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[pleckner]

I think this is my first time asking a question instead of giving the answer!

I would like some input from you all about a very interesting (at least I think so) discussion going on over at eng-tips about whether a non-pressure vessel with a high pressure jacket requires an ASME U stamp. The question is about the vessel, not the jacket. It is accepted that the jacket is ASME stamped but does the vessel need to be stamped?

The original post didn't say anything about how much of the vessel surface is covered by the jacket or what type of jacket or vessel it is.

My stance on this is no, the low-pressure vessel does not need to be U stamped because it doesn't fall within the scope of ASME Section VIII, Div 1.

For one thing, according to Mandatory Appendix 3, Definitions, the MAWP and Design Pressure is referenced to the top of a vessel positioned in its expected operating orientation. In other words, if I have a vertical, cylindrical vessel, the design pressure stated applies to the top of the vessel when it is properly erected into operating position, not laying on its side. The manufacturer is to then take into consideration static head and other forces on the vessel when designing. Therefore, as long as the design pressure at the top of the vessel is less than 15 psig, this vessel will not fall under the scope of ASME Section VIII, Division 1 [U-1©(2)].

Another example, take a 50' vessel filled with 50% causitc at a design pressure of 14.9 psig. I think we would all agree that this does not fall within ASME Section VIII, Div. 1 scope even though the pressure at the bottom could exceed 15 psig when filled to the top.

Some people brough forward some older ASME interpretations that on the surface made me think, but did not convince.

So, what do you think?

[Chris Haslego]

Hey Phil,

I think knowing the relationship between the jacket and the vessel is key in this point. There are two (2) main methods that jackets are applied to these vessels (and I think you know what they are). The jackets can either be clamped on or "integral".

If the jacket is clamped on, then the jacket is self contained and can be treated separately from the vessel (and even removed). People typically use a heat transfer mastic to maximize the lousy heat transfer coefficient between the jacket and the vessel. In this case, the jackets are usually code stamped independent of the vessel.

If the jacket is "integral" to the vessel, this means that one side of the jacket is actually the vessel wall. In other words, one side of the jacket is spot welded (many times) directly to the outside of the vessel and the vessel wall forms the backing plate. This arrangement is better for heat transfer. In this case, I believe that the jacket can still be stamped independently of the vessel, but I'm not 100% sure of this. What makes me 90% sure of this is that it's very common to use these integral jackets on low pressure / atmospheric tanks that probably don't require a code stamp. However, integral jackets can also be used on pressure vessels where both pieces may require a code stamp.