Patent Application
20090266817
1. Field of the Invention
The present invention is directed to an apparatus and a method to abate fugitive emissions from a volatile liquid storage tank. The present invention is directed to an improved floating roof assembly to abate fugitive emissions from a volatile liquid storage tank.
2. Description of the Related Art
Storage tanks are used to store volatile liquids including oil and other liquid hydrocarbons, produced brines, organic and non-organic chemicals, solvents, etc. Storage tanks may also be used to store sewage and other mixtures related to public utilities. In some storage tanks, the roof is not fixed, but instead it is adapted to be buoyantly supported by the liquid stored in the tank, and to rise and fall with the level of the liquid stored it the tank. The floating roof assembly may be buoyantly supportable within the interior of a liquids storage tank. For example, the floating roof assembly may be adapted with isolated pockets of trapped gas to make the floating roof assembly buoyant relative to the liquid in the tank, or it may comprise a roof structure that is made buoyant using pontoons or other buoyant members that may be secured to the roof structure and made a part of the floating roof assembly. A floating roof assembly may be buoyantly positionable by control of the volume of volatile liquid stored within the tank.
Volatile liquids may be introduced into and withdrawn from the storage tank to vary the elevation of the floating roof assembly within the tank. One or more pipes pipe may penetrate the wall of the storage tank, preferably near the bottom, or the one or more pipes may penetrate the floor of the storage tank, to introduce fluid into or to withdraw fluid from the storage tank.
A floating roof assembly may comprise a plurality of adjustable and deployable legs to prevent downward movement of the floating roof assembly beyond a certain height within the tank. For example, but not by way of limitation, the floating roof assembly may continue to descend with falling level of liquid in the tank until the feet located at the bottoms of the plurality of legs come to rest on the floor of the storage tank. In one embodiment, the plurality of legs may extend through the floating roof to prevent the roof from resting on or hitting the floor of the tank, or moving beyond a critical level beneath which the stored liquid is supplied to the tank or withdrawn from the tank. The plurality of support legs may be slidably adjustable in a substantially vertical orientation at right angles to the generally horizontal structure of the floating roof to provide a variable height at which the downward travel of the roof may be stopped. A seal may be used at each adjustable support leg penetration to prevent or abate fugitive gas emissions through or around each leg.
Floating roof assemblies may be used to generally isolate the contents of the liquids storage tank from the atmosphere. A conventional floating roof assembly may comprise one or more peripheral seals to obstruct evaporating gasses from escaping the tank and entering the atmosphere, but it is difficult to seal a conventional floating roof assembly against the interior of a tank that may exceed, for example, 100 meters in diameter. It should be appreciated by those skilled in the art that sealing an internal structure to prevent gas migration around such a large interface (the periphery of the floating roof assembly and the interior bore of the storage tank) is a challenge. The large size of the tank and the movable floating roof assembly makes machining close tolerances prohibitively expensive, and factors such as ambient wind direction and speed, ambient and stored liquid temperature, and uneven solar heating make conventional seals unusable on large floating roof-equipped tanks.
Some conventional floating roof assemblies have peripheral seals comprising gas-impermeable fabric skirts supported by framing on the roof structure. These peripheral seals generally slow and reduce, rather than substantially prevent, fugitive gas emissions.
What is needed is a method of abating fugitive emissions from a volume of stored volatile liquid within a large floating roof storage tank. What is needed is a floating roof assembly for a volatile liquid storage tank with an improved seal to abate fugitive emissions from the liquid stored in the tank. An apparatus and a method are needed to capture fugitive emissions prior to escape from a volatile liquids storage tank so that the fugitive emissions can be directed to a processing apparatus, such as a vessel, a condensation process or an incinerator.
