Factors Influencing the Determination of Pipeline Pressure Ratings

Apart from the basic parameters of design temperature and design pressure used to determine the pressure rating of a pipeline, there are several other factors that will affect the determination of the pipeline’s pressure rating.

1.2.1 Application Standard System Different standard systems have varying nominal pressure series and corresponding temperature-pressure tables. That is to say, for the same design conditions, selecting different application standards results in different nominal pressure ratings. Therefore, before determining the nominal pressure rating of a pipeline, it’s necessary to first establish the applicable standard system.

1.2.2 Materials Different materials possess distinct mechanical properties, which means their corresponding values in the standard’s temperature-pressure tables vary. Therefore, before determining the nominal pressure of the pipeline, the materials of the pipeline and its components should be confirmed. Material selection is determined by design temperature, design pressure, and the operating medium. The materials standards for different components of the pipeline often differ. Typically, pipes use tubing materials, flanges use forging materials, and valves use casting materials. Regardless of the material standards used, they should be of the same grade, meaning they should have equal adaptability and strength for the operating conditions; attention should be paid to compatibility among tubing, plate, bar, and casting materials.

1.2.3 Operating Medium Generally, the allowable pressure of a pipeline at a given temperature must not exceed its design pressure. For media where failure of the pipe or its components would cause serious harm or lead to major accidents, when considering the nominal pressure rating, it should not be determined solely by the temperature-pressure table but should appropriately increase the nominal pressure rating to enhance safety and reliability. Standards such as SH3059 and SYJ1064 provide detailed regulations regarding this matter, for example: For pipelines conveying extremely toxic media, when using the SH standard system, regardless of the operating pressure of the medium, the nominal pressure rating should not be less than PN5.0MPa; when using the JB standard system, it should not be less than PN4.0. For pipelines conveying hydrogen, ammonia, liquefied hydrocarbons, and other similar media, when using the SH standard system, regardless of the operating pressure of the medium, the minimum nominal pressure rating should not be less than PN2.0MPa; when using the JB standard system, it should not be less than PN2.5MPa. For pipelines conveying general combustible media, when using the SH standard system, the nominal pressure rating should not be less than PN2.0MPa; when using the JB standard system, it should not be less than PN1.6MPa.

1.2.4 Medium Temperature and Additional Forces in Pipe Systems Many flange standards provide a note stating that the values in their temperature-pressure tables refer to the conditions where the flange is not subject to shock loads. In reality, when the flange is subjected to additional loads such as bending, vibration, and temperature cycling from the attached piping, it affects its sealing performance and even the reliability of its strength. At this point, these external loads should be converted into equivalent medium pressures to determine the required nominal pressure of the pipeline. The bending load applied to the flange is primarily caused by thermal expansion and contraction of the piping system. Generally, for flanges with a nominal pressure rating of PN2.0, when the operating temperature is greater than 200°C, or for flanges rated PN5.0 and above when the operating temperature is greater than 400°C, the additional loads exerted on the flange by the piping system must be considered; otherwise, the nominal pressure rating of the piping system should be increased.

1.3 Factors Influencing the Determination of Wall Thickness Ratings

1.3.1 Allowable Stress of Materials The allowable stress of materials refers to the value obtained by dividing the material’s strength index by the corresponding safety factor. Mechanical properties of materials include yield limit, strength limit, creep limit, fatigue limit, etc., each reflecting the limit values under different failure states. To ensure the strength reliability of pipelines during operation, the stresses in pipeline components are often limited to a certain value under each strength index, which is known as the allowable stress. When the stress in a pipeline component exceeds its allowable stress value, it’s considered that its strength cannot be guaranteed. Therefore, the allowable stress of materials is a fundamental parameter for determining the wall thickness rating of a pipeline. Different design standards select different allowable stress values for materials. For pressure pipelines, domestic design standards use the allowable stress values specified in GB150 “Steel Pressure Vessels,” while ASTM materials use the allowable stress values determined by the ASME B31.3 “Process Piping” standard.

