Trapped Energy by Simon Curry

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CMA

SAFETY SYMPOSIUM

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

THANK YOU

QUESTIONS

Promoting the excellence of belt conveying in South Africa

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