Pipe Pressure Ratings
The components of pressure pipelines are generally standard parts; therefore, the design of pipeline components mainly involves the selection of these standard parts. Determining the pipeline pressure rating is essentially determining the rating of its standard parts. The pressure rating of a pipeline consists of two parts: The nominal pressure rating of standard pipe fittings expressed in terms of nominal pressure; The wall thickness grade of standard pipe fittings expressed in terms of wall thickness. Pipeline pressure rating: Typically, the parameter that reflects the pressure-bearing characteristics of a pipeline, determined jointly by the nominal pressure and wall thickness grades of the standard fittings used, is called the pipeline’s pressure rating. For simplification, it’s common to refer to the nominal pressure grade of the pipe fittings as the pipeline’s pressure rating. Determining the pressure rating is the foundation and core of pressure pipeline design. It serves as the basis for pipeline layout and stress verification, and it’s also an important factor affecting the initial investment and reliability of the pipeline infrastructure. 5.1 Design Conditions In engineering practice, process operating parameters should not be directly used as design conditions for pressure pipelines. Factors such as fluctuations in process operations, influence from connected equipment, environmental effects, etc., must be considered. A certain safety margin should be added to the process operating parameters to establish the design conditions. The design conditions primarily refer to design pressure and design temperature. Design pressure for pipelines: Should not be lower than the most severe conditions caused by internal (or external) pressure combined with temperature during normal operation. Most severe conditions: Refers to the conditions leading to the maximum wall thickness or highest nominal pressure rating of the pipes and pipeline components. Determining design pressure: Considering factors like hydrostatic pressure of the medium, the design pressure is usually slightly higher than the highest working pressure under the most severe conditions due to internal (or external) pressure and temperature. a. Determination of Design Pressure for Pipeline Components Under Normal Conditions Under normal circumstances, for convenience of operation, the method used for pressure vessels can be adopted, i.e., adding a safety factor to the corresponding operating pressure. Table 5-1 Determination of Design Pressure for Pipeline Components Under Normal Conditions
| Operating Pressure Pw (MPa) | Design Pressure P (MPa) |
|---|---|
| Pw ≤ 1.8 | P = Pw + 0.18 |
| 1.8 < Pw ≤ 4.0 | P = 1.1Pw |
| 4.0 < Pw ≤ 8.0 | P = Pw + 0.4 |
| Pw > 8.0 | P = 1.05 Pw |
※ If the design pressure determined according to this principle changes the pipeline pressure rating, it should be judged whether this operating pressure corresponds to the most severe conditions. If so, the design pressure may be taken as the highest working pressure under these conditions after approval by the relevant technical supervisor, without adding any coefficients.
b. When there is a safety relief device in the pipeline, If there is a safety relief device (such as a safety valve or rupture disk) in the pipeline, it indicates that there might be a possibility of exceeding the normal operating pressure during operation. The purpose of installing a safety relief device is to automatically release pressure when the system exceeds its normal operating pressure, protecting the hardware of the equipment and pipeline. In this case, the design pressure of the pipeline should not be lower than the set pressure of the safety relief device. c. High-head pumps in pipelines, For high-head pumps, especially reciprocating pumps, a high closed pressure can often occur within the pipeline and pump for a short time immediately upon startup. Sometimes this closed pressure can reach a very high value. At this point, the design pressure for the outlet pipeline of the pump should be the maximum closed pressure of the pump. D. Vacuum systems, The pressure that vacuum system pipelines bear is atmospheric pressure from the outside, so their design pressure should be 0.1 MPa external pressure. e. Pipelines connected to towers or containers, For pipelines connected to towers or containers, their design pressure should not be lower than the design pressure of the connected equipment. When there is a significant liquid column height in the pipeline, the static head of this liquid should also be considered. In fact, for pipelines, the forces they experience are more complex than those experienced by equipment, because apart from being subjected to media loads, they often also experience system forces due to thermal expansion and contraction. Therefore, the design pressure of pipelines should generally not be lower than the design pressure of the equipment. 5.1.2 Design Temperature Design temperature for pipelines: Should not be lower than the temperature under the most severe conditions due to internal (or external) pressure combined with temperature during normal operation. Most severe conditions: Refers to the conditions leading to the maximum wall thickness, highest nominal pressure rating, or highest material grade of the pipes and pipeline components. Determining design temperature: Considering the impact of factors like environment, insulation, operational stability, etc., the design temperature should be slightly higher than the highest working temperature under the most severe conditions due to internal (or external) pressure and temperature. a. Determination of Design Temperature for Pipeline Components Under Normal Conditions Under normal circumstances, for convenience of operation, the method used for pressure vessels can also be adopted, i.e., adding a safety factor to the corresponding operating temperature (except for flanges and bolts). Table 5-2 Determination of Design Temperature for Pipeline Components Under Normal Conditions
| Operating Temperature Tw (°C) | Design Temperature T (°C) |
|---|---|
| -20 < Tw ≤ 15 | T = Tw – 5 (minimum -20) |
| 15 < Tw ≤ 350 | T = Tw + 20 |
