Introduction Of Air Blown Micro Cable Technology

Micro cable is a special optical cable whose diameter is less than 1/2 of the ordinary duct cable with the same capacity (hereinafter referred to as “ordinary cable”). Due to the thin diameter，the poor mechanical property, micro cables cannot be laid out by traditional manual methods, but can only be laid out by air blowing in a micro tube with an outer diameter slightly larger than the micro cable.

Currently, in China whether it is trunk lines or metropolitan area networks, underground pipe hole space are very tight. Compared with ordinary cables, the use of air-blown micro cables can save nearly 50% of pipe hole resources. So, is air-blown micro cable technology worth promoting?

Micro cable

The common structures of micro cables are central tube type and layer-stranded type. The central tube type micro cable has good temperature characteristics, but the maximum number of fiber is 24 cores, and the commonly used type is GYCFXTY.

The core number of the layer-stranded micro cable can reach up to 576 cores, but its temperature characteristics are poor. When air blowing over long distances, a certain length needs to be reserved at the starting and ending points of each air blowing section to adapt to changes in cable length caused by temperature. Meanwhile, the sheath of layer-stranded micro cable is thin, if use filled structure, the grease may seep out and affect the air blowing effect after the micro cable are stored for a long time. Therefore, layer-stranded micro cables are usually dry or semi-dry. Formula structure, model is GCYFY.

The ratios of outer diameter and cross-sectional area of micro cables and general cables (taking GYTA cable as an example) with common core numbers are shown in the table below.

Note: * means central tube type, the rest are layer-stranded type.

The allowable tensile force of an optical cable is generally proportional to the cross-sectional area of the cable. It can be seen that the allowable tensile force of a micro cable is significantly different from that of a general cable. The long-term/short-term allowable crushing force of micro cable is 150N/450N, which is less than half of that of general cable (the long-term/short-term allowable crushing force is 300N/1000N).

Micro tube

Micro tube is a round, flexible plastic tube with a maximum outer diameter of no more than 16mm. The material of the micro tube is silicone plastic composite material, and the tube body is usually a certain eye-catching color. The outer wall of the micro tube is generally made of high-density polyethylene (HDPE), the inner wall is a permanent solid lubricating layer (silicon core layer), and the inner wall can be equipped with a flow guide groove.

According to different deployment methods, micro tubes are divided into ordinary types and enhanced types. Ordinary micro tubes have thinner tube walls and are mainly used for air-blowing in existing silicon core tubes. They can also be manually deployed in tube holes without optical cables.

The reinforced micro tube has a thicker wall and is suitable for occasions with high requirements for tensile strength or pressure resistance, such as laying out in existing optical cable tube holes or protecting micro cables in hand holes.

The applicable micro tube specifications for micro cables with different outer diameters are shown in the table below.

Newly built micro tube pipelines generally use bundled tubes. Bundled tubes are also called “bundled cables” and generally contain one or more micro tubes of the same specifications but different colors. The outer sheath of the bundled tube is generally made of HDPE, and the whole body is a solid color.

At present, bundled tubes are commonly used in communication pipelines along some domestic highways. Commonly used bundled tubes contain seven 12/9 micro tubes.

Air blown micro cable

Air blown micro cable uses a mechanical propeller to push the micro cable into a micro tube. At the same time, the air compressor delivers a strong air flow into the micro tube. The high-speed flowing gas in the micro tube forms a forward thrust on the surface of the optical cable, prompting the optical cable to move forward. The maximum segment length of a single air blow can reach more than 1,000 meters. The same principle applies to air blowing micro tubes in silicon core tubes.

Effect of air blown micro cable on pipe hole capacity

Truck pipeline

Trunk pipelines usually use 40/33 silicon core tubes, and each silicon core tube is laid with a general cable by air blowing. If micro cables are used, four 12/10 or five 12/8 micro tubes can be blown into the 40/33 silicon core tube, and can lay out one micro cable in each micro tube by air blowing. Even in a silicon core tube where a general cable has been laid, at least one micro tube can be blown in again. The use of micro cable technology can more than double the capacity of trunk pipelines (the number of optical cables laid).

