Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 2961245 2017-03-17
AUXILIARY BURNER ASSEMBLY AND CONTROL SYSTEM FOR COMBUSTING
VENT GASES IN PETROLEUM PRODUCTION AND PROCESSING
TECHNICAL FIELD
The present disclosure relates to vent gases resulting from petroleum
production and processing and in particular to the combustion of the excess
vent
gases using an auxiliary burner.
BACKGROUND
The venting of hydrocarbon vapors into the atmosphere has been a common
practice at many petroleum production and processing facilities. Often, where
the
amounts are substantial, the vapors are collected, recompressed and used.
There
are many other locations where these vapors are vented to the atmosphere.
Recently, as of 2012/2013, the United States Environmental Protection Agency
(EPA) has placed an upper limit on the amount of volatile organic vapors
(VOCs)
that may be vented. There is also a desire to minimize the venting of methane
gases or gases that decompose to methane to the atmosphere because methane
has a strong greenhouse gas heat trapping effect, being twenty-one times more
effective than carbon dioxide over a 100 year period.
Vent gases originate from petroleum liquids that are stored in one or more
tanks but can come from many sources in the extraction, collection, storage,
processing and transportation of oil and gas. A pressure relief devices that
allow the
gases to escape to the atmosphere if a specified pressure is exceeded. To
prevent
the escape of the vent gases to the atmosphere these vent gases are directed
to a
burner system that consumes the gas vapors at a pressure below the pressure
relief
set-point.
An effective method to deal with the vented gases is to combust the gases
under controlled circumstances. The standard method of combusting these gases
is
to feed these gases to an incinerator unit or flare where a pilot, either
continuous or
started on demand, feeds into the vented gases in the presence of air to
ignite the
gases. In the case of a flare, the vent gases are directed through a vertical
tube or
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pipe and burned as the gases contact air. Since a flare is undesirable from an
environmental and public perception point of view, the general preference is
to
enclose the flame and to regulate the air flow to achieve combustion with good
air-
fuel ratio control. The disadvantage with flares and incinerators is that the
heat
energy from the vapor combustion is lost and not used. In addition, adding a
flare or
incinerator to a site may require additional effort to obtain permission for
installation
and operation by regulatory authorities.
In many of the petroleum production processes fired heater units are
employed for a multitude of purposes. Such heaters are used on an intermittent
basis in response to the process requiring the heat energy and as a result are
not
always available to combust the vent gases. Typically these heaters use a
burner
jet where the fuel is introduced and a horizontal fire tube for directing the
flame and
the hot combustion gases to the medium requiring the heat and then to a
vertical
stack for directing the exhaust gases to the atmosphere. The horizontal
portion may
be simply one pipe or several parallel pipes connected to a stack, a U
arrangement
or a multi-pass arrangement where the pipe or pipes make several passes
through
the medium to be heated. The use of heaters can provide an effective solution
to
using vent gases however varying demand requirements can result in times when
heating of the medium is not required and the vent gases must be dealt with.
Accordingly, systems and methods that enable a novel way of combusting
excess vent gases when a main burner is not in operation remains highly
desirable.
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SUMMARY
In accordance with an aspect of the present disclosure there is provided an
auxiliary burner assembly for combusting excess combustible vent gases, the
auxiliary burner assembly comprising: a first auxiliary burner unit housed
within an
exhaust stack of a main burner assembly above the main burner assembly, the
first
auxiliary burner unit for combusting vent gases received from a vent gas
supply line
having a combustion capacity less than the main burner assembly; and an outer
pipe for supporting the first auxiliary burner unit within a center of the
outer pipe
having a diameter equal to or larger than a main fire tube of the main burner
assembly; and wherein the first auxiliary burner unit combusts vent gases when
the
main burner assembly is inactive.
In accordance with another aspect of the present disclosure there is provided
an a method for controlling an auxiliary burner assembly, the method
comprising:
initiating an auxiliary burner of the auxiliary burner assembly positioned in
an
exhaust stack of a main burner assembly; initiating a main pilot of the main
burner
assembly; initiating the main burner of the main burner assembly; determining
air
flow velocity in the main burner assembly; and shutting off auxiliary burner
assembly
when air flow velocity reaches a pre-determined threshold.
