Patent Publication Number: US-2013247684-A1

Title: Batch-type multiphase flow rate measurement device and flow rate measurement method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application of International Application No. PCT/JP2011/068449, filed on Aug. 12, 2011 and published in Japanese as WO/2012/081279-A1 on Jun. 21, 2012. This application claims the benefit of Japanese Application No. 2010-276733, filed on Dec. 13, 2010. The entire disclosures of the above applications are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a batch-type multiphase flow rate measurement device and a flow rate measurement method. 
     BACKGROUND ART 
     Fluid removed from an oilfield contains crude oil, gas (e.g., methane, ethane, butane, and pentane), and water (e.g., salt water), and may also contain solid (e.g., sand). In order to efficiently transport crude oil via a tanker or a pipeline, it is indispensable to separate such a fluid into gas, water, and crude oil, and determine their flow rates. 
     In the petroleum industry, gas has been mainly separated by using a separation tank-type gas-liquid separator that utilizes the buoyancy of gas. Crude oil is slowly supplied to the separation tank-type gas-liquid separator. Since the separation tank-type gas-liquid separator is a large-capacity tank, there is a sufficient residence time during which the gas is separated from the liquid. Therefore, the separation tank-type gas-liquid separator has a large volume, is heavy, and requires a large installation area. These requirements can be met when the separation tank-type gas-liquid separator is used on shore. However, when the separation tank-type gas-liquid separator is used at an offshore platform which has a limited deck space, it is very important to reduce the dimensions and the weight of the separation tank-type gas-liquid separator. Moreover, the separation tank-type gas-liquid separator increases costs. Since the separation tank cannot be provided for each well from the viewpoint of cost and space, the separation tank is shared by a plurality of wells in order to measure the flow rate. Therefore, production rates from each well is measured only several times (days) a year on most of the offshore platforms. 
     It is important to determine production rates from each well as information for producing the maximum amount of oil from the oilfield. For example, when the amount of oil production has decreased, and the amount of water production has increased, the flow passage of water can be closed by closing the valve of the well to recover the amount of oil production before the flow passage of water is sufficiently formed. Therefore, a small and inexpensive multiphase flow rate measurement device that can be easily installed has been developed in order to determine production rates from each well or more frequently measure production rates from each well. A multiphase flow rate measurement device that does not separate fluid into oil, water, and gas, and a multiphase flow rate measurement device that separates fluid into oil, water, and gas have been known. 
     JP-A-2001-165741 discloses a multiphase flowmeter that does not separate fluid into oil, water, and gas. The multiphase flowmeter disclosed in JP-A-2001-165741 includes a mixer, two rotors that differ in blade angle, and a spring that connects the rotors, and measures the pressure loss from the front side of the mixer to the rear side of the rotor, the rotational speed of the rotor, the spring torque, and the temperature. The measured temperature is used to determine the oil density and the water density. The multiphase flowmeter homogenizes a multiphase flow consisting of oil, water, and gas by using the mixer, and the pressure loss, the rotational speed, and the spring torque are used to solve empirical formulas for the total volume flow rate, the liquid density, and the gas volume fraction (=gas volume flow rate/total volume flow rate). The flow rate measurement algorithm is designed to solve the empirical formulas by using the measured pressure loss, rotational speed, spring torque, and temperature to determine the total volume flow rate, the liquid density, and the gas volume flow rate ratio, and calculate the gas volume flow rate, the liquid volume flow rate, and the water cut (=water volume flow rate/liquid volume flow rate). 
     However, the empirical formula cannot be applied depending on the fluid composition. Moreover, the flow regime may change depending on the flow rate conditions for each phase (i.e., the empirical formula cannot be applied). Therefore, it is necessary to calibrate the empirical formula on the site environment. However, the site data used for calibration must allow an error of ±10% for the gas volume flow rate and the liquid volume flow rate, and allow an error of at least ±3% for the water cut. Therefore, the calibrated empirical formula necessarily includes an error. The measured value also has a mechanical error range, and an error may also occur due to adhesion of solid. The multiphase flowmeter disclosed in JP-A-2001-165741 cannot specify an error in measured value and an error factor. When the empirical formula is not appropriate for the site environment, the water cut determined by solving the empirical formula may be 100% even if the actual water cut is 30%, for example. Therefore, it is very difficult to utilize the multiphase flowmeter disclosed in JP-A-2001-165741 on the site environment. 
