An engineering marvel
It is not just Engineering but ´Imagineering´. Transportation of limestone from Meghalaya in India to a cement plant in Bangladesh is an engineering marvel indeed. Though it was completed almost 10 years ago, the project still stands tall among the top industrial infrastructure projects. Despite facing many hurdles even after the project completion, Lafarge plant in Bangladesh continues to receive limestone from India. Pabak Mukhopadhyay of Larsen & Toubro takes us down the memory lane on how this project was commissioned and the various challenges faced.
After the recent intense earthquake in Nepal and the adjacent geographies of North Eastern parts of India and current severe rainfall in Meghalaya, Larsen & Toubro (L&T) was curious to know about the health of the conveyor system installed, which is the main feeder line to Lafarge´s cement plant in Bangladesh. L&T is extremely delighted to know from Lafarge Surma that the conveyor system-which was conceptualised, designed, constructed and commissioned by L&T-still stands tall despite the calamities, and is performing successfully.
Rich reserve of prime quality limestone in east Khasi hills of India and abundant quantity of low-cost fuel (natural gas) in Bangladesh made Lafarge Surma Cement Limited-a joint venture company of Lafarge France and Cementos Molins, Spain-to envisage an integrated cement plant at Chatak, Sylhet, a bordering town of Bangladesh. Since April 2006, the single flight 17-km long belt conveyor (LBC) built by L&T ECC Division, an engineering and construction company of India, is feeding limestone/shale to the Lafarge Surma cement plant. This engineering marvel was created on a turnkey basis with technology support of Aumund France.
It is concept to commissioning
L&T submitted the feasibility report (FSR) in December 1998 and bid against international competitors in December 2000. The contract was awarded in September 2002 and received notice to proceed in July 2003. When LBC was commissioned in December 2005, miscreants fire damaged a 1.2 km stretch. The conveyor was re-commissioned against tight timelines in April 2006 and the system is running successfully since then.
Trans-Border Conveyor: The complexities
Crossing of an international border has immigration, logistic, commercial, legal and security-related issues. Two countries are governed by different rules and regulation. However, L&T overcame these hurdles by their following innovative solutions:
- The conveyor was installed at an elevated level and without walkway. Two maintenance vehicles run over the conveyor gallery for carrying maintenance personnel, tools/tackles to attend the conveyor. Each vehicle will run from either end up to the international border and will cross with special permission in case of emergency. The gallery bottom is maintained at minimum 5.5 m height from ground level.
- Design the LBC with drive and control at both ends. Provide independent power supply of each country to the two drive station with control located at two ends. The two control system to communicate with each other for controlling the drives as well as the conveyor.
- Supply and construct portion of the conveyor in each country as two separate conveyors. (Special permission was arranged for working in international border area during survey and construction of foundation for few days).
- Erect the portion of the conveyor in each country independently and make final connectivity at border to make it a single conveyor. (Special permission was arranged for working in international border for few days while establishing the continuity of structural gallery, fibre optic cable and joining the steel cord belt).
- Commission the conveyor with data communication between the two control rooms located in two countries thro process control system. All mechanical checking being carried out from maintenance vehicle of each country.
This single flight conveyor is 17 km long, of which 7 km is in India and 10 km is in Bangladesh. The conveyor starts from the hill of Meghalaya and climbs down by 59 m in first 1.5 km and the rest of the conveyor is in plain land. It is a 35-degree trough belt conveyor, capacity 800 TPH, 800 mm wide steel cord belt, 4 m/sec speed, 10 horizontal and vertical curve, two 630 kW drive at dead-end Bangladesh, one 630 kW drive at tail-end India. Belt turn over arrangement at both end provided for better performance. The conveyor civil design takes care of the local soil condition and seismic condition for the geological fault zone. RCC trestles have been provided for the plain land area where as structural steel trestles are provided in hill area for ease of erection and better flexibility. The belt profile of the conveyor is shown below.
Special design features
Feeding arrangement to LBC: A small conveyor (sacrificial conveyor) feeds the material in the same line of the LBC travel direction. Uniform feeding to LBC is ensured by variable speed apron feeder taking material from hopper mounted with load cell. Gallery length of 24.5 m found to be the optimum solution. It was designed for 195 m/hour wind speed, two maintenance vehicle load, conveyor load of 800 tonne/hour and temperature variation of 35 degree. All galleries of hill area is with one end hinge and one end sliding support. In plain area sliding supports provided at every 100 m. The top and bottom chord of each gallery is designed to take care of expansion due to temperature variation. Colour coated roof sheeting and side sheeting provided in the gallery to protect the environment from dust and the raw material from rain water.
Idler: Idlers used are base mounted 35 degree conventional type. The bracket is designed such that in case any roll is dropped belt will not get damaged by any sharp edge of the bracket. At curve zone the wing rolls are longer and idlers are erected skewed. Design also takes care of condensation problem due to high humidity and high temperature variation of 35 degree.