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BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present disclosure will become
apparent from the following detailed description, taken in combination with
the
appended drawings, in which:
FIG. 1 shows a representation of a top view of a burner assembly with an
exhaust
gas stack;
FIG. 2 shows a representation of an auxiliary burner in the exhaust stack of a
main
burner assembly in a side view;
FIG. 3 shows a detailed representation of an auxiliary burner assembly;
FIG. 4 shows a representation of a high capacity auxiliary burner in the
exhaust
stack of a main burner assembly in a side view;
FIG. 5 shows a representation of a control system for controlling an auxiliary
burner
and a main burner;
FIG. 6 shows a representation of a control system for controlling a high
capacity
auxiliary burner and a main burner;
FIG. 7 shows a method for controlling a main burner and an auxiliary burner
for
combusting vent gases;
FIG. 8 shows a method for combusting vent gases using an auxiliary burner;
FIG. 9 shows a representation of an auxiliary burner assembly utilizing
baffles;
FIG. 10 shows a graph of vent gas flow from a petroleum liquids storage tank;
FIG. 11 shows a graph of stack surface temperature when the auxiliary burner
is
operational;
FIG. 12 shows a method of controlling a main burner without using an auxiliary
baffle; and
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FIG. 13 shows a representation of an auxiliary burner with a flame arrestor.
It will be noted that throughout the appended drawings, like features are
identified by like reference numerals.
DETAILED DESCRIPTION
Embodiments are described below, by way of example only, with reference to
Figs. 1-13.
In accordance with an aspect of the present disclosure there is provided an
auxiliary burner assembly for combusting excess combustible vent gases, the
auxiliary burner assembly comprising: a first auxiliary burner unit housed
within an
exhaust stack of a main burner assembly above the main burner assembly, the
first
auxiliary burner unit for combusting vent gases received from a vent gas
supply line;
and an outer pipe for supporting the first auxiliary burner unit within a
center of the
outer pipe having a diameter equal to or larger than a main fire tube of the
main
burner assembly; and wherein the first auxiliary burner unit combusts vent
gases
when the main burner assembly is inactive.
In accordance with another aspect of the present disclosure there is provided
a method for controlling an auxiliary burner assembly, the method comprising:
initiating an auxiliary burner of the auxiliary burner assembly positioned in
an
exhaust stack of a main burner assembly; initiating a main pilot of the main
burner
assembly; initiating the main burner of the main burner assembly; determining
the
air flow velocity in the main burner assembly; and shutting off auxiliary
burner
assembly when air flow velocity reaches a pre-determined threshold.
In accordance with yet another aspect of the present disclosure there is
provided an auxiliary burner assembly for combusting excess combustible gases
from a vent gas tank, the auxiliary burner assembly comprising: a first
auxiliary
burner unit housed within an exhaust stack of a main burner assembly, the
first
auxiliary burner unit for combusting vent gases received from a vent gas
supply line
from the vent gas tank; and an outer pipe for supporting the first auxiliary
burner unit
within a center of the outer pipe having a diameter equal to the exhaust gas
stack of
the main burner assembly; and wherein the first auxiliary burner unit is
utilized to
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combust vent gases from the vent gas tank when the vent gases exceed a first
pressure set-point and the main burner assembly is inactive.
In accordance with an aspect of the present disclosure there is provided a
method for controlling an auxiliary burner assembly for combusting vent gases,
the
method comprising: determining a state of a main burner assembly; determining
a
pressure value of vent gas in a vent gas tank; and initiating the auxiliary
burner
assembly to combust vent gas when the pressure value of gas in the vent gas
tank
exceeds an upper pressure set-point value and the determined state of the main
burner assembly is inactive.
In accordance with another aspect of the present disclosure there is provided
a controller for controlling the combusting vent gas, the controller
comprising: a
processor; and a memory containing instructions which when executed by the
processor cause the processor to: determine a state of a main burner assembly;
determine a pressure value of vent gas in a vent gas tank; and initiate an
auxiliary
burner assembly to combust vent gas when the pressure value of gas in the vent
gas tank exceeds an upper pressure set-point value and the determined state of
the
main burner assembly is inactive.
In accordance with still yet another aspect of the present disclosure there is
provided an auxiliary burner assembly for combusting vent gases comprising: a
pilot
unit; a burner unit housed within an exhaust stack of a main burner assembly
for
heating a medium; and an outer pipe surrounding the burner unit and pilot unit
within
a center of the outer pipe, the outer pipe having a flange for coupling to an
exhaust
gas stack of the main burner assembly wherein the auxiliary burner assembly is
utilized to combust vent gases from a vent gas tank when the main burner
assembly
is inactive.
In accordance with still yet another aspect of the present disclosure there is
provided a computer readable memory containing instructions for controlling an
auxiliary burner assembly for combusting vent gases, the instructions which
when
executed by a processor performing the method comprising: determining a state
of a
main burner assembly; determining a pressure value of vent gas in a vent gas
tank;
and initiating the auxiliary burner assembly to combust vent gas when the
pressure
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value of gas in the vent gas tank exceeds an upper pressure set-point value
and the
determined state of the main burner assembly is inactive.
