Abstract:
Apparatus for controlling vapor so as to increase the rate of flow of liquid while moving from one vessel to a second vessel by replacing pumped off liquid with vapor from the liquid and then depressurize the first vessel. After the first vessel is emptied of liquid, the first vessel is depressurized by pulling the vapor out of the first vessel, condensing the vapor, and routing the condensed vapor to the second vessel. The vapor utilized for balancing the first and second vessels during movement of liquid is vapor from the head space of the second vessel or vapor obtained by vaporizing a side stream of the liquid being pumped from the first vessel to the second. The system is also adapted for assisting with the recovery of vent gas from the vent gas recovery system of the facility.

Description:
This applications benefits 60/153,751 filed Sep. 13, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates methods and apparatus for moving liquids, particularly volatile liquids, from one, vessel to another. In more detail, the present invention relates to methods and apparatus for balancing the vapor pressure of two liquid vessels, such as a storage tank and a railroad car, during the movement of the liquid from one vessel to the other so as to increase the rate of fluid movement and decrease vapor loss, for depressurizing the vessel from which the liquid is pumped, and for assisting with the recovery of vent gas from the vent gas recovery system of the facility. 
     Equipment is disclosed in the patent literature, and a number of systems are currently in use, for loading and unloading liquids from, for instance, a barge to a storage tank or a storage tank to a tank truck. U.S. Pat. Nos. 3,527,384 and 4,014,358 describe equipment and methods for moving liquids from one container to another. Of particular concern in the movement of such liquids are combustible and/or volatile liquids, and the patent literature includes, for instance, U.S. Pat. Nos. 3,783,911 and 5,125,439, directed to methods for collecting and/or controlling vapor emissions during loading and unloading of such liquids. It is also known in the art to inject natural gas or propane into the ullage of the tank from which the liquid is pumped to balance the tanks and then to flare off that gas to depressurize the tank. 
     In addition to the flammability of the vapors and/or the gas used to balance the tanks during loading and unloading, such systems are complicated by the distance the liquid must be moved and/or the physical requirements of the facility at which they are being moved. For instance, it is not uncommon to have a loading/unloading facility at which the liquid must be moved from a barge to a storage tank that may be several hundred, or even several thousand, feet away from the barge. Further, for safety and other reasons, the pumps and electrical equipment for moving the liquid may be located remotely from the barge. In short, the design of such systems is almost always constrained by the specifics of each installation. 
     There is, therefore, a need for methods and apparatus which are adaptable for moving liquids from one container to another for use in many locations, regardless of the distance the liquid must be transported and the available electrical capacity and proximity. There is also a need for methods and apparatus which are safely used for moving combustible and highly volatile liquids from container to container. There is also a need for methods and apparatus which increase the speed with which liquids are moved from one container to another. There is also a need to reduce facility-wide vented vapors which must be separately compressed, condensed and piped to storage tanks. 
     SUMMARY OF THE INVENTION 
     These needs, and others known to those skilled in the art, are met by providing an apparatus for balancing vapor while moving liquid from a first vessel to a second liquid storage vessel comprising a pump for unloading or loading liquid from a first vessel to a second vessel, a vapor source, and a vapor line in fluid communication with the vapor source and the first vessel for returning vapor to the first vessel to replace the liquid pumped from the first vessel to maintain pump suction pressure. A separator with a heating coil and a compressor is provided for pulling vapor off the then-unloaded second vessel as is a condenser (which is in fluid communication with the compressor and the second vessel) for changing vapor from the compressor to liquid and then routing the liquid to the second vessel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a preferred embodiment of a barge liquid unloading and barge vapor depressurizing system constructed in accordance with the teachings of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Chemical plants and/or refineries typically include certain equipment for loading and unloading of vessels such as rail tank cars, tank trucks, and barges. This equipment usually includes storage tanks, pump(s), pipes, valves, and fittings and, in some installations, a vaporizer or equipment for injecting natural gas into the vessel from which the liquid is being unloaded. Regardless of whether the vaporized liquid or natural gas is used to pressurize the vessel from which the liquid is unloaded, the vaporized liquid or natural gas is typically either left in the vessel or flared off. The present invention is designed to interconnect with this on-site equipment to facilitate liquid loading and unloading and subsequent depressurization of the vessel from which the liquid is unloaded. 
