Abstract:
The method of transferring compressed gas at from a first tank to a second tank without decompressing the compressed gas and then re-pressuring the compressed gas comprising filling the second tank with a fluid, connecting a first fluid connection on the first tank to a second fluid connection on the second tank with a first line with one or more first valves, connecting a first gas connection on the first tank to a second gas connection on the second tank with a second line with one or more second valves, opening the first valves and the second valves to allow the compressed gas to pressurize the fluid, and pumping the fluid in the second tank into the first tank, thereby causing the compressed gas in the first tank to be displaced into the second tank.

Description:
TECHNICAL FIELD 
     This invention relates to the method expelling compressed gas from one or more compressed gas tanks, especially as associated with the transportation and delivery of compressed natural gas. 
     BACKGROUND OF THE INVENTION 
     The transportation of natural gas from the supply location to the tanks at the market by ship or truck transportation tanks requires that the gas be highly compressed to make the transportation economic. The expense of high pressure transportation tanks (e.g. 3000 p.s.i.) rather than at atmospheric pressure (e.g. 0 p.s.i.) is more than offset by the fact that about 250 times as much gas product can be transported. 
     A second problem exists that if the tanks at the market have an intermediate pressure such as 600 p.s.i. When the 3000 p.s.i. high pressure transportation tanks are dumped into the market tanks, approximately 1780 p.s.i. will remain in the transportation tanks. This means that approximately 60% of the product transported remains undelivered. 
     Two choices have remained here in the art. First, you can simply leave the gas in the transportation tanks for the return trip and always be transporting this 60% of the volume back and forth from the supply location to the market location. Secondly you can provide gas compression pumps to pump the stranded gas from the ship or truck transportation tanks and deliver all the gas to market. The gas compressors are expensive and expensive to operate. However, the higher cost in many cases is the time tying up the access to the terminal while they are being pumped out. Especially in the case of ocean going ship terminals, the dock time is an expensive charge. However, because of the efficiency of the compressors, residual pressure never comes below about 600 p.s.i. or 20% of the original pressure. 
     Throughout the history of the transportation of natural gas, the balance between the transportation of the stranded gas in the transportation tanks and the cost to pump it out has been studied with various combinations of stranded gas and compression applied. In the case of trucks, the total volume of stranded gas is not large, however, in very large ocean going vessels, the amount of gas stranded by contemporary methods can be very large. 
     Another problem associated with conventional methods of transportation are nefarious thermal issues. If the receiving tank pressure is zero and the transportation tank pressure is 3,000 p.s.i., for example, the instantaneous temperature drop upon opening the valve would be 84 degrees K or 151 degrees F., with very bad consequences if there was any water or foreign gases or liquids in the transportation tank. In addition to substantial thermal risks, the 3000 p.s.i. on the transportation tank and 0 p.s.i. in the receiving tank will average out to be 1500 p.s.i. in both tanks, with half of the gas being delivered. At that point gas compressors would be employed with more and more time and money spent as the percentage of the transported gas is transferred, as was also indicated above. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of this invention is to provide a method of transferring compressed gas from a transportation tank to a stationary tank with little or no gas in it and vice-versa without requiring the use of gas compressors. 
     A second objective of this invention to provide a method of transferring compressed gas from a transportation tank to a stationary tank with little or no gas in it and vice-versa without decompression and recompression. 
     A third objective of this invention is that all of the gas is expelled from the transportation tank so that all the product is delivered to market, rather than a lower pressure residual simply being carried back in the transportation tank for another trip. 
     Another objective of this invention is that as the tank can be totally purged, it can also be disconnected from the other tanks for maintenance, if required, which would be precluded by any residual natural gas in the tanks. 
     Another advantage of this invention is that the connectors can easily be backfilled with either a liquid or nitrogen before being safely disconnected. 
     Another objective of this invention is that there is no transfer of liquid between the two systems, the required power to pump the water would be 5,600 kW with an expenditure of 5.5 metric tons of gas. Gas usage would not really be a problem but power would, as well as regulation of the system. 
     Another objective of this invention is minimizing the transfer differential pressure so that it enables the installation of safety devices on the tanks so that in case of a collision when the piping on top of the tanks is ripped off or any other type of leakage, a safety mechanism can quickly shut down the flow of gas trying to exit the tank through the broken piping, substantially increasing the safety level of the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view of a vessel having the filling method of this invention. 
         FIG. 2  is a view of the vessel of  FIG. 1  with the top deck removed and showing a set of tanks about to be installed. 
         FIG. 3  is a view of the vessel of  FIG. 2  with a full complement of storage bottles installed. 
         FIG. 4  is a schematic of method of the present invention as would be seen when the transportation vessel arrives at the delivery location, valves are opened, but pumping has not started. 
