Whether it’s to Expand, Convert, Modernize or Debottleneck, the Addition and Replacement of
CPVC application for acid handling
liquid pipelines remains a never-ending task in many industries. For service temperatures between 100 and 200 F, such lines are increasingly made of chlorinated PVC piping (CPVC) rather than traditional metals. Despite more than three decades of successful use in the chemical process industry, however, questions still arise from the field about proper design, installation and testing of CPVC piping systems.
When properly applied, designed and installed, CPVC piping provides several operating benefits not available in metal piping:
- Greater corrosion resistance – CPVC offers outstanding chemical resistance to nearly all acids, alkalies, alcohols, halogens, aliphatic hydrocarbons and many other corrosive materials. The resistance of other plastics will vary depending on the respective materials.
- Lower fluid friction – Smoother ID surfaces create less friction loss than in metal pipe, thereby reducing pressure drop. More liquid can be moved with smaller pumps and less pump horsepower; and since the surface stays smooth, a permanent electricity savings results.
- Better thermal stability – Thermal conductivity of CPVC is lower than for metals, reducing heat loss and the need for insulation and jacketing. Fluid temperatures remain more stable and surface condensation is greatly reduced.
- Electrical corrosion resistance – Like all plastics, CPVC does not conduct electricity, completely eliminating any possibility of galvanic or electrolytic corrosion.
- Service life – CPVC piping maintains its integrity over long periods of time with minimal upkeep. In applications within its service limitations for temperature and pressure, properly installed CPVC piping literally will last indefinitely. By contrast, metal rarely lasts the 10 to 12 year economic life of the typical chemical plant.
- Economics – In smaller diameter pipe systems, the economic benefit of CPVC relates to its longer service life; the installed cost saving is minimal. In larger diameters, installed cost of plastic will be less than metal because of the difference in weight. Two men can handle a length of 8- or 12-inch CPVC pipe. In metal, they’d have to use equipment. Installed cost savings vs. stainless steel will be dramatic, because stainless steel is both expensive and requires costly passivated welding.
The principal tradeoff vs. metal piping is a temperature limit of 200 F for CPVC. A second tradeoff is that being less ductile and having a lower impact-resistance than metal, CPVC requires greater care in handling and installation. It shouldn’t be used in applications where severe mechanical shock and impact are likely to occur. A third tradeoff is CPVC’s reduced ultraviolet resistance vs. metals. CPVC piping will retain its appearance and properties better if protected from extended exposure to direct sunlight.
CPVC Design Guidelines
The differences in piping system design and installation necessary with CPVC arise directly from differences in the physical and mechanical properties of CPVC and metal (see table). Lower tensile and flexural strength and higher elastic modulus means shorter spans and more support for CPVC. Higher thermal expansion coefficient drives the need for greater expansion allowances. Lower impact strength prompts the need for greater care in handling.
Compared with metal piping in ambient temperature service, CPVC requires about 25 percent more mechanical support and five times greater allowance for thermal expansion. The need for mechanical support will increase as the temperature increases. Adjustable clevis, ring, or roll hangers and roll stands with broad support surfaces are preferred for horizontal runs, but other acceptable types include pipe clamps, straps and riser clamps. The broader and flatter the support surfaces, the better. For vertical runs, riser clamps should be supported on spring hangers. For short risers, include a saddle at the bottom and perhaps an additional hanger at the top. Long risers may need additional straps or riser clamps along their length. Support all valves, flanges and other points of concentrated loads independently. For higher temperature lines or conveying of hazardous fluids, continuous support is worth consideration.
In general, size the piping diameter such that flow rates remain below 5 ft/s. At this velocity, water hammer should not be a problem as valves or pumps cycle. Higher flows require considerable knowledge and understanding of any valves, pumps, etc. that may affect sudden changes in velocity.
Pipe friction losses will need to be calculated on an application specific basis to predict pumping requirements. As a rule of thumb, they will always be 20 to 50 percent lower with plastic than with metal over the life of the system. The empirical Hazen-Williams coefficients will help predict the reduction in friction losses when switching from metal to CPVC piping. Unlike metal, plastic piping doesn’t develop restrictive scale buildup with age.
Thermal expansion will need to be considered because the CPVC thermal expansion coefficient is about five times higher than it is for metal. A good rule of thumb is to provide one-half inch per 100 feet of pipe run for every ten degrees of temperature change. The more common expansion compensation designs used successfully in CPVC piping include expansion loops, flexible bends and bellows-type joints. In underground lines with diameters up to 21/2 inches, it’s a good idea to “snake” the pipe to compensate for expansion and contraction.
