Hi,


If we talk about a saftey valve, from my understanding we talk about a valve with two rings, one "coneccted" to the disk and one "coneccted" to the nozzle.

The one with the disk is called a blowdown ring and its funtions are:

1.Create a greater reaction force and longer blowdown by lowering the ring.
2.Create less reaction force and longer blowdown by raising the ring.
3. With less reaction force to keep the valve open, it will close at a higher inlet pressure.

hope this will help

Bill

for a PSW blowdown is the difference between lifting and closing pressure.

For a normal spring actuated valve the valve will close at a lower pressure than it lift. Normally the difference is about 7% of the set-P although it may vary.

This is another protection against chattering. If you have a situation where PSV header dP is a problem (its slightly greater than 3%) you may be saved by adjusting blowdown (since pressuredrop in the header will have to be higher before the valve closes). This is actually one of the thing considered when API mentions that the 3% limit can be passed based on engineering judgement!

Best Regards

Morten

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   

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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