Vapor Recovery System for Loading Backs and Storage Tanks

This invention relates to equipment for recovering volatile petroleum vapors, and more particularly to a closed petroleum storage facility the ullage of which is charged with a blanketing gas under accurately controlled pressure conditions together with means for monitoring the system for small quantities of oxygen before being admitted to vapor recovery equipment. The system features high precision self-checking oxygen analyzers for detecting incipient hazardous conditions and for operating control devices to maintain wide-margin safe operating conditions under widely varying operating and atmospheric conditions.

Persons constructing and operating facilities for handling volatile liquids and petroleum products and derivatives thereof are confronted with difficult and challenging problems as respects the control of portions of these products in vapor phase. Such vapors are fugitive and commonly escape to the atmosphere in quantities presenting economic losses and series hazards to the health of humans, animals and plant life. Private and commercial interests have made earnest and costly efforts to minimize these vapor emissions and hazards but the complexities of the problems to be dealt with and the costs involved present obstacles which have not been satisfactorily resolved heretofore. In recent years governmental agencies have endeavored to establish strnginet standards butressed with stiff penalties. Despite these incentives and the diligent efforts of many designers in providing a wide variety of modes of dealing with the problems no satisfactory protective system has yet been devisedmeeting current requirements.

The following patents are typical of the vapor treating and recovery system heretofore proposed, namely, Hartman U.S. Pat. No. 2,765,872, Moragne, U.S. Pat. No. 3,266,262, Bragg U.S. Pat. No. 3,590,559, Battey U.S. Pat. No. 3,714,790, Husa, U.S. Pat. No. 3,783,911, Schonewald U.S. Pat. No. 3,648,436, David, U.S. Pat. No. 3,672,180, Henrix U.S. Pat. No. 3,830,040, McNamee U.S. Pat. No. 3,886,759, and Mair U.S. Pat. No. 3,903,078. These systems are either totally lacking or have inadequate means for preventing the occurrence of flammable mixtures in the recovery system and, in consequence, are subject to hazardous operating conditions. Only Husa U.S. Pat. No. 3,783,911 and McNamee U.S. Pat. No. 3,886,759 utilize oxygen analyzers to monitor vapor for the presence of oxygen but each of these teachings lacks suitable means for utilizing the analyzer to provide protection against flammable vapor conditions. Some prior vapor recovery systems are said to operate with a reasonable degree of satisfaction under limited ambient temperature conditions as, for example, a temperature range of 70.degree.-80.degree. F, but provide unacceptable and unsatisfactory results under commonly encountered higher ambient temperatures, such as 85.degree.-120.degree. F.

Among the vapor control expedients currently in use are those utilizing floating roofs with seals between the roof rims and the interior tank walls. These systems are costly, difficult to maintain, subject to varying vapor emissions under varying structural and atmospheric conditions, which losses are extremely difficult to measure with any degree of accuracy. They also fail to meet the continually more stringent standards promulgated by governmental environment agencies. A predominant cause of vapor leaks from floating roof seals is the out-of-roundness of the tank shell. Such distortions develop over a period of time with earth movements under the tank. Ambient weather conditions also tend to deteriorate the effectiveness of the working floating roof seal.

Closed vapor recovery systems, on the other hand, using a blanketing gas under fixed tank roofs have no working seals, and 100% of liquid vapor emissions are normally recovered. Prior to this invention such systems have been hazardous for lack of any reliable provision to safeguard against explosive conditions owing to the possible entry of oxygen into the system under various operating conditions. Ignition of such combustible gas mixtures can occur from numerous sources, such as electrostatic conditions caused by fluid movements within the storage tanks, tank gauging operations, or the presence of pyrophoric iron sulfide, and others. Also, liquid contaminations have occurred between tankages in a closed system, and constant operator attention has been necessary. Commercial vapor recovery packages commonly are designed for truck loading rack vapors exclusively and use higher compression ratios, sometimes to auto-ignition pressures, refrigeration with attendant icing problems, and/or absorption with up to 10% of the collected vapors being wasted in order to dispose of that which does not absorb.

This invention avoids the numerous shortcomings and disadvantages of prior proposals for safeguarding volatile fluids against vapor losses and combustible mixtures, utilizing fully closed storage and volatile liquid loading systems which are substantially fully automatic and as efficacious as it is reliable and safe. The system has general application to all volatile liquids, and more particularly to the storage and loading of petroleum liquids. Recovery of substantially all vapors from these liquids is assured under conditions which have provided at least a ten-fold safety factor for petroleum storage facilities and a three-fold safety factor for the operation of loading rack facilities. The difference is due to the normal absence of oxygen in the vapor recovery system of storage facilities.

