Natural gas steel pipe design characteristics

In gas systems, steel pipe is used to deliver gas at higher pressures and where external pressures are greater. Pipe size and type vary depending on various design considerations such as flow rate, pressure, and environmental factors. Key factors in specifying steel pipe include geometry, strength, toughness, and weldability. 

Geometry: Pipe geometry specifies the pipe diameter and wall thickness. Diameter affects the volume of gas the pipe can carry, and thickness affects how the pipe can withstand pressure and environmental stress. Transmission pipes are large and thick since they carry large amounts of gas at high pressures. Distribution and service pipes are smaller and thinner since they carry smaller amounts of gas at lower pressures. 

Steel pipe diameter by location/type 

Pipe strength refers to a pipe’s ability to resist deformation that can lead to collapse, bursting, or breaking. The strength of steel pipe is determined by the type of steel used, the diameter, and the wall thickness. The relationship among these three factors determines the maximum allowable operating pressure for the pipe. 

The type of pipe is defined by two factors: the steel grade and the manufacturing technique. The manufacturing technique describes whether the pipe has a welded seam and, if so, what method is used to weld the seam. 

When engineers design a steel gas line, they designate a required pipe minimum yield strength, often called the SMYS. This is the level of stress the pipe must be capable of experiencing without permanent deformation. Manufacturers specify a SMYS for each type and grade of pipe they sell. By doing so, they certify that their pipe can, at a minimum, be safely subjected to the certified SMYS. Minimum yield strength is a factor in determining the MAOP a pipe. It also determines the allowable external loading. The SMYS for a specific pipe is impacted by the grade of steel used and how it was manufactured. Stronger steels are more expensive but can provide higher stress resistance. 


Toughness: Toughness is a measure of the steel’s resistance to rapid cracking under stress. Once a pipe starts to crack, the internal pressure pushes the crack edges out, causing the crack to grow. At low temperatures, cracks can grow to hundreds of yards or even more than a mile.  A pipe can become so damaged that it ruptures. But a pipe with sufficient toughness can stop a crack in a matter of inches.

Weldability: Weldability describes how easily sections of pipe can be welded together. This is important for two reasons. Pipe that is more difficult to weld increases the likelihood of poor welds, which can result in leaks. And secondly, more difficult welding requirements mean higher construction costs. Designers selecting pipe must balance these factors with the cost of various steel options.

The following chart compares steel pipe versus plastic pipe applications: