Chemical Piping Support - Helpful Tools and Resources
Tech Notes - Back Welding Polypropylene
Due to the high chemical resistance of polypropylene, the common method of joining by solvent bonding cannot be used. Hot gas back welding is the most feasible method for fabricating large size fittings and for repairing socket fusion joints. This process is suitable for Brownline, Blueline and Whiteline polypropylene products.
The equipment used consists of an electrically heated hot gas flameless torch, pressure regulator, compressed nitrogen gas and polypropylene welding rod. The use of nitrogen gas prevents surface oxidation and degradation of the material's properties. Temperature control is obtained by adjusting the gas pressure and the distance from tip to weld area. Welding control is then obtained by correct location and movement of the hot gas/weld tip.
Exact conditions of temperature, pressure and movement can only be found with experience. A few sample welds will allow conditions and technique to be evaluated. A quick test is to try to pull off the weld rod after cooling; the weld rod should be firmly adhered to the surface. As a guide, gas pressure should be between 2 and 3 psi, and the gas temperature should be about 570F, about 1/4" from the tip. All weld surfaces, including the weld rod, must be softened by the hot gas, and the weld rod gently pushed into the joint so that all the surfaces fuse together. If the temperature is too high, or the tip is moved too slowly, overheating of the surfaces and weld rod will occur, and it will not be possible to direct the weld rod smoothly into the joint.
Preparation and Welding of Pipe and Fittings
All surfaces must be clean; free from paint, grease, oil etc. Contaminated material must be removed. Use a file, or alternate to offer a fresh clean surface for the weld.
The hot gas is directed simultaneously to the welding rod and the welding surfaces of the pipe and fitting, the tip being held 1/4" from the surfaces and aimed along the V trough formed by the joint (see diagram). The welding rod is held at a 45 to 60 angle back from the torch. As welding progresses, the softened rod is fed into the trough to fuse with the softened surfaces of the pipe and fitting. At the end of a run, the unfused rod must be cut off, so that the next layer will be started against a welded section. Several well packed layers are preferred to a single run. The thicker the surfaces to be joined, the more layers are needed. Normally, three layers (see cross-section next page) will be adequate. The first is in the bottom of the trough, the second between the pipe and the first, the third between the fitting and the first layer.
Back Welding Instructions
Normally when backwelding (for polypropylene drainage), three layers as shown below should be adequate. For extra strength, add layers until weld equals wall thickness. Do not grind the welds; this will weaken the joint. Do not stress welds until they have completely cooled, and have achieved full strength.