Abstract:
A safety device for containers pressurized with gas safeguards the gas side of the working chamber of hydraulic accumulators. A relief apparatus ( 2 ) reduces an elevated gas pressure in the container ( 3 ) caused by heat. The relief apparatus ( 2 ) is a component loaded under the influence of a shear force or compressive force. The shape change of the component under the influence of heat on the safety device ( 1 ) in a space ( 5 ) closed to the outside occurs such that a fluid-conducting connection ( 6 ) from the inside of the container ( 3 ) to the outside in the direction of the surroundings is enabled.

Description:
FIELD OF THE INVENTION 
     The invention relates to a safety device for gas-pressurized containers, in particular for safeguarding the gas side of the working space of hydropneumatic devices, for example, a hydraulic accumulator. 
     BACKGROUND OF THE INVENTION 
     To ensure the safe operation of devices with containers containing a pressurized gas, such as hydraulic accumulators, among other things, all risks which can arise at the installation site of these devices must also be considered. Possible hazardous external effects are important, especially temperature spikes that occur in a fire in the immediate vicinity of these gas-pressurized containers and that can lead to the failure of the container. 
     DE 32 19 526 A1 discloses a safety device for these pressure vessels, in particular for lightweight metal pressure vessels for liquefied gas. The safety device is formed essentially by a fusible screw threaded in a flange of the pressure vessel. The fusible screw contains a metal alloy with a low melting point, with the metal alloy being disposed in an exit channel for pressurized liquefied gas. The fusible alloy is covered by a metal foil ensuring tightness during alternating loads in operation of the pressure vessel. 
     In normal operation, the safety device is disposed in a region surrounded by fluid so that its temperature is defined. In the event of a fire, it can be assumed that the liquefied gas will undergo transition into the gaseous phase and the fusible alloy will thus be heated, as a result of which pressure equalization to the outside is enabled. An explosion of the pressure vessel is avoided in this way. However, the vicinity of the pressure vessel can be adversely affected or endangered by the rapidly emerging hot and liquid fusible alloy. 
     U.S. Pat. No. 6,367,499 B2 discloses a safety device for pressure vessels disposed in a relief opening of a gas pressure vessel. A piston-like closure element which is disposed axially in an outflow opening from the gas pressure vessel to the outside with an energy storage mechanism for opening of the outflow opening. The closure element is pressed against a stop of fusible material. The stop constitutes a heat-sensitive relief device. Under the action of heat, the molten material of the stop can escape radially from the safety device so that the closure element clears the outflow opening. A radial escape of the fusible material constitutes a safety increase in the known safety device. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved safety device for gas-pressurized containers which, when activated, does not constitute a hazard for the vicinity and which has a simple structure. 
     This object is basically achieved according to the invention by a safety device having a relief device which is formed from a heat-sensitive material and disposed in a closed space of the safety device. Through this space, exhibiting an opening, none of the material of the relief device can flow. 
     This design measure prevents a risk to the vicinity of the safety device due to the molten material of the relief device. Moreover, the relief device is pretensioned with shear, preferably for buckling or bulging, so that in principle melting of the relief device is not necessary to clear an outflow opening closed by the safety device. Softening is thus sufficient. The dimensioning of the space surrounding the relief device first enables a buckling or bulging of the relief device. The safety device according to the invention, however, also works when the relief device melts at elevated ambient temperature. In this case, due to the pretensioning and the associated shear forces, the relief device in turn clears the outflow opening closed in the normal operating state of the vessel. This melting away can be expected especially when the relief device is formed of a plastic material with a low melting point. If a ceramic material or a metal material, even in the form of a bimetal, is used for the relief device, buckling or bulging of the relief device can be expected to occur with subsequent clearance of the outflow opening as a result of the pretensioning. 
     The relief device is exposed to shear in the sense of pretensioning by a component which preferably directly closes a fluid-conducting connection (outflow opening). No additional energy storage mechanism pretensioning the relief device is necessary since the gas within the container alone pressurizes the relief device. The component closing the drain opening is held in position on the drain opening, secured with a seal, so that even in the unmounted state, not pressurized by gas, the safety device can be handled and in particular can be mounted without limitation. Under the action of heat, the relief device deforms, preferably along one axis of symmetry, and the component closing the outflow opening can be moved axially within the safety device and can clear the drain opening. The space, having several times the volume of the relief device, allows this reforming of the relief device in the sense of softening or melting. 
     The relief device is preferably formed from a plastic body which can deform or melt away under the action of heat, as described. Depending on the pressure to be maintained in the pertinent container, choosing a fluoroplastic, in particular a polyvinylidene plastic such as polyvinylidene fluoride, with a continuous working temperature range of from −30° C. to +140° C. or polyvinyl fluoride with a continuous working temperature range of from −70° C. to +110° C. can be feasible. 
     The relief device can be a rod-shaped plastic part stressed for buckling or bulging, or a cylindrical one, in the sense of a plastic part designed as a hollow cylinder. 
