## 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.

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.

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

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

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

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

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

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

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.