Urban pipeline

The pipe holes of urban pipelines mainly use seven-hole plum blossom pipes with an inner diameter of 32mm (hereinafter referred to as “plum blossom pipes“) and single-hole plastic pipes with an inner diameter of 100mm (hereinafter referred to as “single-hole pipes“). One sub-hole of the plum-blossom tube can lay 1 to 3 general cables of different cable diameters, and each plum-blossom tube can lay more than 10 general cables of various outer diameters.

Through the combination of micro tubes with different specifications, multiple micro tubes can be artificially placed in one sub-hole of the plum blossom tube. Since optical fiber ribbon cables with a large number of cores are not suitable for micro cable structure, some tube holes need to be left in the plum blossom tube to distribute large capacity optical fiber ribbon cables. Taken together, when mainly laying out micro cables, each plum blossom tube can lay out a total of more than 20 micro cables and general cables.

In single-hole tube, should lay out the sub-tubes before laying out optical cables. If the sub-tubes are made of 40/33 silicon core tubes, then one single-hole tube can lay out about 10 general cables, or about 20 micro cables and general cables.

In actual projects, in order to increase the number of general cables laid in a single-hole tube, general cables are often laid directly without placing sub-tubes. In this way, the number of general cables laid in a single-hole pipe can reach 20 or above. In this case, since multiple general cables are intertwined in the pipe hole, it is easy to cause damage to the general cable sheath when laying out, and it is also inconvenient to replace the general cable, which is not comparable to air-blown micro cables.

Generally speaking, micro-tubes are manually deployed in urban pipelines and micro-cables are air-blown through micro-tubes. The tube hole capacity can be nearly doubled compared to the deployment of general cables.

Cost comparison between micro cable and general cable

Layout in trunk pipelines

If the pipe hole resources of the trunk pipeline are insufficient and not use the air-blown micro-pipe method to expand, the pipeline needs to be re-routed. The construction cost of trunk pipelines is as high as 70,000 yuan to 100,000 yuan/km (3 silicon core tubes), while the expansion cost of air-blown micro-tube technology is less than 2,000 yuan/km (the cost of air-blown micro-tubes and micro-cables minus the number of air-blown concentric cores). Therefore, using air-blown micro cables for capacity expansion in trunk pipelines has significant investment advantages.

Layout in urban pipelines

The cost of micro cables and general cables mainly considers the occupation cost of pipe holes and the construction cost of optical cables. In many cities of China, communication pipe holes are purchased. The purchase price of each plum blossom pipe is about 40,000 yuan/km to 100,000 yuan/km. If each plum blossom pipe can lay up to 10 optical cables, The cost of occupying the tube holes of each optical cable is: 4,000 yuan/km to 10,000 yuan/km. Taking the laying of 48-core optical cable as an example, the unit cost of laying micro-cables and general cables in urban pipelines is shown in the table below (the cost of optical cable construction is estimated based on the 2020 pipeline project cost standard of an eastern province).

Because it is not included in centralized procurement, the unit price of micro cables is higher than that of general cables, and firstly should lay out the micro tubes,which results in the construction cost of micro cables being higher than that of general cables. But after comprehensively considering the cost of duct hole occupation, the cost of micro cables is generally lower than that of general cables. Especially in some metropolitan area networks where pipeline construction or purchase costs are high, the cost advantage of micro cables is even more obvious.

Conclusion

In terms of cost, air-blown micro cables have greater advantages in trunk lines. Due to the use of bundled pipes in some highway supporting pipelines, micro cables have become an inevitable choice.

Although micro cables have certain cost advantages over general cables in urban pipelines, they require construction institution to have cable blowing equipment; in addition, due to the poor mechanical properties of micro cables, their safety in hand holes meet certain problems which led to the current low usage rate of air-blown micro cables in urban pipelines. However, air-blown micro cables can reduce the use of labor several times and multiply the pipe hole capacity. As labor costs rise and pipe hole resources become increasingly scarce, the advantages of air-blown micro cables have become more and more obvious.

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