Selecting pipe and piping materials
- Understand the pros and cons of various piping materials.
- Become familiar with some issues related to materials compatibility.
- Learn about corrosion issues in hydronic and domestic piping systems.
Just as the properties of different pipe materials vary widely (see Table 1), the importance of these properties varies widely between projects. The piping material chosen depends on the application and the water quality. For example, heating systems often employ steel pipe because of its low cost, strength, and resistance to heat-while pure water systems are likely to use virgin polypropylene (PP) or polyvinylidene fluoride (PVDF) pipe.
Basic material properties
Steel is strong, rigid, and has a low coefficient of thermal expansion. It is also heavy (multiple workers may be needed to transport it) and is subject to corrosion. Sometimes it is called carbon steel or black steel to differentiate from stainless and galvanized steel. All steel, by definition, contains carbon.
Steel often is used for closed hydronic systems because it is inexpensive, especially when compared with other materials in systems with high pressures, and corrosion is relatively easily controlled in these systems. It also is a good choice for steam and steam-condensate systems because it handles high temperatures and pressures well, and corrosion is normally not an issue in steam pipes. However, corrosion is an issue in steam-condensate pipes, and many engineers specify schedule 80 steel pipe simply because it takes about twice as long to rust through as schedule 40 pipe.
If amines (commonly cyclohexylamine, morpholine, or diethylethanolamine (DEAE) are fed properly to neutralize condensate pipe pH, condensate pipes can last the life of the building. Some building owners do not want these chemicals in steam that may be used for humidification because of health concerns; however, not using these amines might require a change to stainless steel (SS) piping or adding a separate "clean steam" system for humidification and for sterilization of medical instruments.
Rigidity is important because it determines the distance between hangers. Steel pipe is manufactured in 21-ft lengths, and the hangers can be spaced that widely for large-diameter pipe. More flexible materials, however, may require hangers on as close as 4-ft centers or even continuously. Consult ANSI/MSS SP-58: Pipe Hangers and Supports – Materials, Design, Manufacture, Selection, Application, and Installation for details about hangers and hanger spacing.
A low coefficient of thermal expansion minimizes the need for expansion loops and expansion joints. However, the high rigidity of steel means that although it expands less, it exerts very high forces on anchors.
Galvanized steel pipe is steel pipe that is dipped into a pool of zinc (see Figure 1). Galvanizing has two methods of corrosion reduction:
- It coats the surface like paint, and under most circumstances it forms a very adherent oxide layer like aluminum and SS.
- It provides a sacrificial anode (zinc) to receive corrosion instead of the steel corroding.
Galvanized steel pipe has all the advantages of steel pipe, plus improved corrosion resistance in most environments, although at a slightly higher cost. Galvanizing works almost perfectly in applications where it is wetted and dried periodically (e.g., road signs and guard rails). It can fail in environments with high sodium (e.g., softened water that started out very hard) because the sodium makes the adherent oxide film detach and react more like steel pipe where the oxide flakes off. If galvanized pipe is being welded, the welder needs to be careful to grind down to the raw steel. Repairing galvanizing on the inside of the pipe is difficult or impossible. If the interior needs a continuous galvanized layer, consider mechanical couplings. (More information is available via the American Galvanizers Association.)
Copper pipe often is used in both hydronic and domestic applications, especially for 2-in. and smaller pipe sizes. However, some contractors propose replacing galvanized steel domestic-water pipe with copper up to 6-in. in size, especially in the Midwest. Copper is an expensive material but has the advantage of weighing less than steel and may require fewer employees to install, depending on weight and union restrictions. Also, copper is generally more noble and corrosion-resistant than steel or galvanized steel.
In the HVAC industry, most copper is Type L (medium thickness) hard (tempered) copper, although underground soft (annealed) copper is often Type K (thick). Drain, waste, and vent (DWV) piping is thinner (Type M).
Stainless steel is widely considered to be resistant to all corrosion. This is true in many circumstances, but not all. Anaerobic and chloride corrosion can affect SS. The most common alloy is 304 SS, which adds 18% chromium and 8% nickel to steel. 304L has reduced carbon content to minimize the tendency for SS to corrode at welds. SS with the L designation is recommended for all SS that will be welded and might have corrosion issues, like fume exhaust and some pipe systems. 316 and 316L add molybdenum to reduce susceptibility to chlorides.
In the past decade, we have seen thinner SS being proposed as an alternative to galvanized steel pipe and larger-diameter copper pipe, primarily for domestic potable-water piping. There is one potential problem with this if done incorrectly (see, "Mixing materials may equal trouble").
SS requires some oxygen to build an adhering oxide layer, like aluminum car wheels. This is normally not a problem in hydronic heating/cooling systems or domestic-water systems, but a large chilled-water-storage system could have oxygen levels become low enough to have issues with microbially influenced corrosion (known as MIC).
There are many grades of SS. In general, 300 series alloys are the most corrosion-resistant and are nonmagnetic. 400 series are harder, more resistant to abrasion, withstand higher temperatures, and are magnetic. 200 series alloys are used in sinks and applications where less corrosion resistance is acceptable.
