Patent Publication Number: US-6655418-B1

Title: Drop tube seal for petroleum underground storage tanks

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims priority to the U.S. provisional application No. 60/243,954, filed Oct. 27, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to underground storage tanks for storing gasoline dispensed at filling stations and, more particularly, to an underground storage tank unit having a drop tube sealing system for preventing the escape of underground storage tank ullage vapors. 
     BACKGROUND 
     During the filling of underground storage tanks (UST&#39;s) for fuel, as shown in FIG. 1, a tanker truck generally delivers fuel to a fill riser  26  through a fill adaptor  20  and direct fill spill containment bucket  24 . The spill containment bucket is connected to a riser pipe  28  that extends from the top of the underground tank  30 . Fuel is delivered from the fuel tanker to the fill riser via a fuel duct that attaches to top of the fill riser. A tube  32  (herein referred to as a “drop tube”) that extends from the top of the fill riser to location below the stored fuel level  40  is used to prevent fuel entering the UST from splashing and agitating liquid fuel stored within the tank. The drop tube is concentrically disposed within the fill riser and riser pipe such that an annular channel  37  is formed between them. The annular channel routes any excess fuel that is spilled from the top of the fill riser into the containment bucket to the UST via a drain channel  39  coupled between the bucket floor and the riser pipe. A poppet drain valve  35  disposed in the containment bucket allows the excess fuel to pass through the drain channel. An O-ring seal  36  is provided between the top of the fill riser and the fill adaptor to prevent fuel vapor from rising along the annular channel to the top of the fill riser and escaping into the atmosphere. 
     Filing stations typically utilize a vapor recovery system to capture fuel vapors being displaced from a customer&#39;s tank by the fuel being added to the vehicle tank. Such systems provide a partial vacuum to draw excess fuel vapor from the customer&#39;s tank and route them back to the station&#39;s underground tanks. The captured fuel vapors cause fuel vapor pressure changes in the UST ullage space. As a result, the UST may become pressurized by the cumulative effects of the vapors produced therein, and the sloshing and agitation of the stored fuel during filling of the tank. 
     The higher pressure in the UST ullage space often causes emission of vapors through the annular channel  37  which may leak through defective seals and fittings associated with the spill containment bucket assembly. Additionally, the fumes and vapor pressure that rise from the UST within the annular passage may prevent drainage of excess fluid from the containment bucket, particularly when the fuel is warmed, such as on hot summer days. The drain valve at the containment box is effectively inoperative when excess pressure is exerted on the valve from below. Further, since the spill containment bucket may be exposed to the surface, such as when a manhole cover  22  sealing the containment bucket is removed, often dirt and other foreign matter degrade the seal in the drain valve unit. This problem is exacerbated in sites that use remote UST fills since the entire remote fill piping and spill bucket are exposed to the UST ullage vapors. 
     Attempts have been made to solve a portion of the problem. In one example, the seal between the drop tube and the riser pipe is re-located from atop the intake tube to a location along the riser pipe, just below the containment bucket. While this system has been helpful in preventing vapors from emitting to the atmosphere through the direct fill containment bucket, fuel vapors are still capable of escaping to the atmosphere through piping connected to remote fill spill bucket assemblies. 
     Thus, there is a need for a drop tube sealing assembly which helps prevent any of the UST ullage vapors from escaping out of either the direct or remote fill spill bucket assemblies. Such an assembly should be adapted to be easily installed in existing USTs. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to drop tube sealing assembly for sealing direct and remote spill containment buckets, and associated assemblies, from an underground tank ullage vapor pressure. In one embodiment, the drop tube sealing assembly includes a riser tube having a proximal end with internal threads and an underground storage tank spaced apart from the proximate end of the riser tube. The storage tank contains a threaded inlet which is positioned atop the tank. A standard pipe nipple includes a first end in cooperation with the proximal end, an opposite end in cooperation with the threaded inlet, and an annular inner surface that forms a conduit. The inner surface comprises a female thread section. A drop tube adapter fitting is concentrically disposed within the pipe nipple. The adapter fitting contains an outer surface containing external threads in cooperation with the female thread section. A seal in the form of an O-ring is disposed between the adapter fitting and the pipe nipple. A drop tube having a open end is coupled to the adapter fitting. 