       Handbook of Multiphase Flow Metering , The Norwegian Society for Oil and Gas Measurement and The Norwegian Society of Chartered Technical and Scientific Professionals discloses another multiphase flowmeter that does not separate fluid into oil, water, and gas. Since the multiphase flowmeter disclosed in the  Handbook of Multiphase Flow Metering  also utilizes a measurement algorithm that uses an empirical formula, it is necessary to calibrate the empirical formula on site, and it is difficult to specify an error and an error factor in the same manner as the multiphase flowmeter disclosed in JP-A-2001-165741. 
     U.S. Pat. No. 5,526,684 discloses a multiphase flowmeter that separates fluid into oil, water, and gas. The multiphase flowmeter disclosed in U.S. Pat. No. 5,526,684 utilizes a simple cyclone gas-liquid separator that does not have an internal structure as disclosed in John S. Lievois,  Multiphase Flow Measurement Class  8110, Colorado Experiment Engineering Station Inc. In the multiphase flowmeter disclosed in U.S. Pat. No. 5,526,684, a gas flowmeter is provided in the gas outlet pipe of the gas-liquid separator to measure the gas flow rate, and a Coriolis meter is provided in the liquid outlet pipe of the gas-liquid separator to measure the flow rate of water and the flow rate of crude oil. Therefore, the measurement accuracy of the multiphase flowmeter depends on the separation performance of the gas-liquid separator. U.S. Pat. No. 5,526,684 also discloses technology that combines two gas-liquid separation pipes on the downstream side of the gas-liquid separator in order to deal with a case where it is difficult to fully utilize the performance of the gas-liquid separator. 
     Watanabe T, Ikeda T, and Okatsu H, “Development of Multiphase Flow Measuring System” ( Annual Report  2007, Japan Oil, Gas and Metals National Corporation, pp. 85-88) discloses another multiphase flowmeter that separates fluid into oil, water, and gas. The multiphase flowmeter disclosed in Watanabe T, et al.,  Annual Report  2007 confines the vertical upward flow by using upper and lower valves, measures the liquid surface and the oil-water interface by using differential pressure transmitters, and calculates the water cut. Watanabe T, et al.,  Annual Report  2007 states that oil-water slippage does not occur when the apparent liquid superficial velocity (liquid flow rate/tube cross-sectional area) is 0.5 m/s or more, and measurement can be performed when the error is ±5%. However, when the gas volume fraction is large (e.g., 99% or more), it is necessary to use a sampling tube having a sufficient length for sampling the liquid since the amount of liquid is small. When measuring the flow rate of a multiphase flow having a periodic flow rate, typical water cut cannot be calculated without increasing the number of samplings. 
