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in rebuilding sherwood manifolds, i've noticed a recurrent failure mode 
for the captured viton rings on the two outboard valve stems (the 
isolation knob seems immune). it goes like this:

-tanks get carried by valve knobs
-the valve stems gouge the inner surface of the bonnet
-these gouges cut the captured rings when valve is opened/closed
-gas leaks past the stem seal to the nylon bonnet seal
-the rings get chewed up, the bonnet seal extrudes around the stem
-the knobs become difficult to turn (even though no gas leaks)

soft (O2 compatible) silicone rings are even more susceptable. as pointed 
out previously, rings of high durometer will help, but so will keeping 
all loads off the valve knobs.

regards, em




_____________________________________________________________.sig
Eric Maiken                    email: eapg243@ea*.oa*.uc*.ed*              
Dept. of Physics                   o: 714 824-6621   
U of California                  fax: 714 824  2175
Irvine, CA 92715-4575

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