Patent Publication Number: US-2012037243-A1

Title: Pressure Equalization Assembly for a Storage Vessel

Description:
RELATED APPLICATION 
     This application is a continuation-in-part to U.S. patent application Ser. No. 12/489,886 filed Jun. 23, 2009, which makes a claim of domestic priority under U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/170,442 filed Apr. 17, 2009. 
    
    
     BACKGROUND 
     It is common to collect and store multi-phase fluids, such as liquid and gas phases, in storage vessels or tanks. Such fluids can include oil and other hydrocarbon based fluids, water (fresh or brine), hazardous chemicals and the like. Storage vessels can be buried underground, such as underground fuel storage tanks used in automotive service stations, or they can be located above ground, such as storage tanks used in drilling and refining operations in the oil and gas industry and storage tanks for drinking water in municipal water supply systems. 
     Storage vessels vary widely in size, and are often sized to accommodate thousands and even millions of liquid gallons. Such vessels can be designed to be open or closed systems, with closed vessels used to store various types of pressurized fluids, such as liquefied natural gas (LNG) or other volatile fluids. It is often necessary that closed system vessels be sealed from the ambient atmosphere to maintain the contents under pressure and hence usually in a liquid state. 
     Open vessels, which maintain communication with some source of reference pressure, usually the ambient atmosphere, are susceptible to migration of environmental pollutants, so such vessels may be provided with screened vents or other mechanisms to allow equalization of the vessel&#39;s vapor space with the reference pressure source. For example, it may be desirable to admit an outside fluid, such as atmospheric air to the vessel vapor space as liquid is pumped or gravity fed from the vessel, and it may be desirable to release pressure from the vapor space as additional liquid is accumulated. 
     Ambient conditions can alter the interior pressure of a storage vessel, such as, for example, during a hot summer day solar heating of the storage vessel can substantially increase the internal pressure of the vessel as compared to the ambient atmospheric pressure. 
     Failure to maintain the interior vapor space of a storage vessel within some acceptable pressure range can result in a number of problems, such as reduced fluid flow as efforts are made to transfer liquid to or from the vessel. In some extreme cases, a significant pressure differential may even result in structural damage to a vessel. 
     At the same time, there are a number of reasons why it may not be desirable to maintain continuous venting of a liquid storage vessel to the surrounding atmosphere. For example, a continuously open vent, even if screened, can admit debris or other substances from the external environment, thereby introducing undesirable contaminants to the stored liquid. Thus, it is sometimes required that the vapor pressure be maintained with reference to a reference pressure source, such as an inert blanket gas. 
     Similarly, evaporated vapors or fumes from the stored liquid, such as water vapor or volatile hydrocarbons, may pass through a continuously open vent to the surrounding atmosphere at an unacceptable rate. This can lead to an undesired loss of product or, in some cases, unacceptable levels of environmental emissions. 
     SUMMARY 
     Various embodiments are generally directed to an apparatus for equalizing pressure within a vapor space of a storage vessel. 
     In accordance with some embodiments, a pressure equalization assembly generally includes a base housing member adapted to be mounted to a tank adjacent a tank aperture in fluidic communication with a vapor space of the tank. A canister assembly is adapted for removable engagement with the housing member between a closed position and an open position. The closed position establishes a fluidic seal interface between the housing member and the canister assembly. The open position provides user access to the vapor space through the tank aperture. 
     The canister assembly further has a compound seal assembly with first and second pistons biased against corresponding first and second seal members while the canister assembly is in both the closed position and the open position. A third annular seal member is disposed between the canister assembly and the base housing member to establish the fluidic seal interface. 
     Other features and advantages of the various embodiments disclosed herein will become apparent when the following detailed description is read in conjunction with the drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevational, partially cutaway view of an exemplary storage tank having mounted thereon a pressure equalization assembly constructed in accordance with various embodiments. 
         FIG. 2  is an elevational view of the pressure equalization assembly of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the pressure equalization assembly of  FIG. 2 . 
         FIG. 4  is an enlarged cross-sectional view of a portion of the vapor equalization assembly of  FIG. 3 . 
         FIG. 5  illustrates selected portions of the assembly of  FIGS. 2-4  in a vacuum relief mode of operation. 
         FIG. 6  illustrates selected portion of the assembly of  FIGS. 2-4  in an overpressure relief mode of operation. 
         FIG. 7  is an exploded elevational representation of the assembly of  FIGS. 2-4 . 