An auxiliary burner can be utilized to combust vent gases from petroleum
production or processing when a main burner is not required for heating of a
medium. The auxiliary burner assembly is inserted within the exhaust stack of
an
existing main burner to eliminate the need for additional exhaust stacks to be
added.
A control system consisting of a set of valves, regulators, and sensors that
ensure
the existing main burner achieves its purpose to maintain the desired
production of
heat, while using the vent gas as the source of gases for the main burner.
When the
main burner is not on and vent gas is present, the vent gas is directed to the
auxiliary burner in the stack where the heat from the combustion of the vent
gases
does not add heat to main burner region. A combustion zone of the auxiliary
burner
is less than the main burner if an internal fire tube is utilized. The
diameter of the
auxiliary fire tube is less than that of the exhaust stack the airflow in the
exhaust
stack prevents the combustion zone of the auxiliary burner from creating
excessive
surface temperatures in the existing exhaust stack. At times when the main
burner
is not being utilized and the excess vent gas needs to be combusted one or
more
internal auxiliary burners can be utilized to combust the gases while the main
burner
is inactive.
In conditions where excess vent gas may need to be dealt with and the main
burner is not active, a lower capacity auxiliary burner may not be sufficient
to handle
the excess vent gas. To provide a high capacity auxiliary burner, rather than
using a
containment area with the exhaust stack, external heat shields are provided to
enable larger auxiliary burners. The external heat shields allow full use of
the
existing exhaust stack diameter as a combustion zone for the auxiliary burner
unit.
This enables an existing exhaust stack to be utilized for excess gas
combustion by
using the same airflow as the main burner and the auxiliary burner. No
auxiliary
burner flow regulation is required because the main burner already has an air
flow
regulator. If additional air flow control is required, a limited area
auxiliary burner flow
regulator can be installed.
The vent control system described provides an efficient combustor for all the
components of the tank gases which may include methane, ethane, propane and
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butane and smaller amounts of other volatile gases. If the methane escapes to
the
atmosphere it has a greenhouse effect. Over a 100 year time period the global
warming potential, according to the International Panel on Climate Change
(IPCC)
of methane is some twenty-five or more times that of the same mass of carbon
dioxide. When the methane is combusted 1 kg of methane, equivalent to 25 or
more kg of carbon dioxide is changed to water and 44/16 = 2.75 kg of carbon
dioxide, thus leading to a reduction in the greenhouse effect of the vented
gases. In
addition, the non-methane components of the tank gas may decompose, due to
natural processes in the atmosphere, to products which include methane. The
advantages to such a system where an auxiliary burner is added to an existing
burner assembly is the combustion of volatile organic vapors to meet
environmental
regulations, the reduction in total gases burned, which lessens operating
costs and
greenhouse gas emissions, and elimination of the need to seek approval for the
addition of an additional burner to a oil and gas production site
Figure 1 depicts a top view of a main burner assembly 100 with an exhaust
gas stack utilized in petroleum production and processing facilities. The
enclosed
volume contains the medium 102 to be heated by the main burner, such as
provided
by a heater. A fire tube 104 passes through the medium 102 and contains the
burner flame and conducts the hot gases through the medium 102 to be heated.
Although a single fire-tube 104 in a U configuration is shown in a horizontal
plane,
many other fire-tube configurations are possible in the configuration of the
heater.
The exhaust gases are conducted to a vertical exhaust stack 106 which allows
the
exhaust gases to escape to the atmosphere. The incoming air 114 for the fuel
combustion in the fire-tube 104 passes through a flame arrestor 108, which
prevents
any flame inside the fire-tube 104 from igniting any combustible vapors
outside of
the main burner assembly 100. The flame arrestor 108 may not be required for
areas where there is no likelihood of combustible vapors being present outside
the
main burner assembly 100. The air flow direction through the main burner 100
is
indicated by the arrow 114. An air filter (not shown) may be provided to
reduce the
ingression of contamination to the fire-tube 104. The fuel for the main burner
100 is
conducted to the fire-tube 104 by fuel assembly 110. The fuel assembly 110 may
include a premixing sub-assembly (not shown) to improve the combustion
process.
An igniter assembly 112 is provided for igniting the burner fuel either with a
pilot
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flame or with a spark. Many different designs of fuel assembly 110 and igniter
assembly 112 are available. The fuel is provided from vent gas sources
associated
with the petroleum production and processing facility. The excess gas vapours
must
be vented or stored if not utilized by the main burner. In most cases the
volume of
gas does not warrant capture or transport mechanisms. The auxiliary burner
provides a means of combusting the excess gas when the main burner is not
required without using venting or a separate flare or incinerator.