     The present invention provides an improved method and device for containing underground tank ullage vapor pressure that generally enter the direct and remote spill containment buckets, and associated assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein; 
     FIG. 1 is schematic of an underground storage tank unit of the prior art; 
     FIG. 2 is a schematic of an underground storage tank unit incorporating a drop tube seal system according to one embodiment of the present invention; 
     FIG. 3 is a partial cross-section side view illustrating a drop tube seal assembly according to one embodiment of the present invention; 
     FIG. 4 is a cross-sectional side view of a standard pipe nipple of the drop tube seal assembly in FIG. 3; 
     FIG. 5 is a cross-sectional side view of a drop tube adaptor fitting and drop tube of the drop tube assembly in FIG. 3; 
     FIG. 6 is a cross-sectional side view of an installation tool coupled to the drop tube adaptor in FIG. 5; and 
     FIG. 7 is a schematic depicting an underground storage tank unit having a remote fill that incorporates a drop tube seal system according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In one embodiment of the present invention, there is provided a drop tube seal assembly for sealing direct and remote spill containment buckets, and associated assemblies, from an underground tank ullage vapor pressure. For the purpose of describing the present invention, the term “direct fill” will relate to any components directly connected to the riser pipe (i.e., riser pipe  70  in FIG. 2) extending from the UST, and the term “remote fill” will relate to any fill port or other component indirectly connected to the riser pipe extending from the UST, for example, by a pipe tee fitting. 
     As shown in FIG. 2, a drop tube seal assembly  80  is incorporated into an underground fuel storage unit  50  comprising a direct fill spill containment bucket  56  connected to a riser pipe  70  extending from the top of an underground fuel storage tank or UST  74 . The spill containment bucket is installed within a fill port or manhole  52  and includes an open top  53  for receiving gasoline. The UST in this example contains gasoline and is located directly below the spill containment bucket. A drop tube  72  concentrically disposed within the riser pipe extends into the fuel storage tank to a location below the liquid fuel level. A seal  80  is provided between the drop tube and the riser pipe at a location proximate the top of the storage fuel tank to help prevent the fuel vapors from escaping the UST and migrating into either direct or remote fuel spill bucket assemblies. 
     The spill containment bucket  56  may be of annular construction and adapted to sit within a manhole. The containment bucket may be made from fiberglass-reinforced plastic (FRP), stainless steel or any other corrosion-resistant material, and may include an open top  53  having a shoulder  55  for receiving a manhole cover  54 . The containment bucket may also include a generally slanted floor  61  for draining spilled gasoline into a trough  57 . However, it should be understood that the particular spill bucket design is not part of the present invention, which can be used advantageously with any spill bucket. Indeed, the present invention can be used with a wide variety of above ground fuel sources for underground tanks. 
     Coupled to the slanted floor  61  may be a generally cylindrical fuel intake tube  58  having an inlet end  65  upwardly extending from the floor into the containment bucket interior and an outlet end  67  downwardly extending from the floor towards the UST. The fuel intake pipe generally passes fuel from a fuel tanker truck to the riser pipe  70  through its annular interior. Instrumentation for passing gasoline from the fuel tanker to the storage tank unit, such as a fuel duct, is typically fastened to the intake tube at the inlet end. A fill cap  60  may be secured to the inlet end to seal the fuel intake tube during non-use to prevent any dirt or debris from falling into the UST. The inner surface of the outlet end may be threaded to receive a male threaded end of the riser pipe. 
     A drain valve  64  disposed in the trough  57  allows the drainage of gasoline passing from the containment bucket  56  into a fluid channel  66  in communication with the outlet end of the intake pipe. In a preferred embodiment, the drain valve is actuated by the lifting of the fill lid or manhole cover. In an alternative embodiment, the drain valve is actuated by a manually operated lever, but the drain valve may comprise any suitable mechanical valving mechanism. 
