Note: Descriptions are shown in the official language in which they were submitted.
CA 02463247 2004-04-05
l
FLUID SAMPLING SYSTEM AND METHOD THEREOF
FIELD OF THE INVENTION
The present invention generally relates to the field of fluid sampling and
concerns more' particularly a fluid sampling system and a fluid sampling
method
particularly adapted for use in a gas analysis process.
to BACKGROUND OF THE INVENTION
In all fields using a gas medium or a liquid medium such as air separation
processes, petroleum refining, natural gas production, semiconductor devices
manufacturing, specialty gas laboratories, etc..., all gas being processed or
used in
is one way or another must be analyzed for quality control or process control.
To
perform such an analysis, a gas sample is collected and brought to an
analytical
measuring system. Generally, the gas sample is conveyed through metal tubing,
up
to a sample panel. A plurality of samples may need to be successively
collected,
depending on the complexity of a particular system. The analyzed sample should
of
2 o course be representative of the gas medium being controlled.
The industry has used and still uses various devices and processes to bring
a sample to an analytical system. UVith these sampling systems, contamination
of a
sample often occurs by mixing it with previously selected samples, leaks in or
out of
25 the sampling system or leaky valves.
Another unavoidable source of contamination are the control elements, i.e.
valve, mass flow controller, pressure regulator, etc..., used to select and
control flow
of various sample streams. In U.S. patent No: 5,922,286 and in "Ppt level
analysis
3 0 of UHP Hydrogen", European Semiconductor, 1996, if his been shown that any
on-
line component such as valve, mass flow controller, etc..., will act as a
quasi
continuous source of contamination when attempting to measure very low levels
of
impurities, i.e. ppb (part per billion) and ppt (part per trillion) level.
These components
CA 02463247 2004-04-05
2
wiH outgas some different types of molecules based on material used to
manufacture
them. When measuring particle contamination for process gas in semi-conductor
industries, often, the limit of detection is limited by varicaus types of
control elements.
Sudden change in flow and pressure generates wide variation in readings. For
particle measurement, constant pressure flow is required. As indicated in the
above
mentioned U.S. patent No. 5,922,286 and as well known by people involved in
the
art, transient pressure or flow rate change system equilibrium, leading thus
to an
adsorption or desorption phenomenon. This lead to long purging time to recover
system equilibrium.
to
Presently used in the art, there is a system provided with various sample
lines
made of various tubing material, each bringing a corresponding sample to an
apparatus sample inlet. A plurality of sampling locations may be provided, as
required by the process to be monitored. A bypass rotometer is provided in
each line
for purging a given sampling Line when not selected. The rotometer allows
fixing of
a bypass flow and preferably sets a high flow in the sample line to speed up
the
purge time. The excess flow is vented out of the system: i4 female quick
connector
is provided at the extremity of each sampling line and is adapted to receive a
male
quick connector allowing the sample to flow through a flexible line up to the
analytical
2 o system. To change the selected sample line; the male quick connector needs
to be
removed from the female quick connector and inserted in another one. This
system
makes sure that there is no sample cross contamination from various sample
points,
since the sampling lines are physically isolated. However, this system has
serious
drawbacks. First; each time the male quick connector is disconnected from a
female
quick connector, he gas flow to the analytical system is momentarily
interrupted.
Some analytical systems are affected by the sample flow variation. Also the
female
and mate quick connectors have some internal dead volume that will be filled
with
atmospheric air when disconnected from each other. This air is directed to the
analytical system and serious pollution may occur when measuring H20, 02 or N2
as
3 o impurities in a particular background: Another drawback is that the quick
connectors
tend to wear out with use, resulting in leaks leading to wrong analytical
results.
Another problem with this system is related to the use of flexible tubing.
Often this
CA 02463247 2004-04-05
3
tube is made of various plastic or polymers that exhibit too much permeation
to 02
and H20, thereby polluting the sample. When flexible metal tubing is used, it
must
be replaced often since metal fatigue due to manipulation causes them to
break.
Another drawback is that such quick connectors have a o-ring that is used to
seal
them. The material used to make these o-rings will adsorb or desorb some of
the
impurities to be measured, so it changes the sample composition, making them a
poor choice for low-level measurement.
