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HDPE pipe construction specification
In the application of various buried pipelines , a key factor in the ability of the pipeline to achieve the specified long-term service life is the quality of the paving. The HDPE pipeline has a variety of unique properties to make the pipeline laying safer and faster , and the correct construction design and installation procedures will make these superior performance of the pipeline to a greater extent.
(1) Technical preparation before construction
A. You should be familiar with and master the construction drawings before construction ;
B. Prepare the corresponding construction equipment;
C. On-the-job training for the operators, after the training is qualified, the construction can be carried out ;
D. Accept the inspection of pipes and fittings according to the standard .
(2) Excavation of the trench
The excavation of the trench must be carried out in strict accordance with the excavation route and excavation depth guided by the design drawings or project supervision, and the alteration shall not be carried out without the consent of the relevant department .
HDPE pipes are flexible and lightweight, so long pipelines can be prefabricated on the ground. When local conditions permit, the ground welding of the pipeline can reduce the trench width of the trench. The excavation of the narrow pipe trench can be carried out by using a rotary excavator, a plow or a shovel. Generally, the minimum pipe top cover thickness buried in polyethylene pipes is:
A. The buried depth of the pipe buried under the roadway shall not be less than 0.9 m;
B. It shall be buried under the sidewalk or the pipe branch shall not be less than 0.75 meters;
C. The branch under the green belt or in the residential area shall not be less than 0.6 m;
D. In the permanent frozen soil or seasonal frozen soil layer, the buried depth of the pipe top should be below the freezing line.
In the solid and stable bottom of the trench, the width of the trench is determined by the operating space required for the construction . The size of the space must allow proper preparation of the bottom of the trench and the filling and compaction of the trench material. It is necessary to take into account the cost of trench excavation and the cost of purchasing landfill materials. The width of the trench generally takes into account the specifications of the pipe and the tamping tool used.
The table below gives the corresponding minimum width values:
Pipe nominal diameter (mm) minimum pipe width (m)
Generally, when connecting on the ground, the width of the ditch is D+0.3. When it is difficult to install or trench backfill in the ditch, if the backfill soil compactness requirement is not met, the ditch width is D+0.5, and the total width is not Less than 0.7 meters.
In the trenches of sand or silt, if it is not easy to make an upright trench sidewall , a slope of 45° or a material capable of supporting the trench wall can be made . If the trench is wide, the initial fill material must be compacted in order to support the final fill layer weight.
(3) Preparation of the bottom of the trench
For pressure systems such as water supply, sewage or long-distance transfer lines, unless the design drawings have special requirements, the horizontal accuracy of the bottom of the trench is generally not very high. For gravity drainage systems, the grade of the slope must meet the specified requirements.
If the bottom of the trench is fairly straight and there are essentially no large stones in the soil, then there is no need to level it. Of course, it would be better if it were a bottom layer of the trench that was not disturbed. However, if the bottom of the trench has been disturbed or must be disturbed during the excavation process, the compactness should at least reach the compactness of the surrounding material. The bottom of the trench is generally not more than 50 mm in diameter. Sharp stones with sharp edges and corners are mixed with some sand and clay. All specifications of HDPE pipes can generally accommodate a small amount of local pipe bottom unevenness, but if the backfill material contains sharp-edged stones or hard shale, stress concentration areas may be generated on the pipe surface to cause damage. pipeline.
For excavation in shale and loose rock soil, in order to avoid contact with loose rock, it is necessary to provide a uniform groove bed for HDPE pipeline. The general practice is to dig deeper than the specified level when opening the bottom of the control tank. At least 150mm, then backfill to the specified height with the appropriate landfill material and compact to a density of 90% or higher.
For unstable soils such as wet clay or sand with less supporting strength, the depth of trenching is 100-150 mm deeper than the specified value. The backfill is then carried out with the specified or original excavated material to ensure a uniform support for the HDPE pipe. In unstable organic soils, if the groundwater level at the installation site is so high that the pipeline is submerged, additional weight can be added to the pipeline to resist the buoyancy of the pipeline, but this additional design weight should not exceed the support of the foundation layer. strength.
