Flare gas stack with purge gas conservation system

A flare gas stack is described with a purge gas conservation system adapted to control the flow of purge gas in accordance with the specific gravity of the purge gas, and prevailing conditions including cutting off the purge gas flow when conditions of waste gas flow and wind speed near the top of the stack permit. Provisions are made for visual observation of the waste gas flow, purge gas flow and purge failure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to flare gas stack for waste combustible gas from 
industrial processes including oil refineries and more particularly to a 
purge gas conservation system with provisions for controlling the flow of 
purge gas in accordance with the prevailing conditions including cutting 
off the purge gas flow when conditions permit. 
2. Description of the Prior Art 
It has heretofore been proposed to supply purge gas to a flare stack when 
waste combustible gas is not flowing to the stack and thereby prevent the 
occurrence of explosions in the stack. 
No satisfactory provisions have heretofore been made by others looking to 
conservation of purge gas when conditions do not justify the supplying of 
purge gas. 
In my prior application for Letters Patent filed Feb. 17, 1977, Ser. No. 
769,709, for Flare Gas Stack with Purge Control provisions are made for 
start up, steady state or transient purge gas control and for failure of 
the purge gas supply, and take into account variable wind speed at or near 
the top of the stack and other conditions with provisions for pilot burner 
gas supply and ignition, and indication of the prevailing conditions. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a flare gas stack with a purge gas 
supply has provisions for controlling the purge gas flow, including 
variations of specific gravities of the purge gas, and including shutting 
off the purge gas supply when conditions permit, thereby effecting 
substantial saving of purge gas without the hazard of explosions in the 
stack. The control system includes a waste gas sensor activated by flare 
gas flow to the stack and a wind speed sensor for purge gas delivery 
control, the wind speed, purge supply and purge failure and specific 
gravity preferably being shown on an indicator, and with manual provisions 
for override of the control system. 
It is the principal object of the present invention to provide a purge gas 
conservation system for flare gas stacks which will reduce unnecessary 
delivery of purge gas to a flare stack as measured by the wind speed at or 
near the top of the stack. 
It is a further object of the invention to provide a purge gas conservation 
system for flare gas stacks in which the flow of the flare gas to the 
stack is utilized for control purposes. 
It is a further object of the invention to vary the flow of purge gas 
dependent upon its specific gravity. 
It is a further object of the invention to provide a purge gas conservation 
system which is simple but effective in its action. 
Other objects and advantageous features of the invention will be apparent 
from the description and claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
It is common to utilize a flare stack for the disposal of waste combustible 
gas from chemical and industrial processes and particularly from oil 
refining. Such stacks may be vertical, horizontal or inclined. The waste 
combustible gas is not usually continuously available but is 
intermittently supplied as it becomes necessary to dump such gas. 
It has also been common practice to supply purge gas intermittently to the 
flare stack when waste combustible gas is not being supplied so that air 
backflow in the stack is minimized, the purging of the stack by waste 
combustible gas or purge gas minimizing conditions favorable to an 
explosion within the stack. It is also necessary to take into account the 
failure of the purge gas supply. 
Referring now more particularly to the drawings, a flare stack 10 is 
illustrated having a supply conduit 11 connected thereto for the supply of 
waste gas from a waste gas supply connection past a relief valve 12. The 
waste gas is combustible and is derived from industrial operations and 
particularly from oil refineries. 
The flare stack 10 may be of any desired type, may have a fluidic seal 14 
spaced downwardly from the top to permit free upward movement of gas but 
to provide a substantial obstacle to downflow in the stack 10. A suitable 
form of seal for this purpose is shown in my U.S. Pat. No. 3,730,673. The 
flare stack preferably has a burner 15 on the top or discharge end for 
aiding in the admixture with the waste gas of air for combustion, and with 
or without steam, and may have a hollow cylindrical slotted wind shield 16 
closed at the bottom to protect the pilots 17 and the burners 15 from the 
wind. Suitable burners are shown in my prior U.S. Pat. Nos. 3,730,673; 
3,797,991; 3,822,984 and 3,995,986 but the apparatus of the present 
invention is applicable to a wide range of burners. 
A purge gas supply connection 20 is provided which communicates through a 
strainer 21 and pressure regulator 22 to divided or branch connections 23, 
24 and 29. The purge gas is usually an inert gas, a hydrocarbon gas or a 
combustible gas with an oxygen content too low to support combustion, or 
any other suitable gas with insufficient oxygen content for supporting 
combustion. 