The present invention satisfies one or more of the above-stated needs. This invention provides a fugitive emissions abatement apparatus and method. This invention provides a floating roof assembly to abate fugitive emissions from a volatile liquid storage tank. The present invention also provides, in one embodiment, a method for abating fugitive emissions from the periphery of a floating roof assembly installed within a volatile liquids storage tank by capturing and processing the fugitive emissions. The present invention may also provide, in another embodiment, an apparatus to capture fugitive emissions from the perimeter of a floating roof assembly installed within a liquids storage tank, and to direct the captured gas to a processing apparatus.
In one embodiment, a method comprises the steps of forming an annular gas chamber about a floating roof assembly to generally trap a volume of fugitive emissions within the annular gas chamber, and disposing a first end of a fluid conduit, such as a hose or a pipe, in fluid communication with the annular gas chamber. The method may further comprise the step of disposing a second end of the fluid conduit in fluid communication with an inlet to a gas mover, such as, but not limited to, a positive displacement compressor and/or a centrifugal compressor, such as, for example, a rotary fan. The method may further comprise the step of disposing an outlet of the gas mover in fluid communication with a fugitive gas processing apparatus including, but not limited to, a filter, an incinerator, a heat exchanger or a gas treating vessel, such as an absorption vessel containing a solid and/or liquid absorber.
In one embodiment, the floating roof assembly of the present invention may comprise a peripheral seals disposed about a roof structure to define an annular gas chamber, one or more apertures within the floating roof assembly in fluid communication with the annular gas chamber, and one or more fluid conduits, each having a first end and a second end, and each coupled at the first end to the one or more apertures and coupled at the second end to the inlet of one or more gas movers. In an optional embodiment, the apparatus may comprise a header having a plurality of inlets and at least one outlet, and the header may serve to aggregate a plurality of streams of fugitive emissions delivered to the header from a plurality of fluid conduits that may be coupled at their second ends to the plurality of inlets of the header. The header may be coupled at its outlet to a trunk line to the inlet of a gas mover, and the gas mover may have an outlet to deliver the aggregated fugitive emissions to a processing apparatus such as, but not limited to, a filter, an incinerator, a heat exchanger or a gas absorption vessel.
In an alternate embodiment, the floating roof assembly of the present invention may comprise a plurality of peripheral seals disposed about the roof structure to define a plurality of annular gas chambers, one or more apertures within the floating roof assembly, each aperture in fluid communication with at least one annular gas chamber, and one or more fluid conduits, each having a first end and a second end, and each coupled at the first end to one or more of the apertures and at the second end to an inlet of one or more gas movers. Optionally, one or more valves or other flow restrictors may be used to strategically distribute or allocate gas flow through a plurality of fluid conduits to impart a pressure differential between two or more annular gas chambers. In one embodiment, one or more of the valves may be coupled to an actuator for automatically positioning the valve and for automatically adjusting the rate of withdrawal from at least a portion of the annular gas chamber according to a sensed condition. In one embodiment, the sensed condition may be a differential pressure measured as the difference in pressure between the first annular gas chamber and one of the atmosphere or a second annular gas chamber adjacent to the first annular gas chamber. In an alternate embodiment, the sensed condition may be a concentration of a compound, such as oxygen, nitrogen, or a volatile compound emitted as a vapor from the volatile liquid being stored in the liquids storage tank.
So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. However, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
In one embodiment of the apparatus of the present invention comprises a floating roof assembly comprising a generally disk-shaped buoyant roof structure having a first peripheral seal disposed about its periphery to engage the interior wall of a volatile liquid storage tank and to define an annular gas chamber about the roof structure and generally below the first peripheral seal. The embodiment further comprises at least one aperture penetrating a component of the floating roof tank assembly that is coupled to a first end of at least one fluid conduit. A second end of the at least one fluid conduit is coupled to the inlet to a gas mover, such as a positive displacement compressor or a centrifugal compressor, such as a fan, to continuously withdraw gas from the annular gas chamber.
