US2917201A - Reduction of explosive hazard in nonfloating roof type storage tanks in which combustible liquids are stored - Google Patents

Description

Dec. 15, 1959 2,917,201

H. L.- DRAPER REDUCTION OF EXPLOSIVE HAZARD IN NONFLOATING ROOF TYPE STORAGE TANKS IN WHICH COMBUSTIBLE LIQUIDS ARE STORED 2 Sheets-Sheet 1 Filed June 24. 1957 AIR INVENTOR. H.L. DRAPER FIG. BY: Z

A T TORNEVS 2 Sheets-Sheet 2 /IOI / )4 REuEF VALVE SETTING VACUUM BREAKER SETTING H. L. DRAPER REDUCTION OF EXPLOSIVE HAZARD IN NONFLOATING ROOF TYPE STORAGE TANKS IN WHICH COMBUSTIBLE LIQUIDS ARE STORED so so TANK TEMPERATURE, F

EXPLOSIVE DANGER ZONE Dec. 15,

Filed June 24. 1957 United States Patent Ofice 2,917,201 Patented Dec. 15, 1959 REDUCTION OF EXPLOSIVE HAZARD IN NON- FLOATING ROOF TYPE STORAGE TANKS IN WHICH COMBUSTIBLE LIQUIDS ARE STORED Homer L. Draper, Bartlesville, kla., assignor to Phillips Petroleum Company, a corporation of Delaware Application June 24, 1957, Serial No. 667,605

7 Claims. (01. zzo-ss -Th-is invention relates to a method and apparatus for reducing. the explosive hazard in nonfioating roof tanks in which combustible liquids, the vapors of which form explosive mixtures with air, are stored. In one aspect itrelates to a method and apparatus for reducing the explosive hazard 'in a nonfloating roof type storage tank in which petroleum or petroleum products are stored.

This explosive hazard problem is solved, in many instances, by installation of floating roof type tanks. Such tanks are common and further description thereof need not be made. However, many tanks, particularly older tanks, were constructed with nonfloating roofs. When a petroleum material is withdrawn from the tank and displacedwith air, an explosive mixture may remain in the tank. Such tanks ordinarily have sample hatches and breather vents and it is through these openings that air enters the tanks upon removal of the liquid contents. Furthermore, breathing of tanks admits air into the vapor space. Regardless of how the air enters the tanks, whether it is by breathing or by removal of the liquid contents, a hazardous condition may exist.

The object of my invention is to provide apparatus and a method to eliminate explosive hazard conditions in nonfloating roof type storage tanks used for the storage of liquids whose vapors form explosive mixtures with air.

In tanks which are provided with sample hatches and breather tubes, at certain amount of liquid contents of the tank is lost by vaporization. My method and apparatus is applicable to tanks which are not provided with conventional sample hatches and/ or breather tubes. In other words, my invention is applicable to vaportight tanks." The use of vapor-tight tanks has an added advantage that vaporization of product with its subsequentloss is largely eliminated. The explosive conditions of the vapor contents of the tank, according to my invenform, and partly in section, an arrangement of apparatusparts suitable for use in practicing my invention.

Figure 2 is aplot of tank pressure versus tank temperature illustrating the principle upon which my invention is based.

Figure? illustrates, schematically, one form of a portion of the apparatus of Figure 1.

. Myinvention is directed to an apparatus for eliminating the explosive hazard sometimes present in the vaporcontaining space of a nonfioating roof type tank used for the storage of liquids, the vapors of which form explosive mixtures with air, comprising, in combination, a fluid-tight. tank, means for inlet and outlet of liquid from said tank, a first conduit communicating with said storage tank for venting vapors, a vacuum relief valve communicating with said tank, a motor oper- I troller 14.

ated pop valve assembly in said first conduit for regulation of vapor venting, atemperature responsive device in the normally vapor containing space. of said tank, a temperature responsive controller in operative communication with said temperature responsive device, a reset pressure controller in operative communication with the motor of said motor operated pop valve assembly and with said temperature responsive controller whereby said reset pressure controller regulates fluid pressure to said motor in response to temperature indicated by said temperature responsive device. V

The method of my'invention involves a method for maintaining vaporous contents of a non-floating roof fluid-tight zone free from explosive hazard, a combustible liquid, the vapors of which form explosive mixtures with air, being disposed in said zone, comprising maintaining a mixture of said vapors and air in said zone at a volume ratio of air to vapors below the ratio of air to vapors which form an explosive mixture byregulating the pressure of the mixture of said vapors and air in said zone in response to the temperature of the mixture of vapors and air in said zone.-

My invention is applicable to the storage of liquids having Reid vapor pressures at F. less than atmospheric pressure such that upon withdrawing the liquid from the storage vessel a vacuum valve operates admitting air thereby setting up possible explosive conditions.

