Smart questions
Smart answers
Smart people
Join Eng-Tips Forums
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

Plasmech (Mechanical) (OP)
31 Aug 07 6:58
Hello,

I am designing a 42" diameter pressure vessel that will be used for vacuum service.  Internal pressure will be -13.75 psi.  Spec'ing a tank head for internal positive pressure is pretty straightforward, but what about external pressure?  Where can I find a good write-up about this?  Currently I don't have access to ASME code, I imagine I'll be directed to section 8 division 1.  Maybe there's something online?  Thanks!
JoeTank (Structural)
31 Aug 07 8:16
For ASME VIII, the static head is not included in the MAWP.  MAWP is defined as the pressure at the top of the vessel in it's operationg position.  The vessel designer must add static head of the liquid to the MAWP in order to obtain the design pressure for each part of the vessel.

Joe Tank

jehan17718 (Mechanical)
31 Aug 07 9:27
Plasmech,

A vessel operating below a pressure 15 psi does NOT come under the purview of the ASME VIII-1 Code. Your pressure vessel is most likely to come under the purview of API 620/650. PLease specify the exact design pressures and temperatures [both internal and external].

-jehan
MJCronin (Mechanical)
31 Aug 07 11:52
I believe that jehan is incorrect..

ASME code covers vessels operating above 15 psig and at "significant" vacuum conditions

API-650 covers "atmospheric storage tanks" up to +2.5 psig and at slight vacuum conditions (that they refuse to define)

API-620 covers storage tanks" above +2.5 psig  to ~ 14.5 psig and at sliht vacuum conditions (that they refuse to define). API-620 also covers higher temperatures

In my opinion, the rules of ASME-VII apply to this "-13.75 psi" case. Banu should define minimum MAWP in psia

Now, whether or not the vessel can be designed, fabricated, inspected and certified is another matter..

Where will this component be installed ?....Iran ?

-MJC

  

Plasmech (Mechanical) (OP)
31 Aug 07 12:36
The vessel will have a pelletized product inside, and air.  Absolute pressure will be 50mm Hg, and of course 1.0 atmosphere outside.  Max vessel temp is 350F.  External temp is ambient, figure 50 degrees F minimum.  This will be a product sold to many countries but not Iran (?).  Thanks for the help!
SnTMan (Mechanical)
31 Aug 07 13:20
Plasmech, your imagination is correct. There are design rules in Sec VIII, Div 1 for external pressure (vacuum) design, which you may use even if not a Code vessel. You will not likely find the Code on line for free, but your local library may have a copy.

Regards,

Mike
jehan17718 (Mechanical)
31 Aug 07 14:15
MJCronin,

Sorry, but my views differ from your above reply.

Quote:

ASME Section VIII Div 1 clause U-1(c)(2) Based on the Committee’s consideration,
the following classes of vessels are not included in
the scope of this Division...........
(h) vessels having an internal or external operating
pressure (see 3-2) not exceeding 15 psi (103 kPa)
with no limitation on size [see UG-28(f)];....

Based on the above clause, my interpretation is that the  pressure vessel decribed in Plasmech's 1st post cannot be qualified as as a Div 1 vessel.

Regarding API-650, these tanks are designed either to operate under atmospheric pressure only (vented to atmosphere) or under internal pressures not exceeding weight of roof plates. Higher pressures upto 18 kPA are allowed provided tank conforms to design as per Appendix F of API-650.

Be advised; API standards, have upper temperature limitations.

For API-620, these tanks are designed for a maximum positive pressure of 15 psig and a vacuum (not able to quantify this value right now; been a hard workday today winky smile ).

Irrespective of my above opinion, now that Plasmech has mentioned that the contents are "pelletized" changes the nature of his theread all together.

In my opinion, since this is a bulk product (granular/ powder) dealt with, API standards 620/650 and ASME codes VIII-1 will not suffice. If Plasmech intends to design storage vessel for "pelletized" contents then it is strongly recommended that a standard like DIN 1055 Part 6 pertainnng to design of silos and bins be employed in the desingn.

BULK SOLIDS (POWDERS/ GRANULES, ETC) EXERT LOADS WHICH ARE DIFFERENT IN NATURE (AND HIGHER IN MAGNITUDE) THAN THOSE EXERTED BY A EQUIVALENT LIQUID STATIC HEAD.

Hope this helps. Please give in your views.

regards,

jehan
Plasmech (Mechanical) (OP)
31 Aug 07 19:02
Thanks for the info.  I'm going to try to find free copies of these codes somehwere...wish me luck.

