May 2019

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 duct construction has become increasingly common due to low weight and cost. 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 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.

FM Approval Standard 4922 applies when selecting FM Approved ductwork; however, it is important to note that this Approval Standard utilizes 12 in. diameter ductwork in the testing procedures, so this is the default Approval listing as found on 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.

For new construction, it is recommended that all ductwork (of any size) is noncombustible or FM 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.

If FM Approved ductwork is desired, ensure the duct is FM Approved for the proper diameter and application.

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.