Piping Support

Spring Hanger Design

Select and design variable and constant spring supports for piping systems experiencing thermal displacement, per MSS SP-58, SP-69, and ASME B31.3 requirements.

Launch Calculator Variable Spring Design

Variable Spring

≤ 25% Load Variation

Standard limit per MSS SP-58

Constant Support

0% Load Variation

Constant force through full travel

MSS SP-69

Type Selection

Pipe hanger and support standards

Contents

Use this guide when:

  • Selecting springs for thermally displacing piping
  • Evaluating load variation at spring supports
  • Choosing between variable and constant springs
  • Setting cold load and travel for installation

1. Overview

When piping systems operate at elevated temperatures, thermal expansion causes vertical displacement at support points. Rigid supports cannot accommodate this movement without either lifting off (if the pipe rises) or imposing excessive loads on the pipe (if the pipe drops). Spring supports solve this problem by providing a supporting force that varies with displacement, maintaining pipe support throughout the full range of thermal movement.

Process Piping

ASME B31.3

High-temperature process piping at refineries, gas plants, and chemical facilities.

Power Piping

ASME B31.1

Steam lines, boiler piping, and turbine connections with large thermal movements.

Pipeline Stations

Compressor & Meter

Station piping with vertical runs and equipment connections experiencing thermal growth.

Equipment Connections

Nozzle Protection

Spring supports near equipment nozzles reduce thermal loads on vessels and exchangers.

Key concept: A spring support maintains pipe weight support while allowing vertical movement. The trade-off is that the supporting force changes with displacement (in variable springs), which affects pipe stress analysis. The goal is to keep this load variation within acceptable limits.

2. Support Types

Pipe supports are classified by their ability to accommodate movement and the type of force they provide.

Support Classification

Support Type Movement Load Variation Cost (Relative)
Rigid rod hanger None (vertical) Not applicable 1x (lowest)
Variable spring hanger Limited vertical 0-25% typical 3-5x
Constant spring support Large vertical 0% (constant force) 8-15x
Rigid strut None (any direction) Not applicable 2x
Snubber/shock absorber Slow: free; Fast: locked Not applicable 10-20x

MSS SP-58 Types

MSS Standard Practice SP-58 defines the dimensional and performance standards for pipe hanger and support hardware. Key types include:

MSS SP-58 Hanger Types (Selection): Type 1: Adjustable steel clevis hanger Type 2: Carbon steel yoke pipe clamp Type 3: Adjustable steel band hanger Type 4: Steel pipe clamp Type 5: Pipe sliding support Type 9: Steel pipe stanchion saddle Type 10: Adjustable swivel ring hanger Type 11: Steel turnbuckle Type B: Variable spring hanger (top-mounted) Type C: Variable spring hanger (base-mounted) Type D: Constant support hanger (top-mounted) Type E: Constant support hanger (base-mounted)

MSS SP-69 Selection Guide

MSS SP-69 provides guidance on selecting the appropriate pipe hanger or support type for specific service conditions. It addresses material selection, temperature limits, and load capacity for each hanger type defined in SP-58.

3. Variable Spring Design

Variable spring hangers use helical coil springs to support the pipe. As the pipe moves vertically due to thermal expansion, the spring deflects and the supporting force changes according to Hooke's law.

Variable Spring Fundamental Equations: Spring force: F = k × δ Where: F = Spring force (lb) k = Spring rate (lb/in) δ = Spring deflection from free length (in) Operating (hot) load: F_hot = Dead weight at support Cold load: F_cold = F_hot + k × Δy (pipe moves down) or: F_cold = F_hot - k × Δy (pipe moves up) Where: Δy = Vertical thermal displacement (in) Positive Δy = pipe moves UP (spring extends) Negative Δy = pipe moves DOWN (spring compresses)

Load Variation

Load Variation Calculation: Load Variation = |F_cold - F_hot| / F_hot × 100% Or equivalently: Load Variation = k × |Δy| / F_hot × 100% MSS SP-58 Limit: ≤ 25% Example: Hot load = 2,000 lb Thermal displacement = 1.5 in (downward) Spring rate = 200 lb/in Cold load = 2,000 + 200 × 1.5 = 2,300 lb Load variation = (2,300 - 2,000) / 2,000 × 100% = 15% 15% < 25% → Variable spring is acceptable

Spring Rate Selection

Spring catalogs organize springs by size (load capacity) and spring rate. Within each size, multiple spring rates are available. The goal is to select a spring rate that keeps the load variation within limits while providing adequate travel range.