The term Reid vapor pressure, or RVP, as used throughout this specification and claims, is a measure of the volatility of a nonviscous petroleum product in terms of pounds per square inch at 100? F. as determined by A.S.T.M. method designated as D323-37T.

Referring now to the drawing and specifically to Figure 1, reference numeral 10' identifies a nonfloating.

A pipe 19. is provided for venting vapors from the tank in response to operation of the control apparatus of my.

invention. A thermocouple 12 is illustrated as extending through the roof of tank 10 into the space normally occupied by vapor. The thermocouple can, however, be inserted through the side wall of the tank provided it extends into. the space normally occupied by vapor.

Thermocouple 12 is connected operatively with a tem perature responsive controller 13. A reset flow controller =14 is connected operatively with the temperature responsive controller 13 and with a normally closed motor operated relief valve 15. A pipe 17 conducts air under pressure, from a source not shown, to the reset. pressure controller while a pipe 18 conducts air of regu lated pressure from the reset pressure controller 14 to the underside of a diaphragm 24 of the motor of the valve 15. As mentioned, this valve is a normally closed valve,

the valve being held in a normally closed position by acompression spring 22 arranged as illustrated. A vent.

tube 20 is provided for venting of air in case of leakage through diaphragm 24 and to maintain atmospheric pressure above the diaphragm. An air bleed 21 is provided forbleeding of a small flow of air from the underside of the diaphragm. Reference numeral 23 identifies a re set pointer in conjunction with the reset pressure con- In many operations such reset pointers as pointer 23 are set manually for maintenance of constant outlet pressure in tubes corresponding to tubes or pipes 18; but, according to my invention, the reset pointer is continuously reset by operation of the temperature responsive controller 13. Temperature controller .13, as n 3 V illustrated in Figure 3, includes a coil in which is disposed a movable rod 31 pivoted to a baffle 34 and biased downward by tension spring 32. Amplifier 39 amplifies the very small signal set up by thermocouple 12, the amplified signal moving rod 31 upward in proportion to a temperature increase sensed by the thermocouple. As the rod 31 moves upward, the arrowed end of baffle 34 moves downward on resistor 33 to decrease to a coil in the reset controller 14. Rod 35 in the reset controller coil then moves downward under the influence of decreased in the reset controller coil and tension spring 38 to bias valve 37 toward a closed position. When valve 37 moves toward a closed position, less pressure air from line 17 enters the reset controller 14 and thence pressure via line 18 to the under side of diaphragm 24 is reduced. Upon sensing a decrease in temperature in tank 10 by thermocouple these several elements operate in a manner just the reverse to that just described to open valve 37 and increase the air pressure on the under side of diaphragm 24. Accordingly, the temperature responsive controller 13 operates in response to the temperature as indicated by thermocouple 12 and this controller then continuously resets pointer 23 of the reset pressure controller and this continuous resetting continuously regulates How of air under pressure through pipe 18 to regulate the air pressure on the underside of diaphragm 24.

A valve head 25 is illustrated in Figure 1 as being provided with a seat 26 in such a manner that as gas pressure is exerted in pipe 19, this pressure is also exerted on the underside of valve head 25 to assist in opening the valve against the compression of compression spring 22. The opening of this valve is occasioned by the sum of two forces, one force acting upward on the underside of valve head 25 and the other force is the air under pressure from pipe 18 acting on the underside of diaphragm 24. In the particular example to be described hereinbelow, the compression spring is so adjusted that-a total pressure of 20 pounds per square inch in tank 10 acting on the underside of valve head 25 in the absence of air pressure below the diaphragm opens the valve against the compression spring. Accordingly, when pressure is exerted upward and against the underside of diaphragm 24, less pressure than 20 pounds per square inch is required on the underside of valve head 25 to raise the valve against the compression of spring 22. I

Referring now to Figure 2, which illustrates the principle upon which my invention operates, line 100 is intended to be a plot of the values of pressure versus temperature under which conditions a mixture of vapors of a five-pound Reid vapor pressure gasoline and air will not explode because the mixture is too lean in hydrocarbon vapor or too rich in air under the pressures and temperatures illustrated by line 100. In other words, line 100 represents the lower or lean explosive limit with air of a five-pound Reid vapor pressure gasoline. To the left of this line for this particular product the ratio of air to vapor is too great to be explosive.

