We have two 2" ball valves that tie into a cryogenic line (minus 100 C).   The ball valves have cavity vents.  In between the ball valves is a check valve.  There is a small section (6" long) of piping at the end of "Valve 2", followed by a blind flange.   The cavity vent for "Valve 1" is in the direction of the process line.   The question is what side should the cavity vent for "Valve 2" vent to (ie. vent towards the process line or vent torwards the blind flange)?    Our contractor has put the vent direction of Valve 2 towards the blind flange side.  Is this acceptable?  My opinion is that the vent direction of Valve 2 should be the same as Valve 1 (towards the process line).  

Small ball valves are usually floating-style; meaning that the process DP forces the ball against the downstream seat.
Usually both seats seal anyway, so pressure can be trapped in the ball and cavity.  A vent hole drilled in the ball points upstream so that the pressure in the cavity cannot exceed the pressure upstream of the valve.  

If the vent hole is downstream, pressure might dislodge the ball from the upstream seat and then there would be a continuous leak path by the seat, into the ball and thru the relief hole.  

What concerns me is that the check valve can cause cryo-fluid to be trapped between the ball  valve and the check valve.  If the cryo fluid temperature climbs to ambient over a long weekend, then enough pressure can develop inside the trapped volume to cause a component rupture, with potentially severe consequences.  

Both ball valves should be installed with the vent holes upstream.  Eliminate any other potentially un-vented cavities.  

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