     Referring to FIG. 1, a preferred embodiment of a system constructed in accordance with the teachings of the present invention is shown schematically. The system is comprised of five main components, a pump  10  (which is the pump which is typically found at the installation), compressor  16  (which may be a compressor already in use at the installation), condenser  12 , vaporizer  14  (which may also be the vaporizer which is in use at the installation), and the piping, valves, and controls for operating these components while moving the liquid from one vessel to another. These components are combined in a system such as the system shown in FIG. 1 for loading and unloading liquid product from/to a barge to a product storage tank(s) and subsequently depressurizing the vessel from which the liquid is unloaded. 
     In more detail, the system shown in FIG. 1 is adapted for moving liquid to or from a barge  18  to or from one or more product storage tanks  20 A- 20 D through liquid line  22 . To unload liquid from barge  18 , pump  10  is switched on, valve  24  is closed, and valves  26  and  28  in vapor line  30  are opened. Liquid flows from barge  18  through check valve  32  to storage tanks  20 A- 20 D. As the volume of liquid in the barge  18  is depleted, the rate of fluid flow through liquid line  22  decreases unless barge  18  is vapor pressure balanced by delivering vapor to the ullage  34  of the barge to make up for the displacement loss from the liquid pump off rate. The vapor delivered to the ullage  34  of barge  18  comes from one or both of two sources depending upon the particular installation. The primary vapor source is the tank vapor head space, or ullage, of the product storage tanks  20 A- 20 D. This vapor is returned to the barge  18  by opening valve  42  and insuring that valve  50  is closed. When not being returned to the barge  18 , this vapor is constantly being lost due to tank filling and normal tank liquid vaporization through the pressure regulator  36  to the vent gas compressor system  38  of the facility which, on information and belief, is utilized for the purpose of preventing the loss of vapor to the atmosphere at every such facility. In the particular installation shown in the figure, and in other similar installations, the facility at which the vapor balancing system of the present invention is installed includes a compressor  16  which is used for compressing and returning the vapor from the storage tanks  20 A- 20 D by opening valve  42  to allow tank vapor to enter the separator  44  and on to the compressor  16  through line  46 . A discharge check valve  47  prevents back flow of compressed gas into the compressor  16  when the compressor is stopped. 
     However, vapor from the tank vapor space is normally insufficient to provide adequate vapor pressure to balance the displacement rate from the barge  18 . The present invention therefore contemplates the use of vaporizer  14  for providing vapor flow sufficient to make up the difference to balance the pump displacement rate. Vaporizer  14  receives liquid from the pump discharge/liquid line  22  by opening control valve  15 , which is controlled by the level transmitter  56  to maintain constant liquid level in vaporizer  14 . The vapor pressure of vaporizer  14  is controlled by pressure transmitter  57  which controls the inlet steam valve  54 . It is generally desirable to reduce vapor losses to the vent gas compressor system  38 , so in the preferred embodiment of the present invention a pressure regulator  17  is provided that is set to make up vapor pressure from the vaporizer as required by the system to control the vapor flow as required by the barge pump displacement rate. 
     Compressor  16  has an additional function that confers a significant advantage upon the particular embodiment shown in FIG.  1 . Specifically, compressor  16  is also sized to pull down and compress vent gas from downstream of the pressure regulator  36  through valve  50  and check valve  51 . The pressure regulator  36  is set at a selected set value which, when exceeded, routes vapor to the facility vent gas compressor system  38  at a much lower, but still positive, pressure. By opening valve  50  and closing valve  42 , that same positive pressure vent gas is then re-routed to separator  44  and compressor  16 . The load on the existing facility vent gas compressor system  38  is significantly reduced at no additional cost over the cost of providing the equipment necessary to pressurize the vapor as needed to balance the barge  18  during unloading operations and depressurizing the barge  18  as described below. This operating characteristic and cost savings of the pressure balancing system of the present invention makes the present invention particularly advantageous in the many installations in which, for instance, the facilities production capacity has been repeatedly expanded, thereby increasing the load on the facility vent gas compressor system to a level out or beyond capacity. The addition of the pressure control system of the present invention, when installed in this manner, may prevent the need for additional vent gas compression. 
