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I am designing an experiment where the hydraulic hose is going to be subjected to relatively high internal pressure for prolonged period of time and it is absolutely imperative that the rate of permeation is minimised. I have found one great book that deals with gases but nothing on liquids or liquids under elevated pressure. I am unable to find any data regarding the permeation rates of hydraulic fluids and/or water through the hydraulic hoses or more generally through various polymers.

The length of time is in years and the pressure in tens of MPa so I am assuming that the permeation will not be negligible.

How can I determine the permeation rate of a fluid through the hose before running my experiment and minimise the permeation rate if it is not negligible?

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I have worked on several pieces of equipment in the past so would like to pass along some information I know about their hydraulic systems as it applies to your question.

Equipment is very expensive so to make up for that companies run them as much as possible. Machinery ran at some sites (e.g. mines) go for 24 hours a day and are only 'down' when performing maintenance. The equivalent for hydraulic hoses service time in regards to permeability failure is easily years (which can be seen by the hours meter and maintenance logs of the equipment).

Permeation for your case is likely negligible, here is why.

Most high pressure hydraulic systems (e.g. like that found on a loader) operate at several thousand psi (10's of MPa like your example). They also have to deal with several instances of 'surging' (sudden increase in pressures) throughout their service life.

The fluids also heat up as the machine is used so the hoses have to deal with an elevated temperature environment. This lowers the viscosity increasing permeability through the hose material and of course any chemical reactions that may occur will now happen more quickly. It is not unusual to have temperatures above 80C (not recommended but this is common).

Working machinery also has to contend with constant hose movement (if it is polymer based instead of steel it is because there is movement). This in turn means the hoses also have to fight abrasion where they are tied to and rub against the chassis.

These hoses are also located outdoors where they see all kinds of weather conditions and temperatures fluctuations from frigid cold to over 120F while contending with spilled diesel, motor oil, etc.

Permeation does occur in some instances but it is usually only seen on hoses that are put into service for the wrong application (e.g. wrong pressure rating). Lifetime of the hose (from a permeability standpoint) is easily several years of service life and it is not unusual to see them last substantially longer in this regard (even in harsh environments).

For your experiment I assume this is a static hose rated for these pressures and fluid types. I am also assuming this is in an ambient indoor temperature environment with clean hydraulic fluid and no hose movement.

If this is the case it should be expected that any issues with permeability will be at minimum when compared to the standard service seen by these hoses. Good quality hoses are engineered to last for a long duration in much more severe environments than your lab. If it is installed properly there really should not be a concern.

I suggest making a trip to your local Catapillar, John Deere, Case, etc., etc. dealer and speak with one of their hose technicians. Just be sure to convert your requirements to standard (psi, inches, feet, etc.) and they will be happy to answer any further questions.

Hope this helps!

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    $\begingroup$ This is definitely true for common applications, but the question says that the hose will be pressurized for years, and few systems that operate continuously for year don't require some oil to be added. The permeation though the hose may still be negligible compared to through the fittings, but I could also imagine that it is not. $\endgroup$ – Ethan48 May 28 '15 at 6:07
  • $\begingroup$ Equipment ran at some sites (e.g. mines) go for 24 hours a day and are only 'down' when performing maintenance. This equipment is very expensive so to make up for that companies run them as much as possible. The equivalent for hydraulic hoses service time is easily years (which can be seen by the hours meter and maintenance logs) just like the question asks, with the exception being they have to endure much harsher environments. Permeation issues do occur but usually only on hoses put into service for the wrong application (e.g. wrong pressure rating). Hope this helps! $\endgroup$ – eatscrayons May 28 '15 at 13:31
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    $\begingroup$ Right, but the issue is not about how long the hose lasts, it's about how often the oil has to be topped off. Systems like that can often have oil added even while they are still in service. $\endgroup$ – Ethan48 May 28 '15 at 13:56
  • $\begingroup$ These hydraulic systems certainly do need topped off but the fluid loss isn't through permeability of the hoses :) I can edit if these items are unclear in the answer and appreciate the feedback! $\endgroup$ – eatscrayons May 28 '15 at 14:02
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    $\begingroup$ I completely understand, being thorough is always best (we are the same in our lab). The thing is, hose permeation (with properly selected hoses/fittings) will be negligible in this environment at your rated pressures. It may seem hard to believe but this is not a unique situation in regards to pressure or duration. It would be best to seek out a hose technician (guys who do this for a living that have seen about everything) at any of the big suppliers (listed above) and they will be able to get you the correct hoses for years of worry free service. Hope this helps! $\endgroup$ – eatscrayons Jun 24 '15 at 2:05

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