Solvent welding thermoplastic pipe and fittings PLASTIC PIPE, VALVES AND FITTINGS by Bill Morris 26    august/september 2002 • the iapd magazine Charlotte Pipe and Foundry manufactures a complete line of  PVC Schedule 40 and 80, and Corzan^®  Sche- dule 80 industrial fittings and PVC and CPVC pipe. olvent welding is a commonly used joining technique for thermoplastic pipe and  fittings  in  the  chemical  processing industry. Polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) pipe and fittings are thermoplastic mate- rials suitable for various chemical-process- ing  applications.  They  are  corrosion-re- sistant, can handle both acids and caustics and can withstand high temperatures and pressures. In addition, these materials are lower in cost and are easier to install and maintain than other alternatives. Joining  thermoplastic  pipe  and  fit- tings to provide a tight, leak-free fit can be a simple task — if a few fundamentals are followed. Cross-molecular bond and interference fit Some  installers  believe  a  proper  joint  is made when the solvent cement hardens and  fills  the  void  between  the  pipe  and the fitting. PVC and CPVC plastics, how- ever,  are  softened,  or  “melted,”  by  sol- vent cements. So when a softened pipe is forced into direct contact with a softened fitting, the thermoplastic material at the two  surfaces  forms  a  cross-molecular bond. Once the surfaces cure and harden back to their original state, the resulting weld is permanent. The pipe and fitting must be forced in direct contact with each other to form a cross-molecular bond. For solvent-welded systems, the ASTM dimensional standards stipulate  that  the  pipe  outside  diameter must be larger than the bottom of the fit- ting socket, creating an interference fit. Six steps to a sound joint Creating a sound joint requires more than an interference fit between pipe and fit- ting. Six basic steps help ensure a trouble- free piping system. • Cut the pipe square. Because the joint seals at the very tip of the pipe, an angled cut diminishes the effect of the interfer- ence fit and may prevent the joint from sealing. Pipes should be cut square, prop- erly de-burred and chamfered. • Heed ASTM adhesive standards.  The correct cement and primer must be used, as indicated by the applicable ASTM stan- dard. Specifically formulated cements are available for PVC and CPVC plastics and varying pipe diameters. The wrong ce- ment could provide unsatisfactory results. Primers remove contaminants, and also begin  to  soften  the  thermoplastic  sur- face. Therefore, primer should be used on all  PVC  and  CPVC  Schedule  40  and  80 systems. • Choose the right applicator. To make a proper joint, cement and primer must be applied with an applicator as wide or as long as one half the pipe’s diameter. For example, a 6-inch system requires an ap- plicator that is at least 3 inches wide or long. An undersized applicator prevents an adequate amount of cement or primer from being applied in the time necessary to make a proper joint. • Apply the materials properly. Solvent cements and primers are designed to soften thermoplastics so long as the primer or cement is wet, they will continue to soften the surfaces. If the cement or primer is not properly applied, the pipe or fitting wall  can  be  over-softened,  resulting  in joint failure, particularly in systems with a diameter of less than 2 inches. A  heavier  coat  of  cement  should  be applied  to  the  outside  diameter  of  the pipe; a light trace coat to the inside dia- meter of the fitting socket. If this is done, any excess cement will be forced to the exterior  of  the  socket  when  the  pipe  is inserted into the fitting, forming an even bead of cement at the socket entrance. An inspection of the interior of the assem- bly should show evidence of very little cement or primer. • Join  securely  and  allow  sufficient  cure time.  The cure time is the time required to set a joint before performing a pressure test. Pipe diameter, air temperature, test pressure,  fluid  temperature  and  humidity influence cure times. For a 100-pounds- per-square-inch (psi) test with cold water, cure times can run from 10 minutes for ½-inch  systems  at  60°F,  to  as  many  as eight  days  for  16-inch  systems  at  0°F. Environments with higher test pressures, hot fluids, cooler air temperatures or high humidity all require longer cure times. • Specify pipe and fittings manufactured to the “nominal standard.”  It is possible that  pipe  from  one  maker  and  fittings from another might not fit together prop- erly, even though the materials meet all applicable ASTM standards. That’s because according to ASTM standards, a 2-inch fitting,  for  example,  can  have  an  inside diameter that ranges between 2.365 and 2.375. A 2-inch pipe can have an outside diameter between 2.369 and 2.375, and still be within the standard. If  the  pipe  and  fittings  are  manufac- tured to the middle of the standard (nom- inal), you would have a fitting measuring 2.370  and  pipe  measuring  2.375,  ensur- ing contact during installation for a con- sistently tight or “interference” fit. But, if the  fitting  diameter  is  manufactured  to the high end of the tolerance (2.375) and the pipe is manufactured to the low end S