Note: Descriptions are shown in the official language in which they were submitted.
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10 TITLE
METHOD AND APPARATUS FOR PROCESSING
DILUTED FUGITIVE GASES
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is related to United States Patent
8,235,029 dated August 7, 2012.
INTRODUCTION
This invention relates to techniques used for
controlling and processing diluted fugitive gases used as a
supplemental fuel source for an engine.
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BACKGROUND OF THE INVENTION
The parent case referred to above describes the use of
fugitive hydrocarbon fuels which emanate from various sources
and which fugitive gas can be used as a supplementary or primary
fuel for an engine. In general, the previous applications focus
on using such fuels in their undiluted states; that is, without
the addition of air to the hydrocarbons.
The use of fugitive hydrocarbon gases with air mixed
with the hydrocarbon gas is also useful since, otherwise, such
diluted fugitive gases would be wasted. There are several
sources of fugitive gases where the gases are inherently diluted
such as building gases and gases from dehydrator units.
Additionally, sources of fugitive gases that might otherwise
provide undiluted gases, may have leaks that are difficult to
find or eliminate such as compressor crankcases and these gases
become diluted. Since no check or pressure relief valves are
required for the vent gas collection system used with diluted
fugitive gases, the component cost of a diluted fugitive gas
system is less an undiluted fugitive gas configuration.
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It would therefore be advantageous to allow
diluted fugitive gases to be used as a fuel. To that
end, it is required that the hydrocarbons in the gases
emanating from the sources of such diluted fugitive gases
be estimated.
Engines, turbines and heating units using
natural gas and other gaseous fuels are known and are
used extensively, particularly in locations where natural
gas production takes place. Such engines and turbines
range from 30HP to over 10000 HP and may conveniently be
used in powering gas compressors, pumps and electric
generators and which powered equipment is normally
associated with natural gas production. The heating
units are used in a wide range of industrial processes.
The natural gas or other gaseous fuel is introduced
directly to the cylinder of the natural gas engine or to
the intake manifold. A spark ignitor is typically used
to ignite the combustible natural gas and an air supply
adds the air necessary to support the combustion.
The gaseous fuel used for such engines,
turbines or heating units comes from a fuel source such
as natural gas and the air to support the combustion of
the gas comes from the atmosphere. Normally, the gaseous
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fuel is under pressure and appropriate ducting extends
from the pressurized fuel supply to the engine. A
carburetor, valves or an electronic control mechanism is
used to regulate the quantity of natural gas provided to
the engine and the quantity of air added to the natural
gas for efficient combustion.
Various production processes in natural gas
production result in losses of combustible gases. Such
gaseous losses typically occur from compressors,
particularly where the packing is old or otherwise
deficient, from pneumatic instrumentation utilising
natural gas, from initiating or starting engine procedure
using natural gas, from gas dehydration units, from
engine crankcases and from petroleum liquid storage
tanks. These gas losses, typically called "fugitive
and/or vent emissions", are usually passed to the
atmosphere or to a stack for burning. In either case,
they are lost and the energy content of these gases which
can be considerable, is similarly lost. It is
disadvantageous and energy deficient to lose these
fugitive or vent gases.
It is known to use natural gas as a
supplementary fuel for a diesel engine by adding natural
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gas to the intake air. This natural gas, however, is not a
fugitive or vent gas and the gas is maintained under pressure as
a normal fuel source. The use of such fuel does not lower costs
by using a fuel normally lost or deliberately discarded and such
a fuel is not an emission resulting from venting or escaping
gas. Fugitive gases have been collected and used as a fuel
source but such gases have been collected and put under
pressure. Such gases are not used as a supplementary fuel
source.
SUMMARY OF THE INVENTION
According to one aspect of the invention, there is
provided a method of using a composition of remotely located
hydrocarbon fugitive gases diluted with air as a supplementary
fuel for a natural gas engine, said composition of said
hydrocarbon fugitive gases and air used as said supplementary
fuel having an upper explosive limit and having a lower
explosive limit, said natural gas engine using said composition
of hydrocarbon fugitive gases and air from said remote location
as a supplemental fuel source and further having a primary fuel
source, said method comprising the steps of ensuring said
composition of all of said fugitive gases and air entering said
natural gas engine are outside said upper explosive limit and
said lower explosive limit and maintaining said total volume of
said composition within a predetermined percentage of the volume
of said primary fuel supplied to said natural gas engine.
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BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Specific embodiments of the invention will now be
described, by way of example only, with the use of drawings in
which:
Figure 1 is a diagrammatic illustration of a typical
building housing an engine and a compressor driven by the engine
and which illustrates various sources of fugitive combustible
gases which may be used as a supplementary fuel source for the
engine according to the invention;
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Figure 2 is a diagrammatic illustration of a
typical control circuit used to regulate the input of
fugitive combustible gases to the engine according to the
invention;
Figures 3A-3E diagrammatically illustrate
various control techniques when fugitive gases are used
as a supplementary fuel source for the engine according
to the invention;
Figure 4 is a table illustrating fugitive gas
emissions taken from various sources in a typical
operating environment during experimentation;
Figure 5 is a side diagrammatic view which
illustrates a building which encloses various sources of
fugitive gas emissions which pass into the atmosphere of
the building and are diluted thereby and which are
collected near the ceiling or upper portion of the
enclosed building according to a further aspect of the
invention;
Figure 6 is a diagrammatic schematic view
particularly illustrating a check valve positioned in an
exhaust stack according to the invention;
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Figure 7 is a diagrammatic schematic view of an
accumulator positioned between the exhaust stack of an
engine and the intake of the engine according to the
invention;
Figure 8 is an enlarged view of the check valve
of Figure 6 but illustrating the typical operation of
such check valve;
Figures 9A and 9B diagrammatically illustrate
spark ignited engines with a naturally aspirated engine
shown in Figure 9A and a turbo charged engine being shown
in Figure 9E;
Figure 10 is a diagrammatic view of a check
valve positioned in a fugitive gas vent which is used to
provide a positive pressure head for the fugitive gases;
Figure 11 is a diagrammatic view of an air
filter installed in the engine intake which creates a
negative pressure to allow the flow of fugitive gases
into the engine intake;
Figure 12 is a diagrammatic view of a duct fan
installed in a fugitive gas supply pipe which likewise
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creates a negative pressure and allows the fugitive gases
to be added to the engine air before or after an air
intake filter;
Figure 13 is a diagrammatic view of a venturi
positioned within the engine intake which creates a
negative pressure to draw fugitive gases into the engine
intake and subsequently the engine;
Figure 14A is a typical air pressure-fuel
pressure graph used for engine fuel and is of importance
when an eductor is used to reduce the pressure of the
fuel gas;
Figure 14B is a diagrammatic view of the fuel
and fugitive gas supply system when an eductor is used to
collect the fugitive gases and/or reduce the pressure of
the fuel gas;
Figure 15 is a diagrammatic view illustrating
fugitive gas flow and inlet air when fugitive gas flow is
generally steady;
Figure 16 is a view similar to Figure 15 but
illustrating a control system used for fugitive gas when
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the flow is unsteady;
Figure 17 illustrates a typical control using a
proportion, integral, derivative (PID) algorithm;
Figure 18 illustrates a control system used
with a governor feed forward when the fugitive gas flow
changes rapidly and therefore has the possibility of
changing engine RPM;
Figure 19 illustrates a control system used for
unsteady fugitive gas flow where control of a fuel valve
with feed-forward is not desired;
Figure 20 illustrates a cascade control
associated with a pressure PID and a flow PID having
quick response;
Figure 21A is a diagrammatic illustration of a
flow control using solenoid valves in parallel
association;
Figure 21B is a chart which illustrates the
various flow rates used with a combination of solenoid
openings and closings;
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Figure 22A illustrates a control system used
for fugitive gases diluted with air which control system
is intended to reduce rapid changes to the quantity of
fugitive gases supplied to the engine intake;
Figure 22B illustrates an oxygen PID used to
adjust the opening and closing of a control valve
controlling the quantity of fugitive gases supplied to an
engine;
Figure 23 illustrates a control used for an
eductor which control uses pressure sensors and pressure
set points;
Figure 24A is diagrammatic view of the diluted
fugitive gas being added to the air intake of the engine,
downstream of the engine air filter and the fugitive gas
inlet pipe is connected upstream of the vent atmospheric
outlet;
Figure 24B, appearing with Figure 31,
illustrates the lower explosive limit(LEL) and the upper
explosive limit(UEL) volume percentages for a number of
common gases;
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Figure 25 is a chart which illustrates the
valve flow coefficient percentage opening of the valve;
Figure 26 illustrates engine RPM as a function
of various time increments;
Figure 27 is a table used to calculate the
desired valve opening;
Figure 28 illustrates the piping setup when the
source of fugitive gas is instrument vent gases;
Figure 29 illustrates the piping setup when the
fugitive gas source is a dehydrator without a flash tank
separator;
Figure 30 illustrates the piping setup when the
fugitive source is a dehydrator with a flash tank
separator;
Figure 31 illustrates the piping setup where
the fugitive gas source is a diluted fugitive building
gas or a fugitive as from a coal mine; and
Figure 32 illustrates an example of flow
calculation.
