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29-09-21

9 Látták




  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.

    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

special 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, 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.

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