U.S. patent application number 09/849281 was filed with the patent office on 2002-12-26 for combustible gas measurement apparatus and method.
This patent application is currently assigned to PASON SYSTEMS CORP.. Invention is credited to Skupinski, Wiktor, Taylor, Brian.
Application Number | 20020194936 09/849281 |
Document ID | / |
Family ID | 4168894 |
Filed Date | 2002-12-26 |
United States Patent
Application |
20020194936 |
Kind Code |
A1 |
Taylor, Brian ; et
al. |
December 26, 2002 |
Combustible gas measurement apparatus and method
Abstract
Discloses a system for the analysis and measurement of selected
gases such as combustible gases contained in a gaseous sample to be
analysed including a mixing manifold arrangement to mix the sample
gas with a diluting gas to provide a constant gas output flow rate
to a detector system. The mixing manifold arrangement automatically
reconfigures itself to provide an optimal concentration ratio of
sample and diluting mixed gas to the detector system. A constant
sample gas input rate is preferred. Excess sample gas not required
to maintain a constant mixed sample output rate is discharged.
Inventors: |
Taylor, Brian; (Calgary,
CA) ; Skupinski, Wiktor; (Calgary, CA) |
Correspondence
Address: |
BLAKE, CASSELS & GRAYDON, LLP
45 O'CONNOR ST., 20TH FLOOR
OTTAWA
ON
K1P 1A4
CA
|
Assignee: |
PASON SYSTEMS CORP.
|
Family ID: |
4168894 |
Appl. No.: |
09/849281 |
Filed: |
May 7, 2001 |
Current U.S.
Class: |
73/863.03 ;
137/4; 73/23.31 |
Current CPC
Class: |
Y10T 137/0335 20150401;
G01N 33/0018 20130101 |
Class at
Publication: |
73/863.03 ;
73/23.31; 137/4 |
International
Class: |
G01N 001/00 |
Claims
I claim:
1. An apparatus for mixing gases comprising: (i) a manifold forming
an diluting gas inlet port, a sample gas inlet port and a detector
supply port all in common communication with each other; (ii) a
diluting gas flow control means operable to control a flow of
diluting gas through said diluting gas inlet port in response to a
first control signal; (iii) a sample gas flow control means
operable to control a flow of sample gas to said detector supply
port in response to a second control signal; and (iv) control means
to produce said first and second control signal for the respective
diluting gas and sample gas flow control means, whereby gases
supplied to said manifold are mixed therein and expelled through
said detector supply port in proportions set by said control
means.
2. The apparatus of claim 1 further including detector means in
communication with said detector supply port operable to produce
output signalling representative of the content of a selected gas
of a gas mixture passing therethrough.
3. The apparatus of claim 2 wherein said selected gas is a
hydrocarbon gas.
4. The apparatus of claim 2 wherein said detector is a catalytic
combustion detector (CCD).
5. The apparatus of claim 4 further including threshold means
operatively coupled to said control means whereby said control
means produces control signals for said sample gas and said
diluting gas flow control means to obtain an output signalling of
said detector means within an optimal output signalling level
range.
6. The apparatus of claim 5 wherein said control means sets the
flow of gas through said sample gas flow control means
corresponding to a selected gas flow through said diluting gas flow
control means.
7. The apparatus of claim 6 wherein said control means reduces the
flow of gas through said sample gas flow control means to discrete
predetermined levels between fully open and fully closed.
8. The apparatus of claim 5 wherein said control means operates to
increase the flow of gas through said diluting gas flow control
means for a selected gas flow through said sample gas flow control
means.
9. The apparatus of claim 8 wherein said control means increases
the flow of gas through said diluting gas flow control means to
discrete predetermined levels between fully closed and fully
open.
10. The apparatus of claim 5 wherein said control means operates to
increase the flow of gas through said diluting gas flow control
means and co-operatively to decrease the gas flow through said
sample gas flow control means.
11. The apparatus of claim 10 wherein said control means increases
the flow of gas through said diluting gas flow control means to
discrete predetermined levels between fully closed and fully open
and decreases the flow of gas through said sample gas flow control
means to discrete predetermined levels between fully open and fully
closed.
12. An apparatus for mixing gases comprising: i. a first manifold
forming an inlet port, a common port and a mixer port all in common
communication with each other; ii. a second manifold in
communication with said common port and forming a sample port and a
discharge port all in common communication with each other; iii. a
first gas flow control means adapted to couple to a first
pressurized gas supply and operable to control a flow of gas
through the inlet port of said first manifold in response to a
control signal; iv. a second gas flow control means operable to
control a flow of gas between said first and second manifolds in
response to a control signal; v. a third gas flow control means
operable to control a flow of gas through the discharge port of
said second manifold in response to a control signal; and vi
control means to produce a control signal for each said first,
second and third gas flow control means, whereby any gases supplied
to the inlet port and sample port are mixed and expelled through
said mixer port in proportions set by said control means.
13. The apparatus of claim 12 further including detector means in
communication with said mixer port operable to produce output
signalling representative of the content of a selected gas of a gas
mixture passing therethrough.
14. The apparatus of claim 13 further including display means to
produce a visually perceptible representation of the output
signalling of said detector means.
15. The apparatus of claim 13 wherein said selected gas is a
hydrocarbon gas.
16. The apparatus of claim 13 wherein said detector is a catalytic
combustion detector (CCD).
17. The apparatus of claim 16 further including threshold means
operatively coupled to said control means whereby said control
means produce s control signals for said first and second gas flow
control means to obtain an output signalling of said detector means
below a maximum output signalling level.
18. The apparatus of claim 17 wherein said control means operates
to reduce the flow of gas through said second gas flow control
means for a selected gas flow through said first gas flow control
means.
19. The apparatus of claim 18 wherein said control means reduces
the flow of gas through said second gas flow control means to
discrete predetermined levels between fully open and fully
closed.
20. The apparatus of claim 17 wherein said control means operates
to increase the flow of gas through said first gas flow control
means for a selected gas flow through said second gas flow control
means.
21. The apparatus of claim 20 wherein said control means increases
the flow of gas through said first gas flow control means to
discrete predetermined levels between fully closed and fully
open.
