While calculatiing vessel,Is it justified to add static head in addition to internal pressure to carry out thickness calculations. If YES, than why we only spesify internal preessure as "Design Pressure".

Hello JKS

When you fill the vessel with liquid, you can only fill as far as the top head. Suppose the liquid extends into a nozzle that is installed in the top head. This increases the pressure in the vessel. You can make provision for this extra pressure in the 'Design Constraints' part of the programme. Look for a field called: 'Additional added operating pressure', and add the extra pressure from the static head there.

On the other hand, suppose you set a particular design pressure for a vessel, then you fill it with liquid. The pressure that you require in the calculation is the design pressure plus the static head down to the component being considered. The design pressure for any component is thus P + static head. Remember, the design pressure is actually the pressure at the TOP of the vessel (assuming the pressure gauge is place at the top of the vessel). This is the definition for example given in ASME Section VIII, Division 1.

If Design pressure is 100 Psig and Static head is 5 Psig. Thickness calculated is based on both the pressures (internal + static). Now MAWP is declared as the same as design pressure, which is 100 Psig. Now in this case can we ignore the effect of static head pressure in MAWP, while we are going to specify the MAWP as same as design pressure (100 Psig)?.

Hello JKS

Suppose you set the MAWP to 100 psi. Further, suppose you have a 20 foot high cylinder filled with water of SG = 1.0. When the calculation is performed, the pressure used in the calculation is 108.66 psi, because the MAWP is the pressure at the TOP of the vessel, but the calculation is performed at the depth of the liquid. 20 foot of water has a hydrostatic pressure of 8.66 psi.

Remember, the design pressure, or the MAWP is always the pressure at the TOP of the vessel.

A lot of Thanks Mr. Ray Delaforce,
This means, in this case we can declare design pressure and MAWP both as 100 Psig.

Hi JKS

Yes you can do that. In fact the ASME Division 1 allows you to do that in paragraph UG-99. See note 35 at the foot of the page.