Even though the inner chamber has no pressure, since it is covered by Pressurized jacket, EXTERNAL PRESSURE acts and it is to be considered as a "PRESSURE VESSEL" and has to get U-stamp apart from forcing the manufacturer to increase the thickness of the shell.
I would agree if the entire vessel is covered by jacket but the original post is not clear on this. Again, design pressure of a vessel is taken at the top. If the top is 15 psig or less, I argue the vessel itself will not require a U-Stamp (it no longer falls within ASME Section VIII, Div 1 scope), just the jacket would. The metal under the jacket, or saying it another way, the common wall of the vessel and jacket, would have to be thicker to overcome the pressure of the jacket, but the entire vessel itself need not be stamped.
If I were to have a 40' tall, 12' diameter cylindrical tank designed for 2.5 psig but only 3' of the straight side (from the bottom) is jacketed (jacket design pressure say 25 psig), where do you see the justification for stamping the entire vessel?
If I were to have a 40' tall cylindrical tank designed for 14.9 psig (no stamp required) and it was full of cold 50% NaOH (s.g. about 1.5), then the pressure at the bottom would be almost 26 psig! This vessel still would not require a U-Stamp. However, the manufacturer must still design for the higher pressure at the bottom but the stated design pressure removes the vessel from ASME Section VIII, Div. 1 scope and thus the stamp.
pleckner,
The vessels with 26 psig at bottom is subject of an ASME VIII-1 definition of design pressure in Appeddix 3. The design pressure is not taken at the top, is taken as per the Appendix 3 of the code, at the bottom of the vessel or even lower, at the bottom of the outlet flange face. And 26 psi makes the whole vessel pressure vessel, including the U-stamp.That means the inside vessel from top to bottom is pressure vessel, including the jacket partially covering the internal vessel. The jacket requires U-stamp also because of pressure at or above 15 psig. Please disregard my first reply where I discussed the issue of "foreign" fabricators, qualified to fabricate ASME VIII design vessels, but they are not required to stamp their vessels. That is, the 75% of the world used the code, but cannot endorse the U-stamp beacuse cannot afford a US AIA, doesn't make sence to use US legislation in Korea and the Australian Chartered engineer is just as good as an US AI. Having said that, the U-stamp is mandatory for both the jacket and vessel for 15 psig and above, including any liquid static head.
My interpretation of "design pressure" in Appendix 3-2 is that it excludes the pressure due to the static head of liquid; and it does not mention whether it is referenced at the top or at the bottom of the vessel. On the otherhand, U-1(c)(2)(h) specifically mentions "design pressure at the top of the vessel" as one of the criteria in qualifying a vessel if it is within the scope of ASME VIII-1.
U-1(c)(2)(h) was revised in its entirety when the 2006 Addenda came out. If I remember it correctly, the exemption for Code applicability, prior to the 2006 Addenda, was based on 15 psi "operating" pressure (not design pressure) and there was no reference to where the pressure was located. With the revision on U-1(c)(2)(h), are the previous Code Interpretations still valid? Maybe new Code Interpretations are needed, especially on the cases presented by pleckner.
Thank you doct99660. The code most defintiely excludes the liquid static head in determining if this is a coded vessel or not.
I invite others to read further down the list of definitions in Appendix 3, Section 3-2 where the definition of MAWP is:
maximum allowable working pressure — the maximum gage pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature for that pressure.
And at the bottom of this paragraph, The design pressure may be used in all cases in which calculations are not made to determine the value of the maximum allowable working pressure.
So I go back to my original statement. And if one had to include the static head to determine conformaty with ASME Section VIII, Div. 1 then there are a lot of API style tanks out there that are in violation of the ASME code.
In my copy of the ASME VIII (quite used now), the full definition of MAWP reads a bit more than stated above:
" This pressure is the least of the values for the internal or external pressure to be determined by the rules of this Division for any of the pressure boundary parts, including the static head thereon, using nominal thicknesses exclusive
of allowances for corrosion and considering the effects of any combination of loadings listed in UG-22 which are likely to occur (see UG-98) at the designated coincident temperature [see UG-20(a)]."
Then in accordance with earlier interpretations, there must be another "Maximum allowable working pressure" for the bottom outlet flange, which will include the static head present in the vessel. Is this the intent of the Code? Personally, I'm skeptical of this interpretation.
U-1(c)(2)(h) has had a small change (100 kPa in lieu of 103 kPa), but in essence said the same: any component subjected to in excess of 15 psig (top or bottom), is subject of the ASME VIII regulations. This doesn't exclude, specifically or otherways, the static head. That is, it excludes the interpretation of two different MAWP and different design pressure for the same vessel. The calculation pressure is however, a different issue.