22. The apparatus of claim 17 wherein said control means operates
to increase the flow of gas through said first gas flow control
means and co-operatively to decrease the gas flow through said
second gas flow control means.
23. The apparatus of claim 22 wherein said control means increases
the flow of gas through said first gas flow control means to
discrete predetermined levels between fully closed and fully open
and decreases the flow of gas through said second gas flow control
means to discrete predetermined levels between fully open and fully
closed.
24. A method of measuring a gas mixture comprising: (i). receiving
a sample gas from a source at a predetermined sample gas flow rate;
(ii). supplying a gas mixture to a detector at a predetermined
detector supply gas flow rate; (iii) receiving a detector
signalling produced by a detector monitoring the gas mixture
supplied by step (ii); (iv) periodically comparing the received
detector signalling to a predetermined range; (v) mixing a supply
of diluting gas with a portion of the sample gas flow to supply the
gas mixture at the predetermined detector supply gas flow rate of
step (ii) to maintain the received detector signalling within the
predetermined range; 1. whereby the sample gas received in step (i)
is received at a predetermined sample gas flow rate and the gas
mixture supplied to the detector is supplied at a predetermined
detector supply gas flow rate.
25. The method of claim 24 further wherein the predetermined range
has an upper threshold.
26. The method of claim 25 wherein the upper threshold is 4
percent.
27. The method of claim 25 wherein the gas mixture supplied to the
detector is supplied at a gas flow rate obtained by mixing a supply
of diluting gas with a portion of the sample gas flow to supply the
gas mixture at a predetermined amount selected from one of: (1) 100
percent sample gas mixed with 0 percent diluting gas; (2) 50
percent sample gas mixed with 50 percent diluting gas; (3) 25
percent sample gas mixed with 75 percent diluting gas; (4) 12.5
percent sample gas mixed with 87.5 percent diluting gas; (5) 6.25
percent sample gas mixed with 93.75 percent diluting gas; (6) 3.125
percent sample gas mixed with 96.875 percent diluting gas; and (7)
0 percent sample gas mixed with 100 percent diluting gas.
28. The method of claim 25 further including the steps of: (i)
periodically comparing the received detector signalling to the
upper threshold and when the received detector signalling exceeds
the upper threshold: (i.1) reducing the portion of the sample gas
flow mixed with the supply of diluting gas when the periodic
comparison of the received detector signalling is above a
predetermined, and (i.2) increasing the supply of diluting gas;
whereby continuous receipt of sample gas is obtained at a
predetermined sample gas flow rate.
29. The method of claim 24 further wherein the predetermined range
has a lower threshold.
30. The method of claim 29 wherein the lower threshold is 1.5
percent.
31. The method of claim 29 further including the steps of: (i)
periodically comparing the received detector signalling to the
lower threshold and when the received detector signalling is below
the lower threshold: (i.1) increasing the portion of the sample gas
flow mixed with the supply of diluting gas when the periodic
comparison of the received detector signalling is above a
predetermined, and (i.2) decreasing the supply of diluting gas.
whereby continuous receipt of sample gas is obtained at a
predetermined sample gas flow rate.
32. The method of claim 29 wherein the gas mixture supplied to the
detector is supplied at a gas flow rate obtained by mixing a supply
of diluting gas with a portion of the sample gas flow to supply the
gas mixture at a predetermined amount selected from one of: (1) 100
percent sample gas mixed with 0 percent diluting gas; (2) 50
percent sample gas mixed with 50 percent diluting gas; (3) 25
percent sample gas mixed with 75 percent diluting gas; (4) 12.5
percent sample gas mixed with 87.5 percent diluting gas; (5) 6.25
percent sample gas mixed with 93.75 percent diluting gas; (6) 3.125
percent sample gas mixed with 96.875 percent diluting gas; and (7)
0 percent sample gas mixed with 100 percent diluting gas.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a system for the analysis and
measurement of selected gases and, more particularly, for
measurement of combustible gases such as hydrocarbon gases
contained in a gaseous sample to be analysed.
BACKGROUND OF THE INVENTION
[0002] There is a demand for information indicating the hydrocarbon
content of gaseous mixtures. For example, the return flow drilling
mud material discharged from an oil or gas exploration well can
contain entrained hydrocarbon gases. Detection and measurement of
the hydrocarbon gas content of the well return material can be used
to give an indication of when a certain zone is being penetrated in
the well drilling process. Such data can provide information to the
geology personnel on the drilling project to enable them to form an
assessment or provide an indication as to whether the well drilling
has hit a producing zone. In oil and gas exploration, the primary
hydrocarbon gas of interest is generally methane, although, under
certain drilling conditions, there is also interest in information
relating to some of the other hydrocarbon gases that may be
present.
[0003] The current state of the art uses a variety of apparatus and
methods to quantify and qualify the hydrocarbon content of a gas
sample, that is, to perform analysis of the sample. The simplest
types of apparatus to perform analysis of a gas sample, are systems
that use a "thermal conductivity detector" (TCD). Thermal
conductivity detectors are suitable when the gas to be analyzed by
the detector contains a known gas in a known carrier gas. This is
often referred to as binary analysis of gas. Every gas has a unique
thermal conductivity as one of its properties. Thermal conductivity
detection works best when the carrier gas and the sample gas have
very different thermal conductivities. Typically, the TCD detector
has a Wheatstone bridge arrangement where the detector element
manifests a decrease in resistance with increasing thermal
conductivity of the sample gas. By way of example, U.S. Pat. No.
3,683,671 to Van Swcray entitled Measuring System Including Thermal
Conductivity Detector Means provides an electrical circuit bridge
excited at one power node, by a clamped square wave arm at another
power node by a feedback circuit. The output of the circuit bridge
is fed to a demodulator to generate an output signal representative
of the sample being sensed.
