Patent Application
20190270582
The present invention relates to containers for flammable materials. More specifically, the present invention relates to systems and methods for rendering such containers, such as rail tank cars, more resistant to large, outside heat sources.
Railways have long been used in North America to transport flammable, explosive, and hazardous liquid substances such as crude oil, gasoline, liquefied natural gas, ethanol, sodium hydroxide, etc. These substances are generally transported using railway tank cars. However, in recent years the frequency and severity of tank car accidents resulting in fires or explosions has increased.
Rail accidents may cause fires that can be very hazardous to railway tank cars carrying flammable materials. Such fires may cause a combination of a weakening in the rail tank material and an increase in pressure within the rail tank car, which can lead to explosions. Specifically, rail accidents that lead to derailment may cause a fire, and heat from such a fire may transfer to the flammable material in the tank, causing the temperature of the material to rise. This temperature rise leads to increased pressure inside the tank and such an increase in pressure, combined with a weakening of the tank material, can lead to a rupture of the tank wall. Such a rupture may lead to a release of the flammable material and this can further fuels the existing fire. In some cases, derailment can cause rapid depressurization leading to an explosive vaporization of the flammable liquid, known in the industry as a Boiling Liquid Evaporating Vapor Explosion (BLEVE).
To prevent such fires and explosions, the industry uses specialized tank cars aimed at reducing the heat transfer to the liquid substance. For many decades, double-walled tank cars have been the conventional tank car used in North America to carry flammable and explosive substances.
These double-walled tank cars have an inner steel tank that contains the liquid substance and an outer steel jacket that provides protection to the inner steel tank during an accident. The inner shell and the outer jacket are separated by a gap that provides a protective distance between them when the outer jacket is deformed by impact during an accident.
In some tank cars, the gap is an air gap that provides a small measure of protection against the heat from a fire. Specifically, an air gap reduces the convective and conductive heat transfer between the inner tank and the outer jacket. However, the air gap does not provide effective protection against radiation, which is the dominant mechanism for heat transfer between the inner shell and the outer jacket. As an example, consider one of the standard rail tank cars that is approximately 118 inches in diameter with a 4 inch air gap between the inner tank and the outer jacket. In the middle of a rail accident, the rail car may be exposed to external fires that can heat the inside surface of the outer jacket to 900° C. For standard cars with such a 4 inch air gap between the inner tank and the outer jacket, a 900 degree C. inner surface of the outer jacket can cause the average heat flux across the air gap (from all heat sources) to be 73.4 kW/m2. However, convective and conductive heat transfer mechanisms only account for approximately 2.4 kW/m2 of this average heat flux. The rest of the average heat flux results from heat radiation.
One method to lower this heat flux and to thereby insulate the inner tank from the outside temperatures of a raging fire is to add insulation to the rail tank car. In some tank cars, the gap between the inner tank and the outer jacket is filled with an insulation blanket, such as a ceramic fiber blanket, that acts as an insulating layer between the inner tank and the outer jacket. Regardless of whether these insulating blankets or wraps are placed inside the gap or are on the outside of the protective jacket, they provide better protection against radiant heat transfer. However, insulation blankets can significantly increase the cost of the tank car, particularly when it comes to retrofitting existing tank cars as this requires the removal of all the jackets from a fleet of tank cars. In addition, such measures have mechanical drawbacks as insulation blankets and wraps are prone to shifting due to the heavy vibrations caused by the train's movement. As well, such blankets and wraps, again regardless of whether they are located inside the gap or outside of the protective jacket, may degrade over time. When such protective measures degrade or when insulation shifts, parts of the tank may become unprotected from radiant heat.
Based on the above, there is a need for measures that provide protection against not only conductive heat transfer and convection heat transfer, but also mainly radiation heat transfer. Preferably, such measures are cost-effective and do not degrade over time. More preferably, related measures can be used to retrofit existing rail tank cars such that radiation heat transfer is lowered or minimized.