         FIG. 8  shows an alternative pressure equalization assembly in accordance with various embodiments, the embodiment of  FIG. 8  adapted for use to cover a thief hatch access aperture of a liquid storage tank. 
         FIG. 8A  is an enlarged view of a portion of  FIG. 8 . 
         FIG. 9  shows the assembly of  FIG. 8  in an open position. 
         FIG. 10  illustrates a calibration operation that may be carried out in accordance with various embodiments. 
     
    
    
     DETAILED DISCUSSION  
     Referring to the drawings in general and  FIG. 1  in particular, shown therein is a storage system  100  constructed in accordance with various embodiments. The storage system  100  includes an exemplary storage vessel  102  adapted to hold a liquid  104  above which is a vapor space  106 . Due to the normally sealed nature of the storage vessel  102 , evaporated vapors or fumes from the liquid  104  accumulate in the vapor space  106 . For purposes of an explicit example, the liquid  104  is contemplated as a non-pressurized hydrocarbon liquid, such as crude oil or similar such liquid. Other liquids, of course, especially other hydrocarbon liquids, could as well be stored in the storage vessel  102 . As will be appreciated, a number of factors may result in changes in the pressure of the vapor space  106  over time. Environmental cycling, such as thermal heating and cooling effects, may result in significant changes in the internal pressure of the vapor space  106  within the tank  102 . The pressure of the vapor space  106  may also vary with changes in the amount of liquid  104  in the storage vessel  102 , generally rising as additional liquid is introduced into the vessel, and generally decreasing as liquid is removed. Fluid transfer to and from the vessel  102  is through a transfer conduit  108 . 
     A pressure equalization assembly  110  is mounted on the storage vessel  102  in fluid communication with the vapor space  106 . As described below, the pressure equalization assembly  110  normally operates in a closed mode, that is, in the mode that seals the vapor space  106  to prevent harmful pollutants from being emitted from the vessel  102  into the surrounding environment. When the pressure of the vapor space  106  falls outside a predetermined pressure range, the pressure equalization assembly  110  assumes its open mode, converting the storage vessel  102  to an open system and connecting the vapor space  106  with an outside reference fluid source, which in the present embodiment is the surrounding atmosphere. 
     The aforementioned predetermined pressure range is defined as a pressure range bounded by an upper pressure value and a lower pressure value, and when the vessel pressure exceeds the upper pressure value, the pressure equalization assembly  110  opens to discharge vapors from the vapor space  106 , such as to the atmosphere. Once the pressure in the vapor space  106  has been reduced to equal the upper pressure value, the pressure equalization assembly  110  transitions the storage system  100  back to the normally closed mode. 
     Similarly, should the pressure in the vapor space  106  falls below the lower pressure value of the pressure range, the pressure equalization assembly  110  opens to allow a reference pressure fluid, in the present embodiment, atmospheric air, into the vapor space  106  until the pressure reaches the lower pressure value, at which time the pressure equalization assembly  110  transitions the storage system  100  back to the normally closed mode. 
     While the pressure equalization assembly  110  is shown in  FIG. 1  as mounted to a top plate  112  of the storage vessel  102 , it will be understood that any suitable mounting location on the vessel can be used, so long as the pressure equalization assembly  110  is maintained in fluid communication with the vapor space  106 . 
     As shown in  FIGS. 2 and 3 , the pressure equalization assembly  110  includes an underpressure (that is, a vacuum) relief mechanism and an overpressure relief mechanism, described herein below, that are disposed within a support canister, or upper housing,  114 . The support canister  114  is generally a hollow cylindrical member that is supported by a base housing member (body member)  116 ; the support canister  114  and body member  116  together provide a housing. The body member  116  has a bolting flange  118  at its lower portion. The bolting flange  118  has a plurality of bolt holes  120  ( FIG. 3 ) through which bolts (not shown) extend to engage threaded holes in the top plate  112  of the storage vessel  102 . The treaded holes in the top plate  112  circumscribe an entry port (not shown) that communicates with the storage vessel  102  so that the body member  116  of the pressure equalization assembly  110  will be in fluid communication with the vapor space  106  when mounted to the top plate  112 . 
     The body member  116  has an internal passageway  122  extending the length of thereof, and the support canister  114  is supported on the upper portion of the body member  116  by a circularly extending projecting flange  114 A. A lower portion  114 B of the support canister  114  extends downwardly into the internal passageway  122 , and an upper portion  114 C thereof projects upwardly from the body member  116 . An elastomeric sealing member  115  is disposed within an annular channel in the flange  114 A to sealingly engage the body member  116 . 