[0004] A Wheatstone bridge and visual indicator in the form of
light emitting diodes in a gas analyser arrangement is disclosed in
U.S. Pat. No. 4,028,057 to Nelson. These detectors are used in gas
chromatography where a carrier gas that has a very high thermal
conductivity, such as helium, is used. When a sample that has a
much lower thermal conductivity than helium is introduced into the
carrier gas the output of the detector will show a change relative
to the amount of sample contained within the carrier. A thermal
conductivity detector can be confused, that is produce erroneous
output, if more than one type of sample gas is introduced into the
carrier gas. That is if the thermal conductivity detector is used
to analyse a gas mixture of multiple sample gases. For example, if
one of the sample gases has a higher thermal conductivity than the
carrier gas and the second sample gas has a lower thermal
conductivity than the carrier gas, then the detector output may not
even change for varying constituent gas compositions or
mixtures.
[0005] Thus, a thermal conductivity detector is not well suited to
analysis of hydrocarbon gases entrained in well returns for a
number of reasons. First, it is not feasible to transport large
tanks full of helium to the well site. Consequently, the carrier
gas that is generally used is air. Air has a thermal conductivity
of 1.00 and methane a thermal conductivity of 1.3. This means there
is not a very good signal to noise ratio between the air carrier
and the gas of interest, which makes a thermal conductivity
detector based instrument prone to drifting. Notwithstanding their
drawbacks, such thermal conductivity detectors are in use in
analyzers used in the oil well drilling industry. However, because
of the inherent limitations of using TCD detectors in these
environments, it is not uncommon to need to zero the baseline of a
TCD based system on an hourly basis. Automated baseline adjustment
apparatus have been proposed to compensate for temperature changes
in such systems. For example, the arrangement proposed by Hagen in
U.S. Pat. No. 4,817,414.
[0006] Also, thermal conductivity detectors are, by their nature,
sensitive to ambient temperature. Even a 1 degree shift in ambient
temperature will cause a noticeable shift in the baseline of a
thermal conductivity detector operating in this low signal to noise
ratio configuration.
[0007] Another, somewhat more sophisticated detection apparatus
employs a catalytic combustion detector (CCD) to detect the
presence of hydrocarbons. For example, U.S. Pat. No. 3,607,084 to
Mackey for Combustible Gas Measurement describes passing a stream
of gas containing the combustible gas analytes over a conductive
metal wire coated with a think catalytic coating which is at a
temperature at which oxidation of the gases is initiated. Numerous
other arrangements of CCD apparatus are known for example, U.S.
Pat. Nos. 4,045,177 to McNally, 4,072,467 to Jones, 4,111,658 to
Firth et al, 4,123,225 to Jones et al, and U.S. Pat. No. 4,313,907
to McNally are examples of such CCD detectors. CCD's are sensitive
to anything that is combustible and in an oil and gas well drilling
environment, hydrocarbon gases are the combustible gases that would
be encountered. This means a CCD can be used as to provide a
measurement of the total hydrocarbon content of a gas without
regard to the particular type of hydrocarbon gas. While a CCD will
respond to combustible compounds other than hydrocarbons, it is the
gaseous hydrocarbon compounds that will be of interest in the
sample gases recovered from the drilling mud in a well drilling
environment. A major problem with CCD's is their limited range. If
a CCD is subjected to explosive combustible gas concentrations,
that is concentrations between the upper and lower explosive limits
of that compound, they are destroyed as the gas actually combusts
and coats the detector surface with carbon, rendering it
ineffective after that point. For methane the lower explosive limit
is 5% in air. An air mixture containing methane gas concentrations
greater than the 5% lower explosive limit will result in a mixture
that becomes explosive.
[0008] To obtain the benefit of a stable baseline and wider range
of methane concentrations in a sample, two detector systems have
been produced. Current state of the art two-detector apparatus uses
a CCD sensor to around 4% concentration in the mixture. Above that
point, the sensor apparatus control turns off the CCD sensor and
passes the sensing over to a thermal conductivity sensor. A thermal
conductivity sensor, of course, has all of the problems as
described above. However, a major advantage of a two-detector
analyser is a more stable baseline.
[0009] A combined CCD and thermal conductivity analyzer has some
major drawbacks if a gas other than methane is present in the
sample to be analyzed. For instance, if C2 is the gas being
presented to the CCD, the CCD will detect its presence very nicely.
However, when the analyzer switches over to the thermal
conductivity detector, the C2 gas may not be detected at all. The
system will respond by switching back to the CCD which ultimately
causes the system to keep switching back and forth between the two
sensors and can result in the destruction of the CCD due to
exposure to explosive levels of C2 gas in the sample. An example of
a two-detector system is shown, for example, in U.S. Pat. No.
4,804,632 to Schuck et al which switches from one sensor to another
based on set sample temperatures and holding the sensing devices to
a preset temperature.
[0010] Another gas detection system using a CCD detector, operates
by diluting the sample with air when it exceeds 4% as shown, for
example, in U.S. Pat. No. 3,771,960 to Kim et al. Adding diluting
air to the sample allows such a gas detection system to use a CCD
sensor throughout the entire range. Generally, such gas detection
system apparatus provides preset ranges, for example 0% to 3% which
is the undiluted range and a second dilution range, for example 0%
to 100%. In one prior art arrangement, the dilution is accomplished
by using a manifold with orifices drilled into it that give
approximate volumes of gas for the dilution blending. An on/off
valve is used to control the diluting of the sample with air. This
system requires precise adjustment of needle valves in the factory
before being shipped. A problem with this dilution approach is that
gas concentrations vary considerably with pressure and temperature
and thus are very hard to control precisely enough to give an
accurate reading when there is a switch over from one range to the
other. In addition to the pressure temperature aspects of the
dilution blending problem, a further problem inherent in this
method is that the dilution is very hard to effect without either
reducing the sample drawn from the extraction device or increasing
the amount of sample passed through the detector.
[0011] In conventional combustible gas analysers, a constant flow
rate through the detector is maintained by reducing the amount of
sample drawn from the sample source or extractor. On the other
hand, where a constant flow rate from the sample source or
extractor is maintained, an increase in the flow rate through the
detector is caused by the air added to or blended with the sample
to produce the diluted mixture flowing through the detector.
