What kind of pipelines do the rigid pipes and flexible pipes often mentioned in municipal pipeline engineering refer to? Is a steel pipe a rigid pipe or a flexible pipe? The following is introduced to you according to the definition in GB 50268-2008 "Code for Construction and Acceptance of Water Supply and Drainage Pipeline Engineering".
一、什么是刚性管道（Rigid Pipeline）？
● Definitions
Rigid pipes mainly rely on the strength of the pipe body material to support the external force of the pipe, and its deformation is very small under the action of external load, and the failure of the pipeline is due to the control of the strength of the pipe wall.
● Characteristics of rigid pipes:
- High strength
It relies primarily on the strength of its material itself to withstand external loads and pressures, rather than relying on the soil or other environmental factors surrounding the pipeline.
- Low distortion:
Under force or load, rigid pipes have very limited deformation and usually maintain a relatively fixed shape.
—— Failure of wall strength control:
The structural stability of the pipe is mainly determined by the strength of the pipe wall, and the failure of the pipe is usually due to insufficient strength of the pipe wall, not because of excessive deformation, and the strength of the pipe wall is a key factor in design and use.
—— High impact resistance:
It has good impact resistance and durability, can resist mechanical damage and external impact, and is suitable for high-pressure, high-strength engineering environments.
- Low environmental adaptability:
Due to the low deformation characteristics of pipelines, they are less adaptable to environmental changes such as foundation settlement and earthquakes, and are easily damaged under these conditions.
● Common rigid pipe types:
- Reinforced concrete pipes
Reinforced concrete pipes are pipes made of a combination of steel and concrete, in which steel provides tensile strength and concrete provides compressive strength, which is widely used in municipal and infrastructure projects that withstand high pressure.
—— Pre-stressed concrete pipes
Prestressed concrete pipes are prestressed on the steel bars before concrete pouring, so that the pipes can better resist external loads and internal pressures when used, thereby reducing cracks and deformation.
—— Prestressed steel cylinder concrete pipes
On the basis of the prestressed concrete pipeline, a steel cylinder is added to the inner layer to enhance the impermeability and compression resistance of the pipeline, which is often used in high-pressure water and gas transmission projects.
二、什么是柔性管道（Flexible Pipeline）？
● Definitions
Most of the vertical load of pipes with significant deformation under external load is balanced by the elastic resistance generated by the soil on both sides of the pipeline, and the failure of the pipeline is usually caused by deformation, rather than the failure of the pipe wall.
● Characteristics of flexible pipes:
—— Adaptable:
Flexible pipelines can adapt to geological and structural changes such as land subsidence and building subsidence, so the requirements for the foundation during construction are low.
—— Good bendability:
Flexible pipes can be bent and bypassed obstacles during laying, which is suitable for complex terrain and pipe layouts.
—— Affordability:
When the flexible pipeline is subjected to external loads, it will undergo large deformation, but the vertical load is balanced by the elastic resistance of the soil, which mainly depends on the support force of the soil on the pipeline.
—— Seismic Performance:
Flexible pipelines have good seismic performance and can maintain the stability of the pipeline system in earthquakes and other disasters.
● Common types of flexible pipes:
- Steel pipes
It mainly includes stainless steel pipes, welded steel pipes, seamless steel pipes, galvanized steel pipes, alloy steel pipes, plastic-coated steel pipes, etc.
—— Chemical building materials pipes
According to the GB 50268-2008 specification, chemical building materials pipe refers to glass fiber pipe or glass fiber reinforced thermosetting plastic pipe (referred to as glass fiber reinforced plastic pipe), rigid polyvinyl chloride pipe (UPVC), polyethylene pipe (PE), polypropylene pipe (PP) and its steel-plastic composite pipe.
- Ductile iron pipes with flexible interfaces
Refers to pipes made of ductile iron and connected by flexible connectors (e.g., rubber rings, flexible joints). Ductile iron has excellent mechanical properties and corrosion resistance, and the flexible connection enables the pipe to adapt to a certain displacement when the foundation changes or vibrates, reducing the damage that may be caused by the rigid connection.
3. Comprehensive comparison between rigid pipes and flexible pipes
Finally, a comprehensive comparison between rigid and flexible pipes is made in the form of a table
| characteristic | Rigid pipes | Flexible pipes |
| material with structure | It is made of strong and rigid materials such as concrete pipes, and the structure is stable | It is made of flexible materials (such as PE, PVC pipes, flexibly connected ductile iron pipes) and has a certain bending capacity |
| deformation ability | The deformation capacity is limited, and the structure remains relatively fixed | Able to accommodate a certain degree of deformation and bending. |
| Bearing mechanism | It mainly depends on the strength of the pipe itself to bear the load, and the failure is caused by the rupture or deformation of the pipe wall | Relying on the elasticity of the surrounding soil or environment to share the load, failure is usually due to excessive deformation. |
| Seismic performance | The seismic performance is poor, and failure may occur under dynamic loads such as earthquakes | The seismic performance is good, and it can maintain stability under dynamic loads such as earthquakes |
| construction request | The requirements for the foundation are high, and precise support and fixation are required | The construction has low requirements for the foundation and adapts to the changes in the foundation and complex terrain |
| apply scenario | Occasions with high strength and high stability requirements, such as high-pressure water pipelines | It is suitable for complex terrain and occasions that need to adapt to foundation changes, such as water supply and drainage systems |
There is a problem that according to this specification, steel pipes are classified as flexible pipes, rather than rigid pipes as we usually think, the reason seems to be that this code is mainly based on its mechanical behavior and deformation characteristics under external loads, rather than the traditional meaning of material properties.