Incident alert

LOCATION:
QUARRY
ACTIVITY:
PRODUCTION AND PROCESSING
SUB ACTIVITY:
AGGREGATE PROCESSING
ALERT STATUS:
Normal
DATE ISSUED:
21/07/2006
INCIDENT No:
00078

TITLE
Lost time incident involving Fuller Traylor gyratory crusher
COUNTRY OF ORIGIN
United Kingdom
ACCIDENT / INCIDENT DETAILS
Contractors were undertaking the tenth re-metal of the primary crusher, there being no problems reported with stored energy (hoop stress) within the Manganese concaves (liner plates) during these previous re-metals.
Hoop stress (stored energy) is caused by the plates work hardening and peening (spreading)

The contractors were highly experienced in re-metals and were following the safe method of work as detailed in Fuller Traylors maintenance manual.
Based on the manufacturer manual a detailed risk assessment and method statement had been prepared by the experienced contractors.

Stored energy (vertical/hoop stresses) were identified as a hazard and dealt with in the safe method of work following Fuller Traylors maintenance manual instructions.

The liner plates (concaves) consist of three vertical rows sitting on top of each other. It was the top row of liner plates (16 in total) that were to be involved in a sudden and unexpected release of stored energy (Newton Cradle Affect – refer photo) that resulted in a contractor being hit by a heavy (worn) liner plate weighing 300 kg approx.

Prior to the re-metal of the crusher regular monitoring of stress build up is measured in the body-shells of the crusher and securing bolts.

Any increase in shell gaps triggers a requirement to scarfe (gouge out) the horizontal joints between the liner plates, this being carried out some 4 weeks prior to the incident, the same contractor scarfing (gouging out) the peening (spreading) that had taken place in the joints of the Manganese liner plates.

In the past this stress build up (stored energy) had fractured engineered nuts and bolts.

Normally two dump shutes are used and the spider deflects the stone down either side of the crusher bowl, ensuring that the liner plates wear and stretch uniformly, however in the last 3-4 months only one dump chute was in operation.

The top row of liner plates have a recessed groove on the rear side of the top leading edge which is forced against a raised flange running around the top of the crusher bowl. Metal pegs are then driven between the plates to tighten them up and Nordbak epoxy resin (glue) is poured down the gap at the back to seal them to the crusher bowl.

The current liner plates had been in situ for 18 months and showed signs of peening in the vertical plane once removed from the crusher.

DESCRIPTION OF THE LOST TIME INCIDENT

Following Fuller Traylors maintenance manual procedure, the experienced contractors removed the coverplates and spider and lowered into the crusher bowl a purpose built work platform (tophat) to work from (if required)
Lifting eyes were first welded onto the top row of liner plates (16) to assist in lifting them out using the crusher hoist.
A thermal lance was then used to remove the ‘keystone’ liner plate (normally at right angles to the spider) so as to release any hoop stresses.
To assist in their removal, wedges are driven behind the plates at the top leading edge to break the epoxy resin seal.

Once the ‘keystone’ liner plate was removed a further 3 plates were removed in a clockwise direction.
The fifth plate however was proving difficult to remove, so the experienced contractor entered the bowl of the crusher using a short ladder and standing on the purpose built platform provided walked over to the other free end.

He tapped this ‘free end’ liner plate with a sledgehammer to signal to his work mates that they would attempt to remove it.

On tapping the face of the liner plate (concave) there was a large bang/noise and all the remaining liner plates (12) shifted (shockwaved) in both directions (clockwise/anti clockwise) just like a ‘Newton Cradle Toy Affect’.(refer diagram)

The end two plates on the clockwise ‘shockwave’ where the experienced contractor stood were ‘pinged’ (moved) around the circumference and toppled over, the end plate hitting the contractor on the right forearm as he made his getaway from danger, breaking i
ACCIDENT / INCIDENT IMAGES




LEARNING POINTS / ACTIONS TAKEN
CAUSATION OF THE INCIDENT

No precise causation as been identified by either the manufacturer (Fuller Traylor) or competent re-metal contractors, however I offer the following root causes for consideration.

1/ Only one dump hopper was being used for the last 3-4 months. The spider would have deflected the stone towards the top left/right quadrants which may have resulted in more stored energy/pressure in these plates as the Manganese work hardened and spread (peened)

2/ Though the horizontal peening was mostly removed by scarfing (gouging out), there was visual evidence from the removed liner plates that peening (especially in the vertical plane) still remained. This may have accounted for the retention of stored energy (hoop stress) even though four of the liner plates had been removed. The top edge of the liner plates pushing against the raised retaining flange may also have retained stored vertical stress (energy) through peening.

3/ As the free ends of the liner plates moved ‘shockwaved’ in both directions the epoxy resin backing showed signs of tearing, identifying that the resin backing (glue) still had sufficient strength to hold the remaining plates in situ until a stored energy force moved (‘pinged’) the plates around the crusher bowl circumference. (Newton Cradle Affect)


RECOMMENDATIONS TO PREVENT A REOCCURENCE

Though the manufacturers (Fuller Traylor) identified stress (stored energy) build up in Manganese concaves (liner plates) in their maintenance manual, no one (including the contractor) anticipated a sudden and violent release of stored energy in a classic Newton Cradle Affect as happened in this incident.

Normally the removal of the ‘keystone’plate should have dissipated most of the hoop stresses, the liner plates only moving an inch as they were freed from the backing and then lifted out of the crusher.

1/ Fuller Traylor (manufacturers) to more easily identify the hazards of stress (energy) build up in Manganese concaves in their maintenance manual – action taken ......Ed Kline Safety Director as been asked to review their excellent maintenance manual to more clearly identify (on a separate page) the potential hazards from hoop stress etc (response to suggestion to be monitored)

2/ Another purpose built platform (tophat) needs to be constructed so that fitters can work off it, inside the the top section of the crusher bowl, confident in the knowledge that the steel work would contain any potential movement of the plates due to hoop stress etc.- action taken......Another work platform being constructed by QM&S if a further re-metal is required on this crusher.

3/ consider the provision and use of a modified pecker to chisel away the liner plates prior to them being lifted out. (Preferred option) – action taken.......New primary crusher provided with pecker for this work

4/ Review and Revise the risk assessment and method statement to ensure agreed control measures are incorporated. Make known to all employees and contractors who may be affected by the revised safe work procedure. Action taken......QM&S and Cliffe Hill maintenance team reviewing and revising safe work procedure for this crusher and other similar tasks involving the removal of concaves

5/ Consider undertaking a toolbox talk (team briefing) to all fitters and regular contractors about the dangers of stored/static energy- action taken.....toolbox talk to be prepared by myself and maintenance manager.

6/ Issue a safety alert with regards this incident to the QNJAC for circulation to members such as Aggregate Industries (Bardon/Croft), Foster Yeomans (Glensida), Hanson, Tarmac, Anglo etc – action taken..... This company safety alert to be circulated by e mail to all interested parties

7/ Lost Time Incident to be discussed at all SHE Improvement committees- action taken.......Incident discussed at all units by safety committee improvement representatives

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LEARNING POINTS / ACTIONS IMAGES




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