Sample Taking And Processing - Valmet SMART-PULP Operating And Installation Instructions

SMART-PULP

2.4 Sample taking and processing

It is important that the sample be representative.
This means that the sample should be taken from a
representative point in the line, from a
representative depth in the pipe and over a
representative time period, and that the sample is
transferred in representative condition to the sample
receptacle and to laboratory regardless of the
sampling time. Even if the entire sequence from
process pipe to laboratory result is carried out in the
best possible manner, the margin of error in the
result is rarely better than +5% of the reading. At
3.0% consistency level this means +0.15% Cs for
3% Cs pulp.
1. Representative point in pipe
You should observe the sampler's (e.g. Valmet
NOVE) installation instructions when choosing the
mounting location for the sampler. In particular, you
should avoid installing the sampler on the inside
curve of a pipe bend.
2. Representative depth in pipe
The sampler should reach past the pipe wall towards
the center of the pipe, so as to eliminate the effects
of the water layer near the pipe wall and of its
variation with flow velocity.
3. Representative time period
Since there may be considerable momentary
consistency variations in the pipe, you should try to
obtain a representative average to the sample
receptacle. A large sample volume or several
consecutive smaller portions will help you to obtain
such representative average.
4. Representative sample to receptacle
Critical factors in sample taking include the opening
and closing of the sampler, velocity of flow, and
possible splashing of the sample.
It is recommendable that the sampler be flushed
after each use to avoid clogging and dry lumps.
You should first allow the sample to flow for a few
seconds, and then "scoop" a suitable portion from
the flow e.g. by passing the flexible sample tube
several times over the sample receptacle.
The sample flow should be as high as possible, but
the sample should not splash from the receptacle.
Smart Consistency Transmitter
Too small flow may dilute the sample.
5. Representative sample to laboratory
Sedimentation will occur if the sample is allowed to
remain at standstill even for a short time. Always stir
the sample with special care when separating a
smaller portion from a large sample. Such smaller
portions should be dipped with a ladle, never
poured.
The best result will be obtained by taking several
samples directly from the pipe to separate
receptacles, and by processing all samples
separately and completely. This will prevent the
distorting effect of stirring and of taking a partial
sample.
Keeping the several samples separate from the very
beginning will allow you to determine the time-
related representativeness and efficiency of
sampling from the standard deviation between the
samples. You will get more information from several
small samples than from a single large one.
6. Sample processing
In the laboratory it is important to ensure the
weighing scales' accuracy and proper timing during
and after drying. The TAPPI 240 om-88 Standard
ensures +10% repeatability in determination of
consistency. In actual practice this error can be
about halved. Even then the error may be significant
when evaluating the efficiency of the measurement
or the need for recalibrating the transmitter.
7. Examining the results and recalibrating the
transmitter
The need for recalibrating the transmitter can be
determined if SMART-PULP accepted the samples
and standard deviation between separate analyses
was not too great. Experience has shown that there
is no need to recalibrate the transmitter on the basis
of a single batch of samples if the difference
between the average of the samples and the
transmitter's indication is less than +5% of the
reading.
In recalibration we recommend halving the
difference. The transmitter's output will then
approach the laboratory result by half the difference,
and a random sample/laboratory error will not have
a too large effect.
28
BCs160VA
Nov. 15, 1997