     A hooding cap  124  fits over the upper end of the upper portion  114 C, thereby substantially closing the upper end of the passageway  122 . The hooding cap  124  is connected to the body member  116  by a pair of mounting bolts  126  connected to the external wall of the body member  116  and that extend through holes or slots (not separately numbered) in the hooding cap  124  and secured thereby with hand tightened wing nuts  130 . 
     Each of the bolts  126  has a cross pin member  132  ( FIG. 2 ) at its lower end, the pin member  132  of each bolt  126  pivotally supported in axially aligned bores in a pair of parallel tab member  134  attached on opposing sides of the outer surface of the body member  116 . The pin members  132  are secured in the bores of the tab members  132  by locking clips  136  pressed into appropriately located locking grooves in the pin members  132  in the manner shown. With the bolts  126  loosened and pivoted aside, the hooding cap  124  can be removed to provide access to, and possibly removal of, the support canister  114 . 
     The hooding cap  124  is a weather guard having an annular cap flange member  140  that extends about and is spaced from the outer surface of the upper housing  114 C. The upper housing  114 C has a several spaced apart vent holes  142  that are spatially disposed about the housing  114 C near its top portion and arranged to be partially hooded by the annular flange  140 . As necessary, a wire mesh screen member  144  or the like is positioned to cover the vent holes  142  to prevent debris from entering the internal passageway  122 . 
     A first piston assembly  150  normally seals the vapor space  106  from the surrounding atmosphere by engaging an annular seat  152  that is characterized as an elastomeric O-ring that is supported by the lower portion  114 B, although other sealing configurations can be used as desired. The piston assembly  150  has a piston guide  154  that has a lower portion  156  with a longitudinally extending bore  158 . The piston guide  154  has a cylindrically shaped upper portion  160  that is disposed within a cavity  162  of the hooding cap  124 , as shown. The piston guide upper portion  160  has a pressure equalizing bore  164  extending therethrough and in fluid communication with the bore  158 . 
     A second piston assembly  165 , serving to determine the lower pressure limit of the predetermined pressure range, has a cylindrically shaped shuttle member  166  having a central bore  168  extending there through. The central bore  168  has a large diameter at the lower end of the piston shuttle member  166  and a threaded smaller diameter at the upper end thereof. The outer diameter of the piston shuttle member  166  is dimensioned so that it is slidably disposed for up and down movement in the piston guide bore  158 . 
     A piston shaft, or rod,  170  has a threaded upper end that is engaged with the threaded smaller diameter portion of the central bore  168 , as shown. The distal lower end of the piston shaft  170  connects to a compound seal assembly  172  that serves to seal the vapor space  106 . A compound seal assembly includes a disk shaped over pressure seal member (piston)  174  and a disk shaped under pressure seal member (piston)  176  that together seal the vapor space  106  when the pressure of the vapor space  106  is within the predetermined pressure range mentioned above. The over pressure seal member  174  has an annular central hub  178  through which the lower end of the piston shaft  170  extends, the lower end of the piston shaft  170  threaded for engagement with a centrally disposed bore (not separately numbered) in the under pressure seal member  176 . 
     The over pressure seal member  174  has a plurality of fluid flow holes  180  spaced about and surrounding the central hub  178  to form an outer ring shaped rim portion  182  joined to the central hub  178  by the webbing between the fluid flow holes  180 . The fluid flow holes  180  are sealed by the upper surface of the under pressure seal member  176  when held in contact with the upper pressure seal member  174 . An outer sealing surface  184  of the rim portion  182  is beveled to mate with a beveled annular shoulder portion  186  formed by the lower portion  114 B of the support canister  114 . The above mentioned annular seat  152  is supported in a groove in the shoulder portion  186  to abut and seal against the outer sealing surface  184  of the rim  182 . 
     The rim  182  of the over pressure seal member  174  has a beveled inner surface  188  that mates with a beveled surface of the under pressure seal member  176 , and an O-ring member  190  is supported in a groove in the inner surface  188  to abut and seal against the beveled surface of the under pressure seal member  176 . 
     Returning to the piston guide  154 , connected thereto is an upwardly extending threaded adjusting rod member  192  that has a lower end threadingly engaged with a threaded bore (not separately numbered) in the piston guide upper portion  160 . The adjusting rod member  192  extends upwardly through a bore (not separately numbered) in the hooding cap  124 , and an adjusting nut  194 . As will become clear below, the position of the adjusting nut  194  on the adjusting rod member  192  determines the placement of the piston guide  154  relative to the hooding cap  124 . As desired, a protective cover  196  can be provided over the protruding portion of the adjusting rod member  192 . 