Neither of these situations is optimal. Drawing less sample gas
from the gas trap or sample extractor can cause the concentrations
to rise as the gas trap is extracting gas from the drilling mud at
a certain rate. If the rate of sample extraction is suddenly
reduced, then there will be a build up of sample gas inside the
extractor. On the other hand, if the extraction rate is kept
constant, the addition of diluting gas will cause the volume of the
diluted sample gas mixture produced to increase with a
corresponding increase in the sample flow rate through the
detector. Changes in sample flow rates through a CCD detector will
consequently change the response of the detector, as the detector
response is dependent on sample flow rates to the detector. To give
accurate results, CCD detectors require a precise flow rate. In
operation, a CCD detector actually destroys sample that it comes in
contact with, so, at low flow rates, the readings will drop off as
there is more and more dead sample in contact with the
detector.
SUMMARY OF THE INVENTION
[0012] To overcome these shortcomings, in one of its aspects, the
invention provides a sample gas dilution system to control the
supply of a sample gas to a detector supply port for supply to a
sample detector system. The gas sample dilution system is arranged
with three gas flow controls. A sample gas flow control is provided
to control input sample gas flow to a detector supply port. A
diluting gas flow control is provided to control supply of a
diluting gas to the detector supply port and therefore control
blending of the sample gas with the diluting gas. An exhaust flow
control is provided to control an exhaust flow of excess sample gas
not required by a sample detector system coupled to the detector
supply port. A controller, such as a computer, provides the
settings of the flow controls. In the preferred manner of
operation, the controller operates the flow controls to keep the
input sample gas flow rate into the sample dilution system constant
and the gas flow to the detector supply port constant. That is, the
controller operates the sample gas flow control, the diluting gas
flow control, which controls blending of the sample gas with a
diluting gas supply, and the exhaust flow control which controls an
exhaust flow of the sample gas to maintain a constant input sample
gas flow rate from the gas sample source and a constant output flow
rate to the detector system. Excess sample gas not required for
supply to the sensor block of the sample detector system is
exhausted from the apparatus.
[0013] In the preferred embodiment, each gas flow control has a
proportional control valve responsive to a control signal to
control the flow of gas therethrough. Preferably closed-loop
controlled mass flow controls are utilized to facilitate precise
control of gas quantities and flow rates. In a closed-loop
controlled gas flow control, the gas flow control includes a flow
sensor to produce signalling representative of the gas flow rates
therethrough. The flow sensor provides a feedback signal that is
used in the control of the proportional control valve to facilitate
closed-loop control of the proportional control valve based on
feedback from the flow sensor.
[0014] In another aspect of a preferred embodiment of the
invention, the sensor block or detector system operates in
conjunction with the sample dilution system to allow for several
ranges to be implemented yet keep the signal to noise ratio from
the detector devices at optimum levels. One preferred embodiment
discloses ranges of 0% to 4%, 0% to 8%, 0% to 16%, 0% to 32%, 0% to
64% and 0% to 100%. An algorithm for automatic range selection
permits optimal sensor block utilisation with minimal user
intervention while providing an output representative of
combustible gas concentrations in the sample gas without the need
to configure or reconfigure the instrument manually.
[0015] In one of its aspects, the invention provides an apparatus
for mixing gases comprising a manifold forming a diluting gas port,
a sample inlet port and a detector supply port all in common
communication with each other. A diluting gas flow control means is
provided which is operable to control a flow of diluting gas
through the diluting gas port in response to a first control
signal. A sample gas flow control means is operable to control a
flow of sample gas to the detector supply port in response to a
second control signal. A detector means in communication with the
detector supply port is operable to produce output signalling
representative of the content of a selected gas of a gas mixture
passing therethrough. A control means is provided to produce the
first and second control signals for respective diluting gas and
sample gas flow control means whereby any gases supplied to the
manifold are mixed therein and expelled through the detector supply
port in proportions set by the control means.
[0016] In another of its aspects, the invention provides an
apparatus for mixing gases comprising a manifold forming a sample
gas inlet port, an exhaust port, a diluting gas inlet port and a
detector supply port all in common communication with each other. A
diluting gas flow control means is operable to control a flow of
gas through the diluting gas inlet port in response to a control
signal. A sample gas flow control means is provided to control a
flow of sample gas to said detector supply port in response to a
control signal. An exhaust gas flow control means is provided to
control a flow of gas through the exhaust port in response to a
control signal. A control means includes means to receive a
detector signal output, the control means produces a respective
control signal for the diluting gas flow control, sample gas flow
control and exhaust gas flow control means is included whereby a
constant rate of gas flow through said detector supply port is
obtained. The sample gas supplied to the sample gas inlet port and
the diluting gas supplied to the diluting gas inlet port are mixed
and expelled through the detector supply port in proportions set by
the control means responsive to a received detector signal
output.
[0017] And in yet another of its aspects, the invention provides a
method of measuring a gas mixture comprising: receiving a sample
gas from a source at a predetermined sample gas flow rate,
supplying a gas mixture to a detector at a predetermined detector
supply gas flow rate and receiving a detector signalling produced
by a detector monitoring the supplied gas mixture. Periodically the
received detector signalling is compared to a predetermined range.
A supply of diluting gas is mixed with a selected portion of the
sample gas flow to supply the gas mixture at the predetermined
detector supply gas flow rate and yet maintain the received
detector signalling within the predetermined range.
[0018] Preferred embodiments of the invention will now be described
with reference to the attached drawings. For convenience, like
reference numerals have been used to depict like elements of the
invention throughout the various drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic representation of an embodiment of a
sample dilution system control in accordance with the
invention;
[0020] FIG. 2 is a schematic representation of an embodiment of a
sample dilution system control of FIG. 1 including mass flow
sensors;
[0021] FIG. 3 is a flow chart representation of a control process
of the sample dilution system of FIG. 1;
[0022] FIG. 4 is a schematic representation of another embodiment
of a sample dilution system apparatus incorporating features of the
invention without an exhaust gas flow control.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] For the discussions contained herein, "flow controller" and
"flow control" will refer to any actuator used to regulate the flow
by volume or mass of a selected gas to a predetermined set point.
