Abstract:
A sample system withdraws a sample of unburned flare gas from a feed line and delivers a liquid free sample to a high pressure gas chromatograph for analysis. In one embodiment, the sample system may be fitted with an automatic insertion system. In other embodiments, the sample system may be static or have manual insertion. The sample system includes a liquid separator to separate liquids from the sample. In one embodiment, the sample system is housed in an insulated and heated cabinet to prevent condensation of liquids as the sample moves through the sample system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In a refinery, petrochemical plant or on a drilling rig, it is common to have a flare to safely burn unwanted gases and vapors. If a plant suddenly shuts down due to an emergency, this is referred to as an “upset” in the industry and all combustible gases flowing through the plant may be shunted to the flare until the situation can be brought under control. All these unwanted gases and vapors are collectively referred to as “flare gas.” 
         [0002]    It is common to sample and analyze the unburned flare gas before it comes into contact with the atmosphere and the burner at the end of the stack. Gas chromatographs (GCs) are commonly used to make this type of analysis. In order for the GC to make the best analysis of the unburned flare gas, the sample needs to be dry. Liquids may be entrained in the flare gas itself and liquids may form in the sample system as the sample passes through the system. It is therefore necessary to a) separate/remove any liquids and/or vapors from the sample of unburned flare gas and b) reduce and/or prevent the precipitation of liquids from the sample as it passes through the sample system and before it reaches the GC. 
         [0003]    In some situations, unburned flare gas travels at speeds of from about 400 to about 900 feet per second while passing through the feed line to the flare stack. In some situations, unburned flare gas may range from approximately 200° F. to approximately 250° F. during regular operations of a plant. In some situations, during an upset, unburned flare gas may reach approximately 450° F. or more. 
         [0004]    In some sample systems, a probe may be automatically inserted into the unburned flare gas line and from time to time automatically retracted; this type of system is referred to in the industry as an “automatic insertion system.” Welker, Inc, the assignee of the present invention, owns a number of patents that use automatic insertion systems as follows: U.S. Pat. Nos. 4,117,676; 4,346,611; 4,387,592; 4,631,967; 5,936,168; 6,085,777; 6,338,359. Welker also owns the following patents: U.S. Pat. No. 3,904,176 for a Dump Valve; U.S. Pat. No. 5,579,803 for an Automatic Liquid Shutoff; U.S. Pat. No. 6,764,536 for a Sampling Device with Liquid Eliminator and U.S. Pat. No. 6,818,045 for a Fluid Separator. 
         [0005]    There is still a need for a better sample system for unburned flare gas that will a) separate entrained liquids from the unburned flare gas, b) reduce the formation of liquids while a sample is passing through the sample system, and c) selectively cool the unburned flare gas below the operational maximum of the sample system, during some situations, such as an upset. A delicate balance is required to keep liquids from reaching the GCs. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is a sample system for unburned flare gas. A probe is used to take a continuous sample from the unburned flare gas feed line and deliver it to the sample system. The sample system may be produced in three different embodiments: a) automatic insertion, b) manual insertion and c) static. 
         [0007]    As previously noted, GCs cannot analyze liquids. It is therefore necessary for the sample system to separate entrained liquids from the sample. The present invention includes a liquid separator to stop liquids from reaching the GCs. In one embodiment, the liquid separator may include a filter that will allow gas to pass through the filter, but not liquids. In one embodiment, the liquid separator may include a sump and drain located upstream of the filter to remove any liquids that may accumulate in the separator. In one embodiment, the liquid separator may include a ball-type liquid shut off valve positioned downstream of the filter to stop the flow of liquids and vapors to the GC, should the filter fail. In one embodiment, the sample system may include a sample gas tube formed with unequal diameters with the largest diameter in proximity to the stream of the flare gas. Should the liquid separator fail or merely need to be purged, a blowback system may be included to blow any liquids from the sample system. This blowback system uses an inert gas, such as nitrogen. 
         [0008]    If the sample system is cold, it may precipate additional liquids from the sample as the sample moves through the sample system. A heating system may be provided to help reduce and/or prevent the precipitation of liquids as the sample moves through the sample system. Further, the sample system may be placed in a heated insulated cabinet to help maintain appropriate temperatures for the sample system. This is especially important during cold winter conditions. 
