EoW September 2009

technical article

A set of injection equipment for inter- • mittent spraying of water-blocking gel onto the metal tape with specified length, width and thickness A set of controls based on the line • speed: the original value can be set according to different requirements and the quantity of the gel sprayed onto the tape can be automatically adjusted with the variation of the line speed Figure 2 illustrates the implementation of the method. We can see that this method is easy for operation without complicated equipment alteration. penetration test As mentioned above, the length, width and thickness of the gel sprayed onto the tape can be adjusted in order to achieve the best result. Many experiments were carried out to optimise these parameters. Using the 24mm width steel tape armour, for example, researchers selected 200mm as the length of the gel and chose different values for the width and the thickness of the gel. The gel was injected onto the tape at intervals of 1m. Water penetration test was performed on the cable samples, with different gel parameters, to verify whether they could pass the test or not. Gel thickness was calculated after the value of gel weight, gel length and gel width was known. The penetration test was performed on 2m long cable samples (1m waterhead, 24 hours), which is stricter than that described in international standards. Table 1 shows the water penetration test results for above-mentioned trial cables with different gel parameters. From the table, it can be seen that optimum gel parameters for the cable with 24mm wide steel tape armour are 200×20×0.3mm (if the gel is sprayed at a certain interval of 1m). 4 Water

Cable core filled with ordinary gel

Water-blocking gel sprayed onto steel type intermittently

Figure 2 ▲ ▲ : Implementation of the newmethod

Thus, good water blocking perfor- mance can be obtained over the whole cross section of the cable The gel can be sprayed onto the • metal tape at intervals, as long as the water-blocking performance can be ensured. In this way, the filling compound material can be saved and the production cost can be reduced 3 Implementation of method This new method can be easily implemented on various sheathing lines by the adoption of the following components: Two gel pumps: one is for the • continuous ordinary gel filling in the cable core, while the other is for spraying the water-blocking gel onto the metal tape A set of pressure filling equipment for • ordinary gel used in the interstices of the cable core

Finally, a thin layer of water-swellable • gel is appropriately spread on the surface of the aluminium or steel tape, as shown in Figure 1 The gel can be sprayed accurately onto the metal tape, which means that the length, width and thickness of the gel should be selected according to the width of the tape so that enough gel will be present at the overlap without overflow. The spray of the gel can be carried out either continuously or intermittently (as shown in Figure 1 ). Compared with traditional water-blocking methods used in cables with metal tape armour, the new method has the following features: The interstices in the cable core are • completely filled with ordinary gel, just as in traditional methods. Since the ordinary gel is cheaper than the water-swellable gel, this behaviour can effectively reduce the production cost, which is a best choice for gel- filled cables A rubber eraser, with the inner dia- • meter almost the same as the outer diameter of the cable core, is employed to wipe away the superfluous gel; thus the diameter of the cable core after gel filling can be precisely controlled, which makes subsequent structural design and process control much easier Since it is difficult to ensure the • water-blocking effect at the overlap of the tape as well as the space between the cable core and the metal tape, a thinner layer of water-swellable gel is sprayed onto the tape continuously or intermittently When water penetrates into the • cable, the gel will swell to fill up the interstices between the cable core and the metal tape. Meanwhile, water can be also blocked at the overlap of the tape due to the swelling of the gel

Table 1 ▼ ▼ : Water penetration test results

Gel thickness (mm)

Gel width (mm)

0.2

0.3

0.5

Failed, but water leakage stops after 1 min Failed, but water leakage stops after 0.5min

Failed, but water leakage stops after 0.5 min Passed, but gel is likely to overflow

18

Failed

19

Failed

Failed, but water leakage stops after 1 min Failed, but water leakage stops after 0.5min

Passed, but gel is likely to overflow

20

Passed

Passed, but gel is likely to overflow

Passed, but gel is likely to overflow

22

79

EuroWire – September 2009