Fig:self unloader components ready for operation
These hopper openings are controlled by ‘gates’, which can be hydraulically opened or shut to
regulate the flow of cargoes out the tunnel conveyor. All the sloping sides in the cargo holds are lined
with special UHMW sheeting to enhance the sliding (flow) ability of the bulk cargo being discharged.
The primary functions of the gates are:-
Bulk flow gate
- As the first step in the unloading process i.e. they allow cargo to pass from the cargo holds
onto the tunnel belts.
- As the means by which mass flow rate of cargo is controlled.
- On the existing CSL fleet of Self-Unloaders there are four types of cargo gate in use.
- Bulk Flow Gate (where applicable)
- Basket Gate (where applicable).
- Nordstrom Gate (where applicable).
- N.C.Feeder (where applicable).
These gates consist of two (opposing) moving sections which slide horizontally in the athwartship
direction. Each section is fitted with two hydraulic cylinders, which are supplied from a central
hydraulic power pack located in the engine-room.
Each gate is operated through a “Teledyne” valve mounted adjacent to the gate. When the Teledyne
valve is operated hydraulic oil flows to all four cylinders via a flow divider. The flow divider ensures
that both gate sections receive an equal amount of hydraulic oil and therefore move in equal amounts.
This facilitates better control of the cargo flow. Bulk flow Gates are situated approximately 500mm
above the trough of the tunnel, and therefore cargo falls this distance when the gate is opened.
operator must exercise vigilance when unloading as certain types of cargo (e.g. Gypsum Rocks) may
bounce off the belt and become trapped in the return strand of the belt. This can result in serious belt
damage, as the rocks will jam between the carriage frames and the moving belt causing the belt to
tear. The gates must be left “cracked” open before loading gypsum, if left fully closed, the gates will
not open under hydraulic power alone. When there are flow problems with cargo in the holds it is
customary to work this type of gate opening and closing, to loosen the cargo above. Operators must
however bear in mind the fact that the gates are not to be “worked” too rapidly, as there is a possibility
that the gates may come off their tracks resulting in damage to hydraulic cylinders and fittings.
This design of gate employs two hydraulic cylinders to operate each gate. The gates are constructed in
one piece hinged at the forward end, and open in the fore and aft direction.
They are operated with a teledyne valve, but because the gate is in one piece there is no need for a
hydraulic oil flow divider. The gates must be opened almost fully before cargo begins to flow, and this
places the lip of the gate very close to the tunnel belt during unloading. Only one gate can therefore
be operated on each tunnel at any one time, otherwise cargo coming down from an open gate further
forward will be pushed off the tunnel belt and into the tunnel. This type of gate “places” cargo onto
the belt as the cargo actually slides down the lip of the gate onto the tunnel belt.
These gates are similar in the design to the Basket Gates type, in that they are one piece gates which
operate in the fore and aft direction.
They are operated by a hydraulic cylinder, and are fitted with a rubber flap, and lengths of steel chain
to prevent cargo from bouncing, as it falls from the cargo hold onto the tunnel belts.
N.C.Feeder (Non - consolidated feeder)
This is a patented system, which, instead of having a series of individual gates over each belt, one N.C.
Feeder is mounted per cargo hold over each belt. The
N.C. Feeder consists of a set of stationary openings and a set of reciprocating openings which resemble
The reciprocation of the “ladder” is in the fore and aft plane. The “ladder” is powered by two large
hydraulic rams. The advantages of the N.C.Feeder are that there is a reduction in the amount of
hydraulic machinery required as there are only two hydraulic rams per cargo hold and the system is
easy to automate. The flow rate from an N.C.Feeder is controlled by the speed of reciprocation of the
“ladder”. The action of the reciprocating motion keeps the cargo above in a constant state of flux.
This allows the N.C.Feeder to handle cargoes of a very sticky nature, which might result in problems
with other types of gates.
Hold conveyor belt
The Hold conveyor belt or tunnel belt runs under the cargo holds i.e. under the gates. Two or three are
fitted depending on the width or design of the vessel.
The direction of travel of the belts ‘carrying side’ is determined by the location of the transfer belts
which transfer the cargo to the lift belt/s. All belts are spliced to form endless loops, around the drive
pulley at one end, and a tail pulley at the other. An electric motor powers the drive pulley coupled
through a Fluidrive coupling, and a reduction gear box. . The tunnel belts are tensioned using a
hydraulically operated tensioning carriage, which can be situated at the forward or aft end of the belts
(depending upon the ship design).
Transfer belts are short athwartship running belts, which receive the cargo from the hold conveyor via
the transfer hopper, and convey it to the lift belt via the centre hopper.
Usually there are two transfer belts from the two outer hold conveyors to the centre on either side.
The transfer area is where the cargo changes direction, by being transferred from one belt to another
via hoppers or bins. This is a high risk area as there is a concentration of ‘drive elements’. Protection
to personnel provided by catwalks, guard rails, protective covers and signboards, and is essential.
Water leaks into the tunnel accumulate in the transfer area if it is located aft. Care must be taken to
ensure that the tunnel bilge levels do not rise, and must be attended to. The trash pump bilge wells
are also located beneath the transfer area at the aft of the end vessels. These wells are fitted with
water level alarms, which must be regularly tested to prove that they are operational.
The hoppers serve as transfer points for the transfer of cargo between two belt systems e.g. hold
conveyor and transfer belt: between the transfer belt and the lift belt; and between the lift and boom
The size and design of the boom or hopper depends upon the width and carrying capacity of the belts
they serve, as they regulate and spread the cargo over the belt they feed. The belt carriage sides may
also be fitted with skirtboards for further regulation of cargo. These bins or hoppers are lined witabrasion resistant liners to protect the mild steel construction, and hardened-impact plates on the side
of direction of the cargo flow.
They may also be provided within their structure, with an adjustable
plate to fine tune the cargo regulation, and avoid spillage. These bins or hoppers must be regularly
inspected, for wear of their abrasive lining, and impact plates, and also that there are no other
blockages e.g. previous cargo build-up, some fallen object, or any sharp object which would tend to
tear the belt. During the discharging operation, inspections using a torch through the “viewing-ports’
provided, are to be carried out.
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Operation of sea going bulk carriers involved numerous hazards . Careful planning and exercising due caution for all critical shipboard matters are important . This site is a quick reference to international shipping community with guidance and information on the loading and discharging of modern bulk carriers so as to remain within the limitations as specified by the classification society.
- Various useful terms related with self-unloading bulk carriers operational matters
Function of loop & bucket belt elevators
Self unloaders various cargo handling gears
- Various type boom conveyor belts - How the belt sytem practically works ?
Dealing with self unloaders stalled lift belt
Conveyor belt construction & troubleshoot guide
Conveyor belt installation guide
Conveyor belt repair & maintenence guide
Safe working practice onboard self unloading bulk carriers
- Preventing conveyor belt fire onboard self unloading bulk carriers
- Cargo work safety precautions
- Various bulk cargoes - free flow ability
- Various bulk cargoes & dealing with cargo hang ups
- Navigation in
Ice & safety precautions
- Dust suppression procedure & environment protection
- Preparations for cargo planning, handling & stowage
- Maintaining safe stability onboard self-unloading bulk carriers
- Preparations for loading - self unloaders guideline
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