What is the main difference between the design methods of compensated direct burial and uncompensated direct burial?

According to the traditional pipeline strength calculation method, the stress generated by various loads is not classified for compensated direct burial laying, and is controlled according to the strength conditions of elastic analysis theory or limit analysis theory. Compensators are used to absorb thermal displacement to reduce the temperature stress of straight pipe sections. The design result is that there are many fixed supports and compensators.

The strength calculation of uncompensated buried pipelines adopts advanced stress classification method, which classifies the stress and controls it under different strength conditions. The temperature stress is controlled by stability analysis theory, so that the straight pipe section can meet the strength conditions corresponding to stress analysis without the need to set compensators to reduce the temperature stress of the straight pipe section. The design result is that compensators and fixed brackets are rarely installed on pipelines.

Is the uncompensated cold installation method for directly buried hot water pipelines theoretically reliable?

The strength calculation of uncompensated cold installation directly buried hot water pipelines adopts stress classification method. This calculation method has been applied in the design standards of pressure vessels and steam water pipelines both domestically and internationally since the 1970s. In the fields of steam and water pipelines in thermal power plants, industrial metal pipelines, steel pressure vessels, and urban thermal power grids, China's current design standards also adopt stress classification method and stability analysis theory for strength verification.

The working temperature range and steel pipe material characteristics of thermal pipelines are suitable for using stress classification methods, including overhead laying, pipe trench laying, and directly buried laying pipelines. Due to the soil confinement effect, the strength calculation of directly buried hot water pipelines can be achieved using stress classification method for uncompensated cold installation, which is theoretically safe and reliable.

What are the weak points on the buried hot water pipeline?

There are two main reasons why accidents may occur in directly buried hot water pipelines: water ingress into the insulation layer and damage to the pipeline strength. Water ingress into the insulation layer can cause detachment of each layer of the insulation pipe, resulting in the loss of insulation, waterproofing, anti-corrosion, and transmission functions.

The main locations where water may enter directly buried pipelines are insulation joints, compensators, fixed brackets, etc.

From the perspective of pipeline strength, compensated, uncompensated preheating installation, and uncompensated cold installation of directly buried hot water pipelines are also relatively safe in straight pipe sections. The weaker parts are accessories such as elbows and valves.

4. Do uncompensated cold installations have higher requirements for pipeline materials and construction processes?

Regardless of whether the direct buried hot water pipeline adopts compensated, uncompensated preheating installation or uncompensated cold installation methods, the only difference is the design method. The weak parts of the direct buried pipeline are the same, and the material requirements and construction process for the insulation pipe are also the same. The uncompensated cold installation method has a simple construction process.

Why are compensators and fixed brackets not installed on long straight sections of uncompensated buried hot water pipelines?

Directly buried hot water pipelines use prefabricated insulation pipes that are tightly integrated with steel pipes, insulation layers, and outer protective layers. The binding effect of soil can prevent thermal elongation of the pipeline when it heats up. Within the working temperature range of the hot water thermal network, the temperature stress generated by the obstruction of thermal elongation can meet the strength conditions for stability analysis. Therefore, from the perspective of pipeline strength, compensators and fixed supports can be omitted in long straight pipe sections.

Why is it necessary to install a small number of compensators and fixed brackets for uncompensated buried pipelines?

There is a fundamental difference in the role of compensators and fixed brackets between uncompensated and compensated direct buried laying. The purpose of setting up compensators and fixed brackets for direct buried installation with compensation is to control the temperature stress inside the straight pipe section; The purpose of setting up compensators and fixed brackets for uncompensated direct burial is to control the thermal displacement of pipeline accessories. Because there are branches, valves, corners, reducers and other pipeline accessories on the thermal pipeline, the displacement and stress of these parts of the pipeline are complex. When the calculated displacement or stress exceeds the allowable range of the pipe fittings, different treatment methods can be adopted according to the site conditions, including setting compensators or fixed brackets.

What are the technical difficulties of uncompensated direct burial of hot water pipelines?

The difficulty of uncompensated direct burial laying lies in the high technical difficulty of design. To meet functional requirements and installation conditions, many pipe accessories need to be installed on the hot water pipeline network. The operation status of buried pipelines is complex, and as the temperature of the pipeline changes, the working state of the pipeline may deviate from the design state. Therefore, designers must be proficient in stress analysis methods and have considerable practical engineering experience. Based on the combination conditions of various factors in the pipeline network, they should comprehensively consider and calculate the straight pipe section and each pipeline accessory according to unfavorable working conditions, and take appropriate technical measures.

What is the function of preheating installation? Can preheating installation improve the safety of pipelines?

Preheating is an installation method of uncompensated direct burial laying. Preheating installation increases the installation temperature of the pipeline, thus reducing the displacement during the initial operation of the pipeline, which is suitable for special situations where the initial displacement of the pipeline needs to be controlled.

The strength verification of pipelines is independent of installation temperature and initial deformation, and there is no difference in pipeline strength and service life between preheating installation and cold installation methods. Preheating installation cannot improve the strength of the pipeline, so it is not safer than cold installation.