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DESCRIPTION OF SPECIFIC EMBODIMENT
The terms "fugitive gases" or "fugitive
combustible gases" or "fugitive emissions" or "fugitive
gases" or "vent gases" or "vent emissions" are used
throughout this specification. The terms are used
interchangeably and, by the use of such terms, it is
intended to include combustible gases which escape from
various apparatuses or which are released deliberately
into the atmosphere. Such combustible gases normally
exist at or near atmospheric pressure in the vicinity of
the sources from where they originate. These fugitive
gases are intended to be collected and to be used as a
supplementary fuel supply for an engine which,
conveniently, uses natural gas as its primary fuel supply
and which natural gas is pressurized before entering the
engine. The various apparatuses from which the fugitive
gases may escape include compressor cylinder packings,
instruments, starting gas sources for the engine, gas
dehydration units, crankcases, petroleum liquid storage
tanks and the like.
Referring to the drawings, an engine is shown
generally at 101 in Figure 1. The engine 101 is
conveniently a natural gas powered engine normally
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located at a place of natural gas production. The engine
101 powers a compressor generally illustrated at 102.
The engine 101 and compressor 102 are normally located
within a building 100. As is usual, an outside location
for cooling apparatus 103 assists in drawing cooler air
or cooling water for cooling purposes.
A cabinet 104 for housing various
instrumentation used in support of the engine 101 and
compressor 102 is located near the engine 101. A
petroleum liquid storage tank 110 is also conveniently
located within the building 100.
Emissions of fugitive combustible gases are
shown as originating from four(4) sources in Figure 1. V,
represents the gases released from the petroleum liquid
storage tank 110. V,. and Võ leakages originate from the
compressor 102 which gases are routed into the petroleum
liquid storage tank 110 and leave with leakage V,.
Leakages Via and Vib represent leakages from the various
packings used to seal the compressor 102 thereby to
prevent the escape of gases. V2 represents the fugitive
emissions released from the crankcase of the engine 101.
V3 represents the gases released from the pneumatic
control of a control valve 105 and V4 represents the
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emissions released from the instrumentation used in
support of the engine 101 and compressor 102, housed in
cabinet 104.
Referring to Figure 2, the fugitive gases shown
as being emitted from various locations within the
building 100 of Figure 1 are collected into a collector
source 111 by way of appropriately sized and
appropriately located ducting, piping, tubing and the
like. These collected fugitive gases are fed into
ducting 131 extending to a diverter valve 112 which, in a
first configuration, passes the fugitive emissions to the
normal vent or stack 113 to bypass the engine 101 and
which, in a second configuration, pass the gases to a
flow meter 114 and thence to the air intake 120 of the
engine 101. The fugitive gases and the air enter the
engine 101 from the air intake 120 through a control
valve 133.
Fuel from the normal fuel source 121,
conveniently natural gas in the case of a natural gas
powered engine 101, passes to a fuel meter 122 and,
thereafter, to the engine 101 through a control valve
134. Combustion products from engine 101 are exhausted
through an exhaust stack 123. An exhaust analyzer 130
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may monitor the combustion products from the engine 101
passing through the exhaust stack 123.
Various control techniques are contemplated as
will be explained in greater detail. A control unit 124
is operatively connected to the fuel meter 122 and to the
valves 133, 134 which control unit 124 controls the
quantity of inletted fugitive gases and air and fuel from
the normal fuel supply 121, respectively. Exhaust
analyzer 130 may also be associated with the control unit
124. If, for example, the fugitive gases entering air
intake 120 and engine 101 provide increased richness in
the exhaust stack 123 as indicated by the exhaust sensor
130, the control unit 124 may adjust the quantity of air
passing through valve 133 thereby maintaining the proper
air-fuel ratio for efficient combustion within the engine
101.
OPERATION
With reference to Figures 1 and 2, the
operation of engine 101 is initiated and will be
operating with the normal fuel source 121 and the normal
air supply entering the engine 101. The emissions of the
fugitive gases from the various apparatuses 110, 101, 105
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and 104 as represented by VI, Võ V, and V4, respectively,
will be collected with appropriate ducting and piping at
fugitive emission collector source 111. The fugitive
gases are then conveyed to the air intake 120 of engine
101 through ducting 131, diverter valve 112 and flow
meter 114.
For safety reasons, the diverter valve 112 will
normally divert the fugitive gases through stack 113 when
the engine 101 is not running and the fugitive gases are
still being collected. Alternatively, a holding
container (not illustrated) may store the gases until the
engine 101 commences operation. Or, the fugitive gases
may be diverted to a flare stack (not illustrated) where
they are burned.
Following the startup of engine 101, the
position of diverter valve 112 is changed either manually
or otherwise, so that the fugitive gases flow directly to
the air intake 120 through ducting 132 and flow meter
114. Flow meter 114, located between the diverter valve
112 and the air intake 120, operates to measure the flow
of the fugitive gases entering the air intake 120. The
use of the fugitive gases operates to increase the fuel
supply which enriches the fuel flow to the engine 101
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thereby creating an increased engine speed. A governor
(not illustrated) for measuring and controlling engine
speed is operably connected to the engine 101 and the
valve 134. As the engine speed increases, the governor
will reduce the normal fuel supplied to the engine 101 by
way of partially closing valve 134. This will act to
reduce the normal fuel supplied to the engine 101 and
return the engine speed to that desired. The reduced
normal fuel supplied to the engine 101 will be replaced
with that energy supplied by the fugitive gases thereby
resulting in less use of normal fuel in the engine 101.
Depending upon the quantity of fugitive
emissions available, the rate of flow of such emissions
and the existing air-fuel control method for the
combustion process, a variety of control techniques are
available to adjust the normal fuel supplied to the
engine 101 when the fugitive gases are being used as a
supplementary fuel source.
For example and as previously described, an
exhaust sensor 130 may be operably associated with the
exhaust stack 123. The exhaust sensor 130 monitors the
components in the exhaust of exhaust stack 123. If the
exhaust sensor 130 senses hydrocarbon and/or oxygen
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content greater than desired, appropriate adjustment will
be provided to either the air or fuel supply, the
adjustment changing the percentage of hydrocarbons and/or
oxygen in the exhaust stack thereby contributing to
combustion of increased efficiency.
A further application utilises the techniques
disclosed in United States Patent 6,340,005 (Maim et al),
the contents of which are herein incorporated by
reference. The flow of the fugitive gases added to the
inlet 120 of the engine 101 may be measured by a flow
meter 114 as earlier set forth. As the rate of flow of
the fugitive gases increases, the rate of flow of the
normal pressurized fuel will decrease thereby causing the
normal control system based on the quantity of normal
pressurized fuel relative to the air supplied to deliver
too little air. By combining the fugitive gas flow with
the normal pressurized fuel flow, the control unit 124
will maintain the proper fuel-air ratio in engine 101 to
provide for appropriate and efficient combustion. Thus,
the normal fuel entering the engine 101 through fuel
meter 122 is replaced by the supplementary fuel supply
provided by the fugitive gas emissions and measured by
flow meter 114. The fuel flow meter 114 can also be
calibrated to ensure that the quantity of fuel added to
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the engine 101 by the fugitive emissions does not exceed
the fuel supply required by the engine 101.
Yet a further control application is
illustrated in Figure aA where manual control is used for
the fugitive gases entering the air intake 204. A
diverter valve 201 is provided which allows the fugitive
gases to pass to the normal fugitive gas vent or stack
202 which may vent or burn the fugitive gases. A control
signal 211 may provide that the diverter valve 201 pass
all fugitive gases to the stack 202 in the event there is
an engine failure or an engine shutdown. A three-way
manual valve 203 is provided downstream of the diverter
valve 201. This valve 203 provides for the entry of
fugitive gases to the air intake 204 of the engine and it
can be adjusted to regulate the quantity of fugitive
gases to the air inlet 204 and to the fugitive gas stack
202 through piping 210. A slow addition of fugitive
gases passed to the air intake 204 by adjusting valve 203
will minimize the engine speed change and will allow the
operator to manually adjust the air-fuel ratio to account
for the addition of the fugitive gases. When engine
operation ceases, a control signal 211 moves the diverter
valve 201 so that the fugitive gases vent to stack 202 in
the normal manner. The three-way valve 203 should be
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selected so that the flow path of the fugitive gases is
not blocked in any valve position which would stop the
flow of fugitive gases and contribute to pressure buildup
in the collection system 111 (Figure 2). This technique
is relatively simple and inexpensive and, under certain
gas flow conditions, it is contemplated that the diverter
valve 201 and the three-way valve 203 could be combined
into a single valve.