         FIG. 5  is a schematic of method of the present invention after a first tank of compressed natural gas has been transferred and valves are set up to deliver the second tank of compressed natural gas. 
         FIG. 6  is a schematic of method of the present invention after the second tank of compressed natural gas has been transferred and valves are set up to deliver the third tank of compressed natural gas 
         FIG. 7  is a schematic of method of the present invention after the third tank of compressed natural gas has been transferred and valves are set up to deliver the fourth tank of compressed natural gas. 
         FIG. 8  is a schematic of method of method of the present invention after the fourth tank of compressed natural gas has been transferred and all valves are closed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , an offshore tanker  10  is shown which has a substantial central portion  12  which contains gas storage tanks. 
     Now referring to  FIG. 2 , the offshore tanker  10  is shown with the top cover from the central portion  12  removed and showing a number of storage chambers  20 . A bank of storage bottles  22  is shown with one of the individual bottles identified as  24 . Individual bottles can be of a variety of sizes, for example 24 inches in diameter by 45 feet long. 
     Referring now to  FIG. 3 , offshore tanker  10  is shown with more of the double wall covering from central portion  12  removed and a full set of bottles  22  installed. In this model  576  of the bottles  24  are shown. 
     Referring now to  FIG. 4 , a graphic of the pumping system of this invention is shown. The lower portion of the graphic shows a transportation tank system  50  for transportation of the compressed gases and the upper portion shows a stationary tank system  52 . The transportation tank system  50  will likely be aboard a ship, but can be transported by a variety of means including barges, railroads, and trucks. The stationary tank system  52  is described following as the location to which the transportation tank system  50  delivers the compressed natural gas for distribution and use but can as well represent the location where the transportation tank system is efficiently loaded, whether from a shore based or offshore location. 
     Hose connectors  54 ,  56 , and  58  connect hoses  60 ,  62 , and  64  from the transportation tank system to piping  66 ,  68 , and  70  on the stationary tank system. The connectors  54 ,  56 , and  58  can be one of several styles which are well known in the art. Due to size they will likely be of the remotely hydraulically operated type. Valves  72 ,  74 , and  76  and valves  78 ,  80 , and  82  are on each side of hose connectors  54 ,  56 , and  58  to close off the ends of the hoses or piping when a disconnection is done. Hoses  60 ,  62 , and  64  can be neutrally buoyant with additional buoyancy added to float the valves  72 ,  74 , and  76  also as they move to the shore installation for connection. Alternately the piping  66 ,  68 , and  70  can be floating hoses, or both sides of the hose connectors  54 ,  56 , and  58  can be floating hoses. 
     The floating gas hose would be rated for a working pressure of 4,250 p.s.i. (we plan to work at 2,133 p.s.i.), inside diameter 7 inch, outside diameter 11 inch, minimum dynamic bending radius 9 foot (7 foot static and 6 foot storage), weight 68 lbs. per ft. The liquid hoses would be the same, which enjoys a higher rating of 5,000 p.s.i. There would be 1 gas line and two liquid lines. The 3 hoses will be bundled, except at their end. Fluid flow needs to be 1,000 cubic meters per hour (4,400 GPM), but with little head if the fluid flows between the receiving and the loading station. The system is inherently safe as no pressure control needs to be applied. In some cases, the difficulty of handling the large high pressure hoses may be made more practical by handling them with a crane. 
     When a fully loaded transportation tank system comes into port for unloading, all valves in both the transportation tank system and the stationary tank system will be closed. After the hose connectors  54 ,  56 , and  58  are connected, valves  72 ,  74 , and  76  and valves  78 ,  80 , and  82  are opened as shown. Additionally, valves  86 ,  88 ,  90 ,  92 ,  94 , and  96  are opened. 
     Tank  100  shows bladder  101  which is empty and collapsed to a flat position. Tank  126  shown bladder  127  which is fully expanded against the internal walls of tank  126 . The bladders are resilient balloon like members which separate the fluids and gases which will be in the tanks from time to time. Various means can be utilized to achieve this separation of fluids and gases such as floating piston. In some cases no separating method would be required if the fluid utilized did not tend to absorb the gasses and floats or sonar was used to monitor the level of the fluids in the tanks. 
     All valves in this description are shown as manual valves for simplicity. For rapid and controlled operations, all valves are likely to be remotely controlled. 
     By opening valves  86  and  88  the pressure of the gas in tank  100  will pressurize the fluid in tank  126 . Operating pump  130  will draw fluid out the bladder of tank  126  and pump it through hoses  132 ,  70  and  64  and valve  90  to tank  100 . This will displace the compressed natural gas in tank  100  through valve  88  hoses  60  and  66 , through valve  86  and into the space outside the bladder in tank  126 . As the pressure in the two tanks was equalized, there will not be a head pressure to pump against, but rather simply flowing friction losses will be incurred. 
