APPENDIX GSUGGESTED GOOD PRACTICE REGARDING

PIPING REACTIONS AND DESIGN OF SUPPORTSAND ATTACHMENTS

G-1

A vessel supported in a vertical or horizontal positionwill have concentrated loads imposed on the shell inthe region where the supports are attached. Primaryand secondary stresses due to other loadings, such asthe weight of water present for hydrostatic test, mayexceed that due to normal internal pressure. Calculationsto resist the forces involved are not given here becausethey involve so many variables depending upon thesize and weight of vessels, the temperature of service,the internal pressure, the arrangement of the supportingstructure, and the piping attached to the vessel asinstalled.

G-2

The details of supports should conform to goodstructural practice, bearing in mind the following items(see Manual for Steel Construction,latest edition, bythe American Institute of Steel Construction).

(a) All supports should be designed to prevent exces-sive localized stresses due to temperature changes inthe vessel or deformations produced by the internalpressure.

(b) External stays in ring girders, or any internalframing that may support other internal parts, may alsoexert a stiffening effect on the shell.

(c) Columns supporting field assembled vessels andbearing loads which may produce high secondarystresses in the vessel wall should be so designed atthe attachment to the wall that no high stress concentra-tion can occur near changes in shape, gusset plates ifany, or at ends of attachment welds. It is preferableto use details permitting continuous welds extendingcompletely around the periphery of the attachment andto avoid intermittent or deadend welds at which there

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may be local stress concentration. A thicker wall plateat the support may serve to reduce secondary stressesand, if desired, a complete ring of thicker wall platesmay be installed.

(d) When superimposed forces on the vessel walloccurring at the attachment for principal struts or gussetsand supports of any kind can produce high bendingstresses, and when thicker wall plates do not seemappropriate, an oval or circular reinforcing plate maybe used. The attachment of such reinforcing platesshould be designed to minimize flexing of the plateunder forces normal to the surface of the vessel.

G-3

Vertical vessels may be supported on a number ofposts without substantial ring girder bracing them aroundthe shell, provided they attach to the shell where thelatter is reinforced in an equivalent manner by the headof the vessel or by an intermediate partition.

G-4

Where vertical vessels are supported by lugs, legs,or brackets attached to the shell, the supporting membersunder these bearing attachments should be as close tothe shell as possible to minimize local bending stressesin the shell.

G-5

For large and heavy vertical vessels to be supportedby skirts, the conditions of loading under hydrostatictests, before pressure is applied, or for any possiblecombination of loadings (see UG-22) under the highest

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G-5 1998 SECTION VIII — DIVISION 1 G-9

expected metal temperature in service for the normaloperating pressure, shall be compared in determiningthe best location for the line of skirt attachment. Inapplying UG-22 and UG-23(a) to vertical vessels sup-ported on skirts, the following shall be considered inaddition to pressure effects:

(a) the skirt reaction:(1) the weight of vessel and contents transmitted

in compression to the skirt by the shell above the levelof the skirt attachment;

(2) the weight of vessel and contents transmittedto the skirt by the weight in the shell below the levelof skirt attachment;

(3) the load due to externally applied moments andforces when these are a factor, e.g., wind, earthquake, orpiping loads.

(b) the stress in the vessel wall due to the effectsenumerated in (a) above. Localized longitudinal bendingand circumferential compressive stresses of high ordermay exist in the metal of the shell and skirt near thecircle of the skirt attachment if the skirt reaction isnot substantially tangent to the vessel wall. When theskirt is attached below the head tangent line, localizedstresses are introduced in proportion to the componentof the skirt reaction which is normal to the head surfaceat the point of attachment; when the mean diameterof skirt and shell approximately coincide and a generousknuckle radius is used (e.g., a 2:1 ellipsoidal head), thelocalized stresses are minimized and are not consideredobjectionable. In other cases an investigation of localeffects may be warranted depending on the magnitudeof the loading, location of skirt attachment, etc., andan additional thickness of vessel wall or compressionrings may be necessary.

G-6

Horizontal vessels may be supported by means ofsaddles1 or equivalent leg supports. For other than verysmall vessels, the bearing afforded by the saddles shallextend over at least one-third of the circumference ofthe shell.

Supports should be as few in number as possible,preferably two in the length of the vessel. The vessel

1 See “Stresses in Large Cylindrical Pressure Vessels on Two SaddleSupports,” p. 959,Pressure Vessels and Piping: Design and Analysis,A Decade of Progress,Volume Two, published by ASME.

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may be reinforced by stiffening rings at intermediatesections.2

G-7

Large horizontal storage tanks for gases under pres-sure may be supported by any combination of hangers,with ring girders, stiffeners, and such other reinforce-ment as may be necessary to prevent stresses in theshell in excess of those allowed by UG-23 and toprevent excessive distortion due to the weight of thevessel when the internal pressure is near atmospheric.

G-8

Certain attachments may serve to mount a pump,compressor, motor, internal combustion engine, mixer,or any other rotating or reciprocating equipment upona vessel. Such equipment can cause cyclic forces toact upon the attachment, upon the attachment weld tothe vessel, upon the vessel shell, and upon the vesselsupports. For such cyclic loading, the practices advo-cated in G-2(c) and (d) above are of particular impor-tance. It is important to avoid resonance between thecyclic forces imposed by the equipment and the naturalfrequency of the vessel with the equipment in place.

G-9

Additional guidance on the design of supports, attach-ments and piping reactions may be found in the follow-ing references:

(a) British Standard BS-5500, Specification for Fu-sion Welded Pressure Vessels (Advanced Design andConstruction) for Use in the Chemical, Petroleum, andAllied Industries;

(b) Welding Research Council Bulletin #107, LocalStresses in Spherical and Cylindrical Shells Due toExternal Loadings;

(c) Welding Research Council Bulletin #198, Part1, Secondary Stress Indices for Integral Structural At-tachments to Straight Pipes; Part 2, Stress Indices atLug Supports on Piping Systems;

(d) Welding Research Council Bulletin 297, LocalStresses in Spherical and Cylindrical Shells Due toExternal Loadings, Supplement to WRC-107.

2 See Transactions ASCE, Volume 98 — 1931 “Design of LargePipe Lines.”

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