Line 101 represents the upper or rich explosive limit and to the right-hand side of this line the ratio of air to vapor is too low to be explosive, or, in other words, the mixture is too rich in hydrocarbon vapors to be explosive. Line 101 represents the rich or upper explosive limit. The 20 p.s.i.g. (pressure per "square inch gauge) line 102 represents the maximum relief valve setting. The p.s.i.g. line 103, represents atmospheric pressure, that is, it is the pressure at which the vacuum breaker will operate to admit atmospheric air into the tank when the pressure in the tank is below that of the atmosphere. The tank pressure is intended not to be below this value because of the operation of the vacuum breaker. The shaded area, bounded by lines 103, 101, 102 and 100 (if this latter line were extended to pressures below about 27 p.s.i.g. and temperatures below 0 F.), represents the temperature-pressure region for the 5.0 Reid vapor pressure gasoline of the specific example in which explosive mixtures exist.

According to Figure 2, with the motor relief valve of Figure 1 being set at 20 pounds pressure, such a quantity of a five-pound Reid vapor pressure gasoline is contained in tank 10 that the total fluid pressure in the tank is approximately five pounds gauge pressure. If the tank and its contents are at 40 F., this pressure is indicated on the 40 F. temperature line by point D. Upon pumping an additional quantity of the same gasoline at 40 F. into the tank, the pressure in the tank rises and will follow the 40 F. temperature line upward until such a time as point C is reached. This point C is a point on a dotted line 104 which, as indicated in Figure 2, is positioned on the righthand side of line 101, that is, it is positioned in the area which is too rich in hydrocarbon vapors to be explosive. The temperature responsive controller 13 of Figure 1 still operates when point C is reached, having at point D reset controller 23of the reset pressure controller 14 to admit sufiicient air from pipe 17 through pipe 18 to the underside of diaphragm 24 to open the valve 25 and release pressure from the tank when the 9 /2 pounds pressure in the tank is reached at point C. In other words, if valve 25 did not open at this time, upon further pumping of liquid gasoline into the tank the pressure would follow the 40 F. line upward and ultimately reach a value of 20 pounds at which pressure spring 22 will allow the valve to open. However, from approximately a 9 /2 pounds pressure in the tank up to 20 pounds, the mixture of gasoline vapors and air is within the explosive range (shaded area) and obviously an explosive hazard exists. But, by operation of the temperature responsive controller to reset the reset controller, vapor is vented from the tank when a pressure of approximately 9 /2 pounds per square inch is reached in the tank so that vapor is vented from the tank to maintain the pressure in the tank at a maximum of about 9 /2 p.s.i.g. as long as the temperature remains at 40 F.,- and this 9 /2 p.s.i.g. is outside the explosive range.

Again, if the tank contains such a volume of the abovementioned gasoline at 40 F. that the pressure in the tank is approximately 5 p.s.i.g., as indicated by point D in Figure 2, this point lies in the explosion-free area to the right-hand side of line 101. Upon increase of temperature within the vapor space of the tank from 40 F. to about 70 F., pressure increases, and a plot of temperature versus pressure follows line 107. In this case an explosion-free mixture of vapor still exists in the tank because line 107 did not enter the hatched danger zone area of the diagram.