     To depressurize the vapors from barge  18 , the pump  10  is turned off and compressor  16  pulls vapor from the barge through valve  82  back through vapor line  30  to vapor line  31  by closing valve  26 . Opening valve  72  and closing valves  50  and  42  routes the barge vapor only into separator  44 . Separator  44  is provided with an internal heating coil  45  that vaporizes liquid entering separator  44 . The temperature of coil  45  is adjusted by the operator by modulating valve  78  to divert hot compressed gas through the coil as needed. The gas is then compressed and routed to condenser  12  by opening valves  24  and  28 . Condenser  12 , of course, changes the vapor back to a liquid and the piping  66  routes the liquid on through pressure regulator  64 , which holds the condensing temperature at the optimum point. Depending on whether the cooling media of the condenser  12  is water or air, the regulator  64  is adjusted, if necessary, to raise the condensing temperature. Those skilled in the art will recognize that this adjustment allows an increase in condensing temperature substantially above ambient air or available cooling water temperature which can substantially reduce required surface area, thereby decreasing the cost of condenser  12 . 
     From pressure regulator  64 , the liquid goes through check valve  68  and back to the product storage tanks  20 A- 20 D. Compressor  16  is provided with a differential pressure transmitter  70  for automatically controlling the compressor loading and unloading to match the rate of liquid pump off. To allow this single compressor to handle these multiple flow rates and multiple pressures, the differential pressure transmitter  70  is provided with a site programmable alarm  80  which is programmed to automatically load and unload compressor  16  depending upon the recorded differential pressure as sent to the pre-programmed alarm switch  80 . The programmable pressure alarm can be any commercially available alarm supplied by, for instance, Moore Industries-International, Inc. (California) with a current input and multiple programmable alarm outputs in sufficient number to satisfy the loading and unloading system requirements of the compressor. 
     Different refineries and chemical plants have substantially different separation distances from the storage tanks  20 A- 20 D for barge unloading, and sometimes for rail and/or tank truck unloading. Depending on the separation distance and the chosen pipe size (existing or constructed), the pressure drop of moving the vapor from the vessel from which the liquid is pumped during depressurization to the compressor  16  inlet may be prohibitive. Consequently, the present invention contemplates the use of a second compressor system for overcoming that distance and pressure drop. The minimum requirement for this second compressor system is an inlet separator with heating coil, a compressor, and the required valves, piping and fittings. In more detail, this second compressor system pulls vapor from barge  18  during the depressurization cycle by closing valve  82  and opening valve  84  to route vapor to separator  86 . Heated vapor is then pulled into second compressor  88  and out through the discharge check valve  96 , prevents back flow of compressed gas into the compressor  16  when the compressor is stopped, and through check valve  60  into vapor line  30 . The second compressor  88  is provided with a pressure transmitter  89  and programmable alarm  91  for functioning in the manner described above in connection with compressor  16 , but with the reduced requirement of not compressing the tank vapors or vent gas. The temperature of the coil  92  in separator  86  is controlled using the valve  94  in the same manner as described for separator  44 . The compressed vapor in line  30  passes through open valves  24  and  26  but valve  28  is closed to direct the compressed vapor to condenser  12 . Liquid condensed from the vapor is returned to the storage tanks  20 A- 20 D as described above. 
     From the foregoing description it can be seen that the present invention provides equipment and methods for balancing the vapor pressure of two vessels during the movement of liquid from one vessel to the other, for depressurizing the vessel from which liquid is pumped, and for assisting with the recovery of vent gas from the vent gas recovery system of the facility. Those skilled in the art who have the benefit of this disclosure will recognize that certain changes can be made to the component parts of the vapor control system of the present invention without changing the manner in which those parts function to achieve their intended result. For instance, as noted above, the separator  86  and compressor  88  are not a necessary part of the system of the present invention in installations in which it is not necessary to overcome a substantial vapor pressure drop. Further, in certain installations such as when there are restrictions in the electrical power available or inadequate cooling water, it may be advantageous to connect the two compressors  16  and  88  in series with the final compressor discharging to the condenser  12  and on to storage tanks  20 A- 20 D. All such changes, and others which will no doubt be made clear to those skilled in the art by this description of the preferred embodiment, are intended to fall within the scope of the following, non-limiting claims.