US2786598A - Safety storage system - Google Patents

Description

March 26, 1957 C. L. WEEKS SAFETY STORAGE SYSTEM Filed July 15, 1954 I INVENTOR. CLARENCE L.WEEKS BY firm ATTORNEY SAFETY STORAGE SYSTEM Clarence L. Weeks, Baton Rouge, La., assignor to Ethyl Corporation, New York, N. Y., a corporation of Delaware Application July 15, 1954, Serial No. 443,470

3 Claims. (Cl. 220-439) This invention relates to a storage system for decomposable liquids. More specifically the invention relates to a storage system for such materials including a closed pressure vessel and a safety unloading device in combination therewith.

it is common practice in the process industries to utilize pressure vessels equipped with pressure relief devices for storage of hazardous or decomposable materials. Such pressure relief devices are usually of several types: pop valves and rupture discs. Occasionally these devices are used concurrently, the blowout devices having the advantage that upon rupture the full discharge area of the discharge conduit is made available for release of the material or the decomposition products of the material. Normally the relief devices of this character are at the terminus of a conduit leading from the vapor space almost invariably left above a liquid charge in a pressure vessel. Therefore, these devices are of somewhat limited application in that they would be fully safe primarily for those situations where relief of vapor pressure mitigates the decomposition or explosion hazard. For example, in storage of a chlorinatedhydrocarbon of highly stable character but of a high vapor pressure, provision for re lease of the vapor pressure above the stored liquid is adequate for most purposes.

A different problem is exhibited by a relatively large category of materials which exhibit a decomposition tendency unaccompanied by. an appreciable normal vapor pressure. Materials of this class, if heated in a confined space, would exhibit an increase in vapor pressures, but the increase in vapor pressure is not so great that a gas pressure release device is adequate to prevent explosion hazards. In other words, at the time the vapor pressure is suiliciently large to activate vapor pressure release devices, the temperature of the liquid would be so high that the decomposition or reaction might be uncontrollable. Another aspect of this problem is the fact that if the decomposable liquid is in admixture with volatile, stable liquids then an increase in temperature results in a distillation of the more volatile component. This distillation has the elfect of concentrating the decomposable component and thereby increasing the explosion hazard.

Typical materials exhibiting the above described peculiarities are easily polymerizable monomers, various nitrate, azo or amine compounds, organo metallics such as iron carbonyl or alkyl-lead or other alkylated compounds, and compositions containing these materials.

An object of the present invention is to provide apparatus which is responsive both to pressure and temperature rises resulting in the above described rupture or explosion hazard. A further object is to incorporate with said dual functioning pressure relief device means for accomplishing discharge of the liquid contents of the storage vessel. A further object is to provide such apparatus with essentially no moving parts. Still another object is to provide apparatus which concurrently with removal of the explosion hazard from the vessel itself, provides for discharge of the liquid contents of the vessel 2,786,598 Patented Mar. 26, 1951 to a relatively safe surrounding. appear hereinafter.

Generally, the apparatus of the invention comprises, in a pressure storage system, said storage system including a pressure tight storage vessel, a safety unloading device comprising a double ended conduit having a relief disc positioned at or near the end of said conduit. The term conduit includes a segment of a more extended conduit formed by the addition of a further segment in some instances, as hereafter described. The relief disc is responsive both to pressure and temperature variations, as described hereafter. One end of the conduit is located within the storage vessel near the lowermost point therein. The other end is positioned outside the vessel, and usually at or slightly below the lowermost internal point of the storage vessel. In certain forms of the apparatus, it is used in combination With an external sump containing water or an aqueous solution. This combination is particularly useful in the case of stored materials having a specific gravity greater than water and immiscible therewith. in such instances the liquid discharge is captured below the water and thus effectively isolated from further exposure to elevated temperatures.

A further variation of the apparatus involves the use of plurality of conduits operative in parallel. The disposition of these several conduits is such that a relief flow outlet area responsive to the pressure within the storage vessel is provided. An additional refinement useful in circumstances hereafter described involves the use of a protective sheath in conjunction with the temperature-pressure responsive element.

The relief disc in the unloading conduit is positioned externally of the storage vessel and at a level corresponding to the level of the conduit within the vessel. The conduit passes through the vessel wall at the top or near the top and a portion of the conduit is consequently always above the uppermost level of the liquid stored in the vessel.