However, if, for example, tank 10 contains such an amount of the above-mentioned gasoline that pressure in the tank is about 5 p.s.i.g., which condition is identified by point D in Figure 2, and upon pumping in of a quantity of this same gasoline at a temperature above 40 F., the temperature and pressure of the vaporous contents of the tank may be considered to follow line 110 if the tank were not equipped with my safety vent device. According to my invention, when the vapor contents of the tank as regards temperature and pressure reach point G, the temperature responsive controller 13 operates to reset pointer 23 of the reset controller which, in turn, operates to admit only sufficient air through pipe 18 to the underside of diaphragm 24 to operate in conjunction with the pressure acting on the underside of valve head 25 to open this'valve and vent vaporous contents of the tank. In this manner, with valve head 25 opened at point G, pressure in the tank does not rise above approximately 11 p.s.ig. as long as temperature remains at about 45 F.; and, accordingly, the vapor contents of the tank are not within explosive limits. If applicants invention were not used in this latter case and warm gasoline were continued to be pumped into the tank, the temperature and pressure conditions would continue to follow line 110 until point F were reached under which condition valve 25 would not open until 20 p.s.i.g. (point F) were reached in the tank, and point F represents an explosive mixture. In thetwo cases explained hereinabove in which gasoline was pumped into the tank, if the temperature in the tank does not deviate from 40 F., the pop valve stays open and vents vapors at about 9 /2 p.s.i.g. continuously as long as liquid is pumped into the tank. If warm gasoline is pumped 'into the tank, as mentioned above, the vaporous contents of the tank reach point G and the valve then opens to vent vapors from the tank; but, upon continued'pumping of warm gasoline into the tank, temperatureof the vapors increasesand since the valve is open, the vapor contents of the tank cease to follow line 110 into'the explosive. danger zone and follow line 104 upward andto the right outside of the explosive danger zone until such time as liquid is no longer pumped into the tank. Thus,jin this latter case, the more warm gasoline pumped into the tank the better the reset controller apparatus of my invention operates to maintain a higher pressure in the tank corresponding to the temperature of the vapor in the tank.

The apparatus of my invention operates equally as well upon reduction of the temperature in the tank and upon withdrawal of liquid therefrom. For example, if the tank were filled withthe above-mentioned five-pound Reid vapor pressure gasoline at 70 R, and since the pressure relief valve is set to open at 20 p.s.i.g., point A illustrates this set of conditions. If, for example, a cool thundershower drops water upon the top of the tank, it is obvious that the temperature therein will be lowered and line106 is intended to illustrate the temperature of the vapor. containing portion of the tank as occasioned by cooling and partial condensing of the vapor without removal of any liquid from the tank. Under these condi tions, if cooling continued to 40 F., pressure would also decrease and finally point B would be reached. Point B lies within the explosivearea. According to my invention, upon cooling of the vaporcontents of the tank, the temperaturedecreas'es with a simultaneous decrease of pressure following line 106 until such time as line 106 intersects line 104 at'point B. At this point the tem perature responsive controller-reset pressure controller of my invention operates to vent vapors from the tank so that further cooling will cause a pressure reduction and line 104 will befollowed outside the explosive area until such time as temperature ceases to decrease. If the relief valve 15 did not open upon further cooling of the vapors in the tank, point B would be reached when the temperature of the vapors reached 40 F. instead-of pointC. J

. Beginning at point A, i.e., with the tank containing 7.10 F. gasoline of five pounds RVP at 20 p.s.i.g., if liquid'jcontents .are'removed from the tankalong with simultaneo'us cooling of th'e'vapor contents of the tank, the pressure-temperature conditions of the vapor in the tank may follow line 108. Upon continued following of line 108 it is noted that this dotted line does not enter the explosive danger zone until pressure is reduced to a value below about 11 /2 p.s.i.g. at about 45 F. Thus the pop valve need not relieve pressure of the tank unless temperature is reduced to a value below about 45 F. A reduction of pressure only keep the condition on the rich side of line 101. i

Still another condition is that if liquid at 70 F. were withdrawn from the tank without a simultaneous decrease of the temperature of the vapors in the tank, and beginning at point A pressure in the tank falls and follows line 109 downward. Line 109, as will be seen from Figure 2, is also outside the explosive range. Thus the tank may be entirely emptied of liquid at 70 F. without ever entering the explosive region provided the temperature of the remaining vapors remains at 70 F.

The above example involving a five-pound Reid vapor pressure gasoline was given merely as an example of the operation of the apparatus and method of my invention. The diagram of Figure 2 isfa diagrammatic representation of the pressure and temperature conditions relative to explosive mixtures with air of a five-pound Reid vapor pressure gasoline. Other gasolines, oils, liquid hydrocarbons such as pentane, or hexane or other liquids such as alcohols, acetone, or the like, whose vapors form explosive mixtures with air, can be stored inan explosionfree condition according to my invention. However, with each of these other liquids to be stored a separate and individual explosive danger zone diagram will need to be provided because explosive limits of other combustible liquids are different.

