Note: Descriptions are shown in the official language in which they were submitted.
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This invention lies in the field of gas flaring systems. More
particularly, it concerns flares in which waste gases are flared and in which
steam is used for injection into the flame zone for the purpose of smokeless
combustion of the gases.
Still more particularly, this invention is concerned with the
protection of the steam injection parts from excessive heat due to the
maintenance gas flow, when waste gases are not being flared.
The art of smokeless flare burning of smoke-prone gases, through
the injection of steam in the conventional manner, to the burning zone of
the flare, is now well known. For the portion of the year when ambient tem-
perature is higher than the freezing point of water there is little difficulty
involved in flare operations. However, when the ambient temperature falls
to or below freezing, there is considerable difficulty.
The difficulty arises because, for a very large percentage of
the time, the flare is either in standby condition, for emergency flaring,
or is discharging and burning gases at a minimal rate, which may be of the
order of 1% of design flow capacity. This condition of gas burning, even
though it is a small portion of the flare capacity, is still productive of
enough heat to very seriously damage the steam injection parts, unless there
is constant flow of a coolant medium through and from the steam injection
parts.
A typical coolant medium is steam and when the weather is mild,
cooling by steam flow is quite satisfactory. Although the quantity of steam
flowing is quite small, because the cost of steam is so great, such flow is
expensive. Also, in cold weather, a large portion of the steam condenses to
water, to be sprayed at random over the critical flare discharge areas. At
very low temperatures, ice for~s in or near the flare discharge areas to
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partially or completely block the flared gas passages to the atmosphere.
This can bring about a condit;on of extreme emergency in the process op-
erations, where flare venting of emergency relieved gases demands instant
and unobstructed gas flow to the atmosphere.
There is also another source of water which may ultimately
freeze in and on the flare. This comes from the large capacity steam line
from the source of steam, to a control point at the base of the flare. The
steam line, which is typically hundreds of feet long, and is either at no
flow or very small flow at the time the flare is on standby, is subject to
heat loss despite insulation. At a time when the control means calls for
steam, and flow to the flare from the control point begins> the flow is
initially all the accumulated condensate, to be followed by steam, after the
water is cleared out. Then there is additional water which has accumulated
in low areas of the steam line, which flows as slugs, on arrival at the steam
line up the flare. However water may be delivered to the flare, the freezing
hazard is present and it demands solution in point of sources of water, at
or near to the burning zone of the flare, when the weather can cause freezing
of the ~ater.
Itis a primary object of this invention to provide a system for
cooling the steam injection parts adjacent the burning zone in a flare stack
during the periods when the flare stack is in standby condition, and there is
only minimim flow of gas through the stack.
It is a further object of this invention to provide an alternate
to the flow of steam for cooling the steam parts, which alternate flow is
of a gas of similar specific heat per ~nit volume weight and not per unit
volume, namely air.
It is a further object of this invention to provide a system
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which can supply air from a blower to pass through and cool the steam
injection parts, at the outlet of a flare stack.
According to one aspect of the invention there is provided
in a flare stack system in which waste gas is flared on demand, and in which
the flow rate of waste gases may vary from low flow rates to high flow rates
of up to 100% of the design value of gas flow rate; and including
steam injection means at the top of said flare stack comprising
manifold means connected to a steam riser pipe, and plural injection nozzles
connected to said manifold;
the improved apparatus comprising low pressure air blower means,
and a first controllable air valve for providing controlled on-off flow of
blower air to said steam riser pipe;
steam line means and a second controllable steam valve for
providing controlled on-off flow of steam on demand to said steam riser pipe;
and
control means for said air valve and steam valve for alternate
flow of air or steam to said steam riser pipe, such that under freezing
atmospheric temperature conditions low pressure air is flowed through said
steam riser pipe to said steam injection means when said gas flow rate is
low and steam under substantial pressure is flowed through said steam riser
pipe to said steam injection means when said gas flow rate is high, and
whereby when said steam valve is open said air valve is closed, and vice
versa,
said control means being responsive to said flow rate of waste
gas, to open said steam valve wherever waste gases are being flared at said
high flow rate.