Figure 2 depicts a side view of an auxiliary burner positioned within the
exhaust stack 106 of Figure 1. The auxiliary burner assembly 210 is inserted
between the exhaust gas stack 202 and the fire-tube 104 from the main burner
100
with flanges 204 and 205. The auxiliary burner assembly 210 may comprise of an
inner fire-tube 212, having a smaller diameter than the exhaust gas stack 202,
an
auxiliary burner unit 214 and a pilot assembly 216. The inner fire tube 212,
also
referred to as a heat shield, may extend beyond the flange 205 into the
portion of
the exhaust gas stack 202. The auxiliary burner assembly 210 provides for the
combustion of excess gases when medium 102 does not need additional heat and
the pressure of the vent gas is above an upper pressure set-point value. By
integrating the auxiliary burner assembly 210 into the exhaust stack 106, the
combustion of the vent gas be provided without additional exhaust stacks and
effectively controlling the amount of heat provided by the main burner to the
medium. Burner control logic as described below is provided so that only the
main
burner in the fire tube 104 is active or the auxiliary burner assembly 210 is
active, in
this way the air 114 is available for either burner. The fire-tube 104 is
provided to
reduce the temperature of the external surface of the exhaust stack 106 in the
flame
region for safety and to prevent damage to the exhaust stack 106, however
depending on the particular auxiliary burner 210 configurations the fire-tube
104
may not be required.
Figure 3 depicts a more detailed drawing of the auxiliary burner assembly
210. The auxiliary burner assembly 210 comprises an outer pipe 300 for forming
part of the exhaust stack 106. The outer pipe 300 has flanges 204 and 205
having a
diameter compatible with the exhaust gas stack 202 and fire-tube 104. The
auxiliary
burner assembly 210 provides a burner unit 214 comprising an inlet pipe 302
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coupled to the gas supply via an elbow joint 304 to position the Venturi
burner barrel
310 vertically in the exhaust stack 106. Between the elbow joint 304 and the
Venturi
burner barrel 310, an air adjustment device 306 and an air-fuel ratio mixer
308 are
provided. The Venturi burner barrel 310 has an outlet nozzle 312 positioned
within
an inner fire tube 212, which can also be referred to as an inner heat shield
which is
supported within the outer pipe 300 by one or more supports 330 and 332. A
secondary air adjustment 314 may be provided in the inner fire tube 212 to
control
the air flow through the auxiliary burner assembly 210. The pilot assembly 216
provides a pilot and igniter 320 for igniting gas feed to a pilot flame or
spark by a
tube 324 within the inner fire tube 212 above the burner nozzle 312. The top
portion
of the inner fire tube 212 may extend above a top flange 205 of the outer pipe
300
for insertion into the exhaust gas stack 202. An access door 334 may be
provided
to enable access to air adjustments of the auxiliary burner assembly 210.
Multiple
internal shields can be more effective than a single reflector. A disadvantage
of
metallic shields, made for example from high temperature stainless steel, is
that the
repeated heating and cooling gradually causes the structural deterioration of
the
shield material. A method for shield replacement without a complete
dismantling of
the burner stack is a benefit. The access door 334 may be of sufficient size
to
enable replacement of the heat shield to be performed by removing the cover
334
over a slot in the auxiliary burner stack section to provide access to the
internal
shields where the outer pipe 300 extends above the inner fire tube 212
allowing for
easier direct access. The diameter of the auxiliary burner assembly 210 is
equal to
or larger than the diameter of the main fire tube 104.
The shields 212 may comprises dimpled sheets shaped in cylindrical shapes.
The dimpling is sufficient to provide physical separation by 1 mm or more of
individual layers. Alternatively the fire tube 212 may be provided by a
ceramic lining
within the exhaust gas stack to deal with the higher temperature in the
auxiliary
burner flame zone approximately 0.3 to 2.2 meters above the flange. The inner
lining of the lower stack portion can be composed of a heat resistant material
such
as fire brick reduces the surface temperature of the metal stack..
Figure 4 depicts a side view of a high capacity auxiliary burner assembly
having multiple burners positioned within the exhaust stack 106 of Figure 1.