A further embodiment of the control technology
is contemplated wherein an exhaust gas sensor is provided
which initiates a signal related to the amount of oxygen
and/or unburned fuel in the combustion exhaust.
Normally, this technique would use the signal to control
the air/fuel ratio for the combustion. If the signal
advised that the mixture was too rich, the normal air
supplied to the engine would be increased and if the
signal advised that the air/fuel ratio was too lean, the
normal air supplied to the engine could be decreased.
Similarly, the proportion of fugitive gases could be
increased or decreased relative to the normal fuel entry.
This control technique is generally referred to a closed
loop air/fuel control.
A further control technique is illustrated in
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Figure 3B wherein automatic control of the three way
valve 203 is provided which allows the control system to
control the quantity of fugitive emissions diverted to
the combustion air intake 204. If the addition of
fugitive gases to the air intake 204 through valve 203 is
excessive thereby prohibiting the engine speed from
otherwise being automatically adjusted, a control signal
advises the three-way valve 203 that any excessive
quantity of fugitive gases are to be diverted to the
fugitive gas stack 202. In this embodiment, it is
contemplated that the diverter valve 201 could be deleted
with control of the fugitive gases provided wholly by the
three-way valve 203.
In yet a further control technique illustrated
in Figure 3C, a flow meter 220 is added upstream of the
engine air intake 204 and downstream of three-way valve
203 to measure the quantity of fugitive gases added to
the air intake 204. The information obtained from the
flow meter 220 can be used to determine general operating
characteristics and/or to determine the fraction of fuel
used by the engine which originates with the fugitive
gases. In this embodiment, the diverter valve 201 could
be deleted with control provided solely by the flow meter
220 which would provide appropriate control signals to
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three-way valve 203.
A further control technique using a combination
of fuel flow measurement and manual control for the
fugitive gases is illustrated in Figure 3D. In this
embodiment, as the fuel flow comprising normal fuel and
fugitive gases increases, the control system will
increase the air flow to the air intake 204. If the flow
of fugitive gases is relatively constant, following the
initiation of the fugitive gas flow, the control system
can be adjusted to compensate for the addition of the
fugitive gases. Diverter valve 201 ensures that the
fugitive gases are vented in the event of engine shutdown
or a safety hazard arising. Any changes in the rate of
flow of the fugitive gases will be done manually since no
automatic adjustment of the fugitive gas flow rate is
provided in this case.
A further control technique is illustrated in
Figure 3E. The control system 230 utilises fuel flow
measurement and fugitive flow measurement with the air-
fuel ratio being controlled by the rate of air flow to
the combustion process. To maintain the desired air-fuel
ratio, a fugitive gas flow meter 220 is required. The
fugitive gas flow measured by meter 220 is added to the
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normal combustion fuel flow value and the control system
230 will use the input from flow meter 220 to determine
the proper quantity of air to be added to the air intake
204. In the event, for example, that the fugitive gas
flow is small, the control system 230 is contemplated to
be sufficient to determine the correct air quantity
without the use of flow meter 220. For higher flows of
fugitive gases, however, the fugitive gas flow signal
from flow meter 220 can be used as a feed-forward signal
to adjust the combustion fuel control valve (not
illustrated) coincident with the addition or removal of
the fugitive gases. This fugitive gas flow value is
again useful for operating information and/or to
determine the fraction of fuel used by the engine which
may come from the fugitive gases.
In the interest of full disclosure, experiments
which have been performed by the applicant are set forth
below. In addition, the potential savings thought to be
achievable by using fugitive emissions as a supplement
fuel source are calculated. It is emphasized that these
experiments and calculated possible savings have not been
measured in a technically rigorous manner, nor have they
been corroborated. Rather, the experiments conducted and
the subsequent discussions based on those experiments are
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included here as being corroborative of the advantages
thought to be achievable only. Applicant would not want
to be bound by the experimental results given hereafter
if subsequent measurements or calculations are found to
be more precise or if subsequent experiments and
calculations adversely affect the results described and
the discussions based on those results.
EXAMPLE 1
For a typical 1000 HP natural gas engine, the
amount of methane used would be approximately 1000 x
7500/900 = 8300 scf/h = 139 scf/m where scf/m = standard
cubic feet per minute. The average packing leak as found
in reciprocating compressors is described in "Cost
Effective Leak Mitigation at Natural Gas Transmission
Compressor Stations", Howard at al, Pipeline Research
Council International, Inc., (PRCI) Catalogue No.
L51802e, the contents of which are herein incorporated by
reference. The measurements reveal that the leaks amount
to approximately 1.65 scf/m per rod packing. For a
four(4) throw compressor, this would amount to 6.60 scf/m
or 5% of the fuel required for the above-identified
engine. If natural gas is used for the pneumatic
instrumentation, gas venting can increase to 10 scf/m or
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more. In addition to packing leaks, other sources of
fugitive hydrocarbon gas emissions include the engine
crankcase, the compressor crankcase, glycol dehydrators,
petroleum liquid storage tanks, engine starting systems
and unit blow downs during gas venting operations.
EXAMPLE 2
Other sources of fugitive gases in a typical
operating environment such as the engine compressor unit
located within the compressor building 100 illustrated in
Figure 1 were measured. The results of those
measurements are given and set forth in Figure 4. The
vent flow measurements were taken by a rotometer which
was calibrated for air and then multiplied by a
correction factor for natural gas. It will be seen from
Figure 4 that the total estimated fugitive emissions by
the sources measured amount to approximately 9 scf/m
which is the value used in the calculations given
hereafter. It will further be noted the term 546 I/P
stands for a Fisher 546 model current/pressure
transducer. A current to pressure transducer (I/P) takes
a 4 to 20 ma control signal from the controller and
coverts it to a proportional gas pressure. This gas
pressure then controls a diaphragm on a control valve.
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The fuel flow consumption was approximately 138
kg/h at 932 rpm. The estimated load percentage based on
fuel was 72% by using the manufacturer's specifications
for the maximum load capacity and comparing it with the
actual load for the engine estimated from the operating
conditions. Using a fuel density of 0.79 kg/m3, the fuel
flow is (138 kg/h/0.790kg/m3) x (35.3 ft3/m3)/60 min/h =
103 scf/m. Thus, the fugitive emissions released at this
location amounted to approximately 8.7% (9 scf/m/103
scf/m = 8.7%) of the total engine fuel consumption.
EXAMPLE 3
A test was undertaken to add fugitive gases to
the engine inlet of a Waukesha 7042 GSI engine modified
for lean operation which powered a four(4) throw, two(2)
stage Anal JGK-4 compressor. Only the vent gas N14 from
the instrument cabinet 104 (Figure 1) was used. This was
so because the cabinet 104 used for housing the
instruments provided a convenient source for fugitive
gases from the instrumentation and a convenient place to
connect a rubber hose for conveying the gases first to a
three-way valve and then to the engine air intake. The
three way valve was positioned in the hose between the
cabinet and engine air intake thereby allowing the gas to
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be vented or directed to the air intake and which also
allowed a sample of the gas to be taken. A subsequent
gas analysis confirmed that the fugitive or vent gas
measured was principally a combustible hydrocarbon
mixture. The speed of the Waukesha engine was set to 932
rpm and the measured suction pressure at the compressor
intake remained relatively constant during the test,
ranging between 347 to 358 kPa, which confirmed the
relatively constant engine load during the test.
When the fugitive gases from the instrument
cabinet 104 were initially directed to the air intake of
the engine, the engine speed initially increased and then
recovered to the set point of 932 rpm. The fuel flow
recorded by the engine flow meter dropped from 126.6 kg/h
to 115.2 kg/h which indicated a potential fuel saving of
about 10 kg/h. The exhaust oxygen dropped from 7.6% to
6.6%. The air control valve was then adjusted to bring
the exhaust oxygen percentage back to the approximate
starting value. The decrease in fuel flow for the engine
operating with the same exhaust oxygen percentage was
(126.6 - 118.6) = 8 kg/h which was a decrease of
approximately 6.3%. To check this value, the gas flow
through the vent was measured at a value of 5.6 scf/m
(air) or 6.6 scf/m (gas). Converting this flow to metric
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mass flow gave a value of 8.8 kg/h. This correlated with
the decrease in fuel flow observed with fuel enrichment
by way of the fugitive gas supply to the air inlet.
