Today a majority of one million tonne integrated cement plants have wagon unloading system. It has become a necessity considering the quantum of materials handled at a regular frequency. It is commonly known as wagon tippler.
The material can be received at a plant by two different types of wagons - BOXN wagons and BOBRN wagons. Generally the material will be discharged from the top in BOXN wagons, while in the case of BOBRN wagons, it will be discharged from the bottom.
The hopper will be provided below the ground for receiving the unloaded material from these wagons.
Hence, BOXN wagons need to be tilted for unloading the material into the hopper, while the BOBRN wagons are provided with pneumatically operated gates at the bottom for unloading the material. A majority of our plants have been using BOXN wagons. In most of the cement plants, coal is received and handled through wagon tippler system.
Wagon tippling system
The wagon tippling system consists of a wagon tippler, the wagon positioning equipment, underground hopper, and feeder below the hopper for evacuating the material unloaded into the hopper. The wagon tippler consists of a table for positioning the wagon, wagon holding mechanism, gears and pinions for rotation, drive unit, hydraulic power pack, etc.
The unloading cycle starts when the wagon is positioned over the wagon tippler table and the wagon along with the table rotates and discharges the material into the underground hopper. The time taken for the unloading operation is about 90 seconds. There are two types of wagon tipplers: rota side that rotates about 135-degree, and another rotary type that rotates 180 degree. The rota side wagon tipplers are provided in most of the plants in India. The drive for the rotation will be hydraulic type for smoother operation with the rota side wagon tippler.
Most of the suppliers of the system are claiming 24 tips per hour with the rota side on tippler. However in reality, the plants are getting maximum 18-20 tips per hour. However there is an improvement in the total load handled per tip which has gone to 150 tonne from 110 tonne. In all the entire railway reck is unloaded in about four hours' time.
Wagon positioning equipment
There are different types of wagon positioning equipment like hydraulically operated side arm charger, beetle charger and shunting locos. The hydraulically operated side arm chargers are being used in most of the plants in India as this equipment is much faster compared to the others. The tractive force of the side arm charger shall be suitable for hauling one fully loaded rake.
The hopper provided below the wagon tippler could be either reinforced cement concrete (RCC) type or structural steel fabricated type. In most of the plants, this will be of RCC construction. Suitable liner would be provided for this hopper depending on the abrasiveness of the material handled. Generally steel grids of 250 mm square will be provided above the hopper to avoid higher size of material going through. The higher size material will be removed and broken separately and then passed through the grid. The grid will be sloping outwards for easy removal of such larger size material.
Feeder blow the hopper
The feeder below the hopper could be either vibrating type feeder or apron feeder. The apron feeder would be more suitable for heavy duty application for taking the impact of the falling material. The apron feeder will be driven by hydraulic motor for smoother operation.
Dust control system
Plain water spray type dust suppression system will be provided for suppressing the dust generated during the unloading operation. Spray nozzles will be provided at the top of the wagon tippler and also around the hopper for spraying the water and settling the dust. An enclosed shed will be provided for the wagon tippler so that the dust will be contained within and will not be spread to the other parts of the plant.
A control room will be provided adjacent to the wagon tippler at an elevated position for operation and control of the wagon tippling system. The complete view of the unloading system will be available from this control room. Generally the time taken for unloading the rake would be about 4 hours with one wagon tippler in operation.
Unloading system track hopper: (BOBRN wagons)
The BOBRN wagons will be discharging the material through the discharge gates provided at the bottom of the wagons. This type of unloading system consists of track hopper below the rail track, mechanism for opening the pneumatic gates of the wagons and feeders below the track hopper. The loaded rake will be brought to the track hopper by the main line locomotive and the same locomotive will stay connected to the rake while the unloading operation is in process and the rake will be hauled out of the plant. It is not necessary to disconnect the loco from the rake unlike in the case of wagon tippler system.
Track Hopper: (BOBRN wagons)
The track hopper will be provided below the rail track and will be suitable for holding the material unloaded from the BOBRN wagons. The length of the track hopper adopted in most of the Indian plants is about 200 m and the capacity of this hopper will be suitable for storing one rake material. With this, it would be possible to operate the wagon unloading system independent of the rest of the plant and the rake need not wait in case any of the downstream equipment is not available. There are a few plants with shorter length of track hopper. It would be possible to adopt track hopper of reduced length as the reliability of the conveying equipment is higher. The track hopper will be of RCC construction and suitable liner will be provided depending on the abrasiveness of the material. Steel grid of about 250 mm square will be provided over the track hopper.
Discharge gate opening mechanism: (BOBRN wagons)
The BOBRN wagons have pneumatically operated gates at the bottom of the wagon. These need to be opened for discharging the material from the wagons. The wagons will have hopper like construction with slope at the bottom portion for effective clearance of the material. The pneumatic gates could be opened by energising the solenoids provided on the wagons. The gates could be closed by de-energizing the solenoids. The opening and closing of the gates could be carried out either manually or with the help of line-side equipment. In case of manual operation, about 15 to 20 wagons will be parked on the track hopper for unloading the material. Once this batch of wagons is unloaded, the next batch of wagons will be pushed forward and parked on the track hopper for unloading operation. This process will be repeated till the completion of unloading operation. In case of line-side equipment, the unloading operation could be done while the rake is in motion.
The compressors required for supplying the compressed air could be either mounted on the locomotive used for hauling the rake or could be in a separate room near the track hopper.
Feeder below the track hopper: (BOBRN wagons)
The paddle feeders will be provided below the track hopper for evacuating the material from the hopper. The paddle feeder will be mounted on the table provided below the hopper from where material will be evacuated and fed on the belt conveyor below.
The layout of the rail tracks shall be such that the track will be straight and horizontal for one rake length on the inhaul side and also on the outhaul side. In case it is not possible to have straight length to accommodate one full rake on either side, at least about 250 m on either sides of the track shall be made straight and the balance could be on a smooth curve. It would be preferable to provide Merry Go Round (MGR) system of rail track so that the rake can enter the plant in one direction and go out of the plant without any need of disconnecting the loco from the rake. The Dust Control system and facilities like Central Control system will be same described earlier.
Article by H Ramakrishna of Tata Consulting Engineers Ltd, Bengaluru.