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#33565 - 03/09/10 01:14 AM External Pressure
Shabeer Offline
Member

Registered: 12/26/07
Posts: 266
Loc: india
Dear all,

In pressure vessel design when we have considered design pressure comes with full vacuum condition at how we consider for external pressure. Any code is to be followed.


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#33620 - 03/12/10 09:34 AM Re: External Pressure [Re: Shabeer]
Mandeep Singh Offline

Member

Registered: 12/15/99
Posts: 600
Loc: Houston, Tx, USA
Dear Shabeer,

I not sure if I understand your question. Can you clarify.

The internal pressure and external pressure is done separately. So, in the internal pressure calculation, the external pressure is ignored and vice versa.

There is no differential design for basic shells/heads.
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Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM

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#34206 - 04/08/10 11:28 AM Re: External Pressure [Re: Mandeep Singh]
M.SHUKLA Offline
Member

Registered: 06/05/08
Posts: 55
Loc: INDIA
Dear,

If in Design specification it is written as "design equipment in full vacuum condition" then external pressure shall be atmospheric pressure I.E. 1.055 kg/cm2 or 1.033 kg/cm2 (get value confirmation from client / owner)

=> Internal pressure : your design pressure
=> External pressure : 1.033 / 1.055 kg/cm2

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#34217 - 04/08/10 03:33 PM Re: External Pressure [Re: M.SHUKLA]
Ray_Delaforce Offline
Member

Registered: 01/02/03
Posts: 743
Loc: Houston, TX
Folks

As explained by Mandeep, the Internal and External pressure calculations are done completely independently. When doing the internal pressure calculation, the external pressure is ignored. Conversely, when doing the external pressure calculation, the internal pressure is ignored.

I am curious: Why do you not use the SI Metric system? It always puzzles me.
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Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM

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#34259 - 04/11/10 08:30 AM Re: External Pressure [Re: Ray_Delaforce]
Mukesh Offline
Member

Registered: 11/12/06
Posts: 125
Loc: Ahmedabad, Gujarat, India
Ray,

Why M/s COADE (Now Intergraph)is not adding SI unit stress values in PV Elite which is already published by ASME many years ago? I just want to inform you that other softwares have incorporated the same in their softwares.

Mukesh

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#34269 - 04/12/10 07:26 AM Re: External Pressure [Re: Mukesh]
Ray_Delaforce Offline
Member

Registered: 01/02/03
Posts: 743
Loc: Houston, TX
Hello Mukesh

We are often asked that question. Let me give you an example: Many countries use terrible units such as kgf/mm^2 or kgf/cm^2 or bar, which are not metric units. To use kg as a force is a travesty of the SI Metric system. The question is; where do we find the stress values in those non metric units (kgf/cm^2 for example)? Neither the Imperial table nor the Metric table will give that information.

The principle we adopted was to use the Imperial tables exclusively, and use internal conversion factors to express those values in the user's unit system. ASME unfortunately adopted a 'soft' conversion unit system so the Imperial and Metric table give different stress values.

There is a slight difference (a tiny fraction) when the two tables are compared. There is a code case issued by the ASME code committee which allows either table to used.

_________________________
Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM

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#34275 - 04/12/10 08:32 AM Re: External Pressure [Re: Ray_Delaforce]
Mukesh Offline
Member

Registered: 11/12/06
Posts: 125
Loc: Ahmedabad, Gujarat, India
Ray,

You explaination is good and OK. But at the same time we are getting a lots of problem from competitior softwares which have included MPa stress values in their softwares.

There is no end towards this debate. But one thing is that we are getting a lots of problem because of this

Mukesh

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#34280 - 04/12/10 10:13 AM Re: External Pressure [Re: Mukesh]
Ray_Delaforce Offline
Member

Registered: 01/02/03
Posts: 743
Loc: Houston, TX
Hello Mukesh

ASME introduced this rather un-scientific way of treating units. Actually, the Americans should get into the 21st Century and realise the Imperial system is really dead and buried. Using feet, inches and fractions as well as kips on drawings is counterproductive. Also, other countries should stop using non metric units such as kgf/cm^2. That is un-scientific too.