EXAMPLE 4
Based on the measurements given above, the
savings in fuel would be in the range of CDN$20000.00 to
CDN$30000.00 per year for this engine. Since the
fugitive gas emissions are normally vented and lost, and
assuming the gas price of $5.00/GJ(Giga Joule) =
5.27/MMBTU(million British Thermal Units) =
$4.79/Mscf(thousand standard cubic feet) (GHV(Gross
Heating Value) = 1100 BTU/scf), the lost value of the
vented gas is CDN$3100/year for gas vented at 1 scf/m.
Thus, the value of the vented gas from the compressor
building alone was calculated to be approximately
CDN$25,000.00 per year.
EXAMPLE 5
In this case, the fugitive emissions were mostly
methane. These emissions contribute to greenhouse
gas(GHG)emissions. A calculation reveals that the
fugitive emissions and the engine CO, result in the
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equivalent or estimated GHG emission(CO2(e)) of 4900
Tonnes per year ( = CO2 mass/y + 21 x CH, mass/year). If
the fugitive emissions are used as fuel by the engine,
the CO2(e) would drop to 3010 Tonnes per year, a decrease
of 40% or 1890 Ply. Thus, this is contemplated to
provide a good technique for the reduction of greenhouse
gases.
Many modifications may readily be contemplated.
Although the teachings are specifically directed to a
natural gas engine where natural gas is used as the
normal fuel, the fugitive gases are contemplated to be a
useful supplementary fuel source for other engines,
including diesel and gasoline powered engines and
turbines. Indeed, with appropriate controls, it is
contemplated that the fugitive gases may be usefully
added as a supplementary fuel to virtually any device
using the combustion of air and fuel where the fuel may
be liquid or gaseous so long as the fuel is combustible.
In addition, although the invention has been
described as providing for the fugitive gases to emanate
from a storage tank to an engine and compressor located
within a building, the presence of a building is of
course unnecessary and quite optional. The engine and/or
CA 02685655 2009-11-16
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compressor and/or storage tank may be instead located in
the open.
Yet a further embodiment of the invention is
contemplated where the fugitive gases may have been
diluted by air as is illustrated in Figure 5. Such
fugitive gases may have escaped from various sources such
as block and control valves, pressure relief valves,
regulators, flange connections, compressor seals,
compressor valve stems and valve caps, coal mines,
livestock and sewage treatment and the like without such
list being all inclusive. Sources for such fugitive
gases are described in "Catalytic Solutions for Fugitive
Methane Emissions in the Oil and Gas Sector", Hayes,
R.E., Chemical Engineering Science 59 (2004) 4073-4080.
While Hayes describes the source of such dilute fugitive
gases, he does not contemplate that the dilute fugitive
gases could be used as a supplemental fuel source for an
engine or turbine.
The fugitive gas emissions which are diluted by
air may occur in buildings where the sources of gas
emissions are located. Typically, the air in such
buildings is replaced constantly with the use of vans or
ventilators using atmospheric air which is provided to
CA 02685655 2009-11-16
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the building and which replaces the internal air of the
building together with the escaped fugitive gases. A
fugitive gas of considerable interest is methane which,
being of a density which is lighter than air, passes to
the inside ceiling of the building before being replaced
by external air and evacuated to the atmosphere.
It is contemplated that such methane and other
dilute fugitive gases being of a density lighter than air
can be collected and used as intake air for the engine or
turbine in which the fuel is used and thereby serve as a
supplementary fuel for the engine or turbine similar to
the procedure desired above where an exhaust gas oxygen
sensor is described. The use of methane as a
supplemental fuel source is particularly attractive since
methane is a greenhouse gas. The combustion of such
methane is beneficial to reduce greenhouse gas emissions.
Reference is made to Figure 5 where the
fugitive gases are shown as being emitted from various
locations within the building 300 which gases
particularly will usually include methane and which gases
are shown by the broken lines 176, V6 and V,. The fugitive
gases migrate to the inside ceiling of the building 300
because they will include, typically, methane which is of
CA 02685655 2009-11-16
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a density lighter than air. They are collected there by
a collector 311. These collected dilute fugitive gases
are fed into ducting 311 extending to a diverter valve
312 which, in a first configuration, is positioned such
that all of the engine intake air is drawn via a duct 313
from outside the building. The exhaust fan 324 is turned
on to ensure the dilute fugitive gases are drawn from the
building. In a second configuration, the diverter valve
is moved to draw all or part of the engine intake gases
from the collector 311. The control and inletting of
natural gas or other fuel together with control processes
provided for the collected and dilute fugitive gases is
similar to the embodiments earlier described to obtain
the desired air-fuel control for the engine or turbine
which utilises the dilute fugitive gases as a
supplemental fuel source. The diverter valve 312 is
controlled (manually or by a control system) to achieve
the desired amount of outside intake air and intake air,
which may contain diluted fugitive gases. An exhaust
sensor 330 may conveniently be associated with the
exhaust stack 323 to monitor the components in the
exhaust of exhaust stack 323 as previous described.
It is further contemplated that the animal
husbandry may be a source of methane and that the
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building 300 may be a barn, for example, with cattle or
other animals being located therein. The methane
produced by the animals would be collected in a similar
manner to that described and inputted to an engine or
turbine 303.
Yet a further embodiment of the invention is
illustrated in Figure 6. With the engine in operation,
there is a negative pressure at the intake to the engine
which tends to draw in the collected fugitive gases which
are subsequently used as a fuel source. The negative
pressure tends not only to dram in the fugitive gases but
it also tends to draw in air through the exhaust stack
which otherwise would vent the fugitive gases to the
atmosphere when the engine is not in operation. In
accordance therewith, Figure 6 illustrates an exhaust
stack 600 and a passive check valve 601 which is
installed in the exhaust stack 600. The check valve 601
prevents the ingress of air into the intake ducting 602
which extends to the engine 603 and which otherwise
allows atmospheric air passing through air filter 604 to
the intake ducting 602.
The passive check valve 601 is operable to
maintain a maximum positive pressure at the source 610 of
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fugitive gases of 1 to 5 inches of water (H20) where the
pressures are here stated as inches of water column with
27.7 inches of water column equaling 1 psi or 6,895
kiloPascals.
A control or on/off valve 611 is closed when it
is not desired to use the fugitive gases as a fuel source
such as when the engine 603 is not in operation. The
fugitive gases will thereby pass directly to the stack
600 and vent to the atmosphere and the back pressure
exerted by the check valve 601 is of a value that it will
not adversely affect this passage of the fugitive gases
to the atmosphere through stack 600.
When the control valve 611 is open and the
engine 603 is in operation, the fugitive gases will pass
directly to the engine air intake 602. The slight
negative pressure created by engine operation will
provide additional force on check valve 601 to maintain
it in its closed position thereby preventing backflow of
air through the stack 600 and into the air intake 602.
With the flow of atmospheric air transmitted through the
control valve 611, fugitive gas flow into the engine 602
can be measured and better controlled.
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Yet a further embodiment of the invention
relates to the addition of an accumulator 612 within the
duct 613 extending from the fugitive gas source to the
engine 603 as illustrated in Figure 7. The use of an
accumulator 612 is valuable if the flow of fugitive gas
is variable on a short term basis. The accumulator 612
will smooth out the fluctuations in fugitive gas flow to
the engine 603 thereby obviating excessive instrument and
control variations. The volume of the accumulator 612
selected is calculated based upon the volume of gas flow
from the fugitive gas source 610 and the expected time
variables involved in such flow.
In operation and when the control valve 611 is
open and the engine 602 is in operation, the normal
pressure in the accumulator 612 will be similar to the
pressure in the air intake, typically 3 to 15 " H20 below
atmospheric pressure. If there is a burst of fugitive
gases, the pressure in the accumulator 612 will rise to a
maximum determined by the check valve 601. When the
check valve 601 opens, the excess gas is vented through
stack 600 to the atmosphere. If the fugitive gas burst
is small relative to the volume of the accumulator 612,
the fugitive gases will all be consumed by the engine 603
due to the storage capacity of the accumulator 612.
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The check valve 601 is conveniently better
illustrated in more detail in Figure 8 wherein one
embodiment is shown. A seal 621 is conveniently provided
to prevent the ingress of air and for reliability
purposes, no spring is used and the weight of the movable
member 620 is designed to provide a force equivalent to a
pressure of 1 to 2" 1120 closure force on the check valve
601. A pliable material is conveniently provided to
ensure seal integrity for the small forces involved.
The present application relates to the addition
of combustible fugitive gases to an engine in order to
reduce greenhouse gases and in order to reduce the cost
of fuel for the engine to which they are added. A
variety of sources may have fugitive gas emissions which
are added to the air intake of an engine. The emissions
may or may not be mixed with air prior to their
introduction to the air intake.