According to another aspect of the invention there is provided
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in a flare stack system m wh~ch waste gas i~ flared on demand, and in which
the flow rate of waste gas may vary from a low flow rate to a high flow rate
of 100% of the design value of gas flow rate; and including
steam injection means at the top of said flare stack comprising
manifold means connected to a steam riser pipe, and plural injection nozzles
connected to said manifold; and wherein;
under freezing atmospheric temperature conditions low pressure
air is flowed through said steam riser pipe to said steam injection means,
when said gas flow rate is low, and steam under substantial pressure is
flowed through said steam riser pipe to said steam injection means,when
said gas flow rate is high;
the method comprising the steps of providing a supply of low-
pressure air;
providing a supply of high-pressure steam; and
alternatively controlling the supply of low-pressure air, or
high-pressure steam, to said steam riser pipe, responsive to the flow rate
of said waste gases to said flare stack;
whereby, when said gas flow rate is highJ steam at high pressure
flows through said riser pipe, and said air valve is closedJ and when said
gas flow rate is 1OWJ said low-pressure air flows through said riser pipeJ
and said steam valve is closed.
Although the specific heat of steam is twice that of airJ since
one pound mol of air is almost twice the wei~ht of one pound mol of steamJ
their cooling effect is substantially the same. ThusJ air and steam can be
substitutedJ one for the otherJ as a cooling medium through the steam parts
that are exposed to the flame in the standby condition.
A better understanding of the principles and details of the
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invention will be evident from the follow;ng description taken in conjunction
with the appended drawings in which;
FIG~RE 1 shows the essential parts of the system.
~ GURE 2 illustrates the normal type of steam injection appar-
atus mounted at the top of a flare stack.
Referring now to the drawings, there is shown means for avoiding
the unintentional delivery of liquid water to a steam actuated smokeless
flare at any time. Since the use of steam as a coolant is significant, as
a source of water, its use is undesirable not only because of its cost, but
also because when the ambient temperature is below the freezing point, the
condensation of the steam passing through the steam injection parts, produces
water which may freeze and cause difficulty in the subsequent burning of
the waste~ gases.
FIGURE 1 illustrates one em~odiment of the system in which
steam is applied through a steam line 50, in accordance with the arrow 68,
for the purpose of injection into a steam injection apparatus 16, 18 on the
top of a flare stack 10 shown in FIGURE 2. The steam line includes a conden-
sate trap 62 and condensed water outlet pipe 64, where any condensate carried
by the steam 68 is separated in the form of water flow indicated by the arrow
66.
The steam flow 60 then proceeds through a conduit 50, through
a remote-controlled valve 48, and through a pipe 28 to the base of the flare
stack 10 and to steam riser pipe 22, which passes up along the side of the
stack 10, as shown in FIGURE 2, to the steam injection parts 16, 18 at the
top of the flare stack. These steam injection parts comprise a circular
manifold 16 which surrounds the flare stack at the top 12, a plurality of
small diameter pipes 18 which rise up from the manifold 16, and which have
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~34737
radial openings or orifices, from which the steam sprays out radially across
the combustion zone, immediately above the top 12 of the stack.
In FI~URE 2 the waste gases 14 pass up through the flare stack
10, either as shown, or as in some cases through a spider arm burner, or
similar device, to the flame zone. The steam jets 20 are at high velocity
and are directed inwardly to the hot burning gases, which cause turbulence
and mixing of the steam with the hot gases to provide the desired chemistry
of smokeless burning.
The valves 36 and 48 have operating controls 38 and 46 respect-
ively, which are connected by control leads 44 to a control means, not shown,
bu~ well known in the art. The control means may be pneumatic or electric,
as is well known in the art. The two valves 36 and 48 are connected so that
they are in opposition, one being closed when the other is open, and vice
versa.
