Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02340514 2001-03-12
1
FLUID SAMPLING DEVICE
FIELD OF THE INVENTION
The present invention relates to the field of fluid sampling and concerns a
sampling device or system particularly adapted for use in a gas analysis
process.
BACKGROUND OF THE INVENTION
In all fields using a gas 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 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 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. With these sampling systems, contamination
of
a sample often occurs by mixing it with previously selected samples, leaks in
or
out of the sampling system or leaky valves.
FIG. 1(prior art) illustrates a sampling process frequently used in older
systems and is still in use today. In this system 10, various sample lines 12
made
of various tubing material each bring a corresponding sample to the apparatus
sample inlet 14. A plurality of sampling locations may be provided, as
required by
the process to be monitored. A bypass rotometer 16 is provided in each line,
for
purging a given sampling line 12 when not selected. The rotometer 16 allows
fixing
of a bypass flow and preferably sets a high flow in the sample line 12 to
speed up
the purge time. The excess flow is vented out of the system. A female quick
connector 18 is provided at the extremity of each sampling line 12 and is
adapted
to receive a male quick connector 20 allowing the sample to flow through the
flexible line 22 up to the analytical system 24. To change the selected sample
line
CA 02340514 2001-03-12
2
12, the male quick connector 20 needs to be removed from the female quick
connector 18 and inserted in another one. This system makes sure that there is
no
sample cross contamination from various sample points, since the sampling
lines
12 are physically isolated. However, this system has serious drawbacks. First,
each time the male quick connector 20 is disconnected from a female quick
connector 18, the gas flow to the analytical system 24 is momentarily
interrupted.
Some analytical systems 24 are affected by the sample flow variation. Also the
female and male quick connectors 18 and 20 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 impurities in a particular background. Another drawback is
that
the quick connectors 18 and 20 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 tube is made of various plastic or
polymers
that exhibit too much permeation to 02 and H20 polluting the sample. When
flexible metal tubing is used, it must be replaced often since metal fatigue
due to
manipulation causes them to break.
FIG. 2 (prior art) shows another sample stream selection used in the
industry. This system 10 is similar to the one of FIG. 1, but uses instead of
quick
connectors, a rotary selection valve 26 well known in the industry and
available
from various manufacturers. This system 10 alleviates some of the drawbacks of
the previous one, but introduces cross port flow contamination that increases
with
time. If a sample line 12 has a higher pressure, it will leak through the
valve body
26 and pollute the stream being measured. This valve requires frequent
replacement. Furthermore, leaks can occur from the valve stem.
FIG. 3 (prior art) shows another system used in the industry. In this system
10 each sampling line 12 includes an ON/OFF valve 28 provided downstream the
bypass rotometer 16. However, this system introduces dead volume in the line
section 30 downstream the valve 28. 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
CA 02340514 2001-03-12
3
the response time of the system and causing drifting readings of the
analytical
system 24. Another source of unswept dead volume is the valve itself. The
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
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
system in service. There are some variations of the previously described
systems
but all have similar drawbacks.
Also known in the art is 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 1/4" 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. 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. Finally the
diaphragm
valves used in this system are costly and the total space required for this
system is
quite large.
Other related prior art systems include U.S. patents no. 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.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a fluid sampling system
that prevents cross-port flow contamination between various sampling lines.
CA 02340514 2001-03-12
4
It is another object of the invention to provide such a system that may be
manufactured inexpensively.
]t is a preferable object of the present invention to provide such a system
that is insensitive to valve leaks.
It is also a preferable object of the invention to provide such a system
allowing an evacuation of dead volume from the valves used.
The present invention therefore provides a fluid sampling system for
providing a fluid sample to a fluid processing apparatus. The sampling system
has
a sample outlet connected to said apparatus, and a plurality of sampling
channels.
Each of the sampling channels includes the following elements:
an inlet line receiving a fluid and an outlet line conveying the fluid to
the sample outlet;
a valve connected to the inlet line and to the outlet line, the valve
having a closed position preventing a fluid flow between the inlet line and
the outlet line and an open position allowing a fluid flow between the inlet
line and the outlet line;
a first purge line connected to the inlet line and purging fluid
therefrom, and a second purge line connected to the outlet line and purging
fluid therefrom; and
flow controlling means for controlling a fluid flow in the first and
second purge lines.
