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Education : All about Plastic Pipes
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Butt Welding Guide NEXOR PIPES R&D June 6, 2006
The quality of High Density Polyethylene Piping Systems rests on the intelligent design of the piping system, the quality of piping materials, and the proper installation of pipes. While NEXOR PIPES are high quality pipes - guaranteed to pass and exceed international pipe quality standards, the failure of piping systems still cannot be avoided when poor design and improper installation methods are in play.
This butt welding guide enumerates some of the important pointers in performing butt welding; and this should be in no way a substitute for a complete course in butt welding. The two sections of this guide are the following: Theoretical Course on General Information and Butt Welding. The first section is a general overview of Polyethylene Piping systems including material properties and standard dimensional characteristics of PE pipes. The experienced user may skip the first section. The second portion familiarizes the reader with the Butt Fusion technique. Please be informed that NEXOR pipes claims no liability should the installation of pipes following the pointers presented below lead to piping system failure.
I. Theoretical Course on General Information
a. Characteristics of PE Compounds
HDPE stands for high density polyethylene. It is a type of plastic material. PE pipes refers to pipes that are made of such material – in the same way as PVC pipes refers to the type of pipe material. There are many types of PE – low density PE, Medium density, PE 80 and PE 100… and so on. It must be noted that the fusion of different plastic materials is not allowed. Thus, PP-R fittings cannot be fused with PE pipes.
b. Standardization of HDPE pipes
The standard color of PE pipes is black. The black color is the result of the addition of carbon black – which protects the PE pipe from UV radiation. NEXOR PE pipes are black in color but with blue stripes.
NEXOR PE pipes follow ISO 4427 – wherein the dimensional characteristics of the pipes follow the SDR standard. SDR is standard dimension ratio. The lower the SDR the higher is the pressure rating of the pipe.
II. Butt Welding
a. Principle
Butt welding involves the heating of two pipe ends to fusion temperature and then subsequently joining the two ends by the application of force.
b. Quality Butt Welds
The most usual cause of poor quality butt welds is the inattention of the butt welder to the proper timing, temperature, and pressure involved in each of the five steps enumerated above.
It is important that each of the steps above is monitored for pressure, time, and temperature.
A successful weld is simply described as having a joint equal or exceeding the strength of the pipe itself. Thus, a hydrostatic pressure test of the pipe with the joint must lead to a failure in the pipe section – not in the joint section.
NEXOR PIPES R&D has a complete set of hydrostatic burst test equipment. The people in the R&D department are capable of making quality butt welds and illustrating this with a burst test.
c. Equipment
The basic parts of the equipment are the following: 1. Planing Tool – used to face pipe ends. 2. Heating Plate - used to melt pipe ends 3. Frame – holds on to the pipes to be joined. It has hydraulic cylinders which apply pressure to the pipe joints. 4. hydraulic unit – has the hydraulic pump, tank, pressure gauges, directional valves, pressure regulation valves 5. Data Logger – records pertinent process data such as time, pressure and temperature. The data logger ensures that every joint is made properly.
d. Site Preparation
Before butt welding, the operator must ensure that the machine is situated in a dry area. The equipment may not be exposed to rain.
Appropriate measures must be taken when butt welding during in rainy weather. An outdoor roofing structure must be employed to protect the equipment from rain. The operator must also ensure that the equipment does not sit on wet ground.
e. Pipe Preparation
Pipe ends to be joined must be dry and free from foreign particles.
It is not necessary to perform butt welding in trench. The installation of HDPE pipes has the convenient advantage over other piping materials of above ground jointing. That is, HDPE pipes can be butt weld on ground and subsequently pushing the pipes into the trench.
f. Welding Procedure 1. Securely fasten the pipes to be joined to the butt welding tool. The pipes must be attached square to the axial direction. 2. Face the pipe ends. The process is complete when the shavings are continuous in length and are equal in thickness. The faced ends must be move to touch each other in order to check for square-ness. The operator must not touch that faced ends at this point in order to prevent contamination. 3. Drag force must be accounted. Heating pressure and soak pressure have to be about 0.16 N/mm^2. On the site however, long lengths of pipes being connected one by one presents a varying levels of drag force. The user thus must be able to compensate the drag force so that the effective welding or soaking pressure are maintained at 0.16N/mm^2. It is important that this pressure is attained. V Volume reduction of plastic in its solidification from the melted state would lead to cavitations (air pockets) in the joint if the proper level of welding pressure is not reached. 4. Melt the pipe interfaces. Follow the recommended heating times from the manufacturer. Generally, a set level of bead height must be attained for a given pipe wall thickness. Bead height refers to the melted plastic that forms around the outer circumference of the pipe. Bead height may vary from 0.5mm to more than 5mm. The recommended heating up pressure is 0.16 N/mm^2 (effective). [Again, effective means that drag forces must be compensated] 5. Soak the pipe ends in heat. When the designated heating time is up, the high pressure is relieved but the pipes remain in contact with the heating plate. Heat soak time varies from 5 seconds to several minutes. Heat soak pressure is about 0.02 N/mm^2 effective. 6. Changeover time. After the pipes have been properly soaked, the pipes must be immediately joined together. There is a changeover time – this refers to the maximum amount of time the pipes can be exposed to the atmosphere while removing the heating plate and moving the pipe ends together. Again, follow the recommended changeover time of the manufacturer of the butt welding equipment. 7. Welding (a.k.a. Cooling). After following the correct elapsed time for heating , soaking, and changeover, the pipe ends must be pressed together at an effective pressure of 0.15 N/mm^2. Cooling time under pressure varies from 6 minutes to 80 minutes. Failure to follow the correct cooling times may lead to the formation of air bubbles in the joint.
III. References
Chasis, David. Plastic Piping Systems. 2nd Ed. Industrial Press Inc. New York, NY USA. 1988 ISO 4427 Polyethylene pipes for water supply - Specifications ISO/TR 19480 Polyethylene pipes and fittings for the supple of gaseous fuels or water – Training and assessment of fusion operators |
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