Stress Analysis

Percent SMYS Calculations

Calculate pipeline operating stress as percentage of Specified Minimum Yield Strength, verify regulatory compliance with 49 CFR 192 limits, and perform integrity assessments for pressure uprating and anomaly evaluation.

Launch Calculator Regulatory limits

Class 1 maximum

72% SMYS

Rural areas: maximum allowable hoop stress = 72% of pipe's yield strength per 49 CFR 192.

Typical operation

50-65% SMYS

Most pipelines operate 50-65% SMYS to provide safety margin for pressure surges and anomalies.

Critical threshold

100% SMYS

At 100% SMYS pipe yields permanently. Hydrostatic tests limited to ≤95-100% SMYS.

Contents

Quick start

Try the calculator

Calculate percent SMYS, verify operating limits, or evaluate integrity for pressure changes.

Open calculator →

1. SMYS Definition

Specified Minimum Yield Strength (SMYS) is the minimum yield strength guaranteed by the pipe manufacturer per API 5L or ASTM specifications. It represents the stress at which the material transitions from elastic to plastic deformation—the threshold where permanent deformation begins.

Key Concepts

  • Yield strength: Stress at 0.5% total elongation (API 5L) or 0.2% offset (ASTM)
  • SMYS: Minimum guaranteed value—actual yield is typically 5–15% higher
  • Ultimate tensile strength (UTS): Maximum stress before fracture, always exceeds yield
  • Elastic modulus: 29 × 10⁶ psi for carbon steel (slope of elastic region)

API 5L Pipe Grade Properties

Grade SMYS (psi) SMYS (MPa) Min UTS (psi) Typical Use
Grade B 35,000 241 60,000 Low-pressure gathering
X42 42,000 290 60,000 Distribution systems
X52 52,000 359 66,000 Standard transmission
X60 60,000 414 75,000 Higher-pressure transmission
X65 65,000 448 77,000 High-pressure transmission
X70 70,000 483 82,000 Long-distance pipelines
X80 80,000 552 90,000 Offshore/high-stress
Stress-strain curve for API 5L X52 pipeline steel showing elastic region, SMYS at 52,000 psi, actual yield approximately 10% above SMYS, ultimate tensile strength at 66,000 psi, and 0.5% offset yield method line with safe plastic operation zone
Stress-strain relationship showing SMYS, actual yield, and UTS for API 5L X52 steel

2. Stress Calculations

Hoop Stress (Barlow's Formula)

Internal pressure creates circumferential (hoop) stress in the pipe wall. For thin-walled cylinders (D/t > 20):

σh = (P × D) / (2 × t) Where: σh = Hoop stress (psi) P = Internal pressure (psig) D = Outside diameter (inches) t = Wall thickness (inches)

Percent SMYS Calculation

Percent SMYS expresses the actual operating stress as a percentage of the material's yield strength:

%SMYS = (σh / SMYS) × 100 Substituting Barlow's formula: %SMYS = (P × D) / (2 × t × SMYS) × 100 Rearranged for required wall thickness: t = (P × D) / (2 × SMYS × %SMYS / 100)
Critical distinction: %SMYS is the actual operating stress ratio. The design factor (F) sets the allowable limit—they are independent calculations.

Worked Example

Given: 16" OD × 0.375" wall, X52 (SMYS = 52,000 psi), operating at 1,000 psig

Step 1: Calculate hoop stress
σh = (1,000 × 16) / (2 × 0.375) = 16,000 / 0.75 = 21,333 psi

Step 2: Calculate %SMYS
%SMYS = (21,333 / 52,000) × 100 = 41.0%

Step 3: Verify against Class 1 limit (72%)
41.0% < 72% ✓ Within limits

MAOP Calculation

Maximum Allowable Operating Pressure includes derating factors per ASME B31.8:

MAOP = (2 × S × t × F × E × T) / D Where: S = SMYS (psi) F = Design factor (0.40–0.72 per location class) E = Joint factor (1.0 for seamless/ERW) T = Temperature derating factor (1.0 at ≤250°F)
Pipe cross-section diagram showing hoop stress distribution with 16-inch diameter and 0.375-inch wall thickness, internal pressure of 1,000 psig creating circumferential tensile stress, formula σh = PD/2t with example calculation yielding 21,333 psi, and small inset showing axial stress direction
Hoop stress distribution in pipe wall under internal pressure loading

3. Regulatory Limits

Federal regulations limit operating stress based on pipeline location class and potential consequences of failure. The design factor (F) directly sets the maximum allowable %SMYS.

49 CFR 192 – Natural Gas Pipelines

Class Design Factor Max %SMYS Description Building Count
Class 1 0.72 72% Rural/offshore ≤10 per mile
Class 2 0.60 60% Fringe areas 11–46
Class 3 0.50 50% Suburban ≥46
Class 4 0.40 40% Urban/high-rise 4+ story buildings

49 CFR 195 – Hazardous Liquid Pipelines

Condition Max %SMYS Notes
General areas 72% Standard design limit
HCA (High Consequence Areas) 72% Requires integrity management
Road/rail crossings, casings 60% Reduced limit per §195.110

Hydrostatic Test Limits (ASME B31.8)

Location Class Min Test Pressure Max Test Stress
Class 1, Div 11.25 × MAOP100% SMYS
Class 1, Div 21.25 × MAOP90% SMYS
Class 21.25 × MAOP90% SMYS
Class 3 & 41.40 × MAOP90% SMYS

Class location change: If development increases building count, the pipeline may require pressure reduction, increased wall thickness, or protective measures to meet the lower %SMYS limit. Operators must conduct class location studies annually per 49 CFR 192.5.

