The document summarizes Zick's analysis for stresses induced in large horizontal cylindrical pressure vessels supported on two saddles. It outlines the longitudinal bending stress, tangential shear stress, and circumferential stress at the saddle horn that Zick analyzed. It also discusses the additional stress in vessel heads used as stiffeners. The document provides equations to calculate these stresses and outlines design procedures for locating saddles, including selecting saddle angles and distances. It lists typical saddle part designs including webs, base plates, ribs, and wear plates.

1. Stresses in Large Horizontal
Cylindrical Pressure Vessels on Two
Saddle Supports (Zick Analysis)
MAHENDRA PRABHU SUBRAMANI
PRINCIPAL MECHANICAL ENGINEER

2. Table of contents
 Zick’s Analysis
 Nomenclature
 Assumptions
 Longitudinal bending stress
 Tangential shear stress
 Circumferential stress at the horn of the saddle
 Additional stress in head
 Procedure for locating saddle
 Design of saddle parts
 Reference

3. The Zick analyzed the following stresses induced in the shell because of
saddle support.
1.Longitudinal bending stress in shell
2.Tangential shear stress
3.Circumferential stress in shell at the horn of the saddle
4.Additional stress in head used as stiffener
Zick’s analysis

4. ts – thickness of shell
th – thickness of head
L – tan length of shell
A – distance of saddle center from head
H – depth of head
Ls – distance between saddles
B – distance between vessel centerline to base plate
Nomenclature

5. Assumptions
Horizontal vessels act as beams with the following exceptions
 Loading conditions vary for full or partial full vessels
 Stress varying according to saddle angle and distance ‘A’
 Load due to weight is combined with other loads

6. Longitudinal bending stress

7. Cont…
 The maximum longitudinal bending stress is given by S1 =
3QLK1/pr2t (compression at bottom and tension at the top of shell)
 The tension stress when combined with the axial stress due to
internal pressure should not exceed the allowable stress times
efficiency
 The compression stress should not exceed one-half of the
compression yield point of the material

8. Tangential shear stress

9. Cont….
 The maximum tangential shear at the plane of saddle is given by S2
= K3Q*{L-2A-H}/rt{L+H}
 The tangential shear stress should not exceed 0.8 times of the
allowable stress

10. Circumferential stress at the horn of the saddle

11. Cont…
 The maximum circumferential combined stress should be less than
1.5 times the allowable tensile stress
 Weld seams in the shell should not be located near the horn of the
saddle where maximum moment occurs.

12. Additional stress in the head
 The stresses are caused due to the varying load in heads
 The maximum stress should be less than 1.5 times the average
stress
 For design purpose the combined stress be allowed to be 25%
greater than the allowable tension stress

13. Procedure for locating saddle
 Trial 1: Set A=0.2L and saddle angle = 120
 Trail 2: Increase the saddle angle to 150
 Trail 3: Move saddles near heads A=R/2 and saddle angle=120
 Trail 4: Increase saddle angle to 150 and recheck the stress
 Trail 5: Move saddles to A=0.2L and saddle angle=120 and design
stiffeners in the plane of saddles

14. Design of saddle parts
The typical saddle comprises of
following parts
 Web
 Base plate
 Ribs
 Wear plate
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15. References
 “ Stresses in large horizontal cylindrical pressure vessels on two
saddle supports” By L.P.Zick
 “ Pressure vessel design manual” by Dennis R.Moss
 “ Equipment design “ By Lloyd E. Brownell and Edwin H. Young
 “Pressure vessel hand book” by Megessey
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