Also used in the industry, there is another sample stream selection system
1 o quite similar to the one just described above: This second system uses,
instead of
quick connectors, a rotary selection valve well known in the industry and
available
from various manufacturers. This system alleviates some of the drawbacks of
the
previous one, but introduces cross port flow contamination that increases with
time.
If a sample line has a higher pressure, it will leak through a valve body and
pollute
the stream being measured. This valve requires frequent replacement.
Furthermore,
leaks can occur from the valve stem.
Also known in the art is another system wherein each sampling line includes
an ON/OFF valve provided downstream a bypass rotometer. However, this system
2 o introduces dead volume in the line section downstream the valve. When
switching
from one sample to a new one, the line section of the previously selected
sample is
full of the previous sample. This gas is trapped there and will slowly diffuse
in the
line, slowing down the response time of the system and causing drifting
readings of
the analytical system. Another source of unswept dead volume is the valve
itself. The
2s space surrounding the valves plunger is always filled with sample gas and
slowly
diffuses in the main stream, causing measurement drift and noise. A Diaphragm
based valve may be used to reduce the problem; but it increases the cost of
the
system since most of the time the use of such a valve will involve orbital
welding for
assembly. Furthermore, over time, ON/OFF valves will develop leaks. So an
3 o unselected stream may leak to a selected one, resulting in analytical
error
measurement and apparent drift or noise when the sample line pressure varies
again. As soon as a valve develops a leak it must be replaced, interrupting
the
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system in service. There are some variations of the previously described
systems but
all have similar drawbacks.
Also known in the art is the aboue mentioned U.S. patent No. 5;922,286
(Girard et al). Girard discloses a system that selects individual sample
streams with
the help of a 4-way, pneumatic actuated, VCR 114" connected diaphragm valve.
Even if this system succeeds in eliminating unswept volume present on the
discharge side of the valve and provides some means to have a sample inlet
bypass
flow or purge, it fails to eliminate the problem associated with leaking
valves, i.e.
so crossport flow contamination. The selected sample must flow through all
unselected
valve bodies just around the seat, which is quite large. Therefore, the risk
of
crossport contamination increases with the number of sampling lines in the
system.
Diaphragms having a relatively short useful life; there will eventually be
leaking
across the seat and contamination of the selected sample will occur. Finally
the
diaphragm valves used in this system are costly and the total space required
for this
system is quite large.
Also known in the art is U.S. patent No. 6,637,277 by the same inventor of the
present invention. In this patent, Gamache discloses a system that eliminates
2 o problems related to dead volume and leaking in valves by adding back purge
flow on
the discharge side of the valves, as illustrated in FIGURE 1. Even if this
proposed
system corrects the inherent problems of all other previously described
systems, the
control valve (on-off valve) is still in contact with the process fluid to be
analyzed,
then contaminating it.
Other related prior art systems include U:S, patents Nos. 5,054,309;
5,055,260;5,065,794; 5,239;856; 5,259,233; 5,447,053; 5,587,519 and 5;661,225.
In all prior art referred here, the sample to be introduced in the high
sensitivity
3 o analytical system passes through the control element in some way. So, in
all these
cases the sample may be affected in some manner that leads to modify its level
of
impurities, leading to erroneous measurement.
CA 02463247 2004-04-05
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fluid sampling system
that
5 prevents cross-port flow contamination between various sampling fines and
that does
not contaminate the sample by the product that is out gassed or adsorbed by
the
control element used to control flow of process fluid.
Accordingly, the present invention provides a fluid sampling system including
Zo a plurality of sampling channels. Each of the sampling channels is provided
with a
sampling line having an inlet, a regulated outlet, and first and second
calibrated flow
orifices connected in series between the inlet and outlet. Each of the
sampling
channels is also provided with a controllable derivation line connected
between the
first and second orifices, for deviating fluid from the sampling line: The
fluid sampling
z5 system also comprises a connecting line for connecting together the outlets
of the
sampling lines. The connecting line has a main regulated outlet for providing
a fluid
sample. The fluid sampling system is also provided with control means for
controlling
pressures between first and second orifices of all of the sampling lines by
means of
the controllable derivation lines, thereby increasing pressure between first
and
2 o second orifices of one of the channels which is then selected to provide
the sample
fluid to the outlet of the connecting line, and decreasing pressure between
first and
second orifices of remaining sampling lines to back purge the remaining
sampling
lines.
