Problems and solutions in the process of electrofusion connection of PE pipes

In recent years, with the rapid development of China's economy and urban construction, polyethylene pipes have been widely popularized and applied. With its advantages of excellent sanitary performance, light weight, corrosion resistance, special flexibility (flexible) and weldability, polyethylene pipes have been widely used in municipal and building water supply, gas transmission and distribution and other fields. The connection of polyethylene pipes includes electric fusion connection and hot fusion butt connection, which are widely used in the connection of medium and small diameter pipes. However, due to the short time of electric fusion pipe fittings used in China and the uneven quality management level of pipeline construction units, a large number of electric fusion pipe fittings connection quality problems occur every year. As an important part of the quality management process of polyethylene pipes, especially gas pipes, Strengthening the quality control in the construction process is the key to ensure the safe and stable operation of the polyethylene pipeline system. This paper starts with the phenomenon of the quality problems of electric fusion connection, analyzes the causes of the problems, and discusses the quality control of electric fusion connection from the aspects of construction personnel, electric fusion pipe fittings, welding machines and tools, construction environment and testing.

electrofusion connection is realized by sheathing the electrofusion pipe fitting on the outer surface of the pipe and electrifying and heating it. Electrofusion pipe fittings include sleeve, saddle, reducer, tee and bending head. Electrofusion connection is based on Joule effect. After electrifying, the resistance coil integrated on the inner surface of pipe fittings heats up, which causes polyethylene (PE) near the coil to melt, and the welding pressure generated by material expansion makes the connection interface weld together. There are two ways of electrofusion connection: socket connection and saddle connection.

the key of electrofusion connection technology is to select the pipe fitting that meets the standard, and the gap between the connection interface between the pipe and the pipe fitting is appropriate, and the connections with different specifications and sizes have their own optimal gap values; When scraping the oxide scale on the outer surface of the pipe before connection, the best interval required by the process should be ensured, so as to ensure that when the temperature of the connection interface reaches the specified value, the welding pressure established by the expansion of the material is balanced in the whole welding interface, so that the molecular chain can fully and evenly spread and entangle.

1. The defects of electrofusion joint mainly include bad operation and welding process defects

1. The influence of bad operation on welding quality

Bad operation refers to the operation of pipe assembly, positioning, treatment of oxide layer on the surface of pipe or socket pipe during electrofusion connection, which will adversely affect the interface quality and lead to welding failure, including the following situations:

a)

this problem is common in pipe connection, because the polyethylene pipe will form an oxide layer of about .1-.2mm on the surface of the pipe during the cooling stage in the production process, so the surface oxide layer in the welding area must be scraped before electrofusion connection. Otherwise, it will greatly increase the rate of bad interfaces and even cause 1% welding failure. It is also very important to scrape the surface of the electric heating wire covering line in the welding area for the saddle-shaped pipe fittings.

b) phenomenon that the electric melting pipe fittings are different from the pipes to be welded

When the electric melting pipe fittings are welded, the electric heating wires are electrified to heat the surrounding polyethylene materials. Under normal circumstances, the heating and expansion of the polyethylene materials are uniform, and the electric heating wires heat the surrounding polyethylene materials to a molten state, so that the molten materials will slowly fill the gaps, and there will be no stress phenomenon at the welding interface, so a relatively stable welding quality can be obtained. When the electrofusion pipe and the pipe to be welded are assembled, there is a phenomenon of different axes. The uneven gap produced by the different axes of the assembly causes the molten material to shift from the small gap to the large gap, which eventually leads to the accumulation of the resistance wire until it is short-circuited. This situation often leads to the phenomenon that polyethylene material is sprayed out of the welding interface in liquid state.

the fused pipe fitting is different from the pipe to be welded.

c) The pipe fitting and the pipe are not installed properly when welding.

When welding, the pipe is inserted into the pipe fitting at a position that is not marked before welding, that is, the insertion position is not in place, so that a part of the resistance wire will not contact with the pipe and be exposed in the air, resulting in uneven heating of the exposed heating wire during welding, resulting in smoking or even short circuit, which leads to welding failure.

fusion failure caused by not clamping the fused saddle-shaped pipe fitting. When installing the fused saddle-shaped pipe fitting, the screws of the pipe fitting should be tightened evenly to ensure that the entire welding end face of the fused saddle-shaped pipe fitting is closely attached to the pipe. If there is a gap between the fused saddle-shaped pipe fitting and the pipe fitting during installation, it will lead to empty welding of the saddle-shaped pipe fitting, which will lead to uneven heating of the exposed heating wire during welding and even short circuit, which will lead to welding failure.

fusion failure caused by not clamping the fused saddle-shaped pipe fittings

d) forced cooling with cold water after welding

The fused pipe fittings after welding need to be cooled normally and naturally to obtain the best welding effect. During the cooling stage, the electric heating pipe fittings and pipes can't be influenced by external forces, and cold water and other methods are not allowed to be used for forced cooling, because the cooling process of fused pipe fittings is the process of shrinking polyethylene (PE) materials, and forced cooling will cause uneven local cooling in the welding area, which will lead to welding quality.

