Isokinetic sampling is very
tedious and we, as people who try to serve our clients best interests are looking for
ways to minimise costs, and in doing this we are having a serious look at the
benefits of doing isokinetic sampling.
Questions we ask
-
Is it worth the
expense?
-
What useful
information is obtained?
-
Is there not an
easier way to get similar information?
These
comments are based on the South African Industry,
and experience is predominantly on mines falling
under the Department of Minerals and Energy, Mine
Health and Safety Act.
The British
Standard Test method for Isokinetic sampling is BS
3405. This standard is unfortunately only
available to be purchased and cannot be published
on a web site.
Isokinetic
sampling for some reason seems to have become over
complicated and technical.
The main
question to answer is : What are the reasons for
taking "Isokinetic samples"?
As mentioned
already it is usually done to calibrate
instrumentation or just to determine the dust load
in a stack.
From our
experience the latter is less common, and almost
always isokinetic sampling is required when the
load in the stack or duct is very low, i.e.
when the instrument is reading at the bottom of
its range, or the dust plant is working well.
Any mine
needs to weigh up the costs versus the benefits of
isokinetic sampling. Very often the benefit
side of the scale is a bit on the light side.
The main
difficulty with isokinetic sampling is to try and
make the sample as representative as possible of
the dust in the stack or duct, without
excessive costs.
The arguments
are that the perimeter of the sampling tube will
effect the air flow quite drastically in and
around the tube unless the flow is very similar
past the outside and into the tube.
The above
problem is very significant if the area of the
perimeter is similar to that of the area of the
sample.
However if the sample area, i.e. the diameter of
the tube is larger, then the disturbance caused by
the perimeter is less significant because more
dust is collected.
The arguments
for the disturbance are as follows:
-
If the
sampling velocity in the tube is less than the
velocity in the duct then the tube will become
pressurised and particles will want to be
expelled from the tube.
The particles that are more easily dispelled
are the small particles, and as a result a
greater percentage of larger particles are
collected.
So the particle distribution will not be
representative of that in the stack or duct.
-
If the
sampling velocity in the tube is greater than
the velocity in the duct then the tube will
suck extra particles into the tube, thus
collecting a higher percentage of smaller
particles, also not a representation of the
dust in the stack.
The
disadvantage of having a larger sampling tube of
about 2 cm is that a higher volume of air has to
be sampled. This requires a larger vacuum
pump at higher capital costs.
Practicalities
of taking samples
If you have a
stack and you are required to monitor it for
environmental reasons then you will purchase an
instrument to measure this for you.
Why then
would you ever need to do isokinetic sampling?
Unless the accuracy of the instrument is disputed,
in which case the suppliers should be contacted,
so that they can provide the best method of
testing their instrument.
At the prices
of these instruments they should be accurate
enough, and the suppliers should be able guarantee
accuracy for a certain number of years.
Stacks are
often very high up with access being difficult.
Then there is the problem of having access to two
sides of the duct so that readings can be taken at
90 deg to each other.
This requires
a platform and two holes in the duct. More
often than not there is only one hole in the duct.
There are
many other standards applicable in different
countries, and these all have small variations.
The
uses of isokinetic sampling are quite varied, and
often are just a check to see if the instrument, bag filter, or scrubber is
working. In places where there is no opacity
instrumentation, isokinetic sampling is the only
way to determine the dust load in a stack.
Isokinetic
sampling is almost always performed when the
process is working well, and the emissions are
very low, often tending towards the invisible
range.
This means that very little dust is often
collected, thus making accuracy in the sampling
measurement very important.
The
equipment is very expensive and the methods are
long and very systematic. One has to weigh
up the cost of doing a test with the benefit
obtained from the result.
The
argument of particle size distribution makes
sense, but often the duct is after a bag filter,
where particle sizes are quite constant, unless a
bag is broken or there is some other process
problem in the plant, and then isokinetic sampling
will not be required because the plant is visibly
out of specification.
Comments on
the equipment and method of isokinetic sampling.
The equipment
used for isokinetic sampling is not identical for
every application with many factors such as
temperature, and moisture content, effecting the
method and equipment used.
In each case
an elaborate method of removing the moisture and
compensating for the temperature is used, each
with it's own inaccuracies, and increased costs.
The effects
of temperature and moisture content on the sample
are very significant, and their influence on the
accuracy of the sample should definitely be
considered. The issue is that there needs to be an
easier, cheaper and more practical way.
An average
velocity is taken, either at the beginning or by
sampling throughout the period.
The irony of
the limit set on stacks is that by diluting the
dust, you can achieve your limits. So
instead of a major capital cost, a small variable
cost that puts in clean air into your stack at any
point would lower your readings.
I have heard
of an operation where two stacks were next to each
other, one was above the limit and the other was
below. To solve this problem, the stacks
were simply connected with a horizontal pipe thus
balancing the air flow between them and keeping
both stacks below the legal requirements.
Even though exactly the same amount of dust was
being emitted. This does not make sense, but
it is how it is done.
This also
puts the limit that is sometimes mentioned and
applied of no "visible dust",
under dispute.
