CONVEYORS /ELEVATORS SAFETY DEVICES AN OVERVIEW

BY RAM BAPAT, CHAIRMAN, JAYASHREE GROUP OF COMPANIES

Bulk Material Handling System is the lifeline of all the modern process industries such as fertilizer, steel, cement, mining, power stations and port facilities. The sizes and complexities of these plants have increased enormously during the last decade and the bulk material handling systems have to effectively cope up with the same. As a result, the numbers & lengths of belt conveyors have increased many folds and keeping a vigil on the functioning of the same has assumed critical importance.

Bulk material handling systems are generally semi-outdoor or outdoor installations extending over several square kilometers and continuous manual supervision of the same is next to impossible.

Safety Switches are therefore installed to detect malfunctioning of conveyors and to initiate appropriate warning and control actions to avoid injuries to operating personnel as well as severe damage to the conveyors and consequent loss of production of the plant.

The family of modern Belt Monitoring Switches includes a) Under speed / Zero speed switches (also known as Belt slip switches) b) Pull Cord Switches c) Belt Sway Switches d) De-Interlocking switches. In addition, some installations may call for Heavy Duty Limit switches as well as Belt Loading Monitors to provide additional operating facilities.

All these switches must have certain features in common considering their location in outdoor aggressive environment involving dust, moisture, varying temperatures and even rains.

Besides, these switches, being basically safety devices, are called upon to operate after prolonged intervals of idle period as and when a fault develops or an emergency arises. Therefore, the design should be such that the switches remain in "ready to operate" condition throughout several years of service life.

These features include:

Enclosure: Cast aluminium or Cast Iron enclosures are extensively used to house the switch mechanisms. Also when the bulk materials being conveyed are chemically active i.e. corrosive or alkaline or acidic in nature, it is recommended that enclosures are made from appropriate Industrial Polymer, that resists the particular environment.

Conformity to IP65 grade of protection by providing properly designed long lasting gaskets and seals for the covers, shaft entries, is essential to maintain the switch in good working order.

Covers : The covers should be designed to overlap the housing to improve protection against rain/water sprays. The fixing screws for covers are exposed to aggressive environment and are prone to get rusted and jammed in position in a very short time unless they are adequately protected by suitable surface coating or made from materials such as stainless steel. Further, due to the widely spread out nature of installation and with usually under-trained erection and maintenance staff, is commonly observed that whenever the covers are opened during initial installation or subsequently for inspection, etc, they are left loosely fitted due to missing or damaged screws.. This occurrence is invariably noticed during installation of switches on the inclined conveyors where the workman/fitter is extremely unwilling to walk down the conveyor, pick up the dropped screw and climb back to the spot for continuing the job. As a result desired grade of IP protection is impossible. The effective remedy against this is to ensure that the fixing screws remain captive in covers by adopting special designs of screws or providing circlips etc

Cable Termination Boxes: Providing adequate and satisfactory termination arrangement for control cables is very important feature which is, many times, neglected due to the increased initial cost. The termination of control cables is then done on the body, which incorporates the operating mechanism, allowing little space for secured and convenient joining and looping, resulting in entry of water. This coupled with an indifferent and/or untrained maintenance staff, will seriously hamper the degree of safety provided by the switches.

The recommended practice is to provide built in terminal box of adequate dimensions incorporating suitable studs, which can accept the control cables for termination through compression type cable glands for connecting and looping. In fact it can be very easily seen that in the absence of such a terminal box, the contractor has to provide a junction box in the vicinity of each switch for termination and looping of the cable from switch to switch. Providing such a junction box at the site and the additional prices of cable and glands etc, is much more expensive than the cost of built-in terminal box.

Painting: The enclosures need to be painted with utmost care and with strict adherence to the recommended painting procedures to ensure maximum adhesion and dense structure to withstand the aggressive environment. Special paint and procedures need to be adopted if site conditions involve presence of corrosive vapours, dust etc.

