––Butt Welded PE-X/Al/PE-X Multi-layer Pipe 

Abstract: This paper describes the definition, variety, material classification and advantages of the PE-X/Al/PE-X multi-layer pipe in detail, and compares the thermal properties and cost between the PE-X/Al/PE-X multi-layer pipe and other pipes for hot water transportation. The temperature resistance performance of various hot water pipes is also discussed in this paper. Finally a conclude is acquired that the butt welded PE-X/Al/PE-X multi-layer pipe is the unique pipe for transporting 95℃ hot water for long term.
 

 Key words: butt welded PE-X/Al/PE-X multi-layer pipe, hot water pipe, temperature resistance (95℃) for long term
 

1. PE-X/AL/PE-X MULTI-LAYER PIPE INTRODUCTION
PE-X/Al/PE-X multi-layer pipe takes PE as the internal and external layers and aluminum pipe as the middle strength layer with thermosol adhering each layer together (the structure is showed in Fig.1). This kind of pipe is fiver layers through one step of co-extrusion. The PE-X/Al/PE-X multi-layer pipe integrates the advantages of metal pipe and plastic pipe overcoming the disadvantages of metal pipe, such as heavy weight, easy to corrod and leakage, and easy to deposite, and the disadvantages of plastic pipe, such as poor strength, poor impact resistance, light and oxygen permeation (breading of microorganism). The PE-X/Al/PE-X multi-layer pipe has the advantages of reasonable structure, excellent performance, economical practicality, clean & healthy, easy to construct and install, and low comprehensive cost, etc.

The PE-X/Al/PE-X multi-layer pipe technique was firstly developed by UNICR company in German in 1980’s, and widely produced and spread in German, America, Japan, and China Taiwan in 1990’s. Later the production technique began to be introduced in Foshan Rifeng Company and Anhui Unicr Piping Company in middle of 1990’s. Guangzhou Advance-Tech Co., Ltd, who have been engaging in welding and piping technique from 1988, has successfully developed a whole set of automatic butt weldrf PE-X/Al/PE-X multi-layer pipe production line by himself in 1997, and has provided nearly 40 lines to customers. At present, the speed of up-to-date designed production line reaches above 28m/min.

 Fig.1: Butt welded PE-X/Al/PE-X multi-layer pipe

2. VARITY AND MATERIAL CLASSIFICATION OF PE-X/AL/PE-X MULTI-LAYER PIPE

1. CJ/T210-2005 PP-R/AL Stable Pipe (thermosol for adhering) (PP-R/AL/PP-R, PE/AL/PP-R pipe series are divided into three degree: S4, S3.2, S2.5).
2. GB/T18997.1-2000 PE-X/Al/PE-X Multi-layer Pressure Pipe (overlap welded) (PE/AL/PE, PEX/AL/PEX).
3. CJ/T108-1999 PE-X/Al/PE-X Multi-layer Pressure Pipe (overlap welded) (PE/AL/PE, PEX/AL/PEX).
4. CJ/T159-2002 PE-X/Al/PE-X Multi-layer Pressure Pipe (butt welded) (PE/AL/PE, PEX/AL/PEX).
5. CJ/T193-2004 Internal Welded PE-X/Al/PE-X Multi-layer Pipe (overlap welded) (PP-R/AL/PE-RT, PP-R/AL/PP-R)
6. CJ/T195-2004 External welded PE-X/Al/PE-X Multi-layer Pipe (overlap welded) (PP-R/AL/PE-RT, PP-R/AL/PP-R, PP-R/AL/PE)

3. ADVANTAGES OF PE-X/AL/PE-X MULTI-LAYER PIPE PIPE

3.1 Eximious hygienism: the PE-X/Al/PE-X multi-layer pipe takes special PE as internal and external layers, which is clean and innocuity. If the PE of PE-X/Al/PE-X multi-layer pipe is PE-RT, the material of pipe for hot water is PE-RT 2344E made by American Dowlex Company or PE-RT DX800 made by Korean SK company, and the material of pipe for cold water is made by Ameriican PHILPS company. The thermosol is made by Kuraray company. The aluminum strip is Al alloy 8011 made by Southwest Aluminum (Group) Co, Ltd.

