Hose Length Change and Air Balance
A crew is placing a fiber-reinforced dry-mix mortar on a 4-story parking structure. The gun sits at ground level; the working nozzle is on level 4, and the crew adds two 50-ft sections to the original 100-ft hose to reach the deck. The nozzleman reports the material arrives 'wet and heavy' at the nozzle, the pattern is sputtering, and the finish is slumping on vertical surfaces. The gun operator has not changed the air valve setting since morning. Air compressor output is 185 cfm at 100 psi; the gun requires 150 cfm at the nozzle. Ambient temperature rose from 60°F to 95°F since morning. The foreman wants to 'turn up the water at the gun' to fix the slump.
1. Diagnose why the material behavior changed after the hose was extended, and evaluate the foreman's proposed fix.
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Extending the hose from 100 ft to 200 ft increases friction loss and changes the air-to-material ratio reaching the nozzle. The gun air setting that worked at 100 ft is now undersized for the longer run - air pressure at the nozzle drops, material moves slower, and water added at the gun has more residence time, arriving wetter and heavier. The sputtering pattern is the classic signature of insufficient nozzle air, not excess water.
Turning up water at the gun makes it worse: more water means more slump, more shrinkage, and lower strength. The correct response is to restore nozzle air pressure first (verify with a nozzle pressure gauge, not by feel), then re-evaluate water. If compressor output is marginal at 185 cfm against 150 cfm demand plus other users, the compressor may be the limiting factor.
2. The crew cannot add compressor capacity today. What operational adjustments preserve quality on the vertical surfaces?
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Options in priority order:
- Reduce hose length by relocating the gun to an intermediate level (e.g., level 2), cutting friction loss and restoring nozzle pressure.
- Reduce output rate - a slower material feed matches available air and reduces slugging.
- Adjust nozzle air up to the maximum the compressor can sustain without starving other users, verified by gauge.
- Build vertical passes in thinner lifts and let initial set occur before the next pass, rather than trying to place full thickness wet.
Document the constraint and the adjustments; if nozzle pressure cannot be maintained within the manufacturer's range, the placement should be paused rather than forced, because low-velocity placement produces entrapment and weak, non-uniform material.