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Put CPVC Piping In Its Place

Aug. 27, 2008
Understand where it fits in terms of cost and performance

Savvy specifiers know it’s not just the purchase price that matters when choosing a product that’s designed for long-term performance. When evaluating a piping system this certainly is the case. The key is to identify the best value for today, as well as tomorrow and beyond. And that typically requires a lifecycle cost analysis tailored to your specific plant, to reflect its fluid temperatures, line pressures, chemical environments, etc.

For such an analysis for piping materials, start by calculating the cost to install the system. Don’t focus only on the direct material costs — labor and related installation expenses can account for more than one-half of the total investment made in a piping system.

Then consider the cost to maintain the system and how maintenance requirements affect productivity, downtime and lost opportunity. Also evaluate expected service life — how long the system should last before a total repipe will become financially or operationally necessary — and its impact on profitability.

Here’s how chlorinated polyvinyl chloride (CPVC) piping typically stacks up.

Many choices

Depending on the demands and environment of the operation, you could consider dozens of materials for a plant’s piping system. On the low end of the cost spectrum are PVC, CPVC and carbon steel. At the other extreme are high-performance metals such as nickel, titanium and zirconium and alloys. Combination systems such as steel lined with rubber, polyvinylidene fluoride (PVDF) or glass fall in the middle.

Figure 1. CPVC piping has a proven itself at plants for about 50 years; new formulations extend the applicability of the material.

Metals. Historically metals have dominated the industrial piping market, largely because they’ve been used longer than any other material in such applications. In general, metal offers higher pressure-bearing capabilities than alternative materials. Additionally, metal systems’ pressure-bearing capabilities aren’t significantly reduced by increases in temperature. And metal’s rigidity allows hangers to be spaced farther apart to save on installation costs.

However, metal poses numerous disadvantages — the most serious of which is vulnerability to internal and external corrosion. Certain substances may cause metal to corrode from within, while elements such as salt in the air or low pH levels in the ground (for underground applications) can prompt external corrosion. Even high-end pricey metals such as titanium, which generally resist corrosion, are susceptible to degradation in certain environments.

Metal is also subject to flow-restricting scale buildup, which increases pressure drop and can contaminate the process. In addition, compared to plastic piping systems, metal is heavier and more expensive to install, both in material and labor cost. And, because metal has poor insulating properties, it sweats more when handling cold fluids, can create a burn hazard when transporting hot fluids and is less energy-efficient — all of which create the need to add costly insulation.

Plastics. Today many different plastics successfully serve in industrial environments. In general one of the greatest benefits of plastic pipe is its corrosion resistance. Various types of plastic piping can be buried in alkaline or acidic soils without requiring any paint or special coating. Plastics containing TiO2 for ultraviolet protection strongly resist weathering.

Most plastic pipe isn’t susceptible to scaling — so, such piping systems maintain their full fluid-handling capability throughout their entire service life. This means it’s often possible to downsize the diameter of the pipe when converting from metal, reducing material costs, and to opt for smaller pumps, saving energy.

Of course, the various plastics differ in cost and capabilities. PVC, for example, offers significant economic advantages but can’t handle high-temperature applications. CPVC, on the other hand, provides superior chemical resistance as well as a high heat distortion temperature, due to its molecular structure — large chlorine atoms surround the carbon backbone to protect it like armor plating. So, CPVC has grown in popularity in both corrosive and high-temperature applications.

Hybrids. In recent years manufacturers have been able to increase CPVC’s temperature and pressure-bearing capabilities by wrapping it with fiberglass. Other hybrid systems include various plastic-lined metallic pipe, which combines the advantages of metal and plastic while minimizing many of their disadvantages. This type of piping system eliminates scale buildup concerns while offering the same superior pressure-bearing capabilities as metal. Such pipe is immune to internal corrosion but still subject to external corrosion. A major disadvantage of plastic-lined pipe is cost. In addition, it requires a difficult labor-intensive joining process. And, any break that occurs in the lining can become a source for future pipe failures.

Of course, cost always is an overriding factor in pipe selection. An authoritative study documented that when allowing for direct and indirect costs — material cost, labor, maintenance, productivity, etc. — CPVC was the bottom-line best choice. Its nearest rival, strictly from a total-installed-cost standpoint, was carbon steel. At the extreme high end were PVDF and titanium.

Specifying CPVC

Due to its high heat distortion temperature, chemical inertness and outstanding mechanical, dielectric, flame and smoke properties, CPVC likely can serve a role in nearly any chemical plant today. Indeed, wherever corrosion resistance and mechanical strength are crucial, consider CPVC. Applications extend beyond processing operations — the material often is the most effective choice for cleaning systems involving high temperatures and harsh cleaning agents.

