Dear Peter,
What sort of industry is this valve for? Upstream or downstream oil and gas, pipeline, mining, etc.?
Aside from manufacturing/workshop constraint, metal seated ball valve is so complex, it might scare even the well known manufacturer to reply back to you. Or provide you with price overestimation.
Seems like your client performed mainly desktop review prior deciding they will go for 36” ball valve, butt welded top entry. Criteria such as maintainable, full bore (PIGable), quarter turn (faster opening/closing) by text book will lead mainly to ball valve. While in practice this is not entirely true.
Our engineering contractor (EC) also did similar things for our 12”-20” pipeline, end up with late delivery, metal seated excessive leak rate, and there was no clear guideline for maintenance. If I were you, I would challenge whether this is necessity. Butterfly valve or flanged trunnion mounted pipeline ball valve (not top entry) as bcd suggested would be better option wrt reliability.
Proper maintenance is better if valve can be blocked (by for instance by two gate valves before and after), removed from line for proper workshop repair, install spare valve for the next xyz years. Lifting ball only (for maintenance as your end user suggested) would require at least same effort or even bigger shall compared to lifting the whole valve.
Clarify acceptance criteria to the manufacturer prior your inquiry. PR2 requirement will spook most of them. Or is this just API6D or API598
MOGAS and Cameron have years of experience in metal seated ball valve. brochure below indicates that they can make 36”-900# ball valve.
Not really sure who supplied ball valves for South Stream transport.
PS: Unless there is a fundamental requirement of using biggest size top entry butt welded metal seated ball valve, I personally would avoid this.
Good luck
Regards