Design, Installation and Maintenance of Conveyor Skirting Arrangements at Transfer Points

Transcription

1 Design, Installation and Maintenance of Conveyor Skirting Arrangements at Transfer Points The following document outlines the preferred design of skirting systems at transfer points, the installation standards for same and tips on the maintenance such that performance is optimised and there is limited consequential damage to the conveyor belting through material entrapment. The purpose of skirting systems needs to be understood. Historically they comprise of two (2) elements, a liner plate (sometimes termed hungry board) that is used to centralise the bulk of the material being transferred onto the centre of the receiving belt and a rubber skirt system that is used to minimise spillage and assist in the control of any dust emissions. The liner plate is usually constructed of wear resistant materials and is bolted, slotted or spot welded into place. It is a consumable item and its design and set up should reflect the abrasiveness and size of the material being transferred. The skirting rubber assembly and design is usually reflective of the ore being handled and its abrasiveness. It is usually 12mm thick rubber and it is important to select a rubber quality that is inferior to the rubber quality of the conveyor belt if the skirting system is a contact type skirt arrangement. The reason is that it is most important that the skirt rubber wear preferentially compared to the belt. It is also important to understand the geometry of the transfer system. Dust and spillage through the skirt boards is generated in a transfer by two methodologies: Most airborne dust in a transfer is created by induced airflow across the face of the material being transferred. Airflow is induced by the inertial drag of the material as it flows into the transfer plus air displacement within the transfer by the material being transferred displacing air at the impact point. This induced airflow creates a high pressure zone at the impact point that must be dissipated and the logical point of venting is the skirting system as it represents the shortest distance to the lower (atmospheric) air pressure zone. A secondary but significant additional source is the mechanical pressure generated on the seals owing to the turbulence of the material as its direction is converted from vertical velocity into horizontal velocity in the direction of the belt at the impact zone on the load point. This is a significant issue when using rock box type transfer chutes or simple bash plate type transfers as this creates significant material turbulence. The most common approach to dust control is to fully enclose the transfer in an air tight enclosure with various forms of flexible dust seals. If dust that is being generated through induced airflow is constrained by an enclosure then the air pressure within the transfer will be higher than the atmospheric pressure. This means the excess air, which is dust laden, must escape. If the transfer is fully enclosed it will escape under the skirt rubbers regardless of how they are set up. This is one reason why fully enclosed systems cannot contain dust though the skirt arrangements, instead they can be: 1

2 Vented at the rear of the transfer ideally as part of dust/air collection system such as a baghouse Vented at the front of the transfer through a long enclosure where hopefully a percentage of the dust will settle before the air is vented out the end Be part of a transfer system where the air pressure is equalised and any induced air flow stilled through recycling the air within the transfer. Such systems require a very carefully designed transfer chute system to create such an outcome. The interior of the transfer can be misted or fogged with fine water droplets such that the dust is settled and any air escaping is relatively dust free. Understanding this is important so that skirting systems are not being asked to do the impossible or worse are being set up in a manner that has consequential hazards for the conveyor system. Setting dust aside the ideal skirting arrangement will: Be self cleaning, that is the gaps; spaces set up will limit the likelihood of material being entrapped. Be easy to maintain and adjust It is also important to eliminate mechanical pressure on the skirts that is generated by uncontrolled turbulence of the ore at the point of impact with the receiving belt. In this latter case overloading a conveyor belt or very poorly controlled material flow through a transfer cannot be corrected by the use of a skirt system. In order to minimise maintenance and improve efficiency of the skirting system such things as: Vertical and reverse loading of inclined belts must be avoided as it causes excessive material boil. Use of some form of forward deflector is essential otherwise material will spill out of the rear of the transfer chute causing consequential issues plus conveyor belt wear will be much higher than normal. High direct impact onto a conveyor that causes material to bounce, deflects idlers etc and therefore changes the sealing arrangements set up for the conveyor belt must be avoided. Design Considerations a. Single load points Set up liner plates ideally so they centralise the material load over the centre 2/3rds of the conveyor belt. It is easier to seal conveyors carrying material that is very dusty if 45 degree trough sets are used in the loading area however if this practice is used ensuring the transitions into and out of the transfer are correct plus ensuring loading is not too centralised are important considerations. Internal width 2/3 BW Example 1200mm belt Width is 800mm mm Rubber Seal mm mm

3 Ideally the liner plates and parallel rubber skirts should be set slightly outwards and upwards after the load or impact point so the friction from the belt can drag away entrapped material easily that is, the design is self cleaning (see drawing). b. Multiple load points As above, except the liners and skirts must remain parallel to the belt edge and the distance between the skirt system and the belt remains constant. In such instances the liners should be set a little further apart centralising the load over 75% so there is less pressure forcing material under the liners. Caution, you need to maintain 75mm clearance between the outside of the skirt rubber and the belt edge to cover belt tracking/belt movement problems and still ensure the belt is positively sealed. If the belt is less than 1000mm wide this may not be possible. In such instances adjust accordingly as ensuring there is always a positive seal is paramount. 3