         [0009]    During an upset, the temperatures of the unburned flare gas may spike upwards. A cooling system may be provided to help maintain temperatures below the operational maximum of the sample system. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a section view of the unburned flare gas sample system with an automatic insertion system, with the probe withdrawn from the flare gas feed line, not shown in this figure. 
           [0011]      FIG. 2  is a section view of the unburned flare gas sample system of  FIG. 1  but the apparatus has been rotated 90°. 
           [0012]      FIG. 3  is a section view of the unburned flare gas sample system of  FIG. 1  except the probe has been inserted into the flare gas feed line, not shown in this figure. 
           [0013]      FIG. 4  is a section view of the unburned flare gas sample system of  FIG. 1  except the probe has been inserted into the flare gas feed line, not shown and the apparatus has been rotated 90° from  FIG. 1 . 
           [0014]      FIG. 5  is an enlarged section view of  FIG. 1  to better show the four heaters in the probe. Flow arrows show the direction that the sample moves through the apparatus. 
           [0015]      FIG. 6  is an enlarged section view along the line  6 - 6  of  FIG. 5 . 
           [0016]      FIG. 7  is an enlarged section view of the liquid eliminator of  FIG. 4 . Flow arrows show the direction that the sample moves through the apparatus. 
           [0017]      FIG. 8  is a schematic of the flow path of all gas and liquids passing through the liquid eliminator and other components of the system. 
           [0018]      FIG. 9  is an elevation view of the cabinet with the doors removed. The cabinet is mounted on a block valve which is connected to the horizontal flare gas feed line. 
           [0019]      FIG. 10  is a top view of the cabinet with the doors closed. 
           [0020]      FIG. 11  is a section view of a static probe for the unburned flare gas system. 
           [0021]      FIG. 12  is a section view of a manually inserted probe for the unburned flare gas system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0022]    The automatic insertion flare gas sample system  20  is shown in  FIGS. 1-10 . The static flare gas sample system  22  in shown in  FIG. 11  and the manual insertion flare gas sample system  24  are shown in  FIG. 12 . All three systems are used to sample unburned flare gas. 
         [0023]    A means for automatically inserting a sample probe into a stream of unburned flare gas and withdrawing the sample probe form the stream of unburned sample gas is provided. Referring to  FIGS. 1 and 2 , the automatic insertion flare gas sample system  20  is shown in the withdrawn position with the probe  48  withdrawn from the unburned flare gas feed line, not shown in these figures.  FIGS. 3 and 4  shown the automatic insertion flare gas sample system  20  with the probe  48  in the inserted position in the unburned flare gas feed line, not shown in these figures. 
         [0024]    The automatic insertion assembly is generally identified by the bracket  28  in  FIGS. 1 and 2 . It may also be referred to as a means for automatically inserting and withdrawing the probe from the unburned flare gas feed line, not shown in these figures. The automatic insertion assembly is a double acting piston cylinder well known in the industry. An upper cap  30  is penetrated by a plurality of elongate stud bolts  32  which threadably engage a flange  34 . The flange may be threaded to a block valve, not shown in these figures. The block valve may be threaded to a flange on the unburned flare gas feed line, not shown in these figures. A cylinder  40  is captured between the upper cap  30  and the flange  34 . A piston  42  is slideably mounted in the cylinder and sealed against the cylinder  40  by circumferentially positioned O-rings. The piston defines an insertion chamber  50  and a withdrawal chamber  52  in the cylinder. A fluid, from a source of pressurized fluid, not shown, enters an insertion chamber  50  through the first port  44  to insert the probe  48  into the unburned flare gas feed line, not shown in these figures. A fluid, from the source of pressurized fluid, not shown, enters a withdrawal chamber  52  through the second port  46  to withdraw the probe  48  from the unburned flare gas feed line, not shown in these figures. 
         [0025]    A means for determining the location of the sample probe is provided. A first reed switch  54  is slideably mounted on a rod  56  and a second reed switch  58  is slideably mounted on the rod  56 . Conductors  60  are connected to the first reed switch and conductors  62  are connected to the second reed switch to send signals to and from the reed switches to a remote location. Magnets  64  are attached to the piston  42 . As the magnets and the piston move past the reed switches, they trip the switch sending a signal through the conductors which is a means for determining whether the probe is withdrawn as shown in  FIGS. 1 and 2  or inserted as shown in  FIGS. 3 and 4  in the flare gas feed line, better seen in  FIG. 9 . 
