UG-45 calculation of tb1 - different software results

[marty007]

I was reviewing a client's calculations that were done in DesignCalcs, and came across a UG-45 result that didn't seem quite right. So, I fired up Compress to see how it handled this calculation and got a different result.

For the vessel in question, we have two different corrosion allowances, CA-shell and CA-nozzle, to account for additional erosion in the shell (so CA-shell is higher).

When the two software packages calculate tb1 (UG-45), they use different corrosion allowances:
DesignCalcs - applies CA-nozzle.
Compress - applies CA-shell.

In the end, DesignCalcs would pass a sch std nozzle, while compress would require a sch80 nozzle.

I tend to agree with the Compress interpretation, but would appreciate anyone else's insights.

Cheers,
Martin

 

[Karloss12]

How do you come to your conclusion?

It sounds a bit counter intuative when thinking of adding a corrosion allowance to a stainless steel nozzle that is welded into a carbon steel shell.

But in the step after calculating tb1, the nozzle can not be thinner than the lesser of the new shell (tb1) or stainless steel Schd40 pipe. As a result a large new shell thickness is removed from the minimum nozzle neck calculation.

What is the purpose of UG-45? I interpret is as addressing peak stresses due to small nozzle nozzle loads (and possibly for differential expansion between the nozzle neck and the shell opening diameeter). By ensuring the minimum thickness of the nozzle neck is atleast the thickness of the new shell or the thickness of Schdule 40 pipe, these peak stresses relating to small nozzle loads and differential expansion are addressed.

 

[SnTMan]

marty007 in my opinion the correct way to do this calculations is to assign to each component its corrosion allowance. So for calculation of:
ta, nozzle CA is added
tb1, tb2, the shell CA is added
tb3, nozzle CA is added

Can't "prove" this of course, but generally speaking CA's are handled on a component-by-component basis elsewhere in the Code.

My intrepretation of the purpose of UG-45 is to provide some degree of mechanical strength in piping connections. Note the lesser requrements for manways and inspection openings.

Regards,

Mike