Key Factors Affecting Permeation

Key Factors Affecting Permeation

We recommend you read our article Permeation: An important factor when looking for coated pipe systems. So you can learn more about this topic.


Most of the factors that affect permeation are relatively simple and intuitive, others require a brief exploration of some more subtle aspects of the physical process at work in permeation systems.


Below we will detail, based on our 31 years of experience, 5 of the factors that have had the greatest impact on our manufacturing procedures for fluoropolymer-coated pipes:

to. Vapor pressure

If there is an unobstructed region outside of the fluoropolymer coating, we know that the vapor pressure is directly related to the permeation rate.

This occurs in fluoropolymer-coated piping systems with their corresponding vent hole. If, on the contrary, there is an obstacle in some external region (metal casing without a vent hole, a vent hole closed by paint, etc.). Additional permeation is then the difference in vapor pressure on each side of the coating, and is reduced compared to the initial stages.

When considering severity of service and projected permeation rate, the value of importance when a liquid is present is its present vapor pressure. If, on the contrary, the present fluid is a gas, the vapor pressure is the real pressure.

So for a gas, permeation can be understood as a very small form of leakage.

b. Fluoropolymer Coating Wall Thickness


The liner wall thickness is inversely related to the permeation rate, ie doubling the wall thickness halves the permeation rate.


On the wet side of a permeation system, where proper ventilation is provided, the wall is completely saturated when in its steady state, just inside the metal shell. However, when steady-state conditions are fully reached, the concentration decreases linearly to
Outside, all the permeant molecules are removed by evaporation and the concentration is then close to zero.


In coated pipes the escape is not unlimited, which means that the vapor pressure of the permeant begins to build up in the annular space between the coating and the metal casing and the amount of permeant molecules still inside the coating, waiting to escape to the outside, it becomes larger and larger, increasing the concentration within the liner and consequently reducing the permeation drive.

c. permeation molecule size

In practice, the good selection of the fluoropolymer is the key so that the size of the molecule of the process medium is or is not an indicator of the final permeation rate.


For liquids, permeation occurs because most are soluble to a greater or lesser degree in the amorphous regions of plastics, at the molecular level, which means that after the adsorption step, the liquid migrates by diffusion through the plastic, jumping into the intermolecular voids that exist in all polymers.

d. Temperature

The temperature of a system is a direct measurement of the molecular kinetic energy, a higher temperature means faster moving voids, which provides a higher statistical probability that the permeants will travel, impregnating the fluoropolymer surface, and , even traveling abroad.

This occurs to a greater extent in systems at elevated service temperatures, as a result of the kinetic energy of the fluoropolymer molecules, as opposed to the same process at low service temperatures.

and. crystallinity

Most fluoropolymers exhibit randomly located areas of ordered crystalline structures (seen at the molecular level) scattered throughout amorphous arrangements of molecules, these arrangements inhibit permeant progress, causing penetrating molecules to migrate along. alternative routes, thus decreasing the overall permeation rate.


This factor is relevant, as it happens, it is a cost-effective and practical tool, which helps to achieve significant gains in permeation rates.


The crystallinity of coatings, such as PTFE, can be manipulated for performance and longevity in piping intended for chemical process services.

Conclusions

It was previously believed that the differences between the various lined pipes, mainly PTFE, and the systems on the market were minor, with the method of manufacture or the wall thickness of the coating being the only key factors in performance.

However, today it is known that there are other factors that affect permeation, which represents an important phenomenon that must be considered when making purchase decisions for coated pipe systems.


You may also like…

Permeation: An important factor when looking for coated pipe systems

Search
Other publications
Categories