US2986010A

US2986010A - Purge means for storage tank

Info

Publication number: US2986010A
Application number: US747015A
Authority: US
Inventors: Beckwith Sterling
Original assignee: Conch International Methane Ltd
Current assignee: Conch International Methane Ltd
Priority date: 1958-07-07
Filing date: 1958-07-07
Publication date: 1961-05-30
Anticipated expiration: 1978-05-30

Description

May 30, 1961 s. BEcKwlTH 2,986,010

PURGE MEANS FOR STORAGE TANK Filed July 7, 195s l I l l l l l United States Patent() PURGE MEANS FOR STORAGE TANK Sterling Beckwith, Lake Forest, Ill., assignor, by mesne assignments, to Couch International Methane Limited, Nassau, Bahamas, a corporation of the Bahamas Filed July 7, 195s, ser. No. u747,015

Claims. (ci. 624s) This invention relates to a tank construction for use in the storage of liqueied natural gas and the like hydrocarbons and it relates more particularly to a storage facility of the type described which operates automatically to purge the container of volatile material when the container is emptied of the liquid.

Referring to the storage of liquetied natural gas or methane in large containers at about atmospheric temperature, the vapor continuously given off by the liquid upon absorption of heat from the atmosphere or otherwise represents a gas capable of combustible mixture with air or oxygen. Ordinarily, when liquefied gas is present in the container a slight positive pressure will be maintained in the container to prevent the inliltration of air to form a combustible or explosive mixture. When, however, the liquefied gaseous material is substantially completely removed from the container, as by evaporation or by liquid removal, the means for maintaining positive pressure within the tank will no longer be available so that the tank will begin to breathe air into and out of its interior. Under such circumstances, a mixture of air and gaseous vapors can be formed which Will be capable of combustion.

It is advisable to avoid the build-up of such conditions as an incidence to the storage and transportation of liquetied natural gas or other low boiling liqueiied combustible gases and it is an object of this invention to provide means within the storage tank which is always eective to prevent the development of such undesirable conditions.

It is an object of this invention to purge the tank of combustible gaseous vapors when the tank is emptied or to effect dilution of the vapors and the admixture of an inert gas to form an atmosphere which is incapable of combustion.

A further object of the invention is to introduce an inert gas into the storage space when the tank is emptied of liquid to displace the vapors from the container or form a mixture therewith which is incapable of combustion and it is a related object to bring such conditions into operation automatically in response to the removal or loss of liquid cargo from the tanks.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustraa tion, but not of limitation, an embodiment of the invention is shown in the accompanying drawing in which- Fig. 1 is a sectional elevational plan view of the interior of a liquid storage tank embodying the features of this invention, and

Fig. 2 is an enlarged elevational view partially in section of the purge means within the tank.

The invention can best be described with reference to the storage of large volumes of liqueed natural gas 10 housed within a container 12 arranged Within another container 14 having a spaced relationship therebetween which is packed or otherwise provided with insulation 16. The inner tank 12 is provided with an inlet 18 for lling the tank with the liquid and an outlet 20 having a tube 22 extending to the bottom of the tank for the discharge of liquid. It is also provided with one or more vapor outlets 24 in communication with the interior at the top of the tank for the release of vapors that are formed. It will be understood that the storage tank may be constructed without the inner shell 12 when the insulation layer is capable of blocking penetration of the liquid to the outer wall 14 of the structure.

The liquefied natural gas 10 has a boiling point at about the critical temperature for methane (-258 F. at atmospheric pressure). It will be understood that the concepts of this invention which will hereinafter be described have application also to use with other cold boiling liquefied gases and particularly to such liquids formedv of combustible gases or gases which are capable of forming dangerous or undesirable mixtures with air or oxygen.

Such liquids vaporize upon the absorption of heat from the ambient atmosphere and the vapors normally till the available space within the tank above the liquid level. It is undesirable to have such vapors collect in the tank when the pressure conditions existing within the tank are able to fall to atmospheric pressure or below whereby air can be drawn into the tank for admixture with the vapors. When, as is preferred, the tanks are maintained at slight positive pressure when lilled with the liquid, such undesirable conditions will be capable of development only where for one reason or another the liquid is either removed or allowed to escape from the tank.