In a further embodiment of the invention,
additional techniques are contemplated in ensuring
fugitive gases are properly introduced to engines,
principally by way of the air intake.
In a normal engine, the air comes from the
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atmosphere and the fuel, either in liquid or gaseous
form, is added to the air, either before the duct to the
cylinder (e.g. with a carburetor), in the duct to the
cylinder (e.g. duct injection), or directly into the
cylinder (e.g. direct injection). Normally the speed of
an engine is controlled by the amount of fuel or fuel-air
mixture that reaches the intake manifold. Various control
devices and/or systems ensure the ratio of fuel to air is
maintained to ensure the desired combustion conditions.
When combustible vapors are added to the otherwise pure
air, additional controls must be provided to ensure the
engine speed is still controlled and that the ratio of
fuel to air is maintained. While some sources of these
fugitive vapors provide steady supply, in general, the
amount and quality of the fugitive gases are unsteady.
This occurs because the fugitive vapors may originate
from unrelated or partly related sources and as a
consequence change in composition and amount. The
additional control devices and systems must be able to
adjust for these changes. Since the use of fugitive
combustible vapors displaces fuel, it is economically
advantageous to use as much of the fugitive vapors as
possible. In some cases, since many fugitive vapors have
a more negative environmental effect than the products of
combustion (for example methane gas), there is an
CA 02685655 2009-11-16
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environmental benefit to use as much of the fugitive
vapors as possible.
Reference is initially made to Figures 9A and
98 which each illustrate a spark ignited engine generally
illustrated at 700, 701, respectively, the initial engine
700 of Figure RA being a naturally aspirated engine and
which engine 701 illustrated in Figure 98 is a turbo
charged engine where air is pressurized prior to entry
into the engine 701. In addition to turbochargers,
blowers, compressors and superchargers could conveniently
also be used to increase the air pressure supplied to the
engine.
In the normal configuration the engine 700, 701
has a specific method for controlling the amount of fuel
admitted to the engine. This control may be by a
carburetor 702 and a fuel control 703 such as a fuel
regulator, a fuel valve, fuel pump or other such device.
The amount of fuel delivered to the engine
controls the engine speed, which is set manually or by a
governor. In Figures 9A and 98, air intake speed control
is by a throttle valve 704 which controls the rate at
which the air fuel mixture goes to the manifold 710,
while the carburetor 702 adds an amount of fuel
CA 02685655 2009-11-16
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approximately proportional to the amount of air passing
through it. The air comes directly from the atmosphere or
from a turbocharger 711 or similar device which increases
the pressure of the air delivered to the engine 701 to
greater than one atmosphere.
An air filter 712 is conveniently placed in the
intake air duct to prevent undesired materials from
getting into the engine 700, 701. The air passing
through the filter 712 experiences a relatively small
drop in pressure, depending on the characteristics of the
air filter 712.
The addition of fugitive gases, such as from a
fuel tank or engine crank-case, call for the addition of
these gases after the throttle valve 704 where there is a
significant vacuum due to the presence of a partly open
throttle valve 704. The negative pressure acts to draw
the fugitive vapors to flow to the engine 700, 701. This
method, however, cannot work with an engine where the air
pressure is increased with a turbocharger 711 or other
pressure increasing device. In such a case, the fugitive
gases must be added before the pressure increasing
device.
CA 02685655 2009-11-16
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If combustible gases are added to the intake
air, less normal fuel is required and the governor or
manual operator will adjust the supply of normal fuel to
achieve the desired engine speed. If the supply or
heating value of the added gases changes rapidly, then
the normal control system will be unable to maintain the
desired engine speed. Additional control is therefore
required.
If combustible gases are added to the intake
air, there must be some method to turn-off the flow of
these gases to the intake air if the flow of gases is too
large for the normal speed control devices, or if the
engine is stopped. To enable reliable engine
starting, the intake air duct should be free of
significant combustible gases before starting. This
requires a special starting arrangement.
A further factor to consider is that the air to
fuel ratio is usually regulated to control emissions such
as carbon monoxide (CO) and nitrogen oxides (N0x). With
the addition of the fugitive gases to the engine intake
air, adjustment to the air-fuel control may be
required.
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Several embodiments are therefore contemplated
to add fugitive gases to engines in accordance with such
problems. The embodiments are conveniently divided into
fugitive gas capture techniques, fugitive gas control
techniques, air-fuel control modifications and
measurement methods.
A first technique for capturing fugitive gases
involves the installation of a low pressure check valve
750 placed on the vent pipe 751 where such fugitive gases
are normally vented to the atmosphere as is illustrated
in Figure 10. The check valve 750 provides a positive
pressure head to cause the fugitive gas to flow to the
engine air intake inlet or duct 752. A low pressure is
normally preferred for the check valve 750 in order to
minimize the pressure build-up in the gas collection duct
or piping 753. If there is a sudden burst of fugitive
gases or the control system does not allow the gases to
flow to the engine (not illustrated), the fugitive gases
can escape through the check valve 750 and vent to
atmosphere.
The fugitive gases may also be added after the
air filter 760 of an engine (not illustrated) as seen in
Figure 11 since it is common to use such air filters to
CA 02685655 2009-11-16
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. remove dirt and other foreign debris from the engine air.
The air filter 760 is positioned on the engine intake 761
to remove dirt and other foreign matter from the engine
air. This filter 760 causes the air pressure to be
negative relative to atmospheric pressure between the air
filter 760 and the engine throttle valve or air pressure
increasing device such as a turbocharger downstream. The
negative air pressure can be used as a pressure head to
collect the fugitive gases either from a pressurized
system, as produced by a check valve of Figure 10, or
from the gases at atmospheric pressure.
According to the flow rate from the fugitive
source to the engine intake 762, the gases may or may not
contain fugitive gases. This system advantageously
provides that no additional positive pressure is placed
on the fugitive gas collection system and that bursts of
fugitive gas flow more than is drawn to the engine air
intake can freely vent to the atmosphere through vent
763.
A further embodiment of fugitive gas capture is
illustrated in Figure 12 wherein a duct fan 770 is
conveniently placed in the pipe 771 conducting the
fugitive gases to the engine intake air 772. Such a duct
CA 02685655 2009-11-16
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fan 770 is chosen with regard to the capacity also
ensures that a positive pressure is not required in the
fugitive vent system, and the vent gases may be added to
the engine air before or after the air intake filter 773.
A further technique for fugitive gas capture is
illustrated in Figure 13 in which a venturi passage 780
is placed in the intake air duct 781 of the engine (not
illustrated) so as to create a negative pressure to draw
the fugitive gases to the intake air duct 781. Such a
venturi 780 results in relatively small pressure loss in
the air supplied to the engine and allows the fugitive
gases to be added before or after the engine air filter.
In a further technique used for fugitive gas
capture, an eductor 790 is illustrated in association
with Figure 148. On engines utilising fuel injection,
there is air scavenging of the engine cylinders.
Scavenging occurs when both the intake and exhaust ports
or both the intake and exhaust valves are open at the
same time and for a sufficiently long period to allow the
engine air to pass from the intake to the exhaust
manifold without combustion. In such a case, the addition
of combustible gases to the intake air would result in
these combustible gases reaching the exhaust manifold.
CA 02685655 2009-11-16
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Since these exhaust gases are hot, the presence of these
added combustible gases may create an unsafe condition.
For such scavenging engines, the fugitive gases are
conveniently added to the main engine fuel, which is at a
positive pressure as seen in Figure 14B. A typical air
pressure-fuel pressure curve is given at Figure 14A. The
engine fuel is often derived from a relatively high
pressure source > 100 psig. If the source is a compressor
discharge line, the pressure may exceed 500 psig. One or
more regulators are used to reduce the pressure to a
constant value. A fuel control valve 791 ensures the
pressure of fuel for injection to the engine is of a
value to maintain the desired engine speed. The eductor
790 is used in this embodiment both as a pressure
reducing device and as a means to collect the fugitive
gasses into the fuel stream.
A governor or governor control (not
illustrated) is operable to open or close the fuel
control valve 791. If the engine speed drops below its
set point, the fuel control valve 791 is opened. If the
engine speed is increased, the fuel control valve 791 is
closed. As shown by the chart of Figure 14A, the fuel
pressure supplied to the engine measured by the pressure
sensor PT1 (Figure 14B) is between 5 and 30 psig. The
CA 02685655 2009-11-16
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flow of fuel gas from the high pressure source at 792 to
the lower pressure fuel line 793 through the eductor 790
normally produces a lower pressure for the vent gases.
The flow rate will be dependent on engine fuel
consumption.