25 According to another aspect of the present invention, there is also
provided
a fluid sampling method comprising the steps of:
a) providing a plurality of sampling channels, each of the sampling channels
comprising a sampling line having an inlet, a regulated outlet, and first and
second
calibrated flow orifices connected in series between the inlet and outlet;
3 o b) connecting together the outlets of the sampling lines to provide a main
regulated outlet;
c) providing fluids to the inlets of the sampling lines;
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d) deviating in a regulated manner the fluids from points located between the
first and second flow orifices of the sampling lines; and
e) after the steps a), b), c) and d), increasing pressure between first and
second orifices of one of the sampling lines which is then selected to provide
a fluid
sample at the main outlet, and decreasing pressure between first and second
orifices
of remaining sampling lines to back purge the remaining sampling lines
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the invention will become
apparent upon reading the detailed description and upon referring to the
drawings
in which
FIGURE 1 (prior art) is a schematic representation of a fluid sampling system
known in the art;
FIGURE 2 is a schematic representation of a fluid sampling system according
to a preferred embodiment of the present invention;
FIGURE 3 is a schematic representation of a fluid sampling system according
2o to another preferred embodiment of the present invention, in a first
operating
position;
FIGURE 4 is a schematic representation of the fluid sampling system of
FIGURE 3, in a second operating position;
FIGURE 5 is a schematic representation of a fluid sampling system according
2s to another preferred embodiment of the present invention;
FIGURE 6 is a diagram of flow versus pressure differential, in relation to the
fluid sampling system of FIGURE 3;
FIGURE 7 is another diagram of flow versus pressure differential, in relation
to the fluid sampling system of FIGURE 3;
3 o FIGURE 8 is another diagram of flow versus pressure differential, in
relation
to the fluid sampling system of FIGURE 3; and
FIGURE 9 is another diagram of flow versus pressure differential, in relation
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7
to the fluid sampling system of FIGURE 3.
While the invention will be described in conjunction with example
embodiments, it will be understood that it is not intended to limit the scope
of the
s invention to such embodiments. On the contrary, it is intended to cover all
alternatives, modifications and equivalents as may be included as defined by
the
appended claims.
DETAILED DESCRIPTION OF THE FIGURES
to
In the following description; similar features in the-drawings have been given
similar reference numerals and in order to weight down the figures, some
elements
are not referred to in some figures if they were already identified in a
precedent
figure.
Referring to FIGURE 1 (prior art), there is shown a fluid sampling system 100
for providing a fluid to a fluid processing apparatus 142 presently used in
the art. The
system 100 includes a plurality of sampling channels 146, each having an inlet
line
148 and an outlet line 150 connected by a valve 152. The valve 152 has a
closed
2 o position preventing a fluid flow between the two lines 148, 150 and an
opened
position allowing such a fluid flow. A first and a second purge line 154, 156
are
provided, respectively connected to the inlet and outlet lines 148, 150 for
purging
filuid therefrom. Rotometers 158 or other similar devices are provided for
controlling
each of the purge lines 154, 156. A connecting line 160 is provided for
connecting
2 s each of the outlet lines 150 to each other and to the outlet 144. In
operation, one of
the valves 152 is opened and the others are closed. Fluid flows from the
selected
valve to the apparatus 142 and backwards through the outlet lines 150 of the
unselected sampling channels 146, providing a backpurge of these outlet lines
through the second purge lines 156. In this system 100, the valves 152 are in
contact
3 o with the process fluid to be analyzed and this can therefore lead to a
contamination
of the sample to be analyzed.
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w, ..
8
Referring to FIGURE 2, there is shown a fluid sampling system 10 according
to a preferred embodiment of the present invention: Contrary to the prior art
system
100 just described above with reference to FIGURE 1, the fluid sampling system
10
of the present invention does not contaminate the sample to be analyzed. The
fluid
sampling system 10 provides a fluid sample to at least one fluid processing
apparatus here embodied by an analytical system 30. It is however understood
that
the present system may for example alternatively be used to feed a process
using
fluid samples, or any other type or number of apparatus adapted to receive a
fluid
io from a given medium. Also, in the illustrated embodiments the fluid sample
is of the
gaseous type, but the present invention may just as well be used to sample a
liquid
medium. The fluid sampling system of the present invention that will be
described
thereinafter can advantageously be inexpensively manufactured.