1.3.2 Corrosion Allowance Corrosion allowance takes into account the reduction in pipe wall thickness caused by corrosion of the medium and thus increases the wall thickness of the pipe. Its size directly affects the value taken for the pipe wall thickness, or in other words, directly influences the determination of the wall thickness rating. Currently, China lacks comprehensive data on corrosion rates of various materials under different conditions exposed to various corrosive media; therefore, in engineering practice, the corrosion allowance is mostly determined based on experience. Many engineering companies or design institutes at home and abroad typically classify corrosion allowances into the following four levels: a. No corrosion allowance. This is commonly used for stainless steel pipes. b. 1.6mm corrosion allowance. Often used for carbon steel and chrome-molybdenum steel with moderate corrosion. c. 3.2mm corrosion allowance. Frequently used for carbon steel and chrome-molybdenum steel pipes with severe corrosion. d. Reinforced level (greater than 3.2mm) corrosion allowance. For pipelines under special conditions such as solid particle erosion, the specific value should be determined based on actual conditions.

1.3.3 Manufacturing Wall Thickness Deviation of Pipes and Their Components During the manufacturing process of pipes and their components, there are both positive and negative deviations relative to their nominal wall thickness (also called theoretical wall thickness). Therefore, when determining the nominal wall thickness of pipes and their components, one must consider the possible negative deviation values. The negative deviation values stipulated in various steel pipe standards are not entirely the same. The negative deviation values specified in GB/T8163 “Seamless Steel Pipes for Fluid Conveyance” and GB/T14976 “Seamless Stainless Steel Pipes for Fluid Conveyance” are as follows:

Table 1-3 Negative Deviation Values of Commonly Used Standards

Material Standard Wall Thickness (mm) Deviation Value (%)
GB/T8163 ≤20 +15, -10, +12, -5, -10
GB/T14976 <15 +15, -12.5
≥15 +20, -15

1.3.4 Welding Coefficient The welding process of metals is essentially a metallurgical process with clear characteristics of cast structure. Generally, casting structures have more defects and reduced material properties. For welded pipes and their components with longitudinal welds and spiral welds compared to seamless pipes and their components, an engineering reduction factor (welding coefficient) is commonly given to measure the extent of mechanical property degradation. The values of the welding coefficients are shown in Table 1-4.

Table 1-4 Welding Coefficients of Welded Steel Pipes

Serial Number Welding Method Joint Type Weld Type Inspection Method Welding Coefficient
1 Forge Welding Butt Weld Straight As per Standard 0.6
2 Resistance Welding Butt Weld Straight or Spiral As per Standard 0.85
3 Arc Welding Single-Sided Butt Weld Straight or Spiral Without RT, 10% RT, 100% RT 0.8, 0.9, 1.0
Double-Sided Butt Weld Straight or Spiral Without RT, 10% RT, 100% RT 0.85, 0.9, 1.0
RT: Radiographic Testing

1.3.5 Design Life a. Design life is related to the corrosion allowance of pressure pipelines. For uniform corrosion, once the annual corrosion rate is known, the corrosion allowance required based on the planned design life can be easily calculated. b. Design life is also related to factors such as the number of loading cycles under alternating stress, the incubation period of hydrogen damage, and the growth period of fracture factors. c. It is related to the initial investment in pressure pipelines, the compensation period for capital, and the cycle for technological updates. d. A U.S. magazine recommends a design service life of 5 years for carbon steel and 10 years for chrome-molybdenum steel and stainless steel. The SH3059 standard specifies a design life of 15 years. Some foreign engineering companies specify a general duration of 10 years for EPC projects and 15 years for non-EPC projects to maximize profit margins.

1.4 Commonly Used Design Standards for Pressure Pipeline Materials

  1. GB50316-2000 “Design Code for Industrial Metal Piping”;
  2. GB50251-94 “Design Code for Gas Transmission Pipeline Engineering”;
  3. GB50253-94 “Design Code for Oil Transmission Pipeline Engineering”;
  4. GB50028-93 “Urban Gas Design Code” (Revised in 1998 Edition and Partially Revised in 2002);
  5. GB50030-91 “Design Code for Oxygen Plant”;
  6. SH3059-2001 “General Rules for Selection of Materials for Petrochemical Pipeline Design”;
  7. SH3064-1994 “Selection, Inspection, and Acceptance of Steel General Valves for Petrochemical Industry”;
  8. HG/T20646 “Provisions for Material Design of Chemical Plant Piping”.