The invention system may include any number of gas-blanketed storage tanks for as many different kinds of volatile liquids each typically equipped with an air-liquid conical roof as well as loading rack facilities for trucks, rail cars, ships and barges, all connected to and forming part of a closed vapor recovery system. The closed blanketing system includesa gas holding tank and suitable means for separating out condensed vapors while retaining the remainder captive and mixed with the combustible blanketing gas. This mixture is then utilizable beneficially either on the premises or disposed of for heating value to the utility supplying the blanketing gas. An inert or non-combustible blanketinggas may also be used in those instances where a combustible gas is not economically available. A suitable inert gas may be carbon dioxide, nitrogen, flue gases, etc., as long as it is not detrimental to the liquids being handled or to the mechanisms involved. The vapor condensing equipment typically comprises a compressor, evaportive condenser, and a liquid separator. Strategic portions of the blanketoing system are equipped with high sensitivity oxygen analyzer recording controllers for dtermining the presence of even minute quantities of oxygen together with means responsive to detection of such oxygen to operate automatic controls to rectify the problem as well as warning devices for supervisory personnel. Eachanalyzer is preferably of the type which periodically checks itself for accuracy and sounds an alarm if not in a fully satisfactory operating condition or in need of servicing.

The liquid storage tanks include sensitive valve control means for isolating the liquid storage tanks from contamination by incompatible vapor from one or more other liquid storage tanks, yet permitting the vapor emanating from all tanks to be processed for recovery by the same recovery facilities. For example, one group of tanks may contain sweet liquid and other s sour liquid. Vapor recovered from the loading rack is also processed by the same compressor and condenser facilities but separately from the holding tank for vapor recovered from the storage facilities. If the oxygen analyzers monitoring the gas holder and the holding racks detect an abnormal quantity of oxygen, controls activated by these analyzers operate to discontinue operation of the compressor and condenser and to purge additional blanketing gas. Uncondensed vapor and blanketing gas mixtures have a higher heating value than that of the blanket gas alone. Net exhalations from this invention, therefore, are a better fuel gas per unit volume than the blanket gas, and the valumes exhaled from the invention are greater than those inhaled into it. When the blanket gas is combustible, it may be used as a fuel gas on the premises or sold as such to others. When the blanket gas is non-combustible, net exhalations are flared, or otherwise disposed of safely. "Sour" is meant herein to identify any volatile liquid whose vapors are detrimental in any way to another volatile liquid, identified as "sweet", "Ullage" as used herein is the total volume of the storage tank less the stored liquid volume. "Margin of safety" as used herein means the concentration of oxygen measured divided by the minimum concentration of oxygen required to support combustion.

Accordingly, it is a primary object of this invention to provide a unique and improved vapor recovery system for recovering all vapor emissions occurring in or produced by volatile liquid storage and loading facilities.

Anotherobject of the invention is the provision of a closed vapor recovery system for volatile liquid loading vacilities maintained charged with an inert or a combustible blanket purge gas, and including vapor recovery facilities, the operation of which is monitored and automatically stopped and restarted by oxygen analyzer means, depending upon the presence or absence of oxygen as respects a predetermined percentage of oxygen, without need for interrupting the loading operation.

Another object of the invention is the provision of a fully closed vapor recovery system for volatile hydrocarbon storage facilities wherein portions of the facilities not filled with liquid are chargeable with inert or combustible blanketing gas, including means for monitoring this gas for the presence of oxygen, and including means for condensing portions of the volatile vapor present therein and for routing the remainder of the combustible vapor and gas mixture to a place of beneficial usage therefor.

Another object of the invention is the provision of volatile liquid handling facilities including bulk storage tanks and loading rack facilities for rail cars, trucks, ships, and barges, all connected in a gas blanketing vapor recovery system continually monitored and controlled by oxygen analyzer means.

Another object of the inventin is the provision of a fully cosed vapor recovery system for hydrocarbon storage and handling facilities havinga combustible blanketing gas admitted to the ullage thereof, and to the loading rack vapor collection header, from a public utility, and wherein said blanketing gas plus collected hydrcarbon vapor is returned to the utility after saidvapor and gas mixture has been analyzed and found not to contain harmful contaminents.

