Apparatus for preventing corrosion in storage compartments

May 22, 1956 q. d. colvin, jr„ et al 2,746,684

APPARATUS FOR PREVENTING CORROSION IN STORAGE COMPARTMENTS

Filed May 6, 1948 3 _She_ets__{sheet χ}

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May 22, 1956

O. D. COLVIN, JR., ETAL

2,746,684

APPARATUS FOR PREVENTING CORROSION IN STORAGE COMPARTMENTS

Filed May 6, 1948

Sheets-Sheet 2

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May 22, 1956 o. _D colvin, jr„ etal 2,746,684

APPARATUS FOR PREVENTING CORROSION IN STORAGE COMPARTMENTS

Sheets-Sheet 3

Filed May 6, 1948

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attorneys.

United States Patent Office 2,746,684

Patented May 22, 19S6 the so-called “surface rust” or “blush rust” is also substantially entirely prevented. “Surface rust” as will appear hereinafter can occur in the absence of “sweat.”

The novel method embodying the use of dried air in ⁰ a tank comprises in general ih, sealing the tank to prevent the admission of air, subjecting air from the atmosphere external to the tank to dehumidification, and substituting the dehumidified air for the air previously in the tank, the dehumidification being carried out to a sufficient dell' gree to reduce the aqueous dewpoint of the air directed into the tank to a point less than the temeprature of the contents of the tank and the tank, the dewpoint being reduced.to a value such that within a predetermined i ί change in temperature in and of the tank, the relative humidity of the atmosphere therein will not exceed about 50%.

In another respect the invention deals with the prevention of combustion in cargo compartments of the above 90 ^ch“^acter and in such a way that corrosion difficulties will not be thereby encountered. It has been proposed for example to inhibit combustion by injecting carbon dioxide or similar inert gases, but under the conditions heretofore existing, this has actually increased and accelerated the rusting of the ferrous walls of the compartment. We have also found that if the previously mentioned conditions be maintained in the compartments, combustion inhibiting gases such as flue gas may be utilized without thereby causing corrosion difficulties.

Vessels for transporting and storing certain types of substances, such as petroleum above mentioned, have required extensive repair at frequent intervals or have required a complete renewal of some or all of the storage compartment bulkheads thereof due to extreme corrosion. With particular reference to tankers, which carry °° the so-called “white oils,” such renewal has been necessary in some instances as often as every eight to eleven years. This has been particularly true of the highly volatile “white oils” and also of sour crudes having relatively high sulphur content. Many oils have sulphur or sulphur compounds therein which can, in combination with or in the presence of atmospheric moisture, break down into dilute or weak sulphurous and sulphuric acid which rapidly consumes the bulkheads of the vessel.

4- _ The storage tanks of such vessels, of course, are subject to the above-mentioned corrosive action of certain gases, from and certain substances in the liquids stored therein, and in the case of tanks which are cleaned with salt water, the combined action of such fumes and substances and the salt water can corrode the inner surface of the tank rapidly. Attempts have been made in the past to inhibit such corrosion by coating same with paint or corrosion resistant metal and other materials. However, these coatings have been unsuccessful and are very costly to install and to renew, and in themselves often degrade the cargo. Under some circumstances these coatings when broken often accelerate corrosion as in the case of a zinc coating on steel.

The corrosion of inner surfaces of storage compart60 ^ments may contaminate substances therein, especially liquids, by the products of corrosion and also because of condensation of moisture within the tank. Rust in particular will discolor and degrade an oil cargo such as lubricating oil.. Furthermore, emulsification of a liquid cargo with moisture and/or a colloidal suspension with ⁰⁶ products of corrosion may occur whereby, for example, in the case of oil its lubricity can be spoiled. Such emulsification will degrade such types of cargos as will such colloidal suspension.

One of the objects of the present invention is to re70 duce corrosion of the inner surfaces of storage compartments and communicating pipelines including corrosion due to surface condensate or “sweat,” and also that at2,746,684

APPARATUS FOR PREVENTING CORROSION IN STORAGE COMPARTMENTS

Oliver D. Colvin, Jr., Seattie, Wash., Eugene Norman Fabres, New York, N. Y., and Farris J. Swaney, Essex Fells, N. J., assignors, by mesne assignments, to Oliver D. Colvin, Seattle, Wash.

Application May 6,1948, Serial No. 25,346

Claims. (Cl. 236—44)

This invention in one of its important aspects relates to apparatus and methods for inhibiting corrosion in storage compartments, especially cargo compartments aboard ship where serious corrosion difficulties have long been encountered. To reduce corrosion under the above conditions, it has been long since proposed (see U. S. Patents to Colby et al. No. 2,160,831 and Brinkmann No. 2,379,215) to eliminate so-called “sweat” from the cargo compartment surfaces by circulating through the cargo compartments, air from which the moisture has oeen removed sufficiently to produce a dewpoint lower than the expected minimum temperature of any compartment surface. This method has been employed in actual service to a substantial extent, and while it has been found effective in eliminating the “sweat” and in retarding corrosion to some extent, nevertheless corrosion still persisted sufficiently to remain a serious problem, notwithstanding the elimination of the “sweat.” In so far as we are aware, no methods have heretofore been devised which were capable of eliminating this residual corrosion which persists even after the “sweat” has been eliminated.

We have discovered that if the moisture content of the atmosphere within the compartment be reduced to such a point that at the lowest expected compartment temperature to be reached during service, the relative humidity of the atmosphere therein is not permitted to rise above approximately 50% of saturation, the abovementioned residual corrosion which normally would have occurred even after the “sweat” had been eliminated, is inhibited or prevented so completely as to be no longer a serious problem. All electrolytic action and corrosion is substantially completely prevented under the above conditions even though the cargo be such as petroleum or the derivatives thereof which are particularly prone to cause rapid corrosion.

The present invention embodies, in accordance with one form thereof, means for maintaining a preselected pressure of dried air over such substances as volatile liquids held in storage compartments, whereby the corrosive influence of the agents in the ingredients thereof is substantially eliminated with respect to the compartment surfaces in contact with the dried air. The pressure is preferably slightly in excess of atmospheric pressure but can be less. During and after emptying the compartment, the dried air is also maintained therein at the desired pressure. It is believed that certain of the ingredients of and agents in such liquids as petroleum and many of its derivatives, act as positive catalysts to speed the corrosion of ferrous compartments by reacting with moisture in the air. Also the moisture itself may act as the catalyst with certain agents. Such catalytic action is substantially eliminated by the present invention by reducing the dewpoint of the air, as above mentioned, to a point such that within a predetermined temperature drop of and in the compartment, the relative humidity will be about 50% or below. By so doing not only is the surface condensate known as “sweat” eliminated but also

2,746,684 tributable to the formation of so-called interstitial moisture which is sometimes referred to as “surface rust” or “residual rust” and can occur without the formation of said “sweat.”