     The riser pipe  70  is a generally annular tube that extends between the outlet end  67  of the containment bucket  56  and the underground storage tank  74 . The riser pipe may be constructed from FRP, stainless steel, steel, aluminum or other suitable material. The riser pipe includes a distal end  73  that is externally threaded for engaging the internal threads of the outlet end  67 , and a proximal end  76  that is internally threaded for receiving one end of a standard pipe nipple coupled to the UST. The annular inner wall of the riser tube defines a main fuel line  71  for passing fuel from the intake tube  58  to the UST. 
     The underground storage tank or UST  74  is a generally spherical drum comprising a cavity  69  for storing fuel and a tank inlet  84  disposed at the top of the UST. The UST may be formed from FRP, steel or any other suitable material. The cavity contains fuel to a level indicated at  75 . The tank ullage contains fuel vapors at partial pressure equilibrium. The tank inlet  84  upwardly extends a short distance above the top of the UST and includes an internally threaded bore  85  (FIG. 3) for receiving an opposite end of the standard pipe nipple coupled the riser pipe  70 . 
     As shown in FIG. 3, a drop tube seal assembly  80  couples the riser pipe with UST  74 . The seal assembly generally comprises a standard pipe nipple  90 , a drop tube adaptor fitting  100 , and a drop tube  72 . As depicted in FIG. 4, the pipe nipple is a standard  4  inch NPT pipe nipple comprising generally annular central body  91  disposed between a first longitudinally extending nipple end  92  and a second longitudinally extending nipple end  93 . The pipe nipple is preferably made from ASTM A-106 {Steel}, A-53 4″SCH 120 or 160 pipe, or other suitable material. The central body preferably has a nominal outer diameter of about 4.5 inches and a nominal inner diameter preferably of about 3.6 inches. The pipe nipple preferably spans longitudinally about 5.0 inches in length. 
     The first end  92  has externally tapered threads that cooperate with the internal threads of the riser tube proximal end  76 , and similarly, the second end  93  has externally tapered threads that cooperate with the internal thread of the bore  85  at the tank inlet  84 . The annular construction of the pipe nipple defines a conduit  94  longitudinally extending through the fitting interior. 
     The first end  92  is additionally defined by a counterbore  97  preferably having dimensions of about 3.9 inches in diameter, and a female thread section  96  extending along the inner diameter of the adaptor fitting between the counterbore and the central body  91 . 
     Referring to FIG. 5, the drop tube adaptor fitting  100  is generally cylindrical in shape and extends between an upper end  101  and a lower end  102 . The adaptor fitting is preferably made from cast iron, 300 Series stainless steel, bronze or any other corrosion-resistant material. The adaptor fitting also includes an inner surface  103  and an outer surface  111 . The inner surface defines an orifice  104  whereby fluid passes from the main fluid passage  71  to the drop tube interior. The interior surface is chamfered  110  at the upper end to condition the flow of fluid passing through the orifice and minimize flow disturbances during fuel delivery events. 
     The outer surface  111  contains a land  106  at the upper end  101 . The land is generally dimensioned to mate with the counterbore  97  of the pipe nipple. A pair of installation pins  109  are disposed along the inner diameter of the land at diametrically opposed locations. The installation pins are press fit into the land  106  such that they inwardly extend into the orifice  104 . The installation pins cooperate with indexing slots of an installation tool to assemble the drop tube adaptor fitting withing the drop tube sealing system during installation. 
     The outer surface  111  further includes a machined O-ring grove  105  that is disposed between the land and an external thread portion  107 . The outer surface is machined to a reduced nominal outer diameter in an outer bond surface region  108  proximate the lower end  102 . The outer bond surface is preferably tapered with a coarse screw traced finished and machined to dimension corresponding to the interior diameter of the drop tube  72 . In preferred embodiments, the interior surface of a top portion of the drop tube is bonded to the outer surface of the drop tube adaptor fitting along the bond region. The bonding surface area between the drop tube and the adaptor fitting is preferably about 14 square inches. The adaptor fitting may be bonded to the drop tube by, for example, an epoxy adhesive. The drop tube may also be welded to the fitting. 
