Introduction

​ In the construction of many chemical plants at home and abroad, there will be contact with the installation of a large number of explosion-proof electrical equipment, especially in petrochemical enterprises with many fields, due to the harsh environment, more and more armored cables are used for power transmission and distribution control. It is required to be grounded at both ends of the armored cable, so generally, the power distribution box on site, the inlet of the motor, etc. are all equipped with explosion-proof glands that can install armored cables. However, there is a big problem when installing armored explosion-proof cables, that is, when the armored cable is a steel tape structure, sometimes the gland cannot be screwed tightly, and some even directly damage the seal on the gland. Rubber, when dismantling the Gland, the most common thing is to cut or even smash the second sealing rubber, causing hidden dangers of accidents and greatly reducing the quality of the project. Later, after comparative analysis, it was found that the problem of the installation of the Gland is concentrated on the steel tape armored cable, and it is all when the steel tape wrapping direction and the rotating nut are in the opposite direction. So far, we have studied and analyzed the steel tape itself of the armored cable, as well as the internal structure of the Glan, and gave some solutions and suggestions, and obtained good results in practice.

The role of the explosion-proof gland of the armored cable

​ According to our country’s standard, the armored cable must be grounded at both ends. When the cable enters the junction box at the field end, if the armored metal of the steel tape or steel wire is separately welded to the ground, it is obviously very troublesome in the actual installation process, and there will be false Unreliable situations such as welding occur, so the inlet end of the field junction box is used to directly clamp the steel tape or steel wire, so that the armored metal can be reliably grounded and easy to install.

Armored cable and explosion-proof gland structure types

General Types of Armored Cables

(1) Thick steel tape armor; (2) Thin multi-layer steel tape armor; (3) Steel wire armor.

Domestic general structure of explosion-proof glands for armored cables

(1) Double sealing gland, as shown in Figure 1; (2) Single sealing gland, as shown in Figure 2. (3) Gland with a ring, as shown in Figure 6.

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Analysis of problems encountered in practice

(1) Installation process: take Figure 1 as an example, first fix component A on the box; after stripping the cable, insert components G, F, E, C, B onto the cable; place D between C and E. During the time, screw E, press the armor D; finally screw G and E. (2) This installation process solves the problem of armor grounding extremely simply, and is very practical, but why is it difficult to install armored cables wrapped with steel tape armor in the reverse tightening direction in practice, and it is difficult to tighten or even destroy them? As for the problem of the rubber sealing ring, it can be known from the above structure that when tightening the nut E (take Figure 1 as an example), it is all due to the strong friction between the steel strip D and the oblique components C and E, and the Forcibly drive the steel belt in one direction. If the steel belt is exactly in the same direction, the tighter the rotation will be. If the steel belt is in the opposite direction, the steel belt will loosen, and the more it is rotated, the more arched it will become, and even the components will become more and more arched. E is stuck and cannot be tightened further. If the supervisor needs to check, when dismantling the glands that wrap the steel belt in the reverse direction (the same direction when tightening), some of them will be directly arched to cut and twist the sealing rubber, resulting in the need to replace the new glands. In remote areas or underdeveloped areas abroad, it would be very troublesome. (3) Why is there no problem with the armoring that can be installed with steel wire as shown in Figure 1 and Figure 2? According to the above analysis, the reason is simple. The steel wire can move in two directions, unlike the steel tape armor, which has only one direction.