The present invention provides systems and methods relating to the safety of railway cars carrying flammable materials. A rail tank car having an inner tank and an outer protective wall surrounding the inner tank is provided. The outer surface of the inner tank and the inner surface of the outer protective wall are separated by a gap. A low emissivity coating is applied to one or both of the outer surface of the inner tank and the inner surface of the outer protective wall. Existing rail tank cars with a similar construction can be retrofitted by applying, possibly by means of spraying, the low emissivity coating inside the gap to thereby coat at least one of the inner surface and the outer surface. Preferably, the low emissivity coating has an emissivity coefficient of, at most, 0.8.
In a first aspect, the present invention provides a container for storing at least one material, said container comprising:
In a second aspect, the present invention provides a method of rendering a container for containing flammable material safer from an outside heat source, said container having an inner tank with an outer surface, said container having an outer protective wall surrounding said inner tank, said outer protective wall having an inner surface facing said outer surface, the method comprising:
The present invention will now be described by reference to the following figures, in which identical reference numerals refer to identical elements and in which:
The figures showing the present invention are for illustrative purposes only. A skilled artisan would know that the representations in the figures are not to scale and may not accurately represent the specific placement of the specific elements they refer to.
The present invention relates to a storage container for a flammable or a hazardous material. The storage container has an inner tank and an outer protective wall with a gap between the inner tank and the outer protective wall. At least one surface inside the gap is coated with a surface coating having a low emissivity coefficient. The gap may be empty (i.e. an air gap) or it may be filled with suitable insulation. The surface coating inside the storage container's gap is designed to reduce the transfer of heat from an outside heat source (e.g. a fire) to the inner tank. By reducing the transfer of heat, the flammable material is less likely to become pressurised. In reducing the risk of pressurisation, the likelihood of BLEVE occurring is similarly reduced. The present invention further provides a method of retrofitting existing storage containers having a similar double walled structure. Such existing double-wall storage containers can be retrofitted by applying, such as by spraying, the inside of the gap with the low emissivity coating such that one or both walls inside the gap is coated.
It should be clear that hot surfaces with a low emissivity coefficient effectively reduce the emission of radiation from itself while cold surfaces with low emissivity increase the reflection of incoming radiation. Thus, for the storage container with an inner tank and an outer protective wall, by coating the inner surface of the protective wall, even if the protective wall is heated, the low emissivity coating will reduce the emission of radiant heat into the gap and toward the inner tank. Similarly, if the outer surface of the inner tank is coated with the low emissivity coating, this will increase the reflection of incoming heat from a heated protective wall. Accordingly, by providing or retrofitting double-walled storage containers with internal low-emissivity surfaces, the present invention reduces the possibility of catastrophic consequences by reducing the heat transfer from the outer protective wall to the inner tank.
The retrofitting method may be used on any suitable containers such as double-walled rail tank cars. Accordingly, any rail tank cars that conform to the US Department of Transportation specifications DOT-115 and DOT-117 can be retrofitted according to the present invention. Similarly, rail cars that conform to the Canadian Transport Canada specifications TC-115 and TC-117 can also benefit from the retrofitting method according to the present invention. These rail tank cars are either equipped with an inner tank and an outer protective wall or are configured with an outer tank that contains an inner tank (i.e., a “tank-within-a-tank” configuration).
It should be clear that the present invention is suitable for storage containers holding flammable, explosive, or hazardous materials such as oil, gasoline, crude oil, sodium hydroxide, or any other substance that is flammable or that should be kept away from radiant heat. Similarly, the present invention may be used in conjunction with fire-fighting equipment. The storage containers used to store fire-fighting substances such as fire retardant foams may be constructed with double walls having low emissivity coated internal surfaces. Such storage containers can then be more suited for firefighting purposes.
Referring to
The inner tank 20 is a container tank that is completely or substantially surrounded by the outer protective wall or jacket 30. Between the inner tank 20 and outer protective wall 30 is a gap 50. According to one aspect of the present invention, one or more of the surfaces within the gap 50 is covered by a surface coating 40. The cut-away portion of
It should be clear that the surfaces within the gap 50 are the inner surface of the outer protective wall 30 and the outer surface of the inner tank 20. One or both of these surfaces may be coated with the surface coating 40. As noted above, if the inner surface of the outer protective wall 30 is coated with the surface coating 40, the surface coating 40 would reduce the emissive transfer of heat from the outer protective wall 30 into the gap 50 should the outer protective wall 30 be exposed to an external fire. Similarly, if the outer surface of the inner tank 20 is coated with the surface coating 40, the coating 40 will reflect radiant heat from a heated outer protective wall 30. Thus, only one of the two surfaces within the gap 50 is required to be coated for the rail tank car to be rendered safer against an external heat source such as an external fire. However, maximum safety would be achieved if both of these surfaces where coated with the surface coating 40.
