September 03, 2015

03/09/2015: Ship unloading systems: Mechanical Vs Pneumatic?

by Vincent van der Wijk, Buhler 

First published in Milling and Grain, May 2015 

For efficient unloading of grain and other bulk goods, both mechanical ship unloading systems like the Bühler Portalink and Portalino and pneumatic ship unloaders like the Bühler Portanova are employed worldwide. Each of these system types is founded on sophisticated, powerful technology and is the perfect instrument for specific areas of application. Mechanical and pneumatic ship unloaders serve the same basic purpose: to unload grain or other bulk goods as efficiently as possible. But each system possesses specific advantages that make it the preferable choice for certain areas of application.
Figure 1: The simple design of mechanical unloaders reduces wear and maintenance costs
An often-used rule of thumb is that pneumatic unloaders are an excellent choice for lower unloading capacities up to 600 t/h (some experts even set the limit at just 400 t/h) and lower annual shipping volumes, while mechanical unloaders show their strengths at higher capacities. The comparison that follows is intended to illustrate the most important characteristics, advantages and disadvantages of each of these technologies. Grab systems, which are also commonly used, are not included in this analysis.

Functional principles and technical configuration
Mechanical: The Portalink/Portalino mechanical ship unloading systems transport bulk material to the pier at low speed via a high-performance chain conveyor. This is carried out using two independently run chain conveyors, one in the vertical conveying arm, the other in the boom. The boom and tower designs are standardised and can thus be manufactured economically.

The advantages of this simple design principle with few wearing parts and low conveying velocity are high dependability and low maintenance requirements. The low wear also ensures a constant conveying capacity with no losses in performance over the entire life cycle. Thanks to the minimal maintenance requirement, the amount of downtime of the entire installation decreases while system availability increases.

Pneumatic: The design of pneumatic ship unloading systems requires the inclusion of several components such as telescopable spouts, airlocks, filter systems and the suction nozzle. These parts require more maintenance and thus also lead to higher maintenance costs. The high conveying velocity of the bulk material also contributes significantly to the high level of wear – unexpected failures of the installation are more probable with this system.

Additionally, many of the parts must be designed to be particularly resistant to wear in order to withstand great forces – another cost factor to consider. The frequency at which parts must be replaced is naturally greater, which increases the amount of downtime of the installation and reduces the availability of the system. The higher amount of wear also automatically causes a loss in capacity that reduces the overall efficiency of the unloader. The berthing times of the ships can also increase due to sudden stoppages in operation, leading to higher berthing fees. These disadvantages are less critical at lower conveying capacities, since wearing parts do not need to be replaced as often and maintenance requirements are lower as a result.

Unloading process
Mechanical: Mechanical unloading systems like the Bühler Portalink/Portalino use a so-called ‘kick-in/kick-out system’ for unloading bulk materials. 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 minimises the need for bulldozers for removal of bulk material residues.

Pneumatic: Pneumatic solutions can only adjust the position of the conveying arm vertically or horizontally. This makes more frequent repositioning of the ship or unloading system necessary, which in turn reduces unloading velocity. The bulk material below the rim of the cargo hold cannot be reached, requiring bulldozers to be used earlier and leading to longer unloading times.
Unloading of bulk material residues
Mechanical: As a rule, bulldozers and bucket loaders are used for the removal of bulk material residues from the cargo hold. Many modern unloading systems with mechanical technology such as the Bühler Portalink and Portalino are equipped with a winch integrated in the boom for moving the bulldozer into the cargo hold.

Depending on the capacity of the unloading system, the winch – e.g. in installations with a throughput of 1300 t/h – can offer a high load-bearing capacity of 15 tons. In this case, the use of a single heavy bucket loader is sufficient to remove all cargo residues. However, the bulk material residues must first be placed into the bucket of the loader so that it can then be lifted out of the hold.
By comparison, pneumatic unloading systems come equipped with winches having a more limited load-bearing capacity of no more than 3.5 tons. Winches capable of bearing higher loads are technically feasible; however, they require larger steel constructions which lead to much higher costs as compared to standard systems. Thus, several smaller bulldozers or bucket loaders must often be moved into the hold in order to remove the remaining bulk material.
This takes time and increases unloading time and, in turn, the berthing time of the ships, resulting in an increase in costs. The basic advantage pneumatic systems offer, i.e. being able to remove literally every grain from the hold, is countered by their (physically determined) low residue unloading capacity.