     A helical first spring  200  surrounds the piston assembly  150  and is compressed between the piston guide upper portion  160  and the over pressure seal member  174  of the compound seal assembly  172 . A helical second spring  202  surrounds the piston shaft  170  and disposed in compression between the piston shuttle member  166  and the central hub  178  of the over pressure seal member  174 . The compression (biasing force) of the first spring  200  is set to correspond to the upper pressure value of the predetermined pressure range and the second spring  202  is set to match the lower pressure value of the predetermined pressure range. 
     The pressure from the vapor space  106  within the storage vessel will apply an upwardly directed force upon the compound seal assembly  172 , and this upwardly directed force will be countered by the force of the first spring  200 . The amount of the upwardly directed force will be determined in relation to the pressure of the vapor space  106  and the areal extent of the downwardly facing surface area of the compound seal assembly  172  exposed to the vapor space. This upwardly directed force will be countered by a downwardly directed force by the pressure of the surrounding atmosphere upon the areal extent of the upwardly facing surface of the compound seal assembly  172  as communicated there against via the vent holes  142  in the support canister  114 . 
     When the pressure of the vapor space  106  falls below the lower pressure limit of the predetermined pressure range, the under pressure seal member  176  will be pulled downwardly, separating from the over pressure seal member  174  to open the fluid flow holes  180  and establish communication with the vapor space  106 . Such vacuum relief is generally depicted in  FIG. 5 . 
     So long as the pressure of the vapor space  106  remains within the predetermined pressure range, the first and second springs  200 ,  201  will keep the compound seal assembly together and sealing the vapor space from communication to the surrounding atmosphere. That is, the vapor pressure, countering the force of the second spring  202 , will push the under pressure seal member  176  upwardly against the upper pressure seal member  176  to close the fluid flow holes  180 . The first spring  200  presses against the compound seal assembly  172  so that the upper pressure seal member is pressed into sealing engagement against the annular seat  152 . 
     At such time that the pressure within the vessel space exceeds the upper threshold of the predetermined pressure range, both seal members (pistons)  174 ,  176  will move upwardly, allowing fluid to pass from the vapor space into the interior of the assembly  110  and out to the surrounding air atmosphere. Such overpressure relief is generally depicted in  FIG. 6 . 
     Moving the adjusting nut  194  on the adjusting rod member  192  advances or retracts the piston guide  154  into the support canister  114 , thus increasing or decreasing the compressive force exerted by the first spring  200  on the compound seal assembly  172 , and thereby adjusting the force necessary in the vapor space  106  to move the compound seal assembly  172  away from the O-ring seat  152  (see  FIG. 6 ). 
     The extension of the piston shaft  170  downwardly from the piston shuttle member  166  is adjustable by rotating to increase or decrease the depth of penetration of the upper threaded end of the piston shaft  170  with the treaded bore of the piston shuttle member  166 , thereby increasing or decreasing the force on the second spring  202 . This increases or decreases the negative pressure required on the under pressure seal member  172  to separate it from the over pressure seal member  174  to expose the fluid flow holes  180  to fluid communication with the vapor space  106 , permitting entry of atmospheric air until the pressure in the vapor space is increased to that of the surrounding atmosphere, at which time the vapor pressure on the under pressure seal member  172  will counter the force of the second spring  202  to again seal the fluid flow holes  180 . 
     From the above, it will be appreciated how the pressure equalization assembly  110  stabilizes pressure of the vapor space  106  to remain within the predetermined pressure range. When a sufficient volume of vapor from the vapor space  106  has been vented to reduce the vapor space/exterior pressure differential to a resulting force that is less than the biasing force of the first spring  200 , the piston assembly  150  will automatically reseat the compound seal assembly  172  on the sealing member  164 , thereby closing the pressure relief assembly  214 . Should the pressure in the vapor space drop below the ambient atmospheric pressure to reach the lower limit of the predetermined pressure range, air entry to the vapor space  106  will occur via the fluid flow holes  180  that are opened temporarily by the drawing downward of the under pressure seal member  172 . Once the pressure of the vapor space  106  has reached the lower pressure limit, force of the second spring  202  will be countered by the vapor pressure to push the under pressure seal member  176  upwardly to again seal the fluid flow holes  180 .  FIG. 7  shows an exploded view of the pressure equalization assembly  110  of  FIGS. 2-4 . The upper portion of the assembly  110  housed within or otherwise integral with the canister  114 , hereinafter referred to as a “canister assembly”  204 , can be removed from and reinstalled into the lower body member  116  as shown. This provides easy access to the inner workings of the pressure equalization assembly  110  and, as desired, access to the vapor space  106  of the storage vessel  102 . It will be appreciated that during such removal and reinstallation, the preload forces of the first and second springs  200 ,  202  will remain intact, and the respective piston sealing members  174 ,  176  will remain seated on the associated sealing members  152 ,  190 . 
