1. Overview
Cable entry devices for explosion-proof electrical equipment refer to devices that allow one or more cables or optical fibers to be introduced into the electrical equipment while ensuring its explosion-proof type. Explosion-proof cable entry devices are an important component of explosion-proof electrical equipment; and flameproof cable entry devices are the most demanding and widely used among all explosion-proof cable entry devices. The sealing ring is the most critical component of the flameproof cable entry device, and its performance directly determines the realization of the flameproof function; failures caused by the sealing ring account for more than 90% of all failures of flameproof cable entry devices. This article mainly discusses the sealing ring structure design, rubber material selection, and rubber hardness of flameproof cable entry devices.
2. Introduction to Explosion-proof Cable Entry Device
According to the GB3836.2-2010 standard, explosion-proof cable entry devices can be classified into flameproof type and increased safety type, etc. Flameproof type can be further divided into sealing ring type and filler type; according to the thread type, they can be divided into metric system and NPT system. A typical sealing ring type explosion-proof cable entry device structure is shown in Figure 1.
Figure 1
3. Technical and testing requirements for the sealing ring of explosion-proof cable entry device
Explosion-proof cable entry devices use clamping components to tighten the sealing ring, causing the rubber sealing ring to deform and tightly grip the cable. This prevents the cable from being pulled arbitrarily by tensile or torsional forces and prevents internal explosions in electrical equipment from propagating to the external hazardous environment from the cable entry point. According to national standards GB3836.1-2010 and GB3836.2-2010, the sealing ring of explosion-proof cable entry devices should meet the following requirements:
3.1 Heat and cold resistance aging test of sealing ring
The test should be conducted with the cable under tension. Heat resistance performance should be determined using non-metallic components of the cable entry device (i.e., sealing rings) related to explosion-proof integrity. The heat resistance test is divided into two cases based on whether the maximum operating temperature exceeds 75°C: If the maximum operating temperature does not exceed 75°C, it should be stored for 28 days in an environment with a relative humidity of 90 ± 5% and a temperature at least 80°C, 20 ± 2°C above the maximum operating temperature. If the maximum operating temperature exceeds 75°C, the above-specified four-week period should be replaced by maintaining the sample in an environment with a temperature of 95 ± 2°C and a relative humidity of 90 ± 5% for 14 days, followed by maintaining it in an air chamber with an ambient temperature 20 ± 2K above the maximum operating temperature for another 14 days. After the heat resistance test, the sample should be left to stand at room temperature for 24 hours before undergoing a cold resistance test. The cold resistance performance should be able to withstand an environmental temperature drop of 5–10°C for 24 hours.
3.2 Sealing performance test of sealing ring
The explosion-proof cable entry device should pass a sealing performance test. The sealing performance test method is as follows: First, use a clean, polished, and dry low-carbon steel round mandrel with a diameter equal to the minimum cable diameter allowed by the sealing ring specified by the manufacturer. Then, install the sealing ring inside the cable entry device and apply the torque specified by the manufacturer to the clamping nut to ensure a seal under Class I 2MPa hydraulic pressure and Class II 3MPa hydraulic pressure. After assembly, install it in the hydraulic device, and gradually increase the hydraulic pressure after adding liquid. For Class I, maintain the pressure at 2MPa for 10 seconds, and for Class II, maintain the pressure at 3MPa for 10 seconds. Colored water can be used as the liquid during the test. If there are no leakage marks on the absorbent paper, the sealing requirements are considered met. Finally, a mechanical strength test is performed. The test method is as follows: apply a torque twice the torque required for the sealing test to the clamping element, but the applied torque value is at least three times the maximum allowable cable diameter for round cables or the maximum allowable cable circumference for non-round cables. After the test, remove the entry device and inspect its parts; if no damage to any components is found, the test is considered passed. Note: Any damage to the sealing ring is negligible, as the purpose of this test is to demonstrate that the mechanical strength of the cable entry device meets its operating conditions.
