Exhaust duct material considerations for life-science manufacturing plants

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The ever-changing body of rules and regulations governing operations of the pharmaceutical and biotech industry has required flexibility and adjustment of production facilities in order to comply with current and even proposed mandates. One area where flexibility and innovation can be clearly seen is in new material choices for facility exhaust systems.

The Environmental Protection Agency’s Ruling 40 CFR and the National Fire Protection Association’s (NFPA) Standard 318 pertaining to cleanrooms is affecting the way life-science plant operators specify process ventilation systems. Manufacturers of one of the key components of these systems, the duct itself, are touting new materials of construction and innovations that have been developed over the past fifteen years. The evolution of one such specialized product, fluoropolymer-coated stainless-steel duct, began in the semiconductor industry of the early 1990s. Fires spreading through manufacturing buildings via a ventilation system made of combustible materials resulted in a string of catastrophic losses. Much of the work done at these sites was conducted in “cleanrooms” where corrosive chemicals were used and hazardous vapors were generated. The use of fluoropolymer-coated stainless-steel duct was a revolutionary breakthrough and has now become the material of choice for this industry as it meets the extreme demands of both safety and corrosion resistance. Although coated duct has been in service over fifteen years in the semiconductor industry, it has only recently been discovered by the life-science sector.

Process vent and corrosive exhaust in life sciences

Process vent “piping” is usually associated with manufacturing and pilot-plant operations where chemical synthesis and formulation are conducted. In these areas, the construction materials for reactor vessels, process equipment and process piping are more specialized and include borosilicate glass, glass-lined steel, exotic alloys, and fluoropolymer. It’s here, where these specialized “high-end” materials are required, that fluoropolymer-coated stainless-steel duct and pipe offer a complementary corrosion-resistant vent system. Reactors, filter-dryers and other process equipment in this category are usually not in a cleanroom, but rather in a manufacturing suite. The equipment is usually an ASME pressure-rated type, capable of containing process liquids and vapors up to 150 PSIG, and incorporates a vent nozzle or relief device connection. The manufacturing suite may have a general exhaust (or snorkel vent at individual equipment stations) for the area, but increasingly we see pharmaceutical and biopharmaceutical facilities also incorporating large walk-in reactor enclosures and fume hoods that are tied into the process vent system. In applications where higher pressures and full vacuum are customary, fluoropolymer-coated Schedule 10 pipe (150 PSIG-rated) would be an appropriate choice for corrosive vent lines.

Cleanroom exhaust in life-science applications is typical of the types of systems found in university and corporate research labs where an entire room or specific piece of equipment requires removal of hazardous vapors. These negative- and low-pressure systems are governed by SMACNA HVAC guidelines. In applications where biologic agents are involved, but no corrosive chemicals are present, exhaust is usually passed through a high efficiency particulate air (HEPA) filter and perhaps even a carbon adsorption (CA) filter. In this situation, standard duct materials could be used. But if the process is one where corrosive chemical vapors are produced (which can potentially damage HEPA filters), duct material with a higher level of corrosion resistivity, such as fluoropolymer-coated stainless steel, should be considered.

Potential hazards

As in the semiconductor industry, hazards in the life-science industry are ever-present. They include fire or explosion due to the use of solvents, flammable liquids or dust, and the resulting contamination of production, storage, and cleanroom areas by smoke or other substances released by fire. As stated in the National Fire Protection Association’s Standard 318: 3-3.6 Exhaust duct systems shall be constructed of non-combustible materials or protected with internal sprinklers in accordance with 2-1.2.6. Exception: Ducts approved for use without automatic sprinklers.1 The NFPA standard continues with a hierarchy of material preferences: 3-3.6 Considering fire protection issues only, duct materials listed in descending order of preference are: (a) metallic, (b) approved coated metallic or nonmetallic not requiring fire sprinklers, (c) combustible with internal automatic sprinkler protection.


