For over a century, draught surveys have
been internationally accepted as an accurate
and convenient means of establishing the
weight of bulk cargoes, writes Mark Hopkins
BSc(Hons), C.Eng, FIET, FInstMC, Technical
Manager Procon Engineering Ltd
. They provide
the basis for the preparation of bills of lading
and assessing various charges and port fees.
Based on Archimedes’ buoyancy law, cargo
weights are calculated by assessing changes in
the level or draught of the ship before and
after cargo is unloaded. Unfortunately
accuracies using this methodology can vary
considerably and it is not uncommon for
errors of ±5% or higher to occur. Subjective
rather than qualitative reasoning can play a
major part in this and surveyors may call
upon personal experience to arrive at final
readings. As a result, discrepancies in draught
survey results can give rise to commercial
disputes amongst sellers, vessel operators,
buyers and surveyors relating to which weight values to accept
for payment of the cargo, port fees and other charges.
For cargoes with low tonnage value, these levels of accuracy may
be deemed acceptable, but for companies purchasing higher
value goods, errors can be expensive. This has led companies to
implement other more reliable and accurate methods of
measuring received cargo quantities. A clear advantage of the
draught method is that calculations can be done relatively
quickly, minimizing operational delays in unloading and therefore
minimizing demurrage costs. Therefore any alternative methods
need to be operationally efficient, accurate and provide traceable
results in the case of discrepancies or disputes. It should be
noted that it is important that any weighing systems should be
suitable for Weights and Measures approval so that data can be
used in the case of disputes.
For granular and solid bulk products such as grain and sugar,
belt weighers can be an effective solution for verifying unloaded
quantities. These provide an accurate way of totalizing material
as it is offloaded and systems can be selected to provide the
necessary weights and measures approved weight data.
Belt scales operate by continuously weighing the material as
it travels across a predetermined length of conveyor belt above
the weigh carriage (weight per unit length). This is then
combined with the measured belt speed to give the throughput
weight per unit time (typically in units of tonnes per hour or
tph). Finally the integrated ongoing totalized weight is calculated.
Throughput capabilities of such weighers typically range from
less than 5tph up to 12,000tph.
Belt weigher accuracy depends on a number of factors,
including the number of rollers on the weigh carriage, the
weighing technology used and the uniformity of the material
being weighed. Accuracies of better than ±0.5% can be achieved
for free flowing product. Moisture content measuring systems
used in conjunction with the belt weighers can further improve
Belt weighers play another important role at ports and
terminals on giant mobile stacker-reclaimers. In these
applications, the weighers are fitted within the moving booms
and therefore operate in one of the most arduous and hostile
environments imaginable with little time for maintenance.
Although the weighers are a relatively small part of the giant
equipment, their importance cannot be underestimated. Any
downtime is costly and very inconvenient, and as a result,
reliability is vitally important.
A high proportion of belt weighers use strain gauge load cells
to support the weigh carriage and although these provide good
serviceability, they can be unreliable long term in harsh outdoor
environments. To overcome this shortcoming, weighing specialist
Procon Engineering has continued to develop its unique
resometric belt weighers. These robust weighers have proved to
be significantly more reliable than conventional load cell based
units in arduous applications. The units provide a unique
combination of features that ensure the key factors influencing
belt weigher performance are maintained in service over long
periods of time.
The digital force transducer has no moving parts and the
resonator is pre-loaded to a standard level by the applied force
from the weigh carriage. As the force varies with the load
passing over the carriage rollers, the resonant frequency changes
in proportion to applied load. This change in frequency is
processed by the electronics and combined with the belt travel
speed to provide an accurate weight reading per unit time.
Unlike fixed inclination weighing applications, the belt
weighers on the stacker reclaimers are subjected to constantly
changing angles of operation. This would normally introduce
significant cosine weighing errors due to the apparent change in
the suspended weigh carriage mass. To automatically
compensate for the changing angles of operation, the weighers
are fitted with Procon’s proprietary Astatic Mass Zero
Compensation systems.
The type of method chosen for verifying off-loaded cargo
quantities will depend on the product, its value per tonne and
available port facilities. If weighing systems are utilized, then
these should be suitable for Weights and Measures approval to
ensure traceable accuracy levels in order that the weight data
can be used to clarify legal disputes if required.