The
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high capacity auxiliary burner assembly configuration enables a higher output
auxiliary burner to be utilized enabling the assembly to consume vent gas at a
rate
similar to or greater than the main burner unit. The auxiliary burner assembly
410 is
inserted between the exhaust gas stack 202 and the fire-tube 104 from the main
burner 100 with flanges 404 and 405 for connection to flanges 403 and 406
respectively of the exhaust stack 202. The auxiliary burners 412 and 414 can
be
positioned within the exhaust stack 202 to utilize the existing stack 202 as
part of the
fire tube for the auxiliary burners eliminating the need for additional stacks
or venting
methods. The auxiliary burners 412 and 414 may extend beyond, or above, the
flange 205 into the upper portion of the exhaust gas stack 202. A pilot 426 is
provided to ignite the auxiliary burners 412 and 414. If the auxiliary burners
can be
independently activated, an additional pilot may be provided for each burner
if
required. If there is a need to reduce the heat radiated by the exhaust stack
when
the auxiliary burner is in operation, one or more heat shields provided by
external
shrouds 416 and 418 which are situated on the exterior of the exhaust stack
facilitate cooling extending from the auxiliary burner assembly 210 upwards
along a
length of the exhaust stack. The heat shields may surround the exhaust stack
where heat would be increased due to the combustion zone of the auxiliary
burner
unit(s). The external heat shield enables the full diameter of the exhaust
stack to be
used as the combustion size increases the maximum burner size for the
auxiliary
burner. The external heat shield can dissipate heat output and enable the
burners
to handle similar or higher capacity to the main burner assembly. Spacers can
be
distributed between the external shrouds 416 and 418 and the exhaust stack to
maintain spacing and ensure consistent airflow. Alternately the external
shrouds
can be dimpled to provide suitable spacing without the need for spacers. The
external shrouds 416 and 418 can be made of a suitable metal formed as a tube,
curved segments, bars or corrugated metal wrapping to allow for cooling air
flow to
the exhaust stack. A portion 420, or the entire external shroud, may have
perforations or holes to facilitate cooling between the shrouds and the
exhaust
stack. Depending on the position of the auxiliary burner assemblies 412 and
414
the shroud may start above the auxiliary burner assembly 410 or may encompass
all
or part of the auxiliary burner assembly 410. Although the description and
figures
show two auxiliary burners one or more auxiliary burners may be utilized based
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upon the range of gas flow rates that may be provided by the vent gas dictated
by
the operation parameters of the facility. The size and length of the shroud
will
depend on the size and output of the auxiliary burners, the size of the
combustion
size and the specifications of the exhaust stack.
A high capacity auxiliary burner assembly 410 provides for the combustion of
excess combustible gases when the main burner is not required and inactive and
the pressure of the vent gas is above an upper pressure set-point value. The
high
capacity auxiliary burner size can approach or exceed the main burner size as
no
extra air that flows through the existing fire tube and the flame arrestor is
needed for
internal cooling. By integrating the auxiliary burners 412 and 414 into the
exhaust
stack 106, the combustion of the vent gases can be provided without additional
exhaust stacks at the production/processing site. Burner control logic as
described
below is provided so that only the main burner in the fire tube 104 is active
or the
auxiliary burners 412 and 414 are active at one time, in this way the air 114
is
available for either burner. Each auxiliary burner 412 and 414 may have gas
mixer
assemblies. Although the shrouds 416 and 418 are shown as commencing above
the auxiliary burner at flange 206, they may be part of the auxiliary assembly
410 or
provided in additional to the auxiliary assembly. Similarly the auxiliary
assembly 410
may be integrated in to the exhaust stack or provided in a retrofit
installation and the
shroud installed around an existing exhaust stack.
Figure 5 is a representation of a control system for controlling main and
auxiliary burners for combusting vent gases. The solid lines indicate pipes or
tubes
for gas flow and the dashed lines indicate electrical signals. The main burner
assembly 110 and its pilot assembly 112 are controlled by a Burner Management
System 502 (BMS). This BMS 502 monitors one temperature or more than one
temperature from a medium temperature sensor 580 (for example glycol) and an
emergency shutdown temperature sensor (ESD). When a parameter such as
temperature is below a user-settable medium set-point, gas is turned on to the
pilot
unit 112 by opening the solenoid valve 504. At the same time the BMS 502 sends
pulses to a coil which generates sufficient voltage to generate a spark near
the pilot
unit 112. A few seconds after the spark is generated, the current on the spark
electrode is measured. If a pilot flame is present there will be sufficient
ion current
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to exceed a threshold value. If there is no flame present, the spark will be
repeated
followed by an ion current measurement. Once the pilot flame has been on for a
desired period of time, the BMS 502 turns on the main gas pneumatic valve 506.
A
temporary medium set-point adjustment may be provided by the BMS 502 to
determine when the medium 102 is to be heated in response to a requirement to
relieve vent gas 501, for example when pressure is above an upper pressure set-
point, but the BMS 502 is not yet calling for heat for the main controller
503, the
Main controller 503 may cause the BMS 502 to temporarily increase the medium
set-point to enable the vent gas 501 to burn in the main burner assembly 110
until
the temporarily higher medium set-point is exceeded. By temporarily modifying
the
medium set-point the heat energy from the vent gases from a short-term
pressure
burst can be absorbed by the heat capacity of the medium 102 with minor upset
to
the system requiring heat. The burner unit 214 and its pilot assembly 216 are
controlled by an Auxiliary Burner Control system (ABCS) 510. The ABCS 510
ignites the pilot by opening solenoid shut-off valve 560 and after the pilot
has been
proven, the solenoid operated shut-off valve 512 is opened to bring gas to the
burner unit 214 of the auxiliary burner assembly 210.
