What about effusion and PTFE hose?

Effusion or permeation is the process of migration of one substanceinto and through another. Usually the movement of a gas into andthrough a hose material. The rate of permeation is specific to thesubstance, temperature, pressure and the material being permeated.(Also refer to FAQ #8 regarding .040 wall versus .030 wall.)
Effusion can occur with many types of media, and with many differenthose materials. In every case, certain questions must to be asked:

  1. What is the rate of effusion? Is it constant or intermittent?
  2. How hazardous is the medium being conveyed? Is it noxious? Is it poisonous? Is it flammable? Is it explosive? Is it corrosive?
  3. What is the external environment? Will the product be exposed tosunlight? An open area? An enclosed space? Close to a source of heat orignition? Near potable water sources or where it can create otherhealth risks?

It is best to categorize the application based on the answers to thequestions listed above. Usually you will find some general categoriesof concern. They are:

  • Media that is not corrosive or noxious but may displace breathable air in an enclosed space thus creating a hazard to personnel.
  • Media that will effuse at ‘vapor phase’ if a certain temperatureis reached, i.e. a fluid that boils at 125°F at atmospheric pressure.Such substances may form chemicals with exposure to air. Thesechemicals can be corrosive and/or cause injury to personnel or in someway create a hazard. For example, liquid chlorine can effuse as a gasand form hydrochloric acid in the atmosphere.
  • Corrosive, hazardous or noxious media that is already a gas atatmospheric pressure and at ambient temperature and known to effusethrough PTFE.
  • Flammable or explosive media that may or does effuse underconditions within the scope of the application. Here again, no attemptis being made to cover all possible scenarios.

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