US3782880A

US3782880A - Control system to automatically maintain a smokeless flare

Info

Publication number: US3782880A
Application number: US00290706A
Authority: US
Inventors: E Eubanks
Original assignee: Gulf Oil Corp
Current assignee: Chevron USA Inc
Priority date: 1972-09-20
Filing date: 1972-09-20
Publication date: 1974-01-01
Anticipated expiration: 1991-01-01

Abstract

Waste gases from refinery units are delivered into a vessel having a plurality of outlet lines connected into the vessel at different elevations. The outlet lines are connected into a waste gas flare line for delivery of waste gas to a smokeless flare. A sealing liquid in the vessel can be displaced from the vessel through a line opening from the bottom of the vessel to a sealing liquid reservoir. The level of sealing liquid in the vessel, which is dependent upon the rate waste gases are discharged through the outlet lines, produces a signal to control the rate of steam delivery to the flare to maintain a smokeless flare. As the rate of waste gas flow into the vessel increases, the sealing liquid level in the vessel is lowered to uncover successively lower outlet lines and allow increased rate of flow of waste gas to the flare.

Description

United States Patent [1 1 Eubanks Jan. 1, 1974 CONTROL SYSTEM TO AUTOMATICALLY Primary Examiner-William F. ODea MAINTAIN A SMOKELESS FLARE Assistant Examiner-William C. Anderson [75] Inventor: Elton C. Eubanks, Port Arthur, Tex. AttorneyMel/er Nelshloss et [73] Assignee: Gulf Oil Corporation, Pittsburgh, 57] ABSTRACT Waste gases from refinery units are delivered into a [22] F'led: Sept 1972 vessel having a plurality of outlet lines connected into [21] Appl. No.: 290,706 the vessel at different elevations. The outlet lines are connected into a waste gas flare line for delivery of waste gas to a smokeless flare. A sealing liquid in the [52] US. Cl. 431/4, 431/190 Vessel can be displaced from the vessel through a line [51] III". Cl. F23] p g from the bottom of the vessel to a sealing q Fleld Of Search 90, reservoir- The level of Sealing in the vessel 431/202 23/277 137/1 10 which is dependent upon the rate waste gases are discharged through the outlet lines, produces a signal to [56] References C'ted control the rate of steam delivery to the flare to main- UNITED STATES PATENTS tain a smokeless flare. As the rate of waste gas flow 3,162,236 12/1964 Williams 23/272 C X into the vessel increases, the sealing liquid level in the 2,891,607 6/1959 Webster et al 431/90 vessel is lowered to uncover successively lower outlet 2.80252] 8 al C lines and allow increased rate of flow of wa te ga to 2,779,399 1/1957 Zink et al 23/277 C X the fl 2,761,496 9/1956 Verner et a] 23/277 C X 10 Claims, 1 Drawing Figure PATENTED JAN 1 74 i i i CONTROL SYSTEM TO AUTOMATICALLY MAINTAIN A SMOKELESS FLARE This invention relates to the disposal of waste gas at oil refineries and more particularly to apparatus for controlling the rate of delivery of steam to smokeless flares used to dispose of waste gases.

Some units in oil refineries discharge hydrocarbon or other gases into a waste gas disposal system adapted to carry the gases to a remote area and dispose of them in a manner which will avoid dangerous accumulation of gases and pollution of the atmosphere. Frequently waste gases are discharged into the waste gas system only when a pressure relief valve on a refinery unit blows. The gases collected in the waste gas disposal system may be contaminated with gases diflicult to separate from hydrocarbon gases or may vary so widely in composition and the rate at which they are delivered into the waste gas disposal system that recovery of valuable products from the waste gas cannot be accomplished economically.

One method of disposing of the waste gasesis to deliver them to a flare at the top of a stack and burn the gases in the presence of an excess of air to maintain a smokeless flame. Steam is injected into the waste gas stream at the flare to aspirate air into the waste gases and improve mixing of air with the waste gases. It is desirable to inject the steam at a rate which increases with increases in the rate of delivery of waste gases to the flare to avoid large excesses of air and waste of steam at low waste gas flow rates andyet aspirate sufficient air into the flare at high waste gas flow rates to maintain a smokeless flare. Attempts to measure the waste gas flow to the flare with measuring devices such as orifice plates and pitot tubes in waste gas lines to a flare to develop a signal that can be used to control the rate of delivery of steam to the flare have been unsuccessful, at least partially because of variations in flow line pressure. It is necessary for automatic control of the rate of steam delivery to the flare to develop a signal that is sensitive to small changes in the rate of flow at low flow rates and yet will permit high rates of flow of waste gas tothe flare.

