Installation of Optical fibre Cable by Blowing / Jetting

Comparison between Pulling and Blowing methods

Overall, the blowing method is preferred over the traditional pulling method due to savings in manpower & installation time and improved installation efficiency, particularly in longer ducts with multiple bends and undulations. In this application note, cable installation by blowing method and its best practices are explained.

Pulling methods

Cable installation by using high-speed air flow combined with additional mechanical pushing force is called as "blowing or jetting". Cable blowing is the process of installation of optical fibre cable into a pre-installed duct. Compressed air is injected into the duct inlet after a few hundred meters of cable is pushed into the duct. Compressed air flows at high speed through the duct and along the cable. The pushing force is applied mainly near the cable inlet by a pushing device. Standard optical fibre cables (like uni-tube, multi-tube, unarmoured & armoured ), micro­duct cables, and micro-ducts can be installed by using this method. It is possible to install micro­duct cable using the blowing method in continuous lengths of more than '1000 meters depending upon the duct route. There are two methods for blowing which are discussed below.

Method 1: High air speed blowing
This method requires pushing the cable with a tractor mechanism by traction rollers in a blowing device for a few hundred meters and then introducing compressed air to make the cable inside the ducts remain in floating condition, which reduces friction between the cable outer surface and duct inner wall by reducing the area of contact as shown below in Figures '1 and 2.

Figure: 1

Figure: 2

Method 2: Push/Pull (Piston) blowing
In this type of blowing unit, an air-tight piston is attached to the front of the cable. The air pushes this piston, and the piston "pulls" the cable as shown in Figure 3. Installers should be aware that a pulling force exists in piston blowing, and the pulling force shouldn't cross the maximum cable tension. This method is preferred for larger size ducts (32140mm and 42150mm) and in "straight" trajectories to achieve longer cable blowing distance.

Figure: 3

Key Parameters of Blowing Method
The following three major parameters and their combined effects control how far a cable can be blown in a duct.

• Duct Fill Ratio (cable to duct size ratio)
• Coefficient of Friction (between cable outer surface and duct inner wall)
• Cable Stiffness (related to cable flexibility & undulations in duct route)

Duct Fill Ratio (DFR)
The ratio between cable and duct size is known as the 'Duct Fill Ratio' or 'Fill Ratio'. There are two methods to calculate DFR.
a) The ratio between cross- sectional area of the cable and the inner space of the duct.
b) The ratio between cable diameter and duct inner diameter.

For optimum blowing performance DFR to be kept between 30 to 80%. For conventional cable of diameter ≥ 10 mm: 30 to 50% For micro cable of diameter 1-9 mm: 30 to 80% Higher DFR helps to achieve longer blowing distance particularly in straight route. Higher DFR also prevents cable buckling effect (cable forming a helix inside the duct) particularly for cables with low stiffness. However, high DFR may result in shorter blowing distance in the route having multiple bends. Therefore, an optimized DFR needs to be maintained to achieve the best blowing performance.

Coefficient of Friction (CoF)
Coefficient of Friction can be termed as the drag force (sliding friction) generated during cable blowing due to area of contact between cable and duct inner wall because of the mass of the cable and gravity. CoF mainly depends on elevation, bending in duct route, undulations in duct and properties of cable outer jacket materials. Generally stiffer materials like Polymide, high- density polyethylene (HOPE) give better blowing results as compared to low density polyethylene (LOPE) and flame retardant material like low smoke zero halogens (LSZH). CoF can be reduced by using lubricants during the blowing of the cable. Blowing with lubricants increases efficiency approximately by 50 %. CoF is also influenced by the quality of ducts i.e. inner wall material of duct (smooth & ribbed). Generally blowing distance in ribbed inner wall duct will be more as compared to the smooth inner wall due to less area of contact between cable outer surface & duct inner wall. Various factors that influence CoF, are summarized in Table 2.

