Fire sprinklers have a long and successful track record of protecting lives and property. Developed in the late 1800s, the foundational technology that applies water in the right place at the right time early in a fire has not changed dramatically.
One of the biggest industry developments has been the advent and acceptance of non-metallic (plastic) sprinkler pipe. Its use reduces material and labor costs, provides design and installation flexibility and is inherently resistant to microbiologically influenced corrosion (MIC) and other contaminants. While it may seem illogical to install a product that could soften and fail under high heat conditions, plastic sprinkler pipe must meet rigorous performance requirements of ASTM F442 “Nonmetallic Piping Specification for Special Listed Chlorinated Polyvinyl Chloride (CPVC) Pipe.”
Reliability is the key to successful fire control using any type of materials. Fire protection system components must be listed for their use; design and installation standards describe where the materials are suitable; inspections and tests enhance the likelihood of proper performance; and regular maintenance enhances dependability.
However, the fire protection system standards alone can’t anticipate all misuses or abuses. Nothing in the standards protects from an errant worker drilling a hole into steel pipe. Only responsible installers and inspectors can assure non-metallic pipe is used where it is permitted. And, frankly, unforeseen issues, such as product compatibility, arise over time as new materials enter the market.
Despite its benefits, the compatibility of non-metallic sprinkler pipe with other construction materials must be verified in order to assure the system’s reliability.
CPVC in fire protection
In most markets, CPVC is the contractors’ product of choice for residential fire sprinkler systems. The National Fire Protection Association sprinkler standards for single- and multi-family housing as well as other “light hazard” occupancies allow plastic pipe. CPVC also can be used in rooms rated “ordinary hazard” and not exceed 400 square feet where they are part of light hazard occupancy. This might include spaces such as full-scale laundries or commercial-type kitchens that one might find in a hotel, condominium or institutional facility.
In these applications, the pipe can be installed so it is exposed to the room. In the event of a fire, the automatic sprinklers would cool the atmosphere. Normally, sprinkler systems in these occupancies are “wet pipe” systems having water in the pipe at all times. CPVC works because it’s naturally intumescent, and the water flowing through the pipe keeps its inside cool.
Where a “dry pipe” sprinkler system is needed, CPVC can be used, but it must be protected from the usable space by a thermal barrier consisting of 3/8-inch or thicker gypsum wallboard, ½-inch plywood soffits or a suspended membrane ceiling with lay-in panels or tiles weighing at least 0.35 pounds/square-foot when installed with metallic support grids.1
CPVC is permitted for fire sprinkler risers, cross mains and branch lines (See Figure 1 to the right).
Once installed and the joints are properly cured, the sprinkler pipe must be subjected to at least one hydrostatic test to verify the system’s integrity against leaks in pipe or fittings. If a fire department connection is attached to the sprinkler system, the pipe must be pressure tested to 200 psi for two hours and show no evidence of leaks or lost pressure. If there is no fire department connection, the system can be tested at normal operating pressure.
Understanding product chemical compatibility
Every plastic material has a certain range of chemicals, which can affect it. That’s why there are a variety of industry standards such as ASTM D543 “Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents” and ISO 22088 “Plastics — Determination of resistance to environmental stress cracking (ESC)” that exist to determine whether or not a chemical has some effect on a plastic material.
Three factors — strength, chemical properties and mechanical stresses — all play a role in chemical resistance failures of plastic materials. Like a fire triangle, there has to be just the right combination of these three factors to initiate a crack. A crack can begin when the amount of stress exceeds the strength of the material in a particular area. Two legs of the triangle have to do with the material strength and how it might be reduced, and the third leg has to do with mechanical stresses on the system.
Plastics are polymers; they consist of many thousands of molecules hooked together in big, long chains. The plastic material derives its strength from the tangling together of these long chain molecules, just like a well-twisted rope is stronger than the many individual strands that comprise it. The strength of the pipe or fitting depends not only on the strength of the CPVC itself, which is derived from its molecular weight and chlorine content, but also on how well those molecules are entangled during the manufacturing process.
During manufacturing, the polymer is melted to get its molecules moving around and then shaped into pipe by an extrusion process or into fittings by an injection molding process. A variety of factors, including temperature, degree of mixing and pressure during the forming process, can affect how well the molecules entangle as they are formed into pipe and fittings.
When a piping system is installed and functioning, there are a number of stresses acting on those polymer molecules and trying to pull them apart. There is the obvious stress of the water or air pressure in the system. There is the stress that comes from supporting its own weight between the hangers. There are stresses that come from other sources such as tight hangers or other construction elements impinging on the pipe, or from bending the pipe to get it positioned just right. The interference fit of the pipe and fittings puts a stress on the fitting socket when the joints are assembled.
The phrase a “safety factor of two” often is used to describe a pipe’s pressure rating. This means that the ultimate long-term strength of the material has been determined, and the pressure rating of the pipe has been set at half of that. If an installed system is operating at the maximum allowable pressure, only half of the ultimate strength of the material is being used. The other half remains to account for ordinary installation stresses, normal manufacturing variation and moderate chemical effects that might be experienced on a construction site.