     Providing a self-contained canister assembly such as  204  can provide a number of benefits. The system will generally retain its calibrated pressure range settings after the canister assembly  204  has been removed and reinstalled, thereby ensuring continued operation over the desired pressure range. This also allows an existing canister assembly to be easily removed and replaced with a new, replacement canister assembly that has been calibrated and set to the appropriate pressure range settings. This can facilitate easy field servicing of the pressure equalization assembly, and tank reconfigurations (e.g., changing to a different operational pressure range). 
     The self-contained canister assembly  204  also ensures that the pistons  174 ,  176  remain correctly aligned and seated on the associated sealing members  152 ,  190 , which reduces the likelihood that a seal interface misalignment will occur as the canister assembly is lowered into the lower body member  116 . Finally, the canister assembly can be easily installed into the lower body member  116  to bring the sealing member  115  into contact with the upper surface of the body member  116  without requiring the user to exert a downwardly directed force to overcome the spring forces of the springs  200 ,  202 . Thus, the spring force upon the pistons  174 ,  176  is independent of the insertion force required to install the canister assembly  204  into the lower body member  116 . 
     While the pressure equalization assembly  110  is shown to utilize a pair of opposing threaded fasteners  126 ,  130  to secure the canister assembly  204  to the base housing member  116 , it will be appreciated that such is merely for purposes of showing an illustrative embodiment and is not limiting. It will be appreciated that any number of fasteners can be utilized (e.g rivets, tape, wire, clamps, zip ties, integrated fasteners, rotate/locking flanges, latches, etc.) in order to facilitate the removability and replaceability of the canister assembly  204  relative to the base housing member  116  while maintaining the structural integrity and interchangeability of the canister assembly and the base housing member. 
     An alternative pressure equalization assembly  210  is shown in  FIG. 8 , with an enlargement of a portion of the assembly  210  provided in  FIG. 8A . This alternative embodiment is generally similar in construction and operation to the embodiment of  FIGS. 2-7  and is particularly well suited for attachment over a thief hatch aperture of a vessel. 
     As will be recognized by those skilled in the art, a thief hatch in a storage vessel commonly incorporates a relatively large aperture that is normally closed off by a hinged hatch or other sealing structure. The hatch is opened to allow an individual to lower a cup or other collection member into the vessel to retrieve a sample of the contents therein. In some cases, the thief hatch access may be sufficiently large to allow a user to climb down through the aperture and into the vessel. The embodiment of  FIG. 8  thus serves the dual purpose of providing thief hatch access to the interior of a vessel when opened, and pressure equalization of the vessel when closed. 
     The pressure equalization assembly  210  includes an underpressure (that is, a vacuum) relief mechanism and an overpressure relief mechanism disposed within a housing (canister)  212 . The housing  212  is generally characterized as a hollow cylindrical member having an internal passageway  214 , and mates with a peripherally extending base housing member (flange)  215 . The flange  215  may be provided with a plurality of bolt holes (not shown) through which bolts extend to attach the pressure equalization assembly  210  adjacent a thief hatch aperture in the storage vessel  102  so that the passageway  214  communicates with the vapor space  106  of the vessel (see  FIG. 1 ). 
     A pivotally supported hooding cap, or top cover  216  fits over the upper end of the housing  212 . The hooding cap  216  is connected to the lower flange  215  by a hinge pin  217 A and is latched by a pivotal hook latch  217 B disposed on an opposing side of the hinge pin. With the pivotal hook latch  217 B unlatched and pivoted aside, as before the upper portion of the assembly  210  can be rotated to an open position to provide access to the upper end of the housing  212 , as generally depicted in  FIG. 9 . 
     The hooding cap  216  can function as a weather guard that extends about and is spaced from the outer surface of the upper housing  212 . Several vent holes  218  are spatially disposed about the housing  212  near its top portion and arranged to be partially hooded by the annular cap  216 . As necessary, a wire mesh screen member (not shown) or the like can be positioned to cover the vent holes  218  to prevent debris from entering the internal passageway  214 . 
