Patent Publication Number: US-2021188538-A1

Title: Multiport Thief Hatch for Storage Tank

Description:
FIELD OF THE INVENTION 
     This invention relates generally to systems used in connection with storage tanks configured to liquid hydrocarbons or other potentially hazardous fluids. 
     BACKGROUND 
     Crude oil and other petroleum hydrocarbons produced from oil and gas wells are often temporarily stored in tanks that are located near the well. These tanks are sometimes referred to as “stock” tanks and multiple tanks may be arranged in “tank batteries.” The hydrocarbon products can be transported from the storage tanks to a transport terminal, pipeline or refinery in a tanker truck. The oil is routinely measured and tested at the storage tank. In many cases, the crude oil in the storage tank is tested by opening a door on the lid of the tank often referred to as the “thief hatch.” Opening the thief hatch permits the operator to take measurements to determine the volume of hydrocarbon liquids in the tank, the position of the interface between oil and water liquids, and the quality of the crude oil in the tank. 
     Because crude oil and other hydrocarbons may present personal safety and environmental risks, it is important to properly contain the hydrocarbons within the storage tank, while permitting the assessment of the quantity and quality of those hydrocarbons. In particular, the loss of hydrocarbon gases to the atmosphere may present an environmental emission, a safety risk, and a loss of revenue. At storage tank batteries, the separation, containment and collection of low-pressure hydrocarbon vapors depends on multiple pressure controls on the near-atmospheric storage tanks. Those controls are sequentially set and maintained at graduated settings of very low pressure (often in ounces/square inch). 
     Hydrocarbon fluids may offgas while captured in the storage tank, thereby potentially increasing the pressure of gases trapped in the headspace above the liquid. Additionally, the gas pressures inside the storage tank may fluctuate with changes in environmental temperatures surrounding the storage tank. The proper design of a tank vapor management system requires the incorporation of tank outbreathing and inbreathing protection through pressure/vacuum relief valves (PVRVs) to accommodate changes in internal pressures, as well as high volume venting for fire sizing. Each incorporated PVRV usually requires its own dedicated tank connection, and all added devices must be setpoint-integrated with one another and the aforementioned graduated tank pressure controls to ensure that the entire system vents when, but only when, necessary. 
     Newly constructed tanks can be designed to accommodate the venting requirements by equipping the tanks with PVRV connections initially, but at incremental expense. This, however, does not eliminate the use of the conventional thief hatch and the fugitive emissions of greenhouse gases (GHG), hazardous air pollutants (HAPs) and other volatile organic compounds (VOCs) that may leak at problematic rates from the thief hatch. For many storage tanks already in operation, however, there is a lack of the necessary pressure/vacuum controls and inbreathing/outbreathing is typically performed by pulling air in from, and venting harmful vapors out to, the atmosphere via the thief hatch. With existing thief hatches, there is no way to easily ascertain if the thief hatch or another port on the storage tank has begun to leak and is in need of repair or replacement. In view of these deficiencies, there is a need for an improved system for safely accommodating variations in pressure within the storage tank, while providing a mechanism for safely measuring the volume and quality of fluids in the storage tank. 
     SUMMARY OF THE INVENTION 
     In one aspect, embodiments of the present invention include a multifunction tank access device configured for connection to a storage tank capable of storing crude oil that has an opening for a conventional thief hatch. The multifunction tank access device has a lower assembly with an isolation valve, and an upper assembly removably connected to the lower assembly. The upper assembly includes a central housing, a pressure relief module, a vacuum relief module, and a tank access module. 
     Some embodiments include a multifunction tank access device configured for connection to a storage tank capable of storing crude oil, where the storage tank has an opening for a conventional thief hatch. The multifunction tank access device has a lower assembly that includes a tank mounting flange configured for attachment to the opening for the conventional thief hatch. The multifunction tank access device further includes an upper assembly removably connected to the lower assembly, where the upper assembly comprises a central housing. In these embodiments, at least a portion of the multifunction tank access device is constructed from a material that melts at a temperature below about 450° F. 
     In yet other embodiments, a multifunction tank access device is configured for connection to a storage tank capable of storing crude oil. The multifunction tank access device has a lower assembly with an isolation valve, and an upper assembly removably connected to the lower assembly. The upper assembly includes a central housing, a pressure relief module, a vacuum relief module, and a tank access module. The pressure relief module has pressure relief valve that is configured to open in response to a pressure inside the central housing that exceeds a setpoint pressure established for the pressure relief valve. Similarly, the vacuum relief module has a vacuum relief valve that is configured to open in response to a vacuum inside the central housing. In these embodiments, the tank access module includes a vapor control valve that permits the safe measurement of liquids contained within the storage tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a storage tank and a multifunction tank access device. 
