Hi Peter,
You mentioned elsewhere that pressure transients were blowing your welds? I was wondering how transient they are. We use various flow systems at work (for totally different porpoises), and in some systems we have pressure-buffers such as short bellows sections (metal), or glorified balloons which sit inside the system and can absorb some of the transients. May be completely out of the question for your application, but thought I'd mention it. Might save over-engineering everything to cope with a transient problem.