     A first piston assembly  220  normally seals the vapor space  106  from the surrounding atmosphere by engaging an annular seal  222  that is characterized as an elastomeric O-ring supported on the lower portion of housing  212 , although other sealing configurations can be used as desired. The piston assembly  220  has a piston shuttle member  224  with hollow cylindrical lower portion  226  and a substantially solid cylindrical upper portion  228 . A central bore extends through the upper portion and communicates with the hollow of the lower portion, the lower portion of the central bore being threaded. 
     A hollow, cylindrically shaped piston guide  230  is disposed in a cavity  232  formed by an upwardly extending support dome portion  234  of the hooding cap  216 , as shown. The piston guide  230  can be provided with an annularly shaped shoulder ridge  236  near its medial portion, dividing the hollow interior of the piston guide  230  into an upper chamber and a lower chamber; the shoulder ridge provides a passage channel having an internal diameter dimensioned to permit free passage of the cylindrically shaped lower portion  226  of the piston shuttle member  224 . The piston shuttle member  224  has a disk shaped portion  242  having an outer diameter sized to be slidingly disposed in the upper chamber, the shoulder ridge  236  extending to abut the disk portion  242  and thus limiting the downward travel of the shuttle member  224 . A pressure equalizing bore (not shown) can be provided as desired in the disk portion  242  to equalize the pressure between the upper and lower chambers. 
     A cap member  244  is threadingly connected to the upper end of the piston guide  230 ; and a threaded, adjusting rod member  246  extends upwardly from, and is threadingly engaged with a threaded bore (not separately numbered), in the dome portion  234  of the hooding cap  216 . Preferably, the upper end of the rod member  246  is squared to accept a wrench for advancing or retracting the rod member  246  relative to the dome portion  234 . 
     The lower end of the rod member  246  extends through a bore (not separately numbered) in the top of the cap member  244  and is retained in connection therewith via a cotter pin  248 , permitting the rod member  246  to rotate freely relative to the cap member  244 , thereby raising or lowering the piston guide  230  in the cavity by rotating the rod member  246 . A locking nut  250  on the threaded rod member  246  serves to lock the rod member  246  in position once advanced or retracted as desired. Preferably, a protective cover  252  is mounted over the protruding portion of the rod member  246 . 
     The pressure equalization assembly further comprises a second piston assembly  260  that has a piston rod  262  with a threaded upper end that is threadingly engaged with the lower, threaded end of the central bore of the piston shuttle member  224 . The threaded lower end of the piston rod  262  connects to a compound seal assembly  266 . The compound seal assembly  236  has a disk shaped over pressure seal member (piston)  268  and a disk shaped under pressure seal member (piston)  270  that together seal the internal passageway  114  when the pressure of the vapor space  106  in the vessel  102  is within the predetermined pressure range mentioned above. 
     The over pressure seal member  268  has an annular central hub through which the piston rod  262  extends; the lower end of the piston rod  262  is threaded and engages a centrally disposed threaded bore (not separately numbered) in the under pressure seal member  270 . 
     The over pressure seal member  268  has a plurality of fluid flow holes  274  spaced about and surrounding the central hub to form an outer ring shaped rim portion  276  joined to the central hub by the webbing between the fluid flow bores  274 . The fluid flow holes  274  are sealed by the under pressure seal member  270  when held in contact with the upper pressure seal member  268 . 
     An outer surface  278  of the rim portion  276  is beveled to mate with the above mentioned annular seal  222  (O-ring) which is supported in a lower groove of the housing member  212 . The over pressure seal member  268  has an inner groove that supports a second annular seal  282  (O-ring) abuts and seals against a corresponding beveled outer surface  284  of the under pressure seal member  270 . 
     The first piston assembly  220  has a helical spring  290  that is supported in compression between the over pressure seal member  268  and the shoulder ridge  236  of the piston guide  230 . The second piston assembly  260  has a helical second spring  292  nested within the first spring  290  and disposed to surround the piston rod  262 . As before, the compression force of the first spring  290  is set to match the upper pressure value of the predetermined pressure range and the compression force of the second spring  292  is set to match the lower pressure value of the predetermined pressure range. 
