Engineering guide
Module BS55 calculates horizontal cylindrical vessels on saddle supports to the British pressure vessel standard BS 5500 (1985 edition), taking additional forces into account. This edition of the standard has been withdrawn — its successor is PD 5500 (Specification for unfired fusion welded pressure vessels), and in Europe additionally DIN EN 13445-3 with its clause on saddle supports. For re-rating existing vessels designed to BS 5500 and for comparative calculations, the method remains fully relevant.
The calculation follows the classical approach of saddle support analysis (after Zick): the vessel is treated as a beam on two supports, loaded by dead weight, contents and saddle reaction. From the cylinder length, mean radius, wall thickness, depth of the dished ends, saddle spacing, saddle width and contact angle, the module determines the bending moments at mid-span and at the saddle plane and, from these, the longitudinal stresses (at the top and bottom in each case), the shear stresses and the circumferential stresses at the saddle horn and in the support region — superimposed with the stresses from the design pressure.
Whether the saddles are welded to the vessel enters as a separate boundary condition, since this changes the shear transfer and the local loading at the saddle. This makes it possible to calculate a horizontal vessel on saddle supports and verify the acceptability of all stress components in a single run.
Typical workflow
Capture geometry and support arrangement
The cylinder length including the straight flange, the mean vessel radius, the wall thickness and the depth of the dished ends are entered, together with the position of the saddles (distance to the dished end), the saddle width and the contact angle. It is also specified whether saddle and vessel are welded together.
Define the loads
The saddle reaction follows from dead weight, operating or test contents and any additional forces; the design pressure supplies the superimposed membrane stresses. In the hydrostatic test, the vessel completely filled with water is often the governing load case.
Calculate the bending moments as a beam on two supports
The vessel is idealized as a beam with the dished ends as load-carrying end sections. The bending moment at mid-span (span moment) and the moment at the saddle plane (support moment) are calculated as functions of saddle spacing and vessel length.
Determine longitudinal and shear stresses
From the moments follow the longitudinal stresses at mid-span and at the saddle plane, at the top and bottom in each case, whereby at the saddle plane only the effective portion of the shell participates unless the cross-section there is stiffened by the ends or by rings. In addition, the shear stress in the transition region at the saddle is calculated.
Verify the circumferential stresses at the saddle
At the lowest point of the support and at the saddle horn (edge of the saddle), local circumferential stresses arise that depend on the contact angle, the saddle width and whether the saddle is welded on. All stress components are compared with the allowable values of the standard; exceedances lead to a larger contact angle, wear plates or stiffening rings.
Frequently asked questions
Why is the saddle horn the critical location?
At the edge of the saddle (saddle horn), the supported shell passes abruptly into the unsupported region. The circumferential bending from the local load introduction concentrates there; the stresses rise sharply with smaller contact angle and thinner wall. In practice, saddle supports almost always fail at the saddle horn — remedies are larger contact angles (typically 120°), wear plates with sufficient overhang, or stiffening rings.
What role does the distance of the saddles from the ends play?
Dished ends stiffen the cylinder cross-section. If the saddle is close to the end (rule of thumb: distance less than about half the radius), the end participates in carrying the load, the cross-section at the saddle plane remains round, and the longitudinal and shear stresses are more favorable. Saddles positioned far inboard reduce the span moment at mid-span but increase the support moment and the ovalization — the saddle position is always a compromise between both effects.
What changes if saddle and vessel are welded together?
A welded saddle transfers shear forces continuously into the shell and prevents relative displacements; the standard applies different coefficients for shear and circumferential stresses than for a loosely resting vessel. Note: with two saddles, only one saddle may be welded or fixed as the fixed support — the second needs slotted holes so that the thermal expansion of the vessel does not generate restraint forces.
Is a calculation to BS 5500:1985 still acceptable today?
BS 5500 in its 1985 edition has been withdrawn; it lives on as PD 5500, whose saddle clause is methodically based on the same Zick approach. For new builds in the EU, DIN EN 13445-3 (Clause 16.8) or another recognized code generally applies. For assessing and re-rating existing vessels whose design documentation is based on BS 5500, calculating to the original standard remains sensible and common practice.
Technical content source: LV-Solved-Problems-Content 628d313247e7