Well designed conveyors are very stout and reliable machines. In a CHP operation of a thermal power plant, ensuring 85-90 per cent availability of conveyor system is considered a good scheme of material feeding.
Operations and maintenance (O&M) are closely related functions which can assure optimum performance, dependability and cost saving for conveyors. A trouble free conveyor system is a product of three well executed stages of design - layout finalisation, preparation of conveyor geometry and profile drawings, equipment selections and detailed engineering. There are many operational problems in belt conveyors of Coal Handling Plant (CHP) whose causes and cures have their roots in system design. In this article, the focus is on the design precautions to reduce the commonly faced operational problems in a belt conveyor system of CHP.
Operational problems of a belt conveyor
Belt conveyors, when appropriately designed, manufactured, installed, inspected, commissioned, operated and maintained, shall perform continuously and properly.
Performance of a conveyor can
be judged visually as well as with parameters like current drawn by the drive as shown in the HMI in control room, vibration levels of prime mover and power transmitting elements, feedbacks from the in-built safety sensors etc. Every conveyor system has exclusive performance requirements, operating conditions, design characteristics, number of operating hours etc which are the variables deciding the type and nature of operational problems.
The most common problem encountered during the operation of a belt conveyor is belt sway/ belt running ´off-track´. The sway can happen at tail pulley, head pulley, take up pulley, at a particular zone in the length of conveyor, at a particular set of idlers and in curvature. Belt sway is a scenario in which line along the centers of idlers in the plan view is not straight. The belt will have a tendency to keep straight due to the pull, but relative to conveyor centerline, it is running misaligned. The aftermaths of these operational intricacies are stripping off of belt edges, rubbing of belt against the chute inner portion accompanied by smoke, belt folding , excessive material spillage (especially at the loading zone) and additional loading on the side rollers reducing the bearing life . Even though belt sway switches are provided at regular intervals in conveyor belts, under Indian operating conditions, they are ´forced´ or bypassed on many occasions. It has been shown that, belt centering can be time consuming during the initial phase of operation and multi-month efforts are not unusual.
The phenomenon of belt accumulating behind the drive pulley due to excessive slack is known as belt slippage. The starting phase of a conveyor operation is more vulnerable to belt slip. Belt slip, if not properly taken care in the preliminary stage of operation, leads to high wear of belt, material over loading on the downstream conveyors, undue current consumption of prime mover and excessive loading on the pulley bearings . Zero speed switch- which is normally installed either on the snub pulley or tail pulley of the conveyor-is used to alert the monitoring personnel in control room regarding belt slip.
The belt is an elastic element which withstands the tension in the entire length of conveyor and when continuously subjected to stresses beyond the elastic limit, causes permanent elongation. Excessive belt stretch happens in the conveyor system after some period of operation depending on the rating selected, manufacturing quality and loading on the conveyor. When the take up stroke reaches close to the length available for travel, the belt requires shortening.
The basic strength of a conveyor belt is imparted by carcass which resists all the tensions generated in the belt during the operation by providing stiffness longitudinally as well as laterally. Too often the O&M team experiences the tribulations with belt plies separation, especially in applications where highly abrasive materials are carried by the conveyor. Wear and tear of belt is an un-avoidable hazard which every material handling system experiences. However a sound design can ensure that wear and tear happens after a longer term of operation and does not call for a break down maintenance. Maintenance personnel should remember that for textile fabric belt, even with fire resistant grade top and bottom cover, can have a fabric which can catch fire rapidly. This is the same reason because of which experienced maintenance personnels do not use cut pieces of textile fabric belts as cleaning elements for primary belt scrappers instead of PU blocks.
Efficiency of a material handling system is the ratio of the material fed in the bins/bunkers/ stockyards to the material fed into the conveyor via unloading systems. Invariably, material spillage will happen when using belt conveyor. Apart from lowering the efficiency of the conveying system, this results in stacking of material in the walkways, thereby making the house keeping cumbersome. Further this spillage loads generate additional stresses in the structural members which may not have been considered in the design. Maximum amount of spillage and dust emission is observed in the loading point through the gap between the skirt sealing rubber and belt. Clogged chutes, longitudinal cracks on the cover (top as well as bottom) of belt, belt hardening and subsequent cracks , belt snap and carcass cracks at factory joints are other commonly experienced outfitted hitches in bulk material handling system.
Design considerations to reduce operational problems
Designing a belt conveyor system shall be an attempt to synchronize between two desirable but incompatible features viz reducing operational problems and the effect on total cost of ownership. Stages of design including that of deciding the conveyor geometry, selection of drives, choice of conveyor accessories like idlers and pulleys have effects on the behavior of belt during movement. Few vital points, often deserted by designers which contribute to the operational difficulties are discussed here.
Untimely wear and inadvertent damages may result from loading improper grade and over sized material on to the belt. The foreign materials like spikes, metal scrap pieces, liner plate strips, rocks and stone pieces, timber and wooden materials etc when entrained into the feed can cause expensive shut downs and costly repairs. The client/ owner of the plant should realize that extrinsic factors like moisture content, percentage of over sized particles etc have exhaustive influence on the conveying system.