Another object is to prevent the contamination of sub- 5 stances stored in storage compartments resulting from condensation of moisture therein, from emulsification of such condensate with such substances, and from a mixing or suspension of the products of corrosion with or in such substances. ¹⁰

A further object is to inhibit the formation of a combustible and/or explosive gaseous mixture in the unoccupied spaces of a storage compartment, and at the same time to prevent corrosion.

A still further object is to inhibit any combustion with- 15 in a storage compartment by means which inhibit corrosion.

Various further and more specific objects, features and advantages of the invention will clearly appear from the detailed description given below taken in connection with the accompanying drawings which form part of this specification and illustrate, by way of example, preferred, arrangements of apparatus for carrying out the invention, the latter not being limited to said drawings. The invention consists in such novel methods and the combinations of features and method steps as may be shown and described in connection with the equipment herein disclosed.

In the drawings:

Fig. 1 is a schematic illustration of a form of the apparatus for carrying out the invention wherein dried air is the gaseous medium directed to a storage compartment;

Fig. 2 is a schematic illustration of another form of the apparatus for carrying out the invention wherein a dried combustion inhibiting gas is directed to a storage compartment; and

Fig. 3 is a graphic representation of the dewpoint of dehumidified flue gas directed to a storage compartment by the apparatus of Fig. 2.

Fig. 4 is a side view of an oil tanker indicating schematically the connections between the dehumidifying apparatus and the cargo tanks;

Fig. 5 is a schematic cross-sectional view through one of the cargo tanks;

Fig. 6 is a sectional view taken on line 6—6 of Fig. 5;

Fig. 7 is an enlarged view of a cargo hatch and related conduits employed in connection with a cargo tank.

The forms of the invention illustrated in the accompanying drawings, by way of example, comprise, in accordance with one embodiment thereof, means for automatically maintaining a slight pressure of dry air over a liquid cargo in a tank while in storage or in transit, when loading, or discharging, and also for maintaining such a pressure when the tank is empty. In accordance with another embodiment of the invention illustrated in the drawings, a combustion inhibiting gas, for example, flue gas, is cleaned and dehumidified to a predetermined degree and automatically maintained at a slight pressure within the storage tank over the liquids stored therein.

The term “white oils” as used herein includes within its purview all oils and liquid chemicals which fail to deposit a corrosion resistant residue upon the compartment surfaces, for example, alcohol, acetone, gasoline, kerosene, and certain petroleum distillates. Certain types of acid free crudes, tarry liquids and other substances deposit such protective residues or coatings, and although they may be of considerable corrosive quality in the presence of moisture, the protective coating prevents moisture from reaching the tank surfaces and thus inhibits corrosion.

Method and apparatus employing dehumidified air in the storage compartment

Referring to Fig. 1, means are provided for automatically maintaining air dehumidified to a predetermined extent under pressure within a storage tank for liquids such as petroleum oils. It is well known that tanks for liquids such as petroleum oils must not be filled completely because of the expansion and contraction of the contents. Thus some empty space must be left above the liquid. Such storage tanks in water borne oil tankers, of course, are subject to wide variations in temperature, for example, as caused by intermittent solar radiation, ²³ changes in cargo temperature, and by passing through atmosphere and waters of differing temperature whereby the gases within the tank are caused to expand and contract, thereby causing the tank “to breathe” provided openings to the atmosphere exist. The unoccupied space ³⁰ within the tank in accordance with this embodiment is filled with dehumidified air which is subject to such expansion and contraction. Means are provided for sealing the tank from the outer atmosphere and for conducting thereto and maintaining such dehumidified air at a pre³³ determined pressure whereby inward “breathing” is prevented thus keeping out an inflow of undesired air which may contain too much moisture. When such pressure has become dissipated and reduced by a preselected amount, automatic means are provided for replenishing ⁴⁰ the dry air to the desired pressure.

The air is treated in such a manner that rusting within the storage tank is substantially entirely prevented. Ferrous metals can rust even though the dewpoint of the contacting gases is below the temperature of the metal and 45 even though at the lowest temperatures encountered, the relative humidity of said contacting gases is within a range of approximately 60% to 100%.

Thus it is not sufficient merely to keep the dewpoint below the temperature of the storage compartment and 50 its contents, but the dewpoint must be kept sufficiently low that when the lowest temperature is encountered in the compartment, the relative humidity of the atmosphere therein will be below about 50%.

Apparatus for carrying out one form of the novel 5 method is constituted (Fig. 1) in general by a dehumidifier 10 through which atmospheric air is drawn by means of a blower 11 driven by a motor 12, there being suitable means as at 13 for determining dewpoint comprising, for example, temperature and relative humidity

We have discovered that corrosion of the inner sur- θθ sensitive means. Of course, any suitable instrument can faces of metal storage tanks for liquids, for example petroleum and certain derivatives thereof, proceeds at an accelerated rate in most cases where the atmosphere within the tank contains moisture above certain amounts be used for this purpose. The dried air is directed to a tank 14 which contains, for example, a liquid of the “white oil” type, such as kerosene or gasoline 15. The tank 14 is, for example, employed in an oil tanker and and also deleterious gases including acid gases. Certain elements or agents contained in so-called “white oils” react with such moisture to produce excessive corrosion of the metal storage compartment surfaces. Some “white oils” may wash off or remove protective surface coatings and expose the surface to attack by corrosive agents. Also certain “white oils” may be of less corrosive quality than other substances but in ships where the tank surfaces move there occurs a washing action on the tank surfaces which leaves them in a condition which is more tolerant of corrosion.

thus is subject to the above-discussed temperature changes, whereby over a relatively short period, without proper control of the dewpoint of the atmosphere within the tank 14, serious corrosion can occur.

Control apparatus, which is responsive to the pressure 70 of the atmosphere within the tank are provided for governing (a) the flow of air to the tank, (6) the blower motor 12, and (c) as will be explained below, a reactivation heat supply for the dehumidifier unit 10. Such pressure-responsive apparatus are illustrated as at 16 and 75 17 and operate in a manner to appear hereinafter, to

2,746,684 maintain a preselected pressure of, for example, approximately two and one-half pounds (above atmospheric) of dried air within the tank.

