Thanks again to all who braved my long and overly-detailed posts asking about the IRMS vac system. In case anyone was wondering, the reason I could never diagnose a severe, underlying problem that was causing multiple turbos to fail is that there never was one.
The first turbo, as I had initially determined (before deciding I must have been wrong) likely died because of the missing o-ring on the vent valve. The second turbo died, however...
... because there was no oil wick in it.
The end result was parts of the shaft that weren't supposed to spin freely... spinning freely. So, let this be a lesson: unless you've got a 'warranty void if broken' sticker on the reservoir cover for your turbos, you might want to take a look before installing and make sure someone hasn't removed it for long term storage or to feed another turbo in need of a wick, or that the rebuild company hasn't missed a step.
So, if anyone had become alarmed about 3-way valve positions on pump-down due to all my ranting, well, I imagine you can relax. (As per the manual, you're really NOT supposed to have it set to FV when pumping down a fully vented machine/analyzer, but everyone does it to no apparent ill effect, so, hey.)
Well, at least we've got a shiny new rough pump now.
University of Washington
School of Oceanography