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INTRODUCTION
 Several elements allow the correct operation of a finished installation, among which is the procedure for welding pipes. And, one of the most common methods for joining copper pipes is precisely welds. Depending on the welding temperature there are two types of welds: soft and hard welds. In this opportunity we will first explain the principle of capillary. Further on, we will present information on the two types of welds, the quality of the weld, such as the filler material, the amount of heat applied, and the tools required, as well. Finally, we will present a very useful operational sequence.
1.
CAPILLARY 1.1 The phenomenon of Capillary If two pipes with different diameter are placed in a container with liquid, the level of the liquid in the container will be observed to be the same as in the pipe with greater diameter. However, the liquid is seen to rise in the pipe with smaller diameter due to the surface tension. Likewise, if the smaller pipe is replaced by two pipes placed one inside the other with a very small looseness, the liquid will be seen to rise in the space between them. This phenomenon is called “capillary”. This occurs not only with liquids but also with metals in fusion state, and its application is known as capillary weld.
Capillarity is better produced when the annular space between the pipe and the accessory is less and more regular. 1.2 Capillary weld In summary, the capillary weld consists of the union of a pipe and an accessory by filling a metal into the interstice (annular space) between them, in fusion state due to the heating of the joint. The perfect adjustment between the pipe and the accessory is of fundamental importance to obtain a well welded joint. The attraction force makes the molten weld to penetrate the joint in whatever position it would be, i.e., the weld elevates or depresses without any problem. This procedure has big advantages, particularly when joints have to be made in difficult places or of little accessibility.
2.
TYPES OF WELDS 2.1 Soft welds This term is applied to a series of alloys having in common a solidus point of less than 450°C (even lower than any of the metals being united). Considering that soft welds require lower temperatures than those of the solidus point of the parts to be united, there is low risk of damage.
2.1.1 Filler elements
The filler element is the metal alloy which will favor the union of pipes when melted.
Formerly, capillary weld processes used alloys of 50% tin (Sn) and 50% lead (Pb). Nowadays, it has been replaced with unleaded alloys due to the environmental restrictions regulating the presence of lead in the water. Currently, unleaded soft welds in the market have a high percentage of tin alloyed with other metal, which is added to improve the mechanical resistance properties. Applications requiring higher resistance or having to bear higher work temperatures (of up to 167ºC) use brass capillary welds. Likewise, it is the favorite alloy used in pipes for refrigeration purposes. Commercially, soft welds are found in reels and bars. Reels weight approximately ½ kg [1.33964 pounds] each and develop a length of nearly 7 m [22.965831 feet] with a weld diameter of 3 mm [0.11811 inch], an adequate diameter for sanitary installations.
2.1.2 Use of soft welds
2.2 Hard welds Hard welds are the union of metals by using heat and filler alloys, which solidus point is above 450ºC, lower than the solidus point of the metals to be united.
2.2.1 Filler elements Hard welds in copper pipes are found in the market as uncovered rods and flux-covered rods, which may be classified in two types:
The first class of alloys have a fusion gap according to the Cu, Ag, Zn and Cd alloys or, if not available, Ag, Zn alloys between 600°C - 775°C. The second class of Cu, AgP, and CuP alloys has a fusion gap between 650°C - 820°C. Silver alloyed with other materials equally virgin produce filler alloys that weld safely with high reliability and a very low work temperature. The following chart shows the silver percentages and temperatures for works requiring alloys. For example, the hard weld used in gas installations under intermediate pressure is P-15, containing 15% Ag and a solidus point of 650ºC. P-35 and P-45 with 35% and 45% Ag, respectively, are used under high pressures. Sanitary installers commonly use silver alloys due to its adequate fluidity. The fusion temperature required is obtained with the liquated gas equipment they use.
2.2.2 Use of hard welds
When both the soft and hard welds are applied it is important to consider the flux, the heat source and the accessories additionally to the filler element.
3.
THE
CLEANER When working with copper pipes, it is convenient to apply a welding paste over its polished surface: the flux. This is the metallurgic name received by certain materials capable of accelerating the coating of metals when heated by the filler alloy. The purpose of using flux is to eliminate oxides and other impurities from the areas and joints exposed to the weld action, favoring the fusion of the filler material. A good flux should have several qualities of effectiveness. Among them it is important to highlight the following:
Molten materials must float over the alloy bath so it would not produce slag inclusion. Final residues should be inactive, electrically insulated, and whenever possible, water-soluble.
3.1 Soft weld cleaner In general, this paste is composed by high-purity chemical components, such as: · Petroleum jelly, · Ammonium chloride, · Zinc chloride, · Water, wetting agents and others.
Due to their composition, these fluxes are suitable to be used with tin/lead soft welds, which solidus point range between 180ºC and 312ºC.
3.2 Hard weld cleaner Hard-weld deoxidizers are usually found in the market as powders, which may be dissolved in distilled water, thus obtaining a paste. Once in this state, the flux may be more easily applied with a paintbrush onto the clean surfaces of edges of the pipe and the accessory. Hard weld fluxes are recommended for all the cases requiring silver welds including steels, bronze copper, stainless steel, brasses and nickel-base alloys. They have an excellent deoxidizing capacity from 300ºC. Easily applied, fast dry and water-soluble.
3.3 Criterion for choosing a good cleaner Some of the criteria for choosing the quality of the cleaner are:
4.
HEAT
SOURCE Installers generally use burners or heater electrodes as heat sources.
5.