Atmospheric air is conducted to the dehumidifier unit 10 through a suitable inlet conduit 18. The dehumidifier unit 10 is, for example, of the dual desiccant bed type. In a well known manner, one of the beds is adapted for undergoing an adsorption cycle while the other bed is undergoing reactivation. As shown in Fig. 1, air from the atmosphere is directed to one desiccant bed 19 through a four-way valve 20, the air passing out of the bed through a four-way outlet valve 22, and after-cooler 21, and thence to the blower 11 via a conduit 23. Most of the moisture, of course, is retained in the desiccant bed 19.

While the desiccant bed 19 is undergoing an adsorption cycle, the other bed 24 can be subjected to a reactivation cycle, whereby the moisture previously adsorbed therein is liberated by heat. This is accomplished by directing atmospheric air thereto by means of a fan 25 and a conduit 26 through a reactivation heater 27, from which it passes in a heated condition to the bed 24 via the valve 22. The heated air drives out the moisture which is discharged from the bed 24 through the fourway inlet valve 20 via a suitable discharge conduit schematically indicated at 28.

The four-way valves 20 and 22 are operatively interconnected whereby the shifting thereof will shift the cycles of the desiccant beds in a well known mariner

The after-cooler 21 removes any excessive heat in the air as caused, for example, by the heating up of the desiccant bed due to the heat applied thereto in the reactivation cycle or due to the heating up of the desiccant bed by the heat of adsorption.

In order to prevent the backflow of gases from the tank 14 into the dehumidifier system, a butterfly valve 29 may be employed in a suitable interconnecting conduit, which valve is normally in a closed position under the influence of a spring in an air actuated valve operator 30 which, by means of said spring, will hold the valve in a closed position. Such interconnecting conduit is indicated at Ila, 32 for the blower 11 and the tank 14. The line of division between the two portions Ila and 32 is at the valve 29, the former portion extending from said blower to said valve 29 and the latter portion from the valve to the tank 14. The valve operator 30 is under the influence of a control device 31 (indicated generally at 16) for governing the flow thereto of an actuating medium such as air. Air control unit 31 is in turn under the influence of the fluid pressure within conduit 32. Air under pressure is communicated to air control unit 31 by a conduit 33. When the pressure of the dried air in conduit 11α exceeds, for example, approximately one-half pound above atmospheric pressure, this is communicated to the control unit 31 by means of a conduit 34 and air at a sufficient pressure is admitted to the valve operator 30 to overcome the pressure of the above-mentioned spring and to open the butterfly safety valve 29. When the pressure in said conduit 32 reaches approximately two and one-half pounds above atmospheric, this is communicated to the pressure medium control device 31 by a conduit 35, air pressure to valve operator 30 is cut off, and the butterfly valve 29 is closed. It is to be understood that the invention is not limited to the above pressures but that any desired pressures can be used.

The blower motor control means are constituted by the above-mentioned pressure-responsive unit 17 which governs a flow of the pressure medium (air) to a pressure-responsive blower motor switch 36 and to an air valve operator 37 for controlling a valve 38 in a steam line 39, the latter leading steam to the abovementioned reactivation heater 27 of the dehumidifier 10. When the pressure within the conduit 32 falls below approximately one-half pound above atmospheric pres6 sure, the control unit 17 is so adjusted that air under pressure is directed to the pressure-responsive motor switch 36 and to the air valve operator 37 by a conduit 40. This will open the valve 38, thereby admitting steam to the dehumidifier reactivation unit 27. A suitable electrical connection (not shown) can be provided for the reactivation air blower 25 whereby the latter is energized in response to the energization of motor 12.

The pressure-responsive control unit 17 is so adjusted that when the air pressure in the conduit 32 and hence in the tank 14 reaches a predetermined point, such as two and one-half pounds above atmospheric, the pressure switch 36 will be opened to stop the motor 12 and the valve 38 will be closed by the air valve operator 37.

As above-mentioned, under normal circumstances, the tank 14 would be subject to “breathing” by virtue of the movement of the vessel into waters and atmospheres of differing temperatures and by virtue of changes in cargo temperature. It is, of course, necessary to prevent the “breathing” inwardly of the compartment whereby undesired moisture might enter. Consequently, the tank 14 should be carefully sealed by suitable airtight hatches, risers, etc. By maintaining a slight pressure of dried gases in the tank 14 at all times, the abovementioned inward breathing of the tank is prevented. The tank 14 can be provided with a suitable gastight cargo hatch 14α adjacent which the conduit 32 may be in communication with the interior of the tank. Also a. suitable riser 14b such as ullage riser may be provided having a gastight cover. The pressure within the conduit 32 may be determined visually by a manometer 41 which is located in a suitable place, for example, upon the bridge of the ship. A one way expansion valve 14c is provided in a vent line 14rf, the latter preferably having an outlet at or near the top of a mast to keep explosive gases at a safe distance from the deck.

In carrying out the novel method by means of the apparatus in Fig. 1, the general object is to insure that the relative humidity of the atmosphere within the tank 14 is never in excess of approximately 50% at the temperature to which the tank and its contents is or will be subjected. It is possible to anticipate with some accuracy, for example, during the voyage of a ship, the general changes in temperature to which the ship will be subjected. The dewpoint of the atmosphere within the tank 14 is reduced by means of the dehumidifier to a point such that within a predetermined temperature drop or when the lowest anticipated temperatures are reached, the relative humidity will be below approximately 50%.

Instead of employing air in the above method it is possible to employ any gaseous substance which in the absence of liquid water is non-corrosive with respect to the metal of the storage compartment. The term “liquid water” as employed herein includes water suspended as droplets visible as in fog or in steam under certtain pressure and temperature conditions and also water in microscopically formed droplets, the latter being formed as by adsorptive or capillary action, as upon rust, and such droplets formed by any wetting action as of a metallic tank surface.

Method and apparatus employing dehumidified combustion inhibiting gas

Referring to Fig. 2, means are provided for automatically maintaining a combustion inhibiting gas dried to a predetermined extent under pressure within a storage tank for liquids such as petroleum oils.

.The unoccupied space within the tank in accordance with this embodiment of the invention is filled with dehumidified combustion inhibiting gases which can be expanded and contracted by the above-mentioned changes in temperature.

