Polyethylene pipe welding

FT-IR spectroscopy has been used in the investigation of welding crosslinked polyethylene pipes [23]. Three types of crosslinking systems were used namely, peroxide (PEXa), silane (PEXb) and electron beam (PEXc). Scholten and co-workers [23] observed that only PEXa pipes have a satisfactory electrofusion quality. The strength of electrofusion welds of PEXb and PEXc pipes is not acceptable. The most likely explanation for the differences in weld quality is related to the adhesion theory and more specifically to differences in composition. Figure 5.6 shows the infrared spectra of medium density polyethylene (MDPE), PEXa, PEXb and PEXc. 

Some trouble was also experienced with the welds of 8-inch diameter high-density polyethylene pipe, which was later replaced by rubber-lined mild steel, or acrylonitrile-butadiene copolymers. The latter was used for all the smaller diameter piping, but was not available in the larger sizes (8-inch diameter and above), when the plant was being built. 

Munn. I. J.. and Georgiou. G. A. (1995). Ultrasonic and radiographic NDT of butt fusion welds in polyethylene pipe. Plastic Pipes IX Conf.. Scotland. UK, 18 21 Sept. 1995. 

A weld factor of 1 or greater indicates that the joint is as strong or stronger that the parent pipe. Joints in polyethylene pipes are qualified ranging from bad to excellent, according to Table 2 [10]. 

A Russian ammonia pipeline of nearly 2400 km extends from Togliatti on the Volga River to the Port of Odessa on the Black Sea, and a 2200-km, 250-mm dia branch line extends from Godovka in the Ukraine to Panioutino. The pipeline is constmcted of electric-resistance welded steel pipe with 7.9-mm thick walls but uses seamless pipe with 12.7-mm thick walls for river crossings. The pipeline is primed and taped with two layers of polyethylene tape and suppHed with a cathodic protection system for the entire pipeline. Mainline operating pressure is 8.15 MPa (1182 psi) and branch-line operating pressure is 9.7 MPa (1406 psi) (11). 

It is less resistant to aliphatic hydrocarbons than polyethylene and polypropylene and in fact pipes may be solvent welded. At the same time the resistance to environmental stress cracking is excellent. 

Heat-shrink crosslinked polyethylene DIN 30672 Flame or heat gun in fieid 1-25-2-25 Utiiised for coating of field weld joints and repairs on extruded polyethylene coated pipes. Careful application required to achieve consistent bond. 

Saran (Dow polyvinylidene dichloride) is a tough, chemically resistant plastic available in a variety of forms that are useful in the laboratory. Saran pipe or tubing can easily be welded to itself or sealed to glass and is useful for handling corrosive solutions. Thin Saran film, available commercially as a packaging material, is useful for windows, support films, etc. Mylar (du Pont polyethylene terephthalate) film and other polyester films are also useful for these purposes. Mylar is chemically inert and has excellent electrical properties for electrical insulation and for use as a dielectric medium in capacitors. Much thinner than these are films that can be made in the laboratory by allowing a dilute ethylene dichloride solution of Formvar (polyvinyl acetal) to spread on a water surface and dry. 

An important application for polybntylene is plumbing pipe for both coimnercial and residential use. The exceUent creep resistance of polybntylene aUows for the manufacture of thiimer wall pipes compared to pipes made from polyethylene or polypropylene. Polybutylene pipe can also be used for the transport of abrasive fluids. Other applications for polybutylene include hot melt adhesives and additives for other plastics. The addition of polybutylene improves the environmental stress cracking resistance of polyethylene and the impact and weld line strength of polypropylene. Polybntylene is also used in pack aging applications. 

Inlet pipes are conventionally used to bring the water from the desired depth in the water source to the hatchery if gravity water is used, or to the pumping station if the water has to be lifted to the hatchery (Huguenin and Colt, 2002 Colt et al, 2008 Pulido-Calvo et /., 2008). It is also possible to utilize the tide to pump the water into the hatchery or to place the hatchery in the tidal zone, but water quality control is more difficult in such cases. Materials commonly used in the inlet pipe are polyethylene plastic (PE) and PVC. PE is a low cost material that is easy to handle and weld and does not release toxic substances. When dimensioning the inlet pipes, normal water velocity in the pipe is 1-1.5 m/s, but it is important to find a specific, suitable dimension. 

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