Special pipes

Written on 18 February 2020. Posted in Insights.

Special pipes: by pipe we mean a network of pipes connected together to allow transport of fluids and liquids, even over long distances, or to connect different equipment of a plant.

The pipes are cylindrical metal section bars, generally made with a single sheet of metal (usually steel), which are used for various purposes. The constituent materials of these products, in addition to steel, are metal alloys (such as nickel, chrome, titanium and others), stainless steel, coated steel (Teflon), copper, cast iron, concrete or plastics, such as polyethylene.

What is a pipe system

When choosing the type of material, it is absolutely necessary to take into account some variables, which are: pressure, operating temperatures, corrosiveness and abrasiveness of the substance transported. A pipe system means that each individual component is capable of performing its specific function, contributing to a complex apparatus for the transfer of fluids. In addition to the rigid one, there is also the flexible tube, usually made of silicone or other rubber mixes, which stands out for adapting itself with extreme versatility to different logistical situations.

The different raw materials used for the various types of pipes are conditioned by the internal pressure value of the fluids transported and by their temperature. When there is a need to transfer gas, the pipes must also be watertight and connected by hermetic joints. The diameter of the pipes is also bound by the kind of substance transported, by its relative pressure and by the length of the path.

Pipes are joined together by fittings, in order to connect adjacent pieces or to close them. The valves used to regulate the flow are also part of the pipe system. When designing a special pipe system, it should be preliminary carried out studies relating to the path of each individual pipe within the system. Particular attention must also be paid to the type of fittings and welds, evaluating any deformations caused by the pressure inside the system.

Fields of application of special pipes

Mechanical special components are used in many fields, such as petrochemical, naval, pharmaceutical ones and pressure vessels.

Pipes in the petrochemical sector

In the petrochemical sector, the pipes, which must transport oil, gas and hydrocarbons, are generally made with a multilayer structure equipped with an internal thermoplastic coating. The chosen material for this kind of pipes is high density polyethylene, capable of satisfying even the most critical operating conditions of both fluids (oil) and gas (carbon dioxide).

Due to the nature of the materials transported, pipes must be impermeable to hydrocarbons to avoid deposits and encrustations along the way. Flexibility is a necessary requirement to avoid any blockage in the transfer of raw materials and to ensure a continuous and homogeneous flow. The internal wall of the pipe must guarantee extreme smoothness, to minimize any pressure drop. These pipes, particularly resistant to deposits and incrustations, show a remarkable resistance to the attack of carbon dioxide, corrosion, abrasions, ultraviolet radiation and chemical aggressions.

Features

Thanks to a particular structural reliability, the special pipes of the petrochemical sector ensure high resistance to the propagation of fractures and impacts. Their installation is reliable, simple and fast, even in case of adverse climatic conditions (low temperatures).

Petrochemical plants are characterized by long piping networks that transfer gas and oil over many kilometers and therefore they must comply with high-quality and safety standards. Special pipes for the pharmaceutical industry include flexible tubes, ideal for decanting, transporting under pressure and vacuuming different types of solid, liquid and gaseous chemical products. The concentration of the transferred material is usually medium-high and so it requires an adequate pressure gradient, in order to proceed smoothly.

Often the internal surface of these devices is coated with Teflon, and the external one with stainless steel; in other cases, the material is polyethylene, which guarantees excellent quality standards and a particular longevity. The inside of the special pipes must thermoregulate independently of the external conditions, to avoid damage to the goods they contain. Furthermore, it must be hydrophobic and non-stick, to ensure the transfer of the products in purity.

Pipes in the bio-pharmaceutical sector

In the bio-pharmaceutical sector, the manufacturing of special pipes can only use materials that are qualitatively guaranteed and comply with safety standards. The special pipes in the naval sector are usually semi-finished metal products in copper and nickel alloy, which offer excellent corrosion resistance performance. The products are coated with Teflon, since the materials transported have very different characteristics and different needs; therefore, it is necessary to guarantee perfect internal insulation.

Waterproofing of metal pipes is an indispensable requirement in the naval sector, above all to ensure adequate longevity and responding to a valid quality/price ratio. The pressure equipment includes special pipes, solenoid valves and containers subjected to a relative pressure higher than 0,5 bar and which, falling within the current regulations, are marked with the CE labeling.

Pipe system of pressure equipment, intended for the transport of fluids, includes also gaskets, fittings and dismantling joints. Due to the nature of these systems, safety is the most important requirement, thus pipes must be manufactured with suitable materials which, depending on the case, can be metal alloys or other.

Danger levels depend on the amount of energy stored in the pipe system and of which the special pipes must take into account; the maximum tolerable pressure is also connected to the thermal state of the transported materials.

Among the various characteristics to be evaluated in the transport of fluids under pressure there are:

toxicity
flammability
explosiveness

Materials for special pipes

Welding, which is a highly-important field in special pipes manufacturing, requires absolute competence in both materials and working procedures. Duplex and superduplex steel are stainless materials, highly resistant to mechanical stimulation and corrosion, particularly ductile and with a high chromium content, necessary for the formation of their protective layer.

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Duplex steel, made up of a mixed structure of crystalline grains (50% ferrite and 50% austenite), stands out for its excellent corrosion resistance, good ductility, ideal especially for “U” bends, and excellent weldability. It also has a remarkable mechanical resistance to breakage. It is widely used for special pipes in the petrochemical industry and in the nautical sector, in the presence of particularly corrosive environments.

Superduplex steel, characterized by rather high concentrations of nickel, chromium and molybdenum, has excellent corrosion resistance and is very robust. It is used in manufacturing special pipes to be used in more critical conditions than duplex, especially as regards the impact with corrosion.

As part of these materials, welding plays a fundamental role and must be carried out with great competence and experience. In fact, the indispensable prerequisite for the successful process is to maintain a very high elastic limit. The special pipes must therefore be welded perfectly, because their fields of application require high-quality standards and excellent functional performance.

Safety, however, remains the main requirement to be assessed since the function of these systems is linked to the transport of materials to be treated with extreme caution.