     When the pressure equalization assembly  210  is sealingly mounted to the thief hatch of the liquid storage vessel  102 , the pressure of the vapor space  106  will be communicated by the internal passageway  214  against the compound seal assembly  266  and is countered by the force of the first spring  290 . When the pressure in the vapor space  106  exceeds the upper limit of the predetermined pressure range, the vessel pressure will force the over pressure seal member  268  and the under pressure seal member  270  upwardly, moving the outer surface  278  of the rim  276  away from the annular seal  222 , providing open communication to the vent holes  218  while the pressure exceeds the upper limit. Once the pressure of the vapor space  106  is reduced to within the predetermined pressure range, the first helical spring  290  will force the over pressure seal member  268  and the under pressure seal member  270  downwardly so that the over pressure seal member  268  will seal against the annular seal  222   
     When the pressure of the vapor space  106  falls below the lower pressure limit of the predetermined pressure range, the under pressure seal member  270  will be pulled downwardly against the second helical spring  292 , separating the under pressure seal member  270  from the over pressure seal member  268  to open the fluid flow bores  274  to fluid communication with the vapor space  106 , and surrounding atmospheric air will flow through the vent holes  218  to the vapor space  106 . Once the pressure of the vapor space  106  has risen to a value that exceeds the lower limit of the predetermined pressure range, the second helical spring  292  will extend to lift the under pressure seal member  270  to sealing reengagement with the O-ring seal  282 . 
     As shown in greater detail in  FIG. 8A , the lower base flange  215  includes an annular seal  294 . The seal  300  has a rectilinear cross-sectional shape, although other shapes and styles of sealing members can be used. A lower facing surface  296  of the housing member  212  contactingly engages the seal  300  when the assembly  210  is in the closed position ( FIGS. 8-8A ). This provides the pressure equalization assembly  210  with a “canister assembly”  298  which can be lifted up and away from the lower flange  215 , as shown in  FIG. 9 . The latch  217 B selectively engages and disengages a latch pin  299  ( FIG. 9 ) as the assembly is transitioned between the closed and open positions. 
     As before, an advantage of the self-contained canister assembly  298  of the pressure equalization assembly  210  is that the respective preload forces of the springs  290 ,  292  are maintained upon the pistons  268 ,  270  even when the pressure equalization assembly  210  is transitioned to the open position ( FIG. 9 ). This can help to maintain the operational accuracy of the assembly vacuum and positive pressure setpoints since the pistons  268 ,  270  are not released and reset as the assembly  210  is moved between the hatch closed and hatch open positions. 
     Maintaining the spring preload forces in this fashion further reduces the force necessary to transition the assembly  210  back to the closed position of  FIG. 8 , since the user need not overcome the spring force that normally urges the springs in the closed position in order to engage the latch  217 B. This can be particularly beneficial when relatively larger cross-sectional pressure equalization assemblies, and relatively large spring forces, are used. It will be noted that any number of sizes of apertures can be covered by the various embodiments presented herein, including apertures that are extremely small and apertures that are sufficiently large to enable a user to climb therethrough to gain access to the interior of the vessel. 
     While it is contemplated that the canister assembly  298  will remain permanently affixed to the base flange via the hinge pin  217 A, as desired the canister assembly can be made easily removable and replaceable through the use, for example, of a removable hinge pin or similar. This allows a replacement, pre-calibrated canister assembly (with the same, or different, pressure range settings) to be installed without affecting the existing mounting configuration. 
     On-site field calibration operations can be performed as desired to set the respective upper and lower pressure threshold boundaries.  FIG. 10  provides a simplified functional block diagram of the storage tank  102  of  FIG. 1  in conjunction with the equalization pressure equalization assembly  110  of  FIGS. 2-7 . It will be understood that the functional block diagram of  FIG. 6  applies equally well to other configurations such as the thief hatch embodiment of  FIGS. 8-9 . 
     A user operated manual valve  300 , such as a ball valve, can be connected via a conduit  302  between the tank  102  and the pressure equalization assembly  110 . During an on-site calibration of the system, a pressure/vacuum supply  304 , such as a portable tank, compressor, vacuum pump, etc., and a pressure gauge  306 , preferably with a GUI display (numeric pressure value readout, etc.) can be connected to the conduit  302 . 
     After closing the valve  300 , the user can utilize the supply  304  to set the pressure sensed by the pressure equalization assembly  110  to a first desired level, such as a first vacuum (negative) pressure, and adjust the pressure equalization assembly  110  until it operates to open at this desired level. Such adjustment can be carried out by beginning with the vacuum pressure piston being in a closed position, and changing the spring tension until the piston moves to the open position. The holding pressure of the assembly  110  can be determined via the gauge  306 . For example, the assembly  110  may be set to operate to nominally open at minus 0.4 oz/in 2  and thereafter close and hold minus 0.3 oz/in 2  of vacuum pressure. 