Preferably, the flow control or flow controller has a sensor, that
is, either a flow sensor or a mass flow sensor paired with the
actuator valve arranged and used in a closed-loop fashion. In the
arrangement of the measurement apparatus and method of operation of
it, the concepts of mass flow and volume flow presented herein are
used interchangeably. Gases supplied at a constant pressure can
provide a constant mass flow. At a constant temperature and
pressure the mass of a gas will be linearly proportional to its
volume, so using mass sensors or flow sensors accomplishes the same
thing. A sensor and actuator valve in a paired arrangement and used
in a closed-loop fashion can be used to regulate the flow of a gas
by volume or by mass. Thus it will be understood that mere
rearrangement of the relative positions of an actuator valve and
flow sensor in a gas path, or choosing a different control
algorithm does not depart from the spirit or scope of the invention
as defined in the claims appended hereto. Thus, in the discussion
that follows, "flow" will refer to any quantity of a selected gas,
measured by volume or mass.
[0024] FIG. 1 shows, in a schematic diagram form, an embodiment of
a sample dilution system of a gas analyzer in accordance with the
invention. The sample dilution system is provided with a source of
pressurized clean dry diluting gas, preferably air, for supply to
conduit 10. This air passes through a heater 12 to heat the air to
a predetermined uniform temperature as required, for example,
40.degree. C. The heated air is then passed to a regulator 14 to
obtain a predetermined uniform air pressure. The temperature and
pressure conditioned air is supplied to conduit 16. The gas sample
to be measured is supplied to sample tube 18 where it is delivered
to systems to condition the sample to obtain predetermined or
pre-set uniform properties. A heater 20 heats the sample to a
predetermined temperature, for example, 40.degree. C. A first
filter assembly 22 removes any particulate matter and airborne
liquid or condensed humidity from the sample. A suitable filter for
this purpose is a coalescing filter capable of removing 99.9% of
any oil or water droplets and particulate contamination, preferably
to the 0.01 micron level. A pump 24 is used to draw air from the
sample source into sample tube 18.
[0025] The sample discharged from pump 24 is supplied to conduit 26
and perturbations in pressure caused by operation of pump 24 are
absorbed by ripple chamber 28. The sample is then fed through a
dehumidifier 30 to dry the sample to a dew point approaching
-40.degree. C. A suitable dehumidifier is a counterflow exchange
membrane dryer fabricated from perfluorinated tetraflouroethylene
copolymer membranes, for example, Nafion (trademark) membrane tube
counter flow dryers available from Perma Pure Inc in the MD series
gas dryers can be used to dry the sample air. To operate the
dehumidifier 30, a source of dry conditioned air is provided by air
supply conduit 29 that interconnects the dry conditioned air
contained in supply line 16 to the dehumidifier 30. The dry air
supplied to dehumidifier 30 passes through an inner chamber or
annulus 31 of dehumidifier 30 in contact with the exterior surface
of the Nafion (trademark) membrane tube 33. The dry air picks up
moisture from the sample gas passing though the lumen of membrane
33. The moisture extracted from the gas sample by dehumidifier 30
into the counter flowing dry air flowing in annulus 31 is expelled
to the atmosphere by discharge line 35. The dried sample gas is
output from dehumidifier 30 into conduit 36.
[0026] A liquid filter 32, for example, a micro porous membrane
filter constructed from expanded polytetrafluoroethylene (for
example, Teflon*trade mark) is provided as a failsafe to remove any
particulate matter that may still be present in the sample stream.
A manifold 34, for example a T-junction, forms a sample port 39 to
receive the gas sample. Manifold 34 communicates the gas received
in sample port 39 to two ports each providing a path for the
filtered, dehumidified sample gas to flow along. A first port,
namely, a sample exhaust or discharge port 41, is coupled to a
surplus sample discharge line 37 and the other port, common port
43, is in communication with sample feed or detector supply line 38
to deliver the sample gas toward the detector system. Sample gas
flows through lines 37 and 38 are controlled by gas flow control
means 46 and 48 respectively.
[0027] Control of the flow sample gas through discharge line 37 and
sample feed line 38 is controlled by gas flow control means,
comprising a sample exhaust or discharge valve 46 and sample valve
48 respectively. These flow control means are each responsive to
signalling received from controller 44. In this manner, the sample
gas passing through sample feed line 38 is under complete control
of controller 44. To provide for more accurate and precise
operation and control of the gas flow control valves, a closed loop
feedback is preferably implemented as will be explained with
reference to FIG. 2. Controller 44 controls the flow of the gas
sample exiting from manifold 34 through lines 37 and 38
respectively. The sample flow rates through lines 37 and 38 are set
to provide for a uniform flow rate of the sample into manifold 34
through sample port 39 so as to provide a constant sample draw rate
from the sample gas source, for example, 500 ml/minute.
[0028] The rate of flow of the diluting air, that is the clean dry
air, in conduit 16 is controlled by a gas flow control means 52. A
gas flow control means 52 is operated in response to signalling
from controller 44 to control the rate of flow of the clean dry
diluting air through conduit 16. Manifold 54 forms an inlet port 51
coupled to conduit 16 carrying the diluting air. Inlet port 51 is
in communication with sample supply port 53 and detector supply
port 55, formed by manifold 54. The supply of diluting air in
conduit 16 and sample gas in conduit 38 are combined in manifold 54
and then supplied to the sensor block detector means 56 via
detector supply port 55.
[0029] It is preferable that the gas flow into detector sample
supply line 58 is constant to maintain a constant rate of gas flow
into the sensor block detector means 56. A constant rate of gas
flow results in a more reliable and repeatable reading from the
sensor apparatus. Controller 44 adjusts the mass flow rate, of the
diluting air by controlling the dry air gas flow control means
valve 52 and of the sample gas by controlling the sample valve gas
flow control means 48, to obtain a uniform mass flow rate of the
gas mixture into detector sample supply line 58. For example, flow
control means valves 48, 52 can be controlled to ensure that a
constant flow of gas at the rate of 500 milliliters per minute of
gas is presented to detector sample supply line 58. The gas present
in detector sample supply line 58 is heated to a uniform
temperature by heater 60, for example, to a temperature of
55.degree. C. The heated sample is then presented to sensor block
detector means 56, which produces an output representative of the
hydrocarbon gases detected in the sample. The sensor block output
is supplied to controller 44 on signal line 62 for processing in
controller 44. On processing, the controller 44 may output the
reading to display 47 for example, or, by supplying the reading in
data form on a communications link to a central or a remote
computer (not shown) for logging and display.
