Note: Descriptions are shown in the official language in which they were submitted.
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GAS BLENDING SYSTEM
Back~round of the Invention
Field of the Invention
The invention relates to a gas blending system. More
particularly it relates to a system for the accurate mixing of a tracer gas
with a carrier gas.
Description of the Prior Art
An important industrial service carried out in a variety of
applications is a leak detection service for the detecting of leaks in
pipelines or other fluid r~s~ge or storage systems. Such service
typically utilizes a mass spectrometer or other devices to sarnple the
atmosphere surrounding a point in a system suspected of CO,~t~ i.,g a
leak. Helillm, which is commonly used for such purposes, is used
because it is rare in the atmosphere and rapidly dissipates into the
atmosphere. As a result, a leak in one area does not contribute to a
high leak reading in other areas, as can occur when other tracer gases,
such as fluorocarbons or sulfur hexafluorides are employed. However,
helium is expensive to use in its pure form so an inert carrier gas,
typically nitrogen, is mixed with the heliurn and then injected into the
system to be checked. While the proportions in which the helium will
be rnixed with nitrogen can vary, a typical heliurn/nitrogen lllixt~ e will
comrnonly be in the 1-2% range, although higher or lower proportions
of heliurn can also be employed. Such leak detection requires the
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accurate mi-xing of a tracer gas, such as helium, with a carrier gas, such
as ni~ogen. Two factors complicate such accurate mixin~ of the leak
detection gas components. First, the gas ll~ixl~e is used to pressurize a
system for leak testing. As a result, the pressure of said leak detection
gas is constantly ch~n ing. Secondly, the carrier gas, e.g. nitrogen, is
provided by a pumper unit. As a result, the flowrate of the carrier gas,
e.g. nitrogen, can easily be changed. In order to m~int~in a desired
concentration of the tracer gas in the carrier gas under such variable
circ lmctPnces, it is desirable to have a mixing system that would
respond to such change of carrier gas, i.e. nitrogen, flow and adjust the
tracer, i.e. helium, flow without the need for operator adjustments to
compensate for such variable conditions encountered in practical
cornmercial operations.
One approach taken in the art in light of such operational
problems was to provide an "automatic" control system using flow
meters, control valves and feedback controllers. In this approach, the
flow rate of the carrier gas is measured, and a microprocessor is used
to calculate the required flow of the tracer gas. The actual flow of the
tracer gas is measured and compared to said required flow. A control
valve is then operated in response to a feed signal to adjust the tracer
gas ~ow to match the required flow.
A second approach suggested in the art was to provide a
"manual" control system to achieve a steady state gas mixture. If the
carrier gas flow rate changes, the system is adjusted to change the
tracer gas flow rate. Such a system would require the use of two flow
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regulators and two back pressure regulators in conjunction with control
valves to provide steady state conditions during the desired gas
blending operation.
There ren ~in~, however, a desire in the art for further
advancement in the gas blending field. Thus, a lower cost, less
complex and more reliable system is desired than is provided by said
"automatic" control system or by said "manual" control system.
Increased reliability, decreased m~intçn~nce and ~e absence of a
power source req ~i~elllent vis-a-vis the characteristics of said
"automatic" control system would be advantageous, particularly for the
intended portable use in the field of such gas blending system for leak
detection services. The greater reliability of the "manual" control
system is desired, but with simpler operation providing automatic
adjustment of tTacer gas flow in response to va~ing carrier gas flow
and/or downstreàm pressure conditions.
- It is an object of the invention to provide an improved gas
blenAing system.
It is another object of the invention to provide a gas blending
system capable of accurately mixing tracer gas with a carrier gas, under
gas supply flow and pressure conditions subject to fluctuations,
without the need for complex and expensive control systems.
With these and other objects in mind, the invention is hereina~er
described in detail, with the novel features thereof being particularly
pointed out in the appended claims.
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Summary of the Invention
The invention employs a back pressure re~lt~tor on the tracer
gas line, with the control signal therefor coming from the carrier gas
line. As a result, the tracer gas pressure will match the carrier gas
pressure. Upon being piped into a common discharge manifold, with
properly sized orifices or valves in each line supplying the manifold,
the carrier gas and the tracer gas flows will always remain in the same
proportion independent of flow or pressure.
Brief Description of the Drawin~ ~
The invention is hereinafter described in detail with reference to
the accompanying single figure drawing that is a schematic flow
diagram of an embodiment of the gas blending system thereof.
