PRV ASME I Valves 2

[gguliye]

Dear All,
May be my question is so basic, however, i am doing my hair off, because i couldnt come up to the result why

in ASME section I valves, we have 2 rings. Nozzle ring and guide ring. Why lowering guide ring decreases blowdown of the valve?
In my opinion it has to increase the blowdown. Could someone help me with this question?
Regards,

 

[JAlton]

I have been Testing ASME Section I Safety Valves for 46 years. Consolidated (1811 & 1700 series), Kunkle (300 & 6000) series, Crosby (HS,HSA, HC, HE etc.). All are two ring design Safety Valves. I can tell you from experience, unequivocally, lowering the upper ring does NOT decrease the Blowdown. Lowering the Guide Ring increases the size of the Huddling Chamber which keeps the Steam underneath the Disc longer, thereby increasing the Blowdown.

I have attached a Chart that I use in Pressure Relief Training worldwide. This chart has been adapted by two major PRV manufacturers. I used the chart in National Board VR Training Classes for 25 years from 1990 until 2015. I also attached excerpts from Crosby H Series & Dresser 1540 Series (2 Ring Design, ASME Sec I, Safety Valves). I hope this is helpful.

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. This may where someone has confused the two types of PRVs.

I hope this is helpful.

JAC

 

[don1980]

Think about the physics of what the huddling chamber does, and how it does it. Then it will all make perfect sense. 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. The huddling chamber puts the "pop" into "pop valves". A consequence or trade-off is that it also makes the valve harder to re-close (blowdown).

The huddling chamber accelerates the PSV opening by creating a "secondary orifice". It's easier to visualize this phenomenon if we talk about a gas (compressible fluid) application. As the gas flows through the primary orifice it then flows through the huddling chamber. 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"). 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. Thus, this pressure creates additional lifting force, helping the valve get to a higher lift position.

When the gap at the outlet of the huddling chamber is reduced, the blowdown increases. 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). One can reduce the gap by either lowering the upper piece or raising the lower piece (blowdown adjustment ring).

If the gap at the outlet of the huddling chamber is increased, then the opposite occurs (and blowdown decreases).

 

[gguliye]

don1980
I also think in the way that u think and Jalton explained everything as I imagined. However, the thing is that the book says the different thing. The book says that lowering the guide decreases blowdown, which is wrong right?

 

[JAlton]

To which "Book" are you referring? Is it a PRV Manufacturer's Instruction?

JAC

 

[gguliye]

Jalton,
Emerson PRV handbook yes

 

[JAlton]

Excerpt from the ENGINEERING HANDBOOK, page 4.7, Next to Figure 4-8, "The guide ring position determines the blowdown setting. The lower the guide ring position, the higher the opening force delivered by the flowing steam. This causes an increase in the blowdown. In order to decrease the blowdown, the guide ring is raised."

As I said, Raising the Guide Ring will shorten the Blowdown. Do you see what I mean?

JAC

 

[gguliye]

Jalton,
yes, i see thanks a lot