As fugitive gas is continuously withdrawn from the annular gas chamber, a small vacuum, preferably less than about 0.15 inches of water, is imparted to the annular gas chamber as a result of the continuous withdrawal of gas. The vacuum resulting from the continuing withdrawal of gas from the annular gas chamber may cause atmospheric air to enter the annular gas chamber at locations along the first peripheral seal that may be separated from or inadequately engaging the interior wall of the liquids storage tank. As a result, the gas withdrawn from the annular gas chamber, and routed to the inlet of the gas mover, may be a mixture of air and volatile gas, and may be explosive. Preferably, the gas mover used to draw the vacuum on the annular gas chamber and to route the withdrawn gas to a processing apparatus is of a non-sparking design with an explosion proof driver, and is designed to isolate or prevent sparks or other sources of ignition from the explosive mixture.
In a preferred embodiment, a plurality of apertures penetrate the floating roof assembly, and each aperture is coupled to the first end of a fluid conduit, such as a hose or pipe, that is coupled at its second end to the inlet of the gas mover. As a result, gas may be withdrawn from a plurality of apertures that may be angularly distributed along the periphery of the floating roof assembly. This preferred embodiment may provide a more uniform distribution of the vacuum in the annular gas chamber, and an overall improved abatement of the fugitive emissions escaping the floating roof assembly and entering the atmosphere. It should be understood, however, that localized seal deficiencies may make it more advantageous to unevenly distribute the vacuum to withdraw more gas from portions of the annular gas chamber adjacent to portions of the peripheral seal where the peripheral seal performs poorly, and to withdraw less gas from portions of the annular gas chamber adjacent to portions of the peripheral seal where the peripheral seal performs well. This uneven distribution may be implemented using adjustable valves disposed in fluid communication with one or more of the fluid conduits in embodiments of the present invention comprising a plurality of fluid conduits. It should be understood that, using a gas mover with an inlet coupled to a plurality of fluid conduits, closing a given adjustable valve to constrict and decrease flow through one fluid conduit may cause an increased rate of flow through one or more of the remaining gas conduits that are also in fluid communication with the inlet to the gas mover.
The present invention also provides a method of monitoring and assessing performance of one or more portions of the peripheral seal disposed about the periphery of the floating roof assembly. In one embodiment, a sample of gas may be taken from a first fluid conduit withdrawing gas from a first portion of the annular gas chamber disposed about the periphery of the floating roof assembly, and a second sample of gas may be taken from a second fluid conduit withdrawing gas from a second portion of the annular gas chamber, and the two gas samples may be analyzed, for example, but not by way of limitation, using chromatographic analysis. The chromatographic analysis may reveal, for example, that the first gas sample representative of the composition of gas within the first portion of the annular gas chamber contains a substantially greater oxygen and/or nitrogen content than the second gas sample representative of the composition of gas within the second portion of the annular gas chamber. These results would indicate that the portion of the first peripheral seal adjacent to the first portion of the annular gas chamber performs poorly compared to the portion of the first peripheral seal adjacent to the second portion of the annular gas chamber. Remedial repairs, modifications or further investigations or tests may be made or performed as a result of these findings in order to repair, modify or adjust the first peripheral seal and to thereby abate the release of fugitive emissions into the atmosphere.
In an alternative embodiment, an apparatus may comprise a floating roof assembly comprising a generally disk-shaped buoyant roof structure having a first peripheral seal disposed about its periphery to engage the interior wall of a volatile liquid storage tank and a second peripheral seal disposed about its periphery to engage the interior wall of the volatile liquid storage tank in a spaced-apart relationship from the first peripheral seal to define a first annular gas chamber there between. This dual-seal arrangement may also define a second annular gas chamber between the second peripheral seal and the surface of the liquid. The alternative embodiment may further comprise one or more apertures penetrating the floating roof assembly to facilitate the withdrawal of gas from the first annular gas chamber. In yet another alternative, the dual-seal arrangement floating roof assembly may comprise one or more apertures penetrating the floating roof assembly to facilitate withdrawal of gas from the second annular gas chamber. In yet another alternative embodiment, there may be two or more apertures penetrating the floating roof assembly, one or more to facilitate the withdrawal of gas from the first annular gas chamber, and one or more to facilitate the withdrawal of gas from the second annular gas chamber.
