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Reliability Moment

One of the biggest HDPE Plant in Indonesia located at Cilegon, Banten Indonesia experienced failures on two extruder screw shafts of 1-X-840 (extruder shaft #1) and 2-X-840 (extruder shaft #2).

Reliability Moment

Extruder Process Design Data1. Feed Materials : HDPE or LLDPE 2. Feed Mat’l Melt Index: 0.1 to 50 (LLDPE 0.1 – 30; HDPE 5 to 50)3. Feed Mat’l Water content: less than 700 ppm4. Process Temperature: <240 C for LLDPE or <260 C for HDPE 5. Machine Capacity : 17.4 ton/h6. Screw Speed : 224/156 rpm7. Screw and Barrel : - Feed and Mixing Section 300 mm dia.

- Discharge Section 226 mm dia.- Slot Clearance Adjustment 8 to 70 mm- Operating Pressure : hopper cylinder 5 barg, others 4 barg- Design Pressure: hopper cylinder 9 barg, others 6 barg

Reliability MomentExtruder Zone

Broken Area at Kneading zone

Reliability Moment

Fracture surface

2

• Multiple crack origins

• Initiation 1 created a stable crack

• Initiation 2 did not create a stable crack

2

Reliability MomentExtruder screw shaft #1 Extruder screw shaft #2

Reliability MomentExtruder Operational Data and Failure History1. Installation date : 19912. Both extruder were used to extrude Polyethylene with similar operational data history.3. Processing temp. : Below 240 C4. 1991 – 1998 : 15 - 16 ton/hour operating capacity5. 1998 : Modification on kneading screw6. 1998 – 2002 : Low operating capacity7. 2003 – 2006 : Idle, not operated8. 2006 – current : 21 ton/hour operating capacity9. 2008 – 2009 : Barrel temp. inverter was in troubled10. 2009 : Indication of vibration increased, Gland packing leakage 11. 2009 : Shaft gear pump coupling was broken (extruder #1)12. 2010 : The temperature inverter problem was solved.13. 2010 : Trial of HMW-PE on both extruder, very low MFI (0.02 up to

0.05)14. 9 April 2010 : Extruder shaft #1 was failed, Broken shafts were on the right side

of the extruder (from motor view), After replacement, the gear box was broken15. 2010 : The original screw was re-installed after the extruder shaft #1 was

broken.16. 9 Mei 2011 : Extruder shaft #2 was failed, Broken shafts were on the right side

of the extruder (from motor view)

Reliability Moment The failure point of the two extruder shaft #1 and #2 was

located before kneading/mixing zone, where the high pressure of polyethylene melt was built up and high bending moment on shaft was exist.

Visual examination and SEM data strongly indicates a strong contribution of fatigue bending moment failure with large propagation area and minor contribution of torsion fatigue.

Crack initiation sites on both extruder shafts have multiple crack origins, although only one crack origin developed a stable crack and propagated through shaft cross section and developed large propagation area.

Large propagation area indicated the applied stress after crack initiation was considerably low. The crack initiation was likely occurred when the loading was not distributed evenly to all hexagonal surfaces and during overload period.

Reliability MomentPhysical Cause :• The concentrated load was only possible when the fitness

between shaft surface to inner hexagonal surface of the screw is not zero clearance. Non zero clearance will cause inhomogeneity contact between both surfaces; the torsion moment has not been transmitted homogeneously to all hexagonal surfaces but only been transmitted through the hexagonal shaft corner.

Human Cause :• The root cause of 1-X-840 and 2-X-840 extruder shaft failure was

due to fatigue failure initiated by domination of concentrated load on hexagonal shaft surface during overload period.

Design Machine Capacity 17.4 ton/hHistorical, Machine in could be running at 21 ton/h

• Start and stop operation mode of this machine might contributed to increase fatigue cycle.

Latent Cause :• Inadequate procedure that allows operation of the machine above

design capacity & excessive start-stop mode.

Reliability Moment

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