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PROPOSED GIUDELINE
For the
IMPLEMENTATION
Of
SAFETY & PROTECTION ON BELT CONVEYORS |
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The prime objective of this guide line is to provide the end user with a better understanding of the mechanics of belt conveyor, the weak points in the design, what can, and will go wrong, how to audit the designer decisions with regards to choice of safety and protection on new installations, as well as, how to enhanced the safety and protection on old systems.
However, it must be stressed to the reader, that the write up of this document has no intentions whatsoever to under mined the designer or the engineering company that has been appointed to design the conveyor and all equipment associated with it.
Prepared by: Asher Singer (Co-owner MD)
C.T. SYSTEMS cc
45 Lake Ave Benoni
+2711 8453258
asher@mweb.co.za
INTRODUCTION
Transporting bulk materials by conveyor belts dates back to approximately 1795; most of these early installations handled grain over relatively short distances.
The first conveyor belt systems were very primitive and consisted of leather, canvas, or rubber belts traveling over a flat or troughed wooden bed. This type of system was not an unqualified success but did provide incentive for engineers to consider conveyors as a rapid, economical, and safe method of moving large volumes of bulk materials from one location to another.
The basic advantages of conveyors over other means of transport (such as truck, rail, skip-hoist, and aerial tramway) for bulk haulage are numerous. The following paragraphs indicate why today’s belt conveyors have become the primary method for bulk material handling.
Conveyor belts have no equal in capacity among competing transport means. At belt speed of 5m/s, a 1600-mm-wide conveyor belt delivers more than 100 metric tons per minute of 1000-kg/m³ material.
Conveyors can follow ordinary natural cross-country terrain by virtue of their ability to traverse relatively steep grades (up to and including 18 degrees, depending on the material being carried). With the development of high-tension synthetic fabrics and / or steel cable reinforcing members, one flight can extend for several kilometers.
A belt conveyor system operates on its own “bed” of idlers, thus requiring a minimum of attention. Repair or replacement is both fast and easy, and the cost of route maintenance is minimal.
The smooth ride of long center roll conveyor belt systems produces little degradation of the material being conveyed.
Electrically powered conveyor belt systems are quiet (an important feature in procuring right-of-ways and in complying with the Occupational Safety and Health Act regulations). Belt systems can be covered to help ensure clean air. They can even be buried out of sight for quiet, functional, and aesthetic reasons.
One man per 2 km of line is generally adequate in a properly designed belt conveyor system. Contrast this with the number of drivers on a truck operation handling equal tonnage.
Low weight of load and conveyor structure per linear foot allows simple structural design for bridging gulley, streams, highways, or other similar obstacles. Likewise, a conveyor structure on a hillside requires little excavation and does not invite hazards from earth or rock slides. Because the structure is compact, it requires a minimum of covering for protection.
These capabilities are important in mining or excavation; where two or more digging operations can feed to a central loading point. At the discharge end material can be dispersed in several directions from the main line. Or material can be discharged along any part of any line by a tripper. Pendulum or caterpillar-mounted belts can be swung in a 180-degree arc to follow a digging shovel or can be used on the discharge end for stockpiling.
 THIS PROPOSED GUIDELINE IS AIMED FOR THE ATTENTION OF TECHNICAL PERSONNEL INVOLVED WITH THE SPECIFICATION, ACQUISTION, INSTALLATION, COMMISSIONING AND MAINTENANCE OF SAFETY DEVICES APPLICABLE TO CONVEYOR SYSTEMS.
Each country has its own rules and regulations when it comes to the decision what safety and protection is required on a belt conveyor and these specification form a basic guideline for the designer and the end user.
In South Africa for example, the inspector of mine and machinery addresses three issues with regards to safety and protection for belt conveyor and they are as follows:
- The facility to stop the conveyor at ANY POINT along side the conveyor and not only at certain points along side the conveyor.
Furthermore, in the event that the conveyor is accessible on both sides, these facilities must apply to both sides of the conveyor. If only one side is accessible, attention must be given to the head & tail sections as they are open to maintenance personal.
- An audible and visible pre start warning alarm that can be heard at ANY POINT along side the conveyor and not only at the head and tail of the conveyor.
