One can reduce the gap by either lowering the upper piece or raising the lower piece (blowdown adjustment ring). The reduced gap causes a slightly higher pressure in the huddling chamber, thus increasing the lifting force (which necessarily impedes the re-closing of the valve). When the gap at the outlet of the huddling chamber is reduced, the blowdown increases. Thus, this pressure creates additional lifting force, helping the valve get to a higher lift position. The pressure on the upstream side of this gap (inside the huddling chamber) is acting on all surfaces, but notice that there's a lot more surface area on the upper side than the lower side. At the exit of the huddling chamber the gas is pinched as it passes through the gap created by the blowdown adjustment ring (also referred to as the "nozzle ring"). As the gas flows through the primary orifice it then flows through the huddling chamber. It's easier to visualize this phenomenon if we talk about a gas (compressible fluid) application. The huddling chamber accelerates the PSV opening by creating a "secondary orifice". A consequence or trade-off is that it also makes the valve harder to re-close (blowdown). The huddling chamber puts the "pop" into "pop valves". The whole purpose of the huddling chamber is accelerate the opening of the valve.to get the valve into a high left position more quickly. Think about the physics of what the huddling chamber does, and how it does it. This may where someone has confused the two types of PRVs. In a One Ring Design, ASME Sec VIII, API 526, Safety-Relief Valve (Consolidated 1900 Series or Crosby JOS-E or Farris 2600), Lowering the Nozzle Ring (There is no Guide Ring) will decrease Blowdown. I also attached excerpts from Crosby H Series & Dresser 1540 Series (2 Ring Design, ASME Sec I, Safety Valves). I used the chart in National Board VR Training Classes for 25 years from 1990 until 2015. This chart has been adapted by two major PRV manufacturers. I have attached a Chart that I use in Pressure Relief Training worldwide. Lowering the Guide Ring increases the size of the Huddling Chamber which keeps the Steam underneath the Disc longer, thereby increasing the Blowdown. I can tell you from experience, unequivocally, lowering the upper ring does NOT decrease the Blowdown. I have been Testing ASME Section I Safety Valves for 46 years.
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