     Watanabe T, Ikeda K, Ichikawa M. Kawai M, Yamada M, and Fujiwara K, “Development of Multiphase Flow Measuring System” ( Proceedings of Lectures at  2009  Spring Meeting , Japanese Association for Petroleum Technology, pp. 85-86) discloses another multiphase flowmeter that separates fluid into oil, water, and gas as technology that solves the problem of the multiphase flowmeter disclosed in Watanabe T, et al., Annual Report 2007. The multiphase flowmeter disclosed in Watanabe T, et al.,  Proceedings of Lectures at  2009  Spring Meeting  includes four valves, a gas-liquid separator, a measurement pipe, a differential pressure transmitter that measures the liquid surface and the oil-water interface, a manometer, a thermometer, and a gas flowmeter. The multiphase flowmeter supplies a multiphase flow to the gas-liquid separator, separates the multiphase flow into gas and liquid, measures the gas volume flow rate by using the gas flowmeter, accumulates the liquid in the measurement pipe for a few minutes, measures the liquid surface and the oil-water interface by using the differential pressure transmitter, and calculates the liquid volume flow rate and the water cut (i.e., batch type). In this case, a liquid discharge operation is required after the measurement. The multiphase flowmeter disclosed in U.S. Pat. No. 5,526,684 may suffer from a liquid volume flow rate measurement error due to bubbles that may be mixed into the liquid outlet pipe. Moreover, a measurement error in water cut may occur due to the difference between the oil velocity and the water velocity. The multiphase flowmeter disclosed in Watanabe T, et al.,  Proceedings of Lectures at  2009  Spring Meeting  is configured so that bubbles are discharged to the gas outlet pipe. Moreover, it is unnecessary to take account of the difference between the oil velocity and the water velocity. Therefore, the measurement error factors of the multiphase flowmeter disclosed in Watanabe T, et al.,  Proceedings of Lectures at  2009  Spring Meeting  include a measurement error due to liquid introduced into the gas outlet pipe, and errors of the instrument. Introduction of liquid into the gas outlet pipe can be determined by providing a droplet separator in the gas outlet pipe, and an error factor can be determined (reduced) by returning the gas to a measurement pipe. 
     Various gas-liquid separators have been proposed. The gas-liquid separator disclosed in Lievois exhibits excellent performance within a specific flow rate range. However, the separation efficiency deteriorates when the flow rate is outside the above range. This makes it impossible to prevent a situation in which liquid is incorporated in the separated gas, or gas is incorporated in the separated liquid. For example, the flow rate of gas-containing crude oil may change by a factor of five after the multiphase flow is produced from an oilfield. Therefore, there are considerable problems associated with the gas-liquid separator disclosed in Lievois. 
     U.S. Pat. No. 4,596,586 discloses a structure that may solve the above problems of the gas-liquid separator disclosed in Lievois. Specifically, an inner pipe is provided in a vertical pipe, the upper end of the inner pipe being connected to a gas outlet pipe, and the lower end of the inner pipe being open at a position slightly lower than the entrance of an inlet pipe. The inner pipe serves as a partition wall, and suppresses a phenomenon in which droplets are mixed into the separated gas from the gas-liquid multiphase flow. 
     The cyclone gas-liquid separator also functions as a mist separator. For example, JP-A-2001-246216 discloses a gas-liquid separator that separates droplets dispersed in gas by utilizing a centrifugal force. The gas-liquid separator disclosed in JP-A-2001-246216 includes an inner pipe and a baffle plate in the same manner as the gas-liquid separator disclosed in U.S. Pat. No. 4,596,586. However, the inner pipe extends downward through the baffle plate, and is open downward. The baffle plate is connected to the lower area of the inner pipe to form a ring, and a circular space is formed between the baffle plate and the outer pipe. A mist-containing gas flows into the outer pipe through the inlet pipe attached to the sidewall of the outer pipe in the tangential direction, and moves downward while forming a vortex flow along the inner wall of the outer pipe. The mist is trapped by the inner wall of the outer pipe, flows downward along the inner wall of the outer pipe, and reaches the liquid outlet pipe. The gas moves downward through the circular space formed by the baffle plate around the inner wall of the outer pipe, then moves upward through the center inner pipe, and reaches the gas outlet pipe. A situation in which the mist-containing gas reaches the gas outlet pipe is prevented by the long inner pipe and the baffle plate that is provided in the lower area of the inner pipe and also extends to an area around the outer pipe. This increases the separation efficiency of the cyclone gas-liquid separator. 
     According to Lievois, U.S. Pat. No. 4,596,586 and JP-A-2001-246216, a vortex flow is produced by attaching the inlet pipe to the side of the outer pipe in the tangential direction. JP-A-2000-317212 discloses a cyclone gas-liquid separator that produces a vortex flow based on a different principle. In the gas-liquid separator disclosed in JP-A-2000-317212, the inlet pipe is connected to the outer pipe so that the inlet pipe extends toward the center axis of the outer pipe and is open toward the circular area formed by the outer pipe and the inner pipe. The circular area is closed by a plate-like guide around the opening, so that bubble-containing liquid is guided to an area opposite to the guide to form a vortex flow. 