CPVC pipe can be joined by either solvent-welded or threaded fittings. By far the most widely used method for permanent lines is solvent welding with conventional fittings. Threaded joints should be used only for heavier wall thickness, schedule 80 piping and in low-pressure systems handling non-hazardous fluids. Threading for thinner wall, schedule 40 pipe is not recommended.
Flanged joints are recommended for permanent systems that may have to be dismantled occasionally. Flanges also overcome a joining problem for working environments that prohibit solvent welding at the site. Flanges can be solvent welded to the pipe ends in a suitable environment, then the assembled pipe sections can be moved to the installation site for bolting together.
For mixed systems, a wide variety of fittings are available to accommodate virtually any conceivable interface between metal and plastic in piping systems. For a partial replacement of a metal piping system, there is no reason not to replace the defective metal portion with CPVC provided the service is within the capabilities of the plastic. Transition unions will be simpler to install in a small diameter system than flanges. To reduce the risk of joint failures due to overtightening, avoid joints with plastic threads on the female part. Plastic threads on the male portion can tolerate overtightening better because the stresses are compressive. In any event, there are no long term, adverse consequences for properly installed plastic-to-metal piping.
At the installation stage, proper welding is essential for long term, trouble-free, leak-tight operation. Any deviations from the recommended basic steps may change the chemistry and impair the integrity of the system. The cement manufacturer’s written procedures for joint preparation, insertion, and curing should be followed to the letter.
Eight-inch CPVC schedule 80 pipe for application at 60 psi and 150 F.
Be sure to match the cement precisely to the piping material, the diameter, the process fluid to be conveyed and the installation environment. PVC cement will not work on CPVC joints. Never use all-purpose cements, ABS cements or commercial adhesives as this introduces a non-compatible ingredient. Always follow the instructions of the pipe and fitting manufacturer since the proper cement will trigger the necessary chemical bond. The result is a far stronger joint.
Pressure testing with water and not compressed gas has always been important in metal piping systems. It is especially critical with CPVC because CPVC piping is less ductile than metal and pressure failure with a compressible gas can propel shards throughout the surrounding area. For the same reason, remember to bleed the system of any entrapped air prior to testing and start up.
The importance of pressure testing with water cannot be over emphasized. Despite warnings in virtually all installation and application manuals for plastic piping, pressure testing with compressed air or nitrogen still takes its toll. This warning has been sounded for years in connection with pressure testing of all piping systems. p
Proper CPVC installation can be summarized in 10 key steps:
- Cut the pipe square. Power saws should have blades suitable for cutting plastic. For large diameter pipe, avoid wheel-type cutters that can flare the ends.
- Deburr the pipe ends and add a chamfer to the OD to facilitate proper insertion into the fitting socket.
- “Dry fit” the joint to make sure everything goes together smoothly; pipes should have an “interference” or “net” fit. Make a depth-of-entry mark so you can be sure the parts are fully seated later on, when it counts. Once the cement is applied, it’s too late to correct any problems; the cure has already begun.
- Remove all grease and dirt with a clean cotton rag. If necessary, use an approved chemical cleaner.
- Apply the primer to both mating surfaces. This will soften the material and facilitate the reaction with the adhesive. Once the primer has been applied, move quickly; the bonding reaction’s “clock” has started running.
- While still wet with primer, apply the solvent cement evenly and quickly to the pipe at a width a little greater than the depth of the fitting socket. For speed and correct application, use an approved applicator. Repeat the process on the inside of the fitting.
- Apply a second application of cement to the pipe. The idea is to create enough of a surplus of cement to form a small bead at the end of the pipe once it is home inside the fitting.
- Quickly insert the pipe into the fitting with a quarter-turn rotation to spread the cement evenly to ensure thorough contact. Then stop all movement and hold the joint in place for 30 seconds to 3 minutes depending on size and type of joint, type of cement, and job site temperature.
- Remove all excess cement with a cotton rag.
- Wait at least 24 hours before pressure testing, longer at ambient temperatures below 60 F and for test pressures above 185 psi.
Terry McPherson, technical services manager for Eslon Thermoplastics, has worked in the piping industry for more than 25 years. McPherson is a graduate of the University of North Carolina and is an active member of the ASTM F17 committee for plastic piping.