Napoleon gave a good metric system, which solves many dynamic problems, and folk have tried to add to the system producing a complete mess. Remember: force = mass x acceleration. It is very elegant in the metric system. In any other system the whole thing crumbles into a mess.

Now, let us consider an example. In division 1, the design stress of SA 516 70 material is 20000 psi. In the metric table it is 138 MPa (which is rounded from the truer value of 137.895146 MPa). This represents a difference of 0.076%. Who argues over such numbers? This is not engineering, but is arguing over a hair's breadth.

ASME could solve this problem by dumping the Imperial system altogether.

Now let me pose a question: How do you determine the stress from either table when the units are kgf/mm^2? Which table to you use? Remember, the kgf/mm^2 unit is not a metric unit.

_________________________
Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM

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#34291 - 04/12/10 09:10 PM Re: External Pressure [Re: Ray_Delaforce]
Mukesh Offline
Member

Registered: 11/12/06
Posts: 125
Loc: Ahmedabad, Gujarat, India
Ray,

When kg/cm2 is used, as you said we utilized values converted from psi or MPa.

I am agreed to your all above logical arguments which seem to be true.

But you look at this situation:
When we have freedom to design in any units then we dont have any problem. But when we have to design in SI units then we are asked to use SI stress values in MPa as given in ASME Code. We are forced to use the same values only as it is published by ASME. When something is not published then we can take converted values. But when ASME has published value then Autorised Inspector (AI) and many other clients are forcing to use ASME Values only. You can't imagine how is that situation when you have to do a lots of adjustments...

My suggestion would be:
Initially M/s COADE must incorporate most commonly material database in SI units...When any user needs to design per SI unit then he can choose that one...Else imperial units and other units from conversion are already available. This is also required in heavily competitive market where other softwares already have MPa stress values.

Mukesh

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#34306 - 04/13/10 07:33 AM Re: External Pressure [Re: Mukesh]
Ray_Delaforce Offline
Member

Registered: 01/02/03
Posts: 743
Loc: Houston, TX
Hello Mukesh

I have explained the reason why we use the Imperial table. I also explained that ASME issued a Code Case (2523) that allows either table to be used. We are discussing minute fractions of a percentage point.

When I was a young engineer, I used a slide rule to do the calculations. There were no calculators or computers a slide rule, pencil and paper were all we had. For accurate work, we used 7 figure logarithm tables. In those days, we could not get the 4th decimal place when doing a calculation. Now we have computers, folk are worried about the 5th decimal place.

Quite frankly, this is not engineering anymore. We have lost sight of the objective.

The ASME code issues values in both metric and imperial units. They allow the use of either table. So, what really is the problem?
We really must draw this discussion to a close.


_________________________
Sincerely,
Ray Delaforce
CADWorx & Analysis Solutions
Hexagon PPM

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#34337 - 04/14/10 01:04 PM Re: External Pressure [Re: Ray_Delaforce]
Mandeep Singh Offline

Member

Registered: 12/15/99
Posts: 600
Loc: Houston, Tx, USA
Hi Mukesh,

But, if you refer to ASME Sec. VIII Div. 1 Appendix GG-1 (last line of 1st para in A-09) it allows the use of unit conversion constant with least 3 significant digits to convert from Imperial allowables to Metric allowables.

A more important point in my opinion is the minimum thicknesses/sizes in metric or English sizes,

For the minimum thickness or weld sizes, PV Elite selects the Imperial values or Metric values based on the unit system selected for the job file. But, if you want to change that you can do so from the Tools menu -> Configuration -> Metric Constant Selection (bottom right side).

If the competitor's product has some feature that we do not have, we should review it and based on its merit decide to incorporate in our program.
_________________________
Best Regards,
Mandeep Singh
CADWorx & Analysis Solutions
Hexagon PPM

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#34381 - 04/16/10 12:01 AM Re: External Pressure [Re: Mandeep Singh]
Mukesh Offline
Member

Registered: 11/12/06
Posts: 125
Loc: Ahmedabad, Gujarat, India
Thanks Mandeep.

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