An embodiment of the apparatus of the present invention may comprise a gas mover having an inlet and an outlet. The gas mover may include, for example, one or more positive displacement compressors and/or one or more centrifugal compressors, such as a rotary fan. A plurality of gas movers may be connected in series or in parallel, depending on the volume and the head needed to move the fugitive emissions to the processing apparatus. The inlet(s) to the gas mover(s) may be fluidically coupled through at least one fluid conduit to the at least one aperture that provides fluid communication with an annular gas chamber. The inlet, when placed in communication with the at least one annular gas chamber through a fluid conduit, will draw gas from one or more annular gas chambers. It should be understood that two or more gas movers may be used to obtain the targeted withdrawal rate(s) and/or vacuum(s) within the one or more annular gas chambers. It should further be understood that the use of one or more gas movers may enable the operator to maintain a desired imbalance or balance between any two of the atmosphere and the one or more annular gas chambers. For example, but not be way of limitation, an operator may wish to abate fugitive emissions and also to ensure that air does not contaminate the liquid product. An operator may utilize a dual-seal arrangement as described above for the floating roof assembly to form two generally adjacent annular gas chambers, and to further ensure that a generally continuous pressure differential from the second annular gas chamber defined between the second seal and the liquid surface to the first annular gas chamber between the first seal and the second seal. The operator may use valves, for example, to modulate or limit the rate of gas drawn from the second annular gas chamber while drawing more gas from the first annular gas chamber, and thereby provide a net pressure differential from the second annular gas chamber to the first annular gas chamber to prevent gas from the first annular gas chamber, which may contain more air, from flowing to the second annular gas chamber where it can contact the surface of the liquid stored in the tank. It should be understood that a reversed pressure differential may be established and maintained in the direction from the first annular gas chamber to the second annular gas chamber where, for example, an operator desires to prevent the release of fugitive emissions to the atmosphere, as a reversed pressure differential from the first annular gas chamber to the second annular gas chamber will better ensure that fugitive emissions from the liquid stored in the tank are better drawn into the fugitive emissions abatement system of the present invention rather than entering the first annular gas chamber which may be separated from the atmosphere only by the first peripheral seal.
It should be understood that the structures that comprise the first peripheral seal and the second peripheral seal may vary. It is preferred that the structures that comprise these gas barriers are generally flexible, as opposed to being rigid and unbendable, so that the first peripheral seal and/or the second peripheral seal may conform to the shape of the interior wall of the liquid storage tank. It may also be preferred, in some applications, that the structures comprise materials that will not generate sparks or otherwise ignite a combustible gas mixture. For example, but not by way of limitation, portions of the first peripheral seal and/or the second peripheral seal that may rub against a metal interior wall of the tank may comprise a non-sparking material, such as fiberglass, rubber, plastic, or bronze and some aluminum alloys. Alternately, a non-sparking coating may be used to suppress sparks.
In one embodiment, the first peripheral seal and/or the second peripheral seal may comprise a skirt portion and a lip portion, and the skirt portion may comprise a gas impermeable barrier such as sheet metal, fiberglass, plastic or other material. In an alternate embodiment, the skirt may comprise a permeable and flexible material, such as expanded metal, with a gas impermeable liner, such as rubber, vinyl or plastic. A lip may be disposed along the portion of the first peripheral seal and/or the second peripheral seal that contacts the interior wall of the tank, and the lip may be of a flexible and resilient material, such as rubber. In one embodiment, the lip is releasably securable to the skirt.
