact Risk Logic with any questions you may have regarding property loss prevention for protection/installation of combustible ductwork or ductwork conveying combustible materials.