Well after a day of mucking around with the valve I have finally managed to get it to blow off with a 5psi pressure drop from open to close. It doesn't "jazz" or "buzz" much but definitely opens and closes without leakage which I am happy with. The final cup clearance was between .0155" and .0160". Any more than that and it starts to blow like a conventional valve, but any less and the pressure drop is abour 7psi.
A lighter gauge spring increased the pressure drop.
I had to make a checksheet for changes to critical dimensions as it was easy to lose track of what it was that had changed or what seemed to be a positive result. I ended up number stamping the end of the spindles.
I also played with the underside of the cup on one of the spindles as the shape I have to get 1mm engagement of cup and secondary seat flattens off the bottom of the cup and would be far more streamlined for any escaping gases. The measurement of reality against my CAD file meant the shape was a little different once the ball had bedded into the seat and cup.
I wondered if the shape of the underside being a recessed cup causes drag/eddying with the escaping air or steam, which I thought would push it open easier without increasing the cup diameter. Didn't seem to make much difference by just putting a groove on the underside. Might have a bit more of a think about that one.
I made a measuring cap to check the engagement of the cup and secondary seat while in situ which assists holding a digital caliper vertical.
Happy steaming!
Nigel
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