Cast iron (CI) is used primarily in sewer and stormwater systems. It has very good corrosion resistance in these applications. The disadvantage is that the most common joints are not restrained. Most cast iron joints are either push-on or no-hub. Push-on joints work very well underground where the soil pressure helps stop the pipe from moving. Above ground, however, there are risks that the pipe may separate if there is a blockage and the pressure becomes too high. Galvanized steel, primarily for storm systems, with mechanical couplings or plastic-bonded piping can be specified when a risk of flooding due to pressure seems possible.
Ductile iron (DI) is like cast iron, except that it has a lower percentage of carbon and has annealing and/or additives, such as magnesium, to form a different (nodular) matrix. This makes it stronger and more ductile than cast iron. Its corrosion resistance is very similar to cast iron. DI is commonly used for city water mains. For storm or sanitary sewers, one length of DI pipe passing under footings can be specified so that, if the structure settles, the pipe will bend and not break.
Duriron is nearly out of the market, but it may be seen in remodeling projects. It is cast iron with silicon added for corrosion resistance. It previously was used for laboratory waste systems. Cast iron plumbing vents that "sparkle" on a roof are Duriron. Today it is normally replaced with polypropylene (PP), polyvinylideneflouride (PVDF), or occasionally borosilicate glass.
Polyvinylchloride (PVC) piping is often used in residential applications and is becoming more popular in commercial/industrial applications. It has the advantage of being very resistant to most corrosion, but not to solvents or some oils. Some manufacturers use polyolester (POE) oil to clean HVAC coils, and, in some instances, caused cracking of PVC condensate drain pipes. Chlorinated polyvinylchloride (CPVC) and acrylonitrile butadiene styrene (ABS) also are highly incompatible with POE oils.
One concern about PVC and CPVC is that they contain chlorine. When chlorine burns, it creates mustard gas. While deaths have not been caused by burning pipe in buildings giving off chlorine gas, they have read at least one article about a burning PVC copy machine that resulted in deaths of firefighters. The biggest concerns about PVC are close hanger spacing and not complying with the 25/50 flame spread/smoke developed rating per NFPA 255: Standard Method of Test of Surface Burning Characteristics of Building Materials and ASTM E84: Standard Test Method for Surface Burning Characteristics of Building Materials, which building codes require for materials located in return-air plenums. This is also true of polypropylene and most formulations of CPVC.
CPVC is basically PVC with a cross-linked chlorine molecule added to give it higher temperature resistance. It is commonly used in domestic hot-water systems. One disadvantage of PVC, CPVC, and most plastic and some fiber-reinforced plastic (FRP) piping systems is that they have very short radius fittings, therefore they have higher pressure-drop coefficients.
Polypropylene is known as olefin in the carpet industry, where it is used for indoor/outdoor carpet. PP has the advantage of handling fluids up to 210°F and being very resistant to corrosion. Some firms use it for acid waste and (in no-additives form) for pure water systems. It is also used for some dairy waste piping where 210°F water may go down the drain to clear solidified cheese. In general, PP is the most corrosion-resistant of all materials other than PVDF and other Teflon derivatives.
Polyvinylidene fluoride (PVDF) is a fluoropolymer that is related to Teflon. It is expensive but has excellent properties. It can withstand 212°F liquids, passes the 25/50 flame spread/smoke developed rating for return-air plenums (and is used for the interior liner of city buses because it will not burn like other plastics), and is very inert (i.e., it can be used for the highest-purity water laboratory or microchip systems).
PEX (polyethylene cross-linked) piping has become very popular, especially in residential plumbing systems. It is a clear, flexible piping material and some formulations comply with 25/50 flame/smoke requirements for location in return-air plenums. It is very flexible, requiring frequent or continuous support.
Borosilicate glass was once a popular laboratory waste piping material. It has high resistance to corrosion but is expensive and potentially can have issues if very hot water is poured down the drain. It is not typically used in modern labs.
FRP is useful for applications where corrosion resistance, ultraviolet (UV) resistance, and more rigidity than plastics are desirable. It has varying corrosion-resistance and strength properties depending on the plastic and the fiber used, and how the fiber is oriented. Many products allow choosing various inner coatings to resist particular chemicals. Cooling tower piping is a good HVAC application, provided that the product has low-loss coefficient fittings.
Welding is an old and reliable technology. It basically involves melting the pipes together. Steel and polypropylene employ this method. Welding can be used for galvanized steel, but it is virtually impossible to repair the zinc coating on the interior of the pipes, so mechanical coupling is preferred.
Threading involves screwing the pipes together, usually with a female nipple between two male-threaded sections of pipe. Threading is common for steel and galvanized steel pipes. It is also common for some plastic pipe materials. It is used for SS but requires fresh dies and an anaerobic pipe compound to make leak-tight joints. Threaded joints withstand forces in all directions.
Flanging is expensive but virtually foolproof. Flanged joints can hold any desired pressure and can be dielectric to minimize corrosion (see Figure 2).
