Hosted at AAC
2012 – 10 - 24

CMA SAFETY SYMPOSIUM – Trapped energy
OVERVIEW
• Background
• Scientific approach
• Potential energy
• Holdback requirements
• Safety issue pertaining to holdbacks
• Trapped energy
• Managing trapped energy
• Conclusion
• Recommendations
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Background
• Attended SACEA meeting earlier this year
• Subject discussed safety around conveyors
• Specific case noted was referred to as stored energy problem
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Scientific approach
• Kinetic energy
– Applied energy
• Potential energy
– Energy to be applied
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Potential energy
• Belt mass slope energy
• Material mass slope energy
• Position of the counterweight energy
• Stretch energy in the belting
• The industry refers to this as stored energy
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
BELT MASS SLOPE ENERGY
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
MATERIAL MASS SLOPE ENERGY
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Counterweight mass
H = height
Ground Level
VERTICAL GRAVITY ARRANGEMENT
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Sheaves
TAKE-UP CARRIAGE
Counterweight mass
H = height
Ground Level
HORIZONTAL GRAVITY ARRANGEMENT without winch positioning
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Sheaves
WINCH
TAKE-UP CARRIAGE
Counterweight mass
H = height
Ground Level
HORIZONTAL GRAVITY ARRANGEMENT with winch positioning
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Sheaves
WINCH
TAKE-UP CARRIAGE
Counterweight mass
Ground Level
HORIZONTAL GRAVITY ARRANGEMENT with winch positioning
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
AUTOMATIC
WINCH
TAKE-UP CARRIAGE
Ground Level
HORIZONTAL WINCH TENSIONING ARRANGEMENT
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Holdback requirements
• When a conveyor is switched off it will come to rest
• This is also required on incline conveyors
• When material is loaded on an inclined belt, a condition is reached where it will run back
• The running back of a conveyor will cause serious spillage
• Conveyors are purposely designed with holdbacks to prevent runback
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Safety issue pertaining to holdbacks
• The inherent function of holdbacks results in unwanted energy being generated under aborted start conditions:
– When the tail pulley becomes jammed due to spillage
– When the tail pulley becomes jammed due to seized bearings
• This presentation refers to this unwanted energy as being trapped energy and labels it accordingly
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Trapped energy
• This energy cant be observed
• All the safety precautions may have been taken in good faith but the dangers could still exist
• Example of incident earlier this year
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Tensions the same top and bottom
Ground Level
CONVEYOR IN THE STATIONARY POSITION
Holdbacks fitted
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Tensions higher in the top than the bottom
Ground Level
CONVEYOR IN THE ABORTED START CONDITION
Holdbacks fitted
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Tail pulley jammed
Tensions higher in the top than the bottom
Ground Level
CONVEYOR IN THE ABORTED START CONDITION
Holdbacks fitted
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Managing trapped energy
• Holdbacks are fitted
– Holdbacks prime cause of trapped energy
– Holdbacks considered unsafe under these conditions
• Brakes offered as an alternative to holdbacks but
– Brakes can be tampered with
– Brakes are subject to wear
– Brake application philosophy generate dynamics
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Managing trapped energy
• The best way of handling trapped energy is to release it.
• Trapped energy can only be released on a holdback by turning the holdback backwards
• The releasing mechanism needs to be safe to operate
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Tail pulley jammed
Tensions higher in the top than the bottom
Ground Level
CONVEYOR IN THE ABORTED START CONDITION
RELEASABLE HOLDBACK FITTED
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
Managing trapped energy - cont
• Units are currently available in the market place offering the release facility
• The permanently applied brake is manually released by unscrewing the pressuring mechanism and releasing the “permanent” brake
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
CONCLUSIONS
• Fitting manual type brakes in conjunction with normal holdbacks overcomes the problem of releasing trapped energy
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Trapped energy
RECOMMENDATIONS
• Users to specify releasable holdbacks on all future projects
• Pursue the possibility of retro fitting these units on existing installations with specific reference to high energy applications which are typically relatively long incline applications like shaft conveyors
• This presentation was compiled without consideration of the effect of mono directional idlers on this application.
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
TAIL PULLEYS
• Latest statistics indicate tail pulleys as being the most dangerous area on a conveyor application with specific reference to fatalities
• A review is required to highlight the operational issues which lead to these unsafe conditions
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
TAIL PULLEYS
• Tail pulleys are recirculating type pulleys and are thus not self cleaning
• Being local to feed points spillage is always an issue
• Any spillage along the length of the conveyor landing on the return strand eventually ends up at the tail pulley
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
Pedestals
Spillage
Ground Level
ELEVATION ON TAIL PULLEY ARRGT
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
TAIL PULLEYS
• Spillage are manually fed onto the belt at the tail
• Spillage can be loaded onto the belt by means of skid loaders resulting in “surge loading”
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
SUGGESTIONS – vibrating plate feeders
• Vibrating plate feeders are ideal for load spillage onto conveyors while the belt is running
• Vibrating plate feeders will not cause material surges on the conveyor
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
SUGGESTIONS – design and construction
• Design the tail pulley framework such that there is ready access for maintenance
• Position the tail pulley as far back as possible from the loading section to allow free access
• Fully guard the tail pulley
• Adequate access to return belt ploughs
• Use the support structure as a barrier beam
• Endeavour to fit spillage deflectors over the tail
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
Pedestals Barrier beams
Ground Level
ELEVATION ON TAIL PULLEY ARRGT
Promoting the excellence of belt conveying in South Africa

CMA SAFETY SYMPOSIUM – Tail pulleys
Promoting the excellence of belt conveying in South Africa