API 650 External Pressure design

[BZ29]

Please correct & guide me if my understanding is wrong regarding these two issues:

1) With Ref to API650 5.2.1(b), upto 0.25kpa external pressure, no calculation is required (safety against such vacuum is inherently built in its internal pressure design). Annex-V is for pressure above 0.25kpa only.

I was just going through a design using HEXAGON TANK Software, having 0.25kpa external pressure. It is showing calculations for Annex-V even at external pressure less than 0.25kpa and gave warning that it failed the annex-v calculations, and need stiffeners, although it was safe for internal pressure. I am assuming that i can ignore this calculation & warning, unless that value exceeds 0.25kpa.

2) If intermediate stiffeners are required as per its calculation (supposedly 1 stiffener is required), while stiffeners are also required as per Annex-V due to vacuum (supposedly 2 stiffeners are required). i assume we can size and place them to suit both requirements, and will use 2 stiffeners in total, instead of 3.

 

[JStephen]

For pressures of 0.25kPA and below, external pressure is still considered in the roof load combinations in 5.2.2 and in the shell buckling criteria, see 5.9.6.1(a), last paragraph.
Note that external pressure can be taken as zero for tanks with circulation vents- see 5.2.1(b).
Where the code rules give you a number of stiffeners, you can always use more- but usually, it is most economical to use the fewest possible.
In many cases, it is more economical to thicken the shell rather than add stiffeners, although either option may be allowed.
I'm not familiar with the software, so can't comment on it.

 

[MJCronin]

What did the people at "HEXAGON TANK Software" tell you when you called them ?

MJCronin
Sr. Process Engineer

 

[BZ29]

JStephen:

Thank you for guidance. I was trying to understand it completely before replying which i have understood now to much extent.

There was some confusion, as i was checking this in API650 12 edition, and there it was not mentioned clearly, regarding the addition of 0.25kpa external pressure in maximum unstiffened shell height formula.

0.25Kpa pressure addition was mentioned in velocity pressure formula, but velocity pressure is not being used in maximum unstiffened shell height formula.

so as per 12th edition i am still confused, but in 13th edition, it is explained clearly

So now i can safely ignore any annex-V calculation upto 0.25kpa

 

[BZ29]

MJCronin: No i did not called them.

This was not a software functionality issue, rather it was code understanding issue.

 

[EmilianoS87]

5.2.1. b) Design External Pressure (Pe): Shall not be less than 0.25 kPa (1 in. of water) except that the Design External Pressure (Pe) shall be considered as 0 kPa (0 in. of water) for tanks with circulation vents meeting Annex H requirements


Design Pressure shall not be less than 0.25 kPa.
-If it exceeds 0.25 kPa thank shall be calculated as Annex V.
-For tanks with circulation vents meeting Annex H requirements the design External Pressure shall be considered as 0kPa.

As I understand you shall always consider 0.25kPa or greater unless you meet that exception requirements.

Be aware that when checking annex V application the formula V.8.1.1 has errors