What is the Maximum Speed at which a Cable can be Blown in?

1. What is the Maximum Speed at which a Cable can be Blown in? Willem Griffioen International Wire & Cable Symposium, 29 Sep – 2 Oct 2019, Charlotte (NC)

2. Contents • Introduction • Analysis • Water hammer, sudden cable stop • Example • Water hammer, sudden cable stop, visualization • Crash test • Intelligent blowing machines • Conclusions

3. Introduction • Optical cables are blown in > 3 decades • High production (12 km/day no exception) • Question: what is the maximum blowing speed? • Often 60 m/min was taken for this • But sometimes much higher speeds are seen (180 m/min) • Today even orthodox telecom operators allow high speed • Intelligent blowing machines record cable speed • Stop cable drum quickly after sudden cable stop • Care taken not to damage cable, experience, crash test, visual inspection

4. Introduction • But, what happens to the cable in the duct? • Buckling of the cable after sudden stop • What does inertia, how much will this buckle the cable? • What will be the resulting pushing force? • And is the maximum pushing force specified anyway? • Floating (with water) risk for duct? • For floating water speed > cable speed needed • Will high water speed create water hammer at blocked flow? • And will pressure of water hammer destroy duct?

5. Analysis, water hammer • Water hammer, or fluid hammer • Pressure surge or wave caused by fluid in motion when forced to stop or change direction (momentum change) • When the flow is suddenly blocked • When a valve is suddenly closed • Column of fluid in motion will stop • Not entirely at once (would result in infinite pressure) • Stopped part grows upstream with speed of sound • Like a longitudinal “compressive” wave

6. Analysis, water hammer • Joukowsky formula for pressure p of water • ρ = cable density (1000 kg/m3) • c = speed of sound in water (1500 m/s, PE SDR 11 duct 23%) • v = cable speed • Correction duct expansion: 23% for PE duct SDR 11 • 60 m/min would give 15 bar, with correction even much less (3.5 bar for PE SDR 11) • By far not limiting speed of water flow

7. Analysis, sudden cable stop • Cable suddenly stopped • Also not entirely at once • Also like an upstream wave (analogous to water hammer) • Longitudinal (axial compression) and transversal (buckling) • Relative “absorbed” cable length εs at force Fc: • kc = cable spring constant • B = cable stiffness • Dc= cable diameter • Dd= duct diameter • cb = constant (2.23 for sine buckling, 0 for tensile)

8. Analysis, sudden cable stop • Joukowskyanalogon for Fc at cable stop • mc = cable mass / unit length • vc = cable speed • Shall (at least) be ≤ Fcmax (set by buckling radius)

9. Example • Cable 96 OF • kc = 3000 N / 1% strain • B = 0.2 Nm2 • Dc= 6.5 mm • mc = 42 g/m • Max pulling force = 500 N • Max pushing force = 300 N • Duct (microduct ↔ duct) • Dd= 8 - 33 mm

10. Example • Water hammer • SDR 11 duct can easily handle water pressures of 20 bar, especially for short time • Resulting max water speed is 360 m/min! • For a water pressure of 20 bar, according to Blasius’ law, even in a 40/33 mm duct the length needs to be as short as 250 m (floating lengths are much larger) in order to reach such high water speeds • So, in telecom ducts there is usually not a problem with water hammers

11. Example • Sudden cable stop forces Fc (N) (and speed vs)

12. Example • Sudden cable stop • For 180 m/min only (just) critical for 10/8 mm duct • Surprisingly smallest microduct • No space to buckle, so buckling bend radius no problem • But, less buckling absorption (maximum pushing force just reached) • For largest ducts much higher speeds possible • For 500 m/min only sinusoidal buckling gives problems • Now for too small buckle bend radius • “Tensile stop” (cable lump or blocking of reel) • Maximum speed around 250 m/min (independent duct size)

13. Example, visualized Cable hits obstacle (and blocks flow)

14. Example, visualized • “Buckle wave” and water hammer wave start Cable hits obstacle (and blocks flow)

15. Example, visualized • “Buckle wave” and water hammer wave start • Wave travels upstream until rear cable end Cable hits obstacle (and blocks flow)

16. Example, visualized • “Buckle wave” and water hammer wave start • Waves travel backwards until rear cable end • Buckle wave travels faster than pressure wave • Only after buckle wave reaches pusher it slows down • After pusher comes to standstill its force may increase to set max (when higher than force of buckle wave) Cable hits obstacle (and blocks flow)

17. Crash test • Crash test to evaluate max pushing force • Cannot be used to evaluate max speed • Extremely high speeds are not reached (on a few m) • If max speed reached, than effective Fpush = 0 (Newton 1st law) • Force due to sudden stop not added to machine pushing force

18. Intelligent blowing machines

19. Intelligent blowing machines Blowing machines have their max pushing force set by max pressure or electric current or voltage For small cables and small forces the machines can therefore not run fast Intelligent machines can correct the force by using info about the speed, allowing higher speeds And, of course, intelligent blowing machines can also calculate the maximum allowed cable speed and set this as max value.

20. Conclusions The max cable speed for a sudden cable stop has been derived theoretically Cable speeds up to 180 m/min are still okay in most situations, if not, intelligent machines can set a lower max value A crash test can be used to find the max pushing force for the cable, not the max speed Water hammer effects (with floating) will usually not cause problems in telecom applications

21. Thanks for the Attention!