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Sect. VIII Div. I Over 3000 PSI 1
- Thread starter weeeds
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widla-
As metengr pointed out, you need to meet VIII-1 U-1(d). That paragraph, though, is not very specific so you'll have to apply engineering judgement and experience. I'd suggest that your approach can take two directions depending on what type of firm you're working on:
If you're with an owner/user or engineering firm, I'd suggest that you investigate using Div. 2 or (much less likely) Div. 3. Focusing on Div. 2, find a U2 stamp shop and get quotes for the same vessel built to both Div. 1 and Div. 2. As an owner/user, keep in mind that future repairs and alterations will be more cumbersome with a Div. 2 vessel compared to a Div. 1 vessel.
If you're with a fab shop which doesn't hold a U2 stamp, I'd suggest designing per Div. 1. Then go back and make sure the design details also work for Div. 2 (but using Div. 1 allowable stresses). Go over the proposed design with your AI and you'll probably get a positive response.
jt
As metengr pointed out, you need to meet VIII-1 U-1(d). That paragraph, though, is not very specific so you'll have to apply engineering judgement and experience. I'd suggest that your approach can take two directions depending on what type of firm you're working on:
If you're with an owner/user or engineering firm, I'd suggest that you investigate using Div. 2 or (much less likely) Div. 3. Focusing on Div. 2, find a U2 stamp shop and get quotes for the same vessel built to both Div. 1 and Div. 2. As an owner/user, keep in mind that future repairs and alterations will be more cumbersome with a Div. 2 vessel compared to a Div. 1 vessel.
If you're with a fab shop which doesn't hold a U2 stamp, I'd suggest designing per Div. 1. Then go back and make sure the design details also work for Div. 2 (but using Div. 1 allowable stresses). Go over the proposed design with your AI and you'll probably get a positive response.
jt
I'll echo metengr's and jte's post in getting an experienced PE in pressure vessel design to design or consult. One thing I would like to add is that in choosing such a PE or other that they have experience in design of high pressure equipment otherwise you could incur a lot more expense than necessary. This is especially true with using the higher alloy materials.
- Thread starter
- #6
Having had a chance to think this problem through a little more, I would like to re-phrase the question:
A vessel has a design pressure of 3000 PSI. Based on materials used, the MAWP is calculated to be over 3000 PSI using on the rules of Sect. VIII Div. I. Can the vessel be stamped with a 'U' Stamp and also show the higher MAWP on the nameplate?
A vessel has a design pressure of 3000 PSI. Based on materials used, the MAWP is calculated to be over 3000 PSI using on the rules of Sect. VIII Div. I. Can the vessel be stamped with a 'U' Stamp and also show the higher MAWP on the nameplate?
Your re-phrased question is even more confusing. The design pressure and MAWP are normally considered the same in code space.
Are you looking to re-rate an existing pressure vessel??? If this is the case, you cannot use ASME Section VIII, Div 1. The National Board Inspection Code is the appropriate code.
Are you looking to re-rate an existing pressure vessel??? If this is the case, you cannot use ASME Section VIII, Div 1. The National Board Inspection Code is the appropriate code.
metengr-
Unfortunately, I know exactly what widla is talking about: Most of the vessels I wind up dealing with were built using the footnote to UG-99(b) which states The MAWP may be assumed to be the same as the design pressure when calc's are not made to determine the MAWP. In other words, the vessels are most often not optimized. The process design conditions are taken as mechanical design conditions, tmin is determined and the next thicker plate size is selected. No attempt is made to go back and recalc a "true" MAWP based on the selected plate thickness. Actually, I usually prefer to see the extra thickness counted as corrosion allowance.
