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# Difference between design thickness and UCS-66 governing thickness

## Difference between design thickness and UCS-66 governing thickness

(OP)
Hi,

I just started using COMPRESS software. I'm trying to work on vertical cylinder vessel. Let me write down a formula so it would be more convenient to explain my problem:
-----------------------------------
Design thickness for internal pressure, (Corroded at 122 °F) Appendix 1-4(d)
t = P*Lo*M / (2*S*E + P*(M - 0.2)) + Corrosion
-----------------------------------
While calculating minimum design thickness t, we consider Lo = crown radius (L) + minimum thickness (based on UCS-66). now I'm confused here why do we use a thickness based on UCS-66. Shouldn't it be like this?
Lo = crown radius (L) + minimum thickness (t, one we are calculating) and just need further simplification.

This is my 1st post so if you need further information to understand my question, please let me know.
Replies continue below

### RE: Difference between design thickness and UCS-66 governing thickness

If you are calculating a torispherical head, Compress uses for calculation Lo as the inside Crown Radius you specified on the menu plus the minimum thickness after forming that you also specified on the commented F&D (exclusive of corrosion allowance and your calculations are made based on external dimensions) which is exactly what you commented it should be, i don´t understand where do you see the thickness based on UCS-66 is used? on Compress calcs? because for heads normally the UCS-66 goberning thickness is the straight flange thickness, unless you have a head that requires a thickner crown-knucle region than the straight flange zone.

### RE: Difference between design thickness and UCS-66 governing thickness

(OP)
I'm being told that:
Lo = crown radius (L) + minimum thickness, where min thickness is user defined thickness not based on UCS-66.
But if you see Compress generated report at the beginning, you will find that:

UCS-66 governing thickness = ------ inch

So I got confused. But I guess this number is correspond to user defined thickness. now to my understanding there is some pre-defined minimum thickness, may be based on material or internal pressure, below which you can't define thickness in the input parameter window (even before you perform calculation, this limit is imposed). And I'm trying to figure out what part of this code determines this limit.

Again my first question is if we are calculating a design thickness based on internal pressure, then why do we use another pre-defined thickness in the same calculation. I think, these both thicknesses refer to the same component which is bottom head.

I apologize for this complicated description, but I'm trying my best to explain my dilemma.

### RE: Difference between design thickness and UCS-66 governing thickness

The first sentence in UCS-66 says "......shall be used to establish impact testing exemptions....". I think the governing thickness statement referes to minimum thickness fro impact testing requirements. The minimum thickness in the head equation refer to thickness due to other conditions such as pressure, temperature, material..etc.

### RE: Difference between design thickness and UCS-66 governing thickness

(OP)
@ Bobfromoh,

That's what I thought. But the funny part with this software is that when min thick is lets say 0.50 inch and if we choose it to be 0.52 inch then report shows UCS-66 governing thickness = 0.52 in. In head equations, while designing thickness based on internal pressure why does it consider another thickness?

### RE: Difference between design thickness and UCS-66 governing thickness

"UCS-66 governing thickness" means that when checking ucs-66 impact requirements this thickness is used. That thickness should be the actual thickness of the plate not the required min thk.(read UCS-66 carefully). if you enter .50in compress uses.50in. if you enter .52in compress uses .052in. Plain and simple

### RE: Difference between design thickness and UCS-66 governing thickness

and an addition for first post.

also when calculating Lo actual thickness should be used (minimum thk. after forming, not required minimum thk.)you are not manufacturing head from a required min thk. plate. you use something thicker. And you enter this thickness into compress.

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