Anogther object of the invention is the provision of loading racks with conventional vapor recovery nozzles for tracks, rail cars, ships and barges connected to a closed vapor recovery system automatically supplied with a blanket gas purge during the liquid transfer to such vehicle compartments and which vapor recovery system is monitored by an oxygen analyzer automatically operable to discontinue vapor recovery and to route the gas and vapor mixture to a safe point of disposal upon detection of a predetermined concentration of oxygen therein, and to resume vapor recovery operation as soon as the excess oxygen condition has been rectified, all without interrupting the liquid loading operations.

Another object of the invention is the provision of volatile liquid storage facilities including separate storage tanks for non-compatible volatile liquids in a closed vapor recovery circuit charged with blanketing gas and including valve means for automatically isolatong one or more liquid storage tank from contamination by vapor emanating from another storage tank or tanks.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.

Referring now to the drawing in which a preferred embodiment of the invention is illustrated:

FIG. 1 is a diagrammatic showing of an illustrative embodiment of a closed vapor recovery system for a volatile liquid storage and loading facility incorporating the principles of this invention.

Referring to FIG. 1 there is shown a par of sweet volatile liquid storage tanks 10,10 and a sour volatile liquid storage tank 11. It will be understood that these three tanks are merely illustrative and that a typical storage facility may and usually includes a larger number of storage tanks. Each tank has a fluid-tight cone roof conventionally 3/16 inch thick provided with an automatic pressure control valve 12 for controlling the admission of blanketing gas into the associated tank and a pressure-vacuum relief valve 13 typically adjusted to vent to the atmosphere if the internal pressure exceeds 0.50 osi and to admit air if the internal pressure is below the ambient pressure by 0.50 osi. Relief valves 13 only open upon emergency to prevent structural damage to the fixed roof tanks.

The pressure control valves 12 are of conventional construction having a diaphragm one side of which is subject to the pressure within the tank and the other side to atmospheric pressure and effective to open and close a valve admitting blanketing gas to the tank ullage preferably supplied from a blanketing gas supply, such as a public gas utility distribution system, or a supply of inert gas if combustible gas is not available. This gas is distributed to the tank ullage from piping 14 through pressure control valve 12. The ullage pressure is maintained above 0.25 osi vacuum by blanketing gas admitted by pressure control valve 12, thereby preventing relief valve 13 from admitting air into the tank.

The ullage of each of the storage tanks 10,11 is connected by piping 15,16 to a gas holding tank 17 equipped with a flexible diaphragm 18. This diaphragm is supported by the captive mixture of vapor and blanketing gas and has a weight, such as 0.2 osi, selected to maintain the captive gas in the gas holder at that pressure. In consequence, the slightly higher pressure available in the ullage of the storage tanks is effective to provide an exhalation of vapor and blanketing gas into the gas holding tank 17. If the physical arrangements of tankage piping and the stored liquid volatilities combine to produce pressure drops which cause the pressure-vacuum relief valves 13 to release vapor or gas to the atmosphere, it is necessary that piping 15 be enlarged and/or provided with a motor driven blower 20 for aiding or augmenting gas flow. Alternatively, a greater storage tank pressure can be tolerated by increasing the weight and strength of the roof, and increasing the pressure-relieving set point of valve 13. Piping 15 is provided with a flame arrester 21 in order to prevent any flame that should occur at the gas holder from propagating to tanks 10 or 11.

If the described totally closed vapor recovery system includes one or more volatile liquid storage tanks which contain vapors which are detrimental to the liquid contents of the other tanks, such as those vapors from tank 11, piping 15,16 is provided with differential pressure responsive valve means to prevent the movement of vapor therefrom into the liquid storage tanks 10. Suitable automatic valving for this purpose includes a normally closed butterfly valve 23 in piping 15. This valve remains closed until the pressure on the inlet side of the valve exceeds the pressure on the outlet side. When the inlet pressure in piping 15 exceeds the pressure in piping 16, a sensitive pressure transmitter 24 functions in a manner well known to those skilled in the differential pressure valve art to open valve 23 widely. Differential pressure transmitter 24 is connected by conduit 25 to the upstream side of piping 15 and the other side of which is connected by a conduit 26 to piping 16. A supply of air pressure is transmitted from 24 to the diaphragm of valve 23 to positively actuate the butterfly valve position under the control of the differential pressure transmitter 24.