The following simulation based on a comparison of unloading systems with 600 t/h throughput and a Panamax ship shows just how greatly the use of heavy bulldozers or bucket loaders can impact the efficiency of the entire unloading process. While the unloading time of the simulated ship comes to only 99 hours using the mechanical Portalink unloading system, with a pneumatic unloader the complete process takes a total of 131 hours.
This enormous difference comes about not only through the use of heavy bucket loaders to unload the material residues, but also through the difference in throughput: While the Portalink unloading system continues to unload at constant full capacity as the level of bulk material sinks, with the pneumatic unloading system, the throughput decreases by at least 10 percent as the bulk material level decreases and the conduit pipes are extended fully. The overall efficiency of the mechanical Portalink system is 87 percent, whereas with the pneumatic system efficiency is only 66 percent. Here again, it should be pointed out that at lower conveying capacities, the efficiency disadvantages of pneumatic unloading systems illustrated here can be significantly lower.
Mechanical: With the mechanical Bühler Portalink and Portalino unloading systems, operating staff only occasionally have 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. The static construction of mechanical unloading systems does however have disadvantages whenever there are strong variations in the water level at the pier. In this case, the length of the conveying arm cannot simply be flexibly adjusted to the water level, which means additional repositioning is required.
Pneumatic: Without the benefit of the automatic ‘sink-in’ function, operators of pneumatic unloading systems have to manually adjust the position of the conveying arm depending on the height of the bulk material and reposition the suction nozzle accordingly. On the other hand, the possibility of varying the length of the telescopable spout allows the system to be employed flexibly where water level varies. The disadvantage of this flexibility: As the length of the telescopable spout increases, throughput decreases. In addition, pneumatic systems prove superior with regard to handling, e.g. when two different products are transported in the same hold and are separated only by sheeting or flooring or when tankers are employed as bulk carriers.

Product protection
Mechanical: Mechanical solutions like the Bühler Portalink/ Portalino unload the bulk material at a constant low velocity. This not only reduces wear and the maintenance costs of the system, but also ensures that especially sensitive grain is unloaded in an exceedingly gentle manner. In this way, damage to the product is minimised and financial losses caused by high reject rates are avoided. The overall high product quality leads to higher margins than with pneumatic unloading systems – a financial aspect that should not 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.

Energy consumption
Thanks to a simple design with few components and a lower conveying speed, the energy consumption of mechanical unloading systems like the Bühler Portalink or Portalino is approximately 0.35 – 0.4 kWh per ton. By comparison, the energy consumption of pneumatic systems is roughly 0.85 – 0.9 kWh/t; older systems even require more than 1 kWh/t. Depending on conveying capacity and local energy prices, mechanical unloading systems can save the user tens of thousands of Euros each year.

In the light of the continuing trend toward higher energy prices, it stands to reason that in the future, mechanical unloading systems may also become more attractive for lower capacities – above all in countries where energy is expensive.

High levels of specific energy consumption also have a negative impact on the overall power supply of an installation – and on costs for the provision of energy. The necessary investment costs as well as the annual increases in electricity rates are often overlooked in calculating the overall costs of a system. In this regard, mechanical unloading systems like the Bühler Portalink or Portalino are clearly superior to pneumatic solutions.
Noise pollution
Mechanical: To say that mechanical unloading systems like the Bühler Portalink or Portalino operate quietly would certainly be an exaggeration. Nevertheless, with the exception of necessary safety signals, their constant noise level means they can only be heard in the immediate vicinity. In this way, the system adheres  strictly to environmental and occupational safety guidelines and minimises the impact of noise on workers.

Pneumatic: In contrast to mechanical systems, the air suction blower (roots type) employed in pneumatic unloading systems produces an unpleasant noise frequency that is often perceived as an annoyance, particularly at long distances. While multi-stage fans produce less noise as a whole, at further distances, they result in noise pollution similar to that of roots-type blowers.
Mechanical: Mechanical unloading systems such as the Bühler Portalink and Portalino are capable of handling throughputs up to 1300 tons per hour and beyond. This means even larger amounts of bulk material can be unloaded with just a few unloading systems. At an average of 0.08 €, the specific maintenance costs per ton of unloaded raw material are also low (replacement parts and personnel). Because of their design however, mechanical unloading systems are heavier and must be built larger than pneumatic solutions due to their geometry. This requires greater investments in the pier installations.
Pneumatic: By contrast, pneumatic unloading systems are only capable of handling a maximum of 600 t/h throughput (with one suction nozzle). Larger conveying amounts require the employment of several unloading systems, which in turn drives up investment costs. At approximately 0.14 € per ton of unloaded raw material, the specific maintenance costs are significantly higher. On the other hand, costs for the pier installations are lower than that of mechanical unloading systems.

The advantages of mechanical ship unloading systems like the Bühler Portalink/Portalino lie in the areas of energy consumption, low wear and low maintenance and efficient unloading with short berthing times, in particular at high throughputs starting at 300 t/h or higher volumes of approximately 300,000 – 500,000 tons annually. Continuing increases in energy prices could also make mechanical systems an attractive alternative to pneumatic systems at lower volumes in the future.

Pneumatic systems continue to demonstrate advantages with low material volumes, through their high level of flexibility where water levels vary at the pier, when different products are transported in the same cargo hold or when tankers have been converted into bulk carriers.

In short: Each of these technologies serves a purpose. The specific area of application and volume of bulk material are the deciding factors. Bühler has long experience in both technologies and offers methods and calculation tools to assist any client in his or her decision for the right technology for his or her individual application.
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