What Is Permeation?

In an automotive context, permeation refers to how gases, vapors, or fluids pass through barriers or materials. Fuel vapor is known to permeate through fuel hoses or fuel lines. Hence, fuel hoses are made with materials that are meant to either lessen or completely prevent permeation.

Here is a video dealing with coolant permeation as relates to silicone coolant hoses often used in police vehicles and big trucks:

How Does Permeation Happen?

Permeation happens when fuel vapor molecules pass through the molecules of solid materials, such as hoses, cables, or plumbing. This process happens very slowly. However, when it comes to fuel vapors this gradual leak can eventually be significant over a long period.

The byproducts of combusting fuel are known to be harmful pollutants. That said, fuel vapors are also harmful. Inhaling small amounts of gasoline vapors can lead to nose and throat irritation, headaches, dizziness, nausea, vomiting, confusion, and breathing difficulties.

Given these health risks, the Environmental Protection Agency (EPA) has issued these standards for fuel lines:

• EPA Low-Emission Fuel Lines must have permeation emissions at or below 10 g/m 2/day when measured using their prescribed test procedure.

• EPA Nonroad Fuel Lines must have permeation emissions at or below 15 g/m 2/day when measured using their prescribed test procedure.

• EPA Cold-Weather Fuel Lines must meet the following permeation emission standards when measured using their prescribed test procedure:

Fuel Line Materials and Construction

Fuel lines can be made from a variety of materials. There are hoses made from metal, rubber, nylon, and Polytetrafluoroethylene (PTFE).

What Materials Are Susceptible to Permeation?

Both polybutylene (PB) and polyethylene (PE) hoses are easily susceptible to vapor permeation. PB and PE hoses are typically used for house plumbing. Polyvinyl chloride (PVC) hoses, which is the material traditional garden hoses use, are also permeable. These materials shouldn’t be used as a fuel line because of their permeable nature.

What Is the Best Material For Fuel Lines?

To prevent fuel vapors from permeating through your fuel lines, you should only use fuel lines that use the right materials.

Metal lines are prized for their durability but are difficult to make. This is why they’re only typically used in factories. These lines are typically composed of stainless, aluminum, or coated steel tubing, making them the preferred choice for fuel lines.

On the other hand, rubber hoses are known for their ease of installation. They’re connected using standard barbed fittings and fuel line hose clamps. However, they must meet specific Society of Automotive Engineers standards. Despite their user-friendliness, rubber hoses have drawbacks. Firstly, they typically degrade when in contact with fuel. Some rubber hoses are also known to leak fuel vapor. That being said, a hose specifically designed to carry fuel and/or oil isn’t nearly as permeable as other types of hose not designed to carry petroleum liquids.

Lastly, rubber hoses can be susceptible to cuts. Nylon tubes could be a more suitable option than long stretches of plain rubber fuel hose, but nylon tubes can chafe and develop leaks if they’re not routed properly.

Braided rubber hoses combine the durability of having a braided stainless nylon exterior with a flexible rubber liner. However, they still share some of the durability and fuel vapor concerns associated with standard rubber hoses.

The best option is PTFE Lined Hoses, which boasts a braided stainless or braided nylon exterior, and a PTFE liner that effectively blocks fuel vapors and prevents degradation. It’s important to note that PTFE Braided Hoses require special fittings as standard AN fittings won’t work with them, and they’re less flexible compared to typical braided or rubber hoses.