PNEUMATIC UNLOADERS RESPOND TO GREATER CARGO DEMANDS
Increasing demand for grain storage solutions and silos with larger storage capacities is translating into bigger demand for pneumatic unloaders, as Dave Macintyre discovers
Increased capacity and speed are the main reasons why larger-sized equipment is required for unloading increasingly-larger vessels. At the same time, minimising dust emissions, grain spillages and noise pollution are important considerations in the design of the unloaders.
Progress in design is occurring rapidly, giving ports and terminals increased choice for handling free-flowing agribulk cargoes such as grains and meals, chemicals, wood pellets and animal feed pellets.
For other cargoes, both mechanical and pneumatic unloaders can be fitted with optional devices to break the cargo before it is lifted, such as a cutting nozzle or screw attachment.
Mechanical unloaders may have the edge in terms of speed, perhaps 200 tph (tonnes per hour) up to 1200 tph, compared to 200 tph up to 800 tph per pipe for pneumatic unloaders, but the pneumatic types have versatility – they can work on most quays due to light loadings, can be moved between quays and handle ship-to-barge transfers etc.
With a lifetime of a ship unloader being about 25-35 years, it is critical the customer makes the right choice of equipment. General Manager of Vigan, Luc Sallets, confirms progress in pneumatic technology has been so great that ten pneumatics are now sold for every mechanical unit.
“It is true for Vigan and I think the whole market is the same - for grain and agribulk at least. It is different for cement, iron ore, coal,” he states.
Summarising the respective strength of pneumatic and mechanical unloaders, Mr Sallets says the three most critically-important factors are efficiency, power consumption and investment cost. Pneumatic unloaders have a higher ratio between average capacity at the peak or nominal capacity (how manufacturers describe the equipment).
Nicolas Dechamps, Managing Director of Vigan explains further. “The main reason for such higher efficiency is that the pneumatic unloader is sucking everything up from the bottom of (the) ship. No grain remains in the suction nozzle. The result is that the total time to discharge the vessel ‘berth in – berth out’ is shorter with a pneumatic unloader compared to the same situation with a mechanical unloader of similar capacity.”
He further explains that for any type of mechanical unloader (chain, screw or double-belt type, or even grab cranes), it is necessary to have the bottom of the vertical leg and its feeder buried in at least one metre of product to keep a high unloading capacity. “The last layer of cargo in the bottom of the hatch needs much more time to be discharged with a mechanical unloader, making its efficiency lower than the pneumatic unloader.”
Mr Dechamps adds that a recent study undertaken in Hamburg on the discharge of a vessel with a combination of mechanical and pneumatic unloaders resulted in the mechanical unloader stopping because it was not efficient during the final clean-up.
In terms of power consumption, he says mechanical unloaders (chain, screw, double-belt) have a lower nominal energy consumption at full capacity ranging from 0.3 kWh/t to 0.8 kWh/t.
Pneumatic unloaders have managed to reduce their consumption to lows of about 0.7 kWh/t due to reducing the number of elbows in their pneumatic lines, using a multi-stage turbo blower instead of an old-fashioned roots-type blower and the use of inverters for the drive of the turbo blower unit (allowing power consumption to be regulated automatically according to power required).
However, the higher efficiency of the pneumatic unloader outweighs its higher electric consumption by a factor of 10 in terms of costs. In terms of investment cost, Mr Dechamps says the pneumatic unloader is significantly cheaper to buy. “In addition to the face value, other factors to be taken into consideration in investment costs are the extra cost of quay construction or quay refurbishment due to the higher weight of a mechanical unloader compared to a pneumatic unloader.
Transport costs are extremely high as mechanical unloaders are usually made in Far East countries and transported on expensive heavy-lift vessels, with all downstream equipment (conveyors, elevators) having to be sized according to the nominal capacity of the unloaders.”
He adds that financial analysis confirms that the pneumatic unloader remains globally the most economical solution and that the extra small power consumption is largely compensated by lower investment costs and its higher efficiency.”
Pneumatic unloaders also appeal due to easier maintenance (a pneumatic unloader is an enclosed machine room with pumps and suction pipes and barely any moving parts),
machine weight (less need for new quay construction or refurbishment) and simple controls meaning quicker training for operators
In terms of costs of operation, Mr Dechamps says that based on 0.1US$/kWh, the difference in power consumption between pneumatic and mechanical is +/- 0.2 kWh/t, which represents approximately 0.02 US$/t or as little as US$1200 USD in total for a Panamax size ships.
However, thanks to higher efficiency, a pneumatic unloader of 600tph will unload a Panamax vessel faster than a mechanical of 600tph. Saving one day on average for the
discharge of each vessel means a saving of one day of demurrage (currently around US$20,000) on each vessel.
Tomas Kisslinger, Managing Director of German pneumatic ship unloading specialist Neuero, says there are several reasons pneumatic unloaders are the best choice, including easy and safe operation, key options like automatic lowering of the suction nozzle as the ship is being unloaded, overall efficient unloading including cleaning the ship’s hold, low dust and noise emissions, low-cost maintenance, and the flexibility to adapt to different working conditions than were originally planned.
He gives two examples of this Summer where the Rhine River water levels were lower than normal. Since the ship was sitting lower than originally planned it was easy to simply add more hollow pipe extensions for the pneumatic unloader.
The cost to do this was extremely low. “Additionally, the extensions are easy to remove when the water levels return to normal. For a pneumatic unloader to make these changes is a matter of minutes, not days like it would be for mechanical unloader to make the depth change.”
The second example is where only one hold is to be completely unloaded from start to clean-up. Normally when a ship arrives several holds are partially unloaded in phases to
lower the stress on the ship’s structure. As this is being done the ship is sitting higher in the water. Mr Kisslinger states that “if only one hold is being unloaded from start to finish then the ship is still sitting lower in the water and extensions can be added to the pneumatic unloader to reach the bottom of the ship’s hold for the final cleaning operation. Again, these extensions are added in a matter of minutes.”
In terms of unloading speeds, Mr Kisslinger says the Neuero turbo blowers can operate at variable speeds to automatically adjust to different material requirements. The only time a pneumatic unloader may not look as favourable to the client, he says, involves operational costs, although these are offset by efficiency gains, before explaining further.
“For energy consumption the grab takes the least, followed by the chain. The screw and pneumatic unloaders have similar energy consumption rates. However, the stated energy consumption is normally given at peak production under the optimum operating situation.”
Mr Kisslinger admits that average unloading efficiency needs to be considered (the time it takes to completely unload a ship’s hold). “Grabs operate at 50% efficiency, followed by a chain at 65%, followed by a screw at 70%, and the pneumatic unloader operating at 75% efficiency.
The reason for the grab, chain, and screw having lower efficiency rates is due to the design of these unloaders a front-end loader is required to feed these unloaders when there is still a significant amount of material left in the ship’s hold,” he states, before adding, “A pneumatic unloader operates by suction and does not require a minimum product height to be able to enter the system.”
He agrees that a straight purchase is the normal way to go compared to hiring, because of transport and assembly costs, but adds that it could be possible to make a lease agreement with the client based on a per ton basis with a minimum tonnage agreed to in advance.
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