SHROUDED-COANDA MULTIPHASE BURNER

Abstract

A gas flare disposed at an end of a supply pipe member for directing a gas flow to a head member, the flare including a slot for ejecting the gas flow and a shroud. The shroud surrounds the slot, at least part of the base member and at least part of the head member of the gas flare. The shroud directs the ejected gas flow between the shroud and the head member.

Description

FIELD OF THE DISCLOSURE

The disclosure relates to a gas flare, and in particular, but not exclusively to a Coanda-type gas burner flare used for combustion of waste gas during surface testing of hydrocarbon wells.

BACKGROUND

Hydrocarbons' importance for the world economy cannot be overstated. The discovery and efficient production of hydrocarbons is becoming increasingly more difficult and poses many new technological challenges. Typically, a borehole is drilled down into the earth, on land or in subsea operations, to reach a reservoir containing hydrocarbons. Usually, fluid hydrocarbons take the form of oil, gas, or mixtures thereof found in reservoirs that can be produced by one or more wells.

An important stage in the evaluation of a reservoir is known as well testing. Well testing includes flowing a well and measuring the response of several key parameters such as pressure and flow rate over time. Many different types of well tests are known to those skilled in the art, such as pressure drawdown, interference, reservoir limit tests, etc. Well testing enables the collection of data that help assess the economic viability of the well. However, the cost of testing operations is significant and often times it may exceed the cost associated with drilling the well. It is important, therefore, that testing operations are performed as efficiently and economically as possible.

During a typical well testing operation, the well effluent is separated into its individual phases, i.e., oil, water and gas via the use of one or more well test separators. One of the key functions of a well testing operation is to combust the waste gas flow exiting the well test separator.

Many different flaring systems have been used to burn the waste gas flow. One well known type of flare systems are the Coanda-type flaring systems. Coanda-type flare systems are widely used in the petroleum industry for flaring waste gases of oil refineries or production platforms. Typical Coanda-type flare systems are described in the following patent documents: U.S. Pat. No. 3,709,654, U.S. Pat. No. 3,915,622, U.S. 2006/0105276 A1, U.S. Pat. No. 3,833,37, EP 0054383 and RU 2315240. Coanda flares offer relatively clean combustion by premixing the waste gas with ambient air prior to combustion. Coanda flares also offer generally good, stable flame combustion at high inlet pressure of combustible gas. In addition, shockwaves generated in the vicinity of the gas exit slot of a Coanda flare help to atomize the liquid droplets that are present in the gas flow into a fine mist, thus facilitating combustion and reducing the risk of fall out even in the case of significant liquid carryover in flare line. So, generally Coanda-type flaring systems offer stable and quite clean combustion of multiphase hydrocarbon effluents. Coanda-type flares are efficient and environmental friendly under proper operational conditions.

There are, however, some disadvantages that limit the use of Coanda flares in well test operations such as noise, ejected debris, and sensitivity to flow rate variations. The noise is believed to be mainly associated with shockwaves and is generally excessive and poses safety and environmental issues.

The Coanda-type flare apparatus described in U.S. Pat. No. 4,486,167 comprises a Coanda body, a slot for gas outlet form the supply pipe, a fructoconical shield for noise reduction, and another shield for noise reflection. However, the addition of these shield decreases the air supply to the burner and creates a narrow operational range on flow rates of gas.

Furthermore, there is also a high risk of debris being ejected radially from the Coanda slot in the form of high velocity projectiles. For example, the debris can be sand particles or other particulates present in the wellbore effluent. Another source of debris is pipe scale or the built up of salt deposits around the Coanda slot that can become dislodged once they reach a critical mass. Debris ejected through the Coanda slot poses a risk of injury to the operators as well as damage to the surrounding equipment. There may also be a fraction of the big liquid droplets that do not follow the gas flow and spray sideways from the slot, causing some fallout, especially if the liquid present in gas stream is highly viscous. Moreover, during well test operations there may be frequent and unexpected flow variations or variations in the composition of the hydrocarbon effluent (e.g. the liquid fraction in the gas stream) that can lead to a temporary cancelling of the Coanda effect (gas or liquid droplets not following the Coanda profile). Under these conditions the gas/liquid mix sprays out radially causing unintended side effects and dangers, as well as hydrocarbon spill.

