James,

It may help you to read this paper:

http://www.iomosaic.com/iomosaic/pdf/scdiers.pdf

Also, try to find a copy of ASME B31.1 and review the non-mandatory Appendix II, "Rules for the Design of Safety Valve Installations". This appendix addresses some of your vent pipe issues.

And if you can find a copy of API RP-520 you can find some help there also.

Regards, John.