The BMS 502 continuously monitors the temperature of the medium 102
needing heat by measuring the output of a thermocouple 580 in the medium 102
and subject to the low liquid level switch 584 and the gas temperature 582,
turns on
its pilot if heat is needed. After a delay to confirm the pilot of pilot
assembly 112 is
on, the BMS 502 opens the valve 506 for the main burner 110. The status of the
pilot is sent to the main controller 503 by the digital input to the main
controller 503.
When the pilot of the pilot assembly 112 is on, the main controller 503
controls the
gas flow according to the following criteria:
= If vent gas pressure 501 from a vent gas, measured by the pressure
sensor 530, is below the lower pressure set-point, the solenoid operated shut-
off
valve 532 is opened to allow the process gas 500 to flow to the main burner
110.
= If vent gas 501 pressure is above a specified first pressure set-point,
the solenoid operated shut-off valve 534 is opened to allow the vent gas 501
to flow
to the main burner 110 until the vent gas 501 pressure drops below the lower
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pressure set-point. The specified first pressure set-point can either be the
lower
pressure set-point plus a pressure margin or an upper pressure set-point.
= If the vent gas 501 pressure does not drop below a specified value
within a specified period of time, the BMS 502 is remotely shut-off causing
the pilot
112 and main burner 110 to go off. The auxiliary burner may then be utilized
to
reduce the pressure as the auxiliary burner may have higher burner capacity
than
the main burner.
When the main pilot 112 is off, indicating no heat is needed by the medium
102, the main controller 503 controls the gas flow according to the following
criteria:
10If the vent gas 501 pressure measured by the pressure sensor 530 is
=
below the lower pressure set-point no gas flow valves are opened.
If the vent gas 501 pressure is above the upper pressure set-point, the
ABCS 510 is turned on remotely. The ABCS 510 causes the pilot 216 to ignite
using similar criteria to the BMS 502. When the pilot 216 is confirmed to be
operating, the ABCS 510 opens solenoid operated shut-off valve 512 and
solenoid
operated shut-off valve 534 allowing the vent gas 501 to flow to the auxiliary
burner
unit 214 until the vent gas 501 pressure drops below the lower pressure set-
point.
The main controller 503 monitors the low level pressure switch 540, and does
now allow any parts of the system to operate if the process natural gas 500
pressure
is too low. If the high-level switch 542 in the liquids knock-out container
for the vent
gas 501, the burner is not permitted to burn the vent gas 501. Additional
pressure
control regulators 566 and 568 may be provided in the system to regulate gas
flow
within the system. The ambient temperature may be received by the main
controller
503 from sensor 586 to enable control modifications according to the ambient
temperature or to enable ambient temperature corrections to orifice flow
calculations. Referring to Figure 6, if multiple auxiliary burners 412 414
such as
shown in Figure 5 are utilized, one or more valves 562 may be used to control
initiation of a respective auxiliary burner based upon changes in gas flow
rates. For
example the burners may be of different sizes where one burner is utilized for
a low
flow rate and a second burner is utilized for a higher rate and both burners
are
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utilized when full capacity is required. An optional data-logging device 550
can be
provided to collect process information on the system operation.
It should be understood that there may be variations in the logic details and
the choice and positioning of control and monitoring devices and the
implementation
disclosed should not be considered limiting in any way. The described system
and
method enable a desired amount of heating to be maintained by using as much
vent
gas as possible and ensuring the vent gas vapors are combusted to prevent the
escape of the unburned vent gases to the atmosphere. Although the control
functions described in relation the burner management system, ABCS, and main
controller may be associated with a particular controller containing a
processor, the
functions may be performed by a single controller or processor; alternatively
the
functions may be allocated across multiple controllers or processors. The
control
functions are provided by at least a processor having an associated memory,
the
memory providing instructions for performing one or more functions associated
with
controlling a main burner and auxiliary burner to combust vent gases of a
petroleum
production and processing facility.
