Steel Pipes
Steel pipes are the most commonly used pipes in water supply systems. They are also used in pipelines for natural gas, and sewerage systems. Although comparatively expensive to other pipes, they hold the advantage of being able to withstand high pressures and are available in more convenient lengths, and can also be welded easily, thereby resulting in lower installation and transportation costs. These types of pipes are highly efficient and can be used in small diameters as needed and are 100% recyclable compared to other materials. The pipes can further be melted down and turned into other usable material in industry. Furthermore, the high strength of these pipes and resistance to damage caused by human errors, tree roots, and extreme weather conditions make these pipes the ideal choice for most water and sewerage supply systems.
The disadvantages of steel pipes include thermal conductivity, which is very poor as there is a difference in heat transfer. These types of pipes are usually bonded with aluminum or copper alloy to increase thermal conductivity and improve heat transfer. Cost is another issue, as these pipes are expensive and this is guided by the misconception of being a one-time purchase. However, steel pipes are difficult to fabricate and lack the malleable qualities that other materials have, therefore repairs and replacements of steel pipes are extra difficult.
Galvanized steel pipe has all the advantages of steel pipe, and is used in insulated and coated piping, 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 pipe fittings.
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 seamless steel tube and 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.