         [0026]    A processing assembly is generally identified by a bracket  80  in  FIG. 1  and is described in greater detail in  FIGS. 7 and 8 . The processing assembly includes a GUAT junction box  82 , which is an explosion proof junction box for electrical conductors; a three-way solenoid valve  84  to control an inert gas blowback system described in detail in connection with  FIGS. 7 and 8 ; a cooling system  86  for the sample and a liquid separator identified by the bracket  88 . 
         [0027]    Referring now to  FIG. 5 , a means for heating the sample in the probe  48  is provided. A plurality of heaters may be staggered inside the probe. For illustrative purposes, a first heater  92 , a second heater  94 , a third heater  96  and a fourth heater  98  are located inside the probe  48  to reduce and/or eliminate the formation of liquids in the sample as it passes through the inlet  106  of the probe as indicated by the flow arrows. Conductors  100  connect to the first and second heaters to provide a source of electrical power to these heaters. Conductors  102  connect to the third and fourth heaters to provide a source of electrical power to these heaters. A sample conduit  104  runs the length of the probe  48 . The sample conduit may be of unequal diameters, as shown or it may be of the same diameter, not shown. A large diameter  108  is located proximate the flare gas inlet  106  of the flare gas feed line, not shown in this figure and a smaller diameter  110  is located further up the probe  48 . The purpose of the unequal diameters is to further retard the presence of liquids in the sample as it passes from the feed gas line up the probe. For example, assuming a flare gas feed line with a 48″ diameter, the larger diameter of the sample conduit may have an OD of 0.75 inches and an ID of 0.63 inches; the small diameter of the sample conduit may have an OD of 0.5 inches and an ID of 0.43 inches. 
         [0028]    Referring to  FIG. 6 , the probe  48  is formed from an outer shell  112  which is thick and stiff to tolerate stress, due to the elevated velocity of the unburned flare gas as it passes by the probe in the flare gas feed line. Inside the probe is a first heater conduit  114  to receive the first heater, a second heater conduit  116  to receive the second heater, a third heater conduit  118  to receive the third heater and a fourth heater conduit  120  to receive the fourth heater. The number and location of the heaters is dependent on the length of the probe  48  and the size of the unburned flare gas feed line, not shown in this figure. For example, a flare gas feed line that is 48 inches in diameter may require a probe that penetrates about 22 inches into the flare gas feed line. The inlet for the probe should be in the middle third of the feed line to avoid liquids. Liquids, if any, tend to flow along the walls of a pipe. Therefore, keeping the probe inlet  106  away from the pipe wall is helpful. Each heater conduit has a closed end to isolate the heaters from the sample. An optional thermocouple conduit  122  is optionally located in the probe to receive a thermocouple  124  to measure heat in the probe and environs. A welded bottom  126 , better seen in the preceding figure is placed in the tip of the probe to prevent ingress of the unburned flare gas except through the sample conduit located in the center of  FIG. 6 . 
         [0029]    Referring now to  FIGS. 7 and 8 , the processing assembly and the liquid separator are shown in greater detail. The purpose of the processing assembly is to reduce and/or eliminate liquids from the sample before the sample reaches a GC. The liquid separator  88  has been enlarged in  FIG. 7  and shown schematically in  FIG. 8  with the rest of the processing assembly  80 . 
         [0030]    The sample flows up the sample conduit  104  of the probe  48  when the probe is in the inserted position of  FIGS. 3 and 4  in the unburned flare gas feed line, best seen in  FIG. 9 . The sample passes through an outlet port  140  in the liquid separator and flows through a conduit  142 , best seen in  FIG. 5 , into the solenoid activated three-way valve  84 , through a conduit  144  into the cooling system  86 , through the inlet port  146  of the liquid separator  88  to a Tee  148 . At the Tee, the sample branches into two redundant flow paths. The flow path on the left side of the Tee will be described in detail. The flow path on the right side of the Tee is a mirror image of the other side. 