4 c. Skirt designs Some drawings are attached. The fundamental logic is a system that is easy to adjust and maintain. Gap Setting At load zone At skirt exit Ore size greater than 30mm 25 mm 40mm Ore size between 6mm & 30mm 10 mm 25mm Ore size less than 6mm 10 mm 25mm Internal width 2/3 BW Wear Plate Set Gap for material size Rubber Seal Space for fines min Wear PL gap + 10mm Rubber Seal Wear plate gap max 2 mm Load Zone Taper out 2/3 BW + 50mm 2/3 BW 4

5 Installation Considerations Liner height above belt. o o o ore > 30mm set liner 25mm above belt ore >6mm <30mm set liner 10mm above belt ore <6mm set liner 10mm above belt The logic is to minimise entrapment under a hard liner but to accept that due to the liners position so central to the belt, material pressure will cause some material to flow under the liner (see drawings). Skirt adjustment above belt. Unless designed specifically for a dustless transfer, skirt rubber should not contact the belt but show daylight and up to a maximum gap of 3 mm above the belt. Use 12mm rubber (this is a minimum for transfers that rely on the skirt boards to contain and redirect the flow after vertical impact in the load zone impact) and ideally chamfer the edge to 45 degrees. If the skirt rubber has a square edge, set leading edge so it just touches the conveyor belt. Rubber should be selected so it has much lower physical properties than the rubber used to construct the conveyor belt. Under no circumstances 5

6 should high tensile rubber, urethane or old conveyor belt be used. Skirt to liner distance The distance between the liner and the skirt rubber should always be set so that the distance between them was 5-10mm wider than the clearance under the liner plate. This is so any material getting under the liner plate is unlikely to be physically trapped between the liner plate and the skirt. Dimensions should be inside dimensions (see drawings). Maintenance Considerations a. Liner Remember liners can damage belts so changing them out prematurely is better than too late. Bolted liners are not recommended if the bolts can be subject to impact. Also iron ore can fuse to mild steel bolts also rending them impossible to remove. In such circumstances bolts may need to be cut out. Liners are therefore best installed on some slotting system or by tack welding. Establishing life cycles for the liners assists in inspections as wear assessment requires getting into a confined space. b. Skirt Rubber If the contact pressure with the belt is not consistent wear is sometimes uneven causing the need for constant adjustment in local spots. This then affects the seals performance as the high spots will prevent close adjustment for the whole length. This is usually a result of poor initial installation tolerances of the idlers or belt sag with load. 6

7 . Any adjustment system needs to be secure; robust and simple. Make it easy by ensuring the belt is relatively flat and stable through the loading zone. Install the load zone in position on the conveyor where the trough has been fully developed, normally a transition distance of 2 to 3 times belt width after the tail pulley. Control sag between idlers by a combination of belt tension and idler spacing, maximum sag between idlers should be limited to a maximum of 2 mm at the edges. See typical design cases in the table below for various load and idler spacings. Idler space Belt sag ` Typical Design Conditions Load Idler centre Allowable Sag Belt mass Belt speed Belt load Min belt tension tph Distance mm mm % kg/m run m/sec kg/m run kn % % % % % %

8 Related Issues: In setting up a skirting rubber system that works there are a number related issues that impact on the systems performance. Impact onto the conveyor belt. This causes the belt/impact idlers to deflect changing the gap distances under the liners and thus leading to material entrapment under the liners or between the liners and the skirts that are bigger than the design allowed for. It is also a source of spillage as material flow is un-controlled in such circumstances. Conveyor Belt Tracking. If the belt is not tracking reasonably centrally under the transfer point the belt wander, can in extreme cases, render the skirting system ineffective. This in turns leads to spillage, belt damage etc. Access to impact idlers. Poor idler access can lead to damaged idlers not being changed out in a timely manner and this in turn can impact on belt tracking. Transition set at tail of transfer. If the trough of the belt is not fully developed or the transition is too short then the belt may not sit correctly under the load point therefore changing the set up of the skirts relative to the belt. This in turn can see the belt damaged by contact with the skirting system. Load Zone Taper out 2/3 BW + 50mm 2/3 BW Start Skirtboards after transition Full depth trough developed Set gap at exit to suit material: 40mm for material size over 30mm 10mm for material size under 30mm Min skirtboard length 2 BW Transition length L Lmin = 1.8 BW for 35 Degree Lmin = 2.4 BW for 45 Degree Set entry end gap: 25mm for material over 30mm 10mm for material under 30mm 8

9 All the above contingencies need to be assessed when setting up a skirting system. There are a number of an alternative solutions and options that need to be considered. They include the design of the transfer chute but this is dealt with separately. Access to impact idlers and maintaining appropriate clearances can be address in some instance by use of slider beds. The overall key is not losing focus that impact and material entrapment will damage a conveyor belt. Spillage and dust should not be controlled at the expense of the belt. 9