The fluid sampling system 10. is provided with a plurality of sampling
channels
12. Each of the sampling channels 12 is provided with a sampling line 14
having an
inlet 16, a regulated outlet 18, and first and second calibrated flow orifices
20, 22
connected in series between the inlet 16 and outlet 18. The sampling lines 14
may
come from different sample points in a particular process and therefore, may
have
2 o different length. Thus, the fluid sampling system 10 may advantageously
have
regulating means for regulating the inlets 16 of the sampling lines 14.
Preferably, the
regulating means have a plurality of purge lines 32 respectively connected
between
the inlet 16 and the first orifice 20 of each of the sampling lines 14.
Preferably, each
of the purge lines 32 is provided with a purge flow contrbller 34 connected to
a vent
2 s line 36. The purge flow controller 34 is preferably mounted with a valve.
The purpose
of this flow controller 34 is to allow a large sample flow to react rapidly in
response
to a process change. This can also be useful when the sample point is located
far
away from the sampling line. Each of'the sampling channels 12 is also provided
with
a controllable derivation line 24 connected between the orifices 20, 22, for
deviating
3 o fluid from the sampling line 14: Each of the sampling lines 14 may
advantageously
be provided with a T-type of joint 38 for connecting the corresponding
controllable
derivation line 24 between the corresponding orifices 20, 22. The fluid
sampling
CA 02463247 2004-04-05
9
system 10 is also provided with a connecting line 26 for connecting together
the
outlets 18 of the sampling lines 14: The connecting line 26 has a main
regulated
outlet 28 for providing a fluid sample to the processing apparatus 30. The
fluid
sampling system 10 is also provided with control means for controlling
pressures
s between orifices 20, 22 of all of the sampling lines 14 by means of the
controllable
derivation lines 24, thereby increasing pressure between orifices 20, 22 of
one of the
sampling lines 14 which is then selected to provide the fluid sample to the
outlet 28
of the connecting line 26, and decreasing pressure between orifices 20, 22 of
remaining sampling lines 14 to back purge the remaining sampling lines 14. In
other
1 o words, by controlling the pressure at the connecting point located between
first and
second orifice 20, 22, the fluid of one sampling line 14 is directed to the
fluid
processing apparatus 30 while the fluids of the remaining sampling lines 14
are
vented out through their respective derivation lines 24.
15 As illustrated in this preferred embodiment, the control means may
advantageously comprise a plurality of pressure regulators 40 respectively
connected
to the derivation lines 24 for regulating pressure therein, thereby
controlling the
pressures between orifices 20, 22 of the sampling lines 14. Moreover, the
control
means may advantageously further' comprise an outlet pressure regulator 42
2 o connected to the main outlet 28 of the connecting line 26 for regulating
pressure
therein, thereby controlling an output pressure at the main outlet 28.
Preferably, the
outlet pressure regulator 42 and each of the pressure regulators 40 comprises
a
back pressure regulator 44 having a discharge side connected to a vent line
46. Also
preferably, the control means further comprise a control unit 54 operatively
2 s connected to the outlet pressure regulator 42 and to each of the pressure
regulators
40 for controlling the output pressure at the main outlet 28 and the pressures
between orifices 20, 22 of all of the ampling lines 14. Also preferably, the
connecting line 26 of the fluid sampling system 10 is loop-shaped so that it
allows an
equal purging time for any of the sampling channels 12.
