ship unloader

Extract from Bulk Materials International, May-June 2004 issue
For the entire paper with original pictures, click here

Print this page

Despite far more " energy hungry " than mechanical systems, pneumatic handling plant continues to offer a more flexible solution for low throughput applications

While the debate continues over whether mechanical or pneumatic systems are the most cost effective means of discharging free-flowing bulk commodities, such as agri-bulks, it is interesting to note that some advocates of mechanical discharge systems, such as BMH-Siwertell and Bühler, have, over the past few years, introduced pneumatic unloading systems to complement their mechanical range.

However, specialists in pneumatic unloading, such as Vigan and Neuero, have not crossed over to the mechanical alternative. One unnamed supplier of pneumatic systems compared it to the difference between using a vacuum cleaner and a dustpan and brush, although possibly a Dyson cleaner might be more appropriate as this employs cyclonic separation technology.

Pneumatic conveying through vacuum pipelines has been in use since the late 1800s and although refined, it still operates on the same principle. At the same time, continuous mechanical systems, such as screws, chain convey- ors or double belts and even bucket-elevators, have also been employed since the early 1900s as a mean of increasing discharge efficiency over grab-type solutions.

Grab type discharge still has its supporters, however, to the chagrin of both mechanical and pneumatic unloader suppliers, both of which promote a continuous and en- closed process.

Subic Grain, for instance, has signed a letter of intent with Vigan for two 500 tph gantry type rail mounted pneumatic unloaders but has not ruled out the option of using grab-type unloaders instead. However, it still plans to employ two or three rubber tyred pneumatic unloaders that could handle 1000 tons per day to provide greater flexibility and be able to discharge directly into road vehicles.

Recognising that the energy efficiency of mechanical unloaders is better suited for high throughput ship to silo import applications, pneumatic system suppliers have focused on mobile and more flexible systems where port infrastructure may not be adequate to match cargo throughput and direct truck loading provides the better alternative.

While it is difficult to be precise, due to different installation set-ups, in general, energy consumption for wheat with a density of 0.75 is approximately 0.3 to 0.5 kW-t of cargo throughput for mechanical systems and about 0.6 to 1.0 kW-t using pneumatic unloaders, is effectively double.

In many cases, a stevedore would rather pay a higher fuel oil bill for a low throughput pneumatic system than invest in a more expensive fuel efficient mechanical counterpart, particularly in the developing world where discharging costs, but not plant investment costs, can be paid by the importer or aid agency.

Grab cranes have significantly lower energy costs, but then the advantages of a closed, dust-free environment are lost.

Contamination issue

One area where pneumatic unloaders have a clear advantage over their mechanical counterparts, according to Alain de Visscher, Commercial Manager of VIGAN, is in cross contamination. While previously this may not have been a significant aspect when handling agri-bulks, it is now becoming more crucial with the requirement to keep GM and non-GM modified products separate and also as stricter security requirements must be implemented.

Contamination can be caused by foreign materials getting into the conveying sys- tem, mixing of different products after multiple unloading operations, mainly due to product residues, and contamination of the environment by fugitive dust, or a factor of all three.

Continuous mechanical systems are more liable to be vulnerable to the first two causes, all the more so when they are not well designed over the complete feeder head to discharge outlet path. If wet grain gets into a mechanical system, for example, it can stick to the inside walls of the conveyor or accumulate in "dead" zones inside the mechanical conveyors. The design of these conveyors is such that they never empty out completely as mechanical systems are much less effective at self-cleaning than pneumatic systems due to the various interfaces where spillage can occur inside a closed environment.

Pneumatic systems have several advantages in this respect. They have no dead zones and the large volume of air circulating inside the pipes ensures efficient self-cleaning and also self-scouring of the system. Furthermore, moisture content is removed into the volume of air which transports the commodity.

Closed circuit

Both continuous mechanical and pneumatic systems are closed circuits, so there is no dust contamination along the conveying line, compared to a grab unloading mode, al- though the latter is arguably the most cost effective and flexible alternative.
Pneumatic systems that operate with negative pressure have a further advantage in that, as a large volume of air is "sucked" up during unloading, they can also reduce dust left over in the holds, which reduces the amount of dust escaping into the environment.
Appropriate filters incorporated in pneumatic systems provide zero dust emission from the suction inlet to discharge, which is why many ports, particularly in the US, will only approve pneumatic systems for the handling of very dusty products such as alumina. It is anticipated that this trend will spread to include other cargoes, such as grains.

Cleaning up…

Hold clean-up is another area where pneumatics have a distinct advantage in that there is no mechanical con- tact between the ship's hold and the discharge system, which not the case with a mechanical unloader or a grab. Pneumatic systems also offer higher overall through-ship performance, which is put at between 80 and 85 per cent.

