Dear Shri S.M. Umarye,

You may look into the website of DIN, BS, ISO etc. in this matter. Naturally, the names and their meaning decided by these institutes will be used by the business organiations (i.e. the names coined by these institutes will be adopted by the parties entering into business contract). However, as per the practice in material handling, these are generally understood as below :

Free digging (unloading) rate : Unloading rate which can be sustained for reasonable (defined) time interval, when digging from a specified point in the ship. This point is generally located at the horizontal centre of the ship and vertically at the mean low water for the port. In general it represents the mean (or somewhat below mean) situation for unloading rate that would be achieved by ship unloader. This rate is marginally specific to the port.

Cream digging (unloading) rate : Maximum unloading rate that will occur during certain situation, such as digging from most favourable layer, optimum (minimum) cycle time, high water level etc. The cream digging rate is generally for short time interval.

Average digging (unloading) rate : The average unloading rate can be with reference to one shift or 24 hours or for unloading of entire quantity from the particular ship. The unloading time for entire quantity could be less than shift or could be more than calendar day depending upon the ship / barge size and the unloading rate. For example if this is with reference to complete ship then, its value is total tonnage unloaded from the ship divided by the time taken by the ship unloader to unload the ship. Ship unloader idle time which is not attributed to ship unloader is not counted in the calculations. For example, if there is no power supply for one hour, or there is a idle time of half hour during shift change, or there is idle time for which ship unloader is not responsible, then such interruption time is not to be counted while calculating the average unloading rate. However, it does include all the operational time of the ship unloader such as positioning at particular hatch, operational settings, actual unloading, final unloading as a clearing operations by ship unloader, travelling to next hatch, operation interruptions created by the operation of the ship unloader itself and so on.

It is advisable that concerned parties (buyer and seller of the ship unloader), define (state) the agreed meaning of these names in their business contract to avoid any misunderstanding and subsequent disputes. It is the agreed meaning between the concerned parties which finally matters. These names signify (draw the attention to) the variations in unloading rate that would occur and their implications on unloading time, maximum design load on the receiving system etc.

The ratio of average unloading rate divided by free digging rate is known as unloading efficiency (of ship unloader).

It may be also noted that when the ship is fully filled it will tend to be dipper in the water and this tends to increase the material average distance from the jetty surface. As the material is getting unloaded, the ship tends to come up and this tends to reduce the bottom most material distance from jetty surface. This phenomenon tends to reduce the wide fluctuation in reclaiming depth, with respect to top layer and bottom layer.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916 ■

Dear Shri S.M. Umarye,

As mr Mulani states, it is very important that the parties involved, agree upon the various capacity definitions.

However, setting up those definitions require field knowledge of the unloading operation itself.

A manufacturer should design the unloader with a higher capacity than the guaranteed capacity.This to be certain that the guaranteed capacity is met. This capacity could be named as design capacity.

Then there is the maximum or nominal capacity, which is the unloading capacity for angreed favorable position in the ship’s hold (top of the cargo) for 1 hour.

Unloading capacity or free digging capacity, which is the unloading capacity of one hold under agreed conditions, excluding the clean up phase.

(f.i. approx 75% of the upper part in the hold)

Through The Ship capacity (TTS capacity), which is the ships cargo divided by the total unloading operation time (from the beginning until the end of the unloading), including clean up, shifting, refueling, breakdowns, maintenance, etc. but excluding time losses due to other factors than related to the unloader and the unloading operation.

As this capacity is very much depending on the skill of the operators, this test is normally executed by or under the supervision of the manufacturers’ operators.

This test can also serve as an endurance test.

Also for this test, the parties have to agree on the chosen ship.

Any capacity test must be thoroughly discussed between seller and buyer (preferably in advance)

Also other parameters can be addressed in these tests such as noise, energy consumption, dust emission, velocities, etc.

all for now

teus ■