Selecting the right ship unloading system
Feb. 3, 2014
by Vincent van der Wijk
For efficient unloading of grain and other free flowing bulk goods, several continuous ship unloading systems (CSU) such as a vertical chain, pneumatic and belt conveyer are employed worldwide. Each of these systems is founded on sophisticated, powerful technology and can be the perfect installation for a specific area of application. An often used rule of thumb is that pneumatic unloaders are an option as long as the unloading requirement is maximum 600 tph and does not exceed roughly 300,000 tonnes per year.
All continuous ship unloaders serve the same basic purpose: to unload grain or other bulk goods as efficiently as possible. To make an informed and well-founded decision on which unloader is the best solution for your terminal, basic issues have to be considered and evaluated, such as total costs, energy consumption, availability, safety, etc.
The Technical University of Munich has made a comparable study of three CSU (shown in Figure 1), which Bühler has prepared to show the bottom line implications of the selecting a different CSU.
Key investment criteria are the operational expenditures (OPEX) in combination with the capital expenditure (CAPEX). For an optimal customer solution, these criteria should be in balance. The Bühler system featuring a highly efficient vertical chain conveyer has the lowest OPEX value of the available CSU. With lower overall efficiency and relatively high replacement costs, the belt and pneumatic conveying systems have a higher OPEX value and are therefore less favorable unloading options.
With the monetary advantages of mechanical systems being explained, let’s take a look at the differences in detail, starting with the principle function of the unloaders followed by the consequential energy consumption. The two main influences on efficiency, active working range and rest unloading are followed by unloader operation and maintenance costs. Lastly, product loss is discussed.
Bühler’s Portalink mechanical unloader transports bulk material to the pier at low speed using two independent high-performance chain conveyors, one in the vertical conveying arm, the other in the boom. With a standardized design based on years of experience, the Portalink is optimally configured (Fig. 2, page 109).
Advantages of this design are limited wearing parts, low conveying velocity, high dependability, low energy costs, and reduced maintenance requirements. Low wear also ensures a constant conveying capacity with no losses in performance over the entire lifecycle. Thanks to the minimal maintenance requirements, the amount of downtime of the entire installation decreases while system availability increases. Mechanical unloaders can save ports tens of thousands of euros each year.
Pneumatic systems transport bulk material to the pier at high product speed using telescopable spouts, airlocks, filter systems and the suction nozzle. These additional parts require more maintenance and thus also lead to higher maintenance costs. These parts must be designed to be particularly resistant to wear in order to withstand great forces. The frequency at which parts must be replaced is naturally greater increasing the downtime of the installation and reduces the availability and overall efficiency of the unloader and terminal. The high conveying velocity increase unexpected failures of the installation and increases the berthing time of ships, leading to higher berthing fees.
High energy requirements have a negative impact on installed power supply of an installation — and on costs for the provision of energy. The necessary investment costs, as well as increasing energy prices are often overlooked in comparing the overall costs of a system. In this regard, mechanical systems are clearly superior to pneumatic systems.
The working range and flexibility of the unloader is important. With a larger effective working range, the unloader can unload a broader area with an increase in the overall efficiency. The following comparison between pneumatic and mechanical systems explains the differences between the two working principles and elaborates on the practical implications.
The mechanical Bühler Portalink uses the so-called “kick-in/kick-out system” to move the unloader arm through the hatch to unload with optimal coverage. In this process, the conveying arm moves through the bulk material in a curve. In this manner, it reaches the bulk material below the rim of the cargo hold without requiring repositioning of the ship (with stationary systems) or the unloader (with mobile systems). This flexibility significantly speeds up the unloading process and minimizes the need for bulldozers for removal of bulk material residues.
Moreover, with the Bühler Portalink, operating staff only occasionally has to intervene in operation. The conveying arm automatically sinks into the bulk material and discharges large amounts of cargo at a constant throughput level before repositioning becomes necessary.
Two persistent myths exist regarding pneumatic systems. The first myth is that pneumatic unloaders have a greater reach in the hatch than mechanical systems. The second myth is that with pneumatic unloading, the material left in the hatch after the main unloading phase is removed more efficiently and without the deployment of bulldozers. Pneumatic solutions are often compared with the vacuum cleaners used at home. In theory, this sounds logical, but nothing is further from the truth. Pneumatic unloaders can only adjust the position of the conveying arm vertically or horizontally, there is no reach under the rim of the cargo hold, requiring early use of bulldozers increasing unloading times.
Pneumatic systems do not have an automatic “sink-in” function requiring operators to constantly manually adjust the position of the conveying arm depending on the height of the bulk material and reposition the suction nozzle accordingly.
Many modern CSU`s with mechanical technology are equipped with a winch integrated in the boom for moving large bulldozers into the cargo hold for highly efficient rest unloading. Depending on the capacity of the Portalink, a high load-bearing capacity up to 15 tonnes is offered. This enables the use of a single heavy bucket loader, which is sufficient to remove all cargo residues. The final remaining product is conveniently placed in the bulldozer’s bucket and lifted onto the pier.
Pneumatic unloaders have a lighter construction and are equipped with maximum load-bearing winches of 3.5 tonnes. When winches capable of bearing heavier loads are preferred, larger steel constructions are required, leading to increased costs compared to mechanical systems. Often, several smaller bulldozers or bucket loaders are moved into the hold to remove the remaining bulk material. Increased unloading time and berthing time of the ships results in an increase in costs. The basic advantage pneumatic systems offer, for example, being able to remove literally every kernel of grain from the hold, is countered by their (physically determined) low residue unloading capacity.
Returning to the customer profile and the specific difference of the two systems, a time reduction of at least 25% using a mechanical unloader is achievable attaining an overall efficiency of 87% compared to only an overall efficiency of 66% attained by pneumatic systems.
Mechanical solutions unload bulk material at a constant low velocity. This not only reduces wear and the maintenance costs of the system, but also ensures that grain is unloaded in an exceedingly gentle manner. Damage to the product is minimized and financial losses caused by high reject rates are avoided. The overall high product quality leads to higher margins than with pneumatic systems — a financial aspect that shouldn’t be underestimated in light of increasing grain prices.
The physical properties of pneumatic solutions require the conveying speed to be higher than that of mechanical systems. The resulting higher maintenance costs are not the only disadvantage. Critical disadvantages also include higher reject rates and lower product quality, which in turn leads to lower market prices. Additionally, greater system wear results in reduced unloading throughput.
For efficient unloading of grain and other free flowing bulk goods, several CSU are available. Each system is a good alternative that performs best in specific areas of application. With each of these technologies serving a purpose, the specific area of application and volume of bulk material are the deciding factors.
Pneumatic unloaders are strong when lower capacity and low annual throughput are key factors and when a short return on investment is more important than efficiency and power consumption. Mechanical unloaders like the Bühler Portalink are strong where high unloading capacity and higher annual throughputs are the case. This solution offers low energy consumption, less wear, low maintenance and efficient unloading resulting in short berthing times.
Increasing in energy prices and personnel costs make mechanical unloaders attractive alternatives to pneumatic systems at lower volumes. Pneumatic systems continue to demonstrate advantages with low material volumes, through higher level of flexibility.
Bühler has many years of experience in both technologies and offers methods and calculation tools to assist clients in deciding on the right technology for their individual application.