7 replies to this topic

[jemmy12]

Hello 

 

I need to design a small atmospheric vessel with the characteristic below

 

Diameter 400 mm

length 680 mm

The rang of internal pression : [ - 50 mbar , 100 mbar ]

external pressure : atmospheric

 

When i apply the ASME Viii div 1 rules i find too small value of the thickness

There is another code applicable for this vessel type?

Edited by jemmy12, 25 February 2015 - 05:29 AM.

[Art Montemayor]

What specific section in ASME Section VIII, Div 1 are you referring to?

 

What is the calculation procedure that you have applied?

What do you designate as "too small value of the thickness"?

 

Calculating the required pressure vessel wall thickness for a vacuum condition requires a special procedure.  Are you sure you are using the correct procedure?

[MTumack]

Be aware, vacuum vessels thickness is typically derived from max allowable strain based upon physical tests. the method is very different from that of regular internal pressure vessels.

 

See attached for an Excerpt of "The Pressure Vessel Handbook" by E.F. Megyesy. Maybe this might help you in your work.

Attached Files

•  Excerpt from Pressure Vessel Handbook - External Pressure.pdf   2.27MB   34 downloads

[jemmy12]

The calculation procedure is discribed in ASME Viii div 1 UG-28 ' THICKNESS OF SHELLS AND TUBES

UNDER EXTERNAL PRESSURE ' there are 8 steps to follow . with thickness = 2 mm the 8 steps are checked that, means  i can reduce more the thicknes .
MTumack thanks for your help , but your document is the same in ASME Viii div 1 . I followed the same steps

I think my problem is the small dimensions of the vessel because the vacuum effect is more dangerous in the wide vessel . Diametre (the larger  the less stable) , lenght (the longer the less stable ). 
I have an other question why in  ASME Viii div 1 UG-28  procedure , the value of vacuum is not required as a calculated variable ? 

Edited by jemmy12, 26 February 2015 - 03:12 AM.

[MTumack]

Vacuum vessels fail under strain, and almost never due to stress concentrations. This looks like this:

 

http://www.mankenber...auumschaden.jpg

 

Internal pressure vessels fail due to brittle fracture and excess hoop stresses, this looks like this:

 

http://www.gowelding.../johnthomas.jpg

 

That being said, your strain on your shell is effected mostly by the difference between the internal pressure and the external pressure. If you have one vessel with 30 PSIA outside the vessel and 15 PSIA inside the vessel you will have very similar strains as a vessel with 45 PSIA will have very similar strain numbers because your Bending strain as a result of being "in a vacuum" is the critical mode of failure in most cases; while the difference in compressive strain on the shell will be double, it is kind of a non factor because of how much more the vessel strains in the hoop bending resulting from the undbalanced internal and external pressures

 

Consider the following; you submerge a piece of 1" THK x 6" Wide x 6" Long plate in pressurized fluid bath ~10 gallons of volume @ 300 psig. Would you assume the plate would fail?

 

Technically your internal pressure is to be considered in UG-28, its a nomenclature thing. Technically a full vacuum is not negative pressure, it is zero pressure. so you dont have a lack of pressure on the inside, but a pressure on the outside. Does that make sense?

[jemmy12]

let me explain my situation further

I need a small vessel that i ll use in a test brench for breather valves.The set pressure / vacuum of most valves belongs to this range 

  [ atmospheric pressure - 50 mbar ; atmospheric pressure + 100 mbar ] that 's why i wrote negative pressure. 

[MTumack]

I understand your situation.

 

The code is assuming your vacuum pressure is absolute 0; your external pressure as defined by the code is not the actual external pressure, but in fact the difference between your absolute external pressure and your absolute internal pressure.

 

You asked why the code didn't reference the internal pressure, but it does; the internal pressure is used as a reference 0 point for the external pressure in the code.

[Art Montemayor]

Jemmy12:

 

You are receiving some very good and solid advice from MTumack and I don't want to distract you from absorbing some of the excellent comments and recommendations he gives.  If you have the time and interest, the attached documents might be of some assistance to you and help you apply what you already are receiving from MTumack.

 

I am an avid proponent of helping Chemical Engineers get more and more involved in the technology regarding the design of pressure vessels.  I firmly believe it is in the best interests of every engineering discipline in a project team to have the ChEs firmly on board with complete knowledge of what goes into a successful and safe design of a pressure vessel - be it a steam or vapor generator or an unfired vessel.  For many decades mechanical engineers have led the way in this arena and the ChEs have a lot to contribute in helping and aiding to make positive and profitable contributions with their process background and experience.  Vacuum (or External Pressure design - as MEs would call it) is an important process condition that needs careful and expert design.

 

Pressure Vessel Engineering Ltd. is a great Canadian engineering company that has helped me a lot in the past regarding pressure vessel design.  I highly recommend you visit their web site and download and study all the free engineering information they distribute.

 

 External Pressure Calculations.pdf   1MB   31 downloads

 External Pressure Design for Vessels.docx   2.82MB   23 downloads

 Vessel External Pressure Design Procedure.docx   1.09MB   25 downloads