Stilt referring to FIGURE 2, the following discussion explains in more details
how the fluid sampling system 10 can be operated. Let's assume for the sake of
CA 02463247 2004-04-05
discussion that P1-N is the inlet pressure of sampling line N. P2-N is the
pressure at
the connecting point between orifices 20, 22 and P3-N is the autlet pressure
of
sampling line N. So P1-1, P2-1, P3-1 are the pressures for channel one and P1-
2,
P2-2, P3-2 for channel two and so on. Orifices 20, 22 of channel one are
respectively
5 R1 and R2, orifices 20, 22 of channel two are respectively R3 and R4, and
orifices
20, 22 of channel three are respectively R5 and R6. Let"s now assume that one
likes
to sample channel one. To do so, the following initial condition must be
respected:
P1 > P2 > P3 for channel one, while P1 > P2 < P3 for all other channels. When
this
condition occurs, the process fluid flows through R1 arid R2 and then through
the
l o loop-shaped connecting line 26 and the analytical system 30. At the same
time, the
selected process fluid from sampling channel one flows back in all orifices
22, R4
and R6, of the remaining channels 12 for back purging them and eliminating the
undesirable effects of dead volume.
~.5 According to a typical application for the measurement of N2, O2, H20
particles, etc... by a gas process analyzer, or for directing a process gas to
an APIMS
(Atmospheric Pressure Ionization Mass Spectrometer) or Hydrocarbon
measurement, the selection of the hardware of the fluid sampling system 10 is
determined in view of the characteristics of the application, i.e, sampling
line inlet
2 o condition required to operate the analytical system 30, the numbers of
sampling
channels 12, gas type and the minimum vent flow required to avoid atmospheric
air
back diffusion into the system. Thereinafter; there is described a typical
step-by-step
procedure to determined the specification of orifices 20, 22 of the fluid
sampling
system 10.
Typically, the request comes from a customer who needs a high performance
analytical system. Most of the time; such system will be used where the level
of
impurities to be measured are very low, i.e. ppt (part per trillion), ppb
(part per billion)
or sub ppm (part per million) levels. Such high purity gases are mainly used
in the
3 o electronic manufacturing industries; but it should be understood that the
fluid
sampling system 10 of the present invention may also be used in other fields
of
application. For example, a gas producer will use the system for process
control or
CA 02463247 2004-04-05
11
for quality control of the final product. In such application, the impurities
to be
measured can be H2, Ar, Oz, N2, CH4, CO, C02 and non methane Hydrocarbons
(NMHC) in H2, 02, N2, Ar or He. Typically the analytical system is a high
sensitivity
gas chronograph using different types of detectors manufactured by various gas
s chronograph companies. Typically, such gas chronograph will conveniently
perform
with a sample inlet pressure of 35 Kpag and a sample flow therein of 75 sccm.
These
are typical values for many gas analysers but it should be noted that other
values
could also be conveniently used. Let's also assume that one needs to measure
an
argon gas background. Based on these data, orifices 20, 22 have to be sized in
to relation to pressures and flows of this application. It should also be
noted that
designing a system for argon will also work for other gases of similar
viscosity like 02,
N2, etc ... . So, these values given above are the first operating parameters
of
specifications, but other values could also be convenieptly envisaged as well
known
in the art.
Referring now to FIGURES 3 and 4; there is shown a two-channels fluid
sampling system 10 according to another preferred embodiment of the present
invention. FIGURE 3 shows different fluid directions and pressures when
channel
one is selected while FIGURE 4 shows them when channel two is selected. As
2 o already defined, P-out is 35 kpag and inlet flow to the analytical system
30 is 75
sccm. To maintain the output pressure at 35 Kpag, a back pressure regulator 44
is
used. Its set point is set at 35 Kpag. The fact of adding this back pressure
regulator
44 introduces a dead volume that is between the connecting line 26 and the
outlet
back pressure regulator 44: So to eliminate the undesirable effect of this
dead
2s volume a purge flow has to be performed constantly. It is welt known from
people
involved in the art that a line That vents some gas to atmosphere is subject
to
atmospheric air back diffusion into it. Air back diffusion will be less if the
inside
diameter of the line is small and the length of the line is long. Typically a,
tube having
an outer diameter of 1116" and an internal diameter of 0.030" will need 20
sccm of
3 o gas flowing out of it to prevent atmospheric air back diffusion. So, to be
safe, one can
choose the double i.e. 40 sccm. This new flow through the outlet back pressure
regulator 44 leads to an extra 40 scan. This results to a total preliminary
flow from
CA 02463247 2004-04-05
12
the selected channel 12, which is channel one, be equal to 75 sccm+ 40 sccm,
i.e.
115 sccm.