[0030] In the preferred embodiment, controller 44 operates to
control the sample valve 48 and air valve 52 such that the ratio of
sample gas to diluting is mixed at predetermined amounts. For
example, a first ratio when the sample gas is known to be less than
4% can be used for calibration. For calibration, a known gas
supply, for example 2.5%, is used and fed directly into the gas
sample tube 18. In this calibration configuration, controller 44
adjusts the valves of the sample dilution apparatus to provide 100%
of the sample volume and 0% of the diluting air volume to the
sensor block detector means 56. In this calibration configuration,
air supply valve 52 and discharge valve 46 are completely closed
and sample valve 48 is controlled to allow a fixed flow, for
example 500 ml/min. In this configuration of the dilution
apparatus, none of the sample gas is exhausted and no diluting air
is mixed with the sample gas before it enters the sensor block
detector means 56.
[0031] FIG. 2 shows, in a schematic diagram form, a preferred
embodiment of the sample dilution system of FIG. 1 that further
includes gas flow sensors.
[0032] Control of the flow sample gas through discharge line 37 and
sample feed line 38 is controlled by gas flow control means,
comprising exhaust discharge valve 46 and sample supply valve 48
respectively. These flow control means are each responsive to
signalling received from controller 44. In this manner, the sample
gas passing through sample feed line 38 is under complete control
of controller 44. To provide for more accurate and precise
operation and control of the gas flow control valves, a closed loop
feedback is preferably implemented. In this regard, a discharge
mass flow sensor 40 and a sample mass flow sensor 42, for example,
AWM Series Microbridge Mass Airflow sensors produced by Honeywell,
provide an output proportional to the gas mass flow through each
respective mass flow sensor. The outputs of mass flow sensors 40
and 42 are used to effect closed-loop control in a control loop. If
desired, closed loop control can also be implemented with suitable
processing in controller 44. Controller 44 sets the mass flow of
the gas sample exiting from manifold 34 through lines 37 and 38
respectively. The mass flow rates through mass flow sensors 40 and
42 are preferably selected to provide for a uniform mass flow rate
of the sample into manifold 34 to provide a constant sample draw
rate from the sample gas source, for example, 500 ml/minute.
[0033] The rate of flow of the diluting air, that is the clean dry
air, in conduit 16 is controlled by a gas flow control means 52.
Preferably, the flow of air through the actuator valve of gas flow
control means 52 is measured by a mass flow sensor 50 to obtain the
benefit of closed loop control. In one embodiment, controller 44
effects closed loop control, or, in another embodiment, a local
feedback loop controller can be used for closed loop control. A gas
flow control means 52 is operated in response to signalling from
controller 44 to control the rate of flow of the clean dry diluting
air through conduit 16. Manifold 54 forms an inlet port 51 coupled
to conduit 16 carrying the diluting air. Inlet port 51 is in
communication with common port 53 and detector supply port 55,
formed by manifold 54. The supply of diluting air in conduit 16 and
sample gas in conduit 38 are combined in manifold 54 and then
supplied to the sensor block detector means 56 via detector supply
line 58.
[0034] In the preferred manner of operation of the embodiments of
the invention depicted in FIG. 1 or 2, controller 44 operates to
control the sample valve 48 and air valve 52 such that the sample
air ratio is mixed at predetermined amounts. For example, a fixed
ratio can be configured and used for calibration when the sample
gas is known to be less than 4%. For calibration, a known
concentration gas supply is used, for example 2.5%, and fed
directly into the gas sample tube 18. In this calibration
configuration, controller 44 adjusts the valves of the sample
dilution apparatus to provide 100% of the sample volume and 0% of
the diluting air volume to the sensor block detector means 56. In
this calibration configuration, air supply valve 52 and discharge
valve 46 are completely closed and sample valve 48 is controlled to
maintain a fixed flow, for example 500 ml/min. In this
configuration of the dilution apparatus, none of the sample is
exhausted and no diluting air is mixed with the sample before it
enters the sensor block detector means 56.
[0035] Following is a Dilution Table, which sets out valve
configurations that are set in a preferred method of operating the
sample dilution apparatus. The sample dilution apparatus valve
configuration settings provide an optimal operating range for
supply of sample gas to the sensor block detector means 56. The
optimal operating range has an upper threshold or limit to ensure
that the maximum hydrocarbon gas concentration supplied to the
sensor block detector means 56 does not exceed a maximum threshold
concentration, for example, a 4% concentration. Also the valve
configuration settings of the optimal operating range provide a
lower threshold or limit which increases the mixing ratio of sample
gas to diluting gas when the predetermined minimum concentration of
sample gas supplied to the sensor block detector means 56 falls
below the lower threshold. Reducing dilution of the sample gas when
the detector sensor output falls below a predetermined threshold
facilitates obtaining accurate readings from the sensors.
1 Dilution Table Sample Sample MFS Air MFS Exhaust MFS range %
(ml/min) % (ml/min) % (ml/min) 1 <4% 100% (500) 0% (0) 0% (0) 2
4-8% 50% (250) 50% (250) 50% (250) 3 8-16% 25% (125) 75% (375) 75%
(375) 4 16-32% 12.5% (62.5) 87.5% (437.5) 87.5% (437.5) 5 32-64%
6.25% (31.25) 93.75% (468.75) 93.75% (468.75) 6 64-100 3.125%
(15.63) 96.875% (484.38) 96.875% (484.38) 7 zeroing 0% (0) 100%
(500) 100% (500)
[0036] Each row in the table is consecutively numbered and
identifies mixing ratios and gas mass flows for the particular
mixing configuration. Progressively increasing concentrations of
the hydrocarbon combustible gases in the sample are shown row 1
through 6 of the table. Row 7 shows a special zeroing setting that
completely closes off sample supply valve 48 thereby preventing any
sample from entering to the sensor block detector means 56.
