Atmospheric Tanks Design Pressure (45/FV?) 1

[andy_oohhh]

We have a tank that was specified with a vessel design pressure of "45/FV" in units of psig. I was told that 45 psig was typical for atmospheric tanks. Is there a standard that states this? Why use 45 and not atmospheric pressure?

 

[Latexman]

I have 43 years experience, and I've never seen/heard that for an "atmospheric tank".

Check the spec, fabrication drawing, and nameplate. If it is 45 psig it should have a Manufacturer's Data Report (U-1 Form) registered with the National Board.

Maybe 45" w.c., but in that case definately not FV.

Good Luck,
Latexman

 

[LittleInch]

"I was told...."

By who exactly?
Ask them surely and wait for the blank look.

Total rubbish.

That is a PV design, not a tank spec.

Now there's nothing to stop you using a PV as a tank, but that doesn't make this typical.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[andy_oohhh]

I found it odd and don't disagree. The only thing I can think of that might be driving the pressure is the "full vacuum". Is there an equivalent design pressure that you use when you need a FV rated tank? I don't know if this makes any sense but just trying to reason all possibilities.

 

[Latexman]

For pressure vessels, ME’s have told me in the past that FV results in a MAWP of ~ 50 psig, size and shape matters. Kinda what you said, but not with “atmospheric tanks”.

Good Luck,
Latexman

 

[LittleInch]

How big is this mysterious "tank"?

FV is a lot harder than it looks.

I can only imagine this is some sort of horizontal cylindrical dished end vessel?

But yes you would need 4 to 5 bar to get enough strength for FV.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Try not to conflate tanks and pressure vessels, they are designed to different codes. The max design pressure you'd see on a tank is 15psig, which is the upper limit of application in API620 used for high pressure tanks.

 

[MJCronin]

I agree with all of the wise comments above .... are you trying to re-use an existing tank (a common MBA subterfuge) and have you been assigned to "work out the details"? ....

How big is the tank and what are the other details ?.... More information = Better answers !!

.... Beware of the tribal knowledge that you hear in the men's room ...!!!

FV construction is expensive for all tanks larger than 36" diameter !!!

.....and I would like to point out that if the real process needs were TRULY FOR FV that the component should no longer be called an "atmospheric tank", but should be designed and called a "pressure vessel"... others may disageree

This also means that the flat bottomed/cone or domed roof is probably no longer acceptable (unless the tank is tiny)... it also means that a "MINIMUM DESIGN PRESSURE OF 15 psig or GREATER is advisable.

This tank design pressure topic has been discussed ad nauseam on eng-tips ... perform a few searches and read ....

Tell us more .... Do not "skip and run" like most of the Third World ....

MJCronin
Sr. Process Engineer

 

[andy_oohhh]

Thank you for all the responses above. I asked around and was also told this "Rough rule I heard on a project was FV (external pressure) would get you about 50-60 psig internal pressure. This will also have to do with vessel size so it's just an approximation." How would you calculate the equivalent internal pressure? The tank is 2500 gallon vertical cylinder tank.

 

[TiCl4]

Andy,

That is a VERY rough estimate that usually applies in the opposite direction. E.g. a 50 psig pressure vessel is the minimum pressure range at which you start to see FV naturally occur due to metal thickness. As diameter increases, so does the minimum MAWP required to achieve FV rating. Usually a vessel is designed for a particular internal pressure, and metal thickness or stiffener rings may be added if FV is required. I haven't seen many tanks starting from the opposite end.

If you are wanting a 2,500 gallon storage tank that is FV-rated and not a pressure vessel, you can do that. You'll just need to ask for stiffener rings on the vessel. This will be cheaper than buying a pressure vessel and using it as a tank that only needs FV rating.

Start with the required MAWP of your tank, and add rings or metal thickness as required to achieve FV. You don't start with FV assumption and try to back into a MAWP...

 

[MJCronin]

TiCl4 .... Do you think that we have here a prime example of the hazards of "somebody once told me" engineering ?

Thank you for your "spot on" comments ...

MJCronin
Sr. Process Engineer

 

[LittleInch]

"How would you calculate the equivalent internal pressure?"

In a pressure vessel design you input the size, wall thickness, end shape, materials strength etc etc and if generates a MAWP.

You've stated volume, but not size or ends. So x m diameter by y m long with dished? hemispherical? toroidal? ends?

But like everyone is telling you, what you have is a Pressure Vessel, not a tank, although you may be using it as an atmospheric tank, that's not how it was designed and built.

Why do you want to know?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

Are you worried about external cooling/rapid vapor condensation with tank collapse? Like, a cold rain after a steam cleaning.

Good Luck,
Latexman

 

[0707]

API STD 650 (13th) addresses vacuum design or design for external pressure in Annex V maybe it helps.

Good luck

luis