While the external hardware is continuously subjected to the aggressive environment, the hardware inside the enclosure also gets affected by the prevailing weather due to breathing action. Hence the internal components need to be electroplated or blackodised or painted depending upon their nature and individual functions.

While the above features are common for the entire family of the Belt Monitoring Switches, the individual safety switches have certain features peculiar to themselves for meeting their specific functional requirements.

They can be described in brief as follows:

PULL CORD SWITCH

This is a very important safety device, which enables the supervisory personnel in the vicinity of the belt to initiate a warning signal and/or to stop the concerned belt if any malfunctioning is noticed. As per the usual practice, the Pull Cord switches are installed on one or both sides of the conveyor, depending upon the provision for walking platform. The switches are installed at intervals of 20-25 meters in between and a pull wire rope is run along the length of the belt at a convenient height and connected to the operating handles of the individual Pull cord Switches so that the rope can be pulled from any point along the length of the conveyor.

In the light of the above, the mechanism of Pull Cord Switch should be such as to allow easy pulling by an average individual during an emergency. The pulling operation has also to overcome the frictional resistance offered by the pull rope, which is normally supported on guide hooks spaced at regular intervals of about 1-2 meters. Taking into consideration these factors the Pull Cord Switch should be capable of getting actuated by a pulling force around 2.5/3.5 kgs. The mechanism should also ensure that the switch contacts do not get open momentarily by partial pulling of the rope issuing stopping/warning signal and then reset as soon as the rope is released. This will be a dangerous situation since the remote control room operator may restart the belt without bothering to investigate reasons at site. The mechanism should ensure toggling snap action whereby the contacts actuate with a snap action and the switch mechanism gets locked in the operated position until and unless it is reset by local manual action.

All the Pull Cord Switches installed along an individual belt are electrically wired in series and connected to the control station by a two core cable. Therefore actuation of any one of these Pull Cord Switches will stop the concerned conveyor until the particular switch is manually reset. Some times the Pull Cord Switches are actuated by falling lumps of material being transported or by mischievous unauthorized individuals who may be loitering unnoticed in the area. Some means are therefore to be provided to identify the particular switch that has been operated to facilitate resetting and restarting the belt. A simple arrangement is to provide a mechanical flag with fluorescent painted head on the handle of the Pull Cord Switch, which can be seen by the maintenance person from a distance of 10-15 meters. A more sophisticated arrangement is to provide an indicating lamp which simplifies identification from a long distance, particularly during the night. However, this involves additional cabling, adding significantly to the cost of the installation. Besides the lamp should have a mechanically strong transparent dome with appropriate sealing ring to prevent entry of dust and moisture.

BELT SWAY SWITCH

This is another important safety device which detects the misalignment of the belt during running.

The loaded side of the belt should normally run smoothly over the supporting roller system. However, it tends to run outside ways either due to unequal load of material or surface or bearings of the supporting rollers getting worn out or jammed. Belt Sway Switches are provided to detect such abnormal running of the belt.

Like the Pull Cord Switch , Belt Sway Switches are also installed along both sides of the belt at intervals of about 40-50 meters and are positioned in such a way that their operating handles get deflected by the edge of the swaying belt and initiate warning/stopping signals. However, unlike the Pull Cord Switch, which is operated manually, the handle of the Belt Sway Switch receives the impact of the loaded belt edge which can be as high as 500 kgs. Or more. The operating mechanism, therefore, should have a strong reset spring to absorb this impact so that the switching element actuated by a cam is protected against heavy shock and a force just adequate for actuation is exerted.

The main shaft carrying the impact and it should be normally supported on ball bearings so that it is able to rotate along with the moving belt edge and at the same time, get deflected to actuate the contacts.

Another important requirement for the Belt Sway Switch is that the design should permit over travel of the operating handle, since the extent to which the swayed belt will deflect is uncertain. The arrangement should ensure that the over travel does not impose additional mechanical force on the switch element but gets totally absorbed by the reset spring.

