Air-Blowing Stall at 860 m on a 1,200 m Duct Route
A crew is air-blowing a 48-fiber microcable (8.0 mm OD, 0.9 kg/m) into a proved 40/33 mm HDPE duct along a 1,200 m suburban route. The duct has two 90° swept bends, one with a 4 m radius, and was rodded and foam-pigged the previous day. At 860 m the cable stops advancing. Compressor delivery is 12 bar at the manifold, but the cable feed unit shows rising push force and the cable jacket is warm at the feed wheels. Line speed was 28 m/min; lubricant was added only at the start. Ambient temperature is 32 °C. The crew wants to increase air pressure to 14 bar and raise feed force to clear the blockage. Duct fill ratio is about 6%, and the duct was verified clear. The cable reel has 300 m remaining. No pulling eye is fitted.
1. What is the most likely cause of the stall, and what does the fill ratio tell you about the duct capacity?
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Fill ratio = (cable OD / duct ID)^2 × 100 = (8.0 / 33)^2 × 100 ≈ 5.9%. That is far below a typical overfill concern, so the duct is not too small and a pure fill-ratio blockage is unlikely. The stall at 860 m with rising push force, a warm jacket, and lubricant only at the start points to friction build-up. The 28 m/min speed is high for a long blow with swept bends, and the 4 m bend concentrates sidewall force. Increasing manifold pressure to 14 bar and raising feed force is the wrong first response: it raises buckling, jacket damage, and duct-ovalization risk without fixing friction. The warm jacket is evidence of mechanical rubbing, not a signal to push harder.
2. List the immediate safe recovery steps and how you would verify the duct and cable before resuming.
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Stop the compressor, relieve feed force, and reverse the feed wheels slowly to take compression off the cable. Do not pull back hard through the bend. Inspect the jacket at the feed point for flattening, glossing, or heat damage; if damaged, cut back and re-terminate the end. Re-verify the duct with a foam pig or mandrel, especially through the 4 m bend, and check for ovalization,debris, or a partially open coupling. Apply a compatible cable lubricant at the entry and, if practical, at the bend. Reduce speed to 10-15 m/min and set a defined push-force limit. Restart with steady low-speed feed and watch the pressure/force trend; if the force rises again, stop before the cable takes a set. Record the force and distance so the next attempt is controlled.
3. If repeated low-speed attempts fail, what installation options exist and what are their consequences?
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Do not keep raising pressure. Options include: pull the cable back and re-blow from the opposite end if the route geometry allows; install an intermediate access point and blow in two sections with a splice; use a pulling line with a swivel and tension monitor if the cable and duct permit, staying below the cable tension limit; or replace the difficult duct section. Each has consequences: an intermediate splice adds loss and a closure, reverse blowing may not help if the obstruction is directional, and pulling introduces tension and bend-radius risk. The selected method must be documented and the installed cable tested with an OTDR before acceptance.