| Tw > 350 | T = Tw + (5 to 15) |
※ If the design temperature determined according to this principle changes the pipeline pressure rating or material, it should be judged whether this operating temperature corresponds to the most severe conditions. If so, the design temperature may be taken as the highest working temperature under these conditions after approval by the relevant technical supervisor, without adding any coefficients. Design temperature for flanges and gaskets should not be lower than 90% of the highest working temperature. Design temperature for bolts and nuts should not be lower than 80% of the highest working temperature. b. Jacketed or externally heated pipelines, For jacketed or externally heated pipelines, if the process medium temperature is higher than the heating medium temperature, the design temperature is selected according to the above table. If the process medium temperature is lower than the heating medium temperature, for jacketed heating, take the heating medium temperature as the design temperature, and for external heating, take the higher value between the heating medium temperature minus 10°C and the process medium temperature as the design temperature. c. Safety relief pipelines, Take the highest or lowest temperature that could occur during discharge as the design temperature for safety relief pipelines. d. Pipelines subjected to steam blowing, For pipelines subjected to steam blowing, if the medium temperature is higher than the temperature of the blowing steam, determine the design temperature according to the medium temperature based on the above table. If the medium temperature is lower than the temperature of the blowing steam, the appropriate level should be raised to accommodate the conditions of the blowing medium if the pipeline and its components selected according to the medium temperature cannot withstand the blowing medium. e. Pipelines operating under multiple conditions, For the same pipeline operating under two or more different conditions, the design temperature should be the highest working temperature under the most severe conditions due to internal (or external) pressure and verify against other conditions. f. Hydrogen service pipelines, For pipelines operating under hydrogen service, when consulting Nelson curves, add 30 to 50°C to the design temperature as the curve parameter value. This is because Nelson curves are statistical values, and cases of hydrogen damage have occurred when selecting materials near the curve. g. Pipelines with linings, For pipelines with insulating wear-resistant linings, the design temperature of the metal part of the pipeline should be determined through calculation or measurement. Generally, a design temperature of 250°C is recommended. h. When performing pipeline stress calculations, When performing stress calculations for pipelines with spring supports, the normal operating temperature of the medium is recommended as the calculation parameter. 5.2 Factors affecting the determination of pipeline pressure ratings Apart from the basic parameters of design temperature and design pressure, several other factors will also affect the determination of pipeline pressure ratings. 5.2.1 Application of Standard Systems Different standard systems have different series of nominal pressure grades and corresponding temperature-pressure tables. That is, with the same design conditions, choosing different application standards results in different nominal pressure grades. Therefore, before determining the nominal pressure grade of a pipeline, the application standard system should be determined first. 5.2.2 Materials Different materials have different mechanical properties, and thus different corresponding values on the temperature-pressure table in standards. Therefore, before determining the nominal pressure of a pipeline, the materials of the pipeline and its components should be determined. Material selection depends on the design temperature, design pressure, and operating medium. Materials standards for various components of pipelines are often different. Generally, pipes use pipe materials, flanges use forging materials, and valves mostly use casting materials. Regardless of the material standard used, they should be of the same grade, meaning they have equivalent adaptability to operating conditions and equal strength; attention should be paid to compatibility among pipe materials, plate materials, bar materials, and casting materials. 5.2.3 Operating Media Generally, the allowable pressure of the nominal pressure of a pipeline at a given temperature should not exceed its design pressure. For media that could cause serious harm or significant accidents due to failure of the pipe and its components, when considering their nominal pressure grades, the nominal pressure grades should not be determined solely based on the temperature-pressure table but should be appropriately increased to improve safety and reliability. Standards SH3059 and SYJ1064 provide detailed regulations on this matter, for example: For pipelines transporting extremely toxic media, when using the SH standard system, regardless of the operating pressure of the medium, the nominal pressure grade should not be lower than PN5.0 MPa; when using the JB standard system, it should not be lower than PN4.0. For pipelines transporting hydrogen, ammonia, liquefied hydrocarbons, etc., when using the SH standard system, regardless of the operating pressure of the medium, the minimum nominal pressure grade should not be lower than PN2.0 MPa; when using the JB standard system, it should not be lower than PN2.5 MPa. For pipelines transporting general flammable media, when using the SH standard system, the nominal pressure grade should not be lower than PN2.0 MPa; when using the JB standard system, it should not be lower than PN1.6 MPa. 5.2.4 Medium Temperature and Additional Forces in Pipe Systems Many flange standards note that the corresponding values in their temperature-pressure tables are based on flanges not being subjected to impact loads. In reality, when flanges are subjected to bending, vibration, cyclic temperature changes, and other additional loads from the pipeline, it affects their sealability and even the reliability of their strength. At this time, 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 mainly comes from thermal expansion and contraction of the pipeline system. Generally, for flanges of PN2.0 grade, when the operating temperature is greater than 200°C, or for flanges of PN5.0 and above grades when the operating temperature is greater than 400°C, the influence of additional loads generated by the pipeline system on the flange should be considered; otherwise, the nominal pressure grade of the pipeline system should be increased. 