bill_bill_2000 probably meant to say "shorter" blowdown from raising the ring in item 2 of his repsonse.
Unless you are dealing with a Power Boiler, chances are you have a one ring design safety-relief valve. bill_bill_2000's commnets were germain to a two ring design Power Boiler Safety Valve. On compressible fluids, the adjusting ring in a one ring design Safety-Relief Valve, must perform three functions. First it needs to be close enough to the disc to direct flow against the disc & disc holder in order to create a reaction force which combines with the inlet force to overcome spring force and cause the valve to pop open. Second, it must be far enough from the disc to allow fluid an escape path so that the valve does not remain open below normal system operating pressure, typically 10% below set pressure. Long blowdown can cause other problems in the system. Thirdly, the ring provides a cushioning effect as the disc reseats. Acting like a retro-rocket, the reaction forces keep the disc from slamming into the nozzle and damaging the seating surfaces.
Blowdown Ring is a specific Part Nomenclature for Farris because it controls reseat. It is referred to as a Nozzle Ring by Crosby for its location (threaded onto the o.d. of the nozzle. Dresser calls it an Adjusting Ring and Knukle refers to it as a Warn Ring. Warn is an older term for simmer. The PRV simmers just prior to pop, therefore simmer is also called warn. SO, 4 major manufacturers with 4 different trems for the same part. The blowdown ring is an important part of PRV Maintenance and Testing. It is what sets PRV repair apart from other mechanical disciplines. DIsassmbly, inspection, machining, lapping and reassembly are common to all valves, pumps, etc. But Blowdown Ring Adjustment is unique to Pressure Relief Valves. Many PRV manufactuerers give ring setting positions in their maintenance manuals. Liquid Settings are typically set very close to the disc during reassembly and require no further adjustment during testing. However, for air/gas/vapor service, the Manufaturers Settings will usually yield long blowdown, because short blowdown may result in insufficient capacity and in a two ring design in chatter. You do not typically see ring setting induced chatter in a one ring design. Most of the time, chatter is a result of an installation problem, i.e. reduced inlet piping, extremely long inlet piping, excessive pressure drop at the PRV inlet. I hope this is helpful