To permit return of the vapor and gas mixture to a fuel gas supply system, it is necessary to raise the pressure of this mixture to the system pressure, such as 40 psi for a refinery fuel gas system. Herein this is accomplished by a compressor 35 driven by motor 36. The heat of compression is removed most effectively by an evaporative condenser 50, the water of which is circulated by pumps powered by motor 70. The vapor and gas mixture is conducted to the intake of compressor 35 via piping 37 which is equipped with a butterfly valve 38. This valve operates in the same manner as valve 23 and is controlled by a pressure differential transmitter 39 connected to piping 37 on the opposite sides of valve 38 via conduits 40,41. If the pressure on the inlet side of valve 38 exceeds that on the outlet side very slightly, transmitter 39 operates to open valve 38 widely with supply air pressure.

The purpose and function of differential pressure responsive valve 38 is to prevent the vapors collected from the loading rack facilities from entering the gas holding tank 17. The gas mixture from the loading rack may contain oxygen in varying amounts while that from tankage should contain no oxygen. The presence of valve 38 assures that oxygen analyzer 60 is monitoring exhalations from tankage only. Thus, it can cause an alarm and a flow of blanket gas purge through solenoid valve 66 when only as little as one tenth of the oxygen concentration is detected that is required for combustion. If the oxygen concentration increases, nevertheless, to one third of that required for combustion, oxygen analyzer 60 will cause vapor recovery compressor 35 and evaporative condenser 50 to stop by de-energizing their respective motors 36 and 70. Such will be described in greater detail presently.

Vapor and blanketing gas compressed in compressor 35 exits through piping 48 and enters the evaporative condenser 50 where the heat of compression is removed and some condensation may occur. The fluid then passes via pipe 51 into the separator 52. When the liquid level reaches a predetermined height in separator 52, level controller 53 pneumatically opens valve 54, allowing the liquid to exit through pipe 55. check valve 56, and through pipe 57 back to any selected one of the liquid storage tanks 10, or to some other storage tank. All of the blanketing gas and the non-condensed vapor, or net exhalations, exits from the vapor recovery system via piping 75. This gas and vapor mixture from piping 75 is a high heating value fuel gas utilizable as such in any of many ways.

A critically important feature of the invention vapor recovery system involves the safeguards and equipment employed to prevent the accumulation of dangerous quantities of oxygen in the vapor recovery system. Such protection is provided by three high precision oxygen analyzers each connected to a particular part of the vapor recovery system as will now be described.

A first one of these oxygen analyzers 60 is connected to the gas holding tank 17 via a sampling connection 61 and a threeway valve 62 normally postioned as shown to supply a sample of vapor and gas to analyzer 60. A suitable high-precision oxygen analyzer found to provide most reliable and satisfactory results is obtainable commercially from Teledyne Corporation and is designated by that manufacturer as TAI Model 326B-2X. This analyzer contains an oxygen detector cell having a porous diaphragm exposed to the flow of the gas to be analyzed and functions to provide an electrical D.C. output signal having a voltage level varying linearly with the amount of oxygen present in the gas being analyzed. This output signal is utilized to govern the operation of electrical control circuitry in a manner well known to those skilled in the control art to operate relay circuits governing power supplies to one or more alarms, warning devices and control auxiliaries for the vapor recovery system. Thus when analyzer 60 detects a low concentration of oxygen present in gas holder 17, such as 1.7 volume percent, representative of a ten fold factor of safety before reaching the upper explosive limit of this gas and vapor mixture, the resulting output signal provided by the analyzer is transmitted through control relays over electrical connections 65 to open the normally closed solenoid valve 66 to admit purging blanketing gas via piping 67 to the gas and vapor mixture entering the gas holder. Concurrently, it will sound an alarm (not shown) to acknowledge the attendant that this mitigating action is taking place. If the oxygen concentration increases further to 5.7 volume percent, or one third of the aforementioned margin of safety, the associated higher voltage output signal is utilized to operate power control relays to shut off motor 36 driving compressor 35 and motor 70 circulating water through evaporative condenser 50, and to sound a separate alarm (not shown) to advise operating personnel that such is occurring.

A variable volume gas holder is provided so that the compressor and evaporative condenser need not operate continuously, but only to empty the holder as far as tankage is concerned. When the gas holder diaphragm is about 80% full, its level sensing switch 72 starts the compressor and condenser. Surplus holder volume is provided for the special case where net tank exhalations into the holder plus loading rack vapors exceed the compressor capacity. When the holder is evacuated and the diaphragm drops to a low level, the level switch 72 stops the compressor and condenser unless the loading rack is in operation.

