Here is how we do it at the factory.
1. We test the valve for set point only, using air.
2 We disassemble the valve and ship it to the customer to be welded inline. The valve can not be assembled during the welding process.
3 The valve is then reassembled after hydro.
4 The valve is tested in...
ASME Sec. I PG-73.4 is a requirement for Manufactures and Assemblers only. It spells out the conditions that allow the Manufacturer to attach the Code "V" stamp. Local Government or Insurer will often dictate the type of testing they will allow. Always check with them before you fully decide...
The simple version is the lower ring gets the disc off the seat and the upper ring keeps it off the seat.The further apart the rings the shorter the blowdown and the closer the rings are together, the longer the blowdown
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In a relief valve it allows the blow down to be controlled. The ASME code does not address blowdown for a liquid valve but due to customer's demands a "liquid trim" was developed. Basically the nozzle ring has a different angle and the huddle champer will be different from a vapor...
Try the ERV valve from Valvtechnologies. Although it is designed for steam, it could probably be used for liquid. The ball valve design should help you for the amount of flow you need and the electronic controller will handle the different set points. Good luck.