It is therefore desirable to overcome the limitations of existing Coanda-type flaring systems, especially insofar as surface well testing operations are concerned.

SUMMARY

According to a first aspect of the disclosure there is provided a gas flare that overcomes the above mentioned deficiencies of existing Coanda-type gas flares. The inventive gas flare comprises: a supply pipe for directing a gas flow to a head member; a slot for ejecting the gas flow; and a shroud surrounding the slot, at least part of the supply pipe and at least part of the head member. The shroud directs the ejected gas flow between the shroud and the head member.

Advantageously, the shroud prevents the gas flow from being ejected substantially radially from the gas flare. So while the gas flow is still ejected radially from the slot to provide a Coanda effect and aspiration of an ambient air, such gas flow including any liquid progresses only as far as the shroud and adheres to the head member. By preventing radial gas flow from the gas flare the safety of the device is improved, since any debris contained in the high-speed gas flow will be contained within the shroud.

According to a further aspect of the disclosure there is provided a shroud for a gas flare, the shroud surrounding at least a portion of the gas flare and designed for directing a gas flow ejected from the gas flare to flow between the shroud and the gas flare and for enabling an ambient air flow to be directed to mix with the gas flow ejected from the gas flare.

According to a further aspect of the disclosure there is provided a shroud with an axial airfoil-shaped cross-section with a smooth bottom end and a sharp distal end, so that the air aspired by gas jet coming from a slot is being accelerated from the smooth bottom end to the sharp distal end of the shroud.

According to yet a further aspect of the disclosure there is provided a method of combusting gas ejected from a gas flare; the method comprising: directing the gas flow using a gas flare having a contoured shape; directing an ambient air flow using a shroud surrounding at least part of the gas flare to flow between the shroud and gas flare; mixing the gas flow with an ambient air flow; and combusting the mixture.

The disclosure could be used with other gas flare systems, however, it is particularly advantageous when used in combination with a Coanda-type flare, especially one having a tulip-contoured shape.

According to yet a further aspect of the disclosure, the Coanda gas flare comprises a stalk-shaped base member connected to a tulip-shaped head member. At least part of the stalk-shaped base member is located within at least part of a supply pipeline defining a first passage therebetween for directing a gas flow towards the head member. The Coanda gas flare further comprises a slot located between the supply pipeline and the head member, substantially adjacent to where the head member is connected to the base member. The slot is surrounded by a shroud defining a second passage therebetween for directing the gas flow substantially around the tulip-shaped head member to be combusted.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described solely by way of example and with reference to the accompanying drawings, in which:

FIG. 1 shows a prior art Coanda gas flare; and

FIG. 2 shows a cross-sectional view of a gas flare according to a one embodiment of the disclosure;

DETAILED DESCRIPTION

So that the above features and advantages of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the accompanied drawings. It is to be noted, however, that the drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.

In a typical well testing operation a gas flare is used to burn and, thus, dispose waste gas effluent exiting a well test separator that is used to separate a multiphase hydrocarbon effluent into its oil, gas and water components. Although the effluent to be combusted is typically referred to as gas effluent, it should be appreciated that there is still a fraction of liquid that often remains in the gas flow, which is to be combusted. This liquid fraction is especially high during the flowing of high gas rate fluid at the upper operation limit of separator, or flowing bypass separator on gas-condensate wells.

The Coanda-type flare is often useful to combust this liquid fraction, since the gas flow ejected from the Coanda-gas flare reaches supersonic speeds. This creates shockwaves, which atomize the liquid fraction in the gas flow into a fine mist that is more easily and completely combusted by the burner flame.

