Premise
In sites with risk of explosion and fire we often find the electrical conduit in a metal protective tube (also called conduit). [1] (Figure 1 on the side)
This constructive choice recalls the approach used in the American standard for electrical constructions in hazardous locations, based on the indications of the National Electrical Code in article 500, indicated by the abbreviation NEC 500.
In Europe, the reference plant legislation in the field of environments at risk of explosive atmospheres is the EN 60079-14. According to this standard, the use of the protective tube in places classified as at risk of explosion is mandatory only for single core cables without sheath and for cables with “easy to strip” sheath indicated in the standard as “easy tear”. [2]
Its use, however, is recommended in plants where the production process and the use and maintenance activities can present situations that are dangerous for the integrity of the cables (mechanical risks, exposure to heat or corrosive substances).
The metallic pipe conductors
In “metal pipes” the electrical conductors are inserted in protective galvanized steel pipes, consisting of a closed casing with a circular section, without interruptions.In these conduits the cables are laid and/or replaced by pulling.
The use of these conduit, together with the sealing fittings, the explosion-proof enclosures and the dedicated accessories make up the pipe systems and give them the typical appearance like a plumbing system.
Figure 2: pre-assembled skid with connections between the enclosures made of conduit
Conduits
Conduit suitable for the construction of these pipes must complies with the UNI 7683 [3] standard is considered, which concerns: "seamless and welded galvanized pipes of unalloyed quality steel for conical gas threads, cable holders, for explosion-proof electrical systems", Example of designation: AD PE 1 ½ UNI 7683.
Being a standard issued in 1977, it contains some references to standards that are outdated today. The material indicated is Fe 360, corresponding in the European nomenclature (EN 10025-2: 2004) to the current S235JR steel.
As can be seen in Table 1 on the side, this type of steel tube has an external diameter greater than the measurement in inches that designates it (1 inch = 25.4 mm). This is because the inch designation is conventional and originally referred to the actual inside diameter.
These conduits, such as Elfit BC series conduits, are supplied with conical thread at the end. International standards require that the tapered thread must be made according to the American NPT; however, in Italy the variant with ISO 7/1 thread is widespread and still marketed.
The conduits are supplied after having undergone hydraulic test at 50 bar for at least 10s, tensile test and bending test, they are hot dip galvanized and marked on the length at intervals of less than 1.5m.
It is necessary to pay attention to the choosing of the tube and the necessary electrical fitting to ensure that no metal combinations are created causing galvanic corrosion.
Conclusions
The system with electrical conduits in pipes has recently been joined by more versatile systems in armored and non-armored cable, with entry into the enclosures by means of a cable gland.
Despite this, this type of plant is still the most widespread system in petrochemical sites at risk of an explosive atmosphere where corrosive environments, extreme environmental temperatures and mechanical risks require high mechanical and chemical protection.
It is not uncommon to find mixed systems that integrate skids with metal pipe ducts alongside connections to cable systems.Notes, reference standards and bibliography
[1] CEI 31-108 9.1.2
Electrical conduit: set consisting of one or more electrical conductors and the elements that ensure their insulation, support, fixing and possible mechanical protection.
[2] CEI 31-108: 9.4.1
[3] CEI 31-108: 9.4.2
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