The conveyor belt represents a high proportion of the total CHP system cost. Hence an under-designed belt often proves out to be a grave error as the replacement induces heavy commercial implications and use of the same forces the customer to operate at a lower load thereby adversely affecting the production. Apart from adhering to the guidelines and stipulations given in CEMA design book for conveyors and IS:11592 standard, the conveyor designer should have a ´feel´ of the zone of application while selecting the belt. For instance, a conservative selection of belt rating for yard conveyors often turns out to be a wise decision as many plant operators report frequent belt elongation and failures while operating them even at a lower TPH (tons per hour feed).
Minimum required belt rating of a conveyor is selected on the basis of maximum running tension, starting tension and the percentage of margin over the maximum running tension as mutually agreed between the client and the contractor. However, a thorough cross check of the maximum permissible stress of the belt with that of the stress due to impact at feed points is also a necessity in the design stage. When the height of fall of material is more, it is preferable to explore methods to reduce the impact energy rather than over designing the belt. The belt conveyor designer is often confronted with the problem of accurately determining the starting belt tension in this regard. The acceleration coefficient, which is based on the type of drive is a crucial factor while working out the starting belt tension. Longitudinal belt cuts, failure at factory joints, plies separation at an early stage of operation are few of the operation problems encountered due to the under rated belt.
For a floating type belt take-up system -horizontal gravity take up unit (HGTU) and vertical gravity take up unit (VGTU)- proper calculation of take up weights for tensioning has a significant impact on the performance of belt in motion. A VGTU with less counter weights than required causes majority of the abnormal behaviors like belt sway at tail pulley, belt sway at take up pulley, belt slippage etc. On the other extreme, more counter weights lead to excessive belt stretch and thereby permanent elongation and development of higher tensile stresses. Typically for a VGTU of a conventional conveyor, gravity take up tension (Tgtu) in lbs is calculated by the following formula:
Tgtu= T2- (0.015 x Lgtu x Wb) + Hgtu x Wb - 2 x Tp
T2= Slack side tension in lbs
Lgtu= Length of belt from head pulley to gravity take up centre in ft
Wb= Unit weight of belt in lbs/ft
Hgtu= Vertical distance from the head pulley centre to gravity take up centre in ft
Tp= Belt tension to rotate the two bend pulleys in lbs
The above equation clearly puts across the fact that a conveyor designer should be careful in working out the slack-side tension of the belt inorder to guarantee that take up system ensures the compensation of overall changes in belt length without causing any redundant stress in the belt. The wrap factor for the belt should be selected on a conservative basis while calculating the slackside tension. The allowance required for splice joints, elastic properties of the belt, thermal stress factors etc should be taken into account while determining the take up travel length.
Apart from accurately estimating the counter weights, minor arrangements like providing guide rollers to arrest the belt sway while the belt is wrapping over the take up pulley, can be beneficial during operation. In the pulley shaft design , the system engineer should be vigilant about the probable belt sway out of the pulley face (which ideally is not desirable, but practically happens). Even though IS: 8531 (design standard for the pulleys) does not emphasize any conditions on the arm length of a pulley, the designer should select the value in such a way that, the belt gets enough space between the pulley face edge and the member on which plummer block is mounted. It is observed on many occasions that, the belt folds in the take up pulley after ´climbing´ over the take up pulley frame while moving due to the insufficient space for belt play. Pulley diameter is selected on the basis of type of belt, belt rating and percentage of utilization of RMBT value of the belt. Under-sized pulleys often cause vulcanized splice separation, belt plies separation, belt hardening and cracking.
Self aligning idlers are provided in conveyor belt for training the conveyor to its centre and absorb the sway. In carrying side, it is advisable to provide the self aligning carrying idlers at a pitch of 15-20m and on the return at a spacing of 25-30m. Due to layout requirements and constraints, customarily there will be changes in angle of inclination at various points in a conveyor profile. Selection of adequate radius of curvature at transition points leads to reliable performance and avoids everlasting operational problems. Applying the simple theory of elastic bending to the belt, the conveyor curvature causes stretching of belt in the outer radius and contraction in the inner radius. While calculating the radius of curvature, the designer should caution that the induced tensile stresses in the outermost filament of the belt do not create any permanent elongation and compressive stresses generated in the inner most fibre do not cause net effect zero or negative tension which tends the belt to shift towards inner radius causing lateral belt folding. These two conditions are ´conflicting interests´ and out of the two, one dictates the suitable radius.
Particularly in yard conveyors and tripper conveyors, substantial sway happens at no load and partial load especially after long term of run. The most severe case under consideration for minimum radius of concave curvature is to avoid the belt lift off from idlers . Belt over stress at centre and lack of tension at edge which leads to wayward movement are the other factors which are to be accounted for suitable concave radius selection. In many practical cases, because of layout constraints, conveyor designers are forced to accept the probable belt lift off conditions (like in bunker tripper conveyor and stacker tripper conveyor ) at no load where they provide either belt hold down pulleys or hold down wheels or stub idlers. The height of installation of these hold down devices should be prudently selected, so as to avoid material interference with them during feeding. Convex radius portion of the conveyor is rarely prone to sway and material spillage problems because of their consistent contact with the rollers. However, the pitch of the idlers should be half of the normal idler spacing as the belt with feed exerts more normal force on the rollers due to the centripetal force.