What can be done to minimize mechanical stresses on the system so that they are not using up too much of that safety factor and bringing the system too close to a situation where environmental stress cracking might occur? The manufacturer’s installation instructions should be consulted regarding proper design and installation of the piping system. The instructions contain recommendations for proper joining techniques, appropriate pipe hangers, correct placement of supports, allowable bending deflections, etc. Following these recommendations in the design and installation of the piping system can help minimize other mechanical stresses.
Finally, the third failure mode leg — chemical properties — can interact with the other two to create a crack in the pipe. Let’s start by considering a chemical we’re all very familiar with that has a very strong effect on the plastic — solvent cement.
Solvents penetrate the pipe surface, getting in between the polymer molecules and spreading them out from one another, causing the plastic to swell and soften. The solvent has a very strong effect on the plastic and thus requires little or no assistance from the other two legs of the triangle. When the softened surfaces of the pipe and fitting are pushed together in the joint, the molecules of the pipe and fitting actually entangle with each other, creating a true solvent weld, not merely an adhesive bond.
The cement joint is the only place chemicals should be mobilizing the polymer molecules. There are other chemicals which also have some ability to separate the molecules from one another, just much less effectively than the solvents in cement. These types of chemicals work in conjunction with stresses on the material and/or exploit weaknesses in molecular entanglement to separate the polymer molecules from each other, thereby starting a crack.
These chemicals can have a range of effects on the plastic. Some have a strong effect and thus require less contribution from stresses or molecular entanglement deficiencies, while others have a milder effect and thus require fairly high stresses or part weaknesses in order to present a problem. Sometimes these types of chemicals can be found as ingredients in other types of construction products that might be used in conjunction with the piping system.
Checking product compatibility
When choosing other products that are intended to be in direct contact with the piping system — such as thread sealants, fire- and draft-stopping materials or pipe insulation — the installer should choose products that have demonstrated little or no effect on the piping material. It is always wise to check with the manufacturer of ancillary products to confirm that the products can be used with CPVC.
The success of a plastic piping system can depend upon the strength of the material chosen and the quality of the parts produced from it, the stresses imposed from operating conditions and installation issues, and the chemical substances in contact with both the outside and inside of the system. By appropriately managing the influence of these three factors on the piping system, the risk of pipe failure is reduced.
The Lubrizol Corp., an Ohio-based specialty chemical company, manufactures the feed stock from which most CPVC sprinkler pipe is made. The company has been studying the chemical compatibility issue for more than 30 years. Their research reveals there are many widely used products that are trouble-free with CPVC but many that had caused conspicuous problems. As a result, the company established a System Compatible Program to identify products that are approved for contact with BlazeMaster sprinkler pipe.
Lubrizol conducted extensive testing on these products to measure exactly how much effect each one had. It became apparent that the successful products each had a small measureable effect, but it was much less than the allowable safety factor for pressure-bearing pipe. The unsuccessful products all had a much larger effect, often more than the allowable safety factor. The criteria for compatibility then was set as close as possible to the performance of those known successful products, leaving plenty of safety factor remaining to account for other factors such as normal jobsite stresses and reasonable variance in quality.
How do products get on Lubrizol’s System Compatible Program? Participants must submit their entire proprietary formulation to a third-party laboratory, which will look for ingredients that have been known to cause problems with CPVC in similar types of applications.
The product must undergo testing to demonstrate that its effect on CPVC is within the allowable limits. The testing often occurs in more aggressive conditions than are likely to be encountered in normal building construction. This is done to ensure that any long-term effects that might take five or ten years to appear in normal circumstances can be detected in a shorter-term test. The product manufacturer pays for screening and testing.
Once the product has passed screening and testing, the manufacturer must sign a contract agreeing not to change the formulation without additional testing. They are required to place System Compatible logos on their containers and literature. Finally, the manufacturers must submit to audits of their manufacturing facility and follow-up testing on an annual basis. All of this ensures that the contractor can choose a product from the System Compatible program with confidence that it has been thoroughly tested and verified, and that it will remain compatible from year to year.
Lubrizol also publishes a list of products known to have caused CPVC piping system failures in building installations. Contractors are encouraged to check both lists before choosing construction products that will come in contact with plastic pipe and fittings.
One of Lubrizol’s most frequently asked questions in its System Compatible Program is, “Why aren’t certain types of products like wiring or paint in the program?” Manufacturers of those types of commodities don’t see any benefit to themselves in participating. Advertising their product as compatible with CPVC won’t perceptibly increase their sales volume; so it’s not worth the cost the plant audits and the restrictions on being able to use different ingredients in order to participate in this kind of program.
The use of non-metallic pipe in fire sprinkler applications has been a game-changer for the sprinkler industry. Today it is rare to see a light hazard occupancy sprinkler system that is piped with any other product. In order to assure system integrity and avoid leaks — a building owner’s common complaint about sprinklers — every effort should be made to verify products that touch the pipe are compatible with it.