A strong competitor for steel pipes-reinforced thermoplastic (RTP) composite pipes

I. Introduction

In recent years, the plastic pipe industry has developed rapidly and has formed a large scale. The basis for the steady growth of plastic pipes is that their technology is developing rapidly, and new materials, new technologies and new applications continue to appear.

The scope of application of plastic pipes is very wide, from construction, public facilities to basic engineering, from industry, agriculture, mining to medicine and health, plastic pipes are being used in almost every field, and there are new applications and new needs to promote new development. . It is worth noting that in recent years, developed countries and regions in Europe and the United States have conducted a large number of in-depth experimental research on plastic pipe technology. From raw material development to pipeline system application technology, from basic theory to test standards, there have been continuous innovations in all aspects. Results are emerging. For the plastic pipeline industry to achieve leapfrog development, it is necessary to grasp the latest development trends of the international plastic pipeline industry in a timely manner and fully absorb advanced scientific and technological achievements.

Plastic has many recognized unique advantages, but its lower strength is its obvious disadvantage. Therefore, the application of all-plastic pipes is bound to be limited. The use of other high-strength materials and plastics to make reinforced composite pipes is a topic that everyone has been working hard to explore. The pipes currently used are mainly steel pipes and polyethylene (PE) plastic pipes. Steel pipes have the advantages of high strength and can be used in high pressure ranges, but they have the disadvantages of easy oxidation and corrosion, short life, and complicated construction. The characteristics of PE pipes are exactly the opposite of steel pipes: corrosion resistance, easy construction, but due to their relatively high strength Low, can only be used in the medium and low voltage range. In order to overcome the shortcomings of plastic pipes and steel pipes, composite pipes of various structural forms have emerged at the historic moment. Over the years, many new designs and patents have been proposed, but not many can gain a foothold in the market. Because realizing complementary advantages requires solving many technical problems, as soon as reinforced thermoplastic (RTP) pipes appear, Be sought after by related industries and become a welcome new product.

2. Structure and development of RTP Pipe    

Reinforced thermoplastic composite pipes (RTP pipe) is a high-pressure plastic composite pipe, which is composed of three layers of structural materials: the inner and outer layers are made of PE80 or more, and the middle layer is a reinforcing band made of reinforced materials (see Figure 1). The outer layer can choose different colors (usually white or black) according to the needs, and can use a special outer tube material to resist the erosion of ultraviolet rays; the reinforcing material of the middle layer can choose polyester fiber, aramid fiber or steel wire. The current development is mainly reinforced thermoplastic pipes-RTP (Reinforced Thermoplastic Pipes). These pipes are characterized by high strength and flexibility. They are often called Flexible pipes in the natural gas industry.

The main goal of the early development of RTP pipes is to meet the impossible performance of all-plastic pipes, such as higher working pressure, and less use in the field of lower pressure water and gas pipelines. Early development of RTP pipes focused on saving materials and reducing costs, such as various steel-framed PE pipes. Recent changes are that RTP pipes with high working pressure have begun to be developed. Some companies have started producing aramid fiber-reinforced RTP pipes, and some companies have also produced steel belt-reinforced RTP pipes with a maximum working pressure of 25 MPa. It is certain that the prospect of RTP tube development is very broad. At present, the application range of RTP tubes of various working pressure levels is:

High pressure RTP: 5 ～ 25MPa is mainly used in the field of oil and gas extraction;

Medium pressure RTP: 1.6 ~ 10MPa is mainly used in long distance natural gas pipeline network;

Low-pressure RTP: 0.2 to 1.6 MPa is mainly used in urban and rural water and gas pipeline networks

3. the advantages of RTP tube

The general trend of the global energy structure is the increasing use of natural gas, and the market for polyethylene gas pipelines is constantly expanding. This will in turn promote the application of natural gas in a large area along the line, opening up new market space for polyethylene gas composite pipes.