The use of an eductor to collect fugitive gases
has been described by Goodyear in United States Patents
6,315,000 and 6,418,957. Goodyear describes the use of an
eductor to capture and recover gases into a gas
processing system. However, Goodyear does not teach or
suggest the use of engine fuel gas for the eductor and
for recovering fugitive gases to be introduced to the
engine fuel. In contradistinction, the present
embodiment describes the use of the eductor 790 for
engine fuel delivery as well as being part of an engine
control system. By replacing pressure regulators
previously used to reduce feed gas pressure with an
eductor, there is no need to recompress the gas leaving
the eductor 790 at the reduced pressure.
Goodyear contemplates in the '957 patent that
his eductor system does not use additional energy. See
col 2, line 20. However, Goodyear does not include the
additional energy required by the gas compressor to
CA 02685655 2009-11-16
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recompress the low pressure gas leaving the eductor to
the higher pressure of the gas used by the eductor. In
the present application, there is no need for
gas recompression so no additional energy is required to
collect the fugitive gases which is advantageous.
Reference is now made to Figure 15 wherein a
further technique to control fugitive gas flow is
illustrated. If the fugitive gas flow is known to be
steady, the gases can be applied to a region near the air
intake generally illustrated at 800. A diverter valve
801 may conveniently be used but is not necessary. With
no diverter valve present and when the engine (not
illustrated) is not running, the fugitive gases are
vented to the atmosphere where the gases dissipate. The
fugitive gases will generally not accumulate in the
intake duct when the engine is not running. During engine
startup, the inlet air flow is relatively small, so only
small quantities of fugitive gases will be brought into
the intake air. As the engine speed increases, the rate
of air flow will also increase, causing most or all of
the fugitive gases to be swept into the air duct 802.
The optimum location for the fugitive gas pipe 803 may be
determined by experimentation or by engineering
approximations. Conveniently, if the flow of fugitive
CA 02685655 2009-11-16
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gases from the fugitive gas source 804 is relatively
steady and not too great relative to the normal engine
fuel, the usual engine throttle can maintain desired
engine speed.
To enhance safety, a diverter valve 801 may be
put into place to divert the fugitive gases well away
from the engine air intake region 800 when the engine
speed is below a specified value. The diverter valve 801
is conveniently an off/on type or a proportional type. If
a continuous acting diverter valve is used, the fugitive
gas flow to the intake 800 may be increased or decreased
slowly enough to allow the engine governor to minimize
RPM fluctuations in the engine caused by the change in
fugitive flow.
In the event an off/on valve is used, the
engine speed changes would possibly be too great. A
feed-forward arrangement to decrease or increase the fuel
or air-fuel valve setting for speed control to coincide
with the arrival or disappearance of the fugitive gases
is therefore contemplated.
If fugitive gas flow is known to be unsteady or
might be unsteady, a control system such as that is
CA 02685655 2009-11-16
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illustrated in Figure 16 is contemplated. In this
embodiment, a low pressure check valve 810 is placed on
the normal fugitive gas vent 811. The resulting pressure
is sufficient to cause the fugitive gas to flow to the
engine intake 812. A shut-off valve 813 is conveniently
provided to terminate fugitive gas flow when the engine
speed is below a specified value or such other conditions
exist which may be safety related. If the fugitive gas
flow is greater than the flow delivered to the engine
intake 812, the check valve 810 will open to allow the
excess gas to vent to the atmosphere. The pressure sensor
814 conveniently determines if the pressure is above the
cracking pressure for the check valve 810.
Normally, a control loop will control the
control valve 814 to maintain a pressure just below the
cracking pressure of the check valve 810. If the pressure
exceeds the desired set-point, the control valve 814 will
open and if the pressure is below the desired set-point,
the control valve 814 will close. A typical control with
a proportion, integral, derivative (PID) algorithm is
illustrated in Figure 17.
In the event the fugitive gas flow changes
rapidly, the fugitive flow change to the engine will
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produce an RPM upset before the governor can react to
change the main fuel flow to maintain the set-point speed.
To reduce the RPM upset, the fuel flow measurement signal
can be used as a feed-forward signal to the engine fuel or
speed control valve thereby causing the device controlling
the fuel to the engine to open or close appropriately
according to the flow of the fugitive gases. Such a system
is illustrated diagrammatically in Figure 18.
As shown in Figure 18, the control algorithm,
shown here as a PID 820, compares the RPM set-point 821
with the measured RPM 822 and produces an output based on
the difference. The fugitive flow measurement signal 823 is
delayed at 824, the delay being appropriate to the transit
time between the addition of the fugitive gases and their
arrival at the engine cylinders. An amount of feed-forward
830, which is related to the fuel flow amount, is then
added to the output on the governor PID 820. When the
fugitive flow increases, the feed-forward calculator 830
generates a negative output causing the main fuel control
device 831 to close. The total fuel reaching the engine is
therefore unchanged, even though the fugitive flow has
increased. If the amount of fugitive flow decreases, the
feed-forward calculator 830 will generate an output to
compensate accordingly.
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The fugitive flow measurement may be made
directly or calculated from the measured pressure and the
valve open position using well-known formulas for
calculating flow
from the valve characteristics and the pressure. The
general form of the formula is:
Q = Cv*P1*K*(AP/P1*T))0.5
Where Q is the gas flow rate;
Cv is the valve flow coefficient, which is of the valve
open fraction;
AP is the pressure difference across the valve;
P1 is the absolute pressure of the gas before the valve;
T is the absolute temperature of the gas; and
is a factor that depends on the units, valve fittings,
gas specific gravity and the like
If the flow is known to be unsteady, and
control of the fuel valve with feed-forward is not
desired or possible, then the fugitive control may be
configured to reduce the rate of change of the gaseous
fugitive flow as seen in Figure 19. A pressure signal
840 is fed to a pressure PID 841 and the pressure signal
is also converted to a correction to the valve output. In
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this case, a rapid increase of the pressure signal 840
will cause the main fuel control valve 842 to close a
specified amount, thereby minimizing any rapid flow
upsets.
If there is unsteady fugitive gas flow with
cascade control, a pressure PID 850 is utilised with slow
response in series with a flow PID 851 having a fast
response as illustrated in Figure 20.
In this system, the pressure PID 850 is set to
give a relatively slow response to input changes. When
the pressure is less than the SP 852, the output of the
PID 850 will increase. From the pressure and the flow
coefficient, Cv, corresponding to the valve fractional
opening a desired flow is calculated using the expression
described previously, or by a similar expression for
calculating flow. The maximum flow is set according to
the engine size or the maximum amount of fuel which will
be displaced by the fugitive gases.
The calculated flow 853 used as a set-point is
compared to the actual flow in the flow PID 851 where the
response is relatively rapid. If the actual flow is
greater than the flow set-point, the output of the flow
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PID 851 will decrease to close the flow control valve
854. In this way, the valve 854 can respond rapidly to
short-term pressure upsets and respond to minimize the
venting of fugitive gases. The flow can be measured with
a flow sensor or calculated from the measured pressure
and the position of the control valve 854 in order to
allow a rapid response to changing fugitive gas
conditions.
In place of a control valve 854 of Figure 20,
the flow may be controlled by a number of solenoid valves
860 in parallel, where orifices control the flow across
each of the solenoid valves as is illustrated in Figures
21A and 21B. By sizing the orifices appropriately, the
total flow can be controlled according to the number of
solenoid valves 860 which are open.
Flow control using the three solenoid valves
860 and orifice combinations are labeled A, B, and C in
Figures 21A and 21B. The orifices are sized so that the
flow of Orifice A is about fifty percent (50%) of the
flow in orifice B which, in turn, is about fifty percent
(50%) of the flow in orifice C. The total flow through
the orifices at the expected pressure difference should
then be the maximum flow desired by the system. A
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combination of valve openings and closings can control
the flow to eight(8) different values as is illustrated
by the table of Figure 21B.
In a similar manner, by selection of orifice
sizes and number of parallel legs, the flow can be
controlled in steps of the desired size. The fugitive
flow controller can read the pressure across the
solenoids as at 861 (Figure 21A) and control the flow to
the desired value, using the relationship between flow
and pressure difference across an orifice. Control
arrangements previously described can be used to provide
the desired response. The solenoid arrangement of
Figures 21A and 21B is advantageous when a faster
response is desired than is possible with a control
valve.
When the fugitive gases are diluted with air,
the fraction of air is generally unknown. Further, if
the fraction of air changes rapidly, the change can cause
a short-term RPM excursion. A control arrangement as is
shown in Figures 22A and 22B is advantageous to minimize
rapid changes to the amount of combustible vapors
supplied to the air intake. OxT is conveniently an
oxygen measurement device 870 as is illustrated in
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- 55 -
Figures 22A and 22B although a nitrogen or hydrocarbon
measurement device may advantageously also be used.