     In the cyclone gas-liquid separator disclosed in U.S. Pat. No. 4,187,088, there is not only the guide around the opening but also a guide that extends to the lower end of the opening of the gas-liquid multiphase flow inlet pipe. In the gas-liquid separator disclosed in U.S. Pat. No. 4,187,088, the inlet pipe is connected to the outer pipe so that the inlet pipe extends toward the center axis of the outer pipe (i.e., a flow passage is formed). The front side, the upper side, the lower side, and the side opposite to the desired whirl direction are completely enclosed so that a gas-liquid multiphase flow that enters through the inlet pipe is guided in the whirl direction to form a vortex flow. 
     The pressure of a gas-liquid multiphase flow produced from an oilfield is very high, and changes. Therefore, a gas-liquid separator is designed to withstand a high pressure. When using a configuration in which the inlet pipe is connected to the side of the outer pipe in the tangential direction (e.g., the gas-liquid separators disclosed in Lievois, U.S. Pat. No. 4,596,586 and JP-A-2001-246216, since the connection section is not symmetrical, an unbalanced load may be repeatedly applied to the weld when the pressure of the fluid changes, so that fatigue failure may occur. On the other hand, a configuration in which the inlet pipe is connected to the outer pipe so that the inlet pipe extends toward the center axis of the outer pipe (e.g., the gas-liquid separators disclosed in JP-A-2000-317212 and U.S. Pat. No. 4,187,088) is safe since the connection section is symmetrical. 
     The gas-liquid separator disclosed in JP-A-2001-246216 is used for gas in which droplets (mist) are dispersed, and the gas-liquid separator disclosed in JP-A-2000-317212 is used for liquid in which bubbles are dispersed. Specifically, JP-A-2000-317212 discloses a gas-liquid separator that separates excess ozone contained in ozone water by using a cyclone method. When the cyclone gas-liquid separator is modified in this manner, it can be used for a gas-liquid multiphase flow that differs in gas-liquid ratio. 
     SUMMARY OF INVENTION 
     Technical Problem 
     As described above, a multiphase flowmeter that does not separate fluid into oil, water, and gas makes on-site calibration and specification of an error factor difficult. On the other hand, a multiphase flowmeter that separates fluid into oil, water, and gas suffers from introduction of droplets into the gas outlet pipe of the gas-liquid separator, and introduction of bubbles into the liquid outlet pipe. 
     The gas-liquid separators and the multiphase flow rate measurement devices disclosed in Lievois, U.S. Pat. No. 5,526,684, U.S. Pat. No. 4,596,586 and JP-A-2001-246216 are inferior to the gas-liquid separators disclosed in JP-A-2000-317212 and U.S. Pat. No. 4,187,088 from the viewpoint of safety at high pressure. The gas-liquid separator disclosed in JP-A-2000-317212 cannot apply a sufficient centrifugal force for separating a gas-liquid multiphase flow produced from an oilfield into gas and liquid. The gas-liquid separator disclosed in U.S. Pat. No. 4,187,088 has a complex structure, and the pressure loss by a gas-liquid multiphase flow increases. 
     The invention was conceived in view of the above situation. Several aspects of the invention may provide a flow rate measurement device that has a simple configuration and a small number of error factors, and a flow rate measurement method. 
