In dry climates, dust emissions from cement become even more problematic. The fine particles are easily picked up by wind or stirred up by any sort of impact; cement being dropped or crushed is likely to emit a cloud of cement dust. For this reason, the best way to transfer it is through continuous dry bulk material handling, rather than a series of stops and starts. Neither bucket-chain unloaders or grabs are able to offer this, or environmental protection from dust emissions, and pneumatic systems are also limited.
Pneumatic unloaders have often been thought a good option for cement handling, and it is true that in the right conditions they are capable of high throughput; but, when materials become compacted, these machines lack the digging forces necessary to handle them and efficiency hugely drops off. They also require significant input from payloaders in the hold.
Bruks Siwertell defines unloading efficiency by comparing the actual through-ship unloading capacity against the rated capacity. While most alternative unloading systems offer efficiencies of between 50% and 60%, a Siwertell screw-type ship-unloader delivers efficiencies of 70% or more.
The main reason for this higher overall efficiency, compared with grab cranes for example, is that at the beginning of an unloading operation, the grab only travels a very short distance to reach the cargo. However, as more bulk material is unloaded, the greater the distance the grab has to travel, into and out of the hold. Therefore, the efficiency progressively decreases.
In contrast, a Siwertell screw type unloader maintains continuous unloading at a steady discharge rate, regardless of the level of cargo in the hold, right up to the hold clean up stage. It also picks up material below the cargo surface, avoiding any hold avalanches, with a counter-rotating inlet device. Further adding to its efficiency, a Siwertell unloader can reach right into the corners of a hold, offering an additional advantage over pneumatic unloaders and other systems. This results in quicker vessel turnaround and reduced berth occupancy.
SUPPORTING MARKET GROWTH
The first screw-type Siwertell ship-unloader was supplied to the cement industry in 1975. In 1980, Bruks Siwertell set a new standard for the industry by introducing the 800tph Siwertell unloader to the market and since then, it has continuously set an extremely high bar. Today, Siwertell cement unloaders offer continuous rated capacities in excess of 2,000tph.
Bruks Siwertell has long-standing references for cement handling in the US. In 2004, Houston Cement Company, in Houston, Texas, was beginning to plan a new, larger terminal. The company was expanding and looking for an efficient, environmentally friendly, high-capacity method to discharge increasing volumes. It could sell more cement than it could import, and needed to replace an existing pneumatic ship-unloading system.
In 2006, a rail-mounted Siwertell ST 640-D unloader, designed to offer cement unloading rates of up to 1,500tph, was delivered to Houston Cement. Once commissioned, performance tests revealed impressive results: the ship-unloader met the guaranteed rated capacity and actually exceeded it to deliver 1,646tph. The through-ship capacity was also in excess of the guaranteed rate of 1,050tph and energy consumption tests, achieving 0.39kWh/h, were surpassed as well.
OPTIMUM EFFICIENCY LEVELS
The unloader was also one of the first to feature an auto level mode, now standard. It ensures that the vertical conveyor’s counter-rotating inlet feeder, which effectively forces material into the screw, is kept at an optimum level. By the feeder not digging too deep, or too shallow, unloading efficiency is kept at an optimum level.
Optimum efficiency at the Houston Cement terminal is also ensured by the jetty belt conveyors being longer than the vessels the terminal receives. This allows the ship-unloader to move past the bow and stern of the vessel and for the ship unloader’s conveying arm to pendulate at an angle into the furthest corners of the vessel, particularly in the forward hold, under the deck and hatches.
The entire system is enclosed from the vessel to the jetty conveyors, via a moveable transfer trolley, and through the terminal’s 100,000 metric tonne capacity storage silos.
The unloading rates offered by Siwertell technology at Houston Cement set a new world record, and were so far ahead of the market that the unloader remains extremely competitive today. At the time, it was also the largest Siwertell unloader dedicated to cement.
The unloader has now handled more than 10 million metric tonnes of cement. Its high-capacity capabilities meant a reduction in unloading days of 50%, translating into a 50% reduction in berth occupancy and the possibility of higher annual intakes; return on investment was achieved in less than two years.
A GOOD REPUTATION
The Houston Cement installation has gone on to be heavily influential in Bruks Siwertell securing other cement handling contracts in the US. In 2019, the company delivered a high-capacity Siwertell ST 640-M screw-type ship-unloader to South Texas Cement’s terminal in the US port of Corpus Christi, after being contracted by cement handler GCCM Holdings LCC. It, too, is capable of 1,500tph throughput.
The specifications for the choice of unloader were stringent; aside from the expansion in throughput, GCCM Holdings LCC and South Texas Cement also discussed through-life maintenance costs and electrical demand, both of which were deemed to be lower for a screw-type unloader than a pneumatic system of equivalent throughput.
“When combining all of the deciding factors that led to choosing a mechanical unloader, it was apparent that Siwertell was the best fit for our needs,” said a GCCM Holdings and South Texas Cement spokesperson at the time of the order. “We especially like the high rate of unloading combined with the versatility to handle various ship sizes.”
Last year Bruks Siwertell was awarded a contract to supply the US-based Colonial Group with a new high-capacity Siwertell 490 F-type ship-unloader. The fully enclosed system will deliver dust-free cement handling for the company’s Savannah, Georgia, terminal and supports Colonial’s cement import growth. The unloader is planned for commissioning later this year and will offer a rated cement handling capacity of 800tph, discharging vessels up to Panamax size.
NEW OPPORTUNITIES AHEAD
Not all imports will be handled at dedicated terminals, however, and the complex dynamics of the construction industry leave some operators seeking a more flexible approach. “We have seen an influx of interest in Siwertell road-mobile unloaders, specific to the cement industry, as cement demand continues to rise,” says Upchurch.
Siwertell road-mobile unloader orders were secured in the US last year and this trend is continuing. Most recently, Bruks Siwertell secured a new Siwertell road-mobile ship-unloader order for cement handling operations in the Gulf of Mexico region of the USA. Its new, undisclosed owners already operate numerous Siwertell screw-type ship-unloaders, recognizing their ability to protect the environment from dust and spillage, and to offer a cost-effective dry bulk handling solution, with low operating costs in comparison with other technologies.
“During his election campaign, President Joe Biden pledged to ‘build back better’, and recent reports suggest that there may be as much as US$1 trillion on the table for roads, bridges, rail, and other infrastructure,” notes Upchurch.
“This is very encouraging news for the US, and clearly, it will need a lot of cement to make these plans a reality. Whatever strategy importers adopt for meeting this growth, our US cement handling deliveries, and installations across the world, show that Bruks Siwertell will be with them every step of the way.”
Dry Cargo International