What is a variable spring hanger and when is it used?

A variable spring hanger is a pipe support that uses a coil spring to carry the pipe weight while allowing vertical thermal movement. The support force varies as the spring compresses or extends. Variable springs are used when thermal movement is moderate (variability under 25%) and the changing support load does not adversely affect pipe stress or connected equipment nozzle loads.

What is spring hanger variability and why must it be under 25%?

Variability is the percentage change in support load between hot (operating) and cold (installed) conditions, calculated as (cold load - hot load) / hot load x 100. MSS SP-58 recommends variability below 25% to ensure the support load remains close enough to the design load throughout the thermal cycle. If variability exceeds 25%, a constant support hanger should be used instead.

What is the difference between hot load and cold load?

Hot load is the actual pipe weight that must be supported at operating temperature - it equals the sum of pipe, contents, and insulation weight at the support point. Cold load is the spring force at the installed (ambient) position before thermal movement occurs. Cold load = hot load plus or minus (spring rate times travel). The spring is preset at the factory to the cold load so that when the pipe moves to operating position, the hanger carries the correct hot load.

When should a constant support hanger be used instead of a variable spring?

Constant support hangers should be used when: (1) variability exceeds 25%, (2) thermal movement is large relative to pipe weight, (3) the pipe connects to sensitive equipment like turbines or compressors, or (4) the piping analysis requires constant support force. Constant supports use a mechanism (typically a Trapeze spring with a cam) that maintains nearly constant load throughout the travel range.

Spring Hanger Calculator | Variable & Constant Support Design

Pipe Properties

Nominal Pipe Size (NPS)

Pipe Schedule

Pipe Material

Service Conditions

Pipe Contents

Fluid Density

lb/ft³

Operating Temperature

°F

Installation Temperature

°F

Ambient temperature when spring is locked/installed

Insulation

Insulation Thickness

Insulation Density

lb/ft³

Calcium silicate: 8-14 | Mineral wool: 6-10 | Fiberglass: 3-6

Cladding / Jacketing Weight

lb/ft

Thermal Movement & Support

Vertical Thermal Movement

Positive = pipe moves UP (spring compresses). Negative = pipe moves DOWN (spring extends).

Tributary Span (support spacing)

Half the distance to adjacent supports on each side of this hanger

Support Location

Additional Dead Load (valves, fittings)

Understanding Spring Hangers

Variable vs. Constant
Variable spring hangers allow load variation as the pipe moves. Constant supports maintain nearly uniform load through a cam-and-spring mechanism. Variable springs are used when variability is below 25%.

Key Parameters:
Hot load (operating weight)
Cold load (installed spring force)
Travel (vertical thermal movement)
Variability (load change percentage)

Industry Practice:
Spring hangers are specified per MSS SP-58 (pipe hangers and supports - materials, design, manufacture) and MSS SP-69 (selection and application). The spring is factory-preset to the cold load position and shipped with a travel stop pin that is removed after installation.

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Understand spring hanger design principles, variable vs. constant supports, load calculations, and MSS SP-58/SP-69 standards

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Results

Operating (Hot) Load

lbs

Cold (Installed) Load -

Spring Rate Required -

Total Travel -

Variability -

Support Type Recommended -

Selected Spring Size -

Catalog Spring Rate -

Working Range Position -

Hanger Rod Size -

Formulas

Variability = |F_cold − F_hot| / F_hot × 100%

F_hot = w_total × L_trib (operating load)

F_cold = F_hot + k × δ (installed load)

k = spring rate (lb/in)

δ = vertical thermal movement (in)

Variability ≤ 25% for variable springs

Standards & References

MSS SP-58
Pipe Hangers & Supports — Materials, Design, Manufacture
MSS SP-69
Pipe Hangers & Supports — Selection and Application
ASME B31.1
Power Piping — Support requirements
ASME B31.3
Process Piping — Pipe support design
ASME B36.10M
Welded and Seamless Wrought Steel Pipe

Engineering Notes

Variability limit: MSS SP-58 recommends ≤ 25% for variable springs. Above 25%, use constant supports.
Hot load: Total operating weight at the support point. Must equal the designed pipe weight at that location.
Cold load: Factory preset force. When pipe moves to operating position, spring adjusts to hot load.
Travel stop pin: Spring is shipped locked at cold load. Pin is removed after pipe is installed and before startup.
Hydrotest: Check spring capacity for water-filled weight. May need to lock spring or provide temporary support.
Rod sizing: Hanger rods must support the maximum load (cold or hot, whichever is greater) with adequate safety factor.

Quick Reference — Spring Types

Fig. 82: Variable Spring Hanger (most common)
Fig. 83: Variable Spring Base Support
Fig. 90: Constant Support Hanger (trapeze)
Fig. 92: Constant Support (horizontal)

Spring Hanger Calculator

Pipe Properties

Service Conditions

Insulation

Thermal Movement & Support

Understanding Spring Hangers

📚 Learn the Theory

Results

Formulas

Standards & References

Engineering Notes

Quick Reference — Spring Types

Related Calculators

Pipe Support Span

Thermal Expansion

Bending Stress

Pipe Weight