         FIG. 2  is a close-up perspective view of an exemplary embodiment of the multifunction tank access device of  FIG. 1 . 
         FIG. 3  is a side cross-sectional view of the multifunction tank access device of  FIG. 2 . 
     
    
    
     WRITTEN DESCRIPTION 
     In broad terms, exemplary embodiments provide a multifunction tank access device  100  that replaces the standard tank thief hatch found on existing storage tanks. The multifunction tank access device  100  provides mechanisms for managing accumulating pressures and vacuums inside the storage tank, while also providing a mechanism for allowing an operator to safely and accurately measure and evaluate the fluids stored inside the tank. In some embodiments, the multifunction tank access device  100  is constructed primarily from a composite material with a melting point less than 450° F. In the event of a fire inside the tank, the multifunction tank access device  100  quickly melts to provide a larger port for relieving combustion gases. It will be appreciated that the multifunction tank access device  100  can be retrofitted onto existing storage tanks or provisioned on new storage tanks during the manufacturing or construction process. 
     With reference to  FIG. 1 , shown therein is the multifunction tank access device  100  installed on a storage tank  102 . The storage tank  102  is configured to primarily store liquid hydrocarbons (e.g., crude oil) and mixtures of hydrocarbons, water and other liquids. The storage tank  102  may include one or more inlets  104  and one or more outlets  106 . The storage tank  102  may be standalone or positioned within a tank battery that includes a plurality of storage tanks  102 . The storage tank  102  is generally cylindrical and manufactured from a corrosion-resistant metal or composite material. The storage tank  102  includes a top  108  and a catwalk or ladder  110  that provides access to the top  108 . 
     As depicted in  FIG. 1 , a vapor recovery line  112  is connected to the multifunction tank access device  100 . The vapor recovery line  112  conveys vapors released from the storage tank  102  to a vapor recovery unit, flare or other downstream facility (not shown). Recovered vapor products can be condensed for further processing in accordance with well-established industry practices. 
     Turning to  FIGS. 2 and 3 , shown therein a perspective and cross-sectional views, respectively, or the multifunction tank access device  100  constructed in accordance with an exemplary, non-limiting, embodiment. The multifunction tank access device  100  includes a lower assembly  114  and an upper assembly  116 . 
     The lower assembly  114  includes a tank mounting flange  118  that is configured to be securely fastened to the top  108  of the storage tank  102  with bolts or other fasteners. In retrofit applications, the tank mounting flange  118  can be configured with a bolt pattern that matches the existing thief hatch mounting pattern. In this way, the tank mounting flange  118  can be easily secured to the top  108  of the storage tank  102  once the thief hatch has been removed. The tank mounting flange  118  can be constructed from a durable metal or composite with sufficient strength to withstand the compressive force applied by the bolts or other fasteners used to secure the multifunction tank access device  100  to the storage tank  102 . 
     The lower assembly  114  further includes an isolation valve  120  and an upper flange  122 . In the embodiment depicted in  FIGS. 2 and 3 , the isolation valve  120  is a butterfly valve that can be manually actuated with an external valve lever  124 . The valve lever  124  includes a position lock  126  that allows the operator to secure the isolation valve  120  in a fully closed or fully open position. In other embodiments, the isolation valve  120  is motorized and configured for automated actuation in response to a control signal. 
     When the isolation valve  120  is closed, the isolation valve  120  substantially seals that interior of the storage tank  102  and prevents vapors, gases or fluids from escaping through the multifunction tank access device  100 . Closing the isolation valve  120  permits the upper assembly  116  to be removed or serviced without exposure to gases or fluids inside the storage tank  102 . When the isolation valve  120  is opened, the interior of the upper assembly  116  of the multifunction tank access device  100  is placed in communication with the interior of the storage tank  102 . 
     The upper assembly  116  includes base flange  128  that can be secured to the upper flange  122  of the lower assembly  114  with bolts or other fasteners. In the embodiments depicted in  FIGS. 2 and 3 , the upper assembly  116  includes a central housing  130 , a pressure relief module  132 , a vacuum relief module  134  and a tank access module  136 . In some embodiments, the pressure relief module  132 , the vacuum relief module  134  and the tank access module  136  are each connected to the central housing  130  in fluid communication. In this way, the central housing  130  acts as a plenum or manifold that communicates pressure from the interior of the storage tank  102  to the pressure relief module  132 , a vacuum relief module  134  and a tank access module  136 . 