     When pump  110  is operated, fluid will be drawn from tank  108  through valve  92  and pumped through hoses  62  and  68  into the bladder of tank  124 . The nitrogen gas in tank  124  will be vented through valves  84  and  83 . As the fluid in tank  108  and the nitrogen gas in tank  124  are at atmospheric pressure, there will not be a head pressure to pump against, but rather a simple flowing friction loss will be incurred. 
     This means that the pressure of tanks  100  and  126  will be the same, and will remain the same during the entire gas transfer process at the high pressure of the compressed natural gas. The pressure in tanks  108  and  124  will be a relatively constant pressure at atmospheric pressure plus a small pumping flow loss. This means safety relief valves can be installed on closely controlled conditions rather than trying to compromise on varying pressures of a typical compression process. The ability this provides to quickly recognize a leakage condition or overpressure condition can substantially increase the safety of the systems. 
     Referring now to  FIG. 5 , the results of the pumping in  FIG. 4  is seen. Valves  84 ,  86 ,  88 ,  90 ,  92 ,  94 , and  96  are now closed. Valves  140 ,  142 ,  144 ,  146 ,  148 ,  150 ,  152 ,  154 , and  156  are opened. 
     Operating pump  130  will draw fluid out the bladder of tank  124  and pump it through valve  146 , hoses  132 ,  70  and  64 , valve  150  and into the bladder of tank  102 . This will displace the compressed natural gas in tank  102  through valve  152 , hoses  60  and  66 , valve  142  and into the space outside the bladder in tank  124 . 
     When pump  110  is operated, fluid will be drawn from tank  100  through valve  148 , hoses  112 ,  62  and  68 , valve  144  and into the bladder of tank  122 . 
     The nitrogen gas in tank  122  will be vented through valves  140  and  83 . Nitrogen plant  158  will generate nitrogen and pump it through valves  154  and  156  into the area outside the bladder in tank  100 . 
     Referring now to  FIG. 6 , the results off the pumping in  FIG. 5  is seen. Valves  140 ,  142 ,  144 ,  146 ,  148 ,  150 , and  152  are now closed. Valves  160 ,  162 ,  164 ,  166 ,  168 ,  170 ,  172 , and  174  are opened. 
     Operating pump  130  will draw fluid out the bladder of tank  122  and pump it through valve  164 , hoses  132 ,  70  and  64 , valve  170  and into the bladder of tank  104 . This will displace the compressed natural gas in tank  104  through valve  172 , hoses  60  and  66 , valve  164  and into the space outside the bladder in tank  122 . 
     When pump  110  is operated, fluid will be drawn from tank  102  through valve  168 , hoses  62  and  68 , valve  164  and into the bladder of tank  120 . 
     The nitrogen gas in tank  120  will be vented through valves  160  and  83 . Nitrogen plant  158  will generate nitrogen and pump it through valves  154  and  174  into the area outside the bladder in tank  102 . 
     Referring now to  FIG. 7 , the results off the pumping in  FIG. 6  is seen. Valves  160 ,  162 ,  164 ,  166 ,  168 ,  170 , and  172  are now closed. Valves  182 ,  184 ,  186 ,  188 ,  190 ,  192  and  194  are opened. 
     Operating pump  130  will draw fluid out the bladder of tank  120  and pump it through valve  184 , hoses  132 ,  70  and  64 , valve  190  and into the bladder of tank  106 . This will displace the compressed natural gas in tank  106  through valve  192 , hoses  60  and  66 , valve  182  and into the space outside the bladder in tank  120 . 
     When pump  110  is operated, fluid will be drawn from tank  104  through valve  188 , hoses  112 ,  62  and  68 , valve  186  and into tank  128 . 
     Nitrogen plant  158  will generate nitrogen and pump it through valves  154  and  194  into the area outside the bladder in tank  104 . 
     Referring now to  FIG. 8 , as the compressed natural gas in tanks  120 ,  122 ,  124 , and  126  are exported to users through valve  200 , nitrogen from nitrogen plant  202  will be pumped into the space outside the bladders of tanks  120 ,  122 , and  124  and fluids are pumped from tank  128  into the bladder of tank  126  to be prepared for a subsequent reloading. 
     As the transportation tank system  50  is in transit to the supply location, the fluids in the bladder of tank  106  are pumped into tank  108  and nitrogen from nitrogen plant  158  is pumped into the space outside the bladder of tank  106 . These final pumping operations will return the status of the transportation tank system  50  and the stationary tank system to the status as was shown in  FIG. 4 . 
     Another advantage of this invention is minimizing of the transfer differential pressure is that it enables the installation of safety devices on the tanks. In case of a collision when the piping on top of the tanks is ripped off, a valve mechanism shuts down the flow of gas trying to exit the tank through the broken piping, activated by the differential pressure above a certain predetermined level. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.