Cable Stiffness or flexibility
The stiffness or flexibility of cable also plays an important role in blowing performance. Stiffness depends on cable design construction i.e. metallic or non-metallic, central tube or multi-tube, outer jacket material, etc. Stiffer cable can achieve more blowing distance in straight trajectory and flexible cable can achieve more blowing distance in difficult trajectories or routes with multiple bends and grading. Stiffer cable is preferable as it can blow fast through undulation in ducts and avoids buckling of cable inside the ducts.

Cable Blowing Best Practices
a) Cable Crash Test
Prior to cable blowing it is important to perform the crash test to determine the maximum pushing force that can be applied on cable. Higher pushing force by tractor mechanism can damage cable outer sheath and cable buckling at entry point. This test involves pushing the cable from the blowing head into 5 to 10 meters of a duct that is sealed at the other end. When the cable stops after hitting the sealed end of the duct then the blowing head must stop pushing the cable such that there is no damage to the cable outer jacket and no cable buckling at the entry point. This process is repeated with an increase in push force until cable jacket damage is witnessed. The maximum permissible push force will be the highest push force without any damage to the cable jacket. This test ensures that the tractor will not damage the cable during installation in case of sudden stoppage of blowing.
b) Cable drum pay-off
This feature is very important in a blowing setup to ensure the cable is smoothly pushed inside the duct through the blowing unit. It is always recommended to use cable reel pay-off during cable installation. The cable drum should be kept level to avoid the cable rubbing against the drum flanges. The cable drum orientation should be such that the natural payoff direction is towards the pulling direction. To eliminate possible cable contact with the ground, cable payout should be from the top of the drum. Figure 4 shows a cable drum in a cable drum pay-off and figure 5 shows the do's and don'ts of cable unwinding.

Figure: 4

Figure: 5

Duct Integrity Test (DIT)
The purpose of DIT is to prove that the installed ducts are ready to use for blowing from one manhole to the other. DIT should be performed before blowing to avoid sudden surprises like missing sections of ducts, improper coupling, and kink and blockage in ducts. The DIT is carried out with the help of compressed air and a suitable go-gauge sponge and mandrel of appropriate size, which is pushed inside from one end with pneumatic pressure and is thrown outside from the other end. The three steps outlined below should be performed to conduct integrity.
a) Step 1: Air pressure test
Duct continuity is verified by introducing compressed air from one end of the duct, and checking if air comes out from the other end of the duct. If no air emerges then the fault point has to be identified and corrected before proceeding further. Duct leakage is verified with compressed air by attaching the compressed air unit head to one end of the duct, and attaching the pressure gauge to the other end of the duct. Compressed air at a pressure of 10 bar is applied at the first end for 10 - 15 minutes and air pressure drop at the other end of the duct is measured. If the pressure reading remains the same (not reduced to zero) at other ends for 5-10 minutes then there is no leakage in the duct. If the pressure reading drops to zero then there is leakage in the coupler (that joins two ducts), and the duct is punctured. Figure 6 shows a pressure gauge fixed at one end of the duct to monitor air pressure.

Figure: 6

c) Step 2: Shuttle test The kink in the installed ducts is located by shuttle test. A suitable shuttle of size 70-80% of the duct inner diameter is passed through the duct with the help of compressed air. If the shuttle comes out from the other end then the duct has no blockage and it is considered ready for cable blowing. If the shuttle does not come from the other end, then the duct is either kinked or blocked. To identify the blockage or kinked location, a transmitter is passed through the duct which will get stuck behind the shuttle and then the blockage point in the duct is tracked with the help of the receiver. The blocked portion of the duct is dug out and replaced. Figures 7 and 8 show a shuttle being inserted in a duct.

Figure: 7

Figure: 8

d) Step 3: Sponge Test Very often mud and water are found inside an underground duct. In this test, a sponge double the size of the duct inner diameter is passed through by compressed air to clean the duct. Figures 9 and 10 show a sponge being inserted into a duct.