- Slip detector sensor, in order to detect any sever slip of the belt that can cause friction which may result in fire which can be harmful to personal or even to cause an explosion in a coal mine under ground.
However, the inspector leaves any further protections required on the conveyor and which are of outmost important with the end user and the designer and the end user unknowingly what is required or due to shortage of capital, is left with a system that sooner or later will start to give him problems that could be avoided in the design stages and accommodated for.
With regards to old installations, proper knowledge of how to address repeating problems and how to solve them can reduce down time and increase the availability of the conveyor.
But in order to provide solutions we first have to address the weak areas where problems can develop and how to identify them before they get out of control and the damage is sever and extremely costly in down time and capital.
Areas of concern
- FAIL SAFE SYSTEM
- PRE START WARNING
- POSSIBLE DAMAGE DURING START UP OF CONVEYOR
- MONITORING OF TENSIONING SYSTEM
- DAMAGE TO THE BELT
- TROUGHING & RETURN BELT MISALIGNMENT
- ISOLATION FOR LIGHTNING PROTECTION
- CONTROL & MONITORING OF SAFETY AND PROTECTION EQUIPMENT MOUNTED ON THE CONVEYOR IN ORDER TO MINIMIZE DOWN TIME & INCREASE AVAILABILITY OF THE CONVEYOR.
- GENERAL COMMENTS
- FAIL SAFE SYSTEM
Fail safe by definition, implies that a relay which is mounted and connected in the control circuit of the conveyor drive, is kept energized in a mode of hard wire through the normally closed contacts in each and every safety and protection unit mounted on the conveyor.
In the event that a safety or protection unit has been activated or tripped, this normally closed contact will open and break the hard wire which in return will de-energize the fail safe relay which will trip and stop the conveyor drive thus stopping the conveyor.
PLEASE NOTE!!!
Some end users are using a PLC as a Fail Safe system which is completely wrong.
Semiconductors are not designed or manufactured for use in a device or system that is used under circumstances in which human life is potentially at stake.
A PLC is by definition a SEMICONDUCTOR.
2. PRE START WARNING (PSW)
PSW by definition is an audible and visible alarm that its prime objective is to warn and inform attending personal that maybe working on the conveyor that the conveyor is due to start moving, once this alarm is activated for an adjustable time all activities on the conveyor must be stop and people must evacuate and maintain a safe distance from the conveyor.
However, 2 problems are associated with these procedures; some end users are activating the PSW only at the head and tail section of the conveyor, which is wrong when dealing with long conveyors and other end users which do activate the PSW for the full length of the conveyor, stop the PSW after an adjustable time but start the conveyor after a short delay which can cause confusion that people may assume that the conveyor would not start and accidents may occur.
It is important to keep the PSW activated till such time that the conveyor is moving all the way.
Although some end user have rules that the mains must be switched “off” in the sub-station, prior to any work is to be carried on the conveyor, it is important that the pull-keys which are mounted along side the conveyor should be equipped with the facility for padlocking in the “off” position in order to prevent any accident.
3. POSSIBLE DAMAGE DURING START UP OF CONVEYOR
One common practice the most of the end users are implementing is the usage of a centrifugal switch to monitor the speed of the belt. This centrifugal switch is bridged by a timer which maintains a close contact for an adjustable time in order for the conveyor to reach its speed and by then the contacts of the centrifugal switch will close and the timer will switch off.
The down fall of this procedure is that the operator in the control room does not have a clue of what is happening on the conveyor, for example the belt can be jammed for some reason or other and the belt can be torn in pieces, or the belt can be slipping which can cause another sever damage etc.. And the operator will find out what is happening once the timer has lapsed and the belt is not on speed or it been torn or slipping.
This mode of operation is called static and leaves a-lot to be desire.
The other mode of monitoring the belt during start up is the dynamic mode which is much more reliable accurate and scientific. In this mode the end user can use either the Doppler Effect speed detector or via proximity switch which are linked to the conveyor controller CPU and it is operating as follow:
Once the conveyor drive has been energized a triggering signal is sent to the conveyor controller which start to monitor that the belt is moving even at a speed of 0.1m/s and the belt is not jammed, their after, the controller monitors the acceleration of the belt will comparing speed verses time and confirming that the belt accelerates with the tendency to reach the set speed in the pre set time.