Means are provided for sealing the tank from admission of the outer atmosphere and for conducting thereto and maintaining such dehumidified combustion inhibiting gas

2,740,684 is automatically replenished to the desired pressure. Suitable means are provided for preventing a backflow of gases from the tanks to the dehumidifying unit, as in Fig. 1.

Flue gas is extracted from the boiler uptakes 50 by means of suitable conduits 51 and delivered to a gas cleaning and cooling device 52 which herein is termed a “scrubber-cooler.” The flue gas bubbles up through suitable pans within the scrubber-cooler which contain a coolant such as sea water, thereby removing soot, dirt and water soluble sulphur compounds. Normally the gas will enter the scrubber-cooler at a temperature up to 650° F., usually less, and the scrubber-cooler 52 is designed to cool same to a temperature of approximately 105° F. The sea water coolant is directed to the scrubber-cooler through a suitable conduit 53, the coolant draining from unit 52 through a conduit 54. The cleaned and cooled gases issue from the scrubber-cooler through a conduit 55 at said temperature of about 105° F. which corresponds to its dewpoint, the gas, of course, being saturated at this temperature. A separator 56 interposed in the conduit 55 precipitates any entrained water carried over from the scrubber-cooler and removes same.

In the event that the flow of coolant should be shut off from the scrubber-cooler 52, hot gases would be directed to the remainder of the apparatus and to the cargo tank. This, of course, must be prevented and a suitable safety device is provided as at 57 comprising an automatic valve 58 which is controlled in response to the pressure of the coolant in the conduit 53. A line 59 is provided for interconnecting said automatic valve 58 with the conduit 53. When the pressure of the coolant falls below a preselected amount, the automatic valve closes and thus hot gases are prevented from reaching the cargo tanks. A gauge such as a pressure-responsive water gauge 60 is connected to the coolant conduit 59 in order to provide a visual indication of this pressure. If desired, an audible or visual alarm as at 60α can be associated with said pressure-responsive gauge.

¹ As above mentioned, it is possible to employ either dried inert flue gases or dried atmospheric air in the cargo tanks and to this end an air inlet 61 is provided preferably immediately following the automatic shut off valve 58 through which atmospheric air can be fed to , the system upon the shutting off of the flue gases by said valve 58. Thus the apparatus of Fig. 2 can perform the function of that of Fig. 1. The use of air permits men safely to enter the tank.

It is desirable to employ a means for further cleanI ing the gases flowing through the conduit 55 by means of a so-called eliminator 62 which serves to filter out any residual smoke and dirt to prevent it from reaching the desiccant dehumidifier to be described hereinafter. Moreover the eliminator 62 is effective to remove some i condensation in the line. Said eliminator is constituted by, for example, a series of staggered baffle plates and filter material.

If the saturated flue gases from scrubber-cooler 52 were conducted directly to a desiccant bed type dehumidifier (as shown in Fig. 1), the latter would have to be of impractical size in order to accomplish the purposes herein set forth. Consequently in order to avoid the use of excessively large and expensive dehumidifiers of said desiccant bed type which may consume large amounts of power and add undesired weight to the vessel, we propose to provide a second cooling stage in the drying of the flue gases which is prior to the desiccant bed dehumidification whereby a substantial percentage of the moisture content thereof will be removed by a readily available and inexpensive coolant such as sea water. The first cooling stage occurs, of course, in the scrubbercooler 52. The device for accomplishing this second cooling stage is herein termed a precooler and is indicated at 63. The precooler 63 consists, for example, of a series of coils through which the coolant is passed, the or gaseous mixture at a predetermined pressure. When such pressure has become dissipated and reduced by a preselected amount, automatic means are provided for replenishing the dry inert gas to the desired pressure. There is thus prevented an inflow from the outer atmosphere of air which might increase the moisture content of the gases therein to an undesired degree.

The combustion inhibiting gas employed is obtained, for example, from the flues of the boilers of the abovementioned oil tanker. Such boilers are either of the oil 1 fire or coal fire types, the flue gas therefrom comprising principally a mixture of combustion inhibiting gases such as nitrogen and carbon dioxide with water vapor and a small percentage of uncombined oxygen, the latter being insufficient to support combustion. Instead of flue gas it is possible to employ other combustion inhibiting gases such as exhaust gas from an internal combustion engine or such gases from any suitable source.

The combustion inhibiting gas can be treated in such a manner that (1) sweat upon the inner surfaces of the storage compartment is prevented and (2) not only is said sweat prevented, but the formation of residual rust, as above described, is also prevented.

The combustion inhibiting gases which emerge from the stack of the vessel are usually at relatively high temperature and are directed through a gas cooling and cleaning device which removes the impurities and reduces substantially the temperature thereof. Such a device is termed a scrubber-cooler which can remove solid particles such as soot, fly ash, sulphur and any water soluble compounds. The gas thereafter is acted upon by means for eliminating any entrained moisture and by additional cooling means termed a precooler which using sea water as a coolant greatly reduces the dewpoint thereof and also reduces the size of the desiccant dehumidifier means, the power necessary to operate the system and the cost thereof. It thereafter is passed through said dehumidifier, for example, a desiccant bed type similar to that shown in Fig. 1, which acting in combination with said precooler is capable of reducing the dewpoint of the gas considerably below 32°, if desired (Fig. 3), and the relative humidity can be reduced to a point substantially below 50% with respect to all temperatures likely to be reached within the compartment. The dried gas is then distributed to the cargo tank whereby in the spaces thereof unoccupied by cargo a relatively dry and non-combustible gaseous mixture is maintained which prevents surface condensation, inhibits rust formation, thereby preventing deterioration of the storage compartment and probable contamination of the cargo.

According to present custom, the dry flue gas is delivered to the tank at approximately two and one-half pounds pressure or under regardless of the level of the liquid in the tank. Means are provided for maintaining this pressure head as the liquid level lowers upon discharge of the tank’s contents or upon the filling thereof, and at other times, such as when cleaning.

The term “inert” as employed herein with respect to gases refers not only to the usual inert gases, namely, the members of the so-called “zero group” of the periodic system but also to any combustion inhibiting gas or gaseous mixture, such as the above-mentioned flue gas. In view of the inert nature of the gaseous mixture introduced into the tank, the formation of combustible gaseous mixtures is inhibited or prevented and fire hazard thus greatly reduced.