CPVC piping can handle chemicals that cause process leaks, flow restrictions and, ultimately, premature failure in metal systems. That’s because CPVC withstands most mineral acids, bases and salts, as well as aliphatic hydrocarbons.

However, not all CPVC compounds on the market perform similarly. You can gauge how well your specific CPVC piping system will perform from its cell class, which is defined by ASTM D1784 and certified by NSF International. There’re now two cell classifications — 23337 and 24448. A large majority of CPVC pipe falls into the standard 23337 level. Pipe systems that meet the 24448 classification — all are made from second-generation CPVC formulations — exhibit three times the impact strength of standard CPVC, resulting in fewer breaks and fractures, a lower scrap rate and easier cutting. They also provide a higher heat distortion temperature, 230°F compared to 212°F for standard CPVC. This translates into a lower probability of sagging or bending.

Second-generation CPVC systems also uniquely feature fittings manufactured from pressure-rated compounds. These fitting compounds carry the same pressure-rating classes as the pipe compound. The fittings provide improved creep resistance and can better withstand long-term high-temperature hydrostatic pressure.

It’s important to check with the manufacturer of the pipe and fittings being specified to confirm how well they will perform in a specific application.

Material limitations

No single material is ideal for every application. CPVC isn’t recommended for use with most polar organic materials, including various solvents. CPVC test samples exposed while under stress to surfactants, certain oils or grease have shown signs of environmental stress cracking, softening and swelling.

Figure 2. Solvent cement creates a permanent bond stronger than either the pipe or fitting.

CPVC can safely handle certain organic solvents that are soluble in water, such as alcohols, below a specific concentration. (The acceptable concentration level varies with the type of solvent — consult the CPVC manufacturer for specific recommendations.) However, solvents insoluble in water, such as aromatics, likely will be absorbed by the piping system over time, even when they’re present at very low levels in the water. This may lead to a decreased service life expectancy for the system depending on the operating conditions.

Temperature and pressure also pose restrictions. In general CPVC can safely be used at temperatures up to 200°F in pressure applications and as high as 220°F in non-pressurized applications.

Pipe size and temperature determine the specific pressure rating — the smaller the pipe size, the higher the pressure rating allowed; the higher the temperature, the lower the pressure rating. For example, a ½-inch pipe operating at room temperature can handle up to 900 psi., while a 16-inch pipe operating at the same temperature only can withstand 200 psi. All pressure ratings are based on a 50-year service life with a safety factor of two.

Another limitation relates to hanger spacing. Because CPVC piping systems aren’t as rigid as metal, hangers must be installed closer together, resulting in more hangers. In certain situations, space limitations may preclude the additional hangers required for CPVC installations.

Joining methods

It’s impossible to accurately assess the cost to install a piping system without considering the joining system. That’s because the joining method directly impacts labor costs and productivity.

CPVC offers a number of benefits over metal with regard to joining. Because no welding is involved, no hot work permits or specialized, cumbersome equipment are required. In the majority of situations CPVC pipe is joined with solvent cement. This is a fast, easy and highly reliable process that produces a joint that’s actually stronger than either the pipe or fitting alone. This contrasts with other piping systems, in which the joint typically is the weakest part of the system and the most likely to fail.

Although some people liken solvent cement to glue, it’s very different — the solvent cement actually creates a welded permanent bond. The solvents in the cement soften the surfaces of the pipe and fitting socket. Because the socket is tapered, the softened surfaces bond once they are fit together. CPVC in the solvent cement fills in any gaps that might otherwise exist in the joint. As the solvent cement cures, the solvents flash off.

It’s imperative that only CPVC solvent cement be used. For applications in exceptionally harsh chemical environments, check with the manufacturer regarding the performance of the solvent cement.

Although solvent cementing is the preferred joining method for CPVC piping systems, there are alternatives. If it’s necessary to connect to an existing metal pipe or if the system needs to be disassembled for any reason, the CPVC pipe can be joined via flanges or threading. Full lines of transitions, threaded connections and flanges are available — as are CPVC valves. It’s possible to create an all-CPVC system that facilitates maintenance in the future.

Choose wisely

An analysis of piping should go beyond initial price to consider lifecycle costs. The challenge is choosing a system that meets the performance and budget criteria at installation and over the long term. On that basis, CPVC has proven to be a highly viable option for many chemical processors.

CPVC offers a material cost that is highly competitive with most other options, especially compared with high-end metallics. Its fast and easy joining system reduces labor costs. And its corrosion resistance and ability to withstand harsh chemicals, high temperatures, pressure and impact lead to reduced maintenance requirements, less downtime and greater productivity over an extended service life.

Donald Townley, P.E., is market manager for Corzan Industrial Systems, Lubrizol Corp., Cleveland, Ohio. E-mail him at [email protected].

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