Thus it is desirable to be able to purge the tank of such vapors when the tank is emptied or else to dilute the vapor concentration with an inert gas in an amount which will avoid the development of undesirable mixtures with air. It is desirable also to effect such removal or dilution automatically in response to the development of conditions which would otherwise lead to an inflow of air or oxygen. n

In accordance with the practice of this invention, the tank is purged of the hydrocarbon or other vapors to eiect removal or dilution by the introduction of an inert gas, represented by carbon dioxide, which is released only in response to the increase in temperature in the tank when the tank becomes emptied of the colder boiling liquid. The difference in temperature between the temperature of the liquelied gas and the temperature of the vapor after the liquid is removed operates to provide the control for the release of the purging vapors automatically in response to the development of the required conditions.

For this purpose, carbon dioxide 30 in solid form is housed Within a sealed container 32 xed, as by strips 34, to the bottom 36 of the liquid containing tank 12.` Carbon dioxide is present in the container 32 in an yamount to purge the tank of vapors or otherwise cause dilution to the extent that a safe and inert mixturel is formed when released from the housing 32 into the interior of the 46 of the tank. The outwardly extending portion 48 in communication with the opening and the downpipeisVV provided with a removable cover plate 50 and with a pressure gauge 52 connected by line 54 to an opening in the cover 50 and tted with a gate valve 56. The cover and connections are also provided with sealing members and gaskets for maintaining a desired sealing relationship f throughout the length of the downpipe in communication with the container. The dimension of the cover 50 and L Patented May 30, 1961 the downpipe 40 is such as to permit small pieces of solid carbon dioxide (Dry Ice) to be fed into the downpipe for charging the container 32. A straight line connection is desired to enable use of an elongate ram to effect the desired displacement of the solid carbon dioxide through the feed pipe.

The container is also provided with one or more outlets 60 each of which is fitted with a pressure relief valve 62 adapted to remain closed until the pressure within the container 32 reaches a set maximum, such as 10 p.s.i. The pressure responsive valves 62 have outlets communicating with tubings 64 to distribute the carbon dioxide gas issuing from the container 32 into the interior of the tank 12. In the illustrated modification, two such outlets and pressure relief valves are shown with the tnbings 64 extending about 90 degrees apart from the cover.

In operation, solid carbon dioxide in particulate form is fed into the housing through the inlet 44 at the top and then the cover plate 50 is secured in place to seal the container. The liquefied natural gas in the tank will be at a temperature considerably below the solidification temperature for the carbon dioxide, such as at a temperature of about 240 to -258 F. for liquefied natural gas at atmospheric pressure, or preferably slightly above. Thus the carbon dioxide will be retained within the container in solid form and any carbon dioxide vapors generated will be resolidified as long as cold liquid is in contact with the container to maintain the interior at a temperature below the freezing point of carbon dioxide (-78 F.). However, when insufficient liquid remains in the tank to maintain the desired cold temperature, the temperature in the tank will slowly rise until it reaches a temperature above the freezing point temperature for the solid carbon dioxide. Under such circumstances, the solid carbon dioxide will vaporize and a pressure will soon build up within the container to a level sufficient to open the pressure relief valve 62. Carbon dioxide gas will be released into the vapor filled tanks through the outlet pipes 64 to displace the vapors from the tanks through the relief opening 24 at the top. Carbon dioxide vapors being heavier than the hydrocarbon vapors will tend to stratefy at a level below the hydrocarbon vapors whereby the vapors vented from the tank will be formed principally of the natural gas although some carbon dioxide will be mixed in because of turbulence and diffusion. Nevertheless, the major proportion of the natural gas vapors will thus be displaced from the tank by the released carbon dioxide to render the atmosphere within the tank inert.

When the tank is to be refilled with the liquefied natural gas, the tank can be purged to remove the carbon dioxide but purging is not necessary since an inert atmosphere is desirable. When filled with the liquefied natural gas, or the like liquefied gas, the cold will stop further vaporization of the carbon dioxide and, in fact, it will cause solidification of carbon dioxide vapors remaining within the container whereby the pressure in the container will drop rapidly to close the pressure relief valves 62 communicating the interior of the container with the tank. Thus the tank can be filled with liquefied gas and solid carbon dioxide can be introduced into the container in an amount to make up the solids lost by vaporization.

The pressure gauge 52 can be used to indicate the pressure conditions existing within the container :for purposes of control or for purposes of giving some indication as to conditions existing.

