RELIEF LOAD DEPRESSURIZATION VALVE VS RELIEF VALVE

[wenchop]

HELLO,

IN GENERAL FOR A PRESSURE VESSEL, HOW WOULD YOU CONSIDER THE RELIEF LOAD DEPRESSURIZATION VALVE DESIGN PEAK LOAD VS RELIEF VALVE DESIGN LOAD TO COMPARE TO EACH OTHER? EQUAL? ONE BIGGER THAN THE OTHER? THERE IS NO GENERAL RULE?

THIS IS CASE OF FIRE FOR BOTH (DEPRESSURIZATION DUE TO FIRE EMERGENCY, AND PSV SIZED FOR FIRE CASE). SUPPOSE THE VESSEL IS EQUIPPED WITH BOTH DEPRESSURIZATION VALVE AND PSV.
I WOULD EXPECT THE LOADS TO BE NOT VERY DIFFERENT FROM EACH OTHER.
THOUGHTS?
THANKS,
DTMGO1

 

[LittleInch]

There is no such thing as general for this. The two items address different things. Blowdown is dependent on volume between ESD valves and works on a time limit to reach a certain pressure. PSV includes fire, blocked exit and hence can be a completly different size to a Blowdown valve, or similar or smaller depending on your particular circumstances, fluids and design codes and local regulations.

If there is anything like general I would expect a PSV to have a higher flow, but wholly dependent on Blowdown volume.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Latexman]

I would expect the same heat input from the fire scenario for both cases. The blowdown valve will have the higher flow than the PSV. The blowdown valve should reduce pressure from some pressure below the MAWP to about 1/2 that in a short time (15 minutes?). A PSV has more pressure to work with, 1.21 x MAWP, and only has to maintain pressure below the sizing pressure. All while the fire is still burning. At least, thats the way it looks to me.

Good luck,
Latexman

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[wenchop]

THE PSV MAY BE CONTROLLED BY ANOTHER CASE OTHER THAN FIRE, BUT THIS QUESTION IS ABOUT COMPARING DESIGN LOAD OF PSV VS PEAK LOAD OF DEPRESSURIZATION (VAPOUR MASS FLOW), BOTH FOR FIRE CASE.

IT SEEMS TO ME THAT THE PRESSURE HAS LOW IMPACT ON THE RATE OF VAPORIZATION DUE TO FIRE HEAT FLOW. THE VOLUME OF THE VESSEL IS FIXED AND RELATIVELY SMALL COMPARED TO THE VOLUME OF VAPOURS GENERATED, SO ANY PHASE CHANGE WILL VERY QUICKLY RAISE THE PRESSURE. ANY RELIEF OF VAPOUR WILL VERY QUICKLY LOWER THE PRESSURE UNLESS NEW VAPOUR IS TAKING ITS PLACE.

FURTHERMORE, MASS HEAT OF VAPORIZATION IS NOT A STRONG FUNCTION OF PRESSURE. MOST THINK ERRONEOSLY THAT LIGHTER HIDROCARBONS HAVE SMALLER MASS HEAT OF VAPORIZATION THAN HEAVIER ONES, BUT THE CASE IS THE OPPOSITE.

GIVEN THAT PRESSURE SET POINTS FOR RELIEF HAVE NOT GOT A BIG IMPACT, I EXPECT BOTH LOADS TO BE SIMILAR AS PER ORIGINAL POST.

 

[Latexman]

Please do not YELL at us. Thank you.

Good luck,
Latexman

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[wenchop]

Obviously I did not mean to yell. Why on earth would I yell?

 

[TD2K]

You were using all caps, they are viewed as yelling on the Internet. They are also harder to read, at least in many people's opinion.

http://www.networketiquette.net/core_rules_do_not_use_all_caps.html

Just one link, a quick google will come up with more pages than you likely want to read.

 

[wenchop]

Well, I disagree with that view that caps means yelling, because it enables some people to yell using caps. If nobody held that view, it would be impossible to yell on the internet.
cheers,

 

[25362]

Right. Thre are other cases apart from a fire. For example, hot spots in hydrotreating reactors that may induce a thermal run away.