     Solution to Problem 
     (1) According to one embodiment of the invention, there is provided a batch-type multiphase flow rate measurement device including: 
     a gas-liquid separator that includes a container having a top section, a bottom section, and a hollow body section that connects the top section and the bottom section, an inlet section that supplies a gas-liquid multiphase flow to the container via a side surface of the body section, a liquid outlet section that discharges liquid via the bottom section, a gas outlet section that discharges gas via the top section, an inner pipe that is hollow, an upper end of the inner pipe being connected to the top section, and a lower end of the inner pipe being open at a position lower than a lower end of the inlet section, and a guide plate that is provided on at least one of an outer side surface of the inner pipe and an inner side surface of the body section; 
     a main pipe that includes a gas-liquid multiphase flow inlet section to which the gas-liquid multiphase flow is supplied, a gas-liquid multiphase flow outlet section from which a gas-liquid multiphase flow is discharged, a branch section, and a confluence section, the branch section and the confluence section being provided between the gas-liquid multiphase flow inlet section and the gas-liquid multiphase flow outlet section; 
     an inlet pipe that connects the branch section and the inlet section, and extends toward a center axis of the body section via the inlet section when viewed from above; 
     a liquid outlet pipe that connects the confluence section and the liquid outlet section; 
     a gas outlet pipe that connects the confluence section and the gas outlet section; 
     flow passage switch means that switches a flow passage of the gas-liquid multiphase flow between a first path that dose not pass through the gas-liquid separator and a second path that passes through the gas-liquid separator; 
     first opening/closing means that is provided in the liquid outlet pipe, and opens or closes a path from the liquid outlet section to the confluence section; 
     second opening/closing means that is provided in the gas outlet pipe, and opens or closes a path from the gas outlet section to the confluence section; 
     a pressure measurement section that measures pressure at two or more measurement points that differ in height in at least one of the gas-liquid separator and the liquid outlet pipe; and 
     a gas flowmeter that is provided in the gas outlet pipe, and measures a flow rate, temperature, and pressure of gas discharged via the gas outlet section, 
     the inner side surface of the body section and the outer side surface of the inner pipe being concentric when viewed from above, 
     the guide plate including a guide plate side section that extends in a non-horizontal direction, and a guide plate lower section that extends in a non-vertical direction and is continuous with the guide plate side section, 
     the guide plate side section being at least disposed on the inner side surface of the body section at a position on one side of the inlet section, or on the outer side surface of the inner pipe at a position on one side of an area opposite to the inlet section, 
     the guide plate lower section being at least disposed on the outer side surface of the inner pipe at a position directly under an area opposite to the inlet section along part of the outer side surface of the inner pipe when viewed from above, and a space being formed in at least part of an area between the guide plate lower section and the body section. 
     Since the space is formed in at least part of an area between the guide plate lower section and the body section, it is possible to implement a gas-liquid separator that can separate a gas-liquid multiphase flow that changes in flow rate and gas-liquid ratio to a large extent over time into gas and liquid with a high separation efficiency by using a simple configuration. This makes it possible to implement a multiphase flow rate measurement device that has a simple configuration and a small number of error factors. 
     (2) The batch-type multiphase flow rate measurement device may further include a controller section that controls the flow passage switch means, the first opening/closing means, and the second opening/closing means, and the controller section may perform a first process that causes the flow passage switch means to switch the flow passage of the gas-liquid multiphase flow from the first path to the second path, and causes the second opening/closing means to open the path from the gas outlet section to the confluence section, a second process that causes the flow passage switch means to switch the flow passage of the gas-liquid multiphase flow from the second path to the first path, and causes the second opening/closing means to close the path from the gas outlet section to the confluence section, a third process that causes the first opening/closing means to open the path from the liquid outlet section to the confluence section, and a fourth process that causes the first opening/closing means to close the path from the liquid outlet section to the confluence section. 
     This makes it possible to implement a multiphase flow rate measurement device that suppresses an error due to bubbles introduced (mixed) into the liquid outlet pipe. 
     (3) In the batch-type multiphase flow rate measurement device, the flow passage switch means may include third opening/closing means that is provided in the main pipe, and opens or closes a path from the branch section to the confluence section, and fourth opening/closing means that is provided in the inlet pipe, and opens or closes a path from the branch section to the inlet section. 
     (4) In the batch-type multiphase flow rate measurement device, the controller section may perform the first process that causes the fourth opening/closing means to open the path from the branch section to the inlet section, causes the second opening/closing means to open the path from the gas outlet section to the confluence section, and then causes the third opening/closing means to close the path from the branch section to the confluence section. 
     This makes it possible to more safely measure the flow rate. 
     (5) In the batch-type multiphase flow rate measurement device, the controller section may perform the second process that causes the third opening/closing means to open the path from the branch section to the confluence section, and then causes the fourth opening/closing means to close the path from the branch section to the inlet section. 