[0037] Because CCD sensor elements may suffer damage or burn out
when the hydrocarbon percent gas concentrations are greater than
5%, the sample dilution apparatus is configured at start-up to the
maximum dilution setting, which is that configuration summarized at
row 6 of the Dilution Table. In the configuration of row 6, the gas
mixture supplied to the sensor block detector means 56 is diluted
to a maximum dilution of the sample and consequently supplies the
minimum amount of sample gas to the sensors.
[0038] In the configuration of row 6, 100% concentrations of
hydrocarbon gas in the sample tube 18 will provide no more than
3.125% concentrations of hydrocarbon gas to the sensor block since
3.125% of the sample gas is mixed with 96.875% of the diluting air
to provide a maximum mixed gas ratio of 3.125% to the sensor block.
Thus in this configuration, a sample gas concentration of 100%
hydrocarbon gas will result in a 3.125% concentration of
hydrocarbon gas provided to the sensor block detector means 56.
[0039] The hydrocarbon gas concentrations in the sample tube 18 can
be related to the hydrocarbon gas concentrations provided to the
sensor block as follows:
S=M*X
[0040] Where: X--is the gas concentration of the sample supplied to
the sample inlet tube 18
[0041] M--is the mixing ratio configured, and
[0042] S--is the gas concentration provided to the sensor block
[0043] FIG. 3 shows, in flow chart form, aspects of the preferred
manner of operation of the sample dilution apparatus controller 44.
Setting the maximum dilution setting shown in row 6 of the Dilution
Table is performed on sample start as depicted by process box 90.
In operation of the controller 44, when the readings in the sensor
block detector means 56 fall below a predetermined minimum
threshold, for example 1.5%, controller 44 configures the sample
dilution apparatus to reduce the mixing ratio, that is, to reduce
the amount diluting air mixed with the gas sample. With reference
to the Dilution Table, reducing the mixing ratio moves the mixing
configuration up one row, for example, from the row 6 configuration
to the row 5 configuration. On the other hand, when the mixed
sample gas concentration supplied the sensor exceeds the maximum
threshold concentration, the next higher mixing ratio is configured
by the controller 44. With reference to the Dilution Table,
increasing the mixing ratio moves the mixing configuration down one
row, for example, from the row 1 configuration to the row 2
configuration.
[0044] The lower sample concentration limit in the range indicated
in the Sample Range column of Dilution Table is simply a preferred
range and does not necessarily cause a reconfiguration to a lower
mixing ratio setting, i.e. moving up a row. The lower range limit
is arbitrary and it will be understood that the ranges can and do
overlap. Switching to a lower mixing ratio, that is moving up a row
in the Dilution table, should not occur unless the gas
concentration at the sensor block is less than 1.5% at the time of
the reconfiguration. If the mixed gas sample concentration at the
sensor block detector means 56 is below 2% before a switch to a
lower mixing ratio, this avoids providing too rich a mixture to the
sensor block detector means 56 at the reconfigured reduced mixing
ratio.
[0045] On commencement of sample reading, system start-up or after
system reset, the mixing ratio is set to the maximum dilution rate
by process box 90. A sample reading is obtained from the sensors,
as shown by process box 92 and the reading is output. Each time a
reading is output, the output reading takes into account the
configuration of the sample dilution apparatus to correct the
output amount to the reading obtained from the sensor based on the
formula S=M*X referred to previously. Process box 93 represents the
output of the reading.
[0046] The sample reading obtained is then tested against range
limits to determine if the sample dilution apparatus requires
reconfiguring. At decision box 94, the sample reading is compared
to an upper limit. If the upper limit is exceeded, the "Y" exit is
taken and the sample dilution apparatus is reconfigured to increase
the dilution amount as represented by process box 96 and the next
sample is then taken. If the upper limit was not exceeded, then the
"N" exit of decision box 94 is taken and the sample reading is then
compared to a lower limit at decision box 98. If the sample reading
is below the lower limit, the "Y" exit of decision box is taken and
the sample dilution apparatus is reconfigured to decrease the
dilution rate as depicted by process box 100 and then another
sample reading is taken.
[0047] To provide a higher degree of control over the hydrocarbon
gas concentrations provided to the sensor block, given that a
finite period of time will be required to reconfigure the sample
dilution apparatus (that is, reconfiguration is not instantaneous),
the rate of change of the sample readings can be monitored as well.
At decision box 102 the change in the current reading to the
previous reading is compared to a change limit. If the reading
change shows an increase which exceeds an increase rate limit, the
"Y" exit of decision box 102 is taken and the current reading is
then evaluated to determined if it is near the upper sensor limit
at decision box 104. If the reading is near the upper limit, the
"Y" exit is taken and the sample dilution apparatus is reconfigured
to increase the dilution amount as depicted by process box 106.
This would be equivalent to moving down to the next row in
reference to the Dilution Table.
[0048] Thus, each sample reading obtained is tested against range
limits to determine if the sample dilution apparatus requires
reconfiguring. When the sample gas concentration at the sensor
block detector means 56 is below the set minimum, controller 44
configures the gas dilution apparatus to mix less diluting air with
the sample. The switchover from one mixing ratio to the next is
controlled in response to the sensor reading data received from the
sensor block detector means 56. The sensor block is thus protected
from burnout that would be caused by any percent gas concentrations
greater than 5%. By switching over from one range to the other when
a predetermined threshold, as for example, a 1.5% threshold is
reached, hysteresis problems that might arise when a switchover
from one range to another are minimized. Controller 44 may also
include a sample readings derivative or differential factor to
switch from one range to another when readings appear to be rising
or falling quickly so as to ensure that an out of range condition
does not occur in sensor block detector means 56.
[0049] FIG. 4 shows another embodiment of a sample dilution system
incorporating features of the invention presented in a schematic
diagram form. In this embodiment, no exhaust port is provided in
manifold 54. The sample dilution air is supplied under pressure to
inlet port 51. The sample gas to be diluted is supplied to sample
port 53. The mixed gases exit detector supply port 55 for delivery
to the sensor block detector means 56.