The most important is that the middle layer of the PE-X/Al/PE-X multi-layer pipe is aluminum, which can separate light and oxygen. Everyone knows that the breeded microorganism need light and oxygen. The PE-X/Al/PE-X multi-layer pipe prevent breeding of microorganism and algae.

3.2 Excellent high & low temperature resistance: the PE-X/Al/PE-X multi-layer pipe has no cold-brittleness (brittle point is -70℃). This kind of pipe can be easily installed below -20℃. The long-term service life of PAP composite pipe is 95℃ (50 years, 1MPa), and the maximum service temperature is 110℃. The thermal expansion coefficient of PE-X/Al/PE-X multi-layer pipe is small, which is only one sixth of the plastic pipe. When the temperature changes, the PE-X/Al/PE-X multi-layer pipe only has small deformation. The PE-X/Al/PE-X multi-layer pipe is flexible coil pipe and the pipe itself can consume a certain of deformation, so the pipe system will has no thermal expansion deformation.

The temperature resistance of plastic pipe is poor. For example, the PPR pipe has cold-brittleness (the brittle point is -20℃). Therefore, it must be careful when lay the pipe in winter (below 5℃). And the long-term service temperature of PPR pipe is 70℃ (50 years, 1Mpa), and the maximum service tempterature is 90℃. In addition, the linear expansion coefficient of PPR pipe is large. Therefore, if the external ambient temperature (like summer and winter) or internal water temperature changes greatly, it will has large deform. This will lead to pipe system bend and deform, even leak.

3.3 Long service life: The PE (molecular chain structure is – (-CH2-CH2-)n-) used in PE-X/Al/PE-X multi-layer pipe is a stable structure material in plastics. And the aluminum layer separates the internal and external layers, so the external plastic layer can be added with stabilizing agent for resisting light, oxygen and aging without affecting the hygienism of the internal layer. Therefore, the aging resistance of the pipe is promoted.

3.4 Low comprehensive cost: PE-X/Al/PE-X multi-layer pipe is flexible pipe which can be bended randomly without rebounding, so it can save a lot of joints and the installation time. The installation cost is saved from both materials and manpower, and thereby comprehensive cost is dropped greatly.

4. COMPARISON OF THE PERFORMANCE AND COST OF VARIOUS HOT WATER PIPES
At present, the requirements for the hot water pipe stipulated in the internal standards are to transmit the hot water of 70℃, bearing 1MPa pressure for long term, 50 years service life. The characteristic of plastic pipe is its service life depending on the bearing temperature and pressure. The plastic pipe can bear higher temperature and pressure in short term service. Therefore, when evaluate the performance of pipes, it can not only base on the temperature and pressure bearing.

In rencent years, people has developed multi kinds of pipes for hot water transportation. The common ones are: PEX pipe, PPR pipe, PE-X/Al/PE-X multi-layer pipe, PE-RT pipe, and PB pipe. All these pipes can meet the application and hygiene requirements for indoor hot water pipe, and have corrosion resistance characteristic. All the pipes have international standards, and have been used in industrial civil architecture and housing project.
 

But on the aspects of temperature resistance and cost, these pipes are differrent. People know that “Rifeng pipe for 50 years”.The reasons why Rifeng dare to guarantee 50 years of service life can not separate with characteristics of the long-term service temperature 95℃ and instantaneously reaching 110℃, and the high pressure of PE-X/Al/PE-X multi-layer pipe. The key point of the pressure resistance is the quality of the butt welding seam. The butt welding has advantages of economic material consumptions, uniform transmission force, without obvious stress concentration. The welding type of butt welding is TIG or laser welding, so its welding strength is higher than the ultrasonic welding of overlap welding. The overlap welding can only weld aluminum strip whose thickness are 0.2~0.6mm, while the butt welding can weld aluminum strip whose thickness are 0.3~2mm, even thicker. The butt welding of aluminum pipe can realize process of extruding internal pipe, coating internal adhesive, wrapping aluminum strip and then welding the strip into pipes step by step. The best advantage of Guangzhou Advance-Tech Co., Ltd is to ensure high welding quality with high welding speed. The welding speed can reach 28~30m/min with keeping stability.