When the tank is being filled, the inert gas mixture can be ejected to the atmosphere through suitable one-way vents having outlets preferably high above the deck, thereby eliminating the danger which previously has existed in the neighborhood of such vents during the filling of the tanks. When the tanks are full, the dried gas, upon expansion thereof, can be allowed to escape through said one-way vents, and when the pressure of said gas diminishes to a preselected point, the supply thereof 75

2,746, coils being enclosed within a suitable container whereby the gas is forced to pass over the coils. A conduit 63α is provided for conducting sea water from conduit 53 to said precooler. The precooler under normal circumstances, using the coolant such as sea water, is designed 5 to remove a large percentage of the water vapor in the saturated flue gas, for example, it will remove more than half of this vapor and will thereby reduce the dewpoint from 105° F. to a point lower than 85°. Thus a considerable load is removed from the dehumidifier men- 10 tioned below.

The gas is next passed, for example, through said desiccant bed type of dehumidifier similar to that described in connection with Fig. 1 having two desiccant beds through one of which the gas is passed for mois- 15 ture adsorption while the other bed is undergoing or has undergone reactivation.

The desiccant bed dehumidifier is indicated generally at 64 and consists of desiccant beds 65 and 66.

The saturated gas is admitted through a four-way valve 20 67 which is adapted to direct gas from conduit 55 to either of the beds 65 or 66 and simultaneously to direct wet air exhaust from the bed undergoing reactivation to the atmosphere. Another four-way valve 68 is operatively connected to the four-way valve 67 whereby hot 25 gases are directed to the bed undergoing reactivation. The gas which has just undergone dehumidification through the adsorption bed is directed through said valve 68 to the dry exhaust conduit 68α.

Most of the moisture contained within the gas under- 30 going dehumidification is left in the adsorption desiccant bed and the dried gas is warmed by the heat of adsorption and also possibly by the heat of the bed and the surrounding structure, if any, retained therein from the last reactivation cycle. As necessary, this heat is re- 35 moved by a small after-cooler 69 connected to coolant conduit 53 via the conduit 63α, a suitable conduit being provided to remove the coolant discharge.

Air for reactivation is drawn into the dehumidifier through a duct 70 by means of a suitable fan 70α and the 40 air is heated, for example, by steam in a heater 70b and directed to the bed undergoing reactivation. As this bed is heated, the moisture is evaporated and discharged through valve 67 and a wet air exhaust 71. Steam for reactivating the desiccant beds is led through a conduit 45 70c to the heater 70b. Automatic means for controlling the flow of steam through conduit 70c will appear hereinafter.

The relationship between coolant temperature and dewpoint of the dried gases sent to the hold is indicated in _goFig. 3 and represents the combined action of one type of the precooler 63 and one type of dehumidifier 64 wherein the inert gas temperature is about 105° F.

The dried gases are pumped through the system by means of blower 72 which preferably is interposed in the _ggconduit 68α downsteam relative to the dehumidifying unit 64. Blower 72 is powered by a motor 72α. The fan 70α and blower motor 72α are governed by automatic means analogous to those of Fig. 1 as will appear more fully hereinafter. Fan 70α is preferably electrical- θθ ly connected with blower motor 72α and is energized concurrently therewith.

Blower 72 preferably delivers the dried gas at a pressure of about two and one-half pounds through an outlet conduit or header 55α, 55b to the cargo tanks, one of _ggwhich is indicated at 73. The latter is provided with a cargo hatch 73α with which conduit 55b communicates. Also a vent line 73b is in communication with the tank 73 and is provided with a one-way relief valve 73c. An ullage riser as at 73d, having a gastight cover, is also γθ provided. The conduit portions 55α, 55b have a line of division therebetween at valve 75 in a manner analogous to conduit portions 11α, 32 of Fig. 1.

Suitable means are also here provided for determining the conditions of the output gases of the dehumidifier 64 75

684 ¹⁰ comprising means as at 74 which are in communication with conduit 55α. Such means includes, for example, as above mentioned regarding Fig. 1, a dry bulb thermometer and a relative humidity indicating device, by the inspection of which the dewpoint of the gases directed to the tank 73 can be determined by reference to a psychrometric chart or table.

In order to prevent a backflow of gases from the storage compartment or tank 73 into the above apparatus, the suitable butterfly valve 75 (as in Fig. 1) is employed which is controlled by a valve operator 76. The latter, for example, is provided with a diaphragm which is normally urged towards one limit of its motion by a spring (not shown) through a linkage 77. This spring is adapted for urging the valve 75 to a closed position. However, said spring can be overcome by gas pressure and said valve thus opened. The means for overcoming said spring pressure is constituted by an air pressure control unit 78 which operates in a manner similar to control unit 31 of Fig. 1. Normally the spring within the valve operator 76 holds the valve 75 closed. When the pressure of the gases in conduit 55b reaches a desired value, for example, one-half pound above atmospheric, the air pressure control unit 78 is actuated, this pressure being communicated thereto through a suitable conduit 79, whereby said spring within the valve operator 76 is overcome and the butterfly valve 75 is opened. Thus, if the blower 72 should fail for any reason, the spring within the valve operator 76 is effective to close the valve 75 and prevent the backflow of gases into the apparatus.

The air pressure control unit 78 is also connected to the conduit 55b by means of a conduit 80 which is adapted for communicating pressures for influencing control unit 78 to reduce the air pressure within the valve operator 76 and thereby to close valve 75 when the gas pressure in the conduit 55b exceeds, for example, two and one-half pounds.

The above-mentioned steam for reactivation of the desiccant beds of the dehumidifier unit 64 is led thereto through said conduit 70c which is controlled by a valve 82 under the influence of an air actuated valve operator 83 and an air pressure control device 84, the latter being analogous to unit 78. The coolant passing through the conduit 53 to the scrubber-cooler 52 and the safety valve 57 is governed by a valve 85 controlled by an air actuated valve operator 86. The latter is also under the influence of the air pressure control device 84. Conduits 87 and 88 are employed for interconnecting the air pressure control unit 84 with the air actuated valve operators 83 and 86, respectively. Control unit 84 is operatively associated with the system by means of a conduit 89 interconnecting same with the conduit 55b.

The blower motor 72α is governed in response to the pressure within the conduit 55b by means of a pressure switch 90 which is in communication with the conduit 87 and which is adapted for actuating pressure switch 90 when the pressure within the conduit 55b falls below, for example, one-half pound above atmospheric. For a purpose to appear hereinafter, a delay relay 91 is placed in the control circuit of said motor whereby the blower is under the influence of both the pressure switch 90 and the delay relay 91. The sequence of operations in the control of the blower motor 72α are: (1) closing of the pressure switch 90 by means of air from the control unit 84; and (2) actuation of the delay relay 91 whereby a predetermined time elapses before file energization of the blower motor. The delay relay 91 is effective only to delay the starting up of the blower motor and not the stopping thereof.