     This makes it possible to more safely measure the flow rate. 
     (6) The batch-type multiphase flow rate measurement device may further include a liquid flow rate calculation section that calculates a liquid flow rate, and the liquid flow rate calculation section may calculate the liquid flow rate based on pressures measured by the pressure measurement section at two or more measurement points that differ in height, and an elapsed time when the flow passage of the gas-liquid multiphase flow is the second path by the flow passage switch means. 
     (7) In the batch-type multiphase flow rate measurement device, the first opening/closing means may be provided at a position higher than the height of the confluence section. 
     This makes it possible to easily discharge liquid from the gas-liquid separator. 
     (8) The batch-type multiphase flow rate measurement device may further include a droplet separator that is provided in the gas outlet pipe, and separates droplets from gas that is discharged from the gas outlet section. 
     This makes it possible to easily determine the amount (state) of droplets introduced into the gas outlet pipe. This makes it possible to determine the liquid in gas per total liquid and the measurement accuracy. 
     (9) According to another embodiment of the invention, there is provided a batch-type multiphase flow rate measurement method using the above batch-type multiphase flow rate measurement device, the method including: 
     a first step that includes switching the flow passage of the gas-liquid multiphase flow from the first path to the second path by using the flow passage switch means, and opening the path from the gas outlet section to the confluence section by using the second opening/closing means; a second step that includes switching the flow passage of the gas-liquid multiphase flow from the second path to the first path by using the flow passage switch means, and closing the path from the gas outlet section to the confluence section by using the second opening/closing means; a third step that includes opening the path from the liquid outlet section to the confluence section by using the first opening/closing means; and a fourth step that includes closing the path from the liquid outlet section to the confluence section by using the first opening/closing means. 
     This makes it possible to implement a flow rate measurement method with a small number of error factors by using a multiphase flow rate measurement device having a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100 . 
         FIG. 2  is an exemplary schematic cross-sectional view illustrating the gas-liquid separator  100  taken along the line A-A in  FIG. 1 . 
         FIG. 3  is a partial enlarged view illustrating an example of the configuration of an inner pipe  30  and a guide plate  40  when observing the center axis of a body section  13  from an inlet section  20  in the horizontal direction. 
         FIG. 4  is a perspective view illustrating an example of the inner pipe  30  and the guide plate  40  provided on the inner pipe  30  in  FIG. 3 . 
         FIG. 5  is a partial enlarged view illustrating an example of the configuration of an inner pipe  30  and a guide plate  40  when observing the center axis of a body section  13  from an inlet section  20  in the horizontal direction. 
         FIG. 6  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100   a  according to a second configuration example. 
         FIG. 7  is an exemplary schematic cross-sectional view illustrating the gas-liquid separator  100   a  according to the second configuration example taken along the line A-A in  FIG. 6 . 
         FIG. 8  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100   b  according to a third configuration example. 
         FIG. 9  is an exemplary schematic cross-sectional view illustrating the gas-liquid separator  100   b  according to the third configuration example taken along the line A-A in  FIG. 8 . 
         FIG. 10  is an exemplary schematic cross-sectional view illustrating a gas-liquid separator  100   c  according to a fourth configuration example taken along the line A-A in  FIG. 8 . 
         FIG. 11  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100   d  according to a modification. 
         FIG. 12  is a graph illustrating the results for the liquid in gas per total liquid measured by using the gas-liquid separator  100  according to the first configuration example. 
         FIG. 13  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  1  according to a first embodiment. 
         FIG. 14  is a flowchart illustrating an example of the batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  1  according to the first embodiment. 
         FIG. 15  is a schematic view illustrating a liquid flow rate calculation example. 
         FIG. 16  is a graph illustrating an example of the relationship between the volume V and the height h. 
         FIG. 17  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  2  according to a second embodiment. 
         FIG. 18  is a schematic view illustrating a liquid flow rate calculation example. 
         FIG. 19  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  3  according to a third embodiment. 