In US, Pat. No.'2,89l,607 of Webster et a1., a flare is provided with a water-filled chamber at its lower end. Waste gases are delivered to the flarethrough a small line opening downwardly a short distance below the surface of the water in the chamber and a large line that opens downwardly a greater distance below the surface of the water in the chamber. The pressure drop across an orifice in the small line provides a signal for control of the rate at which steam is delivered to the flare. In the apparatus illustrated in the patent, the seal could be broken by sudden discharge of gases. If that should happen, the normal vacuum under which the flare operates could result in oxygen-containing gases being drawn into the apparatus where it could be mixed with combustible waste gases to form an explosive mixture. In US. Pat. No. 2,792,070 of E. Strunk, a water seal separates the top part of a blowdown drum from the bottom part of the blowdown drum. The top part of the blowdown drum communicates with a vent to the atmosphere and the bottom part of the drum communicates with a flare. When there is an excessive blowdown of waste gases, the water seal is broken and gases are discharged through the vent. The object of Strunk to operate a small flare at substantial distance from the refinery units is accomplished by the provision of a large vent near the refinery units to vent gases when there is excessive blowdown.

This invention resides in apparatus for controlling the flow of steam to a smokeless flare in amounts that will aid in aspirating the amount of air required for smokeless combustion of waste gases delivered to the flare including a vapor control vessel adapted to receive the waste gases. A plurality of outlet lines open from the vapor control vessel at different levels. Each of the outlet lines is connected to a flare line for delivery of waste gases to the flare. A sealing liquid balance line from the vapor control vessel to a sealing liquid reservoir allows sealing liquid to be displaced from the vapor control vessel to the reservoir to successively uncover the outlets as blowdown of waste gases increases to permit flow from the control vessel to the flare. Means are provided for developing a signal responsive to the level of liquid in the control vessel to control the rate of delivery of steam to the flare.

The single FIGURE of the drawings is a diagrammatic flow sheet of apparatus controlling the delivery of steam to a smokeless flare in accordance with this invention.

Referring to the Drawing, a waste gas recovery header 10 into which waste gas lines 12 from a number of refinery units are connected discharges waste gases into a knock-out drum 14. A waste: gas line 16 from the upper portion of knock-out drum 14 is connected into a waste gas control vessel 18. Liquid separated from the waste gas in knock-out drum 14 is delivered through a line 20 from the bottom of the knock-out drum to a suitable processing unit for recovery of valuable constituents that may be in the liquid or treatment of water that may be in the liquid before discharge into a sewer.

In the embodiment of the invention illustrated in the drawing, waste gas control vessel 18 has an upper outlet line 22, an intermediate outlet line 24, and a lower outlet line 26 opening from the vessel 18 at successively lower elevations. Each of the

outlet lines

22, 24, and 26 connects into a waste gas flare line 28 that delivers the waste gas into a stack 30 at theupper end of which a flare is maintained to burn the waste gases. In the embodiment of the invention illustrated in the drawing, each of the

outlet lines

212, 24 and 26 is provided with an orifice indicated by

reference numerals

32, 34, and 36. This invention is not limited to the use of a vessel having three outlets. Vessel 18 can have two or more outlet lines. Waste gas control vessel 18 may have a delivery line 37 from its upper end for supplying waste gas to burners or compressors during normal operation of refinery units connected into the waste gas system.

A loop 38 in waste gas flare line 28 prevents flow of liquid to the flare. As shown in the drawing, flare line 28 and

outlet lines

22, 24, and 26 slope from loop 38 toward the waste gas control vessel 18 to cause liquid to drain from each of those lines to the vessel 18 when the liquid level in vessel 18 drops below the liquid level in the respective outletline. Between the loop 38 and the stack 30 is a check valve 40 loaded to require a pressure on the upstream side of the valve higher than the pressure on the downstream side of the valve to open valve 40 and allow flow through line 28. A pressure drop of at least 5 inches of water to open valve 40 is desirable to maintain a superatmospheric pressure on the control vessel 18.

Waste gas control vessel 18 contains water or other sealing liquid such as a gas oil or anti-freeze solution filling the lower portion of the vessel to a liquid level indicated by reference numeral 42. A sealing liquid balance line 44 extends from the lower end of vessel 18 and is connected into a sealing liquid reservoir 46. Sealing liquid balance line 44 opens into reservoir 46 at an elevation higher than the elevation at which upper outlet line 22 is connected to vessel 18 to maintain a superatmospheric pressure in vessel 18 when outlet line 22 is uncovered. Balance line 44 should be large enough to allow rapid transfer of sealing liquid from vessel 18 to reservoir 46 when there is a sudden surge in the pressure in vessel 18. The pressure drop through balance line 44 should be less than the pressure drop through orifice 32 to prevent surges of sealing liquid over loop 38 and through flare line 28 to the stack.