While I have described my invention as employing a relief valve 15 set at 20 p.s.i.g., it is realized by those skilled in such art that the valve can be set to open at pressures above or below 20 p.s.i.g. as dictated by local conditions. If the safety apparatus is applied me high pressure tank, obviously a higher pressure can be maintained. If a tank is used which will safetly maintain a pressure of only 10 p.s.i.g., then the relief valve should 1. An apparatus for eliminating the explosive hazard sometimes present in the vapor containing space .of afluid-tight tank used for the storage 'of a liquid, the vapors of which form explosive mixtures with air, comprising, in combination, a fluid-tight tank, means for inlet. and outlet'of liquid from said tank, a conduit commun'ieating with said tank for venting vapors, a vacuum relief valve communicating with said tank, a pressure re lief motor valve in said conduit for regulation of venting of vapors from said tank, the motor of this latter valve being a fluid pressure operative motor, means biasing said pressure relief valve normally closed, a temperature s'ens' ing device in the normally vapor containing space of said tank, a temperature responsive controller in operative communication with said temperature sensing device, a reset pressure controller in operativecommunication with said motor and with'said temperature responsive controller, said reset pressure controller being adapted to reset fluid pressure to said motor in response to an increase in temperature in said tank as sensed ,by said sensing device whereby said motor and the pressure 'of said vapor in said tank cooperate to open said pressure relief valve.

against the means biasing said valve closed.

2. The apparatus'of claim 1 wherein'said temperature sensing device is a thermocouple.

3. A method for maintaining the vaporous contents of a fluid-tight zone for storage of liquid free from explosive hazard, a combustible liquid, the vapors of which form explosive mixtures with air, being disposed in said zone, and the vapor pressure of the liquid in said "zone being less than atmospheric pressure at storage temperature, comprising maintaining a mixture of said vapors and air in said zone at a volume ratio of air to vapors too rich to form an explosive mixture by reducing the pressure of the mixture of said vapors and air in said zone by venting vapors and air therefrom in response to a combination of the pressure of the mixture of said vapors and air in said zone and of a valve actuating air pressure, the latter air pressure being inversely proportional to the temperature of the mixture of vapors and air in said zone.

4. A method for eliminating the explosive hazard sometimes present in the vapor containing space in a fluidtight storage zone in which is disposed a combustible liquid, the vapors of said liquid forming explosive mixtures with air, and the vapor pressure of the liquid stored a 7 being less than atomspheric pressure at storage tempera ture, comprising introducing said liquid into said fluidti'ght storage zone, at an ambient temperature, continuing introduction of said liquid into said zone until fluid pressure in said zone reaches a pressure at which a mixture of vapors of said liquid and air forms an explosive mixture at the temperature of vapor in said zone, main taining the mixture of said vapors and air in said zone at a volume ratio of air to vapors below that ratio of air to vapors which forms an explosive mixture at said temperature by venting air and vapors from said zone until the pressure therein is below the pressure at which the air and vapors remaining have said volume ratio, said venting being carried out in response to a combined pressure of said vapors and air in said zone and an extraneous air pressure, said extraneous air pressure being inversely proportional to the temperature of the mixture of vapors and air in said zone.

5. An apparatus for eliminating the explosive hazard sometimes present in the vapor containing space of a fluidtight tank used for the storage of a liquid, the vapors of which form explosive mixtures with air, comprising, in combination, a fluid-tight tank, means for inlet and outlet of liquid from said tank, a first conduit communicating with said storage tank for venting vapors, a vacuum relief valve communicating with said tank, a spring loaded pressure relief valve in said first conduit for regulation of flow of vapors from said tank, said spring biasing said relief valve closed, said relief valve having a valve head and a seat for said valve head, said valve head being so disposed with respect to said seat that fluid pres-- sure from said tank tends to open said valve from said seat, means biasing said valve head closed against said seat, a motor operatively connected with said valve head for opening said valve head from said seat, said motor being adapted to open said valve from said seat against said bias with the assistance of fluid pressure from said tank acting on said valve head, a temperature responsive device in the normally vapor containing space of said tank, a temperature responsive controller in operative communication with said temperature responsive device, a reset pressure controller in operative communication with said motor and with said temperature responsive controller whereby said reset pressure controller resets fluid pressure to said motor in response to temperature indicated by said temperature responsive device and said pressure relief valve being adapted to open against the bias of said spring under the combined influence of tank pressure and the reset fluid pressure to said motor.