     The foregoing steps can be repeated by supplying a positive pressure to the pressure equalization assembly  110  and adjusting the spring force upon the positive pressure piston until the pressure opens it at this second desired level. As before, the holding pressure can be determined via the gauge  306 . For example, the system may operate to open at a positive pressure of about 6.0 oz/in 2  and thereafter close and hold at a positive pressure of about 5.9 oz/in 2 . In this example, the operational pressure differential range would thus be from a negative pressure of 0.4 oz/in 2  to a positive pressure of 6.0 oz/in 2 ; pressures at or beyond this range would result in the opening of the equalization pressure equalization assembly  110 , while the pressure equalization assembly  110  would remain closed for pressure excursions that remained within this range. 
     From the above, it will now be appreciated that the pressure equalization assembly as variously embodied herein can operate to stabilize pressure of the vapor space of a liquid storage vessel to remain within a predetermined pressure range. When a sufficient volume of vapor from the vapor space has been vented to reduce the vapor space/exterior pressure differential to a resulting force that is less than the biasing force of the first spring, the first piston assembly will automatically reseat the compound seal assembly on the annular seal, thereby closing the pressure relief assembly. Should the pressure in the vapor space drop below the ambient atmospheric pressure to reach the lower limit of the predetermined pressure range, air entry to the vessel will occur via the fluid flow holes opened temporarily by the drawing downward of the under pressure seal member. Once the pressure of the vessel has reached the lower pressure limit, the under pressure seal member will be forced to again seal the vessel. 
     Unlike many prior art equalization systems which fail to hold a setpoint pressure differential, the various embodiments disclosed herein can be configured to maintain the storage tank in a continuously sealed (closed) condition until and only at such time that the pressure of the vapor space exceeds the upper limit or falls below the lower limit of the predetermined pressure range, after which the system returns to maintain a sealed condition. The pressure in the vapor space will thus not necessarily equal that of the surrounding atmosphere, but will be within the predetermined range of acceptable pressure differentials. 
     It follows that, depending on the structural integrity of a storage tank, the tank may be able to remain fully sealed against the external environment over a wide range of environmental cycling conditions. For example, a given storage vessel may heat up during a hot day hours and cool off during night hours, and if the pressure excursions can be safely handled by the vessel, no venting to the external atmosphere will occur. This advantageously prevents environmental contamination by eliminating the unnecessary venting of volatile fumes to the surrounding atmosphere, and may prevent the vessel owner from incurring fines or other sanctions from a regulatory authority carrying out on-site “sniffer” type inspections in an attempt to detect emitted vapors. 
     In some embodiments, the respective upper and lower pressure limits can be set in relation to the structural capabilities of the vessel so that, should changes in internal pressure be sufficient to approach such can result in damage, the system will safely vent (or admit) fluid to prevent such damage, but otherwise prevent venting or admitting of fluid in other circumstances. Exemplary structural capabilities of some types of storage tanks may be on the order of about a positive pressure of 6.0 ounces per square inch (6.0 oz/in 2 ) and about a negative pressure of 0.4 oz/in 2 . The setpoint pressure differential thresholds can be set at some prorated percentage, such as eighty percent of these values. 
     The various embodiments presented herein further advantageously operate to provide easy access to the interior of the vessel by removal/pivotal rotation of the operational upper portion of the assembly. The spring preload and seal interfaces remain intact, ensuring correct replacement and operation of the assembly when transitioned back to the closed position. 
     It will be appreciated that the various embodiments discussed herein provide a number of advantages over the prior art. The various embodiments provide both overpressure and underpressure relief at specified levels, while normally closing the vapor space of the storage vessel to the surrounding atmosphere at all other times. The assembly is readily constructed and maintained, and is contemplated to provide reliable operation over a variety of changing environmental conditions. 
     For purposes of the appended claims, terms such as “removably engageable” and the like will be construed consistent with the foregoing discussion to describe a self-contained assembly as exemplified by the canister assemblies  204 ,  298  that can be removed from and subsequently replaced into a base housing member as exemplified by the base housing members  116 ,  215 . 
     It will be clear that the various embodiments presented herein are well adapted to carry out the objects and attain the ends and advantages mentioned as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes may be made that will readily suggest themselves to those skilled in the art and that are encompassed in the spirit of the subject matter disclosed and as defined in the appended claims.