Governing MAWP - Nozzle Pad

First WAWP is to be known, then other things. You should know what MAWP you want.

I disagree, you need to know what Design Pressure you need. Then the vessel Engineer determines what is the MAWP of the vessel. The MAWP must equal or exceed the Design Pressure.

Dear Mechanicalmark,

To differentiate between design pressure and MAWP, please read my explaination:

As per ASME code, the MAWP is defined as the pressure applied at higher point of such a pressure vessel or vessel component. And for certain a pressure vessel, there are a lot of vessel sections at different elevations, and at each elevation there is a design pressure component need to determined to enable us to calculate the thickness of that part.

Suppose you divide your vessel into 3 elevated sections, the design pressure for first section P1 is equal to the summation of MAWP plus the static component of pressure measured at bottom of that section due to the liquid column height H1 (weight per unit area), which depends on density Ω of that liquid.

So,

Design pressure for 1st section, P1 = MAWP + (ρ g/gc) H1

Design pressure for 2nd section, P2 = MAWP + (ρ g/gc) H2

Design pressure for 3rd section, P3 = MAWP + (ρ g/gc) H3

So, the lower sections of an elevated vessel are thicker than when compared with those higher sections for the same MAWP applied to whole of vessel.

Therefore, for a vessel designed under one MAWP, there are a lot of design pressures for each component located at different elevation. And the only pressure related to the vessel is that nominated MAWP which must be included at nameplate.

From which, the design pressure for any vessel component is at least equal to the MAWP plus the static head, and not vice versa.

Note. g = Gravity acceleration & gc = Gravitational constant.

DEAR ABDUL GALALA,

Your explanation is very logical. But I got confused between my theoretical knowledge and the explanation. As per the explanation, MAWP always less than design pressure.

Please correct me if my argument is wrong.

Design pressure- corresponds to the minimum required thickness

MAWP- corresponds to the manufactured vessel thickness.

latter is always more then the former, so MAWP must be larger than design pressure.

clear me..

GA.

One of the most important reasons for using a reinforcing pad with thickness not to be more than the thickness of shell of vessel is to avoid higher MDMT of whole vessel in case of those vessels designed for low and very low temperatures (cryogenic). As we know that MDMT is directly proportion with the type of material and its thickness (see UCS-66), and any increasing in thickness of any vessel component shall badly affect the whole MDMT of vessel.

It has been a while since I have done this, but as I recall, the reinforcing elements must lie within 2x nozzle diameter and 2.5x shell thickness. I think this allows the pad to be thicker than the shell, even if not usually done that way. Whether or not allowed, there seems no good reason to make the reinforcement design any less than the shell MAWP. One reason for thicker pads would be if there are adjacent nozzles where thinner-but-wider pads would interfere with each other.