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(4) In production practice, the structures shown in Figure 1 and Figure 2 are relatively simple and are widely used in the installation of large armored power cables. It is often used in the installation of relatively small instrument communication cables. Armored cables such as instrument communication generally prefer to use steel wire armor, so in the construction of instrument cables, this installation problem will not be encountered no matter what. Similarly, in developed countries abroad, steel wire armored cables are more common, while domestic power cables generally use steel tape armored cables. (5) Comparative analysis of the grating structure: Figure 3, Figure 4, and Figure 5 are a foreign explosion-proof grating structure. As shown in Figure 3, the installation process is to install the first rubber seal first, at this time, the cable can be fixed, and then the depth of the insertion of the armor F between the elements D and E can be easily controlled, and the shape of the armor can also be easily trimmed , After controlling the insertion depth of the armor, it is very easy to tighten G at the end. Also look at the domestic pictures 1 and 2. When fixing the first seal and the armor is a one-time package of a large number of components, it is not easy to The insertion depth of the sheathed sheath is not clear, and it is difficult to control and modify the sheath. There is no first seal in Figure 2, it is difficult to hold the cable, and installation is even more difficult. However, Figure 4 and Figure 5 have a ring. When installing, because of an additional slippery structure, it is relatively difficult to reverse the flanging of the armor. Figure 6 is an explosion structure diagram of a domestic ring with a ring. The structure with ring structure is more complicated, and it is indeed difficult to install armored cables with thick steel tapes. This may be one of the reasons why steel tape cables rarely use Gland with ring structure. If the armor is inserted as shown in F in Figure 5, the nut cannot be tightened at all. (6) According to the above analysis, it can be known that the reason why the steel belt wrapped in the reverse tightening direction cannot be installed well is that, no matter what structure of the grommet is used, first of all, when the armor is installed in the clamping position, if there is no suitable insertion depth, such as: As shown in Figure 7, when it is fixed, the armor will be extended inwards until the flanging is performed as it is tightened; secondly, if the surface is uneven as shown in Figure 8, it will also be flanged during tightening, especially the circled part in the figure. If Using Figure 4 with slip rings can significantly reduce friction and reduce the possibility of buckling and twisting of the opposite strip. Figure 6 shows the products of some explosion-proof electrical appliance manufacturers in northern China. Figure 1 and Figure 2 are typical product structures of various electrical appliance manufacturers in southern China, which are obviously not suitable in practice.

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Problem-solving advice

The key point of the problem is that the wrapping direction of the cable armor is opposite to the rotation direction of the nut. So it has to be solved as follows: (1) Since there is only a domestically made granite with the current structure, the installation task must be completed successfully. Therefore, the solution adopted on site: ① Before installation, use iron scissors to cut the armor evenly without unevenness as shown in Figure 8; ② Use a file Grind it flat, and do not have any cracks, especially sharp holes, as shown in the circle in Figure 8; ③The outer insulation layer of the armor is exposed by about 1.5-3cm, and when the inner bevel ring is inserted into the armor, The skin just exceeds about 3mm of the metal ring (H in Figure 7), and then tighten the nut. In this case, after tightening, the top of the armor can be just between the whole slanted opening, and it will not hurt others if it is disassembled. ④After the armor is trimmed and before tightening, it can be coated with Vaseline, preferably a special conductive paste for electric power, in order to increase lubrication, so as not to loosen the armor in the opposite direction when tightening, or even twist and deform and tighten. or damage the sealing rubber. The above method is for the reverse direction of tightening. In fact, it should also be done for the installation of the steel belt in the same direction as the tightening direction, so that in case of disassembly, it can be smoothly withdrawn without flanging and cutting due to reverse direction. Sealing rubber. (2) Of course, by analyzing the structure, it can be found that if the manufacturer can modify the structure or handle it properly, the installation problem of this type of steel belt can be more fundamentally solved: ① Use D in Figure 6, add slip rings, but It is necessary to easily insert the thick steel strip, so that the nut will not drive the armor when rotating, and try to make the nut and the steel strip slip as much as possible; ② Similarly, if the structure of the slip surface can be modified, so that when tightening or loosening It does not need to drive the steel belt, but it should be suitable for multi-layer thin steel belts. ③Whether the structure like a slip-knot can be used, the steel belt is fixed first, and then docked, which can completely and effectively avoid the steel belt from participating in the tightening or loosening process, which may be the most fundamental solution.