widla-
I appreciate that you are trying to optimize the vessel. If you choose to calc a "true" MAWP instead of using the UG-99(b) footnote and that pushes you over 3000 psi then yes, you need to meet the "over 3000 psi" requirements. With a vessel under this high pressure you probably won't find much excess pressure: If you're selecting a plate thickness based on rounding up to the next 1/16" or 1mm then how much (as a percentage) more pressure can you squeeze out of that rounding up? On the other hand, you may be able to increase the corrosion allowance more significantly: If your CA is specified as 0.125" and you round up 0.060" to the next available plate thickness then you can add roughly 48% more CA. You'll still have to re-work the calc's since by increasing the CA you are also increasing the ID of the vessel for calculation purposes. The third way to optimize would be to increase the design temperature. I don't see that desire all that often, though for a reactor with an exothermic reaction this can give the operator a lot more time to try to bring a runaway reaction back under control before having to shut down the process altogether.
So in your situaiton, and without having the benefit of discussing the situaiton with the operator's process engineer, I'd consider optimizing for pressure a last choice. Just avoid the problems which you'd be bringing upon yourself. My usual preference would be to add CA which can have a big impact on end-of-life considerations (when you figure 0.005 to 0.010"/year corrosion rate is not unusual). I usually optimize for temperature last, but in your case I'd rank it the second option.
By the way, why not take some time and click on the "personal profile" link and fill in a brief description of yourself. It helps us to know a bit of your background when responding. You can see mine by clicking on the jte at the top of this post.
jt
Unfortunately, I know exactly what widla is talking about: Most of the vessels I wind up dealing with were built using the footnote to UG-99(b) which states The MAWP may be assumed to be the same as the design pressure when calc's are not made to determine the MAWP. In other words, the vessels are most often not optimized. The process design conditions are taken as mechanical design conditions, tmin is determined and the next thicker plate size is selected. No attempt is made to go back and recalc a "true" MAWP based on the selected plate thickness. Actually, I usually prefer to see the extra thickness counted as corrosion allowance.
widla-
I appreciate that you are trying to optimize the vessel. If you choose to calc a "true" MAWP instead of using the UG-99(b) footnote and that pushes you over 3000 psi then yes, you need to meet the "over 3000 psi" requirements. With a vessel under this high pressure you probably won't find much excess pressure: If you're selecting a plate thickness based on rounding up to the next 1/16" or 1mm then how much (as a percentage) more pressure can you squeeze out of that rounding up? On the other hand, you may be able to increase the corrosion allowance more significantly: If your CA is specified as 0.125" and you round up 0.060" to the next available plate thickness then you can add roughly 48% more CA. You'll still have to re-work the calc's since by increasing the CA you are also increasing the ID of the vessel for calculation purposes. The third way to optimize would be to increase the design temperature. I don't see that desire all that often, though for a reactor with an exothermic reaction this can give the operator a lot more time to try to bring a runaway reaction back under control before having to shut down the process altogether.
So in your situaiton, and without having the benefit of discussing the situaiton with the operator's process engineer, I'd consider optimizing for pressure a last choice. Just avoid the problems which you'd be bringing upon yourself. My usual preference would be to add CA which can have a big impact on end-of-life considerations (when you figure 0.005 to 0.010"/year corrosion rate is not unusual). I usually optimize for temperature last, but in your case I'd rank it the second option.
By the way, why not take some time and click on the "personal profile" link and fill in a brief description of yourself. It helps us to know a bit of your background when responding. You can see mine by clicking on the jte at the top of this post.
jt
- Thread starter
- #10
Thank you for the input guys but I am trying to understand the magic limit of 3000 PSI.
I am looking at a vessel made of pipe and forged flanges where the pipe is, say, 8" Sch. 160, and the flanges are 1500#. This combination, using the rules of Sect. VIII Div. I, will give us a MAWP of well over 3000 PSI for our design temperature. I can't go to B31.1/3 since the specification calls for a Code Stamp.
Although I do not know much about Div. 2, this part of the Code is named Alternative Rules, implying to me that there may be an aspect of discretion on my part. Furthermore, I have been under the impression that Div. 2 was created to minimize the thicknesses of construction by increasing the NDE/QA. If this is true then can I choose to not take advantage of these Alternate Rules and still work to Div I?
