Filling a 24-Fibre Loose Tube: Cable Jelly, SAP Powder and the Migration Problem

A 2.4 mm inside diameter loose tube holding 24 fibres takes about 4.3 millilitres of thixotropic jelly per metre, and on a 4 km drum that is 17 litres of jelly, 22 kg of it. At 70 C in a roof space the jelly softens, migrates down the tube, pools at the low end of a span and leaves the upper fibres dry. The splice crew finds out two years later when water tracks in through a sheath nick and reaches 40 m of fibre.

Dry water-blocking with polyacrylate powder is not a straight swap for jelly, and cable engineers who treat it as one end up with dust in the tube and blocked filling needles. This article covers the migration test that decides the case, the gram-per-metre numbers, and the process changes a powder line needs.

Jelly Migration at 70 C: The Drip Test That Decides

Hang a 1 m tube vertically in an oven at 70 C for 24 hours with the bottom end open. A good jelly drips less than 20 mm. A marginal one drips 60 mm, and a poor one empties the top third. This is the test that separates a formulation problem from a filling problem, and it takes a day.

Jelly also brings process cost: it has to be heated to 90 to 110 C to pump, it has to be cleaned out of the line at every product change, and it leaves a film on the fibre that can affect splicing. Those costs are what pushed several tube lines toward dry blocking in the first place, not the jelly price.

Where a Dry SAP Powder Replaces Thixotropic Jelly

In a dry-blocked tube, a small quantity of swellable powder is distributed along the tube before or during fibre loading. When water enters, the powder swells and plugs the interstitial space, stopping longitudinal travel within 1 to 3 m. The tube stays air-filled, which slightly improves attenuation stability and makes the tube easier to strip.

The catch is distribution. Jelly fills every void by displacement. Powder has to be metered and then disturbed into place by the fibre oscillation, so the fill pattern depends on line speed, tube tension and the number of fibres. A line running at 120 m/min with 24 fibres needs a powder feed accurate to plus or minus 8 percent, or some sections end up starved.

Grams per Kilometre and the Swell Window

Typical dry-blocking loads run 2.5 to 4.5 g/m for a 2.4 mm tube, which is 2.5 to 4.5 kg per kilometre. The powder is usually a fine cut, 100 to 200 mesh, because coarse granules will not distribute into a 0.9 mm fibre bundle. Swell rate matters more than total capacity: the powder should reach 80 percent of its final volume in under 60 seconds, because a sheath breach in a duct can admit water at 2 bar.

Test the finished tube by injecting water at 0.5 bar into one end and watching how far it travels in 24 hours. Under 3 m is a pass for most utility specifications. Repeat after 7 days at 90 C in water, because a powder that relies on un-crosslinked surface chains will have washed out by then.

Filling Needles, Blockage and Cleanliness

Powder at 100 mesh behaves like a fluid in a vibratory feeder and like a solid in a 3 mm needle. The two failures are rat-holing in the hopper, which starves the tube, and caking at the needle tip when humidity exceeds 55 percent RH. Both are solved with hardware rather than formulation: a 60 degree hopper cone, a 0.5 bar nitrogen blanket, and a needle with a 4 mm orifice and a 2 mm air assist.

Dust is the other issue. A cable line is usually a clean environment and a fine polyacrylate cloud settles on everything and swells on any condensation. Local extraction at the fill point and a weekly wipe-down are cheaper than a filtered clean room, and operators should wear dust masks during hopper changes.

Ask for a grade with a tight particle size distribution and a documented swell rate, not just a capacity figure. The water absorbing SAP powder and granule range includes the fine cuts used in loose tube and tape blocking.