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FAQs on Common Knowledge of PE Pipe

Refer to PE pipe, there are always some common knowledge that we need to know. But what are them? Here we will introduce them to let you know more about PE pipe. Following are the FAQs.

Question: What does SDR mean? What does it have to do with pressure bearing capacity?

Answer: SDR is the standard size ratio of PE pipe, that is, the ratio of nominal outer diameter De to nominal wall thickness en of PE pipe. The specific formula is: SDR = de / en. The greater the SDR, the lower the bearing capacity. On the contrary, the smaller the SDR, the higher the bearing capacity.

Question: From the color of the pipe, how to distinguish the polyethylene pipe produced by PE80 and PE100 mixed ingredients?

Answer: Nowadays, PE pipes produced with PE80 mixed ingredients are basically gas pipes. For polyethylene gas pipes produced with mixed ingredients, yellow is usually used internationally to indicate that the raw material is PE80 and orange is PE100. If it is black pipe, grade PE80 with yellow strip and grade PE100 with orange strip. The purpose of such identification is to facilitate the use and judge the material grade of pipes in future maintenance.

Question: Comparison the raw materials of PE80 and PE100, what’s the difference?

Answer: PE80 appeared in the 1960s and is used to produce medium density polyethylene pipe materials with long-term hydrostatic strength and crack resistance. PE100 appeared in the 1980s. At 20 ℃, it can still maintain the minimum required strength MRS of 10MPa for PE pipe after 50 years, and has excellent resistance and slow crack growth. Its main characteristics are:

  • stronger pressure resistance. For gas pipes, increasing pressure and reducing pipe diameter can be selected at the same time, and the total cost will be reduced;

  • Thinner pipe wall. Under normal operating pressure, the wall of PE water supply pipe produced by PE100 can be thinner than that produced by PE80 raw material;

  • Higher security;

  • Higher hardness.

Question: Why does polyethylene pipe end shrink? How to determine whether the pipe end necking meets the requirements?

Answer: PE pipe end shrinkage is inevitable in the production of PE pipe. This is because the production of PE water supply pipe adopts vacuum sizing spray cooling method. During production, the pipe blank is pressed on the sizing sleeve by the air pressure inside the pipe, and the cooling water is sprayed outside the pipe. Therefore, the cooling of the pipe is from the outside to the inside. Due to the upward hoop stress of the pipe and being bound due to the cooling of the pipe, when the pipe end is sawed, the port stress is released, resulting in the necking of the pipe end. During production, the process shall be controlled as much as possible to reduce the shrinkage degree. Because the larger necking will affect the hot-melt butt joint of PE pipes.

Question: What is the significance of slow crack growth (SCG)?

Answer: The whole system is a complex of internal pressure, cooling stress during pipe production, stress during installation, soil and traffic load. Once the crack has been formed, its development will depend on the dominant stress (driving force) and the ability of the material to resist stress growth. The advantage of flexible thermoplastic pipes such as PE pipes lies in their ability to relax or redistribute these stresses. In the final analysis, the higher resistance to slow crack growth means that the penstock is more safe.

Question: Can pipes and pipes (or pipe fittings) with different wall thickness be directly butt fused?

Answer: It's best not to directly hot melt butt joint. If the pipes and pipes (or pipe fittings) with different wall thicknesses are directly hot-melt butt jointed, due to the hot-melt butt joint, the heating temperature of the heating plate is the same, the cross-sectional area of the hot-melt zone of the pipes and pipes (or pipe fittings) is different, and the applied welding pressure is the same, which can not guarantee the bonding effect between the two and affect the butt joint quality. In this case, flange connection or electrofusion pipe fitting connection can be adopted.

Question: Can polyethylene pipes be used in the open air and how to select materials?

Answer: Polyethylene pipes can be used in the open air, but there are requirements for the raw materials used in the production of pipes, which are required to have good weather resistance. Generally, PE pipe special materials with carbon black content of about 2.5% shall be selected, while polyethylene pipes with low carbon black content and coloring function shall not be exposed to the sun. If possible, it is better not to use in the open air. After all, it will have a certain impact on the service life of PE pipe.

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