kashew Feb 11, 2011:

More ref. material: The blowdown rings found on most ASME type safety valves are used to make fine adjustments to the overpressure and blowdown values of the valves (see Figure 9.1.8). The lower blowdown (nozzle) ring is a common feature on many valves where the tighter overpressure and blowdown requirements require a more sophisticated designed solution. The upper blowdown ring is usually factory set and essentially takes out the manufacturing tolerances which affect the geometry of the huddling chamber.
The lower blowdown ring is also factory set to achieve the appropriate code performance requirements but under certain circumstances can be altered. a
"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

Andrew Hunt (asker) Feb 11, 2011:

Thks everyone your timely input