Aother thing I need is a detailed drawing of an ASME semi-ellipsoidal tank head, .211 min.  Is this online somewhere?  Also material selection, what is the strongest readily available commonly used carbon steel material?  I don't deal in this stuff so it's all new to me, bear with me!  Thanks again.
MJCronin (Mechanical)
1 Sep 07 15:40
jehan,

You are splitting hairs.....

"-13.75 psig" is nearly the same as "15 psig external presure.

Use the rules of ASME VIII, they are much closer to apply than the rules of API-650 or 620.

I would design the component for FV and be done with it.

my opinion only

-MJC

  

jte (Mechanical)
1 Sep 07 18:29
jehan-

I agree with you that a vessel for vacuum service only, not exceeding 15 psi external, is out of scope of VIII-1, as you point out in your quote. However...

Quote (jehan):

Based on the above clause, my interpretation is that the  pressure vessel decribed in Plasmech's 1st post cannot be qualified as as a Div 1 vessel.

...you chose to selectively quote the code, and I don't understand why. The full quote is below:
U-1(c)(2) Based on the Committee’s consideration, the following classes of vessels are not included in the scope of this Division; however, any pressure vessel which meets all the applicable requirements of this Division may be stamped with the Code U Symbol:

I added the bold. So, please provide further explaination as to why you feel that this container cannot receive a VIII-1 U stamp.

For lack of other codes or standards available to design to 13 psi external, I'd go with MJCronin's suggestion, design it to full vacuum and some nominal (say 5 psi) internal MAWP and be done with it.

Now, whether or not to actually stamp the vessel is more dependent on local jurisdictional issues - there may be an advantage to not stamping the container from a repair / maintenance perspective.
jehan17718 (Mechanical)
2 Sep 07 1:38
jte,

I concur with your logic regarding the qualification as per Code but........ my viewpoint is that by simply incrementing the design conditions to "fit" into the Code would be a way a kind of over design. The economic aspect too needs to be looked into. Is it feasible to rate this vessel as per ASME VIII-1 and then bear the additional cost of the increased engineering/ documentation? Is the time, effort and  documentation really worth the "simplicity" of the vessel? We frequently joke amongst ourselves when we encounter the design of a small vessel that the equipment weight will be <100 kgf and the accompanying ASME documentation will be 50 kgf winky smile. A hyperbole?

ASME VIII-1 Code will allow many types of vessels falling under the "grey-areas"/ "battery-limits" of it's scope (example clauses U-2g, 1-5g, 1-8e).

Ok, the "vessel " under the current discussion might be small and maybe the minimum thickness would be adequate in this case.... but the documentation/ inspection etc ?

In addition, as I have previously highlighted in my earlier post,

Quote:

since this is a bulk product (granular/ powder) dealt with, API standards 620/650 and ASME codes VIII-1 will not suffice. If Plasmech intends to design storage vessel for "pelletized" contents then it is strongly recommended that a standard like DIN 1055 Part 6 pertainnng to design of silos and bins be employed in the desingn.

BULK SOLIDS (POWDERS/ GRANULES, ETC) EXERT LOADS WHICH ARE DIFFERENT IN NATURE (AND HIGHER IN MAGNITUDE) THAN THOSE EXERTED BY A EQUIVALENT LIQUID STATIC HEAD.

Bulk solids exert a myraid of loads- switch pressures, filling and discahrging pressures (to name a few as per the DIN 1055 part 6 standard).

Hoping to hear from your end on this issue.

-jehan
jehan17718 (Mechanical)
2 Sep 07 1:48
Just wish to add that my viewpoint above is based on the fact  that certain project specifications and/or national/local laws require MANDATORY U stamping with or without NB registraton for vessels designed as per ASME VIII-1. For an identical equipment the one with a U-stamp (with or without NB registraton) will be higher than the one wthout any stamping/ registration.

MJCronin- You are brutally honest in saying that I am splitting my hairs on this one. I could'nt agree with you more on this smile smile

Plasmech- Can you give more info regarding the nature of the contents being stored and where this vessel is intented to be installed?

Thank you all.

