Mechanical part-stream
sampling
One example of the need for
condition C (part-stream sampling)
occurs when the mechanical
sampling system in a terminal breaks
down or is damaged by debris
entering the system. In these cases, it
is necessary to balance the time it
would take to make the repair versus
the cost of stopping the loading.
At most export terminals, a
limited amount of time is allowed to
make repairs, but there is a manual
sampling protocol in place so that
loading the ship or barge can
continue. This is usually a
part‑stream sample collected
manually with a shovel from a
moving conveyor belt. In almost all
cases, it is temporary and only small
portions of the overall cargo is
collected this way.
The ASTM Standard for this type
of sampling is D 6609. Even though
it is a judgment sample, it is still
important to have rules and use
common sense to overcome, as much
as possible, the inherent challenges
in this type of sampling. For
example, it is good practice,
especially on larger conveyors, to
use two personnel so that both sides
of the conveyor can be reached by
the sampling instruments in order to
account for the segregation of
material – especially in blends – on
either side of the conveyor.
One issue that has emerged in the
mechanical sampling world is that
many terminals, in their pursuit of
economies of scale, are operating at
higher and higher flow rates. To
accomplish this, the conveyor belts
are larger and operate at much
higher speeds. In some cases, it is
simply no longer safe for the back‑up
part‑stream sampling from the
conveyor to be performed manually.
In these ports, there is no back‑up
method other than stopping the
loading or sampling at the stockpile
– which reverts to using the lowest
ranked sampling method and which
places a sample technician in a
position of danger.
To provide an alternative,
Sampling Associates, in
cooperation with a mechanical
sampling system manufacturer,
has developed a sampling device: a
Mechanical Part‑stream Sampler
(MPS: see Figure 2; p. 32). The MPS,
which has a patent pending, is still
a condition C method; it does
not replace condition B
sampling. However, because it is
fully mechanised and removes
the human element, it is a
significant improvement on manual
part‑stream sampling in two
important ways.
The first way is that it is much
safer. No person is exposed to the
moving conveyor parts or harsh
weather. Secondly, it removes the
human element in the timing of the
sample increments and the selection
of material into the sample. An
additional benefit of using the MPS
is that it frees the technician to
concentrate on getting the
mechanical sampler back online,
instead of manually sampling and
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