The floating roof assembly 10 may further comprise one or more stabilizers 40 to maintain a desired minimal stand-off between the roof structure 12 and the tank 4. The stabilizer 40 may comprise a roller 42 that is pivotal on a pin 44 to provide smooth relative movement between the roof structure 12 and the interior wall 4A of the tank 4. In one embodiment, the roller 42 may be extendable from the stabilizer 40 to provide adjustable positioning of the roof structure 12.
The floating roof assembly 10 of
The floating roof assembly 10 shown in
As shown in
As described above, the floating roof assembly 10 of the present invention may be disposed in a very large tank, and the first peripheral seal 70 may comprise numerous seal sections 74 (see
Factors such as material seal degradation, ambient wind or temperature, solar heating or wear may cause certain seal sections 74A to fail or to underperform relative to adjacent or other seal sections 74A. In one embodiment of the method, comparison of gas samples obtained using the sampling ports 80A may indicate poorly performing portions or sections of the first peripheral seal or other peripheral seals to be discussed below in connection with
The first peripheral seal 70 and the second peripheral seal 71 define a first annular gas chamber 60 there between. Like the first peripheral seal 70, the second peripheral seal 71 may be coupled to or disposed on the roof structure 12 to seal the periphery of the roof structure 12 against the interior wall 4A of the tank 4. In the embodiment shown in
The second peripheral seal 71 may, like the first peripheral seal 70, comprise a skirt that may be formed of a thin and flexible material, such as sheet metal, plastic or vinyl. In the embodiment of the second peripheral seal 71 shown in
Returning to
It should be understood that the valves 83 may be actuated valves that are positionable using a valve actuator. In one embodiment, one or more of the valves may be actuated using an actuator that positions the valve in response to a signal generated by an instrument, such as a differential pressure transmitter, a pressure sensor, a gas chromatograph or a temperature sensor. In one embodiment, a valve actuator comprising, but not limited to, an electrically, pneumatically, or hydraulically operated valve, may be used to automatically adjust the rate of gas withdrawal from one or more portions of a first annular gas chamber or a second annular gas chamber, or both. In one embodiment, one or more of the valves may be coupled to an actuator for automatically positioning the valve and for automatically adjusting the rate of withdrawal from at least a portion of the annular gas chamber according to a sensed condition. In one embodiment, the sensed condition may be a differential pressure measured as the difference in pressure between the first annular gas chamber and the atmosphere. In an alternate embodiment, the sensed condition may be a differential pressure measured as the difference in pressure between the first annular gas chamber and a second annular gas chamber adjacent to the first annular gas chamber. In another alternate embodiment, the sensed condition may be a concentration of a compound, such as oxygen, nitrogen, or a volatile compound emitted as a vapor from the volatile liquid being stored in the liquids storage tank. It should be understood that, while differential pressure transmitters, gas chromatographs, pressure sensors and temperatures sensors may be used, standing alone or in combination, to generate signals to automatically control the gas withdrawal rates using automatically positionable actuated valves, the valves may be manually adjusted based on the sensed conditions using these same instruments, and a lack of automation is still within the scope of the claims.
It should be understood that the pressurized and aggregated fugitive emissions discharged from the outlet of the gas mover may flow to an apparatus to recover, incinerate or process the fugitive emissions in a manner that is more substantially less environmentally damaging compared to the release of unprocessed fugitive emissions from the tank. It should be further understood that a variety of structures may be used to create one or more annular gas chambers, and to fluidically couple the annular gas chamber(s) to a gas mover. It should be understood that the annular gas chambers may be divided or segregated, or combined or aggregated to form smaller or a larger chamber without departure from the spirit and purpose of the invention, and without diminishing the effectiveness of the invention.
The terms “comprising,” “including,” and “having,” as used in the claims and specification herein, shall be considered as indicating an open group that may include other elements not specified. The terms “a,” “an,” and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The term “one” or “single” may be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as “two,” may be used when a specific number of things is intended. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.
While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.