Like I said earlier, the characterization about the ASME Code being "strands of spagetti" seems valid.
If educated, knowledgable professionals can have legitimate diferences about fundamental concepts, there must be a problem with the rules.
jte: you asked me....
"Have you volunteered your time to help clarify the codes? Being on a code committee is not an insubstantial commitment."
Nope... and I haven't volunteered to repair all of the refineries in Iraq either. There comes a point where Codes, institutions and governments cannot be salvaged....
There comes a point where old spagetti must be thrown out and something new must be brought in. IMHO, the European Union is doing it right with the new comprehensive PED Directive
I left out the rest of the paragraph because it only brings in more detail but does nothing to change the MAWP reference location, which is my point. As a matter of fact, since we are trying to become more complete, I include the entire definition:
maximum allowable working pressure — the maximum
gage pressure permissible at the top of a completed vessel
in its normal operating position at the designated coincident temperature for that pressure. This pressure is the least of the values for the internal or external pressure to be determined by the rules of this Division for any of the pressure boundary parts, including the static head thereon, using nominal thicknesses exclusive of allowances for corrosion and considering the effects of any
combination of loadings listed in UG-22 which are likely
to occur (see UG-98) at the designated coincident temperature [see UG-20(a)]. It is the basis for the pressure setting of the pressure relieving devices protecting the vessel. The design pressure may be used in all cases in which calculations are not made to determine the value of the maximum allowable working pressure.
Please note the second to last sentence, "It is the basis for the pressure setting of the pressure relieving devices protecting the vessel."
Therefore, my PSV is set for the top pressure even though the pressure at the bottom of the vessel may be several pounds or more higher (static head or whatever; a jacket perhaps?)!!
There is no "second" MAWP. The manufacturer is required to account for these higher pressures in their calculations/design but the stamp MAWP is still based on the pressure at the top of the vessel in its operating position. Note in the definition, "This pressure is the least of the values...".
I haven't volunteered to go to Iraq to repair their refineries either. Though I could probably get the job and be on a plane in a week if I wanted to. I don't have any problem with that. Nor do I disagree that Div. 1 and Div. 2 have grown so much that the "strands of spaghetti" remark is reasonable.
But, I don't take potshots such as "old, well-paid men from entrenched, aging bureaucracies" at the folks who choose to help in the Iraqi refineries… or at the folks who volunteer their time and effort to serve on the committees, and I'll continue to disagree with those who do.
I haven't seen the PED but agree with your point about throwing out the old spaghetti eventually and starting a new dinner (lasagna?). As I understand it, this is exactly what the ASME chose to do with Div. 2 with the Div. 2 Rewrite effort. The resulting new Div. 2 will allegedly be more user friendly. From the few 30,000 ft level overviews I've seen, that seems entirely possible. Hopefully it is a success and they don't stop with Div. 2 but continue and start a Div. 1 rewrite effort. Keep in mind that its probably a 10 year job…
pleckner,
Please note the conflict of your posts dated 3rd and 11th;- the design pressure definition slipped in the MAWP definition "debate". Nobody is disputing that the lowest calculated MAWP for any of the vessel components could be the design pressure which includes the static head. And nobody is disputing the correctness of the MAWP definition in the Appx 3. The issue was the design pressure taken at the top of the vessel, which is incorrect.
And once more, the Code is not a Bible, but a guidance filtrated though the best of the latest engineering advances and the best of our engineering abilities and experience. Try to believe that you can be a better engineer than the PE talking down to you. That will help to interpret the Code and find the cracks (just count the number of interpretations to realise how many confused designers and fabricators built perhaps the wrong vessel because they followed blindly the Code).
Thank you guys for the debate... I love spagetti..
Cheers,gr2vessels
gr2vessels: I don't see any conflicts in what I posted, please elaborate. Not that I wanted to get into a debate on this particular issue but...per ASME, the design pressure may definitely be substituted for the MAWP if the MAWP is not calculated. And since the MAWP is referenced to the top of the vessel then so is the design pressure. Thus I have to take exception to your statement "The issue was the design pressure taken at the top of the vessel, which is incorrect." The design pressure is most definitely taken at the top of the vessel and I don't see how this can be disputed.
I would love to believe that the Code is not the bible but this is not necessarily true. There are only two states in the U.S. that have not made ASME law and one, Texas, is having a lot of problems because of this. The rest of this country adopted ASME as law and this makes it the bible. And going back to what we've been saying, this "law", "bible", "Code", "Guide" is broken and needs fixing.