At the times when the flare is on standby, and the ambient
temperature is below freezing, the blower 42 is operated, and air is supplied
through open valve 36, up pipe 22 to the steam parts 16 and 18, providing
cool air through the hot parts to prevent damage due to the heat of the flame.
In the event that air is not available, for any reason, it is
necessary that there be substitute means for cooling the steam injection parts.
Therefore, the cooling system must permit alternate selection of a cooling
medium. Steam may be selected as the alternate medium, but any gaseous
coolant flow would be equally good. In case steam is desired to provide the
cooling effect through the steam injection parts, while the flare is on
standby, instead of opening the valve 48 a bypass 52 is provided so that a
small part of the steam 60 can go through the bypass 52, through a valve 56
and through a flow limiting orifice 54, and in accordance with arrow 57
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flow back into the steam line 22> and up to the top of the flare stack.
When the ambient temperature is above freezing, of course, eitherair or steam can be used for standby. In one case, the valves 56 and 48
would be closed and the valve 36 would be open, and air would be supplied
from the blower 42. In the other case, the valve 36 would be closed, or alter-
nately the valve 34 can be closed, and the valve 56 opened, providin~ a
maintenance flow of steam for purposes of cooling the parts 16 and 18, etc.
When the flow of waste gases is initiated, the control on 44
then opens valve 48 and closes 36 and permits a full flow of steam 60 through
valve 48 and as arrow 24 to the injection parts 16 and 18, in the well known
manner.
Since the steam pressure applied to line 50 is much greater
than the pressure of the air supplied by the blower 42, it is important
that the valve 36 (or 34) be closed whenever the steam valve 48 is open,
otherwise part of the steam will be bypassed back through the blower causing
serious damage.
What has been described is a system for providing a standby
cooling gas through the steam injection parts in a flare stack, which uses
steam injection for purposes of smokeless combustion, whenever the flare
is on standby, and there is only a minimal flow of gases, whether waste gas
or fuel gas supplied to the stack. When the ambient temperature is above
freezing, any gas can be used such as either air or steam, since there is
no problem of ice formation. If steam is available it may be used, on the
other hand, air from a blower can be supplied at less cost, than can the
steam, and so it may be desirable to use air at all times during the standby
period. Of course, when the valve 36 is closed, the blower is shut off and
the power required for the blower is thus saved. This is shown schematically
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~l34 .~3~
by the dashed line 72.
In cases where the ambient temperature is below freezing, the
preferred gas to use for cooling purposes is air, since in the use of blower-
supplied air, there is no danger of carrying water to the region of the steam
injection parts.
The use of air for cooling the steam injection part corrects
for one source of water at the top of the flare stack. The other source of
water due to the condensation of steam inside the steam supply line is pref-
erably corrected by means of apparatus 62, which provides a separation or
trap, for the condensate, and permits the dry steam to pass on as arrow 60
to the riser pipe 22 and to the stack.
Since the relative cooling effect of a given number of mols of
steam and air is roughly the same, it is clear that economically it is pref-
erable to use blower air for cooling the steam parts, than to use steam,
under all conditions of ambient temperature. In such case, the bypass line
52 would not be needed nor would the valve 34 and the control would simply
open valve 36 and close valve 48 when the flare stack is on standby and it
would close valve 36 and open valve 48 when waste gases are flared and
burned.
In the day-to-day operation of a smokeless flare, where steam
is used for smoke suppression, the flare, on demand, must immediatedly accept,
and burn, gas discharges which vary in volume from 100% of design discharge
rate to less than 0.5% of des;gn discharge volume. When the discharge rate
for flammable gases exceeds a minor portion of gas discharge rate, there is
delivery of steam to the steam injection parts for the cooling of them in
their locations in or immediately adjacent to the flame produced as the
vented gases burn.
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Such steam-induced cooling is vitally necessary to avoid severe
heat damage to the steam injection parts when there is flame at the flare
discharge point, and without reference to the size of the flame produced.