Thus a 500 tph capacity pneumatic unloader should have a through the ship rate of around 400 tph due not only to the faster clean- up operation of the suction effect but also the greater flexibility of the discharge telescopic leg which can accommodate vertical and horizontal movements to cover all areas of the hold under the hatch coamings.

A far more limited area and cargo volume can be reached by the inlet nozzle of a mechanical system, which, combined with less efficient clean-up, can cause the through the ship efficiency of a large screw type unloader to fall to 50 to 60 per cent.

Clean-up operations of a mechanical unit can also be slowed by up/down movements caused by tidal fluctuations and wave motions or the orientation of its vertical leg through its kick-in and kick-out capability.

Many small ports in Africa, for instance, prefer pneumatic unloaders due to their sometimes exposed jet- ties and swell conditions as the marine leg can better cope with these conditions than a mechanical design.

A mechanical unloader normally requires some form of feeder device, which can be static or powered, to generate cargo flow into the marine leg and at some stage during clean- up, this will have to come in contact with the double bottom. In calm conditions with no significant tidal movements or wash from passing vessels, such as in an enclosed dock area, this presents few problems.

If the ship is moving, however, not only can damage occur to the hold, but more importantly from the stevedore's viewpoint, the feeder head can be damaged.

As the marine leg contains the mechanical system to elevate the cargo out of the hold, as compared to the pneumatic system which is simply a pipe, it cannot be telescopic and any vertical movement must be accommodated by the angle of the support jib. The response time for this, however, may not be sufficiently rapid to cope with swell conditions whereas the pneumatic marine leg can be telescopic or the lower section, especially for lower through- put applications, can be fabricated from reinforced rubber tubing to accommodate shock loading contacts.

…and saving up

VIGAN estimates that using pneumatic unloaders could save upwards of US$100,000 annually for each 1 million tons handled by improving turnaround times through greater clean- up efficiency and also the number of workers required during clean-up operations.

During the peak time of hold unloading, both mechanical and pneumatic systems have similar man- power requirements with one operator normally enough but, in practice, one assistant is also generally present. When discharging into trucks and/or railroad cars, an additional supervisor is needed for this operation.

The main difference in manpower requirements is during clean-up, where more workers and more time are needed for mechanical units. The additional costs related to this can be significant for some ports, but for many applications in developing countries, labour costs are not an is- sue and if it is a government sponsored import facility, high manpower levels are normally encouraged, but on a very low wage basis.

A similar attitude prevails to- wards maintenance, although it can be inadequate in developing countries where there may not be sufficient expertise or education.

Keep it simple

It is, therefore, essential that equipment is designed for ease of maintenance and to reduce unscheduled repairs, which increase downtime. Due to their heavy construction, mechanical unloaders are commonly considered to be low maintenance, but because of the high number of moving components, all of which are subject to wear, there is a higher potential for technical failures coupled with the risk of abrasion wear points.

These potential technical failures also account for the many safety de- vices required by mechanical systems, which can in turn lead to more downtime, stoppages and more maintenance checks.

Any repair of a mechanical system can be difficult, due to poor access and weight of the parts as many of the components are housed in the vertical marine leg. The position of the drive at the end of the boom of a mechanical sys- tem can, therefore, cause a lot of difficulties when it has to be changed.

Pneumatic unloaders require less maintenance, while all the main equipment components are located in the machinery room This provides better accessibility to the motors and the auxiliary systems allowing maintenance personnel to undertake checks and increase the probability that the maintenance programme will be carried out properly.

Easier access to the equipment components means, for instance, that it is much simpler to replace a vertical pipe than to repair the vertical arm of a mechanical system.

When choosing and defining the features of an unloader with its supplier, a customer also has to take human factors into account: machines that are too sophisticated are often not well accepted and considered too risky in many countries and/or for local harbour conditions.

Safe and sound

Both continuous mechanical and pneumatic unloaders are considered to be safe.

However, the greater number of moving parts and motors along the conveying line of a mechanical system, requires the installation of appropriate fail-safe devices with limit switches in many places.

With pneumatic systems, many motors are located in- side a machine room, so monitoring can be centralised far more easily.

In mechanical systems, there is a risk of dust explosions as any metallic piece caught between the enclosure and any moving part, such as the screw, chain or bucket, can produce sparks.

For bucket elevators, some authorities recommend an explosion vent every six cubic metres of elevator leg, with one square metre of light relief material panelling de- signed to open automatically under slight pressure.

There is also a risk of explosions with pneumatic systems as while the cargo flow does not come into contact with any moving parts, the large air volumes carrying the commodity can generate static electricity, which can cause a discharge spark. This is easily prevented through the fit ting of earthing straps at strategic locations, but these need to be checked occasionally to see that they still maintain a contact.

