Working Pressure of Seamless Steel Pipe

Basic concepts and application fields of seamless pipe:

Seamless steel pipe (SMLS pipe) is a type of pipe produced without welds by deforming the metal as a whole using piercing and rolling techniques. Compared to welded steel pipe, its core advantage stems from the continuity of the material and the uniformity of its structure, providing superior reliability and safety under high pressure, high temperature, and complex stress environments. This makes seamless steel pipe a fundamental and reliable material in the petroleum, chemical, natural gas, and power industries for critical processes and the transportation of hazardous media.

Working pressure range of seamless pipe:

Steel pipes themselves are not classified as "high, medium, low pressure," but rather their applicable pressure ratings are distinguished according to material standards (such as ASTM A106) and Pipe Schedule wall thickness series (such as Sch40, Sch80). Seamless pipes of the same standard and specifications can be used for both low and high pressure by increasing the wall thickness.

In industrial standards, the classification of low pressure, medium pressure, high pressure, and ultra-high pressure varies depending on the industry and application (for example, 20MPa is considered a conventional pressure for hydraulic systems, while 0.4MPa is considered medium pressure for urban gas pipelines).

Pressure ranges for common application scenarios:

The following pressure ranges for common applications of seamless steel pipes are for reference only and should not be used as a substitute for specific design calculations.

Hydraulic System Pressure Range: 16 MPa - 40+ MPa;
Common Materials: 45#, 27SiMn, ST52.4
Features: High pressure and compact design. Utilizes cold-drawn precision tubing, requiring high cleanliness and precision, and necessitates pulse fatigue testing.

Ordinary Fluid/Structure Pressure Range: 1 MPa - 10 MPa
Common Materials: 20#, Q235B, A53 GR.B, A106 GR.B
Features: Low-pressure transportation or support structures for water, gas, oil, etc. Pressure is determined by cost and economical wall thickness.

Boiler/Power Plant Pressure Range: 4 MPa - 30+ MPa
Common Materials: 20G, 15CrMoG, P91
Features: Temperature is the core variable. Medium and low-pressure boilers (4-14MPa), ultra-supercritical power plants (>25MPa). High-temperature specific standards must be followed.

Petrochemical Industry Pressure Range: 10 MPa - 20+ MPa
Common Materials: 20#, 304, 316L, Alloy Steel
Features: Balances pressure, temperature, and media corrosion. Extremely demanding operating conditions, such as in hydrogenation units.

Oil and Gas Pipeline Transportation Pressure Range: 8 MPa - 15+ MPa
Common Materials: API 5L GR.B-X70
Features: Long-distance pipelines. This field now extensively uses high-performance straight seam welded pipes, not exclusively seamless pipes.

CNG/LNG Pressure Range: 20 MPa - 25 MPa
Common Materials: X70, 304
Features: Vehicle-mounted or station-based storage and transportation. Requires extremely high low-temperature toughness and fatigue resistance.

Factors affecting the working pressure of seamless pipes:

The working pressure of seamless steel pipes depends on various factors, such as their material, wall thickness, diameter, manufacturing process, and operating environment. Ordinary carbon steel seamless pipes can operate at pressures up to approximately 20 MPa at room temperature, while stainless steel seamless pipes, due to their higher material strength, may have a higher load-bearing capacity.

1. Material 

The material of a seamless steel pipe has a crucial impact on its load-bearing capacity. There are significant differences in load-bearing capacity between carbon structural steel, stainless steel, and alloy seamless steel pipes. For example, the allowable stress of A53 and A106 carbon steel is much lower than that of P91 alloy steel. Allowable stress refers to the maximum stress a material can withstand at the design temperature, obtained from tables in standards (such as ASME B31.1).

2. Diameter and Wall Thickness 

The diameter and wall thickness of a seamless steel pipe are also important factors determining its load-bearing capacity. Generally, with the same material, seamless steel pipes with smaller diameters and thicker walls have higher load-bearing capacity. Wall thickness is the most direct variable; the thicker the wall, the higher the pressure it can withstand. The pressure-bearing capacity of seamless steel pipes must be calculated using the minimum measured wall thickness.

3. Manufacturing Process
Manufacturing Process: Hot-rolled pipes and cold-drawn pipes have different performance characteristics.
Quality Coefficient: For seamless steel pipes, due to their seamless integrity, the quality coefficient is taken as 1.0 in standard calculations. This is one of their core advantages over welded pipes (whose coefficient is often less than 1.0).
Defect Control: Internal inclusions, cracks, and other defects significantly reduce the actual pressure-bearing capacity.

4. Operating Environment

The environmental conditions in which seamless steel pipes operate, such as temperature, humidity, and corrosive media, all affect their load-bearing capacity. In harsh environments, the load-bearing capacity of seamless steel pipes may decrease. Therefore, it is necessary to address this by increasing corrosion allowances and selecting heat-resistant materials.

Design Standards: Standards such as ASME B31.3 and GB/T 20801 specify calculation formulas and safety factors.

Comparison of working pressures of seamless pipes of different materials:
‌Ordinary carbon steel seamless pipes‌: The working pressure at room temperature is about 20MPa.
‌Stainless steel seamless pipes‌: Due to the higher material strength, their pressure bearing capacity is stronger. The specific value needs to be determined based on the actual situation and the technical data provided by the manufacturer.

Classification and specifications of seamless steel pipes:

Depending on the manufacturing process, seamless steel pipes can be divided into hot-rolled and cold-drawn types. Among them, hot-rolled seamless steel pipes have relatively low manufacturing costs and faster production speeds; cold-drawn seamless steel pipes, due to more processes and lower yields, generally have higher costs than hot-rolled pipes. The specifications and dimensions of seamless steel pipes are mainly determined by two parameters: outer diameter and wall thickness. Different specifications and dimensions can be selected according to different application requirements.

Calculation of pressure bearing capacity of seamless steel pipe:

Seamless pipes are widely used in the industrial field, and their pressure bearing capacity is a key indicator. Generally speaking, the pressure range that seamless pipes can withstand is between 10 MPa and 80 MPa. However, this is only a rough range, and the actual pressure bearing capacity needs to be evaluated according to specific circumstances.

The pressure-bearing formula of seamless steel pipe: 

Where:
- S: Allowable stress of the material at the design temperature (obtained from standards, unit MPa).
- E: Quality coefficient, taken as 1.0 for seamless pipes.
- W: Weld coefficient, taken as 1.0 for seamless pipes.
- t: Minimum measured wall thickness of the steel pipe (mm).
- C: Sum of allowances for corrosion, wear, machining, etc. (mm).
- D: Outer diameter of the steel pipe (mm).
- Y: Temperature coefficient.

It should be noted that this is only a simple estimation method. The actual bearing pressure may be affected by many factors such as the quality of the pipe, processing technology, and use environment. Therefore, in practical applications, we need to conduct actual tests on seamless pipes of different specifications, materials, processes, and use environments, so that we can obtain more specific pressure bearing data. These data are of great significance for selecting suitable seamless pipes and ensuring their safety in practical applications.

Read more: Schedule 80 Pipe Pressure Rating or Which material is used for boiler tubes?