Engineering guide
This module calculates horizontal cylindrical vessels on saddle supports to DIN EN 13445-3 clause 16.8. It covers vessels without a reinforcement ring on two or more saddles — the standard design for horizontal storage tanks, heat exchangers, and process vessels in plant engineering. The vessel acts as a beam on several supports: self-weight and contents generate global bending moments and shear forces, while the saddles additionally introduce local stresses into the shell.
The standard carries out the verifications separately for the governing locations: between the saddles, the longitudinal stresses from the global bending moment are limited (clause 16.8.6); at the saddle, the combined global and local loads on the shell are verified (16.8.7); and in addition, the load-bearing capacity at the saddle horn and the saddle plate is checked (16.8.8). For thin-walled vessels, stability against buckling under longitudinal compressive stresses must also be considered.
Anyone who wants to calculate a horizontal vessel on saddle supports obtains with this module the complete code-compliant verification to DIN EN 13445-3 — including the influence of saddle position, saddle angle, and, where applicable, a reinforcing plate underneath on the local stresses.
Scope
- Horizontal vessels on saddles
Typical workflow
Enter vessel and saddle geometry
The diameter, wall thickness, and length of the cylindrical shell including the heads are entered, plus the number and position of the saddles (distance from the vessel end), the saddle contact angle, and the width of the saddle plate and of any reinforcing plate.
Determine the loads
Governing is the weight of the vessel with its contents — usually the water fill level during the pressure test as the most unfavorable case — superimposed where applicable with internal or external pressure and additional loads.
Determine the internal forces as a beam
The vessel is treated as a beam on the saddle supports; from this follow the global bending moments at mid-span and above the saddles as well as the shear forces at the supports.
Carry out the verification between the saddles
Per clause 16.8.6, the longitudinal stresses from bending moment and pressure in the shell between the saddles are verified against the allowable stress and against the buckling limit for longitudinal compression.
Carry out the verification at the saddle
Per clauses 16.8.7 and 16.8.8, the local loads at the support are checked: combined longitudinal and circumferential stresses at the governing points of the saddle horn and in the support region, including the effect of a reinforcing plate.
Adjust the design
If limits are exceeded, a larger saddle angle, a wider or thicker reinforcing plate, a saddle position closer to the heads, or a greater wall thickness will help; the verification is then repeated.
Frequently asked questions
Why should the saddles be placed as close to the heads as possible?
The dished heads locally stiffen the cylindrical shell. If the saddle stands within the stiffening influence of the head, the local circumferential bending stresses at the saddle horn are significantly reduced. At the same time, too large a saddle spacing increases the mid-span moment between the saddles — the saddle position is therefore always a compromise, which the verification quantifies.
Which load case is typically governing?
Usually the hydrostatic pressure test with complete water filling, because the fill weight is then greatest — even for vessels that only contain gas or light media in service. In addition, the vacuum case can become critical, since external pressure and saddle-induced longitudinal compressive stresses jointly challenge the buckling safety.
When is a reinforcing plate under the saddle required?
When the local stresses at the saddle horn exceed the allowable values but the wall thickness is not to be increased. The reinforcing plate is effective only if it extends sufficiently far beyond the saddle horn and is welded to the shell; its creditable effect is regulated in the standard. Alternatively, a larger contact angle helps (120° is common).
Does the module also cover vessels with reinforcement rings?
No, the module covers vessels without a reinforcement ring per clause 16.8. Ring-stiffened horizontal vessels, in which the saddle forces are distributed via rings, are a separate application with a different calculation procedure.
Technical content source: LV-Solved-Problems-Content 628d313247e7