There can not be anything magical about the 3000 PSI limit. Is a vessel designed for 3001 PSI that different then one at 3000 PSI?
I feel that I am missing something.
Any additional comments?
Thanks
I am looking at a vessel made of pipe and forged flanges where the pipe is, say, 8" Sch. 160, and the flanges are 1500#. This combination, using the rules of Sect. VIII Div. I, will give us a MAWP of well over 3000 PSI for our design temperature. I can't go to B31.1/3 since the specification calls for a Code Stamp.
Although I do not know much about Div. 2, this part of the Code is named Alternative Rules, implying to me that there may be an aspect of discretion on my part. Furthermore, I have been under the impression that Div. 2 was created to minimize the thicknesses of construction by increasing the NDE/QA. If this is true then can I choose to not take advantage of these Alternate Rules and still work to Div I?
There can not be anything magical about the 3000 PSI limit. Is a vessel designed for 3001 PSI that different then one at 3000 PSI?
I feel that I am missing something.
Any additional comments?
Thanks
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1
- #11
What's particularly magical about the 3000psi limit has to do with the theory around which the Div. 1 (and Div. 2) calculations are based. That theory is thin-shell theory. It assumes a D/t ratio of at least 10 (although even that is really pushing it). Thin-shell theory assumes that in a cylindrical shell, far from any discontinuities, the distribution of stress through the thickness is uniform - resulting in a constant "membrane" stress (or stress intensity in Div. 2).
If you look back in your upper-year solid mechanics textbooks, you would find Lame equations that give a quadratic equation for the distribution of stress in a cylinder subject to internal (or external) pressure loading. Beyond the D/t limit of 10, you will find that the Lame equation is essentially (within a reasonable engineering limit) equivalent to the thin-shell theory formulae.
So, to answer your last question - is a 3001psi vessel different from a 3000psi vessel? - no. However, the Code writers had to pick a limit and that limit is 3000psi. Be forewarned, however, that even vessels with design pressures (or MAWP) in the vicinity of 3000psi _may_ violate (or at least come uncomfortably close to violating) the above-noted D/t limit.
One additional bit of information - the consequences of ignoring the D/t limit is that thin-shell theory becomes UNCONSERVATIVE at low D/t values. Consider this a very seriour warning.
As metengr stated earlier - an experienced vessel engineer will know this information and can help to guide you.
If you look back in your upper-year solid mechanics textbooks, you would find Lame equations that give a quadratic equation for the distribution of stress in a cylinder subject to internal (or external) pressure loading. Beyond the D/t limit of 10, you will find that the Lame equation is essentially (within a reasonable engineering limit) equivalent to the thin-shell theory formulae.
So, to answer your last question - is a 3001psi vessel different from a 3000psi vessel? - no. However, the Code writers had to pick a limit and that limit is 3000psi. Be forewarned, however, that even vessels with design pressures (or MAWP) in the vicinity of 3000psi _may_ violate (or at least come uncomfortably close to violating) the above-noted D/t limit.
One additional bit of information - the consequences of ignoring the D/t limit is that thin-shell theory becomes UNCONSERVATIVE at low D/t values. Consider this a very seriour warning.
As metengr stated earlier - an experienced vessel engineer will know this information and can help to guide you.
When all is said and done, the answer is "yes, Division 1 may be used at pressures above 3000". I see such vessels all the time.
But depending upon the pressure and the size and complexity of the vessel, you may find that Division 2 design will provide a cheaper vessel. Or you might even go to the extreme of using Division 3 (intended for pressures above 10,000 psi, if I remember correctly).
So it all comes down to cost.
But depending upon the pressure and the size and complexity of the vessel, you may find that Division 2 design will provide a cheaper vessel. Or you might even go to the extreme of using Division 3 (intended for pressures above 10,000 psi, if I remember correctly).
So it all comes down to cost.
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