     The pressure relief module  132  includes a pressure relief module housing  138 , a pressure relief valve  140  and a pressure release port  142 . In some embodiments, the pressure relief valve  140  is a mechanical valve that includes a pressure relief valve seat  144  and a pressure relief valve plunger  146 . The pressure relief valve plunger  146  is held in a closed position against the pressure relief valve seat  144  by a pressure relief valve biasing element  148 . In some embodiments, the pressure relief valve biasing element  148  is a weight-based system in which one or more weights are placed on the pressure relief valve plunger  146  to bias the pressure relief valve plunger  146  in a closed position against the pressure relief valve seat  144 . In other embodiments, the pressure relief valve biasing element  148  is a spring-based system in which a spring biases the pressure relief valve plunger  146  against the vacuum relief valve seat  144 . In each embodiment, the closing force can be adjusted to a desired setpoint opening force. 
     When the pressure of fluids inside the central housing  130  below the pressure relief module  132  exceeds the setpoint opening force, the pressure relief valve plunger  146  is forced off of the pressure relief valve seat  144  to vent excess pressure from the storage tank  102 . The pressurized gases are discharged from the pressure relief valve  140  through the pressure relief port  142  to the vapor recovery line  112 . In some embodiments, the discharged gases can be collected for further processing or directed to a flare for combustion. 
     The vacuum relief module  134  includes a vacuum relief module housing  150 , a vacuum relief valve  152  and a vacuum relief port  154 . The vacuum relief module housing  150  is connected to, and in fluid communication with, the central housing  130 . The vacuum relief valve  152  includes a vacuum relief valve seat  156 , a vacuum relief valve plunger  158  and a vacuum relief valve biasing element  160 . The vacuum relief valve plunger  158  is held in a closed position against the vacuum relief valve seat  156  by a vacuum relief valve biasing element  160 . In some embodiments, the vacuum relief valve biasing element  160  is a weight-based system in which one or more weights are placed on the vacuum relief valve plunger  158  to bias the vacuum relief valve plunger  158  in a closed position against the vacuum relief valve seat  156 . In other embodiments, the vacuum relief valve biasing element  160  is a spring-based system in which a spring biases the vacuum relief valve plunger  158  against the vacuum relief valve seat  156 . In each embodiment, the closing force can be adjusted to a desired setpoint opening force. 
     When a vacuum is present inside the storage tank  102 , the negative pressure is communicated to the interior of the vacuum relief module housing  150  directly or indirectly through the central housing  130 . When the pressure gradient across the vacuum relief valve plunger  158  exceeds the closing force applied by the vacuum relief valve biasing element  160 , the vacuum relief valve plunger  158  is forced off the vacuum relief valve seat  156  to allow air or other makeup gas to enter the vacuum relief valve  152  through the vacuum relief port  154  to reduce the vacuum drawn by the storage tank  102 . 
     Once the pressure gradient across the vacuum relief valve  152  is sufficiently mitigated, the vacuum relief valve biasing element  160  forces the vacuum relief valve plunger  158  back against the vacuum relief valve seat  156  to prevent gases in the storage tank  102  from escaping through the vacuum relief valve  152 . As used herein, the terms “vacuum” or “negative pressure” refer to a pressure that is less than the atmospheric pressure surrounding the storage tank  102 . 
     In addition to accommodating the routine expansion and contraction of gases inside the storage tank  102 , the multifunction tank access device  100  also provides a mechanism for releasing rapidly expanding pressurized gases created during a combustion event inside the storage tank  102 . The multifunction tank access device  100  can be partially or entirely constructed from a composite material that melts when exposed to gases produced by a combustion event. In this way, the multifunction tank access device  100  is configured to sacrificially melt in the presence of combustion gas temperatures to provide a large open vent that rapidly releases the pressurized gases inside the storage tank  102  to prevent an explosion. In this way, the cross-sectional area of the open isolation valve  120  provides an auxiliary emergency vent to improve the “fire sizing” of the storage tank  102  when the multifunction tank access device  100  is sacrificed during a combustion event. 
     In exemplary embodiments, one or more of the central housing  130 , the pressure relief module housing  138  and the vacuum relief module housing  150  are constructed from a sacrificial material that melts at a temperature less than 450° F. Suitable materials of construction include polymers, resin-impregnated fiberglass, and low melt point metals, such as Babbitt alloys and other white metals. Suitable polymers may include polyethylene, nylons, poly(vinyl chlorides), poly(ethylene oxide), poly(propylene), and poly(ethylene adipate). For non-metal, non-conductive materials, a grounding strap can be included from the multifunction tank access device  100  to the storage tank  102 . 