Regards,
jehan
Plasmech (Mechanical) (OP)
2 Sep 07 12:54
Whatif we just look at it as air at 350F, forget about the pelletized product for the time being...is there a good program I can get my hands on to analyze this vessel?  Correct me if I'm wrong but predicting what would/could/might/won't happen to a tank head under EXTERNAL pressure is a lot more complicted than internal pressure right?  For example, under internal pressure if the head has a dent or a crease in it, not too big of a deal.  Under external pressure, a dent or crease could cause an implosion.  I'm more of a layman than you guys (I'm envious!), sorry for my simple manner.
gr2vessels (Mechanical)
2 Sep 07 20:25
Plasmech,
Reverting to your original post, you sense it correctly that a container subjected to full vacuum or near full vacuum is considered a pressure vessel and should be designed accordingly. There are many free, unregistered "design software" on the net, most likely not updated for long time, therefore useless and are not supported by the latest edition of any pressure vessel code (ASME, JIS, EN, AS, IS etc..). You won't be able to get design approval, registration or certification either based on those calculations. I doubt it that any decent fabricator will accept a dubious design and fabricate / guarantee the vessel for you. You'll be better off to pay an experienced designer who can guarantee the design and register it with the authorities, including a formal third party design verification, perhaps later an independent third party inspection on your behalf.

Unless you do that, you might have the surprise of someone knocking at your door to take you away...
Cheers,
gr2vessels
jte (Mechanical)
3 Sep 07 17:02
jehan-

Just to be very explicit so we're sure we understand each other: I believe that the vessel as proposed, at ~0.9*FV can be designed and stamped as a VIII-1 vessel. Set some nominal MAWP, perhaps 5 psi, and also place the 13.75 psi on the nameplate and U-1A form. It is code legal. You are not obliged to bump up either the internal nor external pressure to put a U stamp on. Part of my point is that even "FV", or 15 psi, does not exceed the 15 psi limit, thus even a FV vessel may be excluded from the scope. But at the purchaser's option, it may be included provided all of the other conditions are met.

Local jurisdictional issues may be more complicated and/or ornery and are out of scope of this thread - so there!

plasmech
There is a publication out there titled something like Useful Information for the Design of Steel Silos I think published by AISI. Might be useful for you. I think I have a copy in the office; I'll look for it tomorrow to verify title and publisher.

jt
Plasmech (Mechanical) (OP)
3 Sep 07 17:29
Thanks!

Not that this should EVER happen especially since I will likely consult someone with a LOT of experience on this vessel, but if something like this were to implode and someone was standing right next to it, what could happen?  Are we talking simply scaring someone half to death or are we talking major shock wave that could be a fatality?  Like I originally said, vacuum is a whole lot different than positive pressure, which is easy to predict...
vesselfab (Mechanical)
2 Oct 07 18:57
yes, plasmatech, the code covers this and has specific formula and tables for vessels under external pressure.

a small diameter like this is relatively easy as long as the shell does not get too long.  Length of straight shell is much more subceptable to external forces than a formed head.

if you want, I can fax you the formula, but the tables and long formula may be tougher.

don't forget to check openings for reinforcement for external pressure separately from internal.

and Yes, I would not hesitate to apply Sect VIII Div 1 to this vessel.
jte (Mechanical)
3 Oct 07 0:07
Yup, if there's a different national code which could be applied to the vacuum design of this vessel at nearly full vac besides ASME VIII, I'd like to know about it... A 42" dia vessel is relatively small, and a 15 psi internal pressure nearly meaningless, but a 13.5 psi external load can collapse an improperly designed vessel really easily. Ever see photos of API-650 tanks which were inadverdently subjected to a bit of external pressure due to inadequate venting?

vesselfab-

Out of curiosity, have you ever seen a case where you've designed a vessel for which vacuum conditions governed over internal pressure for opening reinforcement? I don't doubt it could happen (it's covered in Div. 1, though simplistically, after all), but I have a hard time dreaming up a case where it would happen in real life.

jt
vesselfab (Mechanical)
3 Oct 07 9:48
well, no, not that I can remember off hand but the code says you must design for it and you don't want the AI or someone up the line finding that you did not.

and tolerances for external pressure vessel are much tighter for roundness to resist it.

SnTMan (Mechanical)
4 Oct 07 16:03
jte, I have on a few occasions had external pressure govern the opening reinforcement calculations. Usually a large, thin (low pressure) shell with a fairly large opening.

It can happen, and needs to be checked out.

Regards,

Mike
SnTMan (Mechanical)
4 Oct 07 16:05
As soon as I hit "submit" I recalled a kind of rule of thumb sometimes used: If your cylinder allowable external pressure is less than 30 PSI, look at reinforcing for external pressure.

Like I said, just a rule of thumb.

Mike

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Back To Forum

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close