The additional blanketing gas purged into the gas holder via solenoid valve 66 and supply piping 67 decreases the concentration of oxygen present in gas holder 17. If so much blanket purge gas is admitted to the gas holder diaphragm that it reaches its maximum capacity before the oxygen concentration is adequately reduced, and in spite of the compressor being in operation, the gas pressure will then rise to a predetermined maximum value, such as 0.5 osi, whereupon a pressure relief valve 70 connected to the gas holder, as via pipe 40, opens and wastes gas to the atmosphere. As soon as analyzer 60 determines that the oxygen concentration in the gas holder has been restored to a lower operating value, the electrical output signal from the analyzer falls below predetermined minimum operating values. Output signals, through the relays and electrical control circuits, activate compressor motor 36 and evaporative condenser water circulating motor 70. Subsequently, when oxygen concentrations reduce from 5.7 volume percent to 1.7 volume percent, control circuits cause solenoid valve 66 to close, and the normal mode of operation is restored.

A second oxygen analyzer 77 of similar construction is employed to monitor the oxygen concentration in the top of separator 52 because oxygen concentrations in piping 75 are greater than those beforehand by the amount of condensation that may occur. For this purpose analyzer 77 is shown as having a sampling connection 76 to piping 75 which conveys the uncondensed vapor and blanketing gas from separator 52.

If analyzer 77 detects an oxygen concentration in piping 75 on the order of 5.7% oxygen, the analyzer provides an output signal transmitted via electrical connections 73 and 65 to deactivate the motors driving the evaporative condenser and the compressor. When either analyzer 60 or 77 detects an oxygen concentration in the order of 5.7 volume percent, the resulting electric signal which de-energizes motors 36 and 70, servicing compressor 35 and condenser 50 respectively, will over-ride any signal from the gas holder diaphragm level switch 72 to energize these motors.

A dual pen integrating flow recorder FR,FR' of any suitable type is connected to monitor, separately, the quantity of blanketing gas entering the invention through piping 14 and the quantity of gas and vapor being delivered from the invention through piping 75. In this manner there is provided a continuous totalizing record of the quantity of vapor emissions recovered, that being the difference between the inhalations or use of, blanket gas and the net exhalations of the system. Thus, the difference in the integrated readings by recording pens FR,FR' represents the net uncondensed vapor recovered from tankage and the loading rack facilities. All condensed portions of the vapor recovered in separator 52 are returned to the storage tanks through piping 57 or to some other suitable liquid collection tanks. Experience has shown that the described system normally recovers substantially all of the vapor originating from the volatile liquid being stored or loaded.

The loading rack facilities, illustrated in the lower left hand corner of FIG. 1, comprise loading racks 80 for tank trucks and 81 for tank cars. The loading racks as shown are intended to apply to any truck, rail car, ship, or barge loading facility with conventional vapor recovery loading nozzles. Typically each loading rack includes liquid supply piping 82 from storage tanks 10,10 and a separate supply pipe 83 from the storage tank 11 connected through suitable control valves to the inlet of a loading rack supply pump 85 driven by motor 87. The outlet of this pump is typically connected by a manifold 86 at the loading facilities 80,81 by conventional auxiliaries. Pump motor 87 is typically started and stopped by manual control switch 88. As soon as the pump 85 starts the resulting flow of liquid to the loading racks will activate the flow sensing switch 96 and send a control signal to start operation of the compressor motor 36 and of the evaporative condenser motor 70, if these two motors are not then in operation. If they are in operation the control signal from flow switch 96 is unnecessary and redundant.

The loading rack filling nozzles are typically provided with vapor collection piping 90,91, equipped with check valves 95, and vapor collection header 92. The header 92 is equipped with a flame arrester 93 and is connected to the compressor by inlet piping 37. Each of the vapor collection pipes 90,91 is provided with a check valve 95 to prevent vapors displaced from one loading compartment to be admitted into another compartment and to prevent the entrance of blanketing gas into the compartments being filled. The blanketing gas is purged into the extremity of the collection header 92 so as to convey the vapor displaced from the compartments being filled to the compressor. As shown, the blanket purge gas is supplied from piping 67 via piping 93 equipped with a check valve 94 and a solenoid control valve 49A. Valve 49A is energized whenever the loading rack supply pump is in operation, such as by a flow responsive switch 96 which responds to the flow of liquid in piping 86 when pump 85 is operating.