To prevent flow of liquid to the flare, loop 38 extends above the maximum liquid level in reservoir 46. A sealing liquid supply line 48 is provided with suitable liquid level control means to maintain the sealing liquid level in reservoir 46 above the opening line of balance line 44 in the reservoir. A vent line 50 from the upper end of reservoir 46 has a valve 52 adapted to close when the pressure within the reservoir drops below atmospheric pressure.

A liquid level indicator 54 is connected by lead lines into waste gas control vessel 18 at elevations above the maximum and below the minimum levels reached by sealing liquid level 42 except in extreme cases of upset operations to generate signals responsive to the liquid level in waste gas control vessel 18. The signal is transmitted through control line 58 to a valve 62 in a steam line 60 to steam jets at the top of the stack. in the apparatus illustrated in the drawing there is an external ring 64 surrounding the upper end of the stack having inwardly directed jets for aspirating air into the waste gas streams. Steam is delivered into ring 64 at periods of high rates of flow of waste gas to the stack.

interior jets

66 and 68 will impart sufficient velocity and turbulence to aspirate adequate air into the waste gas at the flare during periods when the flow rate of waste gases is low. Control of the delivery of steam to

jets

66 and 68 is by a valve 70 that is also operated by a signal from liquid level indicator 54.

In the operation of the apparatus illustrated in the drawings, gas entering the gas waste system flows through header into knock-out drum 14 in which the waste gas is separated from entrained or condensed liquid. Waste gas from the knock-out drum 14 flows through line 16 into control vessel 18. If the sealing liquid level 42 in waste gas control vessel 18 is above the opening of outlet line 22 in the vessel 18, as during normal operations, waste gas flows through line 37 and there is no flow of waste gas to the flare. An increase in the rate of flow of waste gas into vessel 18, such as occurs during upset operations or blowdown of a refinery unit, that exceeds the rate at which gas can be discharged through line 37, will increase the pressure in vessel 18 and displace sealing liquid through balance line 44 into reservoir 46 and thereby lower the surface of the liquid below outlet line 22. Lowering of liquid level 42 below outlet 22 will cause a signal to be sent to valve 70 to open that valve.

When outlet 22 is uncovered, the pressure in the vessel 18 is dependent on both the pressure in flare line 28 and the pressure drop across orifice 32. Orifice 32 is sized to cause, even at low waste gas flow rates and small changes in rates, a significant pressure drop which when added to the back pressure in the flare line develops a signal of sufficient magnitude to allow accu rate control of the rate of delivery of steam to the flare. Further lowering of the liquid level 42 in vessel 18 re sults in a signal by level indicator 54 that actuates valve 62 in steam line 60 to initate and control the flow of steam to ring 64. An increase in the pressure in vessel 18, whether caused by an increase in pressure in the flare line or only by an increased pressure drop across orifice 32, results in still more lowering of the liquid level 42 and a signal to increase the delivery of steam to the flare.

if the pressure in vessel 18 continues to increase, liquid level 42 is lowered until outlet line 24 is uncovered. Waste gas then flows to flare line 28 through both outlet line 22 and outlet line 24. Orifice 34 produces a back pressure in outlet line 24 that will result in further lowering of the liquid level in vessel 18 on further increase in the rate of delivery of waste gas to vessel 18 until the liquid level reaches outlet line 26 or on an increase in the pressure in flare line 28. in a preferred embodiment, orifice 32 is sized to provide the liquid level control up to about one-half of the maximum waste gas flow rate through the vessel 18 at which the flare can be made smokeless and orifice 34 to provide the desired control of the liquid level in vessel 18 through the maximum waste gas rate at which the flare can be made smokeless. Outlet line 26 will take care of all higher waste gas rates including those above the maximum rate at which the flare can be made smokeless. At maximum waste gas flow rates, both valves and 62 are opened wide and most of the steam is delivered through the ring 64 to the aspirating jets. While the orifices in all of the outlet lines are advantageous in adding accuracy and flexibility to the control of the rate of steam injection, the outlet lines would be carefully engineered and the pressure drop through the outlet lines utilized in a manner similar to the utilization of the pressure drop through the orifices to affect the liquid level 42 and thereby the signal delivered by indicator 54. It may be desirable to omit the orifice only from the lower outlet line while using orifices in upper outlet lines. It is preferred that the diameter of the opening in each of the orifices will be less than the diameter of the opening in lower outlet lines.

