Considering the large diameters of the valves, and hence probably a limited number of producers and competitors, the most important qualification factor would probably bee that the producer can show to long-term operation references for the proposed valve, as mentioned in my first post.
At a guess your pressure requirements are low (?????). Even then the pressure for reference valves should at least be the same, and reference valves diameter not too far off.
Even with low pressures one fear would be small seat lekages over time, increasing with grit (and abrasion) as well as possible cavitation if wrong use (strong throtteling) of the valves.
This is one of several reasons that use (and functional requirements) of the valves must be described/studied in detail.
Soft sealings and/or centric construction is not off as an alternative if long-term references can be given. For this diameter, however, you will probably find competitive, more advanced constructions than centric with or without soft seat sealing.
Soft or metallic sealings is an issue in itself.
PU (polyurethane) soft sealings have been used with success for water turbine (hydroelectric plant) inlet valves , with 'sweet' water and larger amounts of abrasive particles, but I am not saying that this is the best for your case.
Even at low pressure for such large valves: do not underestimate solidity of stem and strength of stem fastening to disc. Necessary torque could increase drastically oveer time!
To qualify suppliers at a high level producer should be asked to present (mechanical) strength calculations for the valve construction, and necessary operational forces at normal water speed and for 'emergency/worst case' operation.
Thank you to all for interesting comments and stars!