[0050] In this embodiment, the gas flow rates must change to effect
dilution, and for that reason, this arrangement is not the
preferred arrangement. For example, the rate of sample gas flow
into sample tube 18 must decrease if the rate of mixed gas supply
to the sensor block detector means 56 is to be constant for all
concentrations of hydrocarbons in the sample gas. Or, in another
less preferably method of operation, the rate of mixed gas sample
flow into sensor block detector means 56 must increase if the rate
of sample gas flow into sample tube 18 is to remain constant for
all concentrations of hydrocarbons in the sample gas. The apparatus
is arrange such that a gas flow control means 52 is operated in
response to signalling from controller 44 to control the rate of
flow of the clean dry diluting air through conduit 16. Manifold 54
forms an inlet port 51 coupled to conduit 16 carrying the diluting
air. Inlet port 51 is in communication with common port 53 and
detector supply port 55 formed by manifold 54. The supply of
diluting air in conduit 16 and sample gas in conduit 38 are
combined in manifold 54 and then supplied to the sensor block
detector means 56.
[0051] It is preferable that the gas flow into detector sample
supply line 58 is constant to maintain a constant rate of gas flow
into the sensor block detector means 56. A constant rate of gas
flow results in a more reliable and repeatable reading from the
sensor apparatus. Controller 44 adjusts the mass flow rate, of the
diluting air by controlling the dry air gas flow control means
valve 52 and of the sample gas by controlling the sample valve gas
flow control means 48, to obtain a uniform mass flow rate of the
gas mixture into detector sample supply line 58. For example, flow
control means valves 48, 52 can be controlled to ensure that a
constant flow of gas at the rate of 500 milliliters per minute of
gas is presented to detector sample supply line 58. Similar to the
embodiment described with reference to FIG. 1, the gas present in
detector sample supply line 58 is heated to a uniform temperature
by heater 60, for example, to a temperature of 55.degree. C. The
heated sample is then presented to sensor block detector means 56,
which produces an output representative of the hydrocarbon gases
detected in the sample. The sensor block output is supplied to
controller 44 on signal line 62 for processing in controller 44. On
processing, the controller 44 may output the reading to display 47
for example, or, by supplying the reading in data form on a
communications link to a remote computer (not shown) for logging or
display.
[0052] In the preferred embodiment, controller 44 operates to
control the sample valve 48 and air valve 52 such that the sample
air ratio is mixed at predetermined amounts. For example, a first
ratio when the sample gas is known to be less than 4% can be used
for calibration. For calibration, a known 2.5% gas supply is used
and fed directly into the gas sample tube 18. In this calibration
configuration, controller 44 adjusts the valves of the sample
dilution apparatus to provide 100% of the sample volume and 0% of
the diluting air volume to the sensor block detector means 56. In
this calibration configuration, air supply valve 52 and discharge
valve 46 are completely closed and sample valve 48 is completely
open. In this configuration of the dilution apparatus, none of the
sample is exhausted and no diluting air is mixed with the sample
before it enters the sensor block detector means 56.
[0053] Below is a Constant Mixed Gas Sample Output Flow Rate
Dilution Table (CMGO Dilution Table), which sets out valve
configurations that are set in a preferred method of operating the
sample dilution apparatus. The sample dilution apparatus valve
configuration settings provide an operating range for supply of
sample gas to the sensor block detector means 56 to ensure that the
maximum hydrocarbon gas concentration supplied to the sensor block
detector means 56 does not exceed a 4% concentration. The valve
configuration settings provide a lower range, which ensures that
the minimum concentration of sample gas supplied to the sensor
block detector means 56 does not fall below a predetermined
threshold to facilitate obtaining accurate readings from the
sensors.
2 Constant Mixed Gas Sample Output Flow Rate Dilution Table Sample
Sample MFS Air MFS range % (ml/min) % (ml/min) 1 <4% 100% (500)
0% (0) 2 4-8% 50% (250) 50% (250) 3 8-16% 25% (125) 75% (375) 4
16-32% 12.5% (62.5) 87.5% (437.5) 5 32-64% 6.25% (31.25) 93.75%
(468.75) 6 64-100 3.125% (15.63) 96.875% (484.38) 7 Zeroing 0% (0)
100% (500)
[0054] Each row in the table is consecutively numbered and
identifies mixing ratios and gas mass flows for the particular
mixing configuration. Progressively increasing concentrations of
the hydrocarbon combustible gases in the sample are shown row 1
through 6 of the table. Row 7 shows a special zeroing setting that
completely closes off sample supply valve 48 thereby preventing any
sample from entering to the sensor block detector means 56.
[0055] Because CCD sensor elements may suffer damage or burn out
when the hydrocarbon percent gas concentrations are greater than
5%, the sample dilution apparatus is configured at start-up to the
maximum dilution setting, which is that configuration summarized at
row 6 of the CMGO Dilution Table. In the configuration of row 6,
the gas mixture supplied to the sensor block detector means 56 is
diluted to a maximum dilution of the sample and consequently
supplies the minimum amount of sample gas to the sensors. In the
configuration of row 6, 100% concentrations of hydrocarbon gas in
the sample tube 18 will provide no more than 3.125% concentrations
of hydrocarbon gas to the sensor block as 3.125% of the sample gas
is mixed with 96.875% of the diluting air to provide a maximum
mixed gas ratio of 3.125% to the sensor block. Thus in this
configuration, a sample gas concentration of 100% hydrocarbon gas
will result in a 3.125% concentration of hydrocarbon gas provided
to the sensor block detector means 56.
[0056] In another manner of operation, the controller 44 operates
to control the sample valve 48 and air valve 52 such that the
sample gas concentration in the mixer port 55 which supplies the
mixed sample gas to the sensor block detector means 56 provides an
optimal output, for example a 2.5% concentration. In this manner of
operation, the mixing ratio of sample gas to diluting gas is
continuously variable. The ratio of sample gas to diluting gas is
increased until the desired optimal output of the sensor block
detector means 56 is obtained. When the desired optimal output is
obtained, the mixing ratio of the sample gas to the diluting gas is
known, and, consequently, the concentration of the sample gas is
determined. Naturally the sample gas concentration may be below the
concentration necessary to produce the optimal output of the
detector sensor, in which case, the output of the detector sensor
will be correspondingly reduced.
[0057] Now that the preferred embodiments of the invention have
been described numerous changes and modifications may be made
thereto without departing from the spirit and scope of the
invention as defined in the claims appended hereto.
* * * * *