Micro-valve for Steam

You might try a mod-logic valve from Dynamco. The spool is a precision fit to the body of the valve with no o-rings. It is all metal except for the end caps which are plastic. There is a 150 psi. limit though. At least on the one's I am familiar with. They may have other valves more suited to your application.

If the plate holding the seal is strong and it can be pressed enought to adjust a metal, you could try a ring of the same size as the viton one but made with copper or soft aluminum alloy as those used in gas connections. If it works, saves a lot of efforts.

(I HATE THE TIME OUT ON THIS THING - I HAD A LONGER POST.) VITON is not recommended for steam (see http://www.gtweed.com/GT/Web/GTBase.nsf/vwContentB yKey/N25S3RP3644ELETEN follow elastomer link and look for steam). Try EP/EPDM, but be carefull of other things that could be disolved in the steam. Also, put a "back-up" with soft metal or PTFE to act as a barrier of higher shear strength and close the clearance gap that is allowing the elastomer to extrude through... Have fun!