plastics to seal high pressure

hi can anyone point me in the right direction i have an air bottle of 180 bar now i need to remove the bottle regurlly to fill it and the o'rings just aint lasting....... it screws into an ally block brass thread on bottle into the ally, now the filling adapter i use has a plastic washer inside that seals the bottle for filling the tread has a shoulder so i was thinking of using a plastic/nylon/delrin/acetal washer/seal but what would be the prefered material?

many thanks mike

Edited By michael burton 1 on 04/07/2012 16:32:18

Michael

I think that it will depend on the what the vessel is being filled with and the working temperature range that it is exposed to. As you say it's an air bottle then I take it that it is air that you are filling it with and not some other gas/fluid. If so then is it being used in a critical situation i.e. scuba tank or similar. If this is the case then I would take professional advice and not try to adapt something to fit.

If its not part critical system and the temperature range is limited then I suspect something like nylon would be suitable, delrin/acetal has a tendency to absorb water and change dimension slightly; that said maybe something like PTFE would be suitable.

Cheers

Martin W

sorry should of said no water contact and no heat issue eaither its just to seal the bottle to the pre charged air rifle i designed and yes just filled with air

Martin,

I believe nylon and other polyamides are significantly worse than acetals (including Delrin, which is just a Du Pont proprietary form of acetal) for absorbing water. Agree with your other comments though. Might also consider PTFE (Teflon) as you suggest; it is often used in high-vacuum equipment.

David

Edited By David Littlewood on 04/07/2012 17:15:33

Michael,

To say "no water contact" is rather missing the point; nylon (in particular) will absorb water from the air and may change dimension accordingly.

David

when i said no water contact i meant it wont be submersed or put under water, but i can understand about it asorbing water from the air a fair point

Hi Dave

Thanks for the correction I am not sure why I thought nylon didn't absorb water , must be my age!! Mind you for this application I don't think that any dimensional changes would be significant enough to affect the seal whether it be made from delrin or nylon. The other thing to look at would be how smooth the mating surfaces are and if there are any sharp edges that may be cutting/damaging the O rings.

My initial concern was that this was part of a life support system and as such seals and their reliability are somewhat critical , don't want to see a mass of bubbles from the tank when you are half way to the surface.

Cheers

Martin W

Nylon is significantly hygroscopic. It's use for spaceflight components was banned for that reason since the early days. (The water vents in a vacuum leading to part distortion and possible problems with other equipment).

Delrin is better but does build up machining stresses which may spontaneously release in service causing distortion. To combat this for critical applications, there is a stress releiving process available which can be applied after rough machining. There is an oil bath involved and some minor oil absorbtion may be experienced.

iv done some looking round on a few sites and it looks like delrin is the way to go, its only for a while untill i can get the money togethor for a quick fill kit

I would recommend that you use only parts from the air bottle manufacturer to seal air at 180 bar (over 2600 psi).

If you make your own parts and the seal should ever fail, someone could be seriously injured from high pressure air blast, and you would be liable as the maker of the part. (or it could be you that is injured.) The manufacturer will use only tested certified proven materials and methods to make parts for high pressure air.

2600 psi air is not something to be played or experimented with, and is well outside the scope of model engineering. Even 150 psi air can cause serious injury in the event of a seal or hose failure.

JD

Edited By Jeff Dayman on 04/07/2012 20:16:46

If you make a part from Nylon you can stabilize it by boiling it in water and it will stay at the size and not absorb water.

How I found this out was through an Australian manufacturer of surveying staves that had nylon sliders that would sieze when wet!. They boiled the nylon parts and they were OK after that.

clive

Have a look at a BA set, which firemen use

Ships have them

I recharged bottles all the time in the 1980s up to 2-300 psi(or was it BAR) with zero issues and all metal connections

**LINK**

Diving people will refill air bottles all the time as well

There hasn't been a problem with this issue for decades

siebe gorman made a lot of gear, it screwed straight on and off for recharging using nothing more than hand tight connections, zero issues, ever

If you are going to play about then use high pressure water, not air

Dunlop or someone managed to kill about 5 employees the first time they tested a large tyre to destruction with air pressure. It literally exploded, like a bomb.

They always used water after that incident

The bit that always amazed me was the demand valve, which let you breathe normally even though you were carrying the equivalent of a bomb on your back.

Edited By Ady1 on 05/07/2012 01:17:33

Have a look at the Swagelok site I have used their fittings and valves for high pressure

swagelok.co.uk

Paul

Michael can i suggest you cure the problem and not look for other ways to improve an already established fool proof method thats always been used at 232bar for more than two decades.

Delrin/acetals alright for valves an accepted practice.

Likewise theres no law on testing such vessels below 500cc, very few have ever been tested. Certainly most producton made cylinders from Germany, US, China and UK would not get through the door of the testing station. 200bar operating max should be tested at min 300bar. Some are known for bulging at 245bar so rely on burst discs.

Ady we call them regulators and in Michaels scenario have been in use for 29 years often copied and marketed as their own.