Maximum Allowable Spring Rate: k_max = 0.25 × F_hot / |Δy| This ensures load variation stays ≤ 25%. For the example above: k_max = 0.25 × 2,000 / 1.5 = 333 lb/in Selected rate (200 lb/in) < 333 lb/in ✓ Softer Springs: Lower spring rate → Lower load variation But: Larger spring (more space needed) And: More sensitive to installation errors Typical practice: Select k to achieve 15-20% variation (provides margin below the 25% limit)

Spring Catalog Data

Spring Size Load Range (lb) Travel (in) Typical Spring Rate (lb/in)
Fig. 82 Size 1 50-450 2.5 30-180
Fig. 82 Size 3 130-1,200 3.5 55-340
Fig. 82 Size 6 380-3,500 4.5 130-780
Fig. 82 Size 10 1,200-10,800 6.0 300-1,800
Fig. 82 Size 15 3,000-27,000 8.0 560-3,375

4. Constant Spring Supports

Constant spring supports (also called constant effort supports or constant force hangers) use a mechanism of opposing spring and lever forces to provide a nearly constant supporting force throughout the full range of travel. This eliminates load variation entirely.

Operating Principle

Constant Spring Mechanism: A main helical spring provides the supporting force. A secondary mechanism (typically a Belleville spring stack or cam arrangement) applies a compensating moment that increases as the main spring extends, offsetting the change in main spring force. Result: Supporting force remains constant (±5%) through the entire working travel range. Performance: Load accuracy: ±5% of rated load (typical) Travel range: 2-16 inches (common sizes) Load capacity: 100-200,000+ lb

When to Use Constant Supports

Criterion Variable Spring Constant Support
Load variation ≤ 25% Acceptable Not required (but always acceptable)
Load variation > 25% Not acceptable Required
Large thermal movement Limited travel range Preferred for > 4" travel
Equipment nozzle sensitivity Adds variable load to nozzle Minimizes nozzle load change
Critical stress locations Acceptable if margin exists Preferred for high-stress piping
Cost consideration: Constant spring supports cost 3-5 times more than variable springs of equivalent capacity. Use them where they are truly needed (load variation exceeds 25%, large travel, or critical nozzle loads) rather than specifying them universally.

5. Load Variation Analysis

Load variation analysis determines how the pipe weight is distributed between supports as the system moves from cold (installed) to hot (operating) position. This analysis is fundamental to spring hanger selection.

Weight Balance Method

Dead Weight at Support Points: The dead weight at each support is determined by: 1. Pipe weight (lb/ft) × tributary length 2. Insulation weight (lb/ft) × tributary length 3. Fluid weight (lb/ft) × tributary length 4. Valve and fitting weight (concentrated loads) 5. Tracing and jacketing weight (if applicable) Total support load: W_total = W_pipe + W_insulation + W_fluid + W_valves For carbon steel pipe with standard insulation: 4" Sch 40: ~25-35 lb/ft (insulated, with water) 8" Sch 40: ~60-80 lb/ft 12" Sch 40: ~110-140 lb/ft 16" Sch 40: ~170-210 lb/ft

Thermal Displacement from Stress Analysis

The vertical thermal displacement at each support point is obtained from the pipe stress analysis (CAESAR II, AutoPIPE, or equivalent). The displacement depends on the overall pipe routing, anchor locations, and temperature distribution.

Displacement Sources: 1. Thermal expansion of vertical runs: Δy = α × L_v × ΔT Where L_v = vertical pipe length (in) 2. Thermal expansion of horizontal runs causing rotation at elbows: Vertical displacement at support depends on distance from rotation point 3. Equipment nozzle growth: Vessels and exchangers expand thermally, moving connected pipe support points 4. Anchor or support settlement: Foundation movement (typically small) The stress analysis combines all sources to give net vertical displacement at each support.

