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4: Location of Supports and Restraints

The scope of the Pipe Support function is stated in the relevant clause of the applicable Piping code,repeated here verbatim:

321.1.1 Objectives (From ANSI B31.3 code)

The layout and design of piping and its supporting elements shall be directed towardpreventing the following:

(1) Piping stresses in excess of those permitted in the Code;

(2) Leakage at joints;

(3) Excessive thrusts and moments on connected equipment (such as pumps andturbines);

(4) Excessive stresses in the supporting or restraining elements;

(5) Resonance with imposed or fluid-induced vibrations;

(6) Excessive interference with thermal expansion and contraction in a piping systemwhich is otherwise adequately flexible;

(7) Unintentional disengagement of piping from its supports;

(8) Excessive piping sag in systems requiring drainage slope;

(9) Excessive distortion or sag of piping (e.g. thermoplastics) subject to creep underconditions of repeated thermal cycling;

(10) Excessive heat flow, exposing supporting elements to temperature extremesoutside their design limits.

Placing Dead Weight SupportsGuidelines for placing deadweight supports

Locate dead weight supports using recommended spacing from the code (B31 etc.).

Consider existing support points.

Decrease span by half off equipment.

Decrease span for concentrated loads.

Support concentrated loads.

Support offset loads.

Decrease span for extra lagging or insulation.

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Locate supports at changes in direction (no overhung corners, top or bottom of risers).

Select type (rigid, spring, or constant support) based on thermal expansion analysis.

Preferred Attachment to "Structure"Guidelines for dealing with structures when connected with piping.

Apply loads to columns and beams near main-member intersections to minimize bendingeffects.

Avoid the introduction of unnecessary torsion or lateral bending effects.

Avoid the introduction of movements or transverse loading to slender members (such aswind bracing) and particularly to compression members where instability controls the design.

Confine connections to an independent structure or a foundation when dealing with pipingsubject to pulsating flow or transmitted mechanical vibration, unless a careful andcomprehensive analysis assures that the structures, buildings, etc., are of adequate strength

with nonresonant frequency and sufficient stiffness to control amplitude within the boundsrequired by general comfort level of personnel.

Provide anchors and extremely flexible and nonresonant intervening pipe runs (e.g.,expansion joints) to machinery that introduces mechanical vibrations, in order to isolate theeffect by reducing transmissibility.

Preferred Points of Attachment to PipeGuidelines for selecting preferred attachment points on piping

On a pipe rather than on piping components such as valves, fittings, or expansion joints.Under highly localized loading, flanged or threaded joints may leak and valve bodies may

distort with resulting seat leakage or binding. Attachments to heavy components, however,may be acceptable and even desirable where the effect can be properly provided for.

On straight runs rather than on sharp radius bends or welding elbows, since these arealready subjected to highly localized stresses on which the local effects of the attachmentwould be superimposed. Furthermore, attachments on curved pipe which extend well alongthe length or circumference of the bend will seriously alter the flexibility of the component.

On pipe runs which do not require frequent removals for cleaning and maintenance work.

As close as practical to heavy load concentrations such as vertical runs, branch lines,motor operated or otherwise heavy valves, and minor vessels such as separators, strainers,

etc.

Article Excerpted From SST 101: Piping Stress Analysis Seminar Notes.

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