Saddle Calculation L.P. Zick vs Compress

[Ehiman1]

Hello,
I was studying calculation of stress in vessels on two saddles but I can't understand the first formula!
If I read "Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports" by L.P. Zick you can see the value of longitudinal stress S1 from the following formula:

In Compress this formula is different, because you have to divide "Lenght/12", and I can't understand why!

Does someone can explain me why there is this difference between Compress and L.P. Zick formula?


Many Thanks

 

[JStephen]

I would assume a unit conversion from feet to inches or vice versa, depending on how they define L and R.
Also, there have been some updates made to the original Zick paper, some by Zick himself, I believe, others by various pressure vessel handbook authors, so you may have to compare several sources to make sense of everything. User notes/help on the program may help as well.

 

[Ehiman1]

Thanks JStephen, the problem is I use S.I. metric so I have mm and not feet or inches!
I am thinking Compress formula is wrong! Because if I calculate in S.I. system I don't need to divide L/12.

 

[Geoff13]

In Zick's paper L is the length in feet, so if Compress has L in inches then it looks that's what they're doing.

However, there is very likely a unit conversion hidden in the "3" of Zick's equation. Since L is in feet but S₁ is in psi there should be a 12 inches/foot conversion hidden in the constant. I would run the calc once with your values converted to the units used in Zick's paper, and once with your metric units. It's likely the answers won't match. I would extend this philosphy to the entire Zick paper.

As an example of hidden units in the constants of a formula I would suggest reviewing the shell thickness formula in API 650. In Imperial it's 2.6D(H-1)G/Sd but in SI it changes to 4.9D(H-0.3)G/Sd.

 

[JStephen]

In looking at Zick's paper, it looks like he divides moment in foot-lbs by section modulus in inches-cubed. There is a 12 factor incorporated for the units, which gets broken up into a factor of 3 in the equation and a factor of 4 included in the definition of K1.

 

[Ehiman1]

Many thanks!
I have another question to you. I don't understand how can I find out the following value of K1'

It isn't K1 of S1 and I don't find anything from L.P. Zick.

Did someone find the meaning of this coefficient?

 

[BJI]

You could also review the method in PD5500 Annex G, it is in metric units and includes relevant updates.