Impact Sensitive Safety-Relief System for Tank Cars

Existing tank cars are equipped with pressure relief systems which are intended to prevent excessive internal pressure within the tank shell which could produce catastrophic failure. Under collision and/or derailment conditions, however, tank cars may receive impact loads which may impair the structural integrity of the shell. This may result in a catastrophic rupture of the shell at pressures below the set pressure of the primary relief system.

Where a tank car contains flammable materials such as petroleum products under pressure, a damaged tank car will be a potential bomb in the case of fire due to the flammable contents being under pressure. Even if the pressure relief system was activated, the contents of the tank car would be relieved only to a pressure level dictated by the relief system. While a rupture disc type of relief system guarantees the venting of the contents of the tank car to atmospheric pressure, it would be subject to rupture due to non-hazardous conditions such as overpressurizing the tank car in filling, increased tank pressure due to an increase in ambient temperature, etc. which are readily handled by conventional relief systems.

It is an object of this invention to provide an impact sensitive relief system for depressurizing a tank car in a collision and/or derailment.

It is a further object of this invention to provide pressure relieving structure which is isolated from the pressurized system by a frangible member.

It is an additional object of this invention to provide a secondary relief system for a pressurized tank car which is actuated in a collision and/or derailment and which vents the tank car to atmospheric pressure. These objects, and others as will become apparent hereinafter, are accomplished by the present invention.

Basically, the impact sensitive safety relief system of the present invention complements existing primary relief systems. A safety-relief valve or rupture disc is mounted in a blind port, and is thus isolated from the internal pressure of the tank car under normal transportation conditions. The blind port is provided with a weakened section which is frangible upon collision and/or derailment to thereby communicate the internal pressure of the tank car with the safety-relief valve or rupture disc.

For a fuller understanding of the present invention, reference should now be had to the following detailed description thereof taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a side view of a railroad tank car employing the safety-relief system of the present invention;

FIG. 2 is a sectional view showing the safety-relief system in its normal position; and

FIG. 3 is a sectional view showing the operation of the safety-relief system.

In FIG. 1, the numeral 10 generally indicates a conventional railroad tank car. The tank car 10 has been modified to the extent of locating the safety-relief system 20 of the present invention therein as an additional feature. Safety-relief system 20 has a venting capacity comparable to that of the paired conventional relief valve which is located in the center of a manway cover plate located within dome 14. As best shown in FIG. 2, the safety-relief system 20 includes a closed end cylindrical member 22 which extends through wall 12 of tank car 10 and is secured in place in a fluid-tight manner as by welding. Cylindrical member 22 has a bore 24 therein which extends to a point interior of the tank car wall 12. A preformed circumferential notch or groove 26 is formed in cylindrical member 22 at a point which corresponds to the interior of the tank car 10 and is radially outward of bore 24 and defines a frangible area. The solid portion 28 of cylindrical member 22 has a relatively high mass which is acted upon by the inertial forces developed by impact deceleration. A flange 30 is located at the open bore end of cylindrical member 22 and is integral therewith. Annular member 34 together with flange 30 and a plurality of bolts 36 coact to secure rupture disc 32 across the opening of bore 24. Line 42 communicates with bore 24 and provides a fluid path to exhaust via inspection bleed valve 44 which is a conventional manually actuated valve.

Under normal conditions, as illustrated in FIG. 2, cylindrical member 22 and rupture disc 32 are intact and bleed valve 44 is closed resulting in a closed chamber defined by bore 24 which should be at essentially atmospheric pressure. The safety relief system 20 is designed to open only if the tank car 10 has received an impact of sufficient magnitude that the structural integrity of the pressure shell or wall 12 may have been impaired. To achieve the controlled venting of the tank car 10, solid portion 28 of cylindrical member 22 has a high mass which is acted upon by the inertial forces developed by impact deceleration. The stress induced by deceleration causes the breaking of cylindrical member 22 at the preformed stress raisers defined by notch or groove 26 thereby communicating tank car pressure to bore 24 and rupture disc 32. Venting of fluid from tank car 10 will then occur if internal pressure in the tank car 10 exceeds the set-point of the rupture disc 32 as illustrated in FIG. 3. The set-point of the secondary relief system defined by rupture disc 32 is lower than that of the conventional primary system located in dome 14 to insure that catastrophic failure of a damaged tank car shell 12, which may result in a boiling liquid expanding vapor explosion and "rocketing" of car fragments, will not occur.

Control of break-off impact is obtained by varying the weight of solid portion 28, the moment arm from the groove 26 to the center of mass of solid portion 28, the depth and shape of notch or groove 26 and the material of cylindrical member 22. Since breakage of cylindrical member 22 at notch or groove 26 must be accompanied by a sufficient internal pressure in tank car 10 to rupture the rupture disc 32, it is desirable to test the integrity of cylindrical member 22 at times such as prior to loading tank car 10. Testing can be done by simply manually opening bleed valve 44 or, if desired, a pressure gage can be connected to the outlet of valve 44 prior to opening.

Although a rupture disc has been illustrated and described, a relief valve can be used but it should permit the complete depressurizing of the tank car. Direct release to the atmosphere may also be employed, if no form of pressure control is desired, by simply dispensing with the relief valve or rupture disc. While a railroad tank car has been specifically described as the area of use, the present invention may be employed wherever inertial forces acting on a pressurized system is symptomatic of a condition requiring the depressurizing of the system. Since these and other changes will occur to those skilled in the art, it is therefore intended that the scope of the present invention is to be limited only by the scope of the appended claims.