Flange rating on bottom of tall vertical vessel

[BornTrucker]

Hypothetical situation:

I need to design a vertical vessel with MAWP of 1480 psig designed to handle water. The vessel is 100 feet tall and will have a drain welded to the bottom with a flange. Ballpark numbers tell me during operation at the MAWP when full of water, the vessel will see 1523 psig due to process pressure and static head. The MAWP of the vessel is 1480, am I allowed to use Class 600 flanges? The flanges aren't limiting the MAWP as it is measured from the top of the vessel, not the bottom.

The pressure exceeds the rating for these flanges so I believe the answer is I must use Class 900 flanges.

If this is the case, my suggestion would be to rate the vessel to 1436 psig if class 600 flanges are required. 1480 - 43(hydrostatic head)

Thanks!

 

[racookpe1978]

No, chose a flange that will allow you to build the vessel to its full operating requrements.

The few hundreds dollars of a flange, and the few dozen dollars for the difference between a 600 class flange and a 900 class flange are near-trivial to building a pressure vessel not abl to serve its intended purpose.

 

[BornTrucker]

This is absolutely the practical approach, however, I have seen a few vessels come in rated at the upper limit of the flange rating that don't appear to account for hydrostatic head during hydrotest.

I guess my question is am I allowed to use a Class 600 flange on the bottom of a vessel rated to 1480 psig at 120°F for Group 1.1 materials knowing that the 1480 pressure will be exceeded in that flange? Again, I want to say that because that flange can see pressure exceeding 1480 during normal operation, the vessel would actually end up with a MAWP less than 1480 (1436) because that flange is the limiting factor.

I'm wanting to make sure I'm interpreting the code correctly.

 

[mk3223]

1) Yes, it is correct as stated:

The pressure exceeds the rating for these flanges so I believe the answer is I must use Class 900 flanges.

2) Also, the suggestion as below is okay if the lower MAWP is accepted by the Process. And then, the existing system design and operation need to be revised to meet the lower MAWP requirement.

If this is the case, my suggestion would be to rate the vessel to 1436 psig if class 600 flanges are required. 1480 - 43(hydrostatic head)

It's the Owner's responsibility to make sure the vessels are designed to be complied with the Codes and operated accordingly under the MAWP.