The accompanying figures illustrate, in conjunction with the following description, the principal features of the apparatus and of certain embodiments thereof. Figure l. is a cross sectional view of a storage vessel and a single conduit unloading device in place. Figure 2 is a cross section of part of the external conduit showing the relief device in place. Figure 3 is a cross sectional elevation of a form of the apparatus using a plurality of conduits. Figure 4 is a cross section view of a form of the relief element additionally provided with a protective sheath.

Turning to Figure 1, the vessel shown therein in section is a horizontal cylindrical drum 11 surmounted by a conventional flanged manhole 12. The principal element of the safety unloading device is the conduit 13, here shown including a removable segment or fitting 14, but which may be a single continuous unitary conduit. The internal branch or portion 15 of the conduit extends within the drum 11 to a point near the bottom of the drum 11, so that the conduit is capable of almost completely discharging the vessel. The external end 17 of the conduit is positioned vertically at approximately the same elevation as the internal end 16 and is closed by a pressure and temperature responsive element illustrated more fully in Figure 2. in most instances an additional conduit segment l? continues from the assembly 17 to a safe point below the drum.

Turning to Figure 2, the end assembly 17, in the conduit 13, includes the flange 21 and the matching flange 22 which fastens a rupture disc 23 in place.

The utility of the invention will be apparent from the following description of typical operation of the apparatus. Under usual conditions the drum 11 will contain a liquid charge extending to an upper level 18. If the;

Other objects will atmosphere surrounding the drum increases in temperature, the temperature of the relief disc 23 is aifected almost instantaneously, whereas the average temperature of the drum 11 and the charge therein increases at a much lower rate. Therefore, if the relief disc 23 is of a metal which melts at a temperature which would result in an appreciable rate of decomposition of the liquid charge, the discharge conduit 13 is cleared and open long prior to the attainment of dangerous temperature levels in the liquid charge.

The mere existence of an open conduit for discharge from the vessel does not in itself provide for liquid flow. The only resistance to liquid flow is the elevation head necessary to raise the liquid charge to the uppermost level in the conduit. This resistance is overcome by one of three different routes. The pressure in the vapor space may be raised by direct expansion of the vapor or therein. In addition, in the case of liquid charges which are mixtures of several components, an increase in the liquid temperature can result in a partial fractionation, resulting in vaporization of the more volatile component and consequent increase in vapor pressure providing the necessary carryover pressure. The third route for the pressure which will result in liquid discharge would be the result of moderate amounts of localized decomposition resulting in sufiicient gas evolution to raise the pressure the necessary slight degree.

It is apparent that all the foregoing routes could be occurring concurrently or in sequence, depending on the properties and composition of the liquid charge. In addition, each of these routes necessarily involves a prolonged continuation of the abnormal external temperature condition. In the event that the external heat source is eliminated, the dicharging operation is automatically terminated owing to the absence of any flow of gas or atmosphere into the drum 11.

As already mentioned, the discharge conduit preferably is positioned immediately above a sump pit, or catch basin. In most instances, the sump pit will be charged with water so that the liquid discharge will be immediately cooled. In many instances, the liquid will be of higher density than water, so that a relatively permanent, protective blanket will be provided. In embodiments wherein an extension segment is employed such as the conduit segment 19 in the apparatus of Figure 1, such extension will be terminated below the surface of the water in the pit. The contents of the drum is thus discharged without contact with the atmosphere. Further, in such embodiments, it will be desirable to wrap the assembly 17 with a sheet metal cover (not shown) to form a liquid tight seal. This would prevent liquid loss between the faces of the flanges which normally position the temperature-pressure responsive disc.

The embodiment of Figure 3 is directed to providing an embodiment providing a variable relief for liquid discharge. Instead of a single discharge conduit, a plurality of conduits 31, 32, 33 are provided in a storage tank 34. All of these conduits have an open end within the drum 34, positioned at approximately the lowermost point. The external ends 35, 36, 37 are positioned, similarly to the embodiment of Figure 1, at approximately the same vertical elevation as the interior terminii. As in other forms of the apparatus, rupture discs of a suitable low melting metal are provided for the external closures.

