Introduction to Pressure Bearing Standards for Steel Pipes
Pressure pipeline components are generally standard parts; therefore, the design of pressure pipeline components mainly involves the selection of these standard parts. Determining the pressure rating of the pipeline 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.
1.1 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 1-1 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 = 1.05Pw |

※ 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.
1.1.2 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 1-2 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.