Since steam injection parts demand cooling when flame is present, and since
venting at 0.5% of design (200-1 turn-down) does not cause steam injection
under control, but does create appreciable flame at the flaring point, as
a source of heat damage to injection parts, it has been common practice to
add a by-pass around the steam control valve to assure small steam delivery
to injection parts on a 24-hour basis via the normal steam supply system
after the control valve.
The small steam delivery thus described suitably cools the
injection parts when there is flame present as at 0.5 to 1% of design gas
venting, or even more than 1% of design venting rate, as is practice-proven
in hundreds of cases. But the small steam delivery is productive of an
accessory problem in frigid weather when there is no gas venting, and this
second problem is utterly severe. Due to cold weather, the small quantity
of steam condenses when there is zero venting of gases to the flare and no
flame is present at the flare discharge point. The condensed steam (water)
spatters and freezes on flare parts, including the gas tube to the burning
point, which may become covered with ice. Emergency venting of gases then
becomes impossible, and the process facility is, totally, in danger of ex-
plosion because pressure-relief on process equipment (which is the main
reason for the flare) is impossible.
Such freezing of smokeless flares is quite common in typical
winters, for process plants in areas north of Latitude 45; is far from rare
down to 40 North, and has been known to occur farther South.
The flare must not become inoperable at any time the process
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plant is in operation, in any area where there is freezing danger, as dis-
cussed. Therefore, while steam coolant for steam injection parts is quite
suitable for Spring, Summer, and Fall, it is not at all suited to Winter-
time operation. A coolant which is not subject to freezing is demanded for
very small flare burning and periods when there is no burning at the flare.
Because of its physical characteristics, air, which cannot
cause freezing, is admirably suited as a substitute for the steam coolant,
and at low pressure for just-ample flow. However, steam is demanded for
smoke suppression in the burning of large flow rates of smoke-prone gases as
they are flared, and high steam pressure ~up to 100# or more) is needed for
the steam for injection to the burning zone from the identical parts which
have been discharging the air coolant, and in a substantially instantaneous
manner. Both flows are subject to automatic control for air or steam deliv-
ery sequence. This is to say that, at a time when coolant air is being de-
livered, if it becomes necessa~ to flare-burn smoke-prone gases, steam must
immediately replace the air coolant under control as to quantity. The
specification describes apparatus to perform this service and also to return
to air coolant delivery when there is no longer necessity for gas venting
at large flow ratel and there is no longer need for steam delivery.
A signal comes via 44 to 38 and 46 to cause closure of 36 and
opening of 48. The signal to 38, removes motive power from 42, via 72, and
flow of air 32 ceases, while steam begins to flow through opened 48 via 28
to 22. Meanwhile, 56 is closed. Because 36 is closed, steam must proceed
via 22 to 16 and 18 for the required smoke suppression as long as there is
operating signal via 44.
Automatically, and as the signal via 44 ceases, 38 causes
opening of 36, and simultaneousl~ ~via 72) power is again applied to the
motive source of 42, and air flow 32 begins via opened 36 and 26 to 22,
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~34 J~7
while valve 48 is closed. The cooling air flow to 16 and 18 is thus auto-
matically restored when there is no longer any need for steam. In the
Spring, Summer, and Fall, when there is no danger of freezing, 56 is opened
for passage of steam coolant via 28 and 22 to 16 and 18. But, in the Winter,
56 is closed, and only non-freezing air is used as coolant for 16, 18. In
the Spring, Summer, and Fall, the valve 34, which can be for manual operation,
is kept closed. But, in the Winter, 34 is kept open to permit passage of
air coolant from 36 to 26 and 22 to 16 and 18.
The numeral 68 is for steam flow, as permitted, or required.
Condensate ~water) is typical of lines for saturated steam (which is typi-
cally used), and 62 is a trap means for condensate removal from the steam
system as 66 from 64 to permit flow of dry steam as at 60.