2. Defects in welding process

a, lack of welding or insufficient welding degree

This phenomenon refers to brittle cracking at the welding place between the fused pipe fitting and the pipe during peeling test after welding, which may lead to heating of the pipe fitting itself after welding, ejection of the observation hole and even the pressure test after welding, but catastrophic accidents may occur after long-term operation of the pipeline.

the reasons for this phenomenon are as follows:

a) some manufacturers have the following phenomena: it takes about 4 seconds for the output voltage to reach the set voltage after the start of welding, which is fatal for small-caliber electrofusion pipe fittings with short welding time. For example, the welding time of dn63mm electrofusion sleeve is only 5 seconds, and welding with this welder is equivalent to shortening the welding time of electrofusion sleeve by 1%. In this case, the welding quality cannot be guaranteed.

b) The voltage during on-site welding can't meet the welding requirements of electrofusion pipe fittings due to the influence of welding machine or working environment, resulting in low welding heat and the welding area can't reach the molten state for normal welding;

The welding time is too short during on-site welding, which may be because the setting time is too short or the welding process is interrupted, and the welding heat is lower than the normal welding heat, resulting in welding failure;

c) The gap between the fused pipe fitting and the pipe is too large, and the gap of the whole welding interface cannot be fully filled after the polyethylene (PE) raw material reaches the molten state during the welding of the pipe fitting, resulting in the incomplete fusion of the whole welding interface or the low welding strength;

d) There is sand, water, oil and other pollution on the surface of electrofusion pipe fittings or pipes before welding. When welding, these impurities are directly mixed in the welding interface, which leads to a serious decline in the welding quality of the welding interface, thus causing brittle cracking of the welding surface.

B. Over-melting phenomenon of electric melting pipe fittings

Over-melting phenomenon of electric melting pipe fittings refers to that when electric melting pipe fittings are welded, polyethylene (PE) raw materials overflow or even spray out, and the appearance color of pipes or pipe fittings changes.

There are many reasons for the phenomenon of over-melting, mainly including the following:

a) The voltage during on-site welding exceeds the welding requirements of electric melting pipe fittings, resulting in excessive welding heat. After the welding area reaches the molten state, the polyethylene (PE) raw material is overheated, resulting in flowing overflow or even ejection;

b) The welding time is too long during on-site welding, which may be caused by too long setting time or abnormal welding process, and the welding heat exceeds the normal welding heat, resulting in over-melting.

Over-melting phenomenon After welding, the pressure tester focused on the connection of polyethylene (PE) pipes. The connection methods of PE pipes include: hot-melt butt joint, hot-melt socket and electrofusion connection. Whether hot-melt or electrofusion connection is used, the quality of welding machines and tools is a key factor to ensure the quality of welding interfaces. At present, automatic welding machines can be produced in China, and the verified process parameters can be automatically adjusted according to natural environmental conditions such as ambient temperature. Various process parameter records during welding can be stored and output, eliminating various human factors affecting the quality of welding interfaces, with reliable repeatability and traceability. Auxiliary tools, such as rounder and wall scraper, which are matched with welding machines and tools, are of great help to the welding quality and construction efficiency of pipes.

2. Welding technology

At present, China has comprehensively formulated standards for the products, design, construction and other aspects of polyethylene pipes. The connection technology of electrofusion pipe fittings can be operated according to the relevant standards in TSG D22-26 Technical Rules for Welding of Polyethylene (PE) Pipes for Gas, and the technological parameters of electrofusion connection should be welded according to the manufacturers of pipe components.

3. Environmental conditions for construction

In the original pipeline manufacturer, the environmental conditions are relatively easy to control, especially in the laboratory. However, in the field construction site, the environmental conditions are relatively harsh and vary greatly. If it is not well controlled, it will definitely affect the connection quality of polyethylene (PE) pipelines. In 29, the author dealt with the problem of adhesion caused by overheating welding in Taizhou, Jiangsu Province. In summer, the surface temperature of black polyethylene pipe can reach 6℃ in direct sunlight, and the process parameters of electric melting or hot melting connection, including temperature and time, must be adjusted at this temperature. Therefore, attention should be paid to the working environment of pipeline installation and laying: protective measures should be taken in rainy days, strong winds, low temperature and humidity; Welding process parameters must be revised for construction in high temperature or low temperature weather.

4. Process management

In the aspect of pipeline construction management, due to the short development time in China and the weak basic research work of applied technology, some designers and constructors lack understanding of the characteristics of the polyethylene pipeline system, and they can neither choose suitable materials for specific projects well nor correct construction technology, which makes it difficult to guarantee the quality of some projects. Therefore, while vigorously promoting the new polyethylene pipeline system, pipe fittings manufacturers should actively cooperate with the application design department, formulate practical and operable engineering installation schemes in accordance with the provisions of relevant standards, and constantly accumulate experience in construction, constantly improve and perfect, and strictly implement them to ensure the engineering quality of pipelines. In the process of construction, the construction personnel should strictly operate according to the verified electric melting or hot melting welding process parameters and procedures provided by the pipe and pipe fittings manufacturers, and make process records and self-inspection records of welding interface quality to ensure the stability, reliability and traceability of welding quality.

it should be emphasized here that the welding process parameters and regulations that have been verified by practice must be effectively implemented, which is closely related to the stability of welding equipment performance and the sense of responsibility of operators.