     The multifunction tank access device  100  also provides a safe and reliable mechanism for checking the volume of fluids inside the storage tank  102 . The tank access module  136  includes a vapor control valve  162  that permits the use of a measuring tape without exposing the operator to gases inside the storage tank  102 . In exemplary embodiments, the vapor control valve  162  includes a closing valve  164 , a loading spool  166  and a removable cap  168 . The closing valve  164  and isolation valve  120  can be closed to isolate the loading spool  166  from the central housing  130 . The cap  168  can then be removed to allow the hermetically sealed connection of a tape or other measurement device to the top of the loading spool  166 . Once connected, the closing valve  164  and isolation valve  120  can be opened to permit the tape or other measuring device to extend through the multifunction tank access device  100  into the storage tank  100 , without exposing the operator to gases that would otherwise escape by taking measurements through a conventional thief hatch. Suitable vapor control valves are commercially available from a variety of sources, including MMC International Corporation. 
     The multifunction tank access device  100  optionally includes a plurality of sensors that are configured to monitor and report the state and condition of the multifunction tank access device  100 . An internal pressure sensor  170  inside the central housing  130  reports the instantaneous and continuous pressure inside the central housing  130 . The isolation valve  120  includes an isolation valve position sensor  172  that reports the extent to which the isolation valve  120  is opened or closed. The pressure relief module  132  includes a pressure relief valve position sensor  174  that monitors and reports the position of the pressure relief valve  140  to indicate whether the pressure relief valve  140  has been actuated. Similarly, the vacuum relief module  134  includes a vacuum relief valve position sensor  176  to detect and report the actuation of the vacuum relief valve  152 . 
     Additional sensors may be incorporated within the multifunction tank access device  100  to detect the presence of hazardous gases in the tank access module  136  to alert the operator to take additional precautions before interacting with the multifunction tank access device  100 . An oxygen sensor can be placed within the central housing  130  or vacuum relief module housing  150  to detect oxygen leakage through the vacuum relief valve  152 . A methane detector sensor can be placed in close proximity to the vacuum relief port  154  to detect leakage of gases through the vacuum relief valve  152 . One or more temperature sensors can be deployed inside and outside the multifunction tank access device  100  and configured to alert the operator to changes in the condition of the multifunction tank access device  100 . The internal and external temperature readings can be used alone or in combination to identify or predict ingress or egress of gases from the storage tank  102  or the rapid increase in temperature caused by a combustion event. 
     The measurements produced by the various sensors within the multifunction tank access device  100  can be provided to a central control system  178  to provide operators with live, real-time information about the status and condition of the multifunction tank access device  100 . The multifunction tank access device  100  can be integrated into a local edge computing network that includes a central control system  178 . In other embodiments, the sensor output from the multifunction tank access device  100  is transmitted to a remote central control system  178  through wired or wireless networks using SCADA, MODBUS, or other conventional data transmission protocols. 
     As an example, the central control system  178  can be configured to identify trends of decreasing or increasing pressures measured by the internal pressure sensor  170 . If the central control system  178  determines that the pressures are decreasing or increasing without the actuation of the pressure relieve valve  140  or the vacuum relief valve  152 , the central control system  178  can alert the operator to the presence of an adverse event, such as a potential gas leak or the failure of the multifunction tank access device  100  to release accumulating pressure. This detection system permits the operator to take preemptive action to correct the situation to mitigate risks to the operator or the environment. 
     In some embodiments, the central control system  178  can be configured to use neural networks or machine learning to develop and refine a predictive correlation library  180  based on correlations between the empirical output from the various sensors within the multifunction tank access device  100  and the occurrence of adverse events at the multifunction tank access device  100  or storage tank  102 . As an example, the correlation library  180  can be configured to predict the future occurrence of methane leaks, controlled gas exchange across the multifunction tank access device  100 , or a combustion event based on empirical data produced by the sensors within or around the multifunction tank access device  100 . The central control system  178  can be configured to apply a wide variety of comparative and statistical techniques, including, but not limited to, probability-density based usage indices, multivariate Hotelling T-squared distributions, association rule mining (ARM) algorithms, change point detection algorithms, and Bayesian and neural network-based anomaly detection and classification techniques. 
     Thus, upon its installation on the storage tank  102 , the multifunction tank access device  100  performs most if not all of the necessary venting and vacuum control functions required for the safe operation of the storage tank  102 . The multifunction tank access device  100  also eliminates the need for personnel to be exposed to potentially harmful environments by providing a vapor barrier when any repairs are needed to the upper assembly  116 , as well as providing a vapor control valve  162  to permit the hermetically gauging and sampling of the contents of the storage tank  102 . The multifunction tank access device  100  incorporates one or more sensors to alert the operator to concerning events or conditions and to inform the operator of potential valve malfunctions that indicate a need for valve redress or replacement. The multifunction tank access device  100  can be manufactured from a composite material that will melt in the presence of heat produced by a combustion event inside the storage tank  102 . The multifunction tank access device  100  is thereby sacrificed to open the entire flow area of the thief hatch port for emergency fire venting. 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and functions of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. It will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems without departing from the scope and spirit of the present invention.