Pipe Support Span Design

1. Pipe Support Span Overview

Pipe support span design determines the maximum distance between supports for above-ground piping systems. The support spacing must ensure that bending stresses from weight loads remain within code allowable limits and that pipe deflection does not exceed acceptable criteria for appearance, drainage, and mechanical integrity.

Governing Criteria

The allowable span is determined by the more restrictive of two independent criteria:

• Stress criterion: The bending stress at supports (for continuous beams) or at midspan (for simply-supported spans) must not exceed the code-allowable sustained stress. Per ASME B31.3, sustained stresses from weight and pressure are limited by the allowable stress at temperature (S_h).
• Deflection criterion: The maximum midspan deflection must not exceed the project-specified limit. Common limits are L/240 for process piping and L/360 for piping carrying instruments or connected to sensitive equipment.

For standard carbon steel pipe in gas or vapor service, the stress criterion typically governs for small pipe sizes (NPS 2 and below) while the deflection criterion governs for larger sizes (NPS 6 and above). For liquid-filled lines, both criteria should be checked.

Common Design Scenarios

2. Weight Load Calculations

The total distributed weight load on a pipe span is the sum of all component weights per unit length. Accurate weight calculation is essential because the allowable span is inversely proportional to the square root of the load.

Weight Components

Pipe Weight

Bare pipe weight is obtained from pipe schedule tables. Common values for carbon steel (A106 Gr. B or API 5L Gr. B):

Contents Weight

Insulation Weight

Concentrated Loads

Valves, flanges, and other heavy fittings create concentrated loads. These are typically handled by either reducing the span locally or adding supports adjacent to the heavy item:

Weights are approximate for Class 150. Higher pressure classes are significantly heavier.

3. Beam Formulas

Pipe spans are modeled as beams with uniformly distributed load. The beam end conditions determine the bending moment distribution and deflection. Two models are used depending on the number of spans and support conditions.

Simply-Supported Beam (Single Span)

Continuous Beam (Multiple Spans)

Allowable Span from Stress Criterion

Allowable Span from Deflection Criterion

Modulus of Elasticity at Temperature

The modulus of elasticity decreases with temperature, which increases deflection at elevated temperatures:

4. Support Types

Pipe supports are categorized by the type of restraint they provide: weight support only (rest/shoe), lateral restraint (guide), and full restraint in all directions (anchor). The selection depends on the piping layout, thermal movement, and loading conditions.

Weight Supports (Rests and Shoes)

• Bare pipe rest: Pipe sits directly on a structural member (beam, channel, angle). Only for uninsulated pipe. Allows axial and lateral sliding.
• Pipe shoe: A welded saddle or T-section attached to the pipe that rests on the structural support. Elevates the pipe to protect insulation and provides a defined bearing surface. Standard heights: 3, 4, 6, 8, or 12 inches.
• Pipe roller: A roller bearing support that allows free axial movement with minimal friction. Used for long runs with significant thermal expansion.
• Spring hanger: A variable-spring or constant-support hanger for vertical pipe runs or where differential thermal movement between the pipe and structure must be accommodated. Sized based on hot and cold load conditions.
• Pipe saddle (trunnion): A half-pipe or fabricated cradle welded to the pipe and resting on a support. Common for large-diameter pipe (NPS 16 and above).

Guides and Lateral Restraints

• Pipe guide: Allows axial movement but restricts lateral movement. Essential near expansion loops, bends, and equipment connections to control piping flexibility.
• Line stop: Restricts axial movement in one direction. Used at changes of direction to direct expansion toward expansion devices.
• Snubber (dynamic restraint): Allows slow thermal movement but resists sudden dynamic loads (e.g., slug flow, seismic, relief valve thrust). Hydraulic or mechanical types available.

Anchors

Anchors are fixed points that resist all forces and moments. They divide a piping system into independent sections for flexibility analysis:

• Equipment connections: Pumps, compressors, vessels, and heat exchangers are typically treated as anchor points. Equipment nozzle loads must be checked against allowable values.
• Structural anchors: Welded or bolted connections that restrain the pipe in all six degrees of freedom. Used at changes of direction, branch connections, and midpoints of long straight runs.
• Intermediate anchors: Break long pipe runs into shorter, independent expansion sections. Placed between expansion loops or bellows.

Guide Spacing

Guides are placed to prevent lateral buckling and to direct thermal movement. Typical guide spacing rules:

5. Thermal Movement

Thermal expansion and contraction of piping from ambient to operating temperature creates forces and moments at supports and equipment connections. Support design must accommodate these movements while maintaining structural integrity.

Thermal Expansion Calculation

Expansion Data for Carbon Steel

Expansion Absorption Methods

• Natural flexibility: Changes of direction (elbows, bends) in the piping layout absorb thermal movement through bending. This is the preferred method and should be used whenever possible.
• Expansion loops: U-shaped loops inserted in straight pipe runs to provide flexibility. The loop size depends on the pipe diameter, expansion magnitude, and allowable stress.
• Expansion joints (bellows): Axial, lateral, or universal bellows joints absorb thermal movement in a compact space. Require careful engineering for pressure thrust, guides, and anchors.
• Sliding supports: Allow axial pipe movement on support beams. Low-friction surfaces (PTFE pads, rollers) reduce friction forces transmitted to the support structure.

Expansion Loop Sizing

Friction Forces at Supports

As pipe slides on supports during thermal expansion, friction generates horizontal forces on the support structure:

6. Worked Example

Determine the allowable support span for an NPS 8 Schedule 40 carbon steel pipe carrying lean amine at 180°F, with 2-inch calcium silicate insulation and aluminum jacket.

Step 1: Calculate Total Weight

Step 2: Stress-Based Span (Continuous Beam)

Step 3: Deflection-Based Span (δ = L/240)

Step 4: Governing Span

Step 5: Summary

7. Span Tables & Guidelines

Standard Span Table (Carbon Steel, Std Weight)

The following table provides suggested maximum support spans for standard-weight carbon steel pipe. These are conservative values suitable for initial layout. Verify with project-specific calculations.

Values assume standard weight pipe, no insulation, continuous beam model, carbon steel at ambient temperature. Reduce spans for insulated pipe, higher temperature, or single-span conditions. Add insulation and contents weight for project-specific calculation.

Span Reduction Factors

Practical Design Guidelines

• Coordinate with structural: Pipe support spans should align with structural steel bay spacing whenever practical. Common bay sizes are 20, 25, 30, and 40 feet.
• Pipe rack design: For multi-level pipe racks, supports on the top tier should be designed for the worst-case combination of pipe loads. Bent spacing (support span) is typically 20–30 ft for process plants.
• Minimum span: Do not place supports closer than necessary. Over-supporting a piping system can create thermal restraint problems and increase support costs without benefit.
• Drainage: Ensure that span deflection does not create low points that prevent drainage. This is critical for piping that requires positive slope for gravity flow or pig passage.
• Hydrotest supports: For gas piping that will be hydrotested, check the water-filled condition. Temporary supports may be needed if the hydrotest weight exceeds the span capacity. These are removed after testing.