Figure: 9

Figure: 10

A metal cap is fixed at the head of cable as shown in figure 11, to seal the cable end so that jelly or other elements of the cable will not damage the duct inner wall and cable blowing will be smooth.

Figure: 11

Safety Procedure

This document provides the guidelines for handling, transportation, and storage of Optical fibre cable drums. These guidelines can apply to all 'Outside Plant Optical fibre Cables'. This document provides information for the handling various reels/drums at various places from receiving in stores to shipment to the site for installation. Proper handling of cable reels/drums decreases the probability of accidental damage to cable, material & personnel. All-optical fibre cables are sensitive to damage during handling, storage, and installation. Such damage can degrade cable performance to the extent that replacement becomes necessary. Optical fibre Cable is a valuable product. If handling is not done properly, the drum and in turn the cable may get damaged. At times, the damage might not be discovered until the installation process, when repairing the cable gets extremely difficult and also expensive. There are a number of key safety issues that are important to keep in mind while handling the Optical fibre Cables as follows:

1. Keep the cable drum/reel protected with outer covering till the time it is used at the site.

• dr.com has developed an alternative packing plastic

• Drum covered with wooden lags

• Drum left uncovered

2. Lift the drums without damage

When lifting the drum; use a shaft through the center of the drum and a the spreader beam. In case spreader is not used the rope may damage the flanges of the drum which can cause cable damage.

3. Handling with Forklift

Ensure the drums are moved in an upright position and the fork must be longer than the width of the drum. When moving the drums, tilt the truck mast so that the drum remains in the fork and the points don't touch the ground. Raise the forks of the forklift at least 6-8 inches from the ground surface. Don't release the forks until the truck has stopped completely. Keep sufficient space between the drums so that forks don't damage the other drum.

4. Keep the Drum in an upright position

The drum is designed to be handled in an upright position. It may not sustain if lifted laying flat. When stored in an upright position, the cable-layers do not get entangled during uncoiling.

• a) Drum on flange edge

• b) Drum on the Flange Side

5. Roll the drums only as per the direction marked on the drum flanges

When the drums need to be rolled due to some reason, always roll the drums in the direction of the arrow marked on the drum flanges. This way the cable over the drum will not loosen. However, this does not mean that the drum can be rolled freely for any distance. Limit rolling distance to 5-10 meters. Once placed in position, use a proper stopper to prevent drums from rolling.

• a) Drum on flange edge

• b) Drum on the Flange Side

6. Drum transportation, Unloading, and Storage

A) During transportation, the cable drums should not be kept in a Flat position; it crushes down the layer of the cable, resulting in fibre breakage.

B) The cable drums should always be kept in an upright position and be tied with a Chain or Belt with wooden blocks that should be kept in between the flanges of each drum to avoid any jerks/ movements during transportation.

C) While unloading the truck it is important that the cable drum should not be dropped directly on the floor. The weight of the Drum & cable may cause deflection of Drum flange resulting in flattening, deformation, or damage to the cable.

D) The drum must be rolled from a truck on to receiving platform, which should be at the same height as the tailgate of the truck. An alternate is to use a forklift to unload drums from truck.

Note: If inclined ramps are used, roll the drums over it; but don't allow them to roll out of control. Roll each drum away from the bottom of the ramp before handling the next drum.

E) The drums should always be stored in an upright position. Storage of drums in an alternative position can lead to winding defects.

F) If storage place is limited and it becomes necessary to stack, then stack the completely wrapped on their flanges. Stacking is allowed only for the drums whose flange diameter is 1250mm or less. Do not store drums on Flat flanges

G) Optical fibre cables are winded on wooden drums. Due to the environment surrounding, the drum gets degraded over a period of time. To avoid such degradation in the wood during its storage period, in-house storage is recommended. If drums are stored outside, storing surface should be hard & no moist soil should be in contact with wood, this avoids the generation of harmful insects responsible for wood degradation. During heavy rain, the drums should be closed by polythene to keep the Drum moisture content around 18% to 25%. Copyright

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