6. An apparatus for eliminating the explosive hazard sometimes present in the vapor containing space of a fluid-tight tank used for the storage of a liquid, the vapors of which form explosive mixtures With air, comprising, in combination, a fluid-tight tank, means for inlet and outlet of liquid from said tank, a conduit communi- 7 a 8 eating with the storage tank for venting vapors, a pressure relief valve in said conduit for regulation of flow of vapors from said tank, said relief valve having a valve head and a seat for said valve head, said valve head being so disposed with respect to said seat that fluid pressure from said tank tends to open said valve from said seat, means biasing said valve head closed against said seat, a motor operativelyconnected with said valve head for opening said valve head from said seat, said motor being adapted to open said valve from said seat against said bias with the assistance of fluid pressure from said tank acting on said valve head, a reset pressure controller in operative communication with said motor, a temperature responsive controller in operative conimunication with said reset pressure controller and means responsive to the temperature in the vapor space of said tank to actuate said temperature responsive controller whereby said temperature responsive controller resets said reset pressure controller to regulate fluid pressue to said motor to open said valve.

7. A method for maintaining the vaporous contents of a fluid-tight storage zone free from explosive hazard, a combustible liquid, the vapors of which form explosive mixtures with air, being disposed in said zone, comprising providing a pressure relief valve in operative communication with the vapor space of said zone, said relief valve being set to relieve vapor from within said zone at a safe fluid-tight zone working pressure higher than the vapor pressure of said combustible liquid at storage temperature, a mixture of said vapors and air also being present in said zone, maintaining the pressure of said mixture in said zone at a value below that which is within the explosive pressure range at ambient zone temperature by venting vapors and air through said relief valve, said relief valve being actuated for said venting by impressing fluid pressure upon said valve in excess of said set pressure, the fluid pressure impressed on said valve being a sum of the pressure of said vapors and air of said zone and a valve actuating air pressure, said valve actuating air pressure being impressed on said valve in response to temperature of fluid in said storage zone, said actuating air pressure being decreased as temperature of fluid in said storage zone increases, and said actuating air pressure being increased as temperature of fluid in said storage zone decreases.

References Cited in the file of this patent UNITED STATES PATENTS Colvin, Jr. et al May 22, 1956 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,917, 2Q1 December 15, 1959 Homer L.'Draper It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7, lines 3 and 4, after "temperature strike out "continuing introduction of said liquid into said zone".

Signed and sealed this 9th day of August 1960.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Ofiicer Commissioner of Patents

Claims (1)

1. 7. A METHOD FOR MAINTAINING THE VAPOROUS CONTENTS OF A FLUID-TIGHT STORAGE ZONE FREE EXPLOSIVE HAZARD, A COMBUSTIBLE LIQUID, THE VAPORS OF WHICH FORM EXPLOSIVE MIXTURES WITH AIR, BEING DISPOSED IN SAID ZONE, COMPRISING PROVIDING A PRESSURE RELIEF VALVE IN OPERATIVE COMMUNICATION WITH THE VAPOR SPACE OF SAID ZONE, SAID RELIEF VALVE BEING SET TO RELIEVE VAPOR FROM WITHIN SAID ZONE AT A SAFE FLUID-TIGHT ZONE WORKING PRESSUR HIGHER THAN THE VAPOR PRESSURE OF SAID COMBUSTIBLE LIQUID AT STORAGE TEMPERATURE, A MIXTURE OF SAID VAPORS AND AIR ALSO BEING PRESENT IN SAID ZONE, MAINTAINING THE PRESSURE OF SAID MIXTURE IN SAID ZONE AT A VALUE BELOW THAT WHICH IS WITHIN THE EXPLOSIVE PRESSURE RANGE AT AMBIENT ZONE TEMPERATURE BY VENTING VAPORS AND AIR THROUGH SAID RELIEF VALVE, SAID RELIEF VALVE BEING ACTUATED FOR SAID VENTING BY IMPRESSING FLUID PRESSURE UPON SAID VALVE IN EXCESS OF SAID SET PRESSURE, THE FLUID PRESSURE IMPRESSED ON SAID VALVE BEING A SUM OF THE PRESSURE OF SAID VAPORS AND AIR OF SAID ZONE AND A VALVE ACTUATING AIR PRESSURE, SAID VALVE ACTUATING AIR PRESSURE BEING IMPRESSED ON SAID VALVE IN RESPONSE TO TEMPERATURE OF FLUID IN SAID STORAGE ZONE, SAID ACTUATING AIR PRESSURE BEING DECREASED AS TEMPERA-

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