Still with reference to FIGURE 3, the unselected channel is back purged with
s the fluid sample coming from the selected channel. At all time, there is
some flow in
the derivation line 24 to prevent any inboard contamination and again to
eliminate
the effect of dead volume caused by this connecting point. The back purge flow
of
this unselected channel will be defined by second orifices 22, P-out and P2-N
of the
unselected channel. In fact, by adjusting P2-N, the back purge flow of the
unselected
Z o channel may be adjusted as required. This back purge flow may be increased
when
the user has just switched channels to provide faster purging. This back purge
flow
may then be reduced to a convenient value just to provide enough flow to
eliminate
the undesirable effect of dead volume and to avoid air back diffusion.
15 From the above, one knows that the second orifice 22 of the selected
channel
must have a flow of 115 sccm plus some extra to purge the unselected channels.
The forward flow through orifice 22 of the selected channel is fixed by P2-1
and P-
out. One may set the inlet sample pressure value as needed since, in the field
process, pressure is generally high and has to be reduced by an adjustable
sample
2 o point pressure regulator. The sampling line pressure must be set just high
enough
to provide various flows to the fluid sampling system 10. This allows a higher
velocity
through the sampling line 14, thereby increasing speed of response to process
change and reducing adsorption effect. So one can set intuitively or
arbitrarily the P2-
1 pressure at 75 Kpag as starting point. This gives a pressure differential of
40 Kpa
2 s on orifice 22 of the selected channel. Back purge flow through it may be
easily
determined by simply looking at FIGURE 7, as will be described thereinafter.
From these data's, orifice 22 of the selected channel is tuned to have a flow
of 115 sccm at a pressure differential of 40 Kpa with inlet set at 75 Kpag and
outlet
3 o set at 35 Kpag. When the orifice 22 of the selected channel is tuned, one
performs
flow measurement at various pressure differential to generate a flow curve.
This
curve is shown in FIGURE 7. This is the flow characteristic for orifices 22:
In FIGURE
CA 02463247 2004-04-05
13
6, measurements are done with a back pressure equal to atmospheric pressure
while
in FIGURE 7, measurements are done with 35 Kpag back pressure, the required
pressure for the analytical system 30.
s Now, in order to have a back purge flow through orifice 22 of the unselected
channel, the P2-2 pressure must be lower than P-out. By looking at the curve
illustrated in FIGURE 7, one can see that a pressure differential on orifice
22 of the
unselected channel of about 15 Kpag results in a flow of 45 sccmlmin. This is
more
than enough to eliminate the effect of dead volume. So, for the unselected
channel
1 o two, the back pressure regulator PC-2 is set to have a pressure of 20 Kpag
at P2-2.
Now this leads to a total final forward flow through orifice 22 of the
selected
channel being equal to 75 + 40 + 45, i.e. 160 cc/min. From FIGURE 7, 160
cclmin
is obtained with a pressure differential of 70 Kpa. So this means that PC-1
1~ backpressure regulator will have to be set at 105 Kpag.
The following steps are devoted to selections of orifices 20. So, we know that
we must keep some flow through the derivation line 24 and back pressure
regulator
PC-1. This flow will be supplied by the corresponding orifice 20.
By setting inlet pressure to 170 Kpag and P2-1 at 105 Kpag, one settled the
flow into the derivation line to simply avoid atmospheric air back diffusion
to avoid
contaminating the sample. As mentioned before the sytem is built with a
1116"O.D.
tubing having an internal diameter of 0:030". So a vent of 50 cclmin is a very
safe
2 s value and allows some room for adjustment, as well known by a person
versed in the
art.
This leads that orifices 20 each must have a flow of 160 cc/min + 50 cclmin
at a pressure differential 65-Kpa, i.e. 170 Kpag -105 Kpag. So, these orifices
20 are
3 o tuned for these parameters and their flow curve done at many points. This
gives the
curve shown in FIGURE 9. The flow through orifice 20 of the unselected channel
is
shown from the curve illustrated in FIGURE 8. This is the flow curve of the
orifice 20
CA 02463247 2004-04-05
r~ .
14
done with a back pressure of 20 Kpag and a pressure differential of 150 Kpa
i.e. inlet
pressure of 170 Kpag less 20 Kpag.