         FIG. 20  is a schematic view illustrating an oil density/water density calculation example. 
         FIG. 21  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  4  according to a fourth embodiment. 
         FIG. 22  is a schematic view illustrating a liquid flow rate calculation example. 
         FIG. 23  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  5  according to a fifth embodiment. 
         FIG. 24  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  6  according to a sixth embodiment. 
         FIG. 25  is an exemplary schematic view illustrating an area around the flow passage switch means  130  of the meridian cross section of a batch-type multiphase flow rate measurement device  6  according to the sixth embodiment. 
         FIG. 26  is a schematic view illustrating a slug flow. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Exemplary embodiments of the invention are described in detail below with reference to the drawings. Note that the following embodiments do not unduly limit the scope of the invention as stated in the claims. Note also that all of the elements described below should not necessarily be taken as essential elements of the invention. 
     1. Configuration of Gas-Liquid Separator 
     A batch-type multiphase flow rate measurement device according to one embodiment of the invention includes a gas-liquid separator. The configuration examples of the gas-liquid separator used for the batch-type multiphase flow rate measurement device are described below. 
     1-1. First Configuration Example 
       FIG. 1  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100  according to a first configuration example.  FIG. 2  is an exemplary schematic cross-sectional view illustrating the gas-liquid separator  100  according to the first configuration example taken along the line A-A in  FIG. 1 . 
     The gas-liquid separator  100  according to the first configuration example separates a gas-liquid multiphase flow into gas and liquid, and includes a container  10  that includes a top section  11 , a bottom section  12 , and a hollow body section  13  that connects the top section  11  and the bottom section  12 , an inlet section  20  that supplies a gas-liquid multiphase flow to the container  10  via the side surface of the body section  13 , a liquid outlet section  21  that discharges liquid via the bottom section  12 , a gas outlet section  22  that discharges gas via the top section  11 , a hollow inner pipe  30 , the upper end of the inner pipe  30  being connected to the top section  11 , and the lower end of the inner pipe  30  being open at a position lower than the lower end of the inlet section  20 , and a guide plate  40  that is provided on the outer side surface of the inner pipe  30 . The inner side surface of the body section  13  and the outer side surface of the inner pipe  30  are concentric when viewed from above. The guide plate  40  includes a guide plate side section  41  that extends in a non-horizontal direction, and a guide plate lower section  42  that extends in a non-vertical direction and is continuous with the guide plate side section  41 . The guide plate side section  41  is at least disposed on the inner side surface of the body section  13  at a position on one side of the inlet section  20 , or on the outer side surface of the inner pipe  30  at a position on one side of an area opposite to the inlet section  20 , and the guide plate lower section  42  is at least disposed on the outer side surface of the inner pipe  30  at a position directly under an area opposite to the inlet section  20  along part of the outer side surface of the inner pipe  30  when viewed from above. A space  90  is formed in at least part of an area between the guide plate lower section  42  and the body section  13 . 
     The container  10  includes the top section  11 , the bottom section  12 , and the hollow body section  13  that connects the top section  11  and the bottom section  12 . In the example illustrated in  FIG. 1 , the container  10  is a hollow container that extends in the vertical direction. The horizontal cross-sectional shape of the inner side surface of the body section  13  is circular. In the example illustrated in  FIG. 1 , the body section  13  has an identical diameter from the top section  11  to the bottom section  12 . The center axis of the body section  13  is parallel to the vertical direction. Note that the invention is not limited to the above configuration. For example, part of the body section  13  may have a different inner diameter. 
     The inlet section  20  is provided in the body section  13  of the container  10  as an opening that communicates with the inner space of the container  10 . The inlet section  20  functions as a flow passage that communicates with an inlet pipe  120 , and supplies a gas-liquid multiphase flow to the container  10 . In the example illustrated in  FIG. 2 , the inlet pipe  120  is provided so that the extension of a horizontal cross section along the centerline of the inlet pipe  120  in the supply direction of the gas-liquid multiphase flow intersects the center axis of the body section  13 . It is preferable that the inlet pipe  120  be provided so that the centerline of the inlet pipe  120  intersects the center axis of the body section  13  when viewed from above taking account of the symmetry of the connection area between the body section  13  of the container  10  and the inlet pipe  120  in order to improve safety. In the example illustrated in  FIGS. 1 and 2 , the inlet section  20  has a circular shape. 