Conveyor control systems: The conveyor is driven by three drive motors (630 kW each), two at discharge end and one at tail end. The speed of the conveyor is varied by VSD 6.6/0.69 kV. The conveyor is controlled by the main PCS from central control room located at cement plant in Bangladesh. The data network is through field buses (profibus) Further two PLC´s are located, one close to drive station in Bangladesh and the other close to the Tail Station in Meghalaya. The two PLC´s communicate with each other through fibre optic cable for speed synchronisation of the drives. Safety interlocks and safety switches are provided to make the system full proof. The conveyor can be stopped from either of the maintenance vehicle by pressing the emergency stop push button, which operates on a closed loop system of signal being transmitted through wired network.
Maintenance vehicle: The maintenance vehicle provided in this project is of state-of-the-art design. It can carry five persons (375 kg), and maintenance tools and tackles (200 kg). The vehicle has a variable long travel speed and can move at 5 kmph in hill area and up to 15 kmph in plain land area. Fail safe Rack and Pinion arrangement provided for the drive system for hill area only. The vehicle is powered by two diesel engines, one working and other as standby. The long travel system is through variable speed hydraulic motor with fail safe disc brake. For worst eventuality provision kept to move the vehicle by manual hydraulic pump. The vehicle is fully rain protected and ensures maintenance access below the return belt using fold down side platforms. Safety interlocks provided to disable travel of vehicle when platforms are extended. A diesel generator provided in the vehicle to supply electric power for lighting, welding, material hoisting, power tools. Following tools and attachments are also provided within the vehicle.
- Working bay on both side of the conveyor
- Belt hoisting device
- A microphone system to monitor noise difference between running idlers
- Electric hoist to lift spares/tools up to 500 kg
- Lighting of vehicle including flood light for maintenance and head light
- A safety escape ladder with fall prevention system
- Space for storage of idlers for replacement
- Storm clamp for securing the vehicle with conveyor structures
- Tool box
- Mirror to inspect the return idlers rotation
The Indian portion of the site is very close to Cherrapunji, the highest rainfall area of the world. The monsoon duration is from May to September with an average monthly rainfall of 1.5 m to 2 m and maximum rainfall in a day is around 500 mm. The natural ground level of the site in India is at higher elevation than Bangladesh resulting flow of all rain water to Bangladesh and submerging the conveyor corridor under water for four months. Thus leaving around 7-8 months time for carrying out site activity in a calendar year.
Site welding of the prefabricated gallery and final painting: The gallery was fabricated in workshop to achieve the required tolerance. Then dismantled, prime painted and dispatched in framed packed condition. It is reassembled, final welded and painted before erection at site.
Erection of two long span bridges were the toughest task: Conveyor cross places like rivers, roads, etc., where trestle cannot be put at normal interval, required long span galleries. Since maintenance vehicle moves over the conveyor gallery, bridges were required to support the conveyor gallery with extended span. There were six bridges in India and six bridges in Bangladesh. One long span bridge of 100 m long over Moula River was assembled at ground level with special arrangement and subsequently lifted the entire bridge with the help of strand jack equipment. Long span bridge (100 m) located in geological fault line and weighing about 150 mt was erected by segment launching method successfully.
Transportation of material: Distribution along the corridor for 17 km was the greatest challenge. The Indian portion of the site is a very remote area. The hilly road connectivity leaded to a transportation limitation of 9 tonne weight and 12 m length in any consignment. River route by barge is the only mode of transportation of material to Bangladesh. Non-availability of barge and slushy conveyor corridor affected the receipt of supply materials to site location.
Laying of the belt and belt jointing: The conveyor being first-of-its-kind in the world, the finalisation of the belt laying scheme, equipment selection to carry out the job with a consensus of the designer was a difficult task. Finally the ability to develop the right scheme within the time available is the main reason of the success of this project.
Race against time
The construction schedule: Notice to proceed for procurement and construction was received in July 2003 when the total Bangladesh stretch of the site was submerged with water; however L&T planned the activity such that the conveyor can be commissioned by December 2005, fulfilling customer´s requirement.
Occurrence of natural calamity: It rained unprecedented during March 04 and October 2004 causing flash flood and destroying the construction approach road, damaging construction equipment. This affected the planned programme of the job.
Implementation of catch-up plan: The effect of natural calamity and delay due to the same left almost no time for the belt laying activity. The belt laying plan was totally revised as per the prevailing site condition and belt jointing was carried out at multi-vulcanisation station without compromising the quality. A catch-up plan was drawn and monitored on a daily basis. Additional equipment mobilised to achieve extra productivity. Extra attention during erection, alignment checking of idler paid rich dividends during no load commissioning of the conveyor. Finally the conveyor was commissi¡oned two weeks before the due date. The project scheduled as planned and as carried out is shown below:
The commissioning of Lafarge LBC has established that conveying of raw material to longer distance through trough belt conveyor without environmental pollution is an economically viable solution. L&T has once again proved that they are always ahead when it is a question of Imagineering.
Pabak Mukhopadhyay, Vice President & Head, Larsen & Toubro - Bulk Material Handling, Metallurgical & Material Handling IC, Kolkata. Email: firstname.lastname@example.org