FIG. 1 shows an example of a prior art Coanda flare having a supply pipe 114 connected to a tulip-shaped head member 110 via a base member 112 with pylons. The tulip shape produces the Coanda effect, which is advantageous in that the waste gas flow 116 is directed around the bulge 108 of the tulip which causes an accelerated gas flow in this area and thus produces low pressure. The gas flow is directed, by this low pressure, adjacent to the surface of the tulip-shaped head member, and also causes ambient air flow to be directed toward the low pressure.

Thus, ambient air flow mixes with the gas flow ejected from a slot 104 of the gas flare, making the mixture 117 even more suitable for combustion by the burner flame at the distal end of the head member 110.

The supply pipe 114 and base member 112 provides a first passageway there between having a particular cross-section through which the high pressured gas 116 flows. This high pressured gas flow would typically come from a well test separator gas outlet or directly from a tested well. The slot 104 defines a second narrower passageway for choking the ejected gas flow between the supply pipe 114 and the head member 110. Since the second passageway of the slot 104 is narrower, the gas flow is accelerated to sonic speed within the slot, and furthermore (due to expansion along with the tulip) the flow reaches supersonic velocities; this produces shockwaves that help to atomize the liquid droplets entrained by the gas flow. Then, the tulip-shaped head of the Coanda flare causes ambient air flow to mix with the combustible gas flow. This premixing allows achieving a good fuel-oxidant ratio and makes the flame more stable and clean.

FIG. 2 shows a cross-sectional view of a gas flare according to one embodiment of the disclosure described herein. FIG. 2 shows a Coanda gas flare with base member 12 connected to the supply pipe 13. The base member 12 may be at least partially located within a supply pipe 13. The base member 12 and supply pipe 13 are shaped so as to define a passageway there between through which the gas flow is directed towards the head member 10. As the gas flow reaches the head member, there is a slot 4 between the supply pipe 13 and the head member 10 and base member 12. Specifically, the slot 4 defines a narrowed passageway for accelerating and ejecting the gas flow outside of the gas flare.

The directed gas flow is ejected substantially radially from the slot 4. However, the gas flare has a shroud 14 which surrounds the slot and a portion of supply pipe and the bottom part 11 of the head member 10. More specifically, the shroud surrounds and yet is spaced a distance from the slot 4, to define a further passageway for the gas and aspirated air to flow around the contour of the tulip-shaped head member 10 and to mix with each other,. This allows for shockwaves that aid liquid atomization, but furthermore acts as a shield in preventing particulates in the gas flow from being ejected past the shroud—which may be of danger to working nearby personnel. Furthermore, it is believed that the containment of the shockwaves within the shroud 14 is partially responsible for the observed noise reduction in the backward direction.

In one embodiment, the frusto-conical shroud 14 has an airfoil-shaped vertical cross-section with a sharp top edge 15 and a smooth bottom end 16. This airfoil-shaped geometry of the shroud 14 adds additional functionality to the shroud. The shroud 12 works as an efficient air ejector toward the flow of gas emitted from the slot 4. The smooth and streamlined geometry of the bottom end of the shroud 14 reduces the drag effects of the inlet air. The reduced area of the cross-section defined by the supply pipe 13 and shroud 14 induces acceleration of the inlet air. The high velocity air flow detaches from the sharp top edge 15 of the shroud 14 and removes the portion of liquid droplets produced by atomization in the slot 4, that occasionally separate from the main gas stream.

Additionally, the top edge of shroud 14 may have a serrated rim 20. The elements of serrated rim 20 are known in the designing of jet nozzles as tabs (directed inward the streamlining flow) or chevrons (sharp corner of the structure directed outward the flow). The function of this small-scale serration is for the redistribution and better atomization of liquid jets occurring on the inside surface of shroud 14 and for improving the mixing of different flows.

Although the geometry of tabs is depicted as small sharp triangles, it should be appreciated that other shapes are possible: polygonal elements, combination of rounded elements with polygons, etc.

