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VENT HOLES Webster defines "vent" as a small aperture designed to provide an outlet from a confined space. One lined pipe manufacturer says that vent holes are essential to release gasses which may be generated at elevated temperatures and trapped between the liner and housing before it collapses the liner. They vent all their lined systems. ASTM F1545 for lined pipe says in point 5.3 that each pipe and fitting shall be provided with a venting system that will release any pressure between the liner and the housing. Note 8 states that venting is not required with PVDF, PP, ETFE, or PVDC liners. The key word here is RELEASE. The EPA and other governmental agencies are passing regulations and laws to limit or stop atmospheric releases of volatile organic compounds (VOC's). VOC's are among the most common pollutants emitted by the chemical process industry. VOC's are carbon compounds that react with nitrogen oxides and other airborne chemicals in the presence of sunlight (photochemically) to form ozone, which is a primary component of smog. Reductions of VOC emissions in areas that exceed the current national ambient air quality standard for ozone of 0.12 ppm is mandated under Title I of the Clean Air Act Amendments of 1990. In addition, Title III of the amendments requires reduction of the emission of 189 hazardous air pollutants, most of which are included under the definition of VOC's as well. With all of the above requirements and concerns with cleaning up the air, does it make any sense to vent lined pipe? (About as much sense as drilling holes in the containment surrounding a nuclear reactor.) In the beginnings of Lined Pipe, vent holes were necessary only for PTFE and FEP due to liner thickness of .050"or less which allowed gasses to easily permeate and build up between the liner and the pipe collapsing the liner. Today liner thickness is 3 to 4 times as thick greatly reducing the migration of gasses through the liners even in more easily permeated PTFE.  Heavy wall PFA liners could be manufactured without vent holes, but ASTM F1545 mandates that they be vented. The chance of a gas permeating today's thicker liners in sufficient quantity to collapse the liner we believe is relatively small. The release of those same chemicals into the atmosphere through a venting system would have more far reaching consequences than the collapse of a pipe liner. This is especially true if the liner itself fails and the pipe starts showering acid all over. Most of the information on permeation of PTFE and FEP available today is out of date. It was taken from testing done on thin films and may not reflect what is required in the Chemical Process Industry. PFA liners are by far less easily permeated than the PTFE liners used in the past. We are taking the position that other liner systems should be used instead of vented systems. This will help eliminate one source of fugitive emissions and reduce air pollution. For your information, here are the liners that the industry vents: Resistoflex: Currently vents all of their liner systems. Dow: Currently vents PTFE and PVDF liner systems. 3P: Currently vents FEP, PFA, and PTFE liner systems. Fusibond: Vents only PTFE and PFA. For more information on which liner systems can handle your particular services without vent holes contact your supplier. |
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