One of the trends in the development of polyethylene gas composite pipeline technology in the world is the adoption of higher standards. The new international standard for polyethylene gas pipelines, ISO 4437-2007, has been approved for release. The global standard for plastic composite pipelines will certainly follow. . Another trend is to explore the application of plastic composite pipes to high-pressure transmission pipelines for natural gas, and RTP pipes are powerful competitors who can take advantage of this. Its competitive advantages are mainly reflected in:

1).High use pressure 

There are two major types of reinforced plastic pipes: one is a thermosetting plastic reinforced pipe, commonly known as glass steel pipe; the other is a thermoplastic reinforced pipe RTP. Thermoset plastic reinforced pipes have been developed relatively early and there are many varieties. However, the RTP of thermoplastic reinforced pipes has been developed lately and has received more and more attention in recent years. At present, more RTP pipes are reinforced by aramid fiber. The working pressure of the aramid fiber reinforced RTP pipe can be as high as 9MPa ~ 14MPa, and the burst pressure can be as high as 40MPa, which can basically meet the various pressure requirements of the gas industry and reach the ultimate goal of replacing plastic steel pipes with plastic pipes.

2), easy to lay  

Because softer fibers can be used as the reinforcing material of the reinforcing layer, the RTP pipe can achieve high compressive strength while maintaining coilable flexibility. The outstanding advantage is that it is convenient and quick to lay, especially suitable for mechanized operations and No excavation construction.

3).Low resistance to conveying fluid

The RTP pipe uses polyethylene (PE) material as the inner pipe, and its inner wall equivalent roughness Δ can reach 0.05mm, which is much smaller than the inner wall equivalent roughness of ordinary new steel pipe of 0.19mm and the old pipe's inner wall equivalent roughness of 0.60mm. Therefore, the energy for transmitting gas can be greatly saved, and more gas can be transmitted under the same conditions, which particularly meets the environmental protection requirements of "energy saving and emission reduction".

4), can increase anti-static measures

A major disadvantage of PE pipe is that it is easy to accumulate static electricity due to non-conductivity, which easily causes gas explosion accidents. By adding a low-resistance metal tape to the middle reinforcement layer of the RTP pipe to conduct static electricity accumulated in the gas pipe, the safety of gas transportation is increased.

4. the connection of RTP tube

There are two pipe supply methods for RTP pipes: large pipe diameter (above D200mm) is supplied by fixed-length straight pipe, and small pipe diameter (below D200mm) is supplied by coil. RTP long coiled pipe reduces the connection workload and the need for pipe fittings. The pipe fittings with standard flange end faces have been connected at the factory.

There are two connection methods for RTP pipes: the connection between RTP pipes and RTP pipes; the connection between coiled RTP pipe ends and other pipes and fittings (such as valves, tees, etc.).

There are two types of RTP pipe end connection technology: electrofusion-hot melt connection and mechanical compression connection. The electric fusion-thermofusion connection is to first thermally weld the RTP tube, and then move the special reinforced electric fusion connection sleeve to the connection to be electrically welded (the connection sleeve can bear the axial load). The welding equipment used is similar to that of ordinary polyethylene piping systems, except that the length of the hot-melt sleeve is longer. ) Mechanical compression connection is to insert the socket end of a metal pipe into the RTP pipe, and coat a metal sleeve on the RTP pipe, and mechanically press the metal sleeve, the RTP pipe end, and the socket end of the pipe to ensure the seal and withstand the axial direction. The load joint is similar to the connection of a high-pressure hose.

 5. Outlook

         The advantages of RTP pipes are outstanding, and their application fields will inevitably be further expanded. At present, everyone is working hard to apply RTP to natural gas high-pressure transmission pipelines (at present, the polyethylene pipeline system is only applied to natural gas transmission and distribution pipelines at low pressures, that is, pipeline networks with pressures below 0.7 MPa). The thermoplastic reinforced pipe RTP is systematically tested and is preparing to develop relevant standards.

RTP pipes currently have a market, but the potential market is broader. Internationally, RTP pipe technology has been relatively mature, and specifications have gradually formed from design, manufacturing to construction. On the basis of the existing imported technology, the existing foundation and resources can be used, and this product can be developed without excessive investment. On the basis of mastering RTP pipe manufacturing technology, new products such as flexible multilayer composite reinforced pipelines (offshore oil extraction) can be further developed. At present, the conditions for the development of RTP pipes have been basically mature, and discussions have started on the development of steel wire reinforced polyethylene pressure pipes with a nominal pressure of 2 MPa and a diameter of 500 mm to 1000 mm for the purpose of saving materials and improving pressure resistance. It is expected that the wall thickness of PE100 pipe can be reduced by half, and the cost can be reduced by 20% to 30%. From this point of view, RTP pipes can completely replace steel pipes in higher pressure areas in the future.