Design code - low pressure

[Nobert_47]

Hi,

I'm working on a concept design where I need some guidance:
I'm designing a rectangular box shape (don't want to call it a pressure vessel yet) which under normal conditions are subject to slight under-pressure: 0,8 - 0,95 bara. This pressure is maintained by a control system.
In case of a process failure (pipe rupture or similar) inside this box, we are also equipping it with a PRV opening before the pressure reaches 1,5 bara, and in addition preventing pressure below 0,7 bara.

As weight is an important factor, we would like to design this "box" with minimum thickness.
The box is normally inerted (Nitrogen) with operating temperature approx. 55-75 °C (storage 0-40°C).
It contains process equipment and is placed onboard a vessel.

Question:
1) Is this "box" considered a pressure vessel based on pressure & temp rating shown above?
2) If yes, which code would you recommend to follow to minimize weight?

I have been looking at EN 13445 Unfired Pressure vessel, which becomes valid if design pressure is above 1,5 bara. But it also says it can be used for pressure below 1,5 bara and for vacuum.
Another important factor for us is to keep the PRV setting as low as possible (somewhere between 0,95 - 1,5 bara).

Your comments on this?

Thanks in advance

 

[LittleInch]

Thoughts?

Rectangular boxes are difficult to design as pressure vessels and don't normally work well in terms of design codes which are built around circular vessels and dome ends

Is the box all welded or?

FEA is your friend here as it's all about the stress concentrations at the edges and corners - the sharper they are the worse it gets.

Whether it is a PV depends a lot on your local codes and regulations, but 0.5 barg / 7 psig is almost certainly under most definitions of a PV regulation and requirement to use a PV design code.

ASME VIII is 15psig I believe. Not sure about the the neg pressure.

You also need to think about what your limits are in terms of any max deflection of the big faces as being acceptable, even if you don't exceed the stress limits in the corners.

What sort of size or thickness are we talking here?
Any more information you can tell us?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

In most cases a low pressure enclosure would not be called a pressure vessel (think of a purged electrical box or a glovebox).
But you still need to follow design rules unless you want your box to look pregnant, leak, and/or crack.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[MortenA]

In EU, PED does not consider vessels with a design pressure (MAWP) less than 0.5 barg a pressure vessel. Depending on the fluid and other parameters the vessel can fall into different categories.

Have a look at ISO28300 (aor API 2000 if you in the US - but your choise of units indicates that you are not) wrt to protection again over/under pressure.

Best regards, Morten



--- Best regards, Morten Andersen

 

[Nobert_47]

All, thanks for replying.

@LittleInch
I'm aware that box shaped pressure vessels are not a big hit. But in this particular case, using a circular shape is not an option.
(Sorry, can't go more into details...)

The box is all welded, except for the a lid on the short end. We have done a FEA of the design as it is, but would require more detail analysis once we get closer to the final result. We have large radiuses in the longitudinal corners, and stiffeners around. This helps a lot.
According to EN 13445 pressure below 0,5barg is not covered by the code, and also local rules tells the same. So my thought was that we are not obliged to follow any rules, but we would like to follow some rules anyway, in order to get the best result. The questions is then, is EN 13445 the best choice in order to get the lightest construction?

@MortenA
I've looked at PED also, but since this is marine, PED is not applicable. I will take a look at ISO28300, thanks for the tips.

 

[LittleInch]

The issue is probably that at that low a pressure, there will be minimum thicknesses for things which aren't needed or simply the equations will come up with silly answers.

But if you want to check it I guess it's as good as any other.

I would go with the FEA myself - most of the time the code gives you a simplified design which can be reduced in thickness by FEA, but no one wants to use FEA on every single design of a PV. Plus it's not round...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Nobert_47]

Hi folks,

Just to give you an update:
After consulting with Class society, the (prelim) conclusion is that this is not to be considered a pressure vessel, and does not have to follow those rules or calculated accordingly.
But we will use FEA analysis to check the strength of the design, especially with focus on load carrying and external factors (supporting the internal parts, forces from vessel in motion ++).
Also the container / box will be tested in the end.

 

[DriveMeNuts]

I'm sure you could accurately design this with a hand calculation.
Have a look at adapting the Appendix 13 equations.

 

[XL83NL]

After consulting with Class society, the (prelim) conclusion is that this is not to be considered a pressure vessel, and does not have to follow those rules or calculated accordingly.

Currently dealing with a similar issue. PM said; it's a copy-paste design of a previous job, built it that way. They forgot the previous job was a square (welded) box with 0 over pressure. The current one needs 0.5 bar(g) design. You need to go FEA or App. 13, otherwise, the devil is in the details.

Huub
- You never get what you expect, you only get what you inspect.

 

[chicopee]

You may be better off to have rounded edges and spherical corners instead welded edges and corners

 

[GenB]

If looking for the lightest.
the best bet is to run calcs and analysis per ASME VIII-Div 2
whether is certified or not.



General Blr. CA,USA