Load Case Summary

Load Case Description Spring State
Cold (installed) Ambient temperature, no operating loads Set at cold load, locked with travel stop
Hydro test Filled with water, ambient temperature May need temporary rigid support
Operating (hot) Design temperature and pressure At operating position, travel stop removed
Shutdown (cold) Return to ambient after operating Returns to cold position

6. Selection Procedure

The spring hanger selection procedure follows a systematic approach from pipe stress analysis results to manufacturer catalog selection.

Step-by-Step Selection: Step 1: Obtain from stress analysis: - Hot load at each support (F_hot) - Vertical displacement at each support (Δy) - Direction of displacement (up or down) Step 2: Calculate maximum allowable spring rate: k_max = 0.25 × F_hot / |Δy| Step 3: Calculate cold load: F_cold = F_hot + k × Δy (pipe moves down) F_cold = F_hot - k × Δy (pipe moves up) Step 4: Select spring from catalog: - Find size that includes F_hot in its range - Select spring rate ≤ k_max - Verify available travel ≥ |Δy| × 1.5 - Verify cold load within spring capacity Step 5: Set installation parameters: - Cold load setting (for travel stop) - Expected travel from cold to hot position - Nameplate data for field verification

Multiple Operating Cases

When a piping system has multiple operating modes (startup, normal operation, upset, shutdown), the spring must accommodate all displacement cases. The design case is typically the one with the largest vertical displacement, but all cases must be checked for adequate travel and acceptable load variation.

Common Selection Errors

Error Consequence Prevention
Spring too stiff Load variation exceeds 25% Check k ≤ k_max before selection
Insufficient travel Spring bottoms out or tops out Require 1.5x travel margin
Wrong cold load setting Pipe not properly supported Verify nameplate matches analysis
Travel stop left in place Spring acts as rigid support Inspection checklist during startup
Ignoring hydro test load Spring overloaded during test Check water-filled weight vs capacity

7. Practical Considerations

Installation and Commissioning

Spring Installation Checklist: 1. Verify spring size and type match isometric 2. Confirm cold load setting on nameplate 3. Install with travel stop PIN in place 4. Level pipe to design elevation at cold position 5. Record initial position indicator reading 6. After system reaches operating temperature: - Remove travel stop pins (all springs) - Record operating position indicator - Verify position matches analysis prediction - Investigate discrepancies > 10% of travel

Position Indicator

All spring hangers include a position indicator (travel indicator) that shows the current spring deflection relative to its working range. This indicator is essential for commissioning verification and ongoing monitoring.

Indicator Position Meaning Action
Within middle 80% of range Normal operating position None required
Top 10% (near minimum load) Spring nearly fully extended Investigate – pipe may be lifting off
Bottom 10% (near maximum load) Spring nearly bottomed out Investigate – possible overload
Beyond travel range Spring topped out or bottomed out Immediate action – acts as rigid

Hydrostatic Test Considerations

During hydrostatic testing, the pipe is filled with water which is significantly heavier than gas or most process fluids. The additional water weight may exceed the spring capacity, especially for large-diameter gas lines.

Hydro Test Weight Check: Water weight = π/4 × ID² × 62.4 / 144 (lb/ft) Example for 12" Sch 40 (ID = 11.938"): Water weight = π/4 × 11.938² × 62.4 / 144 Water weight = 48.5 lb/ft If operating fluid is gas (negligible weight), the hydro test load is substantially higher. Options: 1. Size spring for hydro test load 2. Use temporary rigid supports during test 3. Lock travel stops during test (most common)

Maintenance and Monitoring

  • Check position indicators annually during plant turnarounds
  • Verify that travel stops are removed after any maintenance shutdown
  • Look for signs of corrosion on spring housing and coils
  • Check for broken or cracked coils (visible through inspection window)
  • Verify that rod hangers are plumb and not binding
  • Record position readings and compare to previous inspections
  • Investigate any spring that has moved more than 20% from its expected position
Common field issue: Travel stops left in place after maintenance are one of the most common spring hanger problems. When the travel stop is engaged, the spring acts as a rigid support, causing unintended loads on the piping system and connected equipment. Always verify travel stop removal during post-maintenance walkdowns.

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