The loaded portion of the belt runs out in an inclined fashion due to the trough like construction of the supporting roller system. Therefore, the actuating handle of the Belt Sway Switch should be inclined with respect to the base by suitable angle (usually 60? or so) so that the swaying edge meets the roller in a square manner for clean deflecting action. While the roller as well as the main shaft are recommended to be supported on ball bearings, a sleeve type bearing arrangement may be used in some makes/models for reasons of economy. However, this bearing arrangement may gradually deteriorate in aggressive conditions as well as due to the heavy impact of the belt.

The mounting position of Belt Sway Switch is very important for correct functioning. The standard practice is, therefore, to locate the switch in a horizontal position with its inclined actuating handle facing belt edge with an intervening distance of 80 to 100mm. This allows the roller to remain free of contact with the belt edge under normal conditions permitting a sway upto 100mm on either side which is usually acceptable for

safe operation. When the sway exceeds this initial gap, then alone the belt handle comes in contact with roller and actuates the switch.

In sophisticated version, the switch incorporates two switch elements operated by two separate cams so that the first switch element is actuated when the belt sways somewhat beyond the acceptable limit and the other limit switch gets actuated only if the belt sways to a very dangerous extent and initiates stopping command.

While the Pull Cord Switch is designed for manual resetting, the Belt Sway Switch on the other hand is provided with a strong resetting spring so that the handle gets reset automatically to original position as soon as swaying belt assumes its normal position. This self resetting action enables the system operator to restart conveyor system without visiting the site for local resetting of a particular switch.

Like the Pull Cord Switches, all the Belt Sway Switch contacts are electrically connected in series and are led to the control station by a two or four core cable to provide necessary warning/stopping action.

UNDER SPEED/ ZERO SPEED SWITCH

This is a very important safety device which continuously supervises the linear speed of the belt and actuates a contact as soon as speed of the belt drops below the normal working speed by a significant amount (about 15-20%). The belt looses speed mainly due to its elongation after prolonged operation or due to overloading of material. In the worst case, the belt may just snap thereby dropping the speed to practically zero value. In either case, the actuation of the speed switch contact gives warning signal to the control station through a two or three core control cable. It is adequate to have one speed switch per Belt/Drive.

These Speed Switches are manufactured using different principles e.g. so called Centrifugal Speed Switch incorporates governor type mechanism to detect change in speed, whereas Electrodynamic Speed Switch provides the same action by means of electrodynamic torque proportional to the speed exerted by eddy current action. The Centrifugal or Electrodynamic Speed Switches need to be directly coupled to the supporting roller shaft or driven off the belt by means of a friction roller made from rubber or nylon. The setting of the speed switches for actuation at a desired value is established by trial during commissioning of the belt. The most ideal location for these speed switches is near the drive end pulley or the take up pulley on the return side of the belt where the fluctuations of the belt are the minimum, permitting smooth operation and avoiding nuisance tripping commands due to the bumpy movement of the return belt.

The recent addition to the family of speed switches is what is popularly known as Electronic non contact speed switch. This switch has two parts, the sensing probe and the

monitoring unit. The tail end pulley or rotating shaft of any of the rollers supporting the belt, is fitted with recommended numbers of metallic (preferably MS) flags and sensing probes mounted facing this arrangement with a small air gap of 5 to 8 mm. The rotation of the pulley or roller shaft makes the flags move across the sensing face of the probe at appropriate speeds and the sensing probe, usually of inductive type, generates pulses in exact proportion to the number of flags and linear speed of the belt. These pulses are fed by suitable interconnecting cable to the monitoring unit located at a convenient distance away from the sensing probe. The monitoring unit counts these pulses against time base and causes actuation of the output relay at the preset value of speed. This type of arrangement avoids the necessity of mechanical coupling permitting easy installation. It is also capable of sensing low speed conveyors operating below the sensing range of Centrifugal/Electrodynamic speed switches.

The monitoring unit can incorporate a control circuit to provide initial bypass and/or nuisance tripping delay for actuation of the output relay. This will simplify the control arrangement for central control system.