     As illustrated in FIG. 3, the drop tube adaptor fitting  100  can be assembled to the pipe nipple  90  by engaging the threaded portion  107  with the female thread section  96  of the first end  92 . An annular shoulder  88  (FIG. 4) defined by the counterbore  97  cooperates with an O-ring  86  which fits snugly within the O-ring groove  105  to effect a positive seal between the drop tube  72  and the riser pipe  70 . The land  101  provides an O-ring sealing surface  87  (FIG. 5) which acts to compress the O-ring against the annular shoulder when the threaded portion is further engaged with the female thread section. The O-ring is preferably made from a chemically resistant fluorocarbon elastomer, such as Viton®. The seal prevents fuel ullage vapors rising along the drop tube from escaping into the riser pipe and thereby into the direct fill containment bucket. 
     The drop tube  72  of the present invention is partially depicted in FIGS. 2,  3 ,  5  and  7 . The drop tube includes a open end  78  that is coupled to the adaptor fitting  100 , a submerged end  79  (FIGS. 2 and 7) positioned at a desired location in the UST below the fluid level  75 , and an annular wall  77  axially extending downwards from the open end to the submerged end. The drop tube may be formed from aluminum, stainless steel, and more preferably, FRP, or any other material suitable for resisting corrosion. The annular wall  77  forms a fluid passage  95  for passing fuel from the riser pipe  70  to the UST  74  and is preferably dimensioned to a nominal diameter of about 3.5 inches. 
     The drop tube  72  is preferably assembled to the drop tube adaptor fitting  100  by press fitting the lower end  102  of the adaptor fitting into the open end  78  of the drop tube  72  and then bonding or welding it in place. The adaptor fitting may be bonded to the drop tube by applying an adhesive, for example, an epoxy adhesive, to the bond surface  108 . In alternative embodiments, the drop tube may be fixed to the drop tube fitting, for example, by welding and the like. 
     The drop tube may be cut to any length. The drop tube is preferably cut to a length such that the submerged end  79  of the drop tube is sufficiently submerged below the fuel level  75 . In a preferred embodiment, the submerged end is positioned no more than 6 inches from the bottom of the tank. However, the gap between the drop tube and the tank bottom may be regulated by various agencies. The present invention can be advantageously used regardless of the particular gap selected or mandated. The drop tube is preferably cut to a length such that fuel entering the UST via the drop tube does not splash or agitate the fuel stored in the UST. As depicted in FIG. 7, a bottom strike protector  120  may be coupled to the bottom floor of the UST to prevent a dip stick used for manual tank gauging from rupturing the bottom of the UST. 
     The drop tube sealing assembly of the present invention may be easily installed into existing fuel storage tank units. In order to install the system of the present invention, first the riser pipe  70  must be disengaged from the UST threaded bore  85  of the tank inlet. This step may involve partial service station facility demolition if the fill ports are not located in an accessible containment sump, or liquid-tight compartment enclosing a turbine pump and piping connections at the top of an UST. 
     Next, a sealing compound may be applied to the threaded ends of the pipe nipple  90  before the second end  93  is engaged with the threaded bore  85  in the tank inlet  84 . In preferred embodiments, the pipe nipple cooperates with the tank inlet with the female thread portion  96  pointed upward. The riser pipe is then assembled between the first end  92  of the pipe nipple and the outlet end of the direct fill-spill bucket per manufacturers requirements. 
     Once the containment bucket and the riser pipe are assembled to the UST, the drop tube is then prepped for installation. The drop tube fitting may be bonded to the drop tube pipe per FRP manufacturers instructions. In alternative embodiments, the drop tube is pre-assembled with the drop tube fitting, thus eliminating this step. The drop tube may be cut to a length, such that the drop tube extends to a desired or mandated location below the fuel level. In alternative embodiments, a bottom strike protector may be installed to prevent the drop tube from rupturing the bottom of the UST. After the drop tube is prepped, the drop tube may be inserted into the riser pipe. Sealing compounds are preferably not be applied to the external thread portion  107  of the adaptor fitting during this step to allow adjustment and replacement of the drop tube within the riser pipe. 