Figure 1. This 60-inch diameter elbow is ready for shipment to a New Jersey pharmaceutical plant. Photo courtesy of Fab-Tech, Inc.
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When selecting pipe or duct for corrosive exhaust applications, it is also critical to choose a product that is rated by Factory Mutual (FM), an insurance affiliate of FM Global devoted to reducing commercial and industrial property losses and maintaining the continuity of its policyholders’ business operations. Fluoropolymer-coated stainless-steel duct is regulated and approved by FM Research Standard Number 4922. With fluoropolymer-coated stainless-steel duct, structural integrity is maintained in the event of a fire. With extremely low flame and smoke characteristics (flame spread under 10, smoke generation under 15), these systems will not burn, melt or generate large quantities of smoke, an extremely important issue in the life-science environment.

Material of choice: Fluoropolymer-coated pipe and duct

ETFE (ethylene-tetrafluoroethylene) and ECTFE (ethylene-chlorotrifluoroethylene) fluoropolymer-coated stainless steel was developed in the early 1990s. Fluoropolymer-coated duct is 300-series stainless steel that is coated with a two-part (primer and top coat) fluoropolymer system, then heated and “baked” to form a chemical and mechanical bond with the stainless substrate. Some manufacturers utilizing ETFE have developed proprietary primer technology that ensures superior adhesion to the stainless steel, resulting in a coating that will not delaminate. The coating is both visually inspected and spark-tested to guarantee a pinhole-free surface (see fig. 1). ETFE has been evaluated in over 400 chemical applications, and has proven superior to both plastic and fiberglass-reinforced plastic (FRP) when tested for corrosion resistance, concentration and upper temperature limit (see Table 1).

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In evaluating the level of corrosiveness of a given application, it is vital to consider the entire range of chemicals to which the duct system will be exposed. Even more important to consider are the potential combinations. “It is also important to evaluate complex systems with several different types of process streams (e.g., reactor, storage tank, relief stream) discharging into a common header. Where vents from several sources are combined, it is necessary to carefully consider all possible interactions between the different streams in terms of both chemical reactivity and flammability.”2 It’s reassuring that ETFE has a wide corrosion-resistant envelope to handle most chemical situations. Additional reasons for considering ETFE-coated stainless-steel duct include: its robust mechanical strength; light weight; ease and speed of installation using rotating van stone flanges or EZ-type clamps; the ability to be field modified (e.g., shortened, nozzles or drain ports added); and the fact that the stainless-steel exterior will never need painting (see Fig. 2). Unlike FRP and other “glued” plastic systems, there is no need for grinding, sanding, prep work or the use of malodorous epoxies and heating blankets, especially in the cleanroom setting.


Figure 2. ETFE-coated duct being installed on the roof of a biopharmaceutical pilot plant in California. Photo courtesy of Hellwig Plumbing.
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When evaluating relative costs, it is important to look at the total installed cost of the system including the mechanical contractor’s labor and any related piping and sprinkler costs.

Conclusion

In the past, chemical and pharmaceutical plant operators often specified fiberglass, plastics, stainless steel, or even PTFE-lined carbon steel to handle corrosive vent applications. Today, many building codes and insurance companies require vent duct made of stainless steel or other non-combustible materials (see Fig. 3). The intent of the EPA’s Ruling 40 CFR was to reduce harmful emissions and make the air we breathe safer. NFPA Standard 318 is focused on making work environments safer and less prone to the ravages of fire and smoke. Fluoropolymer-coated duct is one step forward in improving the overall safety of both the facility and its inhabitants by providing a reliable corrosion-resistant conduit for the gaseous by-products of manufacturing.


Figure 3. ETFE-coated duct connected to scrubber unit at a biopharmaceutical pilot plant in California. The building was completely renovated from a warehouse to cleanroom research facility. Photo courtesy of G. Cortes, SMW 104.
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If corrosive vapors and fire safety are a concern, fluoropolymer-coated stainless steel could be the ideal solution. Professionals who design, build or manage life-science facilities that contend with corrosive and hazardous vapors can benefit by seriously considering this type of product.

References

  1. National Fire Protection Association (NFPA) 318: Standard for the Protection of Semiconductor Fabrication Facilities.
  2. Tony Ennis, Haztech Consultants, Ltd. “Collect and Destroy Emissions Safely,” Chemical Engineering Progress, May 2004.

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