5.3 Factors affecting the determination of wall thickness grades 5.3.1 Allowable Stress of Materials Allowable stress refers to the value obtained by dividing the strength index of the material by the corresponding safety factor. Mechanical properties of materials include yield limit, ultimate strength, creep limit, fatigue limit, etc., which reflect the limiting values under different states of failure. To ensure the strength reliability of pipelines during operation, the stresses in pipeline components are often limited to a certain value below each strength indicator, which is the allowable stress. When the stress in pipeline components exceeds their allowable stress value, it is considered that their strength is no longer guaranteed. Therefore, the allowable stress of the material is the basic parameter for determining the wall thickness grade of the pipeline. Different design standards select different allowable stress values for materials. For pressure pipelines, the allowable stress value determined by the domestic design standard is based on GB150 ‘Steel Pressure Vessels’, while for ASTM materials, it is based on the allowable stress value determined by the ANSI B31.3 ‘Process Piping’ standard. 5.3.2 Corrosion Allowance Corrosion allowance takes into account the reduction in pipeline wall thickness due to corrosion caused by the medium, thereby increasing the wall thickness of the pipeline. Its size directly affects the value of the pipeline wall thickness, or in other words, directly affects the determination of the wall thickness grade. Currently, China lacks comprehensive data on the corrosion rates of various materials under different conditions when exposed to various corrosive media. Therefore, in most cases, the corrosion allowance is determined empirically in engineering practice. Many domestic and international engineering companies or design institutes typically classify corrosion allowances into four levels: a. No corrosion allowance. This value is commonly used for stainless steel pipelines. b. 1.6 mm corrosion allowance. This value is commonly used for carbon steel and chromium-molybdenum steel where corrosion is not severe. c. 3.2 mm corrosion allowance. This value is commonly used for carbon steel and chromium-molybdenum steel pipelines where corrosion is severe. d. Reinforced corrosion allowance (greater than 3.2 mm). This value is determined based on specific conditions for pipelines subject to solid particle erosion, etc. 5.3.3 Manufacturing Wall Thickness Deviations 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 (or theoretical wall thickness). Therefore, when determining the nominal wall thickness of pipes and their components, possible negative deviations must be considered. The specified negative deviation values in various steel pipe standards are not entirely the same. The following are the wall thickness deviation values specified in GB/T8163 ‘Seamless Steel Pipes for Fluid Conduction’ and GB/T14976 ‘Seamless Stainless Steel Pipes for Fluid Conduction’: Table 5-3 Wall Thickness 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 |
5.3.4 Welding Coefficient The welding process of metals is essentially a metallurgical process, with its structure having distinct casting characteristics. Casting defects are generally more numerous, and material performance is somewhat reduced. For welded pipes and their components with longitudinal and helical welds compared to seamless pipes and their components, an engineering strength reduction coefficient (i.e., welding coefficient) is commonly given to measure the degree of mechanical property decline. The values of the welding coefficient are shown in Table 5-4. Table 5-4 Welding Coefficient of Welded Steel Pipes
| No. | Welding Method | Joint Type | Weld Type | Inspection Type | Welding Coefficient |
|---|---|---|---|---|---|
| 1 | Forging Weld | Butt Weld | Straight | As per standard requirements | 0.6 |
| 2 | Resistance Weld | Butt Weld | Straight or Helical | As per standard requirements | 0.85 |
| 3 | Arc Weld | Single-side Butt Weld | Straight or Helical | Without RT | 10% RT |
| Double-side Butt Weld | Straight or Helical | Without RT | 10% RT | 100% RT | |
| RT—Radiographic Testing |
5.3.5 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 can easily be calculated based on the predetermined design life. b. Design life is also related to the number of load change cycles under alternating stress, the incubation time of hydrogen damage, the expansion period of fracture factors, etc. c. It is related to the one-time investment in pressure pipelines, the capital compensation period, and the technological renewal cycle. d. An American magazine recommends a design service life of: 5 years for carbon steel; 10 years for chromium-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 turnkey projects and 15 years for non-turnkey projects to maximize profits. 5.4 Commonly Used Design Standards for Pipeline Materials
- GB50316-2000 ‘Code for Design of Industrial Metal Piping’;
- GB50251-94 ‘Code for Design of Gas Transmission Pipeline Engineering’;
- GB50253-94 ‘Code for Design of Oil Transmission Pipeline Engineering’;
- GB50028-93 ‘Code for Design of Urban Gas Supply’ (Revised edition of 1998) (Partial revision of articles in 2002);
- GB50030-91 ‘Code for Design of Oxygen Station’;
- SH3059-2001 ‘General Rules for Selection of Materials for Petrochemical Pipeline Design’;
- SH3064-1994 ‘Selection, Inspection, and Acceptance of Steel General Valves for Petrochemical Industry’;
- HG/T20646 ‘Regulations on Material Design for Chemical Plant Pipelines’.
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