The conduits 31, 32, 33 for the embodiment of Figure :3 are preferably of varying size, and in addition, the maximum height of each conduit path varies. The significance of the variation in height is that, if the pressure on the system continues to rise during discharge through the relief conduit providing the lowest level or elevational resistance to discharge, the next highest ele vated conduit will then provide a further discharge port. Thus the system automatically adjusts to the severity or duration of the external heat source. Generally it is preferred that the size of the several conduits be in the order of increasing size with the increase in head resistance provided by the conduit path. In other words, the rank of the sizes corresponds to their rank of their maximum elevation. As a typical example, if the three conduits 33, 32, 31 provide discharge paths having an elevation of, say 2 feet, 2 /2 feet, and 3 feet, respectively, above the average maximum liquid level in the drum, a preferred range of conduit sizes (diameters) is in the proportions of 1:213. It will be noted that this preferred proportions of sizes will provide discharge areas in the proportions 1:4:9. Thus the relief area ratio increases in greater than linear proportion to the pressure on the system.

The modification shown by Figure 4 is intended for service under particularly corrosive conditions, such as are frequentiy encountered with mixtures of components including a volatile halogen compound. These compounds have a tendency to react with the metals used as the pressure and temperature responsive element. The corrosive compounds mentioned have a tendency to condense in the conduits and collect at the external, or dis charge end. In the embodiment of Figure 4, the relief disc 41 is surmounted by a thin film of impervious synthetic resin film such as polyethylene. In the operation of the relief device, because of the relative thickness of the disc 41, at the time of its failure the film 42 automatically falls away from the opening upon melting of the disc.

The height of the discharge conduits above the maximum liquid level of the system is of course established according to the service for which the system is intended, that is, the liquid to be stored. The height will be determined or selected from appropriate consideration of the density of the liquid, its vapor pressure-temperature relationships and the desired rate of discharge.

The metal-s used for the rupture disc will be selected, similarly to the vertical elevation of the conduits, by reference to the properties of the material to be stored. Low melting alloys are available having clearly defined melting points from below C. to 300 C. The following table gives the compositions of a series of alloys and their melting points which are suitable for the present apparatus. Metal and composition, weight percent: Melting point, C. Darcets metal 50 Bi, 25 Sn, 25 Pb 93 Eutectic fusible alloy 54 Bi, 26 Sn, 20 Cd lll Eutectic alloy 50 Sn, 32 Pb, 18 Cd Tinmans solder 67 Sn, 33 Pb 180 Solder, half and half, 50 Pb, 50 Sn 225 Magnolia alloy 90 Pb, 10 Sb 270 Other low melting metals in addition to those listed above are known to the art, so no difficulty is encountered in providing a rupture disc which melts at the desired temperature.

The dimensions of the rupture disc for a particular installation are determined in accordance with standard practices and formulae. These take into account the strength of the material, and the diameter of the conduit in determining the thickness required for a disc to provide a given bursting pressure. In a typical installation, a three inch conduit is provided with a 1 inch thick disc. In some instances, it will be found desirable to provide a grooved circle in the rupture disc, the groove corresponding generally to the diameter of the conduit. This provides for failure by straight shear and admits of relatively precise control.

Having described the apparatus of the invention in some detail, what is claimed is:

1. In a storage system for a decomposable liquid including a pressure tig'ht vessel, a safety liquid unloading device comprising at least one conduit open at one end and normally closed at the other end by a relief element, the open end being positioned within the vessel near the lowermost point therein, the normally closed end being positioned outside and adjacent the vessel at an elevation corresponding approximately to the lowermost point of the vessel, the uppermost point in the conduit being above the uppermost level of the vessel and at a level such that the decomposable liquid is elevatable thereto by its vapor pressure within the vessel at a temperature below substantial decomposition temperature, and the relief element being a pressure rupturablc closure of low melting material.

2. In a storage system for a decomposable liquid including a pressure tight vessel, a safety unloading device including a plurality of conduits, each of said conduits being open at one end and normally closed at the other end by a relief element, the relief element being a pressure rupturable closure of a low melting material, the

open end being positioned within the vessel at approximately the lowermost level therein, the normally closed end being positioned outside and adjacent the vessel at an elevation corresponding approximately to the lowermost level in the vessel, each of said conduits having a portion above the uppermost level of the vessel, but at a different level than the other conduits, whereby a number of discharge paths for liquid discharge is provided responsive to the pressure in the vessel.

3. The apparatus of claim 2 further defined in that the conduits are all of different sizes and the maximum height of each conduit above the vessel corresponds in rank to the size of the conduit.

References Cited in the file of this patent UNITED STATES PATENTS 1,973,182 Shaw Sept. 11, 1934 2,571,961 Smith Oct. 16, 1951

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