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Required thickness tr for the input nozzle pressure exceeds the compon

Required thickness tr for the input nozzle pressure exceeds the compon

Required thickness tr for the input nozzle pressure exceeds the compon

Good Afternoon  

COMPRESS determines the pressure acting at the elevation where the nozzle penetrates the outside surface of the shell; the pressure includes the design pressure plus any static head at that location. To eliminate confusion, the same pressure is applied throughout the entire nozzle analysis (except for an attached flange) for purposes of all applicable nozzle checks, eg: area replacement, UG-45, etc.

Because the pressure used for nozzle design is taken at the outside surface of the shell it will always exceed the pressure at the inside surface of the shell. This contrasts with the MAWP (or MAP) of the shell components being based on the pressure acting on the inside surface of the shell.

In some cases the design of nozzles for the chamber MAWP (or MAP) is logically impossible due to a contradiction: the MAWP is based on the inside surface of the vessel, whereas the nozzle design is to be based on pressure at the outside surface of the vessel. The contradiction can occur, for example, if the component to which the nozzle is attached governs the chamber MAWP and yet the nozzle itself is to be designed for the chamber MAWP. In this case the required thickness "t" of the nozzle's parent shell component under the effect of the MAWP plus the small additional static pressure due to operating liquid acting over the thickness of the shell component will exceed the actual thickness of the shell component on which the chamber MAWP is based. Thus the "logical impossibility" of achieving this condition.

This problem can exist under the following conditions:  (1)  the nozzle design mode is set to "Chamber MAWP" or "Larger of MAWP or MAP",  (2)  there is a liquid level acting on the nozzle, and  (3)  the chamber MAWP (or MAP) is governed by the cylindrical shell or head to which the nozzle is attached. Under these conditions the thickness of the component required to support the nozzle design pressure (which is the chamber MAWP/MAP plus static head to outside surface of the vessel) will be greater than the actual thickness of the component (on which the vessel MAWP/MAP is based, as based on inside surface). In such a case the nozzle cannot be designed for the chamber MAWP/MAP and a different nozzle design mode should be selected.

For example, for your vessel the nozzle mode selected is "Larger of Chamber MAWP or MAP". The chamber governing value is 249.93 psi based on the cylindrical shell. The "Boot" nozzle is located at the bottom of the shell (180°) and must be designed for 254.085 psi to account for the 4.1557 psi static liquid head acting at the outside surface of the cylinder. The required thickness of the cylinder at 254.085 psi, 1.0001", is slightly greater than the corroded thickness actually available: 1.000 - 0.125 = 1.0000". Thus the Deficiency is reported.

To resolve the Deficiency you could select another nozzle design mode or "force" the vessel MAWP to be based on a different component by decreasing the thickness of the head (to lower its MAWP), etc.

Personally, I suggest running with option "Design P, Find Nozzle MAWP & MAP" and compare the nozzle MAWP to the chamber MAWP to determine if acceptable within desired numerical precision (number of decimal places). In this case the MAWP of the Boot nozzle is 249.8343 psi (253.99 psi minus 4.1557 psi static head) versus MAWP of cylindrical shell equal to 249.9293 psi; a difference of about 0.1 psi.

NEW!  Visit the new COMPRESS Users Group at www.eng-tips.com/groups/COMPRESSUserGroup

   Kind Regards for Codeware,

   Tom Barsh, P.E.
   Technical Support Engineer
   Codeware Inc
   Phone: 713-781-6636
   FAX: 713-781-6685NEW!  

Thank you Tom

RE: Required thickness tr for the input nozzle pressure exceeds the compon

Hello Leonard,

Perhaps it is best to put my e-mail response to you (which you have posted) in context for the benefit of other readers. I think it is always best to provide some context for such information drawn from other sources. But this is an interesting technical issue and is worthwhile discussing.

You asked about a Deficiency message reported for a nozzle:

   "Required thickness tr for the input nozzle pressure exceeds the component wall thickness."

The nozzle in question was at the bottom (180°) of a horizontal cylindrical shell. The MAWP of the vessel was governed by the MAWP of this cylindrical shell; the shell also had a liquid level specified. The nozzle design option was set to design nozzles for "Chamber MAWP".

The issue is that the MAWP of the shell (the cylinder in this case, but could be a head or cone) is based on the pressure on the inside surface of the shell. But COMPRESS designs nozzles based on the pressure at the outside surface of the shell. When a liquid level exists there will be a slight difference in static pressure between the inside surface of the shell and the outside surface of the shell; the nozzle calculations are based on this slightly higher pressure.

If (1) the shell component to which a nozzle is attached limits the chamber MAWP, (2) shell contains a liquid level, (3) the nozzle design mode is set to "Chamber MAWP" (or "Larger of Chamber MAWP & MAP"), and (4) the static pressure at the nozzle is greater than that for which the shell was designed, then the nozzle must be designed for a pressure that is slightly greater than the MAWP of the shell component.

In this case, the required thickness of the shell, "tr", in the UG-37 calculations will exceed the actual thickness of the shell...the thickness on which the MAWP was determined! And the shell thickness will be inadequate for this pressure because the nozzle pressure will always be greater than the MAWP of the shell. Thus we have a nice logical conflict going on.

The only time this may not happen is when the nozzle is in the central part of the bottom head and "tr" benefits from the more lenient definition of "tr" given in UG-37(a).

My suggestion in such cases is to change the nozzle design/reporting mode back to "Design P, Find Nozzle MAWP & MAP" and check the nozzle MAWP versus what would otherwise be the chamber MAWP (the smallest of the MAWP of the main pressure components). The difference is likely to be very small (it will be only the slight difference in static head pressure across the thickness of the chamber MAWP limiting component. In your example, the difference was about 0.1 psi. In general, this should be acceptable to the vessel owner.

Tom Barsh
Codeware Technical Support

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