The shroud 14 is also able to absorb high-frequency noise, bands resulting from the share layer and shockwaves, for example by constructing the shroud with a sound absorption structure (porous material or honeycomb) intermediary layer.

Additionally, the flare apparatus is equipped with a back-shield 17 for noise attenuation in the backward direction of the flare apparatus. In the design shown in FIG. 2, the back-shield 17 has a slightly concave geometry for improving the air inlet in the annulus defined by the bottom end 16 of the shroud and the back-shield 17. In other embodiments, the back-shield may be flat plate attached to the supply pipe 13. Preferably, the back-shield 17 is performed from sound-absorbing materials (similar to materials for shroud 14) with a rigid back wall for additional noise redirection towards the flame area.

Thus, the shroud covers a portion of the Coanda flare and is able to perform multiple functions which improve the flare.

In addition to sound absorption, and hence noise suppression, the shroud may act as a protective shield to capture any debris (sand, salt deposits, etc.) that is ejected radially from the Coanda slot. It also prevents any gas or liquid from being sprayed radially in the case of an unexpected sudden flow rate spike or high liquid loading that causes the Coanda effect to break down. The directed and accelerated airflow will ensure that any gas or liquid droplets, not following the Coanda profile, will be redirected in an axial direction into the combustion zone.

It will be understood from the foregoing description that various modifications and changes may be made in the preferred and alternative embodiments of the present disclosure without departing from its true spirit. In addition, this description is intended for purposes of illustration only and should not be construed in a limiting sense.

Claims

1. A gas flare comprising: a supply pipe for directing a gas flow towards a head member;a slot located between the supply pipe and head member for ejecting the gas flow; anda shroud spaced away from and surrounding the slot to form a passage between the shroud and the head member for directing the ejected gas flow therebetween, wherein the shroud has an airfoil-shaped longitudinal cross-section with a smooth bottom end and a sharp top edge.

2. The gas flare of claim 1, wherein the shroud prevents the gas flow from being ejected in a substantially radial direction from the gas flare.

3. The gas flare of claim 1, wherein the shroud prevents debris in the gas flow from being ejected from the gas flare.

4. The gas flare of claim 1, wherein the shroud absorbs noise.

5. The gas flare of claim 1, wherein the shroud absorbs a selectable high-frequency band of the noise as the gas is ejected from the slot at a supersonic speed.

6. The gas flare of claim 1, wherein the shroud comprises a porous inner material layer and a solid external metal lining.

7. The gas flare of claim 1, further comprising: a base member disposed near a bottom of the head member, wherein the base member is at least partially located within the supply pipe defining a passage therebetween for the gas flow.

8. The gas flare of claim 1, wherein the shroud includes a circumferential serrated rim at the distal end.

9. A shroud for a gas flare, the shroud being at least partially disposed around a head member of the flare and comprising: a proximate end having a generally smooth and rounded shape; anda distal end having a sharp edge, wherein the shroud and the head member define a passage for directing a gas flow.

10. A method of combusting a gas flow ejected from a gas flare; the method comprising: directing the gas flow from a supply pipe to a head member;drawing in an ambient air flow into a passage formed by a shroud spaced away from and surrounding at least part of the gas flare;mixing the gas flow with the ambient air flow; andcombusting the mixture.

11. The method of claim 10 further comprising partially disposing the shroud between a shield and the head member, wherein the shield provides a backward noise reduction.

12. The method of claim 11 further comprising forming a smooth passage between the shroud and the shield.

13. A Coanda gas flare for combusting effluent, the gas flare comprising: a stalk-shaped base member connected to a tulip-shaped head member, wherein at least part of the base member is located within at least part of a supply pipeline defining a first passage therebetween for directing a gas flow towards the head member; anda slot located between the supply pipeline and the head member, substantially adjacent to where the head member is connected to the base member, wherein the slot is surrounded by a shroud defining a second passage therebetween for directing the gas flow substantially around the tulip-shaped head member to be combusted.