Percent SMYS
Engineering fundamentals for pipeline stress analysis
1. SMYS Definition
Specified Minimum Yield Strength (SMYS) is the minimum yield strength prescribed by the pipe specification or standard. It represents the stress at which the pipe material begins to deform plastically.
Key Concepts
- Yield strength: Stress at 0.5% total elongation (API 5L) or 0.2% offset
- SMYS: Minimum guaranteed value from manufacturer's specification
- Actual yield: Typically 5–15% higher than SMYS
- Ultimate tensile strength: Maximum stress before fracture (higher than yield)
SMYS by Pipe Grade
| API 5L Grade |
SMYS (psi) |
SMYS (MPa) |
Min UTS (psi) |
| Grade B |
35,000 |
241 |
60,000 |
| X42 |
42,000 |
290 |
60,000 |
| X46 |
46,000 |
317 |
63,000 |
| X52 |
52,000 |
359 |
66,000 |
| X56 |
56,000 |
386 |
71,000 |
| X60 |
60,000 |
414 |
75,000 |
| X65 |
65,000 |
448 |
77,000 |
| X70 |
70,000 |
483 |
82,000 |
| X80 |
80,000 |
552 |
90,000 |
📊 Stress-Strain Curve
Stress-strain diagram showing: Linear elastic region (slope = E), yield point (0.5% elongation or 0.2% offset), plastic region, strain hardening, ultimate tensile strength (UTS), and fracture point. Mark SMYS on Y-axis. Show how actual yield typically exceeds SMYS. Include typical values for X52 steel.
2. Stress Calculations
Hoop Stress (Barlow's Formula)
S_h = P × D / (2 × t)
Where:
S_h = Hoop stress (psi)
P = Internal pressure (psig)
D = Outside diameter (inches)
t = Wall thickness (inches)
Percent SMYS
%SMYS = (S_h / SMYS) × 100
%SMYS = (P × D) / (2 × t × SMYS) × 100
Rearranged for maximum pressure:
P_max = (2 × t × SMYS × %SMYS_allowed) / (D × 100)
Example Calculation
Given: 16" OD × 0.375" wall, X52, operating at 1,000 psig
S_h = 1,000 × 16 / (2 × 0.375)
S_h = 16,000 / 0.75 = 21,333 psi
%SMYS = 21,333 / 52,000 × 100
%SMYS = 41.0%
Combined Stress
Longitudinal stress (restrained pipe):
S_L = ν × S_h - E × α × ΔT + S_bending
Von Mises equivalent stress:
S_eq = √(S_h² - S_h×S_L + S_L²)
Where:
ν = Poisson's ratio (0.3 for steel)
E = Modulus of elasticity (29×10⁶ psi)
α = Thermal expansion (6.5×10⁻⁶ /°F)
ΔT = Temperature change from installation
3. Regulatory Limits
Federal regulations limit operating stress based on pipeline location and consequence of failure.
49 CFR 192 – Natural Gas Pipelines
| Location Class |
Design Factor (F) |
Max %SMYS |
Description |
| Class 1 |
0.72 |
72% |
≤10 buildings in 1-mile section |
| Class 2 |
0.60 |
60% |
11–46 buildings |
| Class 3 |
0.50 |
50% |
≥46 buildings or specific areas |
| Class 4 |
0.40 |
40% |
4+ story buildings prevalent |
49 CFR 195 – Hazardous Liquid Pipelines
| Condition |
Max %SMYS |
| General (non-HCA) |
72% |
| High Consequence Areas (HCA) |
72% (with integrity management) |
| Crossings, casings |
60% |
ASME B31.8 Additional Limits
Hydrostatic test stress limits:
- Class 1: May test to 100% SMYS
- Class 2: Test to 90% SMYS max
- Class 3 & 4: Test to 90% SMYS max
Combined stress limit:
S_L (longitudinal) ≤ 0.75 × SMYS × F × T
⚠ Class location changes: If development increases building count, pipeline may require pressure reduction, pipe replacement, or additional protective measures to meet lower %SMYS limit.
4. Practical Applications
When %SMYS Matters
- New pipeline design: Select wall thickness to meet location class requirements
- Uprating: Verify stress before increasing MAOP
- Class location change: Determine if pressure reduction needed
- Defect assessment: Calculate stress at anomaly locations
- Remaining strength: Evaluate corroded pipe sections
Wall Thickness Selection
Required wall thickness:
t_min = (P × D) / (2 × SMYS × F × E × T)
Where:
F = Design factor (0.40–0.72 per class)
E = Joint factor (1.0 for seamless/ERW)
T = Temperature derating (1.0 at ≤250°F)
Add allowances:
t_nominal = t_min + corrosion allowance + mill tolerance
Pressure vs. %SMYS Chart
| Pipe |
72% SMYS (psi) |
60% SMYS (psi) |
50% SMYS (psi) |
| 12.75" × 0.250" X52 |
1,467 |
1,223 |
1,019 |
| 16" × 0.375" X52 |
1,755 |
1,463 |
1,219 |
| 20" × 0.500" X60 |
2,160 |
1,800 |
1,500 |
| 24" × 0.500" X65 |
1,950 |
1,625 |
1,354 |
| 30" × 0.625" X70 |
2,100 |
1,750 |
1,458 |
5. Integrity Assessment
Pipeline anomalies (corrosion, dents, cracks) reduce effective wall thickness, increasing local stress.
Remaining Strength (ASME B31G)
Safe operating pressure for corroded pipe:
P_safe = (2 × t × SMYS × F / D) × (1 - A/A_o) / (1 - A/(M×A_o))
Where:
A = Defect area = d × L
A_o = Original area = t × L
d = Defect depth
L = Defect length
M = Folias factor = √(1 + 0.8(L²/Dt))
Effective %SMYS at defect:
%SMYS_eff = %SMYS / [(1 - A/A_o) / (1 - A/(M×A_o))]
Defect Severity
| Defect Depth |
Action |
| < 10% wall |
Monitor, no immediate action |
| 10–40% wall |
Calculate remaining strength, schedule repair |
| 40–80% wall |
Evaluate for pressure reduction or repair |
| > 80% wall |
Immediate repair or pressure reduction required |
ILI validation: In-line inspection tools report metal loss as % wall. Always convert to %SMYS at operating pressure to assess safety margin against regulatory limits.
References
- 49 CFR Part 192 – Transportation of Natural Gas
- 49 CFR Part 195 – Transportation of Hazardous Liquids
- ASME B31.8 – Gas Transmission and Distribution Piping
- ASME B31G – Manual for Determining Remaining Strength
- API 5L – Line Pipe Specification