How much do you know about PE pipes?

1. What does SDR stand for? How does it relate to pressure bearing capacity?

Answer: SDR is the standard dimension ratio of PE pipe, which is the ratio of the nominal outer diameter (de) to the nominal wall thickness (en). The specific formula is: SDR = de/en. The larger the SDR, the lower the pressure bearing capacity; conversely, the smaller the SDR, the higher the pressure bearing capacity.

2. How can you distinguish between polyethylene pipes made from PE80 and PE100 grade blends based on their color?

Answer: Currently, PE pipes made from PE80 blends are essentially gas pipelines. For polyethylene gas pipes made from blends, internationally, yellow is typically used to indicate PE80 grade material, and orange is used to indicate PE100 grade. For black pipes, those with a yellow stripe are PE80 grade, and those with an orange stripe are PE100 grade. This marking facilitates handling and allows for easy identification of the pipe material grade during future maintenance.

3. How do PE80 and PE100 grade materials compare?

Answer: PE80 appeared in the 1960s and is used to produce medium-density polyethylene pipe materials with long-term hydrostatic strength and resistance to cracking. PE100 appeared in the 1980s. At 20°C, it can still maintain the minimum required strength MRS of 10 MPa for PE pipes after 50 years, and has excellent resistance to slow cracking and crack growth. The main characteristics of PE100 are: (1) Stronger pressure resistance. For gas pipes, increasing pressure and reducing pipe diameter can be selected at the same time, which will reduce the total cost; (2) Thinner pipe wall. Under normal operating pressure, the wall of PE water pipes produced using PE100 can be thinner than that produced using PE80 grade raw materials; (3) Higher safety; (4) Higher hardness.

4. Why does the end of polyethylene pipe shrink? How to determine whether the end shrinkage meets the requirements?

A: End shrinkage in PE pipes is unavoidable in polyethylene pipe production. This is because PE water pipes are produced using a vacuum sizing and spray cooling method. During production, the pipe blank is pressed against the sizing sleeve by air pressure inside the pipe, while cooling water is sprayed on the outside of the pipe. Therefore, the pipe cools from the outside in. Due to the presence of circumferential stress in the pipe, which is restrained by cooling, the stress at the end is released after the pipe end is sawn, causing end shrinkage. During production, process control should be minimized to minimize the extent of shrinkage, as a large shrinkage can affect the thermal fusion butt welding of PE pipes.

5. What is the significance of slow crack growth (SCG)?

A: The entire system is a combination of internal pressure, stresses from cooling during pipe production, stresses during installation, soil, and traffic loads. Stress can cause cracks in the pipe. Once a crack has formed, its growth depends on the dominant stress (driving force) and the material's ability to resist stress growth. The advantage of flexible thermoplastic pipes like PE pipes lies in their ability to relax or redistribute these stresses. Ultimately, a higher resistance to slow crack growth means a higher level of safety for the pressure pipeline.

6. Can a pipe be directly butted with pipes (or fittings) of different wall thicknesses by heat fusion?

Answer: It is best not to do so. If a pipe is directly butted with pipes (or fittings) of different wall thicknesses by heat fusion, the cross-sectional areas of the heat fusion zones of the two pipes (or fittings) will differ, even though the same welding pressure is applied. This will not guarantee adhesion, affecting the joint quality. In this case, flange connections or electric fusion fittings can be used.

7. Can polyethylene pipes be used outdoors? How should the material be selected?

Answer: Polyethylene pipes can be used outdoors, but there are certain requirements for the raw materials used in the pipe production, including good weather resistance. Generally, a PE pipe material with a carbon black content of approximately 2.5% should be used. Polyethylene pipes with a very low carbon black content, which only serves as a coloring agent, should be avoided from exposure to sunlight. If conditions permit, it is best not to use it outdoors, after all, it will have a certain impact on the service life of the PE pipe.