     An index installation tool  150 , as depicted in FIG. 6, is used to lower the drop tube into adaptor fitting  100  and slowly thread the adaptor fitting into the conduit of the pipe nipple. The installation tool includes a generally cylindrical structure  152  that is fixed to an end of a T-bar handle  154  (partially shown). The cylindrical structure is dimensioned to be received and having a snug fit with the inner surface  102  of the adaptor fitting. The cylindrical structure includes a pair of indexing slots  156  that are machined about its outer diameter, with one of the pair of indexing slots being diametrically opposed from the other. The indexing slots are adapted to receive protruding ends of the adaptor fitting installation pins  109 . 
     As briefly mentioned above, the installation tool  150  is mated with the adaptor fitting orifice  104 , such that protruding ends of the indexing pins  109  are received by the indexing slots  156 . The snug fit between the outer surface of the cylindrical structure  152  and the adaptor fitting inner surface  102  enables the drop tube to be lowered into the riser pipe via the T-bar handle  154  fixed to the cylindrical structure. Once the threaded portion of the adaptor fitting engages the female thread portion of the pipe nipple, the T-bar may be slowly turned counter clockwise until the O-ring  86  (FIG. 3) is seated with the annular shoulder. Preferably, the fitting is tightened ¼ turn after seating the O-ring. 
     Upon installation, the installation tool may be removed from the adaptor fitting by applying upward force on the T-bar handle. 
     In an alternative embodiment, as shown in FIG. 7, the fuel storage unit  50  may include at least one remote fill spill containment bucket  130 . In accordance with this embodiment, the remote containment bucket is connected to the main fluid line  71  via fluid piping  132  and a standard pipe tee fitting  134  coupled between the riser pipe and the drop tube seal assembly  80 . The pipe tee fitting is a generally T-shaped tube having one in-line leg  136  that is connected to the riser pipe  70 , another in-line leg that is connected to the drop tube seal assembly, and a perpendicular leg  138  coupled to the fluid piping. Each of the legs are internally threaded for engaging external threads of the riser pipe, drop tube adaptor nipple and fluid piping, respectively. 
     In this embodiment, the drop tube seal assembly  80  is positioned along the main fluid line at a location below the remote fill pipe tee  134 . As such, tank ullage fuel vapors are sealed and prevented from traveling along the drop tube into the remote fill piping  132  and containment bucket  130 . 
     In a further embodiment, the riser pipe  70  is directly connected to the tank inlet  84 . In accordance with this embodiment, the proximal end  76  of the riser pipe is externally threaded in cooperation with the internal threads of the tank inlet&#39;s threaded bore  85 . An annular seat like that in the adaptor is machined along the inner diameter of the proximal end of the riser pipe. The proximal end also includes an interior female thread section that is disposed along the inner diameter juxtaposed and extending downward from the annular seat. 
     In this alternative embodiment, the drop tube adaptor fitting  100  may be assembled to the proximal end  76  of the riser pipe by engaging the threaded portion  107  of the adaptor fitting with the female thread section of the proximal end. The annular seat cooperates with an O-ring which fits snugly within the O-ring groove  105  to effect a positive seal between the drop tube  72  and the riser pipe  70 . As in the preferred embodiments described above, the land  101  provides an O-ring sealing surface  87  which acts to compress the O-ring against the annular shoulder when the threaded portion is further engaged with the female thread section. 
     In preferred embodiments of the present invention, the drop tube is described as being disposed within the riser pipe and extending from a position proximate the tank inlet into the UST. However, in some instances, the drop tube may extend within the riser pipe from a position proximate the intake pipe into the UST. According to this embodiment, the open end  78  of the drop tube is positioned proximate the inlet end  65  of the intake pipe. An O-ring grove is machined along the inner diameter of the adaptor nipple and the outer diameter of the drop tube cooperates with an O-ring which fits into the O-ring groove to effect a positive seal between the drop tube and the riser pipe. Thus, fuel vapor is effectively sealed from passing along an annular space defined between the drop tube and the riser pipe and into the direct fill containment bucket. 
     The preceding description has been presented with reference to presently preferred embodiments of the invention. Workers skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structure may be practiced without meaningfully departing from the principal, spirit and scope of this invention. 
     Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and illustrated in the accompanying drawings, but rather should be read consistent with and as support to the following claims which are to have their fullest and fair scope.