Regarding the surface coating 40, the coating 40 has a low emissivity coefficient at temperatures of between 500-1000 degrees C. The coating would provide radiation heat transfer protection to the inner tank 20 by either increasing the reflectivity of the inner tank against incoming radiant heat from a heated outer protective wall or by lowering the radiant heat emitted from such a heated outer protective wall. For the surface coating 40 to be effective, the emissivity coefficient must be lower than the emissivity coefficient of the material used in the construction of the inner tank 20 and of the outer protective wall 30. Since the average emissivity coefficient of carbon steel, the most common material used in tank cars 10, is approximately 0.8, a surface coating 40 with an emissivity coefficient of less than 0.8 at temperatures of between 500-1000 degrees C. would provide some radiation heat transfer protection. In a preferred embodiment, the emissivity coefficient at temperatures of between 500-1000 degrees C. of the low-emissivity coating for the relevant surfaces would be 0.1 or less.
Regarding construction, the container according to the present invention can be constructed and assembled according to well-known methods. As such, the rail tank car illustrated in the Figures may be constructed according to methods and means well-known to a person of skill in the art. However, to apply the surface coating to the surfaces within the gap, the surface coating may need to be applied prior to the final assembly of the rail car. Thus, prior to insertion of the inner tank into an assembled outer protective wall or prior to assembling the outer protective wall around the inner tank, the surface coating may be applied. As noted above, the surface coating may be applied to one or more of the inner surface of the outer protective wall and the outer surface of the inner tank. This can be accomplished using well known means for applying coatings including spraying the coating on to the surfaces or applying the coating in a manner similar to paint.
Regarding the gap 50 between the inner tank 20 and the outer protective wall 30, in conventional rail tank cars the distance between the inner tank and the outer protective wall may be approximately 4 inches. For most tank cars, this gap is an air gap, i.e. only air occupies the space between the inner tank and the outer protective wall. Calculations based on experimental data have shown that, when the low emissivity coating is used, an air gap of only 1 inch between the outer surface of the inner tank and the inner surface of the outer protective wall is most effective.
Experimental and theoretical results from calculations have shown that the best results were achieved from coating all the surfaces within the air gap and reducing the air gap to only 1 inch. These results can be seen in the table below. The following table shows the average heat flux across the air gap and into the inner tank under varying conditions. For these results, the surface coating had an emissivity coefficient of 0.1.
As shown in the table above, there is a significant reduction in heat transfer when at least one interior surface of the air gap 50 (i.e., the inner surface of the outer protective wall 30 or the outer surface of the inner tank 20) is coated with a surface coating 40. Specifically, there is an 82.0% reduction in heat transfer when only the inner surface of the outer protective wall 30 is coated with a surface coating and there is an 82.4% reduction in heat transfer when only the outer surface of the inner tank 20 is coated with a surface coating 40. However, when both surfaces were coated, regardless of whether a 4 inch air gap 50 or a 1 inch air gap 50 is used, a heat flux reduction of 89% was achieved.
As shown above, reducing the air gap distance between the inner surface of the outer protective wall and the outer surface of the inner tank has counterintuitive and surprising results. Reducing this gap from 4 inches to 1 inch similarly reduces the heat transfer from the outer protective wall to the inner tank. This reduction in distance suppresses the natural convection heat transfer. However, as the air gap distance gets narrower, the total conduction heat transfer begins to increase. Results show that an air gap 50 of approximately 1 inch provides an optimal balance between reducing convection heat transfer while maintaining low conduction heat transfer. Accordingly, a double walled tank car 10 with a 1 inch air gap 50 that has all the surfaces inside the air gap between the inner tank and the outer protective wall coated with a low emissivity surface coating 40 provides an effective heat transfer barrier.
The gap 310 may be a vacuum gap or an air gap, i.e. the gap may be air filled or a vacuum may be present in the gap. As explained above, a gap 310 reduces convective and conductive heat transfer from a heated outer protective wall to the inner tank.