After the blower 72 has started, the pressure in conduit 55b will rise, open the above-mentioned valve 75, and thereafter will continue to rise until it reaches approximately two and one-half pounds above atmospheric whereby the following will occur:

(1) The air pressure control unit 84 will shut off air

2,740,684 are drawn through the scrubber-cooler 52 where the temperature thereof is reduced, for example, to approximately 105° F., the gas being saturated at this temperature. The separator 56 and eliminator 62 are effective to remove respectively any entrained moisture and smoke particles.

The precooler 63 effects a very considerable saving in size and weight of the dehumidifier 64 by accomplishing a substantial reduction in water content of the gases passing therethrough. The temperature of such gases is, for example, about 105° F. at the inlet thereof and within the normal temperature range of the coolant (sea water) the outlet temperature can be reduced at least to 85° F. or lower. Such a reduction of 20° will remove at least one-half of the vapor in the gas.

The gas thereafter passes through the above-described dehumidifier 64 and the combined action of the precooler 63 and the dehumidifier 64 reduces the dewpoint of the gas to the hold or to the storage compartment 73 in accordance with the dewpoint graph indicated in Fig. 3.

When the output pressure of the blower exceeds the lower limit such as approximately one-half pound above atmospheric, the butterfly valve 75 is opened, and dehumidified flue gases are permitted to flow to the tank 73.

Instead of employing sea water as a coolant in the precooler 63, it is possible to employ any suitable coolant, which can be held at a uniform temperature of, for example, 60° F. whereby the combined action of the precooler 63 and the dehumidifier 64 reduces the dewpoint of the gas to approximately 27.5° F. When the gas pressure in conduit 55b exceeds approximately two and onehalf pounds, blower 72 is stopped by means of pressure switch 90 under the influence of the control unit 84. Simultaneously the reactivation steam is cut off by clos10 to the valve operators 83 and 86 and to the pressure switch 90.

(2) The reactivation steam supply and the coolant respectively flowing in conduits 70c and 53 will be shut off and the blower motor 72α will be deenergized, together with fan 70α.

So long as the gas pressure in conduit 55b, and hence tank 73, remains between, for example, one-half and two and one-half pounds per square inch above atmospheric, the operation of the apparatus will be suspended. However, this gas pressure may not remain within these limits for an appreciable time. Reduction of such gas pressure can occur due to such causes as: (a) leakage in the conduits and tanks; (Z>) contraction of gases due, for example, to temperature drop caused by colder weather. An increase in such gaseous pressure can occur due, for example, to temperature rise caused by warmer weather, in which case the one-way relief valve 73c can be effective to vent the excess volume of gases and, of course, such increase could assist in stopping blower 11 or 72 before the time it might ordinarily stop had the gas pressure increase not occurred.

When the pressure within conduit 55b falls below the lower limit, the pressure switch 90 is closed and the valves 82 and 85 are opened, thereby respectively permitting reactivation steam to flow to the dehumidifier, and the coolant to flow to the scrubber-cooler 52, the precooler 63 and after-cooler 69 (Fig. 2). Also the control circuit of the blower motor 72α will be energized and, after a predetermined time as controlled by the delay 30 relay 91, the blower will start. The reason for delay in the starting of said blower is to permit a flow of reactivation steam and coolant to the system in advance of the movement of the flue gases therethrough. Titus the scrubber-cooler is provided with a sufficient amount 35 ing the valve 82, and the coolant passing,through conduit of coolant at a proper temperature and the reactivation heater of the dehumidifier is heated a desired amount when the movement of the flue gases commences through the system.

is cut off by shutting valve 85. Also the butterfly valve 75 is closed.

When the pressure in the conduit 89 falls to a point approximately one-half pound above atmospheric presa ^summary of the method employed in connection 40 sure, the operation of the system will be recommenced as with the apparatus of Fig. 2 is as follows:

(a) The tank 73 is sealed from the outer atmosphere except for a controlled inlet for dried gases and except for a pressure relief valve (when the tank is being filled, vents 73b can permit escape of the gases during the filling);

(b) An inert gas or gaseous mixture containing moisture is subjected to dehumidification to a predetermined dewpoint, the latter being selected such that over a preselected range of temperature change in and of the storage compartment, the relative humidity of the dried gases therein will not be in excess of about 50%; and (c) The dehumidified gas or gaseous mixture is directed to said tank and is maintained therein preferably at a pressure slightly in excess of atmospheric.

If it is desired to use air instead of flue gas in the system of Fig. 2, a valve 92 in conduit 53 should be closed thereupon main safety valve 57 automatically shuts and coolant is shut off from scrubber-cooler 52. More- _{uuulcul auuuL /v}, _LLlJUI „„ _x . *.xvuuvwm_u»over, a valve 93 in conduit 63α should be closed thereby _{GQ ture content about} 52%_{; an}d f_rom 40° F. to 20° F. about above described.

It is understood that the terms “dewpoint” and “relative humidity” as used herein refer to aqueous dewpoint and relative humidity.

Generally speaking when the dewpoint of a body of air is reduced by approximately 20° F., the moisture content therein is reduced by approximately 50%. This is supported by the following which we have observed:

Dewpoint:

F F F F

Grains of moisture/lb. of air ____________________157 ____________________ 77 ____________________ 37 ____________________ 17

Thus within the range indicated a reduction of 20° F. in dewpoint reduces the moisture content approximately 50% or slightly in excess thereof. That is, a reduction from a dewpoint of 80° F. to 60° F. reduces moisture content about 51%; from 60° F. to 40° F. reduces mois54%.

As has been discussed in detail above, moisture can occur in the interstices of a metal surface producing socalled “interstitial moisture” when the relative humidity 65 of the adjacent atmosphere is in excess of about 50%. Consequently if a 20° F. differential is maintained between the expected terminal temperature in and of the tank (the temperature which occurs after an expected temperature drop) and the dewpoint of the atmosphere inactivating precooler 63.