(4) Laying of pipes in the trench
Before the pipe is placed in the trench, the pipeline should first be inspected thoroughly, and the pipeline is allowed to hang or roll into the trench without any defects. The pipes are usually pre-connected on the ground. Sometimes the pipes may be pre-connected into a number of pipe sections of about 150 meters in length, stored in a certain place, transported to the installation site when they need to be lowered and connected, and then hot-melt connections are used. These sections can be connected by mechanical connection.
Pipes with a nominal diameter of less than 20 mm can be manually pulled into the pipe trench; all large pipes, fittings, valves, fire hydrants and fittings should be carefully placed into the pipe trench with appropriate tools; for long-distance pipe lifting, A nylon rope is recommended.
(5) Final pipe connection and assembly
The hot-melt connection of the pipe in the trench is the same as the hot-melt connection of the pipe on the ground, but it must be ensured that the connected pipe must be cooled to the ambient temperature of the soil before it is connected.
When HDPE pipes are connected to metal pipes, water tanks or water pumps, flange connections are generally used. Flange connections are also available for HDPE tubing when it is not convenient to connect by hot melt. When the flange is connected, the bolts should be evenly tightened in advance, and after 8 hours, retighten.
(6) Pressure test of HDPE pipeline
The HDPE piping system should be pressure tested before it is put into operation. The pressure test includes two parts of the strength test and the water tightness test. Water is generally recommended as the test medium for testing. Strength test
After the air in the pipe to be tested is excluded, the pressure is increased to the required pressure value at a steady boosting speed, and the pressure gauge should be placed as low as possible in the pipe.
The pressure test can be carried out before or after the pipeline is backfilled. The pipeline should be covered with a certain interval, especially for the serpentine pipeline. In the pressure test, the pipeline should be fixed in place. The flange connection should be exposed to facilitate inspection of leaks.
The test pressure of the pressure test shall not exceed 1.5 times the pressure level of the pipe pressure class or the lowest pressure class of the system. At the beginning, the pressure shall be raised to the specified test pressure value and sufficient time shall be allowed to ensure sufficient expansion of the pipe. A process takes 2-3 hours. When the system is stable, raise the pressure to 1.5 times the working pressure, stabilize the pressure for 1 hour, carefully observe the pressure gauge, and patrol along the line if there is no visible leakage or occurrence during the test. With a significant pressure drop, the pipe passes the stress test.
During the pressure test, the pressure drop is caused by the continuous expansion of the pipe. It is normal to produce a certain pressure drop during the test, and it cannot be proved that the pipe system must leak or break.
Water tightness test
HDPE pipes are connected by electric hot melt, which makes HDPE pipes have superior watertight performance compared with traditional pipes. The test pressure of the water tightness test shall not exceed 1.15 times the pressure level of the pipe pressure level or the lowest pressure class of the system. When the pipe pressure reaches the test pressure, it shall be kept for a certain period of time to achieve the test medium temperature and the pipe ambient temperature in the pipe. Consistently, after the temperature and pressure are stable, start timing. Under normal circumstances, the water tightness test should be stabilized for 24 hours. After the test, if there is no obvious leakage or pressure drop, pass the water tightness test.
(7) Backfilling and compaction
In general, the following three types of materials can be used as backfill materials for HDPE pipes;
The first category: 5-40mm angular stone, including a large number of locally available materials, such as coral, crushed slag, gravel and broken shells;
The second category: coarse sand and gravel with a maximum diameter of 40 mm, including different grades of sand or gravel containing a small amount of powder, generally granular and non-sticky, may be wet or dry;
The third category: excellent sand and clay gravel, including fine sand, sticky sand and clay gravel mixture.
Under normal circumstances, the corner and initial backfilling requirements should be at least 90%, and the compacted layer should be at least 150mm from the top of the pipe. Direct compaction should be avoided for less than 300mm from the top of the pipe.
The final backfill may use the original excavation of soil or other materials, but it must not contain frozen soil, agglomerated clay and stones with a maximum diameter of 200 mm.