Detailed Description of the Invention
The objects of the invention are accomplished by a low-cost gas
blen~ling system, as herein described and claimed, that is less complex
and more reliable than the "~l~tom~tic" control system ~felled to
above. It provides all the advantages thereof without the cost and
complexities of the flow meters, control valves and microprocessor
based control ~ltili7e-1 in said "automatic" control system. The gas
blended system of the invention is intended for portable use in leak
detection industrial selvice ac~vities calTied out at job sites in the field.
For such purposes, it offers the added advantages of increased
reliability, decreased maintenance and no source of power requirement.
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While the gas blending system of the invention has the reliability
of the "manual" system, it is a simpler, less complex system than said
prior art system, and is a full automatic system. Thus, while it is
simpler than the manual system, it provides ~lltom~tic adjustment of
tracer gas flow in response to valying carrier gas flow and/or
downstream pressure.
The essence of the invention, as indicated above, is the use of a
back pressure regulator on the tracer, i.e. helium, gas line, with the
signal therefore coming from the camer, i.e. nitrogen, gas line. In the
drawing, said back pressure controlling regulator, represented by the -
numeral 1, is positioned in tracer gas line 2 having a downstream flow
control valve 3. Tracer gas line 2 is piped for ~ow into the cornrnon
discharge manifold 4 from which blended gas is removed in line 5.
Carrier gas line 6 cont~inin~ fixed orifice 7 is also piped for f ow into
said manifold 4. Back pressure controlling regulator 1 is controlled by
signal 8 from pressure sensor 8A capable of sending a control signal
correspon-ling to the pressure in the flow line of said carrier gas
upstream of the positioning of flow control means, e.g. fixed orifice 7,
in the ~ow line for said second gas. As arranged, said back pressure
controlling regulator 1 in tracer gas line 2 senses the carrier gas
pressure in line 6 upstream of carrier gas orifice 7.
It will be understood that the relative flow co-efficient of valve 3
in tracer gas line 2 and of o~ifice 1 in camer gas lille 6 dete.l~ e the
proportion of carrier gas and tracer gas in the resulting blended
ll~ixlu~ e. Control valve 3 is adjustable so as to enable the proportion of
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- 6 -
tracer gas to carrier gas to be adjusted to any desired proportion of the
gases for any particular gas blending operation. However, once set, the
proportions of said gases will remain the same as flow and pressure
conditions change over the course of a given gas blending operation.
Those skilled in the art will appreciate that various changes and
modifications can be made in the details of the invention without
d~ ling from the scope of the invention as set forth in the appended
clairns. Thus, orifice 7 in carrier gas line 6 can be replaced by a
variable orifice, i.e. control valve. If control valve 3 in tracer line 2 is
employed, however, then the use of a variable orifice in carrier gas line
6 offers no real advantage. If control valve 3 is changed to a fixed
orifice in other embodiments of the invention, then the use of a variable
orifice, i.e. control valve, in carrier gas line 6 in place of fixed orifice 7,
would enable the tracer gas/carrier gas mixture ratio to be adjusted for
different application needs. However, thé installing of a variable
orifice in place of fixed orifice 7, in line 6, and a fixed orifice in place
of flow control valve 3 in line 2, would increase the overall cost of the
system, since the carrier gas is usually employed at higher flow rates
than pertain to said tracer gas level, hence, would require a larger and
more expensive variable orifice to be used in line 6 than would be the
case for the plefe"ed practice, as illustrated in the drawing, of using a
vanable orifice in tracer gas flow line 2.
It u ill be appreciated that valve 3 can be replaced by a fixed
orifice so that both lines 2 and 6 contain fixed orifices. The use of such
embodiments would reduce the cost of the gas blending system, but
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would elimin~te the adjustability feature of the ll~ixlure proportions
unless, in conjunction therewith, fixed orifice 7 in line 6 were changed
to a variable orifice as referred to above. The use of valves in both
lines 2 and 6 is also within the scope of the invention, but is not
necess~ry for the reasons above.
While this invention has been described with respect to the
blending of helium and ~itrogen as a commonly employed, desirable
and conveniently detectable tracer gas and inert carrier gas
combination, the invention can also be employed using other tracer
gastcarrier gas combinations, if convenient at a desired job site, and for
other purposes than for the leak detection services referred to herein.
The invention provides a low cost, reliable means for accurately
mixing a tracer gas with a carrier gas or car~ing out other desirable
gas blending operations. The system of the invention provides a
reliable system that, due to its less complex nature than previous
alternatives, is well suited for portable use in providing accurately
mi~ing of a tracer gas in a carrier gas for use of commercially important
leak detection operations.