Mechanical couplings (see Figure 3) withstand forces in all directions and can also hold any desired pressure. Today we see a movement toward either shop-welded assemblies that are connected in the field by mechanical couplings or systems that are fully mechanically coupled, primarily in sizes above 2 in. Both rigid and flexible couplings are available. Some projects also include vertical risers that benefit from the linear flexibility of "flexible" couplings to avoid expansion joints or offsets that increase shaft sizes to prevent pipes from breaking due to shear forces at inflexible shaft walls. Flexible mechanical couplings also can replace flexible connections, depending on the geometry and vibration isolation of the pump or equipment.
Corrosion is very important to address in pipe systems. Generally, hydronic heating or cooling systems employ corrosion inhibitors and perhaps biocides. Nitrites and molybdates are the most common corrosion inhibitors. Some design firms specify only molybdates for chilled-water systems, but it allows either molybdates or nitrites for heating-water systems that raise the water temperature to above 140°F in winter. This is because in cool water, nitrites can be food for microorganisms; microbiological "bloom" can occur in chilled-water systems.
Separate inhibitors are added to protect "yellow metals," such as copper. In glycol systems, most suppliers use a phosphate corrosion inhibitor because it also meets Food and Drug Administration rules for food-grade products, so they only need to make one product for food-grade and non-food-grade glycol.
However, at least one supplier uses nitrates, so each owner must keep records of what is in their building. Data regarding the efficacy of half nitrate and half phosphate treatment is not available; mixing glycols with different inhibitor chemistries is not recommended. Systems that contain glycol must maintain the glycol concentration at between 18% to 25%. Sources vary about the exact limit, but no manufacturer sells premixed glycol below 20% concentration; it is recommend to use nothing below 25%.
If this is not done, microorganisms may multiply rapidly because glycol is food. Glycol is an alcohol, and much like making wine, until the concentration becomes toxic the microorganisms will multiply. Never allow a domestic water makeup connection in a glycol system, or the concentration will slowly decrease until there is a major problem. A feed tank filled with premixed industrial (not automotive) glycol, a pressure switch, and a pump is recommended.
Steel is relatively immune to corrosion if it is in a high pH environment (e.g., steel rebar in concrete). The pH scale is logarithmic and commonly ranges from 0 to 14. It indicates how acidic or basic a solution is, with 0 being the most acidic and 14 being the most basic. A pH of 7 indicates neutrality. A pH range of 8 to 10.5 is commonly used for pipe systems that include steel. Steel is, however, subject to corrosion if the pH is low or individual chemicals attack the steel. Many corrosion-protection schemes rely on high pH, but this is a problem for systems that include boilers that have aluminum heat exchangers because aluminum is not compatible with high pH. The combination of steel pipe and aluminum heat exchangers requires a very narrow pH range in hydronic systems, typically 8 to 8.5.
Surface condensation is another issue. In the Midwest, it is common not to insulate PEX or other plastic pipe materials in some systems because condensation doesn’t form. But from an energy standpoint, PEX loses heat faster than copper pipe. This is because the larger outer diameter of PEX provides more surface area for heat transfer.
Dielectric fittings are controversial today. Dielectric flanges are often a preferred dielectric fitting because if dielectric flanges are specified and the contractor installs non-dielectric flanges, the only correction is to install plastic bolt-isolating inserts-no replacement of the flanges is needed. Today, however, NFPA 70: National Electrical Code (NEC) requires bonding of metallic domestic-water piping, which defeats the dielectric separation provided by dielectric flanges, unions, and perhaps nipples.
Consider carefully the materials you specify for piping systems. Every material has excellent applications in the market, yet each has applications for which it is not well-suited. Pros and cons have been presented here for several commonly used materials, but this article has only scratched the surface of this area of engineering.
Mixing materials may equal trouble: know which piping materials you’re using to minimize corrosion
Over the past decade, mechanically coupled thinner-wall (schedule 10 304 stainless steel, or SS) pipe has become more common for 2.5-in. and larger domestic-water systems. It offers high corrosion resistance and a lower installation cost as compared with schedule 40 galvanized steel, or Type L copper pipe.
The material cost of schedule 10 304 SS is nearly the same as for schedule 40 galvanized steel, but it is half the weight, thus less expensive to install. Copper’s material cost is nearly double that of schedule 10 304 SS in these sizes but has similar installation costs, so it also has a higher installed cost. One issue that has caused problems is that schedule 10 304 SS fittings are about one-third more expensive than schedule 40 galvanized steel fittings, so galvanized fittings are mixed with SS straight pipe, with good intentions.
The thought is that both SS and galvanized steel are corrosion-resistant and the mechanical coupling provides a dielectric separation, which is incorrect. The dielectric corrosion that occurs between the galvanizing (zinc) and the SS is extreme because the materials are at nearly opposite ends of the metal nobility chart. Corrosion of the zinc is rapid and severe (see Figure 4).
Jeff Boldt is a principal with IMEG Corp., where he is director of innovation and quality. He is also a member of ASHRAE TC 3.6 Water Treatment.
Keith Stone is an associate principal and senior mechanical engineering specialist at IMEG Corp., where he is responsible for technical expertise and quality.