confusion of MAWP

[phoenixmoca]

Recently I have been reading API 520 in which its defination is
The maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated coincident temperature specified for that pressure. The pressure is the least of the values for the internal or external pressure as determined by the vessel design rules for each element of the vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure.

So my question is what is actual nominal thickness? It is more than nominal thickness? And why MAWP is greater than design pressure?

 

[e43u8]

phoenixmoca,

The actual nominal thickness is the thickness of the actual plate used in vessel manufacturing and it's obvious it will be selected having not lower thickness than calculated (required) thickness; hence MAWP which is based on actual used plate thickness would be higher than design pressure which is the basis for required thickness calculation.

 

[phoenixmoca]

thank you, e43u8.

So would you like to explain the process of designing pressure vessel from the beginning to the fabrication? Thank you in advance.

 

[don1980]

The phrase "actual nominal thickness" is referring to the thickness after subtracting the corrosion allowance. Corrosion allowance is extra metal thickness that one can tolerate losing. It's the metal thickness that can be lost (corroded away) without impacting the MAWP. If the MAWP calculation included the corrosion allowance, then it wouldn't really be a corrosion allowance.

For your second question, about the difference between design pressure and MAWP, see your previous post. This was explained in my second response.

 

[phoenixmoca]

Thank you, don1980
I have learned a lot for your post.