External Pressure Calculations interpretation.

[rsaeed]

Good Day all,
Can anybody have a look on the attached file page#22 and share please give some comments on this:

1- Firstly, I have checked the user guide of PVElite and found that for external pressure it takes 15psig (103.42kPa) whereas in the calcs shown on pg22 attached they have also added static head pressure which to my understanding is not acting externally. This is something which is acting from inner surface.
Why are they adding static head pressure in this external pressure?

2- In the end there are three cals for Results________:
a) MAEP came out to be 1560kPa. Is this mean that externally we can apply this much pressure whereas as per actual conditions we have only 207.05kPa which means we are applying less pressure than allowed so obviously it is safe. Am I right?

b) Tca is used as 20.708mm. Where did this come from? and MAEP is comming 100.0058kPa which is below the actual applied one. So is that unsafe?

c) Why SLEN (Stiffened Length) is 0.1x10^+30? I assume we have just put a very very large value to see how much max pressure it is allowing us which is comming out as 730.5733kPa and shows that even by taking this much large length b/w stiffners we are safe than without stiffners for 15.6meters wont be a problem. Am I right here.

Please give the point wise answer!
Thanks and best regards

 

[doct9960]

Are you checking a fabricator's calculations?

 

[rsaeed]

Thanks doct9960 for reading and replying on this post. Actually this I am doing to learn it for getting into the world of pressure vessel.

Somehow I have to study old cases and understand them to start than can jump on the software once I get the code basics and know how to read the results.

 

[AWDMIKE]

I can click on the link within the post above, but then I see no link to go to the PDF or whatever type of file was posted. Am I the only one? If not perhaps it can be reposted.

 

[micalbrch]

I have the same problem. I cannot open the link.

 

[rsaeed]

sorry for that and here it is!

 

[SnTMan]

raseed, maybe you could extract the pages of interest. I am not interested enough in answering your questions (assuming I am able) to examine 239 pages of PVE output

Regards,

Mike

 

[SnTMan]

I now see you directed us to page 22, my apologies!

 

[starrproe]

rsaeed,

My views on your queries:

Firstly, I have checked the user guide of PVElite and found that for external pressure it takes 15psig (103.42kPa) whereas in the calcs shown on pg22 attached they have also added static head pressure which to my understanding is not acting externally. This is something which is acting from inner surface.
Why are they adding static head pressure in this external pressure?

Refer UG-32(a), UG-33(a) & UG-33(a)(1)(a).
Probably PV-ELite is misunderstanding "plus heads" as adding liquid static head, which is incorrect.Code intends to denote concave sides of heads as "plus heads" and convex sides as "minus heads".
Static head of liquid has no role in external pressure calculations.

In the end there are three cals for Results________:
a) MAEP came out to be 1560kPa. Is this mean that externally we can apply this much pressure whereas as per actual conditions we have only 207.05kPa which means we are applying less pressure than allowed so obviously it is safe. Am I right?

You are correct.

Tca is used as 20.708mm. Where did this come from? and MAEP is comming 100.0058kPa which is below the actual applied one. So is that unsafe?

20.708mm is the thickness required for shell under external pressure and has been obtained by backcalculating required thickness for external pressure of 100KPa.
Design external pressure for your case is 100KPa only(and not 134KPa, as calculated by adding static head), hence it is not unsafe.

Why SLEN (Stiffened Length) is 0.1x10^+30? I assume we have just put a very very large value to see how much max pressure it is allowing us which is comming out as 730.5733kPa and shows that even by taking this much large length b/w stiffners we are safe than without stiffners for 15.6meters wont be a problem. Am I right here.?

This is not clear to me as well.
PVElite technical support needs to reply to this query.

Starrproe

 

[rsaeed]

Quote:
Tca is used as 20.708mm. Where did this come from? and MAEP is comming 100.0058kPa which is below the actual applied one. So is that unsafe?

20.708mm is the thickness required for shell under external pressure and has been obtained by backcalculating required thickness for external pressure of 100KPa.
Design external pressure for your case is 100KPa only(and not 134KPa, as calculated by adding static head), hence it is not unsafe.

Thanks Starrproe,
I am unable to do backcalculations, as the formula is
EMAP = (4*B)/(3*(D/t))
In this if you want "t" you need B, D and EMAP(which is Max allow external pressure).
We can put EMAP=100kPa D=3500+2*65=3630mm and for "B" we need "A" and for A we have to refer chart which need D/t so again without "t" I cant see the chart for "A" so please tell how to do it. I want to put the values manually and get the results as mentioned in PVElite calcs.

 

[prex]

You need an iterative solution, what the software certainly does: take a tentative value of t, use the design EMAP and calculate a new EMAP: if it is higher than design value, reduce the tentative t and so on.
The large number for SLEN is for sure the equivalent of infinite and means there are no stiffeners on shell. What I don't understand is why they do not take shell length plus a fraction of head depth, as allowed by code. Results wouldn't likely change much, but that is the standard procedure.

prex

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[starrproe]

rsaeed,

You may please refer following message from PVElite regarding your query on addition of static head for extrenal design calculations.

The calculation on page 22 that you noted was added by the analyst and was not produced by PV Elite. Internally, PV Elite uses 1.67 times the specified external pressure (there is no static pressure added) to check the requirements of UG-33.
_________________________
Scott Mayeux
CADWorx & Analysis Solutions
Intergraph Process, Power, & Marine

 

[Mandeep1]

Hi Rsaeed and Prex,

You have correctly stated that this is an iterative procedure. I will clarify little further. In PV Elite there are multiple calculations for external pressure,

1. First the program computes the required thickness from external pressure specified for each section.

2. Then it computes the maximum external pressure that each section can withstand, using the thickness and design length (including the fraction of head depth if applicable as noted by you) of that section.

3. Lastly it computes the maximum un-stiffened length of a section for the specified vacuum pressure and geometry. This gives you maximum distance to the next stiffener. This length can be used place stiffener on the vessel. If a calculated un-stiffened length is more than the design length of that particular section, you do not need a stiffener on that section.

We have already added more output to show UG-33 is really considered in PV Elite for head under external pressure (which it is), but it rarely governs.

I hope this settles it.


best regards,
Mandeep Singh