         [0031]    The sample passes through an aperture  154  in a support  152  and through a filter  150  that will allow gas to pass but not liquid. The filter may be formed from a Teflon® membrane. Teflon is the brand name for tetrafluroethylene (TFE) produced by Du Pont. Other vendors also make other brands of TFE which may be suitable for use in this invention provided that they achieve separation of gas from liquids. Other products may also be suitable for use as a filter  150 . For example, Tyvek® brand material from Du Pont de Nemours, E.I. Company may also be suitable as well as Millipore four micron filter paper from Pall Specialty Materials in Charlotte, N.C. After the sample passes through the filter  150 , the sample should be dry, but to protect the expensive GCs from damage, a ball-type shut off valve  156  is provided in the exit of the liquid separator. If the filter fails or is working improperly, liquids or vapors will cause the ball  158  to rise and engage an O-ring  160  thus sealing the outlet port  162 . In  FIG. 7 , the ball  158  is shown in the open position allowing gases to pass through the outlet port  162  from the liquid separator. These dry gases pass through a conduit  164  through a three way valve  166  to the first GC  168 . The opposite side of the liquid separator  88  is identical. 
         [0032]    A second outlet port  169  is formed in the opposite side of the liquid separator. Dry gases pass through the second outlet port, through a conduit  171  and into the second GC  172 . 
         [0033]    Optionally, a first sump  174  is formed on the left side of the liquid separator to collect liquids, if any. A drain  176  provides a means to remove accumulated liquids, if any from the first sump in the liquid separator. In one embodiment, a small vacuum is pulled on the drain  176 . This small vacuum encourages liquids to move down the drain and away from the liquid separator. The right side of the liquid separator has a similar arrangement including drain  184 . 
         [0034]    If the membrane fails, the ball-type shut off valve will become vapor locked. In this situation, the system is shut down and the first vacuum breaker valve  178  and the second vacuum breaker valve  179  are opened. 
         [0035]    Referring now to  FIG. 8 , electrical conductors  180  enter the GUAT junction box  82 . Some of these conductors  182  pass from the junction box  82  to the 3-way solenoid operated nitrogen blowback valve  84 . Some of the conductors  100  and  102  connect with the heaters  92  and  96  to provide heat to the probe. As best seen in the next figure, the electrical conductors  180  droop down in a J-shape to better move up and down with the processing assembly  80  as the probe is inserted and withdrawn from the flare gas feed line. 
         [0036]    A sample from the unburned flare gas feed line, not shown in this figure, enters the sample conduit  104  and passes to the 3-way valve  84 , through the cooling system  86 , and into the liquid separator  88 , the sample passes through the pair of filters to remove any liquids and exits the separator at outlet ports  162  and  169 . The dry gas from outlet port  162  passes through the 3-way valve  166  and into the first GC  168 . The dry gas from the outlet port  169  passes through the 3-way valve  170  and into the second GC  172 . 
         [0037]    Optionally, the liquid separator  88  may be equipped with drains  176  and  184 . An eductor  194  is T-shaped and connects to a motive line  196  for inlet gas, a suction line  198  to pull the vacuum and a discharge line  200  for the nitrogen and any liquids that may be drained/pulled/educted from the liquid separator and the drains  176  and  184 . 
         [0038]    The 3-way valves  166  and  170  are used to feed calibration standards into the GCs as is well known in the art. When the calibration standard is being fed the GCs, the sample feed is shut off. And conversely, when the sample feed is on, the calibration standard is off. 
         [0039]    Optionally, the liquid separator  88  may be equipped with a nitrogen blowback system. If the liquid separator, cooling system and/or probe are fouled with liquids or otherwise needs to be cleaned out, nitrogen from a source of pressurized nitrogen, not shown, is blown through the 3-way valve  84  and the sample conduit  104  back into the feed gas line. 
         [0040]    In one embodiment of the present invention, flare gas temperatures range from approximately 200° F. to approximately 250° F. in the feed lines. In this embodiment, the sample system includes a heating system to warm the sample conduit to prevent condensation as the sample leaves the flare gas feed line and passes through the sample conduit  104 . The setting for the heating system will vary according to each installation. Various factors include the ambient temperatures, the length of the probe  48 , the temperature range of the feed gas line and other factors. In this embodiment, the heating system operates at a range of from approximately 212° F. to approximately 230° F. During an upset, temperatures of unburned flue gas may spike upward and need to be cooled. The purpose of the cooling system  86  is to keep the temperature of the sample below the operational maximum, which for this embodiment is approximately 400° F. The filter fails at sustained temperatures in excess of 400° F. In this embodiment, the cooling system is pre-set to reduce the temperature of the sample to about 385° F. before it enters the liquid separator  88 . 