The branch connection 23 has a solenoid control valve 25 therein and the 
branch 24 has a manually operable valve 26 therein. The branch connections 
23 and 24 extend to a supply line 27 with a purge failure flow alarm 28 
therein which is moved to a closed condition when there is no flow in the 
conduit 27, as hereinafter pointed out. The branch connection 29 has a 
proportional control valve 31 responsive to wind speed and specific 
gravity, as hereinafter explained. The conduit 11 has a flow responsive 
switch 30 inserted therein, which may be of any desired type, but which is 
closed when there is gas flow through the conduit 11 and open if there is 
no flow. 
The stack 10, preferably adjacent to its discharge end, has a wind speed 
responsive impeller 32, preferably an anemometer, which drives a signal 
source 33 for supplying a wind speed signal for utilization as hereinafter 
explained. 
A source 34 of power is provided connected by a conductor 35 to the flow 
switch 30 and therethrough by a conductor 36 to the winding 37 of a relay. 
The power source 34 is also connected by a conductor 38 to the contact 39 
actuated by the winding 37, when energized by closing of the flow switch 
by waste gas flow, to a downward position to establish a circuit through 
conductor 40 to activate a signal light 41 at the indicating panel 42 to 
indicate that waste gas flow is occurring and that no purge is needed. 
If no waste gas flow is occurring so that the flow switch 30 is in an open 
position the winding 37 is deenergized and the contact 39 is in its upper 
position. Power from the source 34 is available through conductor 38, 
contact 39, conductor 44 for the wind speed responsive signal source 33, 
through conductor 45, conductor 43 to the specific gravity meter 65 to be 
described, and conductor 58 to purge failure flow alarm 28, through 
conductor 59 to purge failure light 60. 
The conductor 45 is connected to a manual meter selector switch 46, and to 
a wind speed signal and specific gravity converting unit 48 
The selector switch 46 has a contact 49 which is connected by a conductor 
47 through indicating meter 50 for making available a wind speed 
indication through a pointer needle 51 at an appropriate scale in miles or 
kilometers per hour when the contact 49 is directly connected to the meter 
50 and to conductor 62. 
The contact 49 of selector switch 46 can be connected to a conductor 61 for 
specific gravity. 
The purge gas may vary in its specific gravity with respect to air so that 
if it is lighter than air there is a tendency to rise more rapidly in the 
stack 10. This makes it desirable to increase the flow for purge gas 
specific gravities less than one and to decrease the flow for specific 
gravities greater than one. 
In order to determine the specific gravity of the purge gas a specific 
gravity meter 65 is provided in a pipe 67 connected to the supply conduit 
11 with a motor driven pump 66 drawing gas from the conduit 11 and 
returning the same to the conduit 11 through a return connection 68. 
The specific gravity meter 65 and the pump 66 are activated by contact 39 
and through conductor 43 when there is purge gas flow and shut off 
whenever there is waste gas flow. 
The meter 65 supplies a signal through a conductor 69 which is connected to 
the signal converting unit 48 to modify the purge gas delivery inversely 
to the specific gravity. 
The signal converting unit 48 modifies the windspeed signal and the 
specific gravity signal to provide a purge flow rate signal in standard 
volumetric units per hour in accordance with the formula 
EQU S.sub.m = K.sub.1 V.sup.n.sbsp.1 + K.sub.2 SG.sup.n.sbsp.2 + K.sub.3 
where 
S.sub.m is the modified signal in terms of purge gas flow 
K.sub.1 is a constant dependent upon the dimensions and other 
characteristics of the components and takes into account a minimum wind 
velocity 
V.sup.n is the nth power of the wind velocity at the wind driven impeller 
33, and n.perspectiveto.2, 
K.sub.2 is a correction constant dependent upon the dimensions and other 
characteristics of the components for specific gravity 
SG is the specific gravity of the purge gas as compared to air, and n.sub.2 
.perspectiveto. 1, and 
K.sub.3 is a correction constant dependent on the dimensions and other 
characteristics of the components 
The purge gas flow signal is available through a conductor 52 and the 
contact 49 when positioned to the right for activating an appropriate 
scale of the purge rate range. 
A conductor 54 also extends to the proportional control valve 31 with a 
signal from unit 48 so that the purge rate may set the proportional valve 
26. 
The conductor 44 also has a conductor 56 extending therefrom to an 
indicating lamp 57 which is illuminated when purge gas is required. A 
conductor 58 extends from the conductor 56 to the purge failure flow alarm 
28 for controlling the indicating light 60 through conductor 59 from the 
alarm 28. The indicating light 60 operates when there is purge failure at 
the same time that there is no flow of waste gas acting on the switch 30 
and through the relay coil 37 and contact 39.