3.3 Cable clamping test with sealing ring
The sealing ring of the explosion-proof cable entry device should pass the cable clamping test. Test method: First, install the sealing ring on a cable with the minimum diameter specified by the manufacturer, or install the sealing ring on a clean, polished, and dry low-carbon steel circular mandrel. The diameter of the mandrel should be equal to the minimum allowable cable diameter specified by the manufacturer, and the maximum surface roughness Ra of the mandrel should be 1.6µm. Then, apply the torque specified by the manufacturer to the clamping element to tighten the sealing ring to prevent cable or mandrel slippage. Ideally, install the prepared test sample on a tensile testing machine and apply a tensile force of 20 times the cable or mandrel diameter to the cable or mandrel. Maintain this position at an ambient temperature of 20 +/- 5°C for 6 hours. If the displacement of the cable or mandrel does not exceed 6mm, the sealing ring cable clamping test is considered合格 (qualified).
4. Design Analysis of Sealing Ring for Explosion-proof Cable Entry Device
4.1 Analysis of Key Design Considerations for Sealing Rings in Explosion-proof Cable Entry Devices
The clamping force of the cable assembly comes from the compression of the sealing ring by the clamping element, causing the sealing ring to deform and thus grip the cable tightly. Therefore, to ensure the sealing performance of the explosion-proof cable entry device, the sealing ring must generate sufficient clamping force on the cable, while also meeting stringent heat and cold resistance tests. We know that elastic sealing rings are typically made of vulcanized rubber; however, vulcanized rubber ages faster due to temperature and humidity, gradually losing its elasticity and affecting the cable's sealing performance. Therefore, the rubber's operating temperature range, aging resistance, and tear resistance are all important considerations in our design.
Furthermore, the deformation of the sealing ring is achieved by applying torque to the clamping element; therefore, torque is also a key concern. Typically, we need to determine a reasonable minimum tightening torque through testing to ensure the cable entry device is installed and used correctly and reliably.
4.2 Analysis of the clamping force of the sealing ring in the explosion-proof cable entry device
When we tighten the clamping element to a given torque, the clamping element pushes the gasket, applying a pressure F to the sealing ring. According to the law of action and reaction, the sealing ring also applies an equal and opposite reaction force F' to the gasket. Because the sealing ring is elastic, according to the pressure formula: F=PS (since the contact area S is proportional to the cable diameter, the thicker the cable, the larger the contact area; and the pressure P is constant), the thicker the cable, the greater the clamping force of the sealing ring on the cable.
Therefore, we only need to consider whether the minimum permissible cable meets the sealing test; if the minimum sealed cable meets the sealing performance test, then the largest cable, due to its larger diameter, will have a greater clamping force of the sealing ring on the cable.
4.3 Analysis of the sealing ring structure of the explosion-proof cable entry device
Currently, there are two common sealing ring structures in explosion-proof cable entry devices: 1) positive pressure sealing ring; 2) displacement sealing ring.
1) Positive pressure sealing ring
2) Displacement type sealing ring
Positive pressure sealing rings: These typically have a V-shaped groove on the outer ring. When the sealing ring is compressed, the V-shaped groove deforms and clamps the cable due to its weaker strength. These types of sealing rings usually have the following drawbacks: 1. They have a relatively small sealing area; 2. When sealing the largest cable, the clamping force of the sealing ring on the cable is often excessive, thus affecting the cable's service life.
Displacement-type sealing rings: These typically have a tapered tip, and the inner bore can also be tapered. This ensures that the contact point between the sealing ring and the cable differs when sealing cables of different diameters. This prevents the cable from experiencing excessive clamping force, which could affect its insulation performance and extend its service life. Because the inner bore is also usually tapered, the cable sealing range of these sealing rings is generally larger than that of pressure-type sealing rings.
5. Conclusion
The structural design of the sealing ring and the selection of rubber materials are key design considerations for explosion-proof cable entry devices. Currently, to meet customer needs in high-temperature and extremely cold conditions and for a wide range of cable sealing applications, high-temperature resistant, tear-resistant silicone rubber with a displacement-type sealing structure is generally used.
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