Trapped Energy by Simon Curry

Report
CMA
SAFETY SYMPOSIUM
Hosted at AAC
2012 – 10 - 24
CMA SAFETY SYMPOSIUM – Trapped energy
OVERVIEW
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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
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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
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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
THANK YOU
QUESTIONS
Promoting the excellence of belt conveying in South Africa

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