Root Cause Analysis, Operating Envelopes

1.  
M&R Day
Iwan - Abadi

2. 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).

3. Extruder Process Design Data
1. 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 ppm
4. Process Temperature: <240 C for LLDPE or <260 C for HDPE
5. Machine Capacity : 17.4 ton/h
6. Screw Speed : 224/156 rpm
7. 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

4. Extruder Zone
Broken Area at
Kneading zone

5. Fracture
surface
2
• Multiple crack origins
• Initiation 1 created a
stable crack
• Initiation 2 did not
create a stable crack
2

6. Extruder screw shaft #1 Extruder screw shaft #2

7. Extruder Operational Data and Failure History
1. Installation date : 1991
2. Both extruder were used to extrude Polyethylene with similar operational data history.
3. Processing temp. : Below 240 C
4. 1991 – 1998 : 15 - 16 ton/hour operating capacity
5. 1998 : Modification on kneading screw
6. 1998 – 2002 : Low operating capacity
7. 2003 – 2006 : Idle, not operated
8. 2006 – current : 21 ton/hour operating capacity
9. 2008 – 2009 : Barrel temp. inverter was in troubled
10. 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 broken
15. 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)

8.  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.

9. Physical 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/h
Historical, 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.

10. Thank You