How Is Steel Pipe Graded?
Steel pipe grades are overseen by a number of organizations, so which standard you follow is often dictated by the industry in which you operate. The American Petroleum Institute, ASTM and ASME and American Iron and Steel Institute all have grading systems you may have to understand.
Because steel pipe is a versatile and widely used material and is vital in many industries, there are a number of different bodies that issue steel pipe grades. There's a degree of overlap between them, as some organizations share grading systems and have very similar specifications.
There are several grading systems for steel pipe, including the American Iron and Steel Institute, the Society of Automotive Engineers and the American Iron and Steel Institute and Society of Automotive Engineers. The most widely used pipe specifications are set by the American Society of Testing and Methods and American Society of Mechanical Engineers.
The American Iron and Steel Institute and Society of Automotive Engineers have a numbered system of grading, which applies only to the kind of steel used but is often cited in a specification for steel pipe. The American Petroleum Institute has its own set of standards and grades because of the unique and exacting demands of the petroleum industry.
The most widely used pipe specifications are set by the American Society of Testing and Methods and American Society of Mechanical Engineers. Their specifications share a numbering system and are largely similar.
AISI/SAE Steel Grades for Pipe
If you manufacture steel piping, or if your clients specify the type of piping you use in your jobs for them, you may see a four-digit identifier in the piping spec. This is the AISI/SAE grade for the specific type of steel you're using. The first two digits spell out the type of steel and alloys – 1-series numbers for carbon steel, 2-series for nickel, 3-series for nickel-chromium and so on – while the second pair of digits express the percentage of carbon in the steel.
For example, 1020 indicates a plain carbon steel with no alloys and carbon content at 0.20 percent. A number of 8140 would indicate a nickel-chromium-molybdenum steel with 0.40 percent carbon. Pipe grading charts breaking down this numbering system are widely available online.
The number identifies the steel only and says nothing about whether it's heat treated or how it's manufactured and tested.
API Pipe Grading System
The API publishes multiple standards for piping within the oil industry, but the one you'll see most often is API 5L, which covers carbon steel pipes. The standard itself is just the starting point, so to supply or receive the correct pipe, you'll need to be more specific.
The next identifier under the API system specifies the pipe's steel and can be either PSL 1 for grade A carbon steel or PSL 2 for grade B. After this, there's a further alphabetic or alphanumeric identifier, which represents the delivery condition and the actual grade of the pipe. For example, PSL 1 pipe might be graded as A or B or in varying grades starting with the letter X followed by a two-digit number.
If your spec called for API 5L PSL1 X42, for example, that would indicate a pipe manufactured from grade A steel with a maximum yield strength of 42,000 psi and a maximum tensile strength of 60,000 psi. This is a common pipe within the petroleum industry.
ASTM/ASME Steel Pipe Grades
The ASTM's grading standards for steel pipe are laid out in a handful of specifications. The most important include A 106, A 53, A 500 and the A 530/A 530M spec, "General Requirements for Specialized Carbon and Alloy Steel Pipe," though many others can be applicable or prevalent within a given industry. The ASME uses the same naming convention but adds the letter S at the beginning – SA 523 rather than A 523 – to differentiate between them.
Within a given standard, pipe may be specified in multiple grades with different characteristics as well as by size and wall thickness. Most manufacturers can provide you with pipe grading charts to spell out the differences between them.
Pipe Grade Calculator
If you aren't a supplier or manufacturer but an end user or builder, and you need to know which grade of pipe to spec for your own purposes, you can use a pipe grade calculator. These use an equation known as Barlow's formula to calculate the allowable, internal and bursting pressure for pipes in a given application.
Armed with that information, you can turn to grading charts to determine which pipe you should use. If there's any question about which grade is appropriate in a given situation, your supplier should be able to explain it to your satisfaction.
- American Piping Products: Understanding Steel Grades for Piping Procurement
- The Engineering ToolBox: AISI/SAE Steel - Numbering System
- Continental Steel & Tube Company: ERW Standard and Line Pipe Grades
- Total Materia: Introduction to the ASTM Designation System
- Total Materia: ASTM Standard List - Structural and Constructional Steels Standards, Part Three
- American Piping Products: API 5L Seamless & Welded Pipe
- The Engineering ToolBox: Barlow's Formula - Internal, Allowable and Bursting Pressure