When the valve 871 is closed, the oxygen sensor
870 will measure a relatively high value, equal to or
approaching the normal oxygen concentration in air. To
initiate flow, the valve 871 must be opened by an amount
sufficient to start a small flow used to displace air.
Then, oxygen control set point 872 (Figure 22B) will be
at a higher value, so that the PID control system will
cause the control valve 871 to open, drawing some more of
the fugitive gases to the engine. As the flow increases,
some air from the atmospheric vent will be drawn past the
control valve 871. When the oxygen percentage from this
air reaches the PID set point, the control PID will
regulate the valve 871 to maintain the desired percentage
of air. If the oxygen SP 872 is relatively low, say 1%,
then a rapid increase in fugitive gas flow will be
largely vented to the atmosphere until the control loop
causes the valve 871 to open, increasing the flow to the
engine. Since the normal amount of air in the flow to the
engine is relatively small, any upset to the engine will
likewise be small.
As earlier set forth, the sensor can be
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sensitive to oxygen, nitrogen or combustible gases. With
a combustible gas sensor, the direction of action of the
PID controller would be opposite in sign and a strategy
for starting from a no-flow condition would not be
required. The gas sensor could also be placed before the
control valve with minor changes to the control strategy.
For use with an eductor, the eductor must be
sized appropriately according to the fuel consumption of
the engine. Reference is again made to Figure 14B. The
pressure of the fugitive gas is measured by the pressure
sensor 794. This pressure is dependent on the check
valve (not illustrated) and the flow of the fugitive
gases. The controller ensures that the fugitive control
valve 795 can open only if the pressure at 796 is less
than the pressure at 794. If this condition is met and
the pressure at 794 is greater than the pressure set
point 880 (Figure 23), the control PID 881 will open the
fugitive control valve 882 to allow fugitive gases to be
added to the engine fuel. If pressure at 796 rises above
the fugitive gas pressure measured at 794, the valve 882
will close, causing the fugitive gas to be vented to the
atmosphere through the check valve.
If there is no automatic air fuel ratio
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control, then the addition of fugitive combustible gases
to the intake air can affect the air-fuel ratio. A
control system is therefore required to ensure the
correct air-fuel ratio is maintained with the addition of
fugitive gases other than when an eductor is used to
bring the fugitives into the main engine fuel supply as
has been described.
The use of air fuel ratio control with an
exhaust oxygen sensor does not distinguish the source of
the combustible vapors. Existing air-fuel ratio control
systems that use an exhaust oxygen sensor will therefore
not require significant modification.
Certain prior art systems use engine fuel flow
and other parameters to determine the required amount of
air for the desired air to fuel ratio. For such systems,
a fugitive flow measurement or calculation is required.
The calculated flow may then be added to the measurement
of the main engine fuel flow to ensure there is no upset
to the air-fuel ratio as the amount of fugitive flow
increases or decreases.
The fugitive flow measurement is required for
control purposes and to estimate the result of combusting
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the fugitives instead of directly venting them to the
atmosphere by way of a vent or by way of a flare stack.
The fugitive gases may also be measured with a flow
measurement device.
When fugitive gases are mixed with air, the
measurement of the quantity of fugitives, which might be
required for an estimate of emissions, cannot be obtained
with a flow meter only. A flow meter used with the
oxygen (or nitrogen or combustible vapors) percentage in
the gas flowing to the engine may be used to calculate
the fugitive quantity when the composition of the
fugitive gas and air is known.
An alternative method, if main fuel flow is
measured, is to periodically turn the fugitive flow off
and on while measuring the difference in the main engine
fuel flow. While this method works best when the engine
load is steady during the measurement period, a more
complex technique where the engine load, or associated
parameters, is monitored during the switching of the
fugitive flow to compensate for changes to the engine
fuel consumption.
Yet a further embodiment of the invention
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relates to specifically using diluted fugitive gases and
reference is now made to Figures 24A through 32.
Referring initially to Figure 24A, the pipe 901
carrying the diluted fugitive gases emanating from a
source of fugitive gases 902 and which gases are intended
to be introduced to an engine 903 is connected to the air
intake 904 of the engine 903 after the filter 910 as will
be seen in Figure 24A with the air filter 910 causing a
small negative pressure of 2 to 15 inches of water (0.5
to 3.75 kPa or 0.07 to 0.54 psi.). In addition, the duct
or pipe 911 carrying the fugitive diluted gases will be
connected to the vent or atmospheric exhaust point before
the vent exhausts to the atmosphere 912. A filter 915
may conveniently be positioned in the flow path of the
fugitive gases and may be used for removing undesired
contamination from the fugitive gas and air source.
If there are short duration bursts of gas from
a fugitive gas source, the flow bursts are intended to be
vented to atmosphere. The piping setup for other sources
of diluted fugitive gases are illustrated in Figures 28-
31.
Diluted fugitive gases emanating from a packing
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vent source 902 pass through the packing vent source pipe
911 which is connected to an oil collection unit 913 so
that any condensed liquids will drain to the oil
collection unit 913. The control valve 914 and its
associated piping 920, 921 is sized for the expected flow
as in known in the art. A typical size for the control
valve 914 would be 1 inch and the piping 920, 921
connected to the control valve 914 would conveniently be
1.5 inches. A pressure transducer 922 measures the
pressure drop across the control valve 914.
A temperature sensor 923 is conveniently
connected to the oil collection unit 913. The
temperature sensor 923 will originate an alarm if the
packing vent fugitive gas flow exceeds a predetermined
safe value such as might happen if the pack vent source
902 springs a large leak.
For hydrocarbons, the source gases together
with the added air should fall outside the lower
explosive limit (LEL) and the upper explosive limit
(UEL). It is therefore intended to use as much of the
source gas as possible. This amount is conveniently
estimated using an RPM reduction technique. Initially,
the piping size will be determined according to the
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nature of the source which sources, for example, include
building with methane sources and coal mines.
Vents from dehydrators may contain a
substantial quantity of water vapor. If the percentage
of water vapor can be estimated, it is possible to
calculate the amount of source gas, appropriately
modified for water condensation and with a modified ratio
for allowed flow relative to the combustible amount which
is determined by the RPM reduction technique.
The table illustrated in Figure 248 provides
the LEL and UEL volume percentages for a number of common
hydrocarbon gases. Hydrogen and acetylene are not set
forth in the table and are not intended to be used as
fuel sources with the present technique.
The diluted fugitive gas flow maximum Faid(max)
is conveniently set at 10% of the engine fuel Fm plus F8,.2
where F is the undiluted flow, if present.
The flow of gas through a valve is described by
a valve sizing component C. The component depends on the
physical characteristics of the valve and the fraction
opening of the valve under operation. Conveniently, the
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valve manufacturer will supply a graph giving Cv as a
function of percentage valve opening. Alternatively, the
Cv as a function of the percentage opening of the valve
can be determined by direct measurement under the
expected conditions of use.
The flow through the valve, Fv, is determined
from the valve C, using the controller percentage open
signal provided to the valve actuator. Fisher, "Control
Valve Handbook", Edition 3, Fisher Controls International
Inc. 1999, page 119 utilises the formula:
Flow = Cv*N7*Fp*Y*P1*((AP/P1)/SG*T*Z)"
where:
N7 = 4.17 for pressure in Kpa, flow in m3/h, and
temperature in deg X;
P1 is the upstream absolute pressure in kPa;
AP is the pressure drop across the valve in
kPa;
SG is the specific gravity of the gas relative
to air;
Fp is the piping geometry factor and is
dimensionless;
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Y is the expansion factor which is
dimensionless;
T is the absolute temperature of the gas in
degrees K; and
Z is the gas compressibility and is
dimensionsionless.
In the present application, Fp is set equal to 1 by a
requirement that there be no elbows, reducers, or
constrictions near the valve. Y is set to 1 as the
expected pressure drop across the valve will be less that
0.5 psig and the upstream pressure is approximately 1
atmosphere. Z, the compressibility, is set to 1 as the
pressures are almost equal to 1 atmosphere.
Hence, the formula can be written as follows for the
present application:
F, = K * C,*(g * AP/(SG*(T, + 273)) =5
where:
Fõ is the flow in m3/hz;
K is a constant = 4.17;
C, is the valve flow coefficient (manufacturer's
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curve of C, vs. % open);
AP is the pressure drop across the valve in
kPa;
g is the upstream absolute pressure in kPa;
SG is the specific gravity of the gas relative
to air;
Tl is the temperature of the gas in deg C;
Ako and Al are the unit conversions factors ("Air
Fuel Control Expressions-Ver 5.7.doc") for
temperature;
E is the unit conversion factor for volume; and
F is the unit conversion factor for pressure
g = 101.3 - 0.0111 * Alt
where:
Alt is the site altitude in meters.
