Threaded Steel Pipe Pressure Ratings

Standards for Pressure Ratings

• Pressure pipe generally carries fluids under pressures ranging from 10 to 15 atmospheres. Because of the safety issues involved, piping used to contain pressurized fluids must adhere to recognized standards. The standard for pressure pipes and fittings in the United States is called ASME B31 or ANSI B31.

  The B31 standards are derived from the ASME (American Society of Mechanical Engineers) Boiler and Pressure Vessel Code. ASME B31 covers pressure piping for several industries, including section B31.1 for power piping, section B31.2 for fuel gas piping and section B31.3 for process piping. To ensure safe operation of a pressurized piping system, all aspects of the production and installation of the piping must meet quality standards.

Threaded Pipe Sizes

• There are different standards for threaded pipe sizes. They vary according to location and type of industry. In general, a pipe size is specified by two numbers--one for the outside diameter, or OD, of the pipe, and another for the wall thickness of the pipe. In the U.S., the pressure piping outside diameter size is called "nominal pipe size." The wall thickness is called the "schedule." For example, you might see piping designated as 1-inch schedule 10 or 4-inch schedule 40.

Standards for Pipe Threads

• Steel pipe threads either are parallel type or tapered type. The tapered type is considered the best option for leak-proof connections. Using pipe sealant or pipe tape to fill voids between two threads enhances tapered connections.

  The most common pressure pipe thread standards in North America are the National Pipe Thread (NPT), also known as the American Standard Pipe Taper Thread, and the Dryseal version of pipe taper thread standards (NPTF).

  Pipe threads are specified by the number of threads per inch, plus the diameter. For a male thread, the outside diameter is used. For a female thread, the inside diameter is used.

Threaded Pipe Sizes

• There are different standards for threaded pipe sizes. They vary according to location and type of industry. In general, pipe size is specified by two numbers--one for the outside diameter, or OD, of the pipe, and another for the wall thickness of the pipe. In the U.S., the pressure pipe's outside diameter is called "nominal pipe size," while its wall thickness is called the "schedule". For example, you might see piping designated as 1 inch schedule 10 or 4 inch schedule 40.

  Threaded connections for steel piping as large as 36 inches in diameter sometimes is used, but piping larger than 4.5 inches in diameter also is normally joined together by welding instead of threading.