Composite pipe for 95¡æ hot water for long term-butt welding PAP composite pipe
LI Xuemei Development Department, Guangzhou Advance-tech Co., Ltd. Research
Abstract: this paper introduces the definition, variety, material classification and advantages of the plastic-aluminum-plastic (PAP) composite pipe in detail, and compares the thermal properties and cost between the PAP pipe and other hot water pipes. The temperature resistance performance of various hot water pipe is also discussed in this paper. Finally a conclude is acquired that the butt welding PAP composite pipe is the unique pipe for hot water of 95¡æ for long term.
Key words: butt welding PAP composite pipe, hot water pipe, long term 95¡ætemperature resistance
1. PAP composite pipe introduction
PAP composite pipe takes PE as the internal and external layer and aluminum pipe as the middle strength layer with thermosol adhering to each layer (the structure is showed in Fig.1). This kind of pipe is fiver layers through one step of co-extrusion. The PAP pipe integrates the advantages of metal pipe and plastic pipe. Its advantages are reasonable structure, excellent performance, economical practicality, clean & healthy, easy to construct and install, and low comprehensive cost, etc.
The PAP 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 introduce 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 automatic butt welding PAP composite 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 welding PAP composite pipe
Item PEX pipe PPR pipe Butt welding PAP pipe PE-RT pipe PB pipe Long term service temperature (¡æ) ¡Ü90 ¡Ü70 ¡Ü95 ¡Ü90 ¡Ü90 Short term service temperature (¡æ) ¡Ü95 ¡Ü95 ¡Ü110 ¡Ü110 ¡Ü95 Softening temperature (¡æ) 133 131 133 123 124 Linear expansion coefficient (mm/(m¡¤¡æ)) 0.2 0.18 0.025 0.19 0.13 Thermal conductivity (W/m¡¤K)) 0.41 0.24 0.45 0.4 0.33 Oxygen barrier N N Y N N Hot melt N Y N Y Y Cost Low Low Middle High Highest
(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 PAP composite pressure pipe (overlap welding) (PE/AL/PE, PEX/AL/PEX).
(3) CJ/T108-1999 PAP composite pressure pipe (overlap welding) (PE/AL/PE, PEX/AL/PEX).
(4) CJ/T159-2002 PAP composite pressure pipe (butt welding) (PE/AL/PE, PEX/AL/PEX).
(5) CJ/T193-2004 Internal welding PAP composite pipe (overlap welding) (PP-R/AL/PE-RT, PP-R/AL/PP-R)
(6) CJ/T195-2004 External welding PAP composite pipe (overlap welding) (PP-R/AL/PE-RT, PP-R/AL/PP-R, PP-R/AL/PE)
3. Advantages of PAP composite pipe
3.1 Eximious hygienism: the PAP composite pipe takes special PE as internal and external layers, which is clean and innocuity. The most importance is that the middle layer of PAP composite pipe is aluminum, which not only can obstruct light but also oxygen. It can stop breeding of microorganism and algae plants, which the all-plastic pipe can not compare with.
3.2 Excellent high & low temperature resistance: the PAP composite pipe has no cold-brittleness (brittle point is -70¡æ). This kind of pipe can be easily installed below -20¡æ. The long-term service life of PAP composite pipe is 95¡æ (50 years, 1MPa), and the maximum service temperature is 110¡æ. The thermal expansion coefficient of PAP composite pipe is small, which is only one sixth of the complete plastic pipe. The PAP composite pipe is flexible pipe, so it will not lead to deformation of the pipe system because of thermal expansion.
3.3 Long service life: The PE used in PAP composite pipe is a stable material in plastic structure. And the aluminum layer separates the internal and external layers, so the external plastic layer can be added with light, oxygen and aging resistance stabilizing agent without affecting the hygienism of the internal layer. Therefore, the aging resistance of the pipe is promoted.
3.4 Low comprehensive cost: PAP composite 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 by the internal standards are to transmit the hot water of 70¡æ, bearing 1MPa pressure for long term, 50 years service life. Multi kinds of hot water pipes which can meet the above requirements are developed in the world for these tens of years, and common ones are: PEX pipe, PP-R pipe, butt welding PAP composite pipe, PE-RT pipe, PB pipe, etc.
People know that ¡°Rifeng pipe for 50 years¡±. The butt welding PAP composite pipe performance of long-term 95¡æ service temperature and reach 110¡æ at instantaneous time as well as high pressure bearing capacity makes Rifeng dare to guarantee 50 years of service life. The key point of the pressure proof is the quality of the welding seam. The butt welding has 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 0.2~2mm, even thicker. 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.
Table 1 is the comparison of performance-price ratio of various hot water pipes. From table 1 we can see that PAP composite pipe and PE-RT pipe can be used in short-term high temperature condition, while only butt welding PAP pipe can be use in long-term high temperature (95¡æ) condition.
Table 1 Comparison of performance-price ratio of various hot water pipes
Note 1: the external laye of the PAP composite pipe in the table is PEX.
Note 2: the data are exacted from the related papers in periodical journals.
The comprehensive cost of the butt welding PAP composite pipe is low. The PAP composite pipe as small indoor pipe (Dn¡Ü32) 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 and compensate, no cool and heat compensation measures need to be taken when use for hot water transmission(1).
The PB pipe has been widely used in USA. But used POM fittings by error, and the POM is damaged by the chlorine containing in the water leading to serious quality accident. Because the PB pipe itself has no problem, the standards and laws do 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 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¡æ (the maximum temperature reaches 110¡æ). The service life of this kind of pipe is 50 years and the pressure is 0.2~1.0MPa. 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¡æ(4)
Fig.3 is the constant strain and creep characteristic curve for various pipes at 70¡æ. From Fig.3 we can see that the pressure bearing strength of the PAP pipe is the highest in the above four kinds of pipes. And the service life curve of the PAP 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 PAP pipe, which is only one half of PAP 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¡æ is similar with the PB and PPR, so it can not used in hot water for long term.
Everybody know 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¡æ, 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 are loose and in plasticity mode in normal temperature, so the creep of the plastic depends greatly on the time and temperature, its mathematic representation is¦Å=¦Òƒ£¨T•t£©(3). That is to say, 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.
In addition, the aluminum or the aluminum alloy in the PAP pipe has light specific gravity (about 2.7g/cm3), high specific strength (¦Òt=110¡«120MPa), good ductility (cracking elongation rate ¡Ý20%) being randomly bended without rebounding. The strength weakening temperature of aluminum alloy is above 250¡æ, which does not exceed the actual service temperature inside the butt welding PAP composite pipe (100¡æ), so the strength of aluminum and aluminum alloy keeps constant. Thereby it provide a reliable support for the long term high temperature (95¡æ) resistance performance of butt welding PAP composite pipe.
From the above descriptions and discuss on the performance characteristics and cost comparison among various hot water pipes, we can find the PAP composite pipe is the optimal for transmission hot water of 95¡æ. At present, there are some bad PAP pipes in the market, which use wire drawing material replacing pipe material, and the middle aluminum pipe is not welded. Use plastic pipe wrapping aluminum pipe imitating PAP pipe. In addition, use the PAP pipe transmitting gas without safety certification. Or use PEX-Al composite pipe transmitting hot water. Or test the butt welding PAP pipe by overlap welding PAP pipe. All these bad butt welding PAP pipes affect the excellent performance of the butt welding PAP 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
