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FIGURE 3. In particularly dirty liquid applications, prefilters are common and highly recommended
reach its setpoint may also be due
to a lack of a proper inlet pressure.
Therefore, before concluding that
there is a clog, one should first confirm
that the inlet pressure is sufficient.
In the event of a clog, the filter
should be replaced (not simply removed).
If the controller can now
achieve set point, then the filter
caught the clog, and it was time to
replace the filter.
However,
if
the
problem recurs
a short time later and the controller
again cannot achieve set point, it
makes sense to filter the process fluid
exiting the system with filter paper
and inspect it under a microscope for
particles. If particles are discovered
after passing through prefilters, inline
filters and the Coriolis flow controller,
the process fluid should be further
analyzed. Many liquids react chemically
with the moisture found in air.
The products of these reactions can
then clog either the filter or the valve.
If the filter is not clogged but the
controller cannot reach the established
process set point, that indicates
that the clog occurred after the
filter. This is characteristic of clogs
caused by a chemical reaction or
O-ring/seal degradation. Check the
controller documentation and the
fluid's material safety data sheet to
assess possible reactions with moisture,
as well as to confirm whether
the process liquids being transported
are compatible with the seal
materials and will not swell with the
fluid. This is most common with elastomer
O-rings and common solvents
like toluene, tetrahydrofuran (THF),
acetone and similar solvents.
Filter, orifice & particle sizes
Effective particle filtration requires an
understanding of filter pore sizes and
their relation to the size of particles
and orifices in Coriolis flowmeters
and controllers. Filters are typically
categorized by the maximum size of
particle that can pass through it, as
measured in microns. Inline filters are
typically offered in dimensions of 1,
10, 20, 30 or 40 µm.
Let's compare this to an orifice inside
the valve of a low-flow Coriolis
mass flow controller. The smallest
orifice offered in this industry has
an internal diameter (ID) of 0.001 in.
This means that the orifice has an
ID of 25.4 µm, which is much larger
than the largest particle that passes
through the 1-µm filter. Table 1 compares
the filter, orifice and particle dimensions
for a typical low-flow Coriolis
mass flow controller application.
It is recommended that all orifices
smaller than 0.007 in. use at least
a 10-µm filter. The worst-case scenario
would then be to use a 40-µm
filter with a 0.01-in. orifice diameter
(D) (254.0 µm). The ratio of the orifice
diameter to the largest particle
to pass through the filter would be
254.0/40 = 6.35. In other words, it
would require more than six particles
to jam into the orifice before the orifice
would clog in one axis. However,
the orifice is a hole with an area of
π(D/2)2
= π(254.0/2)2 = 50,670.7
µm2. Now the area occupied by the
particle is π(40/2)2 = 1256.6 µm2. To
completely block the orifice, there
would need to be 50,670.7/1,256.6
= 40 particles jammed into it.
Table 2 shows the calculations of
the number of particles that could
pass through a given filter to completely
clog the area of an orifice. It
should be noted that these calculations
are estimates of the number
of particles needed to clog an
orifice. They simply compare the
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