Static Head Omitted from Jacketed Spherical Vessel Design Calcs?

[Breitenstein]

Hello -

I am reviewing the design calculations for a jacketed spherical vessel with a 5 ft ID built to 1989 ASME Section VIII Div. 2, 1989 Addenda. The vessel was built for LOX. Back in the 90s, it was changed to isopropyl alcohol service. I need to confirm the MAWP for isopropyl alcohol.

The original calculations state that the MAWP is 9,000 psig, and the value of P or design pressure used to calculate the minimum wall thickness of the vessel is 9,015 psig. For an outlet nozzle, they included a 3 psi static head for LOX (resulting in P = 9,018 psi), but static head was not taken into consideration for any other component including the vessel wall.

My understanding of Code is that static head needs to be included in the design pressure P. Is there anything permissible in the Code that would allow the designing engineer to ignore the LOX static head for the vessel?

Thank you in advance for your assistance!

Breitenstein

 

[JStephen]

Where'd the extra 15 psi come from? If that was included in all the calculations, wouldn't that cover it?

 

[Breitenstein]

15 psi full vacuum

 

[SnTMan]

Breitenstein, yes, static head must be accounted for either directly or perhaps indirectly, i.e. MAWP of each component exceeds DP plus static head.

Assuming the full vac exists in the jacket, then design pressure at the top of the sphere would be 9015 psig, if full vac can exist at full internal pressure. Static head would add just under 3 psig for either of the fluids you mention, also for water, for 9018 psig at the bottom of the vessel.

It appears the nozzle reinforcement was so designed. Not familiar w/ Div 2, but in Div 1 shell would usually have joint efficiency of 1 in nozzle reinforcement calcs, and if excess shell thickness exists to contribute to reinforcement, then the shell in effect has MAWP exceeding the 9018 design pressure.

Of course the efficiency may be less than 1 in shell calcs. If you have detailed calcs, examination of them may provide the answers, be aware the fabricator may have assigned completed vessel MAWP equal to 9000 psig when in reality it may be higher.

Also note MAWP is generally not dependent on the contained fluid.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand