Abstract:
A sampling cylinder for LNG (liquefied natural gas) is provided. The sampling cylinder includes a piston cylinder arrangement having opposite end members closing the chamber inside the cylinder. Various porting arrangements are provided for allowing fluid to be fed into a chamber inside the cylinder on opposite sides of a piston. A sample chamber is provided at one end member of the cylinder. The end member containing the sample chamber is also provided with a heat exchanger that may be connected to a source of coolant and preferably the LNG to be sampled to effect cooling of the end member at least partially prior to the sample chamber having a sample of LNG injected thereinto.

Description:
BACKGROUND OF INVENTION 
     The collection of samples of LNG for subsequent analysis is well known. Analysis is typically performed to determine the BTU value of the natural gas as its component compounds can vary and stratify. Historically, LNG and other forms of natural gas were sold by volume disregarding the BTU content of the natural gas. Natural gas is comprised of several component compounds and is not a “pure compound”. It may contain methane, ethane, propane, moisture and other components. Each of the components has its own BTU value, if any. As the value of hydrocarbons used to produce heat has increased, it is important to know its BTU value in order to properly price the product both from a seller perspective and a consumer perspective. LNG is transported by tanker ships, limited distances of pipeline and the like. When loading to or offloading LNG from a ship, it can be substantially continuously sampled and analyzed on site during the loading/offloading process to accommodate variation in the makeup of LNG. This process historically involves the vaporization of the LNG prior to a sample being taken or analyzed. A sample can also be taken at this point and sent away for laboratory analysis. The samples are usually taken using a sampling cylinder into which the LNG, which has been vaporized into a gas, is injected, stored and transported to an analytical facility for analysis. However, this precludes the ability to take a sample of the LNG in the liquid state, eliminating any question about what might have been altered in the vaporization process. It is important to obtain representative samples of the LNG. Because of the different components making up LNG, the obtaining of a representative sample can be difficult since the LNG can stratify if stored static, can fractionate by allowing lower boiling point fractions to evaporate or flash off from the sample. Further, if the LNG vaporizes, its volumetric increase from liquid to vapor can be on the order of a 600 fold increase in volume and result in a dramatic increase in pressure. 
     LNG is typically maintained at a temperature of approximately −256° F. and at a gage pressure of 40 psi to maintain it in liquid form. If this pressure is reduced or the temperature increased, vaporization of one or more components can occur. Vaporization of the various components will occur at different temperatures and pressures because they are different chemical compounds. Sampling cylinders of the piston type are well known in the art, see for example, U.S. Pat. Nos. 4,628,750 and 4,922,764, both assigned to Welker Engineering Company. The entire disclosures of these patents are incorporated herein by reference. See also U.S. Pat. No. 6,422,737 also assigned to Welker Engineering Company. This latter patent discloses the use of an integral mixing pump to maintain a more homogenous sample. The entire disclosure of this patent is also incorporated herein by reference. 
     While sampling cylinders are known in the art, an improvement therein is desirable to effect a more uniform and representative sample during the injection of the sample into the sampling cylinder. The present invention involves an improved sampling cylinder which allows for directly sampling LNG (liquid state) into the cylinder. 
     SUMMARY OF INVENTION 
     The present invention involves the provision of a LNG sample cylinder useful in the gathering, storing and transport of an LNG sample. The cylinder includes a tubular member defining a sidewall of a cylinder chamber and having first and second ends. A piston is movably positioned in the chamber and selectively moves toward and away from the first and second ends. A first end member is mounted to the tubular member and covers a first end of the tubular member. A second end member is provided and is mounted to the tubular member and covers the second end of the tubular member. A sample chamber is provided in the second end member and is in flow communication with the cylinder chamber. At least two sample conduits are in flow communication with the sample chamber. The second end member also has a heat exchanger associated therewith and isolated from the sample chamber to preclude flow therebetweeen. The heat exchanger is operable to cool the second end member and sample chamber and at least two heat exchange fluid flow conduits are in flow communication with the heat exchanger. 
     The present invention also involves the provision of a method of sampling LNG. This method allows for sampling and trapping a portion of the LNG as LNG, not as a vaporized gas. The trapped portion of the LNG can be maintained at temperature if desired, or it can be allowed to vaporize and expand in the security of the sample cylinder without the potential for any component to escape during the sampling and vaporization process. This method also allows for trapping a known volume of LNG in the liquefied state and comparing component percentages to that known fixed or adjustable volume of liquid. The method includes connecting a sampling chamber to a source of LNG for flow communication between the source and the sampling chamber. The sampling chamber and at least a portion of the structure defining the sampling chamber is cooled to a predetermined temperature with LNG from the source. LNG is injected from the source into the cooled sampling chamber. After the sample is received in the sampling chamber, it is sealed so that the LNG sample is contained therein. The sampling chamber cooling is ceased so that the sampling cylinder can be transported to a site for analysis. The sampling chamber is cooled prior to conveying LNG from the source to the sampling chamber as a sample to be stored. The heat exchanger is separated flow wise from the sampling chamber. 
    
    
     
       DETAILED DESCRIPTION OF DRAWINGS 
         FIG. 1  is an isometric view of a sampling cylinder. 
         FIG. 2  is a longitudinal cross-sectional view of the sampling cylinder. 
         FIG. 3  is an end view of a sampling end of the sampling cylinder with certain flow paths shown in phantom lines. 
         FIG. 4  is an enlarged transverse sectional view of the sampling end member of the sampling cylinder taken along the line  4 - 4  of  FIG. 3 . 
     
    
    
     Like numbers throughout the Figures designate like or similar parts and/or construction. 
     DETAILED DESCRIPTION 
     The reference numeral  1  designates generally a sampling cylinder that is operable to receive and store LNG (liquefied natural gas) from a source  2  of LNG ( FIGS. 1 and 2 ). The cylinder  1  is selectively connectable to (for flow communication with) and disconnectable from the source  2  as with suitable connecting devices (not shown) to receive and store LNG. The source may be a fixed volume container or a pipeline through which LNG flows. LNG is typically stored at a cold temperature rather than a high elevated pressure to maintain the natural gas in a liquid state. The temperature and pressure at which natural gas liquefies may be easily found from a phase diagram. However, the temperature and pressure may vary by the relative amounts of the various components that make up natural gas as contained in the natural gas. Natural gas is a blend of various components, for example, propane, ethane, methane and other components including moisture (or ice). Typically, LNG has water removed therefrom as is known in the art. 
     As best seen in  FIGS. 1 and 2 , the cylinder  1  includes two end closure members  4 ,  5  each positioned on an opposite end of a cylinder tube  6 . Ends  4 ,  5  may be held in sealing engagement and securement to the tube  6  for example, by tie rods  7  positioned in a circumferential array about the tube  6 . The tie rods  6  may be threaded into one end member for example end member  5  and extend through holes in end member  4  and secure the end members  4 ,  5  in place with threaded fasteners such as hex nuts  9 . The end members  4 ,  5  can be sealed to the interior surface of the tube  6  as with O ring seals  11  mounted on protuberances  12 ,  13  extending into the interior of the tube  6 . Positioned inside tube  6  is a piston  15  that is movable between the end members  4 ,  5  as determined by the pressure differential on opposite sides of the piston. The piston is mounted for sealing engagement with an interior surface of the tube  6  as with suitable sliding seals  17 . The piston  15  divides the chamber  18  formed by the tube  6  and ends  4 ,  5 , into two chambers  18 A,  18 B. The chamber portion  18 A is typically referred to as a precharge chamber and the chamber portion  18 B is typically the sample chamber. 
     The end  4  is provided, in the illustrated structure, with two flow paths  20 ,  21 . The flow path  20  provides communication between a port  22  for the connection of a pressure gage or pressure sensor  24  as seen in  FIG. 1 . The flow path  20  provides flow communication between the chamber  18 A and the port  22  so that the pressure gage  24  can sense pressure of fluid contained within chamber  18 A and display the pressure. The flow path  21  provides flow communication between a flow control valve  25  and the chamber  18 A. The valve  25  is preferably a manually operated on/off valve that allows for bleeding of fluid from the chamber  18 A or the injection of the fluid into the chamber  18 A to precharge the chamber  18 A with a pressurized fluid such as pressurized nitrogen. Preferably, the fluid in the chamber  18 A is a gas. In a preferred embodiment, the usable volume of the chamber  18  is on the order of about 1000 cc&#39;s which does not include the volume taken up by the piston  15 . 
     The end member  5  is adapted for the injection of a sample fluid into and the discharge of a sample fluid from the chamber  18 B and a sample chamber  27 . The sample chamber  27  has a fixed volume whereas the chamber  18 B has a variable volume which volume depends upon the position of the piston  15  within the chamber  18 . The sample chamber  27  includes a sidewall  28  and bottom wall  31 . The sample chamber  27  has an open side  26  opening into the chamber  18 B. The piston  15  overlies the open side  26  when bottomed out. The position of the piston  15  in the chamber  18  will be determined by the relative pressure of the fluids on opposite sides of the piston  15  during operation of the cylinder  1 . The details of the construction of the end member  5  are best seen in  FIGS. 2 ,  3  and  4 . Two flow paths  29 ,  30  are provided in the end member  5  to provide flow communication to and from the chamber  27 . Shutoff valves  32 ,  33  are provided to selectively permit flow through the flow paths  29 ,  30  as described below in more detail in the description of the operation of the present invention. The valves  32 ,  33  are also operable to completely seal the sample chamber  27  from inflow and outflow once an appropriate sample is captured. Ports  34 ,  35  are provided for connecting the sample chamber  27  in flow communication with the source  2  through appropriate conduits and for venting the sample chamber  27  during a purge step in the sample taking. In a preferred embodiment, the sample chamber  27  has a volume in the range of between about 10 cc and about 50 cc. 
     The end member  5  is provided with a fluid flow type heat exchanger adapted for receipt of a heat exchange medium which is preferably derived from the source  2  in the form of cold liquid LNG. Preferably, the heat exchanger  37  is isolated from the sample chamber  27  to prevent flow of fluids therebetween. By having the heat exchanger  37  in the end  5 , the entire sampling cylinder  1  need not be cooled in order to obtain a representative sample and prevents or reduces the chance of vaporization of a sample or a portion of the sample during the collection process. An access port  38  is provided for the placing of a temperature probe  50  in temperature sensing or measuring relationship to the end member  5  and heat exchanger  37 . As seen, the heat exchanger  37  is a single cavity provided in the end member  5  wherein cold heat exchange fluid will contact the surfaces and cool the end member  5  and hence the chamber  27 . As shown, the chamber  39  portion of the heat exchanger  37  is formed as a pocket in the end member  5  by a suitable machining or casting technique having its open end covered by a plug  40  that can be welded as at  41  in place on member  5  to provide a fluid tight seal. The plug  40  is preferably provided with an inlet  43  and an outlet  44  that provide for simultaneous flow communication into and out of the chamber  39 . The inlet  43  and outlet  44  may be provided with suitable connectors for connecting them to the source  2  of LNG and to a discharge collection system as is known in the art (not shown) that preferably provides a back pressure on the chamber  39  to keep the LNG liquid. A pressure gage  48  may also be provided by being connected to a port  49  that is flow communication with the chamber  27  to measure or sense the pressure of the fluid within the chamber  27 . The temperature probe  50  by monitoring the temperature of the end  5  can provide an indication of the temperature of the fluid both in the heat exchanger  39  and that contained in the chamber  27 . 
     The sampling cylinder  1  can be provided with means to indicate the position of the piston  15  therein. This can be done as disclosed in above-referenced U.S. Pat. Nos. 4,628,750 or 4,922,764 which are incorporated herein by reference. Position of the piston  15  can also be indicated by the use of a sealed rod that will project from the end  5  or any other suitable means. 
     The present invention is better understood by a description of the operation thereof. When it is desired to sample LNG, a sampling cylinder  1  is provided. A precharge pressurized fluid is provided in the chamber  18 A. The pressure is on the order of about 1,000 psi for a typical LNG sampling. By pressurizing the chamber  18 A, the piston  15  moves to a bottomed out position in the chamber  18 B resting against the protuberance  13 . The charging (pressurizing) of the chamber  18 A and moving of the piston  15  is done with the chamber  27  in flow communication with the exterior of the cylinder  1  leaving the chamber  27  basically at atmospheric pressure. The chamber  18 A is then closed to maintain the pressure. The heat exchanger  37  is then connected via the inlet  43  to a source of liquefied LNG such as the source  2 . The LNG will flow into the heat exchange chamber  39  and can be discharged through the outlet  44  to a suitable collection system or can be repressurized and injected back into the source  2 . Preferably, after cooling of the end  5  has been completed to the desired temperature for example, less than about −150° F. and preferably the temperature or the approximate temperature of the extracted LNG, for example, less than approximately −256° F., the flow of liquefied LNG can be ceased or can be continued during the charging of the sample to the chamber  27 . The inlet port  34  and its flow path  29  are also connected to the source  2  of LNG preferably with the valve  32  closed. The outlet port  35  and its flow path  30  are then connected to the collection system as was the outlet  44 . After the flow connections of the ports  34  and  35  are effected, the valve  32  may be opened along with the valve  33  to allow, because of the pressure differential across the ports  34 ,  35  for LNG to flow into and out of the chamber  27 . After a suitable amount of purging time, the valve  33  may be closed to stop outflow, allow the chamber  27  to fill to a predetermined pressure and then the valve  32  may be closed to seal the chamber  27 . The piston  15  will move toward the end  4  once the chamber  27  is pressurized. It is to be understand that while a purging process has been disclosed, it is to be understood that the chamber  27  could be filled with a known gas for example nitrogen which can be accommodated for in the analysis. After the sample LNG has been captured, the cylinder  1  may be transported to an analytical 716919.98 facility where the sample LNG is extracted by being discharged either through the port  34  or  35  to analytical equipment such as a gas chromatograph or calorimeter for analysis. Once connected, the appropriate valve  32  or  33  may be opened to permit discharge of the LNG sample. In the event the temperature of the LNG sample rises in the chamber  27 , for example during transit, it may vaporize increasing the pressure in the chamber  18 A. This would then move the piston  15  further toward the end  4 . It is preferred to transport the cylinder  1  with the temperature of the sample below about −150° F., preferably below about −200° F. and most preferably below about −256° F. with the sample being maintained as a liquid. 
     Thus, there has been shown and described several embodiments of a novel invention. As is evident from the foregoing description, certain aspects of the present invention are not limited by the particular details of the examples illustrated herein, and it is therefore contemplated that other modifications and applications, or equivalents thereof, will occur to those skilled in the art. The terms “having” and “including” and similar terms as used in the foregoing specification are used in the sense of “optional” or “may include” and not as “required”. Many changes, modifications, variations and other uses and applications of the present invention will, however, become apparent to those skilled in the art after considering the specification and the accompanying drawings. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention which is limited only by the claims which follow.