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# Dimple Jacket

## Dimple Jacket

(OP)
How do you calculate the required thickness of a tank shell or head under external pressure from steam in a dimple jacket.

### RE: Dimple Jacket

Please read UW-19(c) which will take you to UG-47 (braced and stayed surfaces), UNLESS you are designing to Appendix 17 or are conducting a UG-101 proof test. Also see Appendix 17-5(b) for guidance.

There are three kinds of people in this world; those who can count and those who can't.

### RE: Dimple Jacket

(OP)
CodeJackal,

I guess I am the third kind.  Is the pressure in the equations in UG-47a and App.17-5b the pressure in the tank or the pressure in the jacket?

### RE: Dimple Jacket

First, the counting thing is just my signature (because I like it so much)
Let me describe it this way......
First let me assume this is an ASME Section VIII Division 1 design. If it is not,.....never mind....

Dimpled or Embossed Assemblies, which consist of a dimpled (embossed) plate welded to another like or plain plate (un-dimpled) and for which the welded attachment is made by fillet welds around holes or slots, shall be constructed in accordance with the requirements of UW-19(c). The rules of Appendix 17 are similar but are applicable only to “weld through” processes. UW-19 (c) utilizes the following for calculating the thickness of plates of “fillet welded” embossed assemblies.:
1.    Dimple-to-Dimple Plate thickness, per UW-19(c)(2), established in accordance with UG-101 [Proof Test]
2.    Plain-to-Plain Plate thickness, per UW-19(c)(3), established in accordance with UG-47 (a) [Braced and Stayed Surfaces]
3.    Dimple to Plain Plate thickness, where the dimpled plate (external jacket) is established per UW-19(c)(2) and the Plain Plate (inner shell) established per UG-47 (a). Of course this is in addition to the UG-27 calculations which may govern. Many variables and configurations exist.

Depending on the configuration you may want to perform UG-28 calculations. When calculating the inner shell thickness for either a negative pressure or for differential pressure, such as may occur during operation of the vessel when the jacket has a higher pressure than the vessel, you may take credit for the stiffening resulting from the jacket welds (plain or dimple). This is accomplished through U-2 (g) and may need to be invoked to address the loadings specified in UG-19 (a) and UG-22 thus complying with the requirements of UG-28 and UG-29. Dimple jacket external pressure design is described in UG-101 (p)

There are three kinds of people in this world; those who can count and those who can't.

### RE: Dimple Jacket

(OP)
It sounds like you are telling me the pressure used in UG-47 (a) is the jacket pressure. What type of configuration would require UG-28 calculations?

### RE: Dimple Jacket

Yes, UG-47(a) is the jacket pressure.

UG-47 (c) clears it up by stating the following:

(c) If a stayed jacket extends completely around a
cylindrical or spherical vessel, or completely covers a
formed head, it shall meet the requirements given in
(a) above, and shall also meet the applicable requirements
for shells or heads in UG-27(c) and (d) and
UG-32. In addition, where any nozzle or other opening
penetrates the cylindrical or spherical vessel, or completely
covered head, and the jacket, the vessel or
formed head shall be designed in accordance with UG-
37(d)(2).

So in addition to the internal pressure calculations of UG-27 and UG-32 etc… the UG-47(a) calculation is also required. A dimple jacket is classified as a stayed jacket in UW-19 (c).

I will have to agree that this is a very difficult and confusing area of the VIII-1 Code. If it only were more clear. The interpretations on UW-19 I have found are quite old and out of date. It has been a while since I looked at this and I would appreciate others who are currently designing and building dimple jackets for Section VIII-1 vessels to chime in. I have seen manufacturers perform UG-28 external pressure calculations in addition to all the above and in most every case, it was not the governing thickness. But, I would not know what configuration would cause these rules to go out of whack. Is there such a case? Should it be clearer? Perhaps the way the dimple jackets stayed construction loads the inner shell does not affect its wall thickness? This is my personal interpretation the code. If there is disagreement, I would welcome the discussion.

There are three kinds of people in this world; those who can count and those who can't.

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