Similar Names, Different Duties
A106 Grade B and API 5L Grade B are both carbon steel pipes with a minimum yield strength of about 35 ksi, but they are governed by different standards written for different services. ASTM A106 is the specification for seamless carbon steel pipe for high-temperature service in plants and refineries. API 5L is the specification for line pipe used to transport oil, gas, and water in pipeline systems. Understanding the difference matters because the applicable design code will not accept one standard in place of the other without review.
Scope and Standard Structure
| Aspect | A106 Grade B | API 5L Grade B |
|---|---|---|
| Primary service | Plant piping, high temperature | Pipeline transmission |
| Manufacturing | Seamless only | Seamless or welded |
| Product specification level | Single level | PSL1 and PSL2 |
| Hydrostatic test | Mandatory on each pipe | Mandatory on each pipe |
Chemical Composition Compared
| Element | A106 Grade B | API 5L Grade B PSL1 |
|---|---|---|
| Carbon, C, maximum | 0.30% | 0.26% |
| Manganese, Mn | 0.29% to 1.06% | 1.20% maximum |
| Silicon, Si | 0.10% minimum | Not specified |
| Phosphorus, P, maximum | 0.035% | 0.030% |
| Sulfur, S, maximum | 0.035% | 0.030% |
Testing Requirements
Both standards require hydrostatic testing of every pipe. A106 additionally permits non-destructive electric testing as a purchaser option, and its tensile and flattening tests are defined in the standard. API 5L PSL2 adds mandatory Charpy impact testing, stricter chemistry limits, and more demanding non-destructive examination than PSL1. When a pipeline design calls for low-temperature or sour service, API 5L PSL2 with supplementary requirements is normally specified.
Lengths and Tolerances
A106 pipe is commonly supplied in random lengths of about 6 to 7 meters or 20 to 24 feet, or in cut lengths as required. API 5L pipe is produced in the same length categories, with tolerances defined by the standard and the product specification level. Dimensional tolerances are similar for both, and the applicable standard governs the acceptance criteria.
Welding Considerations
Both grades weld with standard arc processes such as shielded metal arc, gas tungsten arc, and gas metal arc welding. Because the carbon content is low, preheating is generally not required for thin walls, and post-weld heat treatment is specified only when the design code requires it. Welding procedures are qualified per ASME Section IX or the applicable code.
How to Choose
The choice is made by the design code, not by preference. If the pipe is inside a plant or refinery and carries high-temperature fluids, A106 Grade B is the typical selection. If the pipe is part of a cross-country pipeline or a transmission system, API 5L is the governing standard. For line pipe that must also meet plant requirements, the project specification usually states the applicable standard explicitly.
FAQ
Can API 5L Grade B be used instead of A106 Grade B?
Only if the design code permits it and the project specification allows the substitution. The two standards differ in chemistry, testing, and acceptance criteria, so substitution requires engineering review and documentation.
Is A106 stronger than API 5L Grade B?
Both have the same minimum yield strength of 35 ksi, equal to 240 MPa. The differences are in the standard requirements, not in the basic strength level.
What is PSL in API 5L?
PSL stands for product specification level. PSL1 covers standard requirements for general service, while PSL2 adds mandatory impact testing, stricter chemistry, and more demanding non-destructive examination for critical service.
Why does A106 require a silicon minimum?
The minimum silicon content of 0.10 percent ensures that the steel is killed, meaning fully deoxidized, which produces a dense and sound structure for pressure service. API 5L does not require this because its scope and production controls differ.
Are the two pipes interchangeable in stock?
In an emergency, an engineer might approve a specific substitution with documentation, but the two are not automatically interchangeable. Certificates, chemistry, and test records should be reviewed before any substitution is made.
Which standard is used for offshore pipelines?
API 5L is the standard for offshore oil and gas pipelines, normally at PSL2 with additional project requirements for sour service, fracture toughness, and non-destructive examination.
