CA3025992A1

CA3025992A1 - Improved flare stack

Info

Publication number: CA3025992A1
Application number: CA3025992A
Authority: CA
Inventors: Kent Stormoen; Paul Chouinard; David Speed
Original assignee: MAXIMUM EROSION MITIGATION SYSTEMS Ltd
Current assignee: MAXIMUM EROSION MITIGATION SYSTEMS Ltd
Priority date: 2018-11-30
Filing date: 2018-11-30
Publication date: 2020-05-30
Also published as: US20200173656A1

Abstract

In one aspect there is provided a gas flare comprising a hollow cylindrical member having a bottom end, a top end, a discharge end at said top end, and defining an interior volume having a mixing region. The gas flare further comprises a gas inlet to receive waste fluids, an air inlet to receive and direct air into the interior volume, and an internal riser having an outlet. The mixing region is located above the air inlet and below the discharge end. The internal riser fluidly and sealably connects to the gas inlet and directs all waste fluids from the gas inlet, out through the outlet, into the mixing region.

Description

"IMPROVED FLARE STACK"
FIELD OF THE INVENTION
The field of present invention relates generally to flare systems used at oil and gas well sites and, more particularly, to an improved flare stack.
BACKGROUND OF THE INVENTION
The background information discussed below is presented to better illustrate the novelty and usefulness of the present invention. This background information is not admitted prior art.
A variety of apparatus for flaring combustible waste fluid streams have been developed and used in the past. Such apparatus are often referred to as flares, gas flares, or flare stacks. Flares dispose of waste fluids, such as hydrocarbon gasses, in an environmentally compliant manner through the use of combustion.
Flares are commonly located at production, refining and other processing plants. They are a critical component of a system design intended for safely disposing of combustible wastes or other combustible streams, such as hydrocarbons from pressure-relieving and vapour-depressurizing systems.
Referring to Figure 1, a flare stack is generally a hollow cylindrical member that may include a flare section having a discharge end which, during operation, is positioned some distance or height above the ground to direct the combustion products and combustion gasses high up into the atmosphere, so that they may be more readily dissipated into ambient air by prevailing air currents. The flare may be connected to stack by means of a hinged or flanged connection, so as to further increase the flare's height above the ground during operations.
Flare stacks may be hundreds of feet in height, and a hinged connection facilitates transport of a flare stack on a truck, trailer or the like, such as when it needs to be transported in a collapsed state between job sites.
Continuing to refer to Figure 1, the flare and stack (together referred to as a flare stack) are typically hollow tubular members constructed of metal and are substantially air/gas tight so as to direct the hydrocarbon gasses from a gas inlet (which is generally located substantially adjacent the bottom of the flare stack), through the flare's interior volume, and then out of an outlet at the discharge end (which is generally located substantially adjacent the top of the flare stack). A drain and drain valve may be provided near the bottom of the flare stack to facilitated draining of condensate and other liquids that may build up in the flare's interior volume over time.
Unfortunately, the efficient flaring of methane or other flammable waste gasses in oil and gas field operations has been a problem for many decades.
Gas compositions can change greatly, including from having hydrogen sulfide (H2S) in the gas stream to containing varying degrees of light ends. Light ends become more dominant in high temperature flow backs. Research has been done to determine the combustion efficiency of methane gas. But it is very difficult to simulate actual flow conditions on a well site. Conditions of the flow parameter will also change as the well cleans up from different operations, such as fracturing,

2 acidizing or clean out operations. Even as the well goes from shut in to flowing conditions, the gas properties will change to some degree. Burn efficiencies will also change with the gas rate being flared. Research indicates that higher gas rates based on flare stack diameter will burn at a higher degree of destruction than lower rates. Research also indicates that wind conditions affect the destruction quality of the burn.
Attempts have been made to resolve the flaring issues. Most involve tip design, or injection of gas or air. Injection of air or gas requires a pumping device of some sort and would, in order to be efficient, have to vary injection rates to follow .. varying gas flow rates. If methane were the required injection source, then a pipeline or production facility would have to be relatively close by and piping would be required along with process equipment to regulate the required flow, as flow parameters changed.
Most flares are designed with a shroud or can at the top of the flare stack around the discharge end; see Figure 1. The purpose of the shroud is to, in some cases, allow for an air draw from the bottom of the shroud and also to partially protect the flame during windy conditions. In the cases where the air draw is designed into the stack via a shroud, such drawn air is usually not in sufficient volumes to ensure a good burn. Moreover, the intake air is typically concentrated to the outer portion (or periphery) of the flare's interior volume and waste gasses being burned. This prevents a good portion of the inner gas flow not to have the adequate air/gas mixture to ensure total combustion. Such incomplete combustion or burning of the gases then unduly contaminates the ambient atmosphere, increases smoke

3 emissions, and reduces flame luminosity. Therefore, what is needed is an improved flare stack that does not suffer from these disadvantages and inefficiencies.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings, several aspects of the present invention are illustrated by way of example, and not by way of limitation, in detail in the figures, wherein:
FIG. 1 is a diagrammatic, sectional, side elevation view of a PRIOR
ART flare stack;
FIG. 2 is a diagrammatic, sectional, side elevation view of one embodiment of a flare stack of the present invention; and FIGS. 3a ¨ 3c are perspective, side views of the flare stack of FIG. 2 alongside a prior art flare stack, both shown in operation and both having a flame plume.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description is of preferred embodiments by way of example only and without limitation to the combination of features necessary for carrying the invention into effect. Reference is to be had to the Figures in which identical reference numbers identify similar components. The drawing figures are not necessarily to scale and certain features are shown in schematic or diagrammatic form in the interest of clarity and conciseness.