QUALITY OF WELDS The quality of a weld as a final product depends on several factors participating during the entire welding process. In effect, the probability of obtaining a good weld is related to: The specialist A good joint is the result of the efficiency of the specialist, who knows the materials and the procedure to be applied. Adequate material An optimum finishing is obtained when the filler alloy is used adequately and there is the knowledge of its yield temperature. Proper tools Using adequate tools offers the possibility of making perfect cuts and adjustments, which are very important to produce a well welded joint. Correct method according to the type and situation The variation between a well developed and a deficient technique may reflect the difference between a good-quality and a failed joint. Among the aspects characterizing a good-quality weld, the following should be considered: Weld strength or adhesion on the joint surface. Compact sealing and porosity-free preventing the leak of fluid from inside. Weld aesthetics by homogeneous application piling-free. The application of an appropriate yield temperature without weakening the pipe’s characteristics.
6.
OPERACIONAL
SEQUENCE 6.1 Soft welds 1. Right-angle cut to pipe. Be sure that the pipe’s cut is 90º.
2. Eliminate flashes Check inside the pipeline so there would be no flashes since they could cause possible oxidation. 3. Recalibrate edges This operation is necessary when pipe edges are damaged due to inadequate transport, stroke or fall. It is convenient to apply this process on annealed pipes. 4. Pipe cleaning and sanding This process is necessary before applying the flux. The area to be welded should be meticulously sanded, preferably with a metal sandpaper Nº 120. 5. Cleaning of accessory Great care in the accessory is also required to produce a good quality weld. 6. Application of the deoxidizer on the pipe and accessory. It should be used distinguishing between soft or hard welds.
7. Coupling of pieces Pieces should be thoroughly coupled. Before heating, the excess of flux should be removed with a clean cloth or absorbent paper. 8. Joint heating The burner flame should be controlled allowing a non-oxidizing calorific flame (blue, not yellow). Heat should only be applied on the area to be welded and at a proper distance.
9. Weld filling The filler material is applied once the flux starts boiling. The flame should be removed. 10. Elimination of residues Residues should be removed with a damp cloth.
6.2 Hard welds The first four steps of this sequence are almost the same as in the previous case. 1. Right-angle cut to pipe Be sure that the pipe’s cut is 90º. 2. Eliminate flashes Check inside the pipeline so there would be no flashes since they could cause possible oxidation. 3. Pipe cleaning and sanding This process is necessary before applying the flux. The area to be welded should be meticulously sanded, preferably with a metal sandpaper Nº 120. 4. Cleaning of accessory Great care is required in the accessory to produce good quality welds. 5. Cleaner application Three types of cleaner are used in hard welds:
6. Heating The heating process begins once the joint assembly is made. It is necessary to use whether propane or oxy acetylene burners to make the pieces reach the yield temperature of the filler metal.
When using an oxy acetylene burner, the flame should be controlled until slightly differential, showing a fine 7-8 mm blue tongue inside, near the burner edge. A special nozzle will be required to distribute evenly the flame’s calorific power. Initially, the flame is oriented only over the pipe for initial heating (at 2- 2.5 cm from the accessory). Keep the flame continuously moving perpendicularly to the pipe’s axis so the entire circumference will be covered and local reheating avoided. Continue until cleaner starts to melt, which occurs when it shows a transparent aspect. Direct the flame with a continuous movement towards the accessory to be heated evenly until it is also transparent. Then, direct the flame to the front and to the back in the direction of the joint’s axis thus avoiding local heating. In case of tubes with big diameter, it will be difficult to heat the entire joint at the same time. Therefore, it is necessary to use a burner with several heads. Likewise, it is advisable to preheat the entire accessory following the same instructions as for the pipelines with standard diameters. In case it is not possible to obtain an adequate temperature simultaneously in the entire joint, a part should be heated and bonded. The weld is vacuumed at an appropriate temperature into the interstice and then the burner is moved to the adjacent area continuing with the operation until completing the circle. 7. Weld application Once the joint is heated and without removing the flame to keep the temperature level, the weld alloy is filled in bringing the rod closer to the accessory’s edge. When the temperature reaches adequate levels, the filler material rapidly penetrates by capillary into the interstice between the pipe and the accessory. When this joint is filled, a continuous small cord will be seen near the pipe on the accessory’s edge. In horizontal joints it is better to apply first the weld in the lower part of the union, then on the sides and finally in the upper part. In vertical joints, the initiation point is not important. If the derivation of the accessory is directed downwards, it is very important not to reheat the pipe because the weld alloy could drip outside the accessory throughout the pipe. In which case the heat source should be kept away, leaving the alloy to solidify and further on restart the operation. If the molten weld alloy does not distribute regularly through the joint’s interstice and tends to create drops, it means that the surfaces should be welded, they are not deoxidized and are not allowing the alloy to wet them or are not sufficiently hot. On the contrary, when the alloy does not penetrates into the interstice but drips over the exterior surface, there has been insufficient heating on the joint’s tap or socket elements. 8. Cooling and cleaning When the weld is completed, molten parts may be sharply cooled down with cool water, producing the separation of most part of welded and vitrified powder. Residues of the cleaner may be eliminated with a wet cloth if it is soluble or with a metallic brush. This operation is performed when the joint is already cold.
Sources
and References: • Fresh water and Copper [El Agua Potable y el Cobre] - CEDIC – Spain • The Copper Pipe in Construction Installations [El Tubo de Cobre en las Instalaciones de la Edificación] - CEDIC – Spain • The Copper Tube Hand Book, Copper Development Association – USA • Chile Piles [Tubos de Chile], Procobre Chile. • Uses of Copper in Sanitary Installations [Usos del Cobre en las Instalaciones Sanitarias], Procobre Chile • Copper Pipes and Accessories [Tubos y Accesorios de Cobre] - CEDIC - Spain
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