It will be apparent from the foregoing that I have provided a simple but effective means for automatically controlling the atmospheric conditions existing within an enclosed space to effect removal of vapors under predetermined conditions. The principal operation is quite simple and effective in that it relies upon the use of an inerting material in the form of a liquid or solid which is capable of remaining in the liquefied or solid from at the temperatures of the cargo liquid stored in the tank but which is converted to a gaseous state when the tank rises to a temperature above the vaporization temperature of the inerting material at the pressure for which the valve is set. Thus, the inerting material remains for an indefinite period of time within the container and is available at all times to inert the tank when, for one reason or another, the temperature rises within the tank, usually upon the removal of the liquid cargo.

These conditions can be met in the use of liquefied nitrogen as the inerting liquid when the liquid cargo in the tank is maintained at a temperature below the boiling point temperature for nitrogen at the pressure conditions existing. Similarly, other liquefied gases can be used as the liquid cargo in combination with solid carbon dioxide and the like inerting or purging material as long as the described conditions are met.

It will be understood that changes may be made in the details of construction, materials and operation without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. In the storage of a cold liquefied gas, an insulated tank having an inlet and an outlet for filling and discharging the liquefied gas and a vapor outlet for bleeding vapors from the tank, a compartment of small dimension in the bottom of the tank and sealed from the tank, pressure responsive means commimicating the interior of the compartment with the interior of the tank, and means for introducing a material into the compartment having a critical gasification temperature above the temperature of the liquefied gas in the tank whereby the material is maintained in a condensed state within the compartment when liquefied gas is in the tank and whereby it is converted to a gaseous state when the tank rises in temperature upon removal of the liquefied gas to activate the pressure responsive means enabling the escape of the released gases into the tank to purge the tank of vapors therein.

2. In the storage of a cold liquefied gas, an insulated tank, means for filling and discharging liquid from the tank and means in communication with the top of the tank for venting vapors from the tank, a housing of smaller dimension in the bottom of the tank and sealed from the interior thereof, means responsive to pressure within the housing for communicating the interior of the housing with the interior of the tank, and means for introducing a material into the housing having a critical gasification temperature above the temperature of the liquid whereby the material is maintained within the housing in a stabilized state when liquid is in the tank and is converted to a gas when the tank rises in temperature upon the elimination of liquid therein to increase the pressure within the housing whereby the formed gas is introduced into the tank for purging vapors therefrom.

3. A storage tank as claimed in claim 2 in which the means for introducing the material into the sealed housing comprises an outlet of substantial dimension in the housing and a tubular connection between the outlet and the exterior of the tank and means for closing the tubular member for effecting a sealing relationship upon the introduction of material.

4. A storage tank as claimed in claim 2 in which the housing is formed with a plurality of openings and in which pressure responsive valves communicate the open ings with the interior of the tank, said valves being normally closed and which open in response to predetermined maximum pressure to communicate the interior of the housing with the tank.

5. A storage tank as claimed in claim 2 in which the liquefied gas comprises liquefied natural gas and in which the material introduced into the sealed housing is formed of an inert gas whereby the release of the gas from the housing into the tank operates to inert the tank.

6. A storage tank as claimed in claim 2 in which the liquid is a liquefied natural gas having a temperature below about 240 F. and in which the materialA introduced into the housing comprises solid carbon dioxide.

7. In the storage of a cold liquid in an insulated tank of large capacity having an inlet and outlet for lling and discharging the liquefied gas from the tank and a relief opening for the passage of vapors from the tank, providing a housing in the bottom portion of the tank which is sealed from the tank but which has a pressure responsive means communicating the interior of the housing with the interior of the tank, and introducing a material into the sealed housing having a critical gasification temperature at a temperature above the liquefied gas in the tank whereby the pressure within the housing remains below the predetermined pressure for opening the valve when liquefied gas is present in the tank but wherein the pressure within the housing rises to the predetermined pressure for opening the valve when liquefied gas is no longer in the tank and the housing heats up to the critical temperature for the material whereby the latter is converted to a gaseous state and introduced into the tank for purging vapors therefrom.

8. The method as claimed in claim 7 in which the cold liquid comprises a liquefied natural gas.

9. 'Ihe method as claimed in claim 7 in which the material introduced into the housing for gasification comprises a material formed of an inert gas.

10. The method as claimed in claim 9 in which the material introduced into the sealed housing comprises solid carbon dioxide.

References Cited in the file of this patent UNITED STATES PATENTS 662,217 Brady Nov. 20, 1900 1,858,610 Banning May 17, 1932 1,885,769 Schlumbohm Nov. 1, 1932 2,501,611 Nicholson Mar. 21, 1950 2,650,478 Brown Sept. 1, 1953 2,668,419 Mapas Feb. 9, 1954