In the event that the controller establishes during acceleration period that the belt is moving but not accelerating it means that the belt is slipping and the controller will stop the conveyor immediately and would not wait for any timer, when the controller receives the triggering signal that the belt is due to move, but after a pre set short delay there are no movements of the belt, the controller will stop the drive and prevent any damage to the belt.
Once the belt reached its set speed in its set time the controller will monitor the speed of the belt for under speed conditions.
4. MONITORING OF TENSIONING SYSTEM
The force required to drive a belt conveyor must be transmitted from the drive pulley to the belt by means of friction between their two surfaces. The force required to restrain a downhill regenerative conveyor is transmitted in exactly the same manner. In order to transmit power, there must be a difference in the tension in the belt as it approaches and leaves the drive pulley. This difference in tensions is supplied by the driving power source.
It should be noted that if power is transmitted from the pulley to the belt, the approaching portion of the belt will have the larger tension, T1, and the departing portion will have the smaller tension, T2. If power is transmitted from the belt to the pulley, as with a regenerative declined conveyor, the reverse is true. Wrap is used here to refer to the angle or arc of contact the belt makes with the pulley’s circumference.
The wrap factor, Cw, is a mathematical value used in the determination of the effective belt tension, Te that can be dependably developed by the drive pulley. The Te that can be developed is governed by the coefficient of friction existing between the pulley and the belt, wrap, and the values of T1 and T2.
The following symbols and formulas are used to evaluate the drive pulley relationships:
Te = T1 – T2 = effective belt tension
T1 = Tight-side tension at pulley
T2 = slack-side tension at pulley
f = coefficient of friction between pulley surface and belt surface.
We identify 3 basic type of tensioning systems:
- Screw Take-up
- Winch Take-up (automatic)
- Gravity Take-up
The Screw Take-up is used mainly on short conveyor which maintains a fixed tension for loaded or unloaded belt and do not have any moving parts.
The automatic and the gravity take-up play a very important role in the operation of conveyors, insofar as providing the required tension on drive pulley for the friction required to drive the belt, as well as collecting the sage of the belt between the idlers etc..
However, they are also an issue of concern and special attention must be given when the belt has to be spliced due to damage, or when the return belt is turned up side down in certain applications and the tensioning is incorrect it can cause the return belt to fold like a pipe conveyor which will require to cut the belt and re-splice it.
It is important to monitor the movement of the counter weight and to add weight if the splicing of the belt was done incorrectly.
Further more both systems have a trolley for the take-up which may and will cause sever misalignment if the trolley does not move smoothly on its tracks.
5. DAMAGE TO THE BELT
Damage to the belt can be described as follows:
- An elongated TEAR in the side wing of the belt (on the right side or the left side).
This type of damage is usually caused due to misalignment on the return belt, it varies in width from 100mm to 300mm and it can be observed as a piece of belt flopping on the idlers. This type of damage can be detected via a TEAR detector which will trip and stop the conveyor before grater damage can occur.
- An elongated RIP in the center section of the belt.
This type of damage usually happens at the tail section of the conveyor, when a sctsystems foreign object traveling via the chute penetrates the belt and gets stuck between the impact idlers and while the belt is moving, it cuts the belt like using a knife.
This type of damage is impossible to prevent, but with the use of a RIP detector the damage can be reduce reasonably.
- An elongated RIP/TEAR in the center section of the belt.
This type of damage usually happens at the head section of the conveyor on the return belt immediately after the head pulley due to a belt scraper blade that gets dislodged from place and penetrates the return belt.
This type of damage is impossible to prevent, but with the use of a RIP/TEAR detector the damage can be reduce reasonably.
- Ugly holes in the belt starting at the tail section by the tail pulley and carries onthe Troughing belt.
This type of damage is caused by a foreign object such a small piece of angle iron,
That travels on the return belt and enters between the return belt and the tail pulley
Penetrates the belt and falls back on the return belt and repeats the damage.
Although there is a plow before the tail pulley that is suppose the prevent such
Damages, after a period of time these plow is not there any more.
Foreign object detector mounted before the tail pulley can detect and stop the
Conveyor and reduce the damage reasonably.
