Electrofusion welding is a form of resistive implant welding used to join pipes. A fitting with implanted metal coils is placed around two ends of pipes to be joined, and current is passed through the coils. Resistive heating of the coils melts small amounts of the pipe and fitting, and upon solidification, a joint is formed. It is most commonly used to join polyethylene (PE) and polypropylene (PP) pipes. Electrofusion welding is the most common welding technique for joining PE pipes. Because of the consistency of the electrofusion welding process in creating strong joints, it is commonly employed for the construction and repair of gas-carrying pipelines. The development of the joint strength is affected by several process parameters, and a consistent joining procedure is necessary for the creation of strong joints.
Advantages and disadvantages
Advantages of electrofusion welding:
Simple process capable of producing consistent joints
Process is entirely contained, reducing the risk of joint contamination
Process allows repair without the need to remove pipes
Disadvantages of electrofusion welding:
A special sleeve is required, so it is more expensive than other pipe joining methods such as hot plate joining Implanted coils make recycling of parts more difficult
Equipment
Electrofusion welds are performed by attaching a controlled power supply to the electrofusion fitting. There are typically two modes of operation.
Constant voltage
Constant current
Constant voltage is typically used for high pressure pipelines such as mains gas and water. Fittings are fitted with a barcode specified to an ISO standard. Typically fittings will be welded at 39.5v, but manufacturers can choose voltages in whole numbers from 8 to 48v. The welding time is specified on the label in seconds or minutes
Accessories
Electrofusion welding employs fittings that are placed around the joint to be welded. Metal coils are implanted into the fittings, and electric current is run through the coils to generate heat and melt part of the pipes, forming a joint upon solidification. There are two possible fittings used in electrofusion welding: couplers and tapping tees (saddles). Coupler fittings contain two separate regions of coils, creating two distinct fusion zones during welding. The inner diameter of the coupler is typically slightly larger than the outer diameter of the pipes. This is to increase the ease of assembly in the field and allows for minor inconsistencies in pipe diameter. Proper insertion of the pipes in the coupler is critical for the creation of a strong joint. Incorrect placement of the coupler can cause the coils to move and lead to the extrusion of molten polymer material from the joint, reducing the joint's strength. Tapping tees, or saddles, are less common but operate under the same principles as a coupler. They require clamping to ensure a proper fit up with the pipes.
Fitting installation
Installation of couplers and tapping tee fittings require slightly different procedures. Common installation steps for each are given below.
Couplers
Wash pipe ends to create clean surfaces for joining
Square pipe ends to facilitate optimal fit-up
Clean area where coupler will be placed with isopropyl alcohol
Mark the pipes slightly beyond half the length of the coupler, to indicate where scraping will take place in later steps Mark the area to be scraped Scrape pipe in marked areas to remove surface layer, allowing clean pipe material to contact the coupler Examine scraped area thoroughly, making sure that fresh pipe material is exposed throughout area Insert pipe ends into coupling to appropriate depth Secure coupler using clamp Connect fitting to control box using electrical leads Apply fusion cycle Allow joint to be undisturbed for the entire prescribed cooling time Pressure test pipe Back fill pipe with appropriate contents Begin service
Tapping tee
Wash pipe area to be joined to create clean surfaces for joining
Clean area where tapping tee will be placed with isopropyl alcohol
Mark the pipes slightly beyond the edges of the tapping tee location
Scrape pipe in marked areas to remove surface layer, allowing clean pipe material to contact the tapping tee Examine scraped area thoroughly, making sure that fresh pipe material is exposed throughout area Place tapping tee onto joint Secure tapping tee using clamp Connect fitting control box using electrical leads Apply fusion cycle Allow joint to be undisturbed for the entire prescribed cooling time Pressure test pipe Back fill pipe with appropriate contents Begin service