Estimation of transition length (distance from flat to trough portion of belt and trough to flat) which is wide off the mark, is a major cause of belt sway at head end and tail end of the conveyor. Transition length is a function of belt width, troughing angle, belt modulus and RMBT (Recommended maximum belt tension) value. The level difference between the central roller and pulley should be selected in such a way that interference with skirtboard is avoided.
Pattern of material entering into the belt via feeding chutes need not be inline with the belt travel direction and its speed may not be matching with belt speeds. A conveyor designer should always investigate the probable effects of material loading pattern on the belt through the chutes. Offset feeding is perhaps the most common and evident cause of belt going off track. The off- centre feeding of material on to the belt results in a difference in the line of traction force generated by the drive and conveying resistance During the commissioning period, site engineers fix deflector plates at the feeding point to ensure central feeding. However during PG tests as well as operations, problems occur due to these deflector plates as the flow area gets constricted due to deflector plates. Even though experience shows that deflector plates are ´unavoidable´, a careful study of chute valley angles during the detailed engineering can reduce the requirement of these plates. For instance, a common location where deflector plates are often called for is the transfer point in which incoming and outgoing conveyors are orthogonal. Use of striker plates or stone boxes at strategic locations inside the chute is a clever method of obtaining central feed in such cases. The correct analysis of material trajectory outline and suitable positioning of the face of the chute are also pre-requisites for ensuring lesser chute clogging and central feeding.
The consequences of improper analysis of coasting time results in piling up at transfer points and excessive fugitive dust emission. Brakes are normally provided for conveyors whose deceleration time is less than that of the conveyor feeding on to it. In many cases, the brakes are provided wherever the coasting time is more than 5 seconds. However in many plants, the brakes are removed due to problems like unbalancing of brake drum, erroneous actuation of thruster, improper movement of members of the linkage of the brake etc which happen very frequently.
Excessive wear and tear of belt cover and longitudinal crack breeding of the conveyors are often the implication of improper functioning of belt cleaners. External belt cleaners (primary and secondary) and internal scrappers (plow or diagonal) are commonly used belt cleaning devices in CHP these days. Tension resulting from belt pull required for belt scrappers is around 5 lbs per inch of length of contact between belt and cleaning device. In the general arrangement drawing of scrappers, the position of primary and secondary scrappers should be decided in conjunction with the wrap of belt over the discharge pulley in order to obtain maximum scrapping efficiency. The system engineer should remember that the position of secondary scrapper is very critical, as the tungsten carbide tip may slice the belt when the contact pressure is improved. In recent times, high speed side of conveyor HT drive is provided with scoop type fluid coupling inorder to avoid frequent tripping of motor and provide smooth start for the conveyor. The ´scoop-in´ engagement time of conveyor has a vital role to play in the probability of conveyor fed to get jammed. Dual speed motors are often provided for the scoop actuator drive, so as to obtain variable speed during scoop in and scoop out. The field operators and commissioning engineers should understand that scoop couplings are effective tools for gradual speed build up and reduction of conveyors. Hence the belt centering and tuning during initial trials can be suitably carried out with this methodology. Application is extremely helpful for the trial run of reversible yard conveyors which are prone to persistent belt sway.
Automatic means for measuring performance and controlling functions such as weighing (belt weigh scale), removing tramp iron particles (suspended magnets and inline magnetic separators), detection of non-ferrous particles (metal detectors) are employed in CHP aiming at increasing the life of belt conveyor. Experience shows that many of the plant owners do not operate these accessories due to interlock failures and frequent stoppage of the system under wrong feedbacks. Interestingly, many of these malfunctioning issues are results of incorrect positioning of equipments and failure to foresee some basic site issues.
Commonly observed operational problems in a belt conveyor system can be avoided by accounting for crucial design aspects and with a watchful and effective maintenance schedule. Even a jammed idler can prove catastrophic for the belt when its edge wears out and turns out to be a spike. Hence it is obvious that even the bearing life calculation of rollers and the shell thickness calculation considering the material weight as a uniformly distributed load over the length of the roller are critical to avoid operational difficulties at site.
Few of the fundamental facets which are to be taken care in this regard are:
i) Belt selection suitable for the application considering the maximum running tension and starting tension
ii)Suitable selection of counterweights and take up tension for VGTU and HGTU
iii)Apposite analysis of conveyor curvature and providing suitable hold down devices in cases of compromise.
iv)Appropriate chute design taking into consideration the material properties and repose angle
v)Apt selection of brakes and coasting time calculation
vi)Proper design and positioning of belt cleaning devices
vii) Determining the scoop actuator speed inline with the requirement
viii) Correct sizing of pulley diameter and shaft length
Author: Jishnu V, Senior Engineer-Projects,BHEL-ISG