         [0041]      FIG. 9  is an open view of the cabinet  186  showing the auto insertion flare gas sample system  20 . All electrical connections in, on and in proximity to the cabinet must be “explosion proof” or “intrinsically safe”. In other words, the sample system, cabinet and associated electrical connections should be compliant with Class 1, Div. 1, Group C &amp; D of the National Electrical Code. The probe  48  is shown in the inserted position. The probe  48  passes through a block valve  188  and into the flare gas feed line  190 . 
         [0042]    A plurality of heaters  192  are placed in the insulated cabinet to help keep the system warm and reduce the precipitation of liquids into the sample. In one embodiment, these heaters are designed to operate at a set point of about 230° F. This temperature is above the boiling point which retards formation of liquids as the flare gas passes through the sample system. The heaters may be in use 12 months a year. The temperature range of the heaters depends on the elevation of the installation site, the size of the cabinet and typical weather conditions, among other factors. (Elevation is relevant because the boiling point increases at higher elevations.) A first port  44  and a second port  46  allow fluid to enter the double acting piston/cylinder which moves the probe  48  in and out of the flare gas feed line. During insertion of the probe, the port  44  acts as an inlet and the port  46  acts as an outlet. During withdrawal of the probe, the port  46  as an inlet and the port  44  act as an outlet for the fluid. 
         [0043]    The optional drains work with an eductor  194 . One educator that may be suitable for use in this invention is a Mini-Eductor Part Nos. 611210-093, -060, -030 and -015 from Fox Valve Development Corporation of Dover, N.J., www.foxvalve.com. The educator  194  is a simple venture, with an inlet and outlet for nitrogen or some other inert gas. The educator is T-shaped and connects to a motive line  196  for the inlet gas such as nitrogen, a suction line  198  to pull the vacuum from the drains  176  and  184  and a discharge line  200  for the nitrogen and any liquids that may be drained/pulled/educted from the drains in the liquid separator as best seen in the preceding figure. Typically, the nitrogen passing through the eductor and any liquids from the optional drains are directed back into the feed gas line. The eductor has a needle valve, not shown, upstream to enable precise adjustments of the vacuum. Typically the amount of vacuum is adjusted in the field, after installation of the apparatus. If too much vacuum is pulled on drain, no sample will reach the GCs. This eductor may pull from zero to about 30 inches of mercury in the drains  176  and  184 . 
         [0044]      FIG. 10  is a top view of the insulated and heated cabinet  186  for the unburned flare gas sample system. A front insulated door  220  is removably connected to the cabinet  186  by hinges  221 , pins or other connecting means. A rear insulated door  222  is connected to the cabinet  186  by hinges  223 , pins or other connecting means. In one embodiment, each insulated door is about 9 inches thick. The cabinet is about 18 inches thick. In one embodiment, the insulation in the doors and in the cabinet provides about 0.19 Btu-inch/hour-foot squared-degrees Fahrenheit of insulation. 
         [0045]      FIG. 11  is a section view of a static sample system  22  for unburned flare gas. The primary difference between the sample system  22  and the preceding sample system in  FIGS. 1-11  is the absence of the automatic insertion assembly  28 . All other components are substantially the same. The probe  48  is welded to the flange  34  to secure the probe in an inserted position in the flare gas feed line. 
         [0046]      FIG. 12  is a section view of a manual insertion sample system  24  for unburned flare gas. The primary difference between the sample system  24  and the sample system in  FIGS. 1-11  is the absence of the automatic insertion assembly  28 . The sample system  24  is manually inserted into the flare gas feed line and depending on the pressure in the flare gas feed line is manually withdrawn. A plurality of stud bolts  226  extend from the flange  34  parallel to the probe  48 . A flange  230  is adjustably clamped to the probe  48 . The flange  230  has a plurality of apertures, not shown, for the plurality of stud bolts  226  to pass through. A plurality of nuts  228  are then threaded on the stud bolts to hold the flange  230  and the probe in place. The nuts may be unscrewed to withdraw the probe from the flare gas feed line.