During the loading period of any truck, rail car, ship, or barge, air trapped in the compartments being filled will flow with the displaced vapor into header 92 and mix with the blanket purge gas entering this header from supply piping 67,93. Samples of this gaseous mixture are continuously bled from header 92 via sampling connection 97 to the oxygen analyzer 98 via the three-way valve 99. If the oxygen concentration is in the order of 5.7% oxygen or higher by volume, analyzer 98 will provide an output signal that will sound the alarm 68 and activate the warning light 69, both located at the loading rack. If the attendant is not in the immediate vicinity of the loading rack, he will go to the rack to turn off the alarm. However, the warning light will remain on until the oxygen concentration is rectified. Concurrently, a signal will be transmitted over connection 100 to deactivate the compressor motor 36 and the condenser motor 70, and to open solenoid valve 105 so that the gas mixture containing excess oxygen is vented to a safe place of disposal, such as a flare, not shown. Concurrently, the output signal from analyzer 98 is transmitted to and opens the solenoid valve 49B in parallel with the then open solenoid valve 49A. The combined flow capacities of the two open valves 49A and 49B provide an increased flow of blanketing gas in order to reduce the time required to dilute the oxygen concentration to acceptable limits.

The flow of the gas collected from the loading rack to a disposal flare does not include gas from holder 17 because the high pressure now prevailing on the outlet of the differential pressure valve 38 in pipe line 37 causes differential pressure transmitter 39 to close valve 38 and keep it closed until solenoid valve 105 to the flare recloses when analyzer 98 determines that the concentration of oxygen in line 92 is acceptable. When analyzer 98 detects an oxygen concentration in the order of 5.7 volume percent, the resulting electric signal which de-energizes motors 36 and 70, servicing compressor 35 and condenser 50 respectively, will over-ride any signal from the gas holder diaphragm level switch 72 to energize these motors.

Analyzer 77 is located in the system after condensations are removed because such removals increase oxygen concentrations in the gas phase. This analyzer backs up analyzers 60 and 98 by recording the percent oxygen volume in gases and vapors from both tankage and loading racks. Analyzer 77 is regulated to activate the alarm and control circuits when the oxygen concentration is one half of that of the upper explosive limit when the loading rack and tank facilities are combined.

To safeguard against continuing to use an oxygen detection cell forming a critical component of the oxygen analyzers if defective or beyond its fully effective service life, each of the analyzers 60, 77 and 98 is provided with a timer-controlled solenoid-driven three-way valve and a compressor 107 operable to check the operating condition of the associated cell. Normally, the three-way valves 62, 78 and 99 are positioned as shown in full lines in FIG. 1 wherein they are effective to sample the vapor and gas mixture for the presence of oxygen. Periodically, such as once per hour, the timer T energizes the solenoid 102 driving the associated three-way valve to supply air from the atomsphere via piping 108 to the associated one of the oxygen analyzers 60, 77, or 98. This air is compressed by a continuously operating compressor 107 and circulated past the cell for a brief period, such as three minutes. If the analyzer determines the normal percentage of 21% oxygen of atmospheric air is present, it is known that the analyzer cell is functioning properly and no warning signal is given. At the end of the brief checking period timer T operates to restore the three-way valve 99 to its normal or former position wherein the analyzer checks the vapor and gas mixture for oxygen. If the cell finds that the ambient atmospheric air being tested contains less than the normal 21% of oxygen, the cell is not functioning properly and the analyzer automatically activates special alarms, not shown, which identify the analyzer at fault and the extent to which it is faulty. It will be understood that each of the analyzers 60, 77, 98 is equipped with the same self-analyzing facilities and that oxygen concentration set-points are adjustable. These facilities include recording strip charts and reset panel alarms which are typically located in a control house occupied by supervisory personnel.

The strip charts include a graphic recorder for displaying the percentage of oxygen detected by the oxygen sensitive cell. Once each hour a high pip representing approximately 21% oxygen found present in the ambient air is recorded. The remainder of the recording will reflect a very low oxygen content in the gas being analyzed. The recorder provides a graphic picture of operating conditions and inspires confidence in the reliability and safety of the entire volatile fluid handling facility. The compressor motor 107 and timer T for oxygen analyzer 98 are deactivated by electric circuit 65 while the loading rack charge pump 85 is not in operation, since the loading rack may not be in operation for extended periods. Oxygen analyzers 60 and 77 and their ancilliaries, on the other hand, are normally in constant service because net tankage breathing and working exhalations continuously occur.

All electrical and pneumatic instrumentation are designed for fail safe operation wherein the failure of the power or the air supply places instruments in a position such that vapor or blanketing gas is contained in a manner that prevents any admixture of air into it.

While the particular vapor recovery system for loading racks and storage tanks herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention and that no limitations are intended to the detail of construction or design herein shown other than as defined in the appended claims.