The SG for the gas rather than the gas-air
mixture is used as the expected mixture will be mostly
gas. If the gas temperature is measured, then that
temperature should be used; if the temperature is not
measured, the a default value of 20 deg C should be used.
A physical requirement for the valve location and piping
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is that an elbow, reducer or other flow disturbance item
shall not be placed adjacent to the valve.
The value of C, is determined from a set of
ten(10) values for valve opening increments of 10% from
10% to 100% inclusive. The C, for a 0% opening is assumed
to be O. A typical set of C, values is shown in Figure
25.
If, as is common with a valve controlled by an
I/P actuator where there is a "lift-off"% open value,
then this lift-off %, LO%, shall be included in the
calculation to provide an actual Valve%a as follows
Valve%. = 100*(Valve% LOW(100 - LO%)
Where Valve% is the signal sent by the controller to the
actuator expressed as %. LO% is the signal required to
initiate flow through the valve. For a digital valve
controller(DVC) the LO% is normally O.
For a particular controller output, Valve$, to
the valve the Cõ can be determined by linear
interpolations as follows:
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If Valve% <= LO%, C, = 0
Starting at Step i = 1, if Valve%.< %Openj, increment i
and repeat.
If Valve%. > %Openj, then calculate
C,.= + (C., Cy(i_,))/ (%Openj %Openi_
- %Open)
Then the flow of gas through the valve is
F, = K * C,*(g * AP/(SG * (T+273))"
The temperature of the gas will be assumed to be the
temperature of the gas if measured, or a reference
temperature of 20 degC or 68 degF.
As noted, the gas flowing is a mixture of air
and combustible vapors. To measure the combustible vapor
amount the RPM reduction method will be used.
RPM Reduction to Estimate Fugitive Flow
In this method, the reduction in engine RPM
caused by the closure of the diluted fugitive gas control
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valve is measured. The RPM change rather than the change
in main fuel flow is used because the RPM can be measured
with more accuracy.
The steps in the process are set out below:
1. The fugitive gas control valve will be open at
a known percentage(%).
2. The engine RPM and undiluted fugitive flow
stability during the period tl, as determined by
the change governor valve position, GV%c(t1-.0)
is compared to a user-determined value, 5%.
3. If the change in governor valve position
GV%c(t2-0) is less than the user determined
value S%, the average RPM is determined for the
period t2 of 2 to 10 seconds.
4. Thereafter the governor and undiluted fugitive
gas control valve are placed in manual and the
diluted fugitive gas control valve is closed as
quickly as possible.
5. After a delay of t3 to allow the RPM to again
become stable, typically about 2 seconds, the
RPM is determined over the next period t4, which
may be 2 to 10 seconds.
6. The average main fuel flow, F., and undiluted
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fugitive gas flow, F.. arre determined over the
interval t2-1-t3+t4 and summed.
7. The governor PID and undiluted fugitive gas PID
are returned to Auto and the diluted fugitive
gas control valve is opened to the original
value at a user-defined rate.
8. After a delay period ts of 2 to 5 seconds for
the engine to come to the new speed, the main
governor valve position is measured again at
the beginning of period t6.
9. If the governor change, GV%.(t6-t1) is less than
the user-defined value, S%, then the fugitive
gas diluted flow amount, Fsd, is calculated.
10. The flow amount Fsd, is used to set the diluted
fugitive gas control valve opening Valve% to
the desired value as described below.
11. If the criteria for GV%0 are not met, then, at
the end of period t6 the process started again
at Step 1 above. If the flow estimation was
successful the diluted flow estimation process
is rechecked after a user-defined interval.
The times are shown schematically in Figure 26.
If the governor valve% (GV%) change during the period ti
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is less than a user specified value S%, the fugitive gas
flow test is allowed to proceed. If not, the governor
valve change is checked every minute until the condition
is met.
Governor valve change criteria, GV%.(tx-t) is
GV%c(tx-t) = 1100* (GV% (t = t7) - GV% (t =
) /GV% (t=tx) I
where t = 0 at the beginning of the test
If GV%,,,<S% as defined above, the fugitive diluted
combustible gas flow is then equal to:
F.d = (F.+ Fõ)*(1 - RPMõre) /RPM20õ,
Fed 0 if RPM4off > RPM
where:
Fx. = EFõu*At/(t2 + t + t4) and the main fuel samples
15 are acquired during the intervals t2, t3 and t,i;
At, is the main processor loop time;
= EFõi*At./(t2 + t, +t4) and the undiluted fugitive
gas flow samples are acquired during the intervals
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Ã2 t 3 and t4;
At. is the fugitive gas processor time;
RPM40, = ERPMileAt3/t4 and RPM samples acquired during
period tõ;
RPM2o1 = ERPM,IcAts/t2 and RPM samples acquired during
period t2.
The actual measurement period is t2 t3 + t, which
will be in the range of 6 seconds to 24 seconds according
to the times specified. It is contemplated that t2 and t,
will be equal. The period chosen will depend on the
accuracy required for the diluted fugitive gas flow.
The period during which the governor PID is in Manual is
t2 + t3 t, which will be in the range of 6 to 24
seconds.
If during the period the governor PID is in Manual, a
shutdown signal occurs, it shall override the PID manual
output.
If other speed control functions make an RPM set-point
change of greater than a specified value, AR11#4 during the
period the governor is in manual, the test will be
aborted and the governor returned to Auto mode and if the
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fugitive gas control valve is closed, it will be re-
opened to the previous value before the start of the
test.
Diluted Fugitive Gas Valve Position
For source gases that have little or no air present
the desired diluted fugitive gas Valve% is a Valve% that
will provide a flow 10% to 30% more than the flow
determined by the RPM test. This ensures that the gas
mixture in the fugitive gas piping is greater than the
Upper Explosive Limit(UEL). It further ensures that if
there is any sudden increase in the source gas flow, the
increase will not cause a large upset to the engine
because most of the excess will be vented through the
unobstructed vent pipe.
To adjust the position of the diluted fugitive gas
Valve56:
1. Determine the diluted flow valve coefficient C,
from the Valve% signal from the controller
using the method described.
2. Calculate the diluted fugitive gas flow F.1,
using the formula
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Fatly = Fv = K * Cv* (g*AP/(SG*(111+ 273))"
3. Compare this valve flow, F1 to the diluted
slipstream flow, Fsd, from the RPM reduction
test.
4. Calculate the desired diluted fugitive gas flow
Valve% according to the table set forth at
Figure 27 and:
C', = * (g *AP/(SG * (T1+ 273))")
Then from this new value, calculate the adjusted
valve open%. Referring to Figure 27 for the open% and C,
Starting at Step i=0, if C',> Cvi, increment i and repeat.
If C', < C, then calculate
Valve%. = %Open" + (%Openi - ckopeni_1)*(C'v -
/(Cõi - Cv(i-1))
Then the desired controller signal to the valve is:
Valve% = LO% + Valve%. * (100-LOW100
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Starting the Diluted Flow Stream
The diluted fugitive gas valve shall be closed if
the engine RPM is below the fugitive gas threshold RPM.
The diluted fugitive gas flow maximum, Fõd(max) shall be
set at 10% of the engine fuel F. + F.u. To start the
diluted fugitive gas flow, the delta pressure across the
diluted fugitive gas valve with a controller output of 0%
is measured. Then, using a starting undiluted slipstream
flow, Fadvl, of 1% of the engine fuel flow, F. + F the
valve C, is determined from the expression
Cval (K*(g*AP*F/(SG*((T + A,)*A.+273))) =5)
From this desired adjusted Valvelsai can be calculated as
described.
From this the Valve output for the controller, accounting
for the liftoff%, LO%, is:
Valve% = LO% + Valve56.1 * (100 - L0%)/100
Initially, the valve is ramped slowly (e.g. 2% per sec)
to this value. Then the RPM reduction test is performed
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to determine F.d and the method described previously is
used to establish the desired fugitive gas diluted
Valve%. It is contemplated that this process will
require a period of time to achieve the desired diluted
fugitive gas valve open%.
Once the initial fugitive gas valve opening as been
obtained, the fugitive gas flow shall be checked
according to the user-specified period and the valve
opening adjusted as a result of the RPM flow test.
The value of FEid shall be compared to Fsd(max) every
processor cycle and, if necessary, the diluted fugitive
gas valve opening adjusted as required. In addition,
when the diluted fugitive gas is operating with an
undiluted leg, the flow override may force the valve to
close to ensure the overall fugitive gas maximum is not
exceeded.
Many further modifications will readily occur
to those skilled in the art to which the invention
relates and the specific embodiments herein described
should be taken as illustrative of the invention only and
not as limiting its scope as defined in accordance with
the accompanying claims.