 
The methods for adhering each layer of PE-X/Al/PE-X multi-layer pipe can be physical or chemical compounding. The PE is in high-temperature melting status, and the aluminum pipe is in heating status, compounding a layer of adhesive between the PE and adhesive. Therefore the PE-X/Al/PE-X multi-layer pipe has five layers: PE, adhesive, Al pipe, adhesive, PE. All the five layers are melted together through high-temperature and high pressure. This multi-layer pipe integrates the advantages of plastic pipe and metal pipes with service temperature of 95℃.

5. DISCUSS
Table 1 lists the comparison of performance-price ratio of various hot water pipes. From table 1 we can see that the both the PE-X/Al/PE-X multi-layer pipe and PE-RT pipe have high service life in short term. But only the the PE-X/Al/PE-X multi-layer pipe has 95℃ service life in long term. In addition, the PE-X/Al/PE-X multi-layer pipe has low expansion coefficient, small thermal deformation, large thermal conduction coefficient, which can transmit the heat quantity in maximum degree.

Table 1 Comparison of performance-price ratio of various hot water pipes

Note 1: the both internal and external layers of the PE-X/Al/PE-X multi-layer pipe in the table are PEX.
Note 2: the data are exacted from the related papers in periodical journals.

In addition, the comprehensive cost of the butt welded PE-X/Al/PE-X multi-layer pipe is low. As small indoor pipe, the PE-X/Al/PE-X multi-layer pipe (Dn≤32) has obvious performance and price advantages: at first, the pipe whose diameter below Dn32 can be supplied by coil, so as to reduce the transportation cost and expensive fittings. Secondly, because its linear expansion coefficient is small and the flexible pipe can automatically flex for compensation, no cool and heat compensation measures need to be taken when use for hot water transmission(1).
 

Temperature resistance performances of serveral kinds of hot water pipes are discussed as follows:

The PB pipe has been widely used in USA, and the applied quantities are 1400 myriameter in domestic water pipe domain (in domestic water pipe domain, the consumption of PEX pipe was 50 myriameter, the PVC-C pipe was 1200 myriameter, and PE-X/Al/PE-X multi-layer pipe did not being used at that time). But POM (polyoxymethylene resin) fittings was falsely used, and the POM is damaged by the chlorine containing in the water leading to serious water quality accident. Because the PB pipe itself has no problem, the standards and laws did not restrict to use PB pipe system in USA. But this accident made the related manufacturing enterprises paid for 0.95 billion US dollars, and this fact made the PB pipe system exit American market.

In the documents “DRINKING WATER SYSTEMS” of Dow Chemical official website indicates that the temperature resistance range of the PE-RT pipe of 12-50mm for cold and hot water transmission is 20~90℃ (the maximum temperature reaches 110℃). Therefore, it must be careful when use this pipe transmittting hot water of 95℃. Fig.2 is the reference curve of predicating hydrostatic strength of PE-RT.

Fig.2: Reference curve of predicating hydrostatic strength of PE-RT

Fig.3: Constant strain and creep characteristic curve for various pipes at 70℃(4)

Fig.3 is the constant strain and creep characteristic curve for various pipes at 70℃. Under the condition of constant pressure, when the water temperature raises, the pipe life will drop at the rate of 2.5 times every 10℃ temperatue raising.

From Fig.3 we can see that the pressure bearing strength of the PE-X/Al/PE-X multi-layer pipe is the highest in the above four kinds of pipes. And the service life curve of the PE-X/Al/PE-X multi-layer pipe is a straight line without keen point, which is similar with characteristics of the metal pipe. Although the service life of PEX pipe is a straight line, its strength is lower than that of the PE-X/Al/PE-X multi-layer pipe, which is only one half of PE-X/Al/PE-X multi-layer pipe basically.

The service life curves of PB and PPR pipes pipe are broken lines having keen points, which indicate that the performance of the pipes will drop after using for a certain of time.

From the Fig.2, we can see that the service life curve of PE-RT at 70℃ is similar with the PB and PPR, so it can not used in hot water for long term.