Now, if one has to design a system with more sampling channels 12, one may
adjust the system inlet pressure to allow enough gas flow to the unselected
channel
or readjust P2-N, or simply re-size the orifices 22. This is more a rule of
thumb, than
a strict mathematical relationship and this is well known in the art. The idea
is to have
enough flow in all parts of the system to eliminate the effect of any dead
volume and
prevent air back diffusion. Furthermore, if one allows more flow through PC-3,
it
1o automatically allows room for variation in back purge flow of unselected
channels.
For example,:one may want to add one or more sampling channels to an existing
system wherein orifices 20, 22 have already been sized. The flow through PC-3
can
then be modified in acting on PC-3 while substantially keeping the output
pressure
constant for a convenient working of the analytical systerr~ 30.
Each system can be tuned for a particular application to take into account the
molecular weight of the gas. (f a system is tuned for Argon and H2 is used
instead,
the volumetric flow of all vents and back purge will be higher. Pressure may
be
reduced to reduce total volumetric flow or maintained at same point if gas
supply is
2 o not an issue.
Referring now to FIGURE 5, there is shown another preferred embodiment of
the fluid sampling system 10 of the present invention. In this configuration,
the
pressure controllers PC-1, PC-2, PC-N have been replaced by a combination of a
2 5 first calibrated derivation orifice 48 connected in series with an on-off
valve 52 and
these two elements are connected in parallel with a second calibrated orifice
50.
When the on-off valve for channel one is closed, there is no flow through
orifice 48,
only in orifice 50. Orifice 50 is tuned to have a flow of 50 cclmin at 105
Kpag like
derivation line flow of sampling channel one in FIGURE 3: All other channels
have
their on-off valves opened. So their flow in their derivation line 24 is the
sum of the
flow through their orifices 48 and 50. Both orifices 48 and 50 are tuned to
have a flow
of 436 cc/min at 20 Kpag. This is the same derivation line flow of the
unselected
CA 02463247 2004-04-05
channel two of FIGURE 3. So all channels having their on-off valves opened
have
P2 pressure less than P3. In this condition, their orifices 22 are backpurged
and their
inlet process fluid is vented through: the derivation line 24. The channel
having its on-
off valve closed has P2 pressure higher than P3. So, its process fluid is
forced
s through the connecting line 26 and to the analytical system 30.
According to another aspect of the present invention and with reference to
FIGURES 2 to 5, there is also provided a fluid sampling method preventing
cross-
port flow contamination between various sampling lines and not contaminating
the
1 o sample by the product that is gassed out or adsorbed by the control means
used to
control flows of process fluids. Such a method can be used to sample a liquid
or a
gas. Accordingly, the method comprises the steps of:
a) providing a plurality of sampling channels 12, each of the sampling
15 channels 12 comprising a sampling line 14 having an inlet 16, a regulated
outlet 18,
and first and second calibrated flow orifices 20, 22 connected in series
between the
inlet 16 and outlet 18;
b) connecting together the outlets 18 of the sampling lines 14 to provide a
main regulated outlet 28;
2 o c) providing fluids to the inlets 16 of the sampling lines 14;
d) deviating in a regulated manner the fluids from points located between the
first and second flow orifices 20; 22 of the sampling lines 14; and
e) after the steps a), b), c) and d), increasing pressure between first and
second orifices 20, 22 of one of the sampling Fines 14 which is then selected
to
2 s provide a fluid sample at the main outlet 28; and decreasing pressure
between first
and second orifices 20, 22 of remaining sampling lines 14 to back purge the
remaining sampling lines 14.
Preferably, the step a) comprises a step of calibrating the orifices 20, 22.
3 o More preferably, the step of calibrating further comprises a step of
generating a
calibration curve for each of the orifices 20; 22, as explained therein above.
Also,
CA 02463247 2004-04-05
16
preferably, in the step e), the increasing and decreasing of pressures is
performed
according to the calibration curves.
Although preferred embodiments of the present invention have been
s described in detail herein and illustrated in the accompanying drawings, it
is to be
understood that the invention is not limited to these precise embodiments and
that
various changes and modifications may be effected therein without departing
from
the scope or spirit of the present invention.
