Axial Compressive Stress in Horizontal Vessel

[LC-24]

I am trying to design a horizontal pressure vessel that is supported on each end. It will be designed and stamped to ASME Section 8 Division 1. My concern is with the longitudinal axial compressive stress when loaded.

It’s my understanding that I am supposed to use UG-28 and solve for the “B” value and then check if it is greater than the allowable stress of the material. If “B” is less than the allowable stress, then buckling is governing. I then do a beam analysis to determine whether my maximum longitudinal compressive stress exceeds the “B” value.

The “B” value I have gotten for my design is very low at about ¼ the allowable stress of the material. Therefore buckling is a concern. I have added longitudinal reinforcement along the length of the top of the vessel to help, but I am still not quite below the “B” value. The load I am using in my calculation for the stress is 2x the loaded tank, based on the use of this vessel. The 2x loading is not constant, but rather an extreme and intermittent load. Would this loading be considered “primary bending stress” and if so can I then use 1.5x the “B” value?

 

[Geoff13]

Mainly questions at this point. Hard to know what's safe to say with so little information.

• What are your vessel dimensions? Length, diameter, thickness, design temp & pressure?
• Once the vessel is full how can it temporarily reach 2x load?
• Why are your supports at the ends? Moving them inwards can significantly reduce the mid-span moment.
• Have you reviewed Leonard Zick's "Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports"?
• What do you mean by "I have added longitudinal reinforcement along the length of the top of the vessel". Can you provide a sketch? Even without a sketch I can confidently say I would NEVER do this.

I assume you're trying to use the UG-24(c) 1.5x stress increase. If you read carefully that is for through thickness bending, which you do not have. The mid-span bending moment will only create axial tension in the bottom and axial compression in the top.

 

[LC-24]

Thank you for taking the time to reply.

- General dimensions are 70" OD, 432" seam to seam length, 0.12" thick, MDMT -320 F w/ max temp of 100 F, 38psig MAWP.
- This is a vacuum insulated transport vessel, hence the 2x loading situation to design for.
- Supports must be at end. I don't want to go too much into it.
- Thank you, I have not, but I will.
- It is common in these vessels for designers to add pieces of angle or similar metal extrusions on each side of the top of the vessel that span the length of the vessel to help mitigate the issues that arise from the longitudinal compressive stress.

Okay, I see. That is why I have added the longitudinal reinforcements to the top, since the "B" factor is limiting the longitudinal compressive stress. By adding these, the area moment of inertia is obviously increased and the extreme fiber in compression to the centroid is decreased, helping to reduce the compressive bending stress.

 

[TGS4]

I don't understand how you can achieve a 2X load. Please explain.

The bending occurring due to self weight, that is effectively tensile on the bottom and compressive on the top is a primary general membrane stress. Therefore, an increase in the allowable stress is not permitted. That stress is not primary bending (which typically only occurs in a pressurized flat plate).

 

[KevinNZ]

Are we discussing both the pressured and unpressured (but full) cases?