The Pull Cord Switch, Belt Sway Switch and Under Speed switch are the most important field mounted Belt Monitoring switches and the safety of the conveyor system is totally dependent on the reliability as well as proper installation and commissioning of these switches.

De-Interlocking Switch is another safety device mounted in the vicinity of the conveyor. One such switch is adequate per conveyor. The switch is a manually operated cam type safety device having a built in actuation program to allow the electrical isolation of the particular conveyor from the centrally interlocked and controlled conveyor system. It also incorporates starting and stopping push buttons so that the de-interlocked belt can be started/stopped by local manual operation of the push buttons. The provision of such a switch enables maintenance staff to attend to the defective belt in a safe manner while the material handling systems continues to operate through suitably arranged alternative conveyor routes.

Recent addition to the family of Belt Monitoring Switches is the Belt Loading Monitor. The switch has a swinging arm with a roller. The switch is mounted in such a way that the surface of the roller is below the underside surface of the loaded side of the belt with a small gap of few millimeters. Therefore during the empty running, the roller does not come in contact with the belt. As soon as the belt is loaded with material, the central portion of the belt sags and comes in contact with the roller at a predetermined level of loading. The roller wheel then gets pressed downwards and actuates an inductive proximity sensor which in turn energises a control contactor. This contactor switches on the solenoid controlling water sprinklers mounted along the side of the belt. The sprinkling of water over the loaded material prevents the pollution of the entire area by causing the fine dust to settle down in a slurry form. The water spraying system is not

activated during the empty running of the belt so that wastage of water as well as wetting of belt and causing water pools is avoided.

Heavy Duty Limit switches are also required for some material handling installations to perform the function of detecting the position of special loading mechanism which moves across the belts to load the desired belt with material.

The above describes the features of safety switches, which are used in normal outdoor or semi outdoor conditions. However switches with special enclosures and operating components are required for installations dealing in aggressive chemicals such as fertilizers, caustic soda, common salt and so on. The material and painting /coating for the enclosures as well as for other hardware are critically selected for trouble free service.

Certain material handling installations are located in explosive/ inflammable areas and a very special execution under the category "Flameproof enclosures" is required for the Belt Monitoring switches to be used in such installations. These enclosures are required to be certified by the appropriate government agency before they can be accepted for installation.

INSTALLATION:

The Belt Monitoring Switches need to be installed strictly in accordance with the manufacturer's instructions to ensure that they perform reliably and accurately under the arduous conditions. The recommended locations and positions are shown in Fig.3 which shows optimum installation positions for the Belt Sway, Pull Cord and Under Speed Switches.

Utmost care needs to be taken to ensure that the control cables are terminated carefully using the recommended compression type cable glands and appropriate sealing washers provided with the switch/gland are in place. Any lapses on this account will permit entry of dust and water along the cable and put the switch out of operation in a very short time.

Similarly, the covers of the switches /terminal boxes opened for installation/inspection should be carefully refixed by using and properly retightening the screws so that the suitable gaskets provided thereon sit in perfect position to prevent entry of dust, water etc. Countless instances have been known where negligence on the part of the workmen in leaving the covers of partially erected switch open while going off the duty, results in finding the switch full of dust/water the next day morning. Any switch even with the best of designs cannot survive such negligence.

Bad/careless installation will thus render the switches ineffective even before commissioning. Serious accidents are known to have taken place due to nonfunctioning of safety switches installed in such careless and improper manner, involving loss of life and damage to the installation and consequently loss of production.

STANDARDS

At present, there are no Indian Standards to cover the manufacture and installation of any of the Belt Monitoring Switches. Therefore each customer/consultant lays down his own specifications while preparing the tender documents. Such specifications are more often based on some past experience with a particular make or extreme theoretical considerations without due attention to the functional requirements, making it very difficult for the manufacturers to build standardized products. Some of the instances where rethinking on the part of the consultants or users is called for are mentioned below:

Some specifications call for the Belt Monitoring Switches to be capable of say 5 to 10 millions of operations, as recommended for switches used in the general engineering industry. Actually the site conditions and operating requirements for the Belt Monitoring switches are entirely different. For example, it is likely that a particular Pull Cord or Belt Sway Switch may not be operated during its entire service life of several years, whereas some may be operated somewhat more frequently, say once in a shift or so. To lay down a duty of millions of operations for these switches is therefore ridiculous. The actual requirement is that the switches should remain in "ready to operate" condition even after prolonged idle period and the mechanism should therefore be designed in an appropriate manner to meet "circuit breaker" duty rather than " contactor " duty.