A sudden blowing of pressure relief valves could blow all of the sealing liquid through vessel 18, reservoir 46, and vent 50. Valve 52 provides protection against the normal vacuum on the stack drawing air into vessel 18 by closing if the pressure in vent line 50 should drop below atmospheric pressure. Since the waste gases in refineries are principally hydrocarbons, entry of air into vessel 18 or reservoir 46 would create a serious danger of explosion. Sealing liquid is delivered through line 48 into reservoir 46 to fill balance line 44 and re-establish the sealing liquid level 42 in vessel 18.

This invention provides apparatus for controlling the rate of delivery of steam to a waste gas flare to maintain a clean flame at the flare over a wide range of waste gas flow rates and pressures. Deriving a signal from the liquid level in the control vessel corrects for variations in the pressure in the flare line and facilitates control of steam flow to the flare in proportion to the rate of delivery of waste gas to the flare.

I claim:

1. Apparatus for controlling the rate of injection of steam into a smokeless flare in response to the rate of delivery of waste gas to the flare comprising a waste gas control vessel into which waste gas is delivered, said waste gas control vessel having a sealing liquid therein a delivery line to the flare, a plurality of outlet lines from the waste gas control vessel connected to the delivery line, said outlet lines opening from the first vessel at different elevations, a sealing liquid reservoir, a sealing liquid balance line connecting the lower end of the waste gas control vessel and the sealing liquid reservoir through which sealing liquid flows in response to pressure variations in the waste gas control vessel to change the liquid level in said waste gas control vessel, signal means responsive to the liquid level in the waste gas control vessel, a steam line to the flare, and valve means in the steam line to the flare actuated by a signal from the signal means to increase the opening in the valve means with lowering of the liquid level in the waste gas control level 2. Apparatus as set forth in claim 1 including an orifree in at least the upper outlet line.

3. Apparatus as set forth in claim 1 including a check valve in the delivery line to the flare, said check valve adapted to open to permit flow to the flare when the pressure differential across the valve exceeds 5 inches of water.

4. Apparatus as set forth in claim 1 in which the sealing liquid balance line opens into the sealing liquid reservoir at an elevation higher than the upper outlet line.

5. Apparatus as set forth in claim 1 in which a vent line opens from the upper end of the sealing liquid reservoir, and a check valve in the vent line prevents flow through the vent line into the sealing liquid reservoir.

6. Apparatus as set forth in claim 1 including an upwardly extending loop in the delivery line of a height adapted to prevent flow of sealing liquid through the delivery line to the flare.

7. Apparatus as set forth in claim 6 in which the outlet lines and the delivery line between the loop and the waste gas control vessel slope downwardly toward the waste gas control vessel.

8. Apparatus as set forth in claim 1 in which a plurality of outlet lines have an orifice therein.

9. Apparatus as set forth in claim 1 in which there are three outlet lines, the size of orifice in the uppermost outlet line is adapted to control the flow of steam for approximately one-half the range of waste gas delivery rates through which the flare can be made smokeless, and the size of orifice in the next to uppermost outlet line is adapted to control flow of steam up to a maximum rate at which the flare can be made smokeless.

10. A method of controlling the delivery of steam to a waste gas flare in proportion to the delivery of waste gas to the flare comprising delivering the waste gas to a waste gas control vessel containing a sealing liquid and having a balance line from its lower end to a sealing liquid reservoir through which sealing liquid flows in response to pressure variation in response to pressure variations in the waste gas control vessel to change the liquid level in said waste gas control vessel, discharging the waste gas from the waste gas control vessel into an outlet line opening from the lower part of the vessel, passing the waste gas through an orifice in the outlet line and then to the flare, and deriving a signal from the level of the sealing liquid in the waste gas control vessel to control the rate of delivery of steam to the flare with the rate of delivery increasing with lowering of the level of the sealing liquid in the waste gas control vessel.

Claims

1. Apparatus for controlling the rate of injection of steam into a smokeless flare in response to the rate of delivery of waste gas to the flare comprising a waste gas control vessel into which waste gas is delivered, said waste gas control vessel having a sealing liquid therein a delivery line to the flare, a plurality of outlet lines from the waste gas control veSsel connected to the delivery line, said outlet lines opening from the first vessel at different elevations, a sealing liquid reservoir, a sealing liquid balance line connecting the lower end of the waste gas control vessel and the sealing liquid reservoir through which sealing liquid flows in response to pressure variations in the waste gas control vessel to change the liquid level in said waste gas control vessel, signal means responsive to the liquid level in the waste gas control vessel, a steam line to the flare, and valve means in the steam line to the flare actuated by a signal from the signal means to increase the opening in the valve means with lowering of the liquid level in the waste gas control level

2. Apparatus as set forth in claim 1 including an orifice in at least the upper outlet line.