The fluid sampling system further includes a connecting line for connecting
the outlet lines of the sampling channels to each other and to the sample
outlet,
and valve controi means for selectively setting the valve of one of the
sampling
channels in the opened position and the valves of the remaining sampling
channels in the closed position.
Other features and advantages of the present invention will be better
understood upon reading the following description of preferred embodiments
thereof with reference to the appended drawings.
CA 02340514 2001-03-12
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(prior art) is a schematic representation of a first fluid sampling
system known in the art.
FIG. 2 (prior art) is a schematic representation of a second fluid sampling
5 system known in the art.
FIG. 3 (prior art) is a schematic representation of a third fluid sampling
system known in the art.
FIG. 4 is a schematic representation of a fluid sampling system according to
a first embodiment of the invention.
FIG. 5 is a schematic representation of a fluid sampling system according to
a second embodiment of the invention.
FIG. 6 is a side elevation view in transparency of a valve adapted for use in
the embodiment of FIG. 4.
DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Referring to FIG. 4, there is shown a system 40 according to a first
preferred embodiment of the invention. The system provides a fluid sample to
at
least one fluid processing apparatus, here embodied by analytical system 42.
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 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 system 40 includes a plurality of sampling channels 46. In FIGs. 4 and
5, only three sampling channels are illustrated, but it is of course
understood that
as many channels as needed may be provided. Each channel has an inlet line 48
receiving the fluid from the medium to be sampled, and an outlet line 50
conveying
this fluid to a sample outlet 44, connected to the apparatus 42. A valve
connects
the inlet line 48 to the outlet line 50. Each sampling channel 46 also
includes a first
purge line 54 connected to the inlet line 48, and a second purge line 56
connected
to the outlet line 50.
CA 02340514 2001-03-12
6
When the valve 52 is in a closed position, it prevents a fluid flow between
the inlet line 48 and outlet line 50. When opened, it allows such a fluid
flow,
effectively selecting the corresponding sampling channel for analysis. As with
the
prior art model shown in FIG. 3, the first purge line 54 purges fluid from the
inlet
line 48. The second purge line 56 purges fluid from the outlet line 50. Its
function
will be better explained later.
Each sampling channel 46 also includes flow controlling means, here
embodied by standard rotometers 58 ("falling ball" flowmeters), for
controlling the
fluid flow in the first and second purge lines 54 and 56. In this manner, the
fluid
flow in each of the inlet and outlet lines is also adjusted as needed. Any
other
devices allowing to visualize and adjust fluid flow may alternatively be used,
such
as for example electronic flow sensors and electronic proportional valves
connected to a microprocessor based system.
The system 40 further includes a connecting line 60 connecting the outlet
lines 50 of the sampling channels 46 to each other and to the sample outlet
44.
Preferably, the connecting line 60 has a first segment 62 connected to the
sampling channel's outlet lines 50, and a second segment 64 having one end 66
connected to the first segment 62 and another end 68 connected to the sample
outlet 44. The first segment may form a loop, as shown in FIG. 4.
To select a sample from a particular sampling line for analysis, the
corresponding valve 52 must be opened while the others are closed. For
example,
in FIG. 4, the valve of sampling channel 1 is open, while sampling channels 2
and
N are closed. For this purpose valve control means are provided, allowing to
selectively opening the valve 52 of one of the sampling channels 46 and
closing
the valves 52 of the remaining sampling channels. These control means may
include a manual actuator 74 provided on each valve 52 as shown in FIG. 5, or
an
electrical actuator 72 as shown in FIG. 4. The valves 52 may also be
pneumatically actuated. It is understood that any appropriate means of
controlling
a valve may be used in the present system without departing from the scope of
the
invention.
CA 02340514 2001-03-12
7
With particular reference to FIG. 4, the operation of this illustrated
embodiment of the invention will now be explained in more detail. The arrows
on
FIG. 4 indicate the gas flow paths in the system. In the present case, the
sample
valve 52 of sampling channel 1 is open, allowing a fluid flow therethrough.