- Belt top cover is pilling off due to hard minerals hammering the top cover which is made soft rubber.
Water start to enter inside the carcass as well as other dirt and very soon the top cover start to swell up.
The TEAR detector can detect this problem and stop the conveyor thus reducing the damage.
6. TROUGHING & RETURN BELT MISALIGNMENT
Misalignment of the Troughing belt can be listed as follows:
- Excessive amount of water on the belt due to rain etc..
- Dislodgment or seizing of idler due to poor maintenance
- Unequal loading of material on the belt
- Incorrect splicing of the belt
- Mounting of idlers in wrong angle on curve section of the conveyor
However, all the above reasons can not cause damage to the belt accept to spillage of material.
Therefore, it is recommended to mount one misalignment detector at the tail immediately after the chute in order to verify that the belt left the loading area correctly, and to mount one misalignment detector at the head in order to insure that the belt entrance the discharge pulley correctly.
When a conveyor has a curve in its formation it is important to mount one misalignment detector before the curve and one after the curve in order to confirm that the belt enter the curve correctly and left the curve correctly.
Misalignment of the return belt is much more complicated and it is vulnerable to countless problems as the list will show:
- Dislodgment or seizing of any snub pulley
- Loose take-up trolley
- Dislodgment or seizing of return idler
- Wear of pulley lagging
- Dislodgment of cleaning equipment that may put pressure on the belt
- Dislodgment of tail pulley
Misalignment of the return belt has much more sever consequences, it damages the belt
(tear in the side wing of the belt), it causes to the belt edge to chip off while touching the structure legs and it may in some cases even cut into the legs.
Furthermore, one must remember that misalignment of the return belt will increase exponentially on the Troughing side for the simple reason, once the belt enter the chute in a misalignment condition the material loaded on the belt will increase the misalignment.
It is recommended to mount one detector on the take-up trolley connected via a festoon system, one detector after the last snub pulley on the return side and one detector before the tail pulley in order to insure that the belt rides on the tail pulley correctly.
On curved conveyor it is recommended to install one detector before the curve and one after the curve to verify that the return belt enters the curve correctly and lefts the curve correctly.
7. ISOLATION FOR LIGHTNING PROTECTION
Lightning forms a severe problem for any safety and protection system mounted on the conveyor, there is no protection for a direct strike of lightning and for an indirect strike there is a limited and not very reliable protection available.
One of the solutions to provide a form of protection is isolation e.g. isolation from the conveyor structure (in the form of galvanic isolation) as well as communicating with the PLC in the form of potential free contacts.
The PLC by its nature is earthed on one side and lightning can hit the PLC via any of the components that it monitors in the field.
Any intelligent system mounted on a conveyor should and must be of a floating type and never earthed.
8. CONTROL & MONITORING OF SAFETY AND PROTECTION EQUIPMENT MOUNTED ON THE CONVEYOR IN ORDER TO MINIMIZE DOWN TIME & INCREASE AVAILABILITY OF THE CONVEYOR.
The prime objective of the end user is to provide the conveyor with maximum reasonable protection and as much as possible safety for the people that have to look after this expansive
Capital Investment.
When taking into account the complete net-work of protection and safety mounted on the conveyor it requires the involvement of different maintenance authorities to look after the complete system such as: fitters, electricians, instrumentations technicians, belt people, operators and so on.
It becomes extremely important that the system will be able to identify each and every piece of equipment that is incorporated in the system, its location and to have the facility for self fault diagnostics in order to minimize down time and to increase the availability of the conveyor.
Furthermore, it is recommended to incorporate in the system all other pieces of equipment that can cause the conveyor to stop, such as padlock-able emergency switches which are mounted next to each drive, fluid coupling fusible-plugs, metal detectors, magnet separators,
belt turnover detectors, high-high bin level detectors block-chute detectors etc..
Furthermore, the system must be capable to transmit all data to the central control room in any mode preferred by the end user.
9. GENERAL COMMENTS
It is the end user responsibility to inform the OEM what IP rating is required for the equipment to be supplied, in what range of temperature the equipment is to operate,
Furthermore, when metal brackets are required the end user must specify if it is mild steel or stainless steel and what grade like 304 or 316 etc. |