Plan view map showing pipeline location class determination per 49 CFR 192.5 with 660-foot buffer zone on each side of pipeline, 1-mile sliding window, and color-coded sections showing Class 1 rural (≤10 buildings), Class 2 fringe (11-46 buildings), Class 3 suburban (≥46 buildings), and Class 4 urban (4+ story buildings)
Sliding mile method for determining pipeline location class per 49 CFR 192.5

4. Practical Applications

When %SMYS Calculations Are Required

  • New pipeline design: Select wall thickness to meet target location class
  • MAOP uprating: Verify stress remains within limits before increasing pressure
  • Class location change: Determine pressure reduction requirements when building count increases
  • Integrity assessment: Calculate stress at anomaly locations for repair prioritization
  • Hydrostatic testing: Verify test pressure won't exceed stress limits

Wall Thickness Selection

For new construction, calculate minimum wall thickness for target operating stress:

Minimum wall for target %SMYS: tmin = (P × D) / (2 × SMYS × %SMYStarget / 100) Minimum wall for MAOP (includes factors): tmin = (P × D) / (2 × SMYS × F × E × T) Nominal wall (add allowances): tnominal = tmin + corrosion allowance + (0.125 × tmin) for mill tolerance

Quick Reference: Maximum Pressure by Pipe Size

MAOP (psig) at different %SMYS limits for common pipe configurations:

Pipe Size Grade 72% (Class 1) 60% (Class 2) 50% (Class 3) 40% (Class 4)
12.75" × 0.250" X52 1,467 1,223 1,019 815
16" × 0.375" X52 1,755 1,463 1,219 975
20" × 0.500" X60 2,160 1,800 1,500 1,200
24" × 0.500" X65 1,950 1,625 1,354 1,083
30" × 0.625" X70 2,100 1,750 1,458 1,167

Values assume E = 1.0 (seamless/ERW) and T = 1.0 (≤250°F).

Joint Factor (E) Values

Pipe Type Joint Factor (E)
Seamless1.00
ERW (Electric Resistance Welded)1.00
Flash-welded1.00
Submerged arc welded (DSAW)1.00
Furnace lap welded0.80
Furnace butt welded0.60

5. Integrity Assessment

Pipeline anomalies (corrosion, dents, cracks) reduce effective wall thickness, increasing local stress. %SMYS calculations are critical for evaluating remaining strength and prioritizing repairs.

Remaining Strength per ASME B31G

For metal loss defects, calculate safe operating pressure using the modified B31G method (parabolic defect profile):

Safe operating pressure (Modified B31G): Psafe = (2 × t × SMYS × F / D) × [(1 - ⅔(d/t)) / (1 - ⅔(d/t)/M)] Folias factor (bulging correction): M = √(1 + 0.8 × L² / (D × t)) Where: d = Defect depth (inches) L = Defect length along pipe axis (inches) D = Outside diameter (inches) t = Nominal wall thickness (inches) Effective %SMYS at defect (reduced wall): %SMYSeff = (P × D) / (2 × (t − d) × SMYS) × 100

ILI Anomaly Response Guidelines

Metal Loss Depth Typical Response Timeline
< 20% wall Monitor, no immediate action Next ILI cycle
20–40% wall Calculate remaining strength, schedule excavation 1–2 years
40–60% wall Priority dig, evaluate for repair 180 days
60–80% wall Immediate excavation, likely repair 60 days
> 80% wall Immediate investigation, pressure reduction Immediate

Timelines per 49 CFR 192.933 and operator integrity management programs.

Converting ILI data: In-line inspection tools report metal loss as % wall depth. To assess safety margin, calculate %SMYS at the reduced wall: σh = (P × D) / (2 × tremaining).
ASME B31G metal loss defect geometry assessment showing isometric cutaway view with corrosion defect dimensions L (axial length) and W (circumferential width), plus cross-section through defect showing original wall thickness t, defect depth d, and remaining wall t-d, with example dimensions D=16 inches, t=0.375 inches, d=0.150 inches representing 40% wall loss
Metal loss defect geometry and ASME B31G assessment parameters

Standards References

  • 49 CFR Part 192 – Transportation of Natural and Other Gas by Pipeline
  • 49 CFR Part 195 – Transportation of Hazardous Liquids by Pipeline
  • ASME B31.8 – Gas Transmission and Distribution Piping Systems
  • ASME B31G – Manual for Determining Remaining Strength of Corroded Pipelines
  • API 5L – Specification for Line Pipe (46th Edition)
  • API 1160 – Managing System Integrity for Hazardous Liquid Pipelines

Ready to use the calculator?

→ Launch Calculator