The
fluid sample coming out of this valve flows through the corresponding outlet
line
50, sweeps the looped first segment 62 in both direction and is then directed
to
second segment 64 up to the sample outlet 44 and the analytical system 42
where
the sample is analyzed. The role of the loop in the first segment 62 is to
allow an
equal purging time for any of the selected sample channels.
Incoming fluid in the inlet line of the unselected channels is vented out of
the system through the fist purge line 54. This "bypass" purge line is
preferably
located as close as possible to the valve seat. For this purpose, the inlet
line 48
may pass through the valve body or, if valves are manifold mounted, it may be
welded on the manifold just under valve seat. This provision advantageously
allows the inlet portion of the valve body to be swept at all time by the
sample fluid.
The first purge line rotometer may be adjusted to set the bypass flow to the
proper
valve, in accordance with the length and pressure of the sample channel.
The outlet lines 50 of the unselected sampling channels 2 and N are
backflushed with the fluid coming out of the selected sampling line 1. This
has the
effect of purging out the system, through second purge line 56, any sample
previously selected, virtually eliminating the dead volume effect caused by
the
outlet lines of the sampling channels in previous systems.
Preferably, the second purge line of each sampling charinel is connected to
the outlet line of the corresponding sampling channel through the
corresponding
valve, so that it may purge dead volume from the valve body. A valve model
particularly adapted to such an embodiment is shown in FIG. 6. The valve 52
includes an inlet conduit 76 connected to the inlet line, and an outlet
conduit 78
connected both to the outlet line and to the inlet conduit 76. A plunger 80 is
translationally mounted in stem 81, for selectively blocking a flow of fluid
between
the inlet and outlet conduits 76 and 78. A purge conduit 81, defining the stem
of
the valve, is also provided and is connected to the second purge line and to
the
CA 02340514 2001-03-12
8
outlet conduit 78. The plunger 80 has a lower position where it blocks fluid
flow
from the inlet conduit 76, therefore closing the valve 52 as explained above.
When
the plunger 80 is in an upper position, fluid is allowed to flow from the
inlet conduit
76 to the outlet conduit 78, defining the open position of the valve 52.
In this embodiment, the backpurge fluid flow is brought to the second purge
line through the stem 81 of the valve. This feature of the invention is
advantageous
in that dead volume in the valve itself is also evacuated. Even more
advantageously, the plunger 80 preferably has an octagonal cross-section 84,
whereas the opening 82 has a circular cross-section 86, providing a backpurge
of
the valve stem 81 even when the valve 52 is open. In this manner, even when a
given sampling line is selected the dead volume in its stem 81 is pushed in
the
second purge line and vented out of the system. Preferably, the second purge
line
rotometer is set to purge many times per minute the dead volume in both the
valve
stem and the outlet line. A typical flow value for example ranges from 20 to
50
cc/min.
Referring to FIG. 5, there is shown a second embodiment of the invention
where the valves 52 are standard ON/OFF valves and the first and second purge
lines are connected through discreet "T" fittings. In this embodiment, the
first
segment 62 of the connecting line 60 is not looped but simply has various
connecting points to the sampling channels 42.
As shown in FIG. 5, a back pressure regulator 70 may be added in
connection to the second segment 64 of the connecting line to maintain
delivery
pressure constant. As mentioned, the valves could be manually, pneumatically
or
electrically actuated. The various rotometers can also be replaced by
electronic
flow sensors and electronic proportional valves and connected to a
microprocessor
based system. All the systems can also be mounted in a purged enclosure
eliminating any inboard contamination or making it usable in hazardous areas.
An important advantage of the system of the present invention is that
leaking valves can be tolerated since any leak from a closed valve will be
vented
through the second purge line. It will therefore not reach the connecting line
since
there is a net reverse flow through the outlet line. This effectively extends
the
CA 02340514 2001-03-12
9
usefulness of the valve before replacement. When the time comes to select
another sample, the previously selected one is first closed and the desired
one is
then open. Any type of gas or liquid may be sampled as long as they are
compatible with the hardware like tubing, valves, etc.
Of course various modifications could be made to the embodiments above
without departing from the scope of the invention as defined in the appended
claims.
