MAOP – Maximum Allowable Operating Pressure

Engineering fundamentals for gas pipeline design

1. MAOP Definition

MAOP is the maximum pressure at which a pipeline may be operated under federal regulations. For natural gas pipelines, 49 CFR Part 192 governs MAOP determination. The operating pressure must never exceed MAOP during normal operations.

Regulatory Framework

Regulation/Standard	Application
49 CFR Part 192	Natural gas transmission and distribution (federal law)
49 CFR Part 195	Hazardous liquids (oil, NGL, CO₂)
ASME B31.8	Gas transmission and distribution piping systems
ASME B31.4	Pipeline transportation of liquid hydrocarbons
API 1111	Offshore pipelines

MAOP vs. MOP: MAOP is the maximum allowable pressure based on design and testing. MOP (Maximum Operating Pressure) is the actual highest pressure expected during normal operations, which should be less than MAOP to provide margin.

2. Design Formula

The fundamental equation for MAOP is based on Barlow's formula modified with design factors:

49 CFR 192.105 / ASME B31.8: P = (2 × S × t × F × E × T) / D Where: P = Design pressure (MAOP), psig S = Specified Minimum Yield Strength (SMYS), psi t = Nominal wall thickness, inches D = Nominal outside diameter, inches F = Design factor (based on location class) E = Longitudinal joint factor T = Temperature derating factor

Rearranged Forms

Solve for wall thickness: t = (P × D) / (2 × S × F × E × T) Solve for hoop stress: S_h = (P × D) / (2 × t) Percent SMYS: %SMYS = S_h / SMYS × 100 = P × D / (2 × t × SMYS) × 100

Common SMYS Values

Grade	SMYS (psi)	Specification
Grade B	35,000	API 5L, ASTM A53/A106
X42	42,000	API 5L
X46	46,000	API 5L
X52	52,000	API 5L
X56	56,000	API 5L
X60	60,000	API 5L
X65	65,000	API 5L
X70	70,000	API 5L
X80	80,000	API 5L

3. Design Factors

Location Class (Design Factor F)

Location class is determined by population density within a "class location unit"—a 1-mile segment with a 220-yard corridor on each side of the pipeline.

Class	F	%SMYS	Description
Class 1	0.72	72%	≤10 buildings; rural, undeveloped
Class 2	0.60	60%	11–46 buildings; fringe areas, farms
Class 3	0.50	50%	≥46 buildings or within 100 yards of buildings with ≥20 people
Class 4	0.40	40%	≥4-story buildings above ground

📊 Location Class Determination

Plan view showing: Pipeline running horizontally, 1-mile class location unit along pipeline, 220-yard corridor on each side (total 440 yards wide). Count buildings within this rectangle. Show examples: Class 1 area (empty fields), Class 3 area (subdivision). Annotate distances and building counts for each class threshold.

Joint Factor (E)

Pipe Type	E Factor
Seamless	1.00
ERW (Electric Resistance Welded)	1.00
Flash Welded	1.00
DSAW (Double Submerged Arc Welded)	1.00
Furnace Lap Welded	0.80
Furnace Butt Welded	0.60

Temperature Factor (T)

Temperature (°F)	T Factor
≤250	1.000
300	0.967
350	0.933
400	0.900
450	0.867

Typical gas pipelines: Most operate below 250°F, so T = 1.0. Use modern pipe (E = 1.0). The design factor F is usually the controlling variable based on location class.

4. Pressure Testing

New pipelines must be pressure tested before operation. The test pressure validates the MAOP and ensures construction quality.

Test Pressure Requirements (49 CFR 192.619)

Class	Test Medium	Minimum Test Pressure
Class 1	Air, gas, or water	1.1 × MAOP
Class 2	Air, gas, or water	1.25 × MAOP
Class 3	Water (hydrostatic)	1.4 × MAOP
Class 4	Water (hydrostatic)	1.4 × MAOP

Test Duration

Hydrostatic (water): Minimum 8 hours at test pressure
Pneumatic (air/gas): Minimum 24 hours (more stringent due to stored energy hazard)

⚠ Wall stress during test: Ensure test pressure does not exceed 100% SMYS in the pipe wall. Calculate: S_test = P_test × D / (2 × t). If S_test > SMYS, reduce test pressure or the pipe may yield permanently.

Example: Test Pressure

Given: 12.75" OD, 0.375" wall, X52 pipe in Class 2 location

MAOP = (2 × 52,000 × 0.375 × 0.60 × 1.0 × 1.0) / 12.75
MAOP = 1,835 psig

Test pressure = 1.25 × 1,835 = 2,294 psig

Check stress: S = 2,294 × 12.75 / (2 × 0.375) = 39,020 psi
39,020 / 52,000 = 75% SMYS < 100% ✓ OK

5. MAOP Determination Methods

49 CFR 192.619 allows several methods to establish MAOP, depending on available records:

Method 1: Design Formula

Use the design formula (Section 2) when all parameters are known from records:

Pipe specification (SMYS, diameter, wall thickness)
Seam type (joint factor)
Location class

Method 2: Test Pressure

MAOP based on highest test pressure:

MAOP = Test Pressure / Test Factor Test Factor by class: Class 1: 1.1 Class 2: 1.25 Class 3: 1.4 Class 4: 1.4

Method 3: Highest Actual Operating Pressure

For pipelines in service before certain dates with incomplete records:

MAOP = highest actual operating pressure during 5 years before July 1, 1970
Subject to additional requirements (ILI, ECDA, etc.)

Method 4: Pipe Replacement

For replaced pipe segments, use design formula with known specifications.

MAOP Reconfirmation: PHMSA requires operators to reconfirm MAOP for pipelines with incomplete records using methods in 49 CFR 192.624 (transmission lines in HCAs and Class 3/4 locations).

References

49 CFR Part 192 – Transportation of Natural and Other Gas by Pipeline
ASME B31.8 – Gas Transmission and Distribution Piping Systems
PHMSA Advisory Bulletins on MAOP Verification
API 1160 – Managing System Integrity for Hazardous Liquid Pipelines