Figure 7 shows a method for controlling a main burner and an auxiliary
burner assembly for combusting vent gases. The medium coupled to the system is
monitored (702) by the BMS 502 by using one or more parameters to determine
heating requirements such as but not limited to medium temperature, ambient
temperature, a low liquid level indication and gas temperature. If heat is
required for
the medium (YES at 704) the pilot of the main burner is initiated, as
discussed
previously, and it is confirmed that the pilot of the main burner is active
(706). If the
pilot is not successfully initiated a fault condition may be identified. A
pressure
sensor of the vent gas 701 is monitored (708) to determine if the pressure
value is
below a lower pressure set-point value. If the pressure value is below a lower
pressure set-point value (YES at 710) a valve to flow process gas to the main
burner
is opened (712). If the pressure is not below the lower pressure set-point
value (NO
at 710) it is determined if the pressure is above a first pressure set-point
value
greater than the lower pressure set-point value. If the first pressure set-
point value
for the pressure is exceeded (YES at 714), a valve to provide gas from the
vent gas
to the main burner is opened (716), and otherwise (NO at 712) the pressure of
vent
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gas is monitored (708). If the pressure does not drop below a specified value
within
a specified period of time (NO at 718) the pilot and main burner is shut off
(720).
The pilot and main burner may be shut-off as a safety criterion to prevent
over-
heating of the exhaust stack or the fire-tube and to meet regulatory
requirements.
The capacity of the combined auxiliary burners may be larger than the main
burner
assembly capacity; therefore if the main burner capacity is insufficient to
reduce the
pressure then the vent gas is switched over to the higher capacity auxiliary
burners.
If the pressure does drop to a specified value within a specified period of
time, (YES
at 718) the heating of the medium continues (702) until requirements change.
If the main burner and pilot assembly are not active, the auxiliary burners
can
be initiated if required. If the gas pressure is below the lower pressure set-
point
(YES at 722) no valves are opened (724) and the auxiliary burners are not
initiated.
Alternatively, if the auxiliary burners are already active, the associated
valves are
closed to shut-off the auxiliary burner assembly. If the gas pressure is below
the
upper pressure set-point (NO at 726) no valves are opened (724) and the
auxiliary
burner is not initiated. If the gas pressure is above the lower pressure set-
point (NO
at 722) and is above the upper pressure set-point (YES at 726) the auxiliary
pilot is
initiated (728) as discussed above. Once it has been confirmed that the pilot
is
active the valve from the vent gas to the auxiliary burner(s) is opened (730)
and the
decision process starting at (702) is repeated until the upper or lower vent
gas
pressure set-points are exceeded. Alternatively, if the medium heating
requirements
change (702) the auxiliary burner assembly may be turned off (705) and the
main
burner initiation process commenced (706). If
multiple auxiliary burners are
provided the selection of which of the auxiliary burners that are required may
be
determined based upon gas flow rates and capacity ranges of each of the
auxiliary
burners. If there is a need to quickly switch the vent gas flow from the main
burner
to the auxiliary burner, the auxiliary pilot can be on at all times.
Figure 8 shows a method for controlling an auxiliary burner for combusting
vent gases. The auxiliary burner is located within the exhaust stack
associated with
the main burner assembly where only one burner is utilized at a time. The
integrated auxiliary burner assembly enables the same exhaust stack as the
main
burner assembly while effective combusting exhaust gases and providing heating
of
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a medium when required. A controller determines a state of a main burner
assembly (802). The main burner assembly is associated with a medium which is
heated either by the process gas or by the vent gases provided by the
petroleum
production and processing facility. The medium is heated when parameters
associated with the medium meet specified requirements. For example, the main
burner assembly may be activated when a medium/temperature set-point
associated with the medium is reached. When the main burner assembly is not
required to heat the medium, or inactive, a pressure value of vent gas is
determined
(804). When the pressure value of gas in the vent gas exceeds an upper
pressure
set-point value, and the state of the main burner assembly is inactive, the
auxiliary
burner is initiated to combust the excess vent gas (806).
Figure 9 shows a representation of an embodiment of the auxiliary burner
assembly having baffles. Within the fire-tube 104 a main baffle 910 provides
sufficient air flow resistance to ensure the hot gases of combustion from the
main
burner 110 preferentially flow through the fire-tube 104 and the exhaust stack
202.
Also the baffle 910 can be partially opened or closed to ensure the air to
fuel ratio
for the main burner 110 is suitable for optimum combustion conditions. The BMS
starts and stops the main burner 110 to ensure the temperature of the process
heater, in this case a glycol bath, is maintained near to a desired
temperature set-
point.
In some cases the maximum capacity of the auxiliary burner is insufficient
to meet or exceed the rate at which vent gases are generated by other
processes.
An example of the variability in vent gas flow rate from a petroleum liquids
storage
tank is shown in the graph 1000 of Figure 10 where the vent gas flow from the
petroleum liquids storage tank varies from 0 to 10 m3/h over a period of a few
hours.