To retrofit the existing tank car, the low emissivity coefficient surface coating is used to coat one or more surfaces within the gap between the inner tank and the outer protective wall. This may be done using any number of well-known methods such as spraying a liquid version of the low emissivity coating inside the gap. Of course, to gain access to the gap, holes 320 may be bored into the outer protective wall. Suitable spraying equipment can then be inserted into the holes so that the surface coating can be sprayed on to the internal surfaces of the gap. It should be clear that if holes are to be created on the outer protective wall, care should be taken to not damage the inner tank 20 as such incidental damage may prove dangerous.
As an alternative to creating holes in the outer protective wall, the outer protective wall may already have suitably placed holes or other access points so that access may be had into the gap. Should such access points exist, it is preferable that these be used instead of potentially weakening the structural integrity of the outer protective wall by adding holes to the wall. In the event such holes or other access points need to be created, these must be closed up or patched using well-known means and methods. Patches may be welded on or otherwise applied so that the structural integrity of the outer protective wall is not compromised and direct access is not provided to an external fire.
In the event such holes are necessary, the number and placement of such the holes 320 may depend on the type of surface coating 40 and on the type of surface coating applicator used. As an example, if a high viscosity surface coating 40 is used, several holes 320 may be required as the internal friction of the surface coating 40 may prevent the surface coating 40 from flowing along the interior surfaces of the air gap 50. As well, for such a high viscosity surface coating, a high pressure surface coating applicator may be necessary. However, if a low viscosity surface coating 40 is used, fewer holes may be necessary (e.g. one hole 310 per section) as the low viscosity coating may simply flow across the internal surfaces. If a high pressure surface coating applicator is used with such a low viscosity surface coating, better results may be achieved. The surface coating applicator may be any suitable type of sprayer or atomizer. Examples of such suitable devices include a hydraulic sprayer, a compressed air sprayer, a backpack sprayer, an electrostatic sprayer, a thermal fogger and a mechanical fogger. The surface coating applicator may include a hand-held gun or wand with a narrow nozzle. Such a narrow nozzle allows for the holes 320 to be small and may help in maintaining the structural integrity of the outer protective wall 30.
Once access to the gap is possible, whether through boring holes or finding a suitable access point, in one aspect of the invention, a nozzle or spraying end of the surface coating applicator is inserted one of the holes 320 or into one of the existing access points. To assist the surface coating in adhering to the surfaces inside the gap, a primer may be used. If such a primer is used, this primer is first sprayed into the gap to coat the surfaces within. Once the primer has dried, the low emissivity surface coating is then sprayed into the gap to thereby coat the surfaces within.
It should be noted that the coating may also be applied to the outside of the outer protective wall. Similarly, a silver colored coating may also be applied to the outside of the outer protective wall. Of course, such outer coatings may not be necessary and are optional.
From the above, it should be clear that the surface coating 40 may be any type of liquid or semi-liquid low-emissivity coating. In a preferred embodiment, the surface coating 40 is a low-viscosity liquid, e.g. a painted-on coating. Preferably, the surface coating 40 is low cost, has low weight, has a high temperature tolerance (up to 1,500° C.), adheres well to metal alloys such as carbon steel and is durable. One possible coating may be the LO/MIT or LO/MIT-II coatings from SOLEC-Solar Energy Corporation™. As an alternative to the above, a foil or a metallic substance that has the desired properties may be deposited on to or adhered to the surface being coated. Of course, many painted-on coatings, foils, and other types of coatings may be used with the present invention.
The description and the figures discuss and show a method of retrofitting a double-wall tank car using an inner tank and an outer protective wall. A skilled artisan would understand that the concepts of the inventive method and the inventive double-walled tank car would equally apply to a “tank-within-a-tank” style tank car. As well, the skilled artisan would understand that the method of retrofitting the double walled tank car would equally apply to any double walled structure.
A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.
This application is a non-provisional patent application which claims the benefit of U.S. Provisional Application No. 62/638,819 filed on Mar. 5, 2018 which is incorporated by reference in its entirety herein.
| Number | Date | Country | |
|---|---|---|---|
| 62638819 | Mar 2018 | US |