     In order to ensure reliable response of the relief device to heat, choosing the space around the relief device to be about four times as large as the volume occupied by the relief device itself is feasible. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1  is a schematically simplified side elevational view in section of a hydropneumatic piston accumulator provided with a safety device according to an exemplary embodiment of the invention; and 
         FIG. 2  is an enlarged, side elevational view in section of the safety device of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows, in a schematic longitudinal section, not to scale, a container  3  constituting a hydraulic accumulator in the form of a piston accumulator in a conventional design. The hydraulic accumulator is provided with a hollow cylindrical accumulator housing  10  closed pressure tight on the end sides by a first cover  11  and a second cover  12 . A cup-shaped piston  13  can move axially in the accumulator housing  10  and separates a fluid side, in particular oil side  14 , from a gas side  15 . The oil side  14  can be connected in the conventional manner to a hydraulic system (not shown) via an oil port  17  coaxial to the longitudinal axis  16  of the container  3 . On its opposite side, in the cover  12 , viewed coaxially to the longitudinal axis  16 , a gas loading port  18  is provided to supply the gas side  15  in the conventional manner with a working gas, such as, for example, nitrogen, with a predetermined loading pressure. 
     In  FIG. 1 , the piston accumulator is provided with an exemplary embodiment of the safety device according to the invention as shown in  FIG. 2 . The safety device  1  is screwed directly into the gas loading port  18  of the cover  12 , so that the safety device  1  directly borders the gas side  15  of the piston accumulator. 
       FIG. 2  shows in a schematic longitudinal section, not to scale, the safety device  1  for the container  3  which as a whole forms a kind of screw-in body for screwing or threading into the gas loading port  18 . For this purpose, the safety device  1  has an external thread  19 , which can be screwed into a corresponding internal thread of the cover  12  in the region of the gas loading port  18 . The external thread  19  is disposed on a cup-shaped housing  21  of the safety device  1 . The housing  21  is provided with a cover  20  crimped in the opposite direction to the external thread  19 . The housing  21  on its outer periphery can have a hexagonal projection (not detailed) for screwing the housing  21  into the cover  12 . A hexagonal recess (not shown) can be provided in the cover  20  of the housing  21  for being able to effect this screwing-in. 
     Viewed from the region of the housing  21  bearing the external thread  19  to roughly an axial center, the housing  21  is penetrated by a through bore  22  which widens in diameter upward and undergoes transition into a perpendicular cross channel  23  extending radially through the housing  21 . 
     Both the cross channel  23  and the through bore  22  are closed by a plug-shaped or piston-shaped component or piston component  7  in the initial state of the safety device  1  shown in  FIG. 2 . The piston-shaped component  7 , fits in the through bore  22  with an extension  25  of almost the same diameter as bore  22  and with a sealing element  26  (not shown) along its radial periphery  27 , and is dimensioned like the diameter d 1  of the through bore  22 . The extension  25  then widens to about twice the d 1  diameter in a portion of the piston component  7  closing the cross channel  23  in the illustrated initial state of the safety device  1 . With this diameter d 2  of that portion, the piston component  7  projects into a space  5  of the housing  21  and then in turn widens in the shape of a plate to a diameter d 3  that is slightly smaller than the inside diameter of the cylindrically shaped space  5 . 
     Viewed in the axial direction toward the cover  20 , the diameter of the piston component  7  in turn diminishes incrementally so that the cross section is provided with two diameter offsets for the piston component  7  in the shape of steps. 
     A relief device  2  in the form of a thick-walled, cylindrical component  4 , has an outside diameter greater than its axial length, is placed or clamped between the piston component  7  and the cover  20  and is held at its top and bottom in both directions by respective centering surfaces  28  on the cover  20  and the piston component  7 . 
     The relief device  2  is loaded by shear resulting from the gas pressure of the gas side  15 . The space  5  has a significantly larger inside diameter than the outside diameter of the relief device  2 . In principle, when shear increases, the relief device  2  then can buckle or bulge. The relief device  2  is preferably formed as a plastic body  9  from a polyvinyl fluoride plastic having a temperature of continuous use of from −40° C. to +140° C. Particularly when a temperature of +140° C. is exceeded by the action of heat in the event of a fire in the vicinity of the safety device  1 , the relief device  2  under the shear of the component  7  then deforms away from its axis  8  of symmetry and is deformed away, for example, by bulging or buckling into the space  5  or melts away altogether without further orientation. Since the relief device  2  is a hollow cylinder, such a bulging or buckling motion into the interior of the hollow cylinder is also in principle enabled. In addition to the initial bulging or buckling process, as a result of material softening at high temperature, at correspondingly even higher temperatures, the relief device  2  melts, and the plastic material, as shown, is displaced into the closed space  5 . These processes take place comparably when the relief device  2  is formed of a metallic material, such as a bimetal. 
     In this case of failure, the piston component  7 , which until then keeps closed the fluid-conducting connection  6  formed by the through bore  22  and the cross channel  23 , moves under the overpressure arising under the action of heat on the container  30  on the gas side in the through bore  22  such that it moves partially into the space  5 . As a result of this movement a controlled outflow of the gas, such as, for example, the working gas nitrogen, is enabled via the cross channel  23  to the outside so that an explosion or other damage of the container  3  cannot accidentally occur. Since the cross channel  23  is being continuously cleared, slow and controlled pressure relief occurs on the gas side. 
     Provided that, as shown in  FIG. 1 , the cross channel  23  with its two opposite cross channel segments discharges into a transition space  29  spaced on the edge side by a projection  30  of the wall of the accumulator housing  10 , deflection of the gas overpressure jet upward takes place. A further reduction of the overpressure, which may be in the region of 1000 bar or more, can occur, so that individuals in the vicinity of the container definitely cannot be endangered. The collar solution shown in  FIG. 1 , in which the container  30  is encompassed, forming a protective collar or protective jacket, can also be omitted without the safety function of the device being adversely affected. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.