Discussion on some problems in the design of low p

Discussion on some problems in the design of courtyard low-pressure PE pipe

1 preface

pe (polyethylene) pipe is a new type of gas material. In terms of its performance, it has many incomparable advantages over steel pipe. At present, PE pipes are used more and more in gas systems. In practical engineering, there are also great differences between the design of PE pipe and steel pipe. In this paper, through physical and chemical methods such as blending, copolymerization or surface modification, the extruded board containing flame retardant additives can prevent accidental fire caused by small fire. Some specific problems encountered in the design of courtyard low-pressure PE pipes are discussed, and our experience and experience are discussed for reference

2 PE pipes are laid naturally

because the linear expansion coefficient of PE pipes is more than ten times higher than that of metal pipes, according to the flexibility of PE pipes, they can be laid serpentine or along the terrain. However, the flexibility of PE pipe has its own limitations. After bending, there is compressive stress on the inner side and tensile stress on the outer side, and it will also break under the action of additional pressure for a long time. Therefore, the bending radius of PE pipe should not be less than 25 times the nominal diameter, but when there are bell fittings on the bending pipe section, the allowable bending radius should not be less than 125 times the nominal diameter

3 when the PE pipe is buried under the non carriageway, the minimum soil cover thickness of the PE pipe buried under the non carriageway shall not be less than 0.6m. As the strength of PE pipe is less than that of steel pipe, the influence of external load on PE pipe should be considered for the community where PE pipe is laid first and then courtyard road is built. For example, at the construction site in Xin'an County, the designed soil covering thickness of PE pipe under the non carriageway is 0.6m, and the pressure test is qualified upon completion. Later, when the roads in the new community were built, they were tamped with a roller, and then tamped again during ventilation, and the strength test was unqualified. Find the air leakage point at the tee connection, which is the weak link in the PE pipe system. Therefore, when laying PE pipes under the non roadway in the new community, the soil cover thickness of the pipes should be appropriately increased

4 design of lead-in tube

there are three methods for the design of lead-in tube

(1) steel pipe is introduced, that is, the main pipe in front of the building is PE pipe. The three-way pipe is connected with a straight pipe steel plastic conversion joint, and then introduced with steel pipe. The steel pipe must be treated with corrosion prevention. The premise of this is that the lead-in pipe and the heat pipe are staggered to avoid vertical intersection with the heat pipe

(2) the PE lead-in pipe is led in and connected to the elbow steel plastic adapter. The joint part is below the soil freezing line, and the steel pipe must be subject to anti-corrosion treatment

(3) above ground steel plastic conversion can also be considered, because the underground is full of PE system, which can avoid underground metal corrosion and reduce the adverse impact of opposite pipes due to ground subsidence, but the above ground PE pipes should be protected by casings

5 connection of steel plastic conversion joint

steel plastic conversion is divided into integrated steel plastic conversion joint and flange steel plastic conversion

when the steel pipe section of the steel plastic transition joint is welded with the steel pipe, it is strictly forbidden to transfer the high temperature of welding to the PE end, so that the prestress release will cause leakage. Cooling measures should be taken in the steel plastic transition section. The usual practice is to wrap the steel plastic transition steel pipe end with a wet towel, and add water on the wet towel to cool it continuously during welding, so as to reduce the temperature and ensure the welding quality. After the steel plastic conversion joint is connected, the joint shall be protected against corrosion and can be directly buried

flange type steel plastic conversion is actually a component of PE flange and steel flange. PE flanges are usually manufactured by injection molding process, steel flanges are generally galvanized or plastic sprayed for corrosion prevention, and gaskets are generally nitrile rubber gaskets. All metal parts such as bolts can be buried directly after effective corrosion prevention. It is best to make a flange well for inspection

6 selection of butt joint between electrofusion and hot melt

electrofusion connection is one of the best methods for PE pipe connection. It is not only convenient to operate, but also safe and reliable, but the cost of electrofusion pipe fittings is high. The connection of electrofusion pipe fittings can be used for the connection of PE pipes of different grades and pipes of different wall thicknesses

hot melt butt joint is easy to be used for the connection of pipes with a wall thickness of more than 6mm. For pipes with too small butt wall thickness, the out of roundness of the pipe and the misalignment of the center line will greatly reduce the area of the welding interface and affect the welding quality. Hot melt butt joint technology is generally used for pipes or fittings with similar melt index, and the thickness of pipes is also the same. In the design, it is generally stipulated that PE pipes with a nominal diameter less than or equal to 90mm adopt electrofusion connection, and PE pipes with a nominal diameter greater than 90mm adopt hot-melt connection

7 laying of trace line

PE pipe is an electrical poor conductor, which cannot be found by traditional pipeline positioning method, but can be found by ground radar, but the cost is too high. At present, metal trace line is usually laid with the pipe. The tracing line shall be laid close to the top of the pipe, and fixed with non-conductive materials at a certain interval. The tracer line should be kept continuous, and the joint should be clamped with tools to ensure that the conductivity will not be affected after connection. The insurance company should also consider the 70 year term of residential land. The exposed part of the joint is wrapped with insulating rubber tape to ensure good insulation. The lead-out point of the trace line should be located in the surge tank, surge tank or the ground with the lead-in pipe, and the lead-out trace line should be reserved for 1 ~ 2m. The length of the tracer line should be controlled within a certain range to ensure good conductivity. The tracer wire should be copper wire with s ≥ 25mm2

8 layout of PE pipes

the layout of PE pipes shall comply with the provisions of GB and CJJ. Due to the complexity of courtyard pipes, effective protective measures should be taken when it is difficult to arrange due to terrain constraints. If the vertical clear distance between PE pipe and water pipe cannot meet the requirements, use 40mm thick cement board to divide the gas pipe and water pipe. It is especially pointed out that when the horizontal and vertical clear distance between PE pipe and thermal pipe is not enough, especially for the directly buried thermal pipe with poor insulation, we should pay more attention to that the soil temperature should be below 40 ℃, and effective thermal insulation measures should be taken. If concrete casing is added, thermal insulation materials shall be filled in the casing if necessary. When PE pipes must pass through the thermal pipe trench, the pipes are usually wrapped with 50 ~ 100mm thick thermal insulation materials and then sleeved, and the two ends of the casing are blocked

if the gas used is wet gas, the PE pipe should be buried below the soil freezing line, and the condensate tank should be set at the low position of the pipe. The slope of the gas pipeline to the condensate tank should not be less than 0.003

references

[1] code for design of urban gas GB50028—2006. It is known as the industry "green card"

[2] Technical Specification for polyethylene gas pipeline engineering CJJ63—2008. (end)