The ‘Ex d’ protection method: the function of the conduit sealing fittings
Sealing fittings are often considered only as secondary or minor components, but the safety of an installation with metal electrical conduits and related explosion-proof enclosures depends on them. The main function of a conduit sealing fitting is to prevent the propagation of an explosion from the inside of the ‘Ex d’ flameproof enclosure to the outside.
by Andrea Battauz, R&D Project Engineer of Cortem Group
Premise
Conduit sealing fittings as the necessary constituent elements for the safety of the explosion-proof electrical system. They perform an essential function of the ‘Ex d’ protection mode, they allow the conductors to enter the explosion-proof enclosures, preventing any internal explosion from spreading along the metal pipe ducts.
Their name therefore recalls their function: the blocking of the explosion.
Figure 1: the conduit sealing fittings in operation, the explosion of the housing below does not propagate in the system
The use of the conduit sealing fittings
The main function of a conduit sealing fitting is to prevent the propagation of an explosion from the inside of the ‘Ex d’ flameproof enclosure to the outside, through the metal conduits of the electrical system. In figure 2 we see the typical explosion-proof equipment of a site classified as an explosion risk area colored in yellow. In the same figure, the purple color makes us appreciate how each single cable entry or exit from the explosion-proof enclosure is made using the sealing fitting. The enclosures thus sealed do not allow the explosion to escape.
Another non-secondary function of the sealing fitting is to limit the volume of the explosion-proof electrical construction to a value close to that for which it was tested, thus preventing pressures higher than those verified from causing it to collapse during the explosion. [1]
At the boundary between the classified area and the safe area, the locking fitting is placed in order to ensure that dangerous gases cannot migrate to areas classified as safe.
Finally, more rarely the sealing fitting is used to separate the part of the electrical system in the pipe from a possible part made with exposed cable.
Figure 2: sealing fittings in purple, explosion-proof enclosures, and equipment in yellow, where the ignition and subsequent explosion of the explosive atmosphere could occur
The assembly of the sealing fittings
In the Elfit catalog the sealing fittings are identified by the acronyms EZS and EYS. These acronyms are also common to other manufacturers and accepted commercially. Then there are the EYD which basically are the same fittings equipped with a valve to let out any condensate formed in the pipes. EZS are referred to as horizontal sealing fittings, EYS as vertical sealing fittings.
As reported in Table 1 here above, the EYS fittings can only be mounted in a vertical position and that in the EZS fitting the lid must be vertical with the cap upwards. [2]
The castings that make up the bodies of the sealing fittings are internally drilled and threaded. The threads mate on the metal pipes used in the pipes. They are therefore mostly equipped with conical NPT or ISO 7/1 threads.
For mounting on explosion-proof enclosures or devices, nipples or, alternatively, three-piece unions must be used. The latter allow free rotation, necessary in certain situations to be able to fully tighten the conical threads on both sides or to have the possibility of giving a certain orientation to an accessory or equipment.
To fully tighten the threads, it is common practice to start tightening the components on one side and the others in sequence. The mounting frames of the enclosures are provided with larger slots and holes for this purpose.In plant engineering regulations, it is required that the sealing fitting be positioned immediately at the entrance to the explosion-proof enclosure. In particular, it must be fixed to the enclosure directly using the least number of accessories for coupling and in compliance with the manufacturer's instructions.
The sealing fitting must be close to the flameproof enclosure wall, providing at most one interposition accessory. The distance between the face of the sealing fitting closest to the enclosure “d” and the wall of the enclosure itself must be less than the size of the pipe and, for conduit of more than 2”, less than 50 mm. (Fig. 3 here next) [3]
It may happen that you are faced with assemblies where these rules are not perfectly respected, as the distance of 50mm is scarce or more accessories are interposed; these configurations can be compliant if they are included among those validated by the manufacturer of the enclosures as a "certified assembly". [4]
The passage of the cables inside the sealing fitting
The English name "conduit sealing device" in English and "dispositif d'opturation antideflagrant" in French suggests its use in metal protective tube called conduit systems. These are cast bodies with female threaded inlets carrying free volumes inside to be used as small vessels. Once inside these volumes a substance prepared from cement-like powders was thrown. In modern times, is used a two-component resins. We can define this filling material as a blocking mixture.The resin is mixed separately and then thrown using a special filling cap present in the fitting, see figure 4.
The same figure shows the glass fiber which must be placed at the base between the interstices left free by the cables so that when throwing the resin it does not run into the cable holder tube.
Figure 4: section of a locking fitting
Conclusions
Sealing fittings are often considered only as secondary or minor components, but the safety of an installation with metal electrical conduits and related explosion-proof enclosures depends on them.
Often the safety of the system can be nullified by an unsealed or improperly sealed connection and it is unfortunate to note that this situation is one of the anomalies most often encountered during plant inspections.
Notes, reference standards and bibliography
[1] The use of locking joints in explosion-proof electrical equipment (Ex d IIB, Ex d IIB + H2e Ex d IIC): the importance of sealing www.cortemgroup.com - January 2017
[2] Fig. 7.11 "ELECTRICAL SYSTEMS IN PLACES WITH DANGER OF EXPLOSION" TUTTONORMEL editions
[3] CEI EN 60079-14: 2015-04 par. 14.4
[4] CEI 31-108 par. 14.1
Publication date: 2022-03-24
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