Bellows compensator is a kind of compensation device that uses the effective expansion and contraction of elastic elements in the bellows to absorb the size changes of pipelines due to thermal expansion and cold contraction. In the direct buried heating pipeline, the external pressure single axial bellows compensator is widely used because of its characteristics of large compensation, long service life and good economy. However, it has high requirements for installation. It needs to be constructed in strict accordance with the design and technical specifications to ensure that the compensator is coaxial with the front and rear directly buried pipes. There should be no corner within 12 meters from the compensator. Before installing the elbow, the bottom of the pipe trench should be tamped and leveled, otherwise the service life will be shortened or even the pipe will become deformed and invalid.
The total length of the hot water pipeline of the centralized heating project of a company's coal mine shantytown reconstruction project is 19 kilometers. The whole line is directly buried. The geological conditions are complex, and the design temperature is 110 ℃. A total of 560 external pressure single axial bellows compensators are used. Due to various reasons, many compensators were damaged during the pipeline pressure test, resulting in large economic losses and affecting the construction period. According to the actual situation of the project, this paper discusses several situations that the axial bellows compensator is prone to deformation and failure during pressure test, and puts forward preventive measures for the majority of colleagues to pay attention to and prevent during construction.
The strength of the fixed pier does not meet the specified requirements. Due to the tight construction period, the strength test will be carried out on the third day of the fixed pier pouring in the test section. When the test pressure reaches 1.85MPa (the design water supply working pressure is 2.4 MPa, and the strength test pressure is 1.5 times of the design working pressure, that is, 3.6 MPa), the compensator will be stretched and scrapped. At the same time, the fixed pier will lose its effectiveness due to displacement, which requires rework.
According to the construction specifications, the concrete of the fixed pier shall reach the design strength before the pressure test, and the upper and surrounding parts of the fixed pier have been backfilled according to the specifications. The front side of the fixed pier is a 90 ° elbow, which bears the blind plate force during the pressure test. However, the fixed pier does not reach the design strength, and there is no backfilling around and at the top of the pier, thus causing deformation and damage to the fixed pier and the compensator.
2、 The division of pressure test section is unreasonable. When dividing the test section, the position of the compensator was not considered. A blind plate was directly added near the compensator for pressure test. There was no fixed pier between the compensator and the blind plate. The free end of the blind plate was not reinforced or was not properly reinforced. During the pressure test, the compensator was stretched under internal pressure and scrapped. In this project, a section of pressure test pipeline is 10 meters away from the non pressure test pipeline, and there is no fixed pier between the compensator and the blind plate. To ensure the safety of pressure test, the compensator is reinforced, but not in accordance with the manufacturer's requirements. Three 16 channel steels are used to connect the pressure test pipeline and the non pressure test pipeline, and three steel plates are used to fix the channel steels, When the test pressure of the return water pipe rises to 1.5MPa (the design return water working pressure is 1.6Mpa, and the strength test pressure is 1.5 times of the design working pressure, i.e. 2.4Mpa), the compensator suddenly stretches 180mm, and the channel steel is twisted into fried dough twist.
Long distance heat supply pipelines need to be pressure tested by sections. In addition to dividing the test section according to the construction team or construction team, the location of the compensator and the fixed pier should also be considered. Generally, there should be no free end, let alone the compensator as a free end. Otherwise, the pipe end or the compensator should be reliably reinforced to make it unable to move freely. Generally, the pressurizing section and non pressurizing section shall not be too far apart, and the two sections shall be connected with a pressurizing short pipe, which is generally no more than 1m, so as to facilitate disassembly. See Attached Fig. 1. The pipe end reinforcement shall adopt the steel pipe with the same diameter to be placed between two sections of pipes, so that the pipe end can be rigidly fixed without displacement, thus the compensator can avoid bearing internal pressure. If the pipe end cannot be rigidly fixed, the compensator shall be reinforced to make it rigidly connected. The reinforcement method of compensator is shown in the following section.
3、 There is an elbow at the front of the compensator. As the front end of the compensator is an elbow, the compensator is pressed without reinforcement, which causes the compensator to bear axial internal tension and deform and damage.
In the case of elbows, compensators should not be set up, because the elbows themselves have the thermal compensation function, and the compensators bear the blind plate force during the pressure test and pipeline operation. Therefore, in this case, the compensator should be cancelled in consultation with the designer. In case of pressure test without cancelling the compensator, the compensator shall be reinforced first, and then the pressure test can be carried out. Taking DN700 compensator as an example, as shown in Figure 2, 6 ∠ 100 * 63 * 8 angle steels or 10 channel steels shall be used for reinforcement, and uniformly distributed and welded along the circumference. One end of the angle steel is welded to the external protective pipe of the compensator, and the other end is welded to the connecting pipe through 100 * 250 * 10 steel plate. The weld length at both ends shall not be less than 200mm to ensure the welding strength. After the pressure test, the angle steel and steel plate shall be cut in time. Stubble shall be left at the welding seam during cutting, and the corrugated compensator shall not be damaged.
4、 The compensator itself has quality problems. Before installation, the compensator shall be checked and accepted together with the manufacturer, the owner and the supervisor to see whether its model and specification are consistent with the design, whether the size deviation is within the allowable range of the specification, and the product certificate shall be checked. Before installation, the model, installation position and installation direction of each compensator shall be checked according to the design drawings to prevent rework losses caused by unqualified products installed on the pipeline.
In addition, during the pressure test of the pipeline, the pressure shall be raised slowly to exhaust the air in the pipeline; The pressure holding time shall be strictly in accordance with the design or specifications and shall not be extended at will; The pressure shall not drop too fast during pressure relief to prevent negative pressure from forming, otherwise the fatigue strength of the compensator will be reduced. As the corrugated pipe is made of stainless steel, the chloride ion content in the water for pressure test shall not exceed 25PPM.
To sum up, the damage of the compensator is caused by the construction not in strict accordance with the construction specifications and design requirements. Therefore, we must carefully study and study the specifications and design drawings during the construction process, strictly implement the inspection application procedures and the equipment and materials mobilization acceptance procedures, and strictly follow the specifications and design requirements to avoid the damage of the compensator, reduce unnecessary economic losses, and ensure the construction quality and duration.