In operation, the system can be initially actuated, for example, by manually governing the air pressure control unit 84, whereby valves 82 and 85 are opened and steam is directed to the dehumidifier unit 64 for heating the reactivation element 70Z> thereof. Also the coolant is directed to (a) the scrubber-cooler 52, (b) the safety valve 57 opening same, and (c) via conduit 63α to the precooler 63 and after-cooler 69. Simultaneously with the __________ opening of the valves 82 and 35, the pressure switch 90 γθ within the compartment (subject to the provision discussed below) the relative humidity of such atmosphere will be not in excess of about 50% after such drop and the formation of interstitial moisture will be prevented.

____ If the temperature of such atmosphere in and the temdrawn from the flues at for example 650° F. or less and 75 perature of a storage compartment is below 32° F., it is is closed, thereby actuating the delay relay 91 which, after a preselected time, for example eight minutes, starts blower 72. Flue gases are then withdrawn from the flues 50 and moved through the conduit 51. These gases are with2,746, clear that there can be no aqueous or liquid water condensate occurring within the compartment. Any precipitation of moisture will be i;i a solid form which is incapable of ionization and incapable of corrosion. Thus if the temperature in and of the tank is below 32° F., the 6 dewpoint of the atmosphere therein does not have to be 20° F. below such temperature in and of the tank. Nevertheless, as a practical matter, such 20° F. differential should occur because as the expected terminal temperature in and of the tank gradually reduces, for example, 10 from 60° F. down to 30° F., it is necessary to maintain the 20° F. differential between it and the dewpoint of the compartment’s atmosphere in order to prevent such interstitial moisture. Thus if the expected terminal temperature in and of the tank is say 33° F., it is clear that 15 there can be no freezing of the condensate at this temperature. Thus interstitial aqueous condensate may occur unless the relative humidity of the atmosphere is not greater than about 50%, as above discussed. Under such circumstances the dewpoint of the atmosphere within the 20 tank should be substantially 13° F., that is, 20°.F. less than such expected terminal temperature.

In view of the fact that water can be in either the solid or liquid state at 32° F., the dewpoint within the tank should be substantially 12° F. when the expected 25 terminal temperature is 32° F. However, when the temperature in and of the tank goes below 32° F., the aforementioned 20° F. differential between it and such dewpoint is no longer necessary. But such differential, as a practical matter, will have already been effected dur- 30 ing the gradual reduction down to 32° F.

Two important principles can be drawn from the above relationships as follows:

(1) If the expected terminal temperature in and of the compartment is 32° F. or above, the dewpoint of the 35 atmosphere within the storage compartment should be about 20° F. less.

(2) If such expected terminal temperature is below 32° F., the dewpoint of the atmosphere within the compartment need by only less than such 32° F. but, as a 40 practical matter, said dewpoint will be about 20° F. less than such expected terminal temperature since during the movement downwardly of such expected terminal temperature in and of the tank from above 32° F. to below 32° F., such dewpoint will have been reduced at least to 12° F. in order to prevent the occurrence of interstitial ⁴⁵moisture.

Control of frequency of operation of dehumidifying system by automatic control of valve for storage compartment 50

From the above it will be seen that the frequency of operation of the dehumidifying system depends, for example, upon the two following elements:

(1) The degree of gastightness of the conduits and 55 tanks of the system; and (2) The changes in the gas pressure within the tank or cargo space caused, for example, by changes in temperature. The change in temperature, of course, will cause the gases in the tank and its connecting conduits go to expand or contract.

It is, of course, conceivable that the degree of gastightness of the system may be so high that the system will not be actuated due to reduction of pressure caused by gas leakage. And, moreover, it may be possible that gg a change in temperature, such as a drop in temperature, is not adequate to contract the gas a sufficient amount to actuate the dehumidifying system. Under such conditions, it is desirable to have other means for actuating the dehumidifying system which, for example, functions 70 directly in accordance with changes in temperature.

A novel device for controlling the frequency of operation of the dehumidifying system in response to temperature change is indicated in Figs. 1 and 2 and is constituted by a thermostatic control for a valve of the tank, for 75

684 example, the relief valve. The thermostatic control may be operatively connected to a valve of the tank where by opening the latter the pressure within the tank may be diminished to a point where the dehumidifying system will come into operation, for example, at every 10° drop in temperature. This, of course, can occur regardless of leakage or gas contraction. Such a thermostat may be adjusted to actuate the dehumidifying system intermittently in response to increments of temperature drop, for example, when the temperature of the atmosphere has fallen from 90° to 80° and thereafter from 80° to 70° and so forth. The system can also be actuated in response to increments of change in temperature of the water through which a vessel passes. The thermostatic control for the relief valves are indicated in Figs. 1 and 2 at 107 and 108, respectively. These thermostatic controls preferably act in combination with a timing device which has the effect of maintaining the valve in an open condition, for example, for a predetermined time, such as fifteen minutes during which time a continuous purging takes place until a dewpoint is reached of sufficiently low degree whereby during the next increment of temperature change, say 10°, the relative humidity of the gases within the tank will not rise above about 50%. Thereafter such automatic opening of the valve is terminated and the valve resumes its normal function, of course, until again so opened by the thermostat means. A suitable timing device of the above nature is indicated as at 109 for acting in combination with the thermostatic valve control 107 in Fig. 1, and an analogous timing device as indicated at 110 of Fig. 2 for acting in combination with the thermostatic control 108.

It is to be understood, of course, that the thermostatic control device for the relief valve acting in combination with the timing device is not necessarily limited to the tanks of an oil tanker or to any particular type of storage compartment, but can be employed advantageously, for example, upon so-called “tank farms” or in any type of stationary storage compartment. The thermostatic valve control plus the timing device will be advantageous especially under circumstances where it is not practical to have personnel interpret meteorological data upon which depends the operation of a dehumidifying system.

The dehumidifying systems indicated in Figs. 1 and 2 are described as being automatically operated by the pressure-responsive apparatus described in connection therewith and, of course, such systems can be operated by manually controlling the pressure-responsive devices, such as 17 and 84 (Figs. 1 and 2, respectively) which control the pressure switches for governing the blower motors. Such manual actuation of the dehumidifying systems can occur in response to interpretation by personnel of meteorological indications as obtained from, for example, the relative humidity indicators and dry bulb thermometers described in connection with the devices of Figs. 1 and 2. However, it is desirable under some circumstances not to have to depend upon interpretation of such data by personnel who possibly may not be especially trained for that purpose. Thus the automatic control of the frequency of operation of the dehumidifying systems, as effected by the thermostatic valve control in combination with the timing device, eliminates such interpretation and provides a sure and accurate control of frequency of operation and thus of the proper accomplishment of the desired low dewpoint and relative humidity.