Sound of silence

In addition to dust pollution, there is now a growing awareness of noise pollution and while mechanical systems are considered quiet due to the low running speed of the moving parts, pneumatic discharge systems can be considerably noisier.

In the case of pneumatic equipment using fan blower type systems, proper insulation of the machinery housing can be very efficient in reducing the high frequency noise produced.

This does not apply for unloaders employing Roots type blowers, however, as the low frequency noise produced by the back flow during the rotation of the lobes is very difficult to reduce.

The Roots blower is a positive displacement design and consists of two rotors in- side a housing rotating in opposite directions. As the electric motor drives the rotors, air is trapped between them and the housing to create a suction effect at the inlet to generate the air flow to carry the cargo.

While fan type blowers are quieter, however, the Roots blower still remains the most effective.

(This point of view is definitively not shared by VIGAN and other pneumatic equipment manufacturers ' contact VIGAN for more specific explanations)

Sensitive issues

Products being unloaded must keep their characteristics during their transport and handling, particularly delicate bulk cargoes such as barley for breweries, malt, cocoa beans, rice, alumina, soda ash etc, al- though some stevedores will quite happily discharge bulk cocoa with a grab.

In mechanical systems, the contact between the product and the moving parts of the unloader, such as the screw, and along the metal walls, can cause physical degradation to the cargo.

In pneumatic systems, potential damage to cargo can be significantly reduced if the manufacturer uses the most appropriate technology and if the equipment is designed properly. For example, when handling delicate cargoes, it is important to avoid the separation of air and product by using a cyclone.

One solution developed by VIGAN employs vertical and horizontal telescopic pipes with small diameters at the suction nozzle, which allows the product to be conveyed into the pneumatic system with a low energy consumption. The pipe diameter is then gradually increased to reduce product speed, and the inclined horizontal pipe enables the product to keep moving and to flow gently down to the airlock.

A large receiving hopper, with a properly sized filter, ensures good separation of the product and the conveying air.

Growing up

Another area where the pneumatic unloader scores over its mechanical counterpart is in its ability to be extended to handle larger ships should circumstances change. Mechanical unloaders do not incorporate telescopic and horizontal loading arms, which limits their flexibility. Mechanical unloaders are best suited to a certain range of vessels but not for all ship sizes. In certain circumstances, for example, a small mechanical unloader will be unable to access all areas of the holds of a large vessel.

Pneumatic systems, on the other hand, are available in a great variety of models, sizes and diesel or electrical versions, and can easily be adapted to handle a full range of vessels sizes. They may be mounted on rubber tyres or on rails and can even be trailer mounted and placed on the deck of a ship to discharge directly into trucks on the quayside.

Furthermore, as many of the components are standard, order to delivery times are short while on-site assembly time is also shorter than for mechanical unloaders and consequently less costly.

………
Unlike Bühler, VIGAN, part of the Van de Wiele group, specialises solely in pneumatic unloaders and with some notable success. Last year, for instance, it delivered 30 mobile units, mainly in the 140-250 tph range as well as gantry and tower installations.

The compact mobile ma chines are essentially highly flexible self-contained trailer- mounted unloaders, which can be deployed in a number of different ways.

Five of these machines were delivered to the World Food Program last year for operation in Iraq where they are used to discharge trucks into flat storage distribution warehouses as there is a short- age of tipper trucks to shuttle grain from the docks to the inland warehouses. The same machines are also used to load trucks for distribution.

Five similar 145 tph machines were also delivered to China Grain Reserves Corp earlier this year for barge to truck operations.

Also this year, VIGAN delivered a type 120 SF unloader rated at 170 tph for handling barges and river vessels in Gdynia, again showing the versatility of pneumatic unloaders.

For this contract, the unloader was mounted on a gantry with the land side end supported by the silo immediately behind the quay. Discharge to the machinery house is pneumatic, while horizon tal transfer to the silo is via a chain conveyor fitted to the top of the gantry.

Multi-tasking

This combination of different types of onward conveying systems, while increasing the mechanical component number, does mean that the commodity can flow directly into the reception hopper or truck.

For example, a type 200 VIGAN unit delivered last year to Algeria is lifted onto the ship's deck for discharge of the adjacent hold at some 250 tph (tons per hour) and then delivers grain to a belt conveyor to load trucks on the jetty.

The trailer- mounted unloader is lifted onto a matching gantry, which is equipped with an under lung covered belt conveyor. This deck-mounted solution does not require such a long boom and can, therefore, work a wide range of ship sizes.

However, if maximum ship size is known, restricted by depth alongside or jetty length, than an alternative solution for direct truck loading is to mount the unit on a gantry incorporating loading hoppers.

VIGAN employed this configuration in Pusan, Korea, where a 250 tph trailer- mounted unloader is placed on top of a rubber tyres mobile hopper under which trucks can load directly.

<< back