Considerations on the verification of saddles according to the Zick method

The calculation of saddles for horizontal vessels in the ASME code is based on Division 2 chapter 4.15.3 SADDLE SUPPORTS FOR HORIZONTAL VESSELS. This method is in turn taken from the publication of 1951 "Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports" by LP Zick.

Following several reports received from many customers we realized that the code is not very clear in identifying two values, namely b (width of contact surface of the cylindrical shell and saddle support) and b1 (width of the reinforcing plate welded to the cylindrical shell at the saddle location). It is not clear if b represents the width of the cradle (wear plate) that is normally present in the saddle or if this value is represented by b1. We have therefore observed that many customers in doubt consider the cradle as if it were the saddle reinforcement plate and insert its dimensions into the properties related to the reinforcing plate. In this way, however, a further doubt is introduced, that is, which dimension to consider for the value of b between the width of the main rib (the so-called web plate) and the length of the crosspieces perpendicular to the latter when present (ribs).

Figure 4.15.2 unfortunately does not help to clarify exactly what the code means:

Our opinion is that b1 represents the width of the reinforcement plate and not of the wear plate. The code requires that b1 be at least equal to b + 1.56√(Rm)t, which would lead to a minimum width of the cradle too large if b is the length of the perpendicular ribs. Moreover, if the reinforcing plate is also present, it can not be taken into account in the calculation, while the angle θ1 is clearly identified as "opening of the cylindrical shell arc engaged by a welded reinforcing plate". Since version 2018.0 we have therefore decided to modify the program so that it is clearer for users and we have expressly introduced two new properties, the width of the cradle and its thickness. The width of the cradle will be used as the dimension of b, while b1 will continue to be the width of the reinforcement plate, if it is enabled. In addition, the cradle is now also displayed in the 3D model, so that these changes are also visually clear. Since in the versions prior to 2018 b was the width of the central rib (web plate) these changes could lead to a validation error of width b1 when items created with previous versions are opened. In this case it will suffice to consider the above and disable the reinforcement plate in case it has been enabled to represent the cradle, then enter the dimensions of the cradle in the two new properties.

Author: Sergio Lidonnici
Permalink: KB082797
Published on: 05 Feb 2018

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