The dehumidifying systems described in Figs. 1 and 2, for example, for shipboard use, should not be of excessive size or weight because of obvious limitations placed thereupon by the demands of ship construction. Thus with a dehumidifier of relatively low capacity it may not be possible to accomplish a desired reduction in dewpoint by a single operation of the system but rather by intermittent operations which can give successive reductions of

2,746,684 ¹⁵dewpoint as the temperature falls. The dehumidifying systems in Figs. 1 and 2, in actual physical size are relatively small because adapted for ship use but are capable of producing the desired result because they are actuated at intervals whereby there is obtained the benefit of progressively lower sea water temperatures and air temperatures. Thus the apparatus progressively reduces dewpoint by increments to diminish it to the desired minimum. At no time, of course, is the relative humidity of the atmosphere within the tank greater than 50% with respect to the lowest temperature within the next predetermined increment of temperature drop.

By means of the above-described thermostatic valve control in combination with the timing device, the dehumidifying systems can be actuated by any desired increment of temperature drop. The length of time of operation of the system as caused by the thermostatic valve control and timing device, is, of course, a function of, among other things, the size of the increment of temperature drop, the time of operation being longer as the increment increases.

Apparatus as associated with vessel of Fig. 4

Shown in Fig. 4 is an oil tanker having, for example, nine main cargo tanks. Each main cargo tank is subdivided usually into three separate parts, namely, a port and a starboard tank and a center line tank. Each of these three tank subdivisions have at least one cargo hatch and a vent line. A cross-sectional view of such a main tank is indicated in Fig. 5 wherein the starboard tank is shown at 95, the center line tank at 96, and the port tank at 97. Any one of the latter three tanks can be considered as analogous to either the tank 14 (Fig. 1) or tank 73 (Fig. 2). Consider, for example, center line tank 96 which is provided with a cargo hatch 97 with which the dry gas conduit 55a is in communication. A vent line 98 is in communication with the center line tank 96 and has provided therein a one-way relief valve 99 which is connected to, for example, the upstanding riser 180 (Fig. 4) having an outlet at 100α high above the deck. Thus combustible gases are permitted to escape from the system through the vent line 98, 100 at a safe distance from the deck.

The tank 96 also is provided with a hatch 101 which is commonly known in tanker construction as a “Butter- 45 worth” hatch or cover. The latter cover can be opened, for example, for the gas-freeing process for the tank.

As shown in Fig. 6, a second cargo hatch 97a may be provided through which additional liquid can be passed to the tank in order to expedite the filling thereof. A 50 pair of ullage risers 102 and 103 are indicated in Fig. 6 which can be employed for taking soundings or for the entry or exit of atmospheric air, the latter being permitted to enter, for example, during the ventilation of the tank.

The novel system of Fig. 1, generally indicated at 104 55 in Fig. 4, automatically distributes dry air to the tanks of the vessel shown in Fig. 4 through conduits 105, 105α, a slight positive pressure thereof being maintained within the cargo spaces except when gas-freeing, cleaning or, for example, carrying ballast.

Regarding the ventilating of the tanks, a system of conduits is employed in the vessel for assisting this purpose known as “products lines.” A product line 106 is indicated in Fig. 6 which extends fore and aft of the vessel substantially along the center line thereof near the bot- 65 tom of the tank. Associated with the fore and aft product line 166 is a device known as a “bellmouth” 107 which has the mouth thereof open very near the bottom of the tank. The upper portion of the “bellmouth” is in communication with the fore and aft products line 106. 70 In Fig. 5, a products line 108 is indicated which extends athwartship and reaches, respectively, toward the starboard and port tanks wherein each has associated therewith its respective bellmouth.

¹⁶ can be employed for assisting in the ventilation of the tank 96 with the aid of a suitable so-called gas exhauster (not shown) which is in communication with line 106 and which pumps or sucks the gases from the bottom of the tank to the atmosphere. In connection with the ventilation of the tank 96 after cleaning or discharging ballast, ventilation is required to dispel the high moisture content air and to assist in the drying of the tank. This is accomplished by opening the cover 101 and the ullage risers 102 and 103 and actuating the above-mentioned gas exhauster whereby atmospheric air is drawn through the tank and the moisture-laden air is discharged to the atmosphere. Gas exhausters and/or blowers can be used for gas-freeing, ventilating and also to expedite the drying of tanks after cleaning or discharging ballast. A supply of air dehumidified as above explained is pumped into the tanks while the gas exhauster and/or blower is in operation, thus reducing the normal time required for drying or gas freeing.

With respect to the purging of the tank with dry air, when the air in the tank is at atmospheric conditions, it is necessary further to reduce the moisture content thereof in accordance with the above set forth invention. This is accomplished by injecting dry air from the dehumidifier tank. The dry air then circulates, and accelerates the drying process in the tank. The purging air can be discharged to the atmosphere, for example, through the ullage riser.

With respect to so-called “normal pressure operation” of the system, when discharging a liquid cargo, dry air from the dehumidifier is introduced into the tank as the cargo level drops. This leaves the tank full of dry air after discharging of the cargo has been completed. When the tank is empty, the normal expansion of the air therein is relieved by the pressure relief valves. Contraction of air will be indicated by subsequent reduction of pressure and when the pressure drops, additional dry air will be introduced as above set forth. When loading cargo, the volume of dry air or gas in the tank will be displaced 40 by the cargo except (after filling is complete) for that occupying the above-mentioned space over the cargo. Air and gases will escape through the above-described vent lines. During this loading operation cargo hatches should be kept closed and soundings should be taken • - through the ullage riser. When loading cargo in the tank or tanks, the so-called one-way pressure relief valves may not provide an adequate orifice through which the excess volumes of gases can escape. Consequently it may be desirable mechanically to open such one-way relief valves entirely and also to open any manually controlled valves in the vent line in order to provide maximum and unobstructed passage of the gases which are expelled as the result of the loading operation. On completion of loading, of course, all the space above the cargo must be filled with dry air or combustion inhibiting gas. At this time the one-way pressure relief valves should be readjusted for their normal automatic operation. Of course, when the tank is filled substantially to the top with the exception of the above-described space which 60 is necessary thereabove, the expansion and contraction of the gases above the cargo will be handled similarly as when the tank is empty, except that only those gases which are above the cargo are subject to the novel method.

While the invention has been described with respect to certain preferred examples which have given satisfactory results, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended therefore in the appended claims to cover all such changes and modifications.