" Contactor " duty involves very frequent repetitive actuations and therefore, accurate and satisfactory operation after millions of switching cycles is called for. A circuit breaker on the other hand is not an operating device requiring millions of operations but a safety device remaining in a "ready to operate" condition and operate on those very few and rare circumstances involving emergency of system mal-function. It should, in these situations, operate reliably & positively.

Another example of over specifications is the insistence on providing contact combination of 2NO + 2NC whereas in most of the installations, only 1NO + 1NC contact combinations are usually wired. The underlying theoretical consideration for providing 2NO + 2NC contacts is perhaps that one set of NO/NC contacts may malfunction and therefore a spare set of contacts is required. In actual practice, the switch mechanism itself is more likely to go bad so that whatever may be the contact combinations (1NO + 1NC or 2NO + 2NC) they will all be rendered inoperative and the concept of additional safety is not at all realized. Besides, these switches are field mounted devices and therefore locations are spread throughout the area covering several square kilometers. To interconnect switches with (1NO + 1NC) contacts, a three core control cable is adequate. For wiring additional contacts, multi core cables are required to be installed and the higher costs of such cabling may be exorbitant.

Therefore, the practice of recommending more than one set of NO/NC contacts does not justify the site requirements in terms of safety as well as reliability.

Another prevailing practice is to ask for contact ratings exceeding 10 Amp. Or in some instances15 or 20 Amps. The actual current requirements during normal operations do not exceed few milli Amps and the rating of the contacts need not exceed 6 Amps. (specified contact rating for control switches & devices as per IS 13947)

On the other hand many buyers/consultants, do not realize the importance of providing a built in terminal box and during the price negotiations. The buyer opts for the switches without terminal box which appears to be cheaper at first cost but the result is a less reliable safety device and actually higher installation costs for providing separate junction boxes and cabling.

It is therefore extremely important to evolve a uniform set of specifications to be used for belt monitoring switches, clearly defining the site conditions such as dust, humidity, presence of chemical/ explosive atmosphere and calling for standard termination arrangements to ensure the long term reliability of protection rendered by the switches to the installation.

A DESIGNS

Historically, the designs of Belt Monitoring Switches have been based on electromechanical techniques. However, with the advent of modern electronics, new trends are evolving in the designs of belt monitoring switches. This trend is reflected in preference for Electronic non contact speed switch using the Inductive proximity sensors and associated control circuitry. These switches are found to be ideal for monitoring of extremely low speeds for use in hazardous areas and also where mechanical coupling is difficult or inaccessible. The reliability and repeat accuracy of these Electronic speed switches are now well established and majority of conveyor systems are using such speed switches.

The same trend is also visible in new designs of Pull Cord & Belt Sway Switches incorporating magnetic type proximity/reed switches providing non contact actuation.

However, switches with this design do not appear to have any specific advantages over the conventional switches except perhaps for better compatibility with PLC controlled material handling systems.

B. REMOTE DISPLAY

With the increasing complexity of the Material handling systems in terms of Numbers, Lengths and operational requirements of the conveyors, a remote audio- visual display system is being increasingly used in new installations & also as a retrofit in existing installations.

The most popular arrangement being a two wire system connecting all the sensor modules located in individual Pull Cord/ Belt Sway switches to a central control/display unit which provides visible and/or audible alarm, pin pointing the particular safety switch causing the actuation.

Such a system needs to be installed by using only two wires either from the existing cabling or by a fresh two-core cable and may be extended to include other safety/operational features that are being added to modern material handling installations.