A limitation in the maximum capacity of the auxiliary burner is the
temperature of the stack portion adjacent to the flame zone. The maximum
temperature is determined by the material properties of the stack and the self-
ignition temperature of an air-fuel mixture that may exist adjacent to the
stack. A
vertical temperature profile of the stack surface temperatures observed when
the
auxiliary burner is operational is shown in graph 1100 of Figure 11. The
height
above the flange is the position of the temperature measurement above flange
205
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in figure 2. For this test the inner pipe 212 was not present. Figure 11 shows
a
graph 1100 which demonstrates that the highest temperature portion is limited
in
length.
The removal of the inner heat shield 212, removal of the auxiliary baffle
920, and/or an increase in the stack diameter may be used to increase the
auxiliary
burner capacity. However these modifications do not increase the amount of air
available that must be drawn in through the existing fire-tube and flame
arrestor. It
is well known that an increase in stack height does increase the air flow, but
there
are practical and economic reasons that preclude a large increase in stack
height.
A further method of providing the additional air required by the auxiliary
burner is the
addition of a fan between the flame arrestor 108 and the main burner 110. The
fan
can provides the additional air required by the auxiliary burner.
Alternatively, Figure 12 provides a method of controlling a main burner
without using an auxiliary baffle. Removal of the main burner baffle 910,
shown in
figure 9, will remove some of the air flow resistance when the main burner 110
is off
and the auxiliary burner 214 is on. The method may be implemented may for
example when initiating the main burner (706) in Figure 7. To ensure the
desired
direction of the air flow in the main fire-tube 104 prior to the ignition of
the pilot flame
for the main burner and the start of the main burner, the auxiliary pilot
(1202) if not
already on and, if necessary, the auxiliary burner (1204) is started before
the start of
the main burner pilot (1206). After a suitable time delay for the
establishment of air
flow up the stack (1208), the main burner pilot is started and once proved to
be
satisfactory (YES at 1208) the main burner fuel is turned on to start the main
burner
(1210). The air flow velocity can be calculated from the flow rate of the
burner fuel
and the exhaust oxygen concentration by well-known methods (1212). A suitable
delay period can be determined from the air flow velocity when the auxiliary
burner
and/or its pilot are on (Yes 1214). Just after the main flame is ignited, the
auxiliary
burner and, if desired, its pilot can then extinguished by turning the fuel
off (1216).
An initial estimate of reduction in flow resistance caused by the elimination
of the
main burner baffle is 40%. This, in turn, enables more air to be available for
the
auxiliary burner and allow for an increase in the auxiliary burner capacity.
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Figure 13 shows a representation of an auxiliary burner with a flame arrestor.
In order to provide additional air for the auxiliary burner a flame arrestor
1302 can be
added to the stack just below the auxiliary burner position. The additional
air
provides a means of absorbing more combustion heat from the auxiliary
burner(s)
and thus increasing the capacity of the auxiliary burner(s). When the
auxiliary
burner 214 is on, the flame will draw air from both the main flame arrestor
108,
shown in Figure 1 and air 1314 from the secondary flame arrestor 1302. When
the
main burner 110 is on, the flow resistance for the exhaust gases is less to
flow up
the stack than to flow through the flame arrestor such that the secondary
flame
arrestor 1302 does not act as a conduit for the main burner exhaust gases. In
fact
some air is added to the exhaust gases from the main burner 110. An
electronically
activated shutter 1304 may be inserted to further ensure the exhaust gases for
the
main burner will not be diverted through the secondary flame arrester 1302.
In some embodiments, any suitable computer readable media can be used
for storing instructions for performing the methods described herein to be
executed
by a processor. For example, in some embodiments, computer readable media can
be transitory or non-transitory. For example, non-transitory computer readable
media can include media such as magnetic media (such as hard disks, floppy
disks,
etc.), optical media (such as compact discs, digital video discs, Blu-ray
discs, etc.),
semiconductor media (such as flash memory, electrically programmable read only
memory (EPROM), electrically erasable programmable read only memory
(EEPROM), etc.), any suitable media that is not fleeting or devoid of any
semblance
of permanence during transmission, and/or any suitable tangible media.
Although the description discloses example methods, system and apparatus
including, among other components, software executed on hardware, it should be
noted that such methods and apparatus are merely illustrative and should not
be
considered as limiting. For example, it is contemplated that any or all of
these
hardware and software components could be embodied exclusively in hardware,
exclusively in software, exclusively in firmware, or in any combination of
hardware,
software, and/or firmware. Accordingly, while the fore-going describes example
methods, systems and apparatus, persons having ordinary skill in the art will
readily
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appreciate that the examples provided are not the only way to implement such
methods and apparatus.
The scope of the claims should not be limited by the preferred embodiments
set forth in the examples, but should be given the broadest interpretation
consistent
with the description as a whole. In the design of the auxiliary burner
section, one or
more of the described embodiments may be used to meet the desired maximum
vent gas flow rate to the auxiliary burner. It is recognized that variations
in the
described innovations are possible.
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