deepsea nylon valve 1

[dangnm]

I need to machine up 4 plastic/nylon valves to transport a mixture of oxygen/helium (mixed gas) @ a pressure of 150 p.s.i. (approx) @ ambient pressure equal to an ocean depth of 500 feet for an intermitant period of 12 hrs @ a time.
The "differential pressure" across the valve will not exceed 10 p.s.i.(ie. the pressure of the gas increases & decreases in sympathy with the deph/press. of the water)
The valves will take the form of Quarter Turn Deep Barrel Plug Cocks,have an internal bore of 1-1/4" & the barrel will be 2-1/4" diameter.
Body dimensions are 5" long X 3" x 3" with an axial bore of 1-1/4".
1)Of the plastic/nylons economicaly available;
What type would be best suited for the design parameters of;Salt water imersion for 12 hr periods,depth/pressure of 500 f.s.w./250 p.s.i.,heli/ox gas etc.
2) Should the barrels be tapered to a)eliminate the need for a sealing medium & b) by tightening the barrel into the body would automaticaly make up for any wear etc. OR machine the barrels with parrallel sides & fit them with a mapped o ring to facilitate sealing.I'm concerned the mapped o ring concept may lead to extrusion/displacement of the o ring.
Any help would be greatly appreciated
regards, deepdiver

 

[troppo]

Hi
I would be going for something a little more dimensionally stable in water. i take it that the above valve will be going in sea water and relieving pressure when ascending to the surface.nylon has a problem with absorbing water which in turn changes its dimensions. if you want to use nylon, place some in some water and measure before and after sizes.
A plastic which a lot of marine equipment makers use is acetal(delrin). Its water absorbtion is low and it is designed to be easily machined and holds close tolerances well. it is susceptable to UV but if you are using it underwater this should not be a problem (black is better for UV than white)and you will have to check if it can handle helium.
In regards to the seat why don,t you try an o'ring seat similar to the oxy/acet gas bottle regulator seat or if you want a fat face seat machine a bore for your o'ring to fit into and machine a collar with a taper on the o'ring side to allow the o'ring to move when squeezed and to encaptulate the o'ring.i haven,t used the above method so it's an experiment.
hope this helps?