Grabs play a crucial role in many bulk terminals, and choosing the most appropriate model can help operators significantly raise their handling productivity.
Dry bulk terminal operators are always looking for ways to increase terminal efficiency. This is important for optimising shareholder profits and increasing their competitive edge.
Situated on the upper reach of the River Rhine, the Swiss port of Auhafen Muttenz Basel has started using a completely new system for biomass unloading and storage in large silos.
The terminal operator specified Stemm’s 4CH-16000-1,1 clamshell grabs, with acapacity of 16 m3 , designed to handle all kinds of bulk commodities and biomass, up to a density of 1.1 t/m3.
The start-up of the installation was carried out by Stemm’s technical team in collaboration with engineers from Stephan, a Swiss company from Freiburg that specialises in all types of lifting equipment, bridge cranes and special equipment for combustion of wood and biomass sectors, wood burners and boiler feed, and silos.
Trial and error
Biomass traffic on the Rhine has grown considerably in recent years. Several techniques and processes in handling these combustibles have been tried out, but discharging biomass from inland waterway vessels for storage of the material in tall silos requires the application of state-of-the-art equipment, according to Stemm.
On this occasion, Stemm installed an electro-hydraulic clamshell grab powered by four hydraulic double acting cylinders that work directly on the grab’s shells, applying a powerful and uniform penetration.
Each stroke of the shells ensures full filling, especially when handling this type of material, which can be irregular in shape and grain size.
The grabs can work in any position within the ship, allowing them to work even a sloping position in an almost horizontal profile. Stemm says that its grabs work for 24 hours non-stop, and transmit important data to the automation system, such as position of the valves (closed or open), incline, operational pressure, oil and motor temperatures.
For vessels carrying highly compacted materials, due to their hold conditions or weather circumstances, Stemm grabs have their own system called ‘scratching’ that operates automatically and performs a prior and/or simultaneous scratch when handling the product inside the hold. That way they obtain superior filling ratios.
According to Nemag, grabs play a crucial role in raising terminal efficiency. Thus, the company claims that its nemaX model is the most productive grab for iron ore.
In the company’s latest white paper following the launch of the nemaX, Nemag claims the grab can increase productivity by more than 10%, thereby guaranteeing the most efficient unloading process.
One of the main reasons for this is highly efficient unloading at all stages of the process. The latest white paper explains the elements in this process.
There are four phases involved in unloading a bulk carrier for each hold – cream digging, free digging, intermediate digging, and cleaning up.
In the transhipment sector, cream digging and free digging rates are generally seen as the key figures for the unloading capacity of a crane. The crane capacity is almost always expressed as a free digging rate, which is the productivity of the crane and the grab as measured from a specific point in the hold, multiplied by the number of cycles per hour. This provides the most reliable measurement.
Some 65% of the unloading time is spent on the last 30% of cargo. Nevertheless, most of the unloading time is not spent on the free digging phase, but on the intermediate and clean-up phases.
This is down to four factors. First, during the unloading process, the cycle time slowly runs out because the grab path continues to get longer.
Second, as the grab reaches deeper into the hold during the intermediate phase, extremely deep holes are created in the load, as a conventional grab makes it hard to reach under the hatch coaming. The deep holes create steep slopes on both sides of the hold. Conventional grabs, like clamshells, tend to tip over on steep slopes due to their relatively high centre of gravity, which results in lost
Third, the filling factor of standard clamshell grabs is lower on slopes because the grab has an extremely vertical digging direction. When a grab like this is used on a steep slope, it will fill unevenly.
Finally, steep slopes in the hold increase the risk of avalanches, which in turn leads to production losses.
The result is that the rates achieved during the intermediate and clean-up phases are reduced by as much as to 20-25% of the free digging rate. Increased productivity is therefore preferred in all three phases in order to save time.
Nemag says it drew on years of experience to develop a grab that increases productivity in the free digging, intermediate and clean-up phases. Thanks to its properties, the nemaX is claimed to achieve the highest productivity in the final unloading phases.
The nemaX features a deadweight efficiency of only 25-28%, and a grab ratio of 2.5 to 3. This means the nemaX is about 15% lighter on average than any comparable clamshell grab on the market, the firm says.
Speed is also important. The optimised closing mechanism enables a 20% shorter closing time and maintains a closing force on the lip plates at high levels.
The nemaX grab also has a horizontal closing path and a 30% larger footprint than a clamshell. This allows it to dig more smoothly and more evenly than conventional grabs, resulting in minimal time losses caused by avalanching. With its lower centre of gravity, the nemaX is also more stable on slopes than clamshell grabs. This reduces the risk of tipping, and ensures that the grab maintains an optimal filling factor on steep slopes.
In the clean-up up stage, whereas standard clamshell grabs require a lot of trimming equipment, the nemaX can grab more bulk material in clean-up, thanks to a 30% larger footprint and horizontal digging path. A major benefit is that operators can save on expensive and time-consuming trimming equipment.