Anyone got some Kel-F (PCTFE) to spare?

[pescador775]

Thanks for the offer, PM sent.

Hacker is right about teflon for the most part. It seems to depend on the type of orifice. Used against a smooth, "bullnose" orifice the stuff works well. However, teflon deforms against other types such as found in tank valves which indent the plastic or against a flat orifice with large drilled hole which requires a lot of pressure to shut off. Turning the screw harder to shut off leaks just causes teflon to bulge and is ineffective over time.

 

[cool_hardware52]

Thanks for the offer, PM sent.

Hacker is right about teflon for the most part. It seems to depend on the type of orifice. Used against a smooth, "bullnose" orifice the stuff works well. However, teflon deforms against other types such as found in tank valves which indent the plastic or against a flat orifice with large drilled hole which requires a lot of pressure to shut off. Turning the screw harder to shut off leaks just causes teflon to bulge and is ineffective over time.

PTFE can be machined successfully. It does require a bit of planning, but it's along way from impossible.

The real problem with unfilled PTFE as a material for scuba valve seats is cold flow.

Under load PTFE will flow like Ice in a Glacier.

Tobin

 

[captain]

PTFE can be machined successfully. It does require a bit of planning, but it's along way from impossible.Tobin

I have machined valve stem seals from PTFE.

 

[cool_hardware52]

I have machined valve stem seals from PTFE.

Yup, all sorts of seals and packings are produced from PTFE via machining.

Tobin

 

[pescador775]

So, after looking at teflon (PTFE), HDPE, and PEEK, all can be useful for controlling or containing high pressure gases. Plain, soft teflon is the old workhorse for valve stem seals and in certain valve seats. When used in this application a super soft seat is formed which can turn off the flow of high pressure with two fingers. The plastic has to be fully contained within the carrier socket to prevent bulging or "flow". The orifice nozzle has to have a rounded shape which is smaller than the carrier lip to prevent interference when the valve is closed.

The other plastics, HDPE and PEEK are fairly tough, harder and less subject to mechanical deformation. They can be used with nozzles which have smooth interfaces or rims which have a certain "bite". The materials can be installed in the carrier leaving a slight protrusion if necessary. This can be on the order of 1/32-1/16 inch. If applied against a smooth or round nosed nozzle, more force is necessary to shut off high pressure due to the hardness of the plastic. Three finger plastics, so to speak.

It appears that Teflon and PEEK are relatively inert in high pressure oxygen. However, PEEK is the only one I've found so far that burns without producing undesirable gases other than CO2 and CO. I have not ordered the book on high pressure oxygen effects on materials, but open source information from the "burn index" (attempting to ignite materials in low pressure, pure oxygen) puts PEEK in a favorable position. It is harder to ignite and burns relatively cleanly. The plastic, HDPE has the lowest burn index which suggests it might not be a suitable material for HP oxygen. I have no information on burn products but there is no motivation to look at this time since it may be potentially flammable in HP oxygen and which might disqualify it for that use.