4 A first embodiment of the flare stack 10 of the present invention is shown in FIG. 2. The flare stack 10 is preferably a hollow cylindrical member having a bottom end 10b, a top end 10t and defining an interior volume 10v.
The flare stack 10 may include a flare section 12 having a discharge end 18 at the top end 10t which, during operation, is positioned some distance or height above the ground R to direct the combustion products and combustion gasses high up into the atmosphere. The flare 12 is preferably mounted on top of a stack section 14 by means of a hinged or flanged connection 16, so as to further increase the flare's .. height above the ground R during flaring operations. A gas inlet 19, to receive waste fluids and gases G to be combusted, is preferably provided at the bottom end 10b. A conventional shroud 20 is preferably provided at the discharge end 18.
Air A may travel through the shroud 20 via inlets 21 at the shroud's bottom 20b in a conventional manner. A conventional drain with drain valve 22 is preferably provided at the bottom end 10b.
The flare stack 10 further comprises an internal riser 30 having an outlet 32. The riser 30 is a tubular member which fluidly and sealably connects to the gas inlet 19 so as to direct all waste gas G, that enters the flare stack 10 via inlet 19, out through the outlet 32 and into a mixing region 10m within the interior volume 10v. The outlet 32 may be provided with a nozzle 32n to adjust, e.g.
raise or lower, the rate of waste gas G exiting out of it. The riser 30 is of a smaller diameter than the interior diameter of the flare stack 10. For example, in an

5 embodiment where the flare stack 10 has a six (6) inch diameter, a suitable diameter for the riser 30 and outlet 32 is three (3) inches for most common flaring operations. Further, in such an embodiment, a nozzle 32n to reduce the outlet down to two (2) inches, or increase it to four (4) inches may be provided to adjust the flow of gas G out the outlet 32 as may be desired and in relation to particular well and gas flow conditions. The outlet 32 may be provided with a threaded end (not show) and the nozzle 32n may also be provided with a matching threaded end (not shown) so that the nozzle 32n may be easily installed on and/or removed from the outlet 32.
The riser 30 is preferably mounted substantially centrally and co-axially to the flare stack 10 within the interior volume 10v, so as to create an annulus 40 between the riser 30 and the flare stack's interior wall 10w. The riser 30 may be supported and centralized by means of mounting brackets 35 or the like.
In an alternate embodiment (not shown), the riser 30 may be mounted peripherally against one side of the flare's interior wall 10w. In such a case, the region between the riser 30 and the flare stack's interior wall 10w will not be an annulus, but have some other cross-sectional shape. The riser 30 may be constructed of metal pipe or the like.
The riser's outlet 32 is preferably located some distance from the flare's bottom end 10b. For example, in an embodiment where the flare stack 10 is sixty (60) feet tall, and the connection 16 is at the half-way point, at thirty (30) feet, the riser 30 preferably extends up along the interior volume 10v so as to place the riser's outlet 32 just below the connection 16. As such, the gas G that

6 conventionally would enter the interior volume 10v a few inches above the bottom end 10b (via the inlet 19) is now directed to enter the interior volume 10v at approximately the half-way point up the flare stack 10 (i.e. almost thirty feet up above the bottom end 10b, in a sixty feet tall flare stack 10).
Advantageously, by positioning the outlet 32 just below the connection 16, the top part of the flare stack (i.e. flare 12) may be disconnected, and pivoted away, from the bottom part (i.e.
the stack 14) so as to then provide an operator with easy access to the outlet 32 to install or change any nozzle 32n.
10 The flare stack 10 further comprises an air inlet 50 to direct air A into the annulus 40 (or similar region between the riser 30 and interior wall 10w).