Everybody knows that the plastic has obvious viscoelasticity, that is to say, the plastic has prominent “creep” and “stress relaxation”. For example, take the 315mm PE100 pipe for constant stress test at 20℃, its constant stress is below 8MPa, its strain is 0.7% in 0.05 hour, and the strain increase to 2% after 1000 hours, and the strain will be 3.5% at 50 years(2). It is obvious that the “creep” and “stress relaxation” are not only prominent but also rapid.
Stress relaxation refers to the inside stress of an object reduces along with the time expanding when the object strain keeping constant.

Creep refers to the material shape change increases along with the time expanding at a certain temperature and constant external force. As the large molecules of plastic pipe are loose and keep in plasticity mode in normal temperature, so the creep activity of the plastic depends greatly on the time and temperature, its mathematic representation is ε=σƒ（T•t）. That is to say, the deformation of plastic pipe will enlarge along with the temperature increasing and time expanding(3). In other words, the pressure bearing capacity of the plastic will drop along with the temperature increasing and time expanding.

The high polymer plastics are thermoplastic and thermosetting. Thermoplastic is a kind of melt, which has no fixing melting point, can melt after forming, and its creep period is short. The thermosetting plastic is cross linkage structure, which will not melt after forming, and its creep period is long with its performance being constant in 100 years. In the hot water pipe being common used at present, the PP-R, PE-RT, PB belong to thermoplastic pipe, while the PEX belongs to thermosetting plastic pipe. Therefore the butt welding PAP composite pipe using for hot water has support on having long creep period by using PEX as internal and external pipes.

Hense, the high molecular plastics are divided into thermalplastic and thermalsetting plastics. From the theory, the thermoplastic plastic is a kind of melt, which has no constant melting point. It can also be melted after forming. Its creep period is short. While the thermosetting plastic belongs to crystals, and it will not melt after forming. Its creep period is very long, and it nearly has no change in 100 years. In the common plastic pipes at present, PP-R, PERT and PB pipes belong to thermoplastic pipe, while PEX belongs to thermosetting pipe. PE-X/AL/PE-X multi-layer pipe takes PEX as the internal and external pipes, which is the support for PE-X/AL/PE-X multi-layer pipe transmitting 95℃ hot water in long term.

In addition, the aluminum alloy in PE-X/AL/PE-X multi-layer pipe has light specific gravity (about 2.7g/cm3), high specific strength (σt=110～120MPa), good ductility (cracking elongation rate ≥20%) being randomly bended without rebounding. The strength weakening temperature of aluminum alloy is above 250℃, which does not exceed the actual service temperature inside the butt welded PE-X/AL/PE-X multi-layer pipe (below 100℃), so the strength of aluminum alloy keeps constant. Thereby it provide a reliable support for the long term high temperature (95℃) resistance performance of butt welded PE-X/AL/PE-X multi-layer pip.

From the above comparison on the performance among various hot water pipes, we can find the PE-X/AL/PE-X multi-layer pipe is the optimal for transmission hot water of 95℃. In addition, the PE-X/AL/PE-X multi-layer pipe has some advantages which ensure it becoming the hot point in the market, suh as high pressure resistance, high sealing reliability, stable and reliable production process, safety, high production speed and efficiency, etc.

It is worthy to presnet that there are some bad PE-X/AL/PE-X multi-layer pipes in the market at present, which use wire drawing material replacing pipe material, and the middle aluminum pipe is not welded. In addition, use the PE-X/AL/PE-X multi-layer pipe transmitting gas without safety certification. Or use non PE-X/AL/PE-X multi-layer pipe transmitting hot water. Or test the butt welde PE-X/AL/PE-X multi-layer pipe by overlap welded PE-X/AL/PE-X multi-layer pipe. All these affect the excellent performance of the butt welded PE-X/AL/PE-X multi-layer pipe. Therefore, it must give your attention to choose this kind of product.

References:

1. Baiqian Li, Development situation and market foreground of PAP composite pipe industry
2. Lars-Eric Janson, Plastics Pipes for Water Supply and Sewage Disposal
3. Manjun, He, Polymer Physics, Fudan University Press
4. Zhenhua Na, Guoqing Yu, Application of plastic tube in heating system and its evaluation