Preferably, the inlet 50 further comprises a valve 52 to allow an operator to control if / when air A can enter the annulus 40. In an embodiment of the flare stack 10 where the stack is a six inch diameter, sixty feet tall cylindrical flare stack, where the gas inlet 19 and riser have a diameter of three inches, then the air inlet 50 preferably has a four inch diameter.
During operations when the valve 52 is opened and air A is allowed to enter the annulus 40, then such air A will typically fill the annulus 40 and rise up towards the riser's outlet 32. If gas G is moving through the inlet 19, up the riser 30, out the outlet 32 and expanding into the interior volume 10v, then a venturi effect is created drawing up more air A into the annulus 40 via the air inlet 50. In an embodiment where the flare stack has a 6 inch diameter, a 60 feet total height, wherein the riser 30 has a 3 inch diameter and the outlet 32 is positioned just below

7 the connector 16, the inventor(s) expect that during typical oil field flaring operations the venturi effect will generate approximately 5 PSI of vacuum; which the inventor(s) calculate out to create in excess of 1 mmcf/d of additional air A draw into the annulus 40. Preferably a vacuum gauge 45 is provided to measure any such vacuum that is created in the annulus 40.
Valve 52 can also be utilized during operations, e.g. to be closed prior to shut in of an oil- or gas-well being flared, to reduce the amount of oxygen within the interior volume 10v, and thereby prevent or limit any flame or internal combustion within the interior volume 10v. During normal operations, however, the flare stack 10 and nozzle 32 are designed to ensure enough gas G velocity to create the desired venturi effect and to prevent an internal burn. Also note that when the gas G flow rate dies down (e.g. due to a shut in situation) the venturi effect lessens and the air A draw through inlet 50 shuts down as well. In a situation where a flare stack 10 is left unattended, it could be fitted with an electronic air inlet valve 52 set to open and shut at predetermine vacuum values (e.g. as measured by vacuum gage 45). This will enhance the safety aspect of the flare stack 10 in the event of a well shut in when unattended.
As can be seen and during normal operations, when gas G moves up the flare stack 10 and out the outlet 32, and as air A is drawn up into the annulus 40, both the gas G and air A will thoroughly mix in a mixing region 10m that is situated between the outlet 32 and the discharge end 18. While a mixing region 10m of only a few inches in height (e.g. 3 inches along the flare stack's height,

8 below the discharge end 18) will allow for gas G and air A to mix, preferably the mixing region 10m extends from just below the connector 16 all the way to the discharge end 18, thereby allowing for a very thorough and complete mixing of waste gas G with air A (note that waste gas G is illustrated in the figures by solid arrows, while air A is illustrated by hatched arrows). Preferably, a sampling port 47 is provided to allow an operator to withdraw or sample some of the gas G and air A
mixture in the annulus 40.
As will now be appreciated, this mixture of gas G and air A will burn much more efficiently once ignited at the discharge end 18 as compared to a conventional flare stack where, at best, only some air is introduced only at the periphery of the gas flow by a conventional shroud 20. Advantageously, this mixture of gas G and air A is created by the flare stack 10 without the need for additional mechanical blowers. Note that, while referring to "bottom" and "top" ends of the flare stack, the embodiments of the invention also contemplate horizontal flare stacks, wherein these terms will then refer to inlet and discharge ends respectively.
Preferably, the flare stack 10 further comprises a drain with a drain valve 22 at the bottom end. More preferably, and in a vertical flare embodiment, the drain 22 is located below the air inlet 50 so as to create a sump region 60 to receive any condensates or fluids that might condense within the interior volume 10v and travel down the annulus 40. For example, if the flare stack 10 of the present invention is use on a "hot" well, where the gas G has a higher temperature coming

9 out of the well than the ambient air, the inventors have observed that the air A
moving up the annulus 40 has a chilling effect to that gas G (within riser 30) and will cause an increase in condensate forming within the interior volume 10v once the gas G enters the mixing region 10m. This condensate will then fall and drain into sump region 60 at a higher rate as compared to conventional flare stacks.
Advantageously, such condensate falling and draining into the sump region 60 will reduce the amount of black smoke that will otherwise be emitted by a conventional flare stack (because less condensate is being burned). Preferably, and in cases where a significant amount of condensates and fluids are collected during flaring operations, the bottom end 10b of the flare stack 10 may be expanded or enlarged to provide an increased sump region 60. Likewise, the air inlet 50 may be positioned a higher up the stack 14, up above the drain 22, to create a greater sump region 60 and/or to reduce the chilling effect that such air A has on the riser 30. More preferably, a vacuum system (not shown) is connected to the drain 22 to facilitate emptying the sump region 60 of such condensate and fluids (e.g. during times when the flare stack is non-operational, or in such a manner so as not to interfere with any venturi effect that is created at the outlet 32).
Now with reference to Figures 3a ¨ 3c, the inventors have observed that when using the flare stack 10 of the present embodiment along-side a similarly sized conventional flare stack 70, both stacks 10, 70 operating with the same flow rate of waste gas, the flare stack 10 of the present embodiment results in a bright (higher luminosity) flame plume 80 with little, if any, black smoke 82. This indicates that the flare stack 10 of the present embodiment provides a more efficient burn of the waste gas G than conventional flare stacks 70.
Those of ordinary skill in the art will appreciate that various modifications to the invention as described herein will be possible without falling outside the scope of the invention. In the claims, the word "comprising" is used in its inclusive sense and does not exclude other elements being present. The indefinite article "a" before a claim feature does not exclude more than one of the features being present.