     The liquid outlet section  21  is provided in the bottom section  12  of the container  10  as an opening that communicates with the inner space of the container  10 . The liquid outlet section  21  functions as a flow passage that communicates with a liquid outlet pipe  121 , and discharges liquid separated by the gas-liquid separator  100  from the container  10 . In the example illustrated in  FIG. 1 , the liquid outlet section  21  is provided at the center of the bottom section  12 . Note that the invention is not limited to the above configuration. For example, the liquid outlet section  21  may be provided at a position offset from the center of the bottom section  12 . In the example illustrated in  FIGS. 1 and 2 , the liquid outlet section  21  has a circular shape. 
     The gas outlet section  22  is provided in the top section  11  of the container  10  as an opening that communicates with the inner space of the container  10 . The gas outlet section  22  functions as a flow passage that communicates with a gas outlet pipe  122 , and discharges gas separated by the gas-liquid separator  100  from the container  10 . In the example illustrated in  FIG. 1 , the gas outlet section  22  is provided at the center of the top section  11 . Note that the invention is not limited to the above configuration. For example, the gas outlet section  22  may be provided at a position offset from the center of the top section  11 . In the example illustrated in  FIGS. 1 and 2 , the gas outlet section  22  has a circular shape. 
     The inner pipe  30  has a hollow tubular shape. The upper end of the inner pipe  30  is connected to the top section  11  of the container  10 . In the example illustrated in  FIG. 1 , the upper end of the inner pipe  30  is seal-tightly connected to the top section  11  of the container  10 . The lower end of the inner pipe  30  is open at a position lower than the lower end of the inlet section  20 . The inner pipe  30  communicates with the gas outlet section  22 . 
     The cross-sectional shape of the outer side surface of the inner pipe  30  is circular. As illustrated in  FIG. 2 , the inner side surface of the body section  13  of the container  10  and the outer side surface of the inner pipe  30  are concentric when viewed from above. In the example illustrated in  FIGS. 1 and 2 , the horizontal cross-sectional shape of the inner side surface of the inner pipe  30  is circular, and the inner side surface of the inner pipe  30  has an identical diameter. Note that the invention is not limited to the above configuration. For example, part of the inner pipe  30  may have a different inner diameter and/or a different outer diameter. 
     In the example illustrated in  FIGS. 1 and 2 , the guide plate  40  is provided on the outer side surface of the inner pipe  30 . As illustrated in  FIG. 2 , the guide plate  40  has a partial circular shape when viewed from above. It is preferable the guide plate  40  and the outer side surface of the inner pipe  30  be closely joined by welding or the like in order to ensure sufficient mounting strength and seal-tightness. 
       FIG. 3  is a partial enlarged view illustrating an example of the configuration of the inner pipe  30  and the guide plate  40  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction.  FIG. 4  is a perspective view illustrating an example of the inner pipe  30  and the guide plate  40  provided on the inner pipe  30  in  FIG. 3 . 
     As illustrated in  FIGS. 2 and 3 , the guide plate  40  includes the guide plate side section  41  that extends in a non-horizontal direction, and the guide plate lower section  42  that extends in a non-vertical direction and is continuous with the guide plate side section  41 . The guide plate side section  41  is at least disposed on the outer side surface of the inner pipe  30  at a position on one side of an area opposite to the inlet section  20 . The guide plate lower section  42  is at least disposed on the outer side surface of the inner pipe  30  at a position directly under an area opposite to the inlet section  20  along part of the outer side surface of the inner pipe  30  when viewed from above. 
     The guide plate  40  allows a gas-liquid multiphase flow supplied via the inlet section  20  to flow through the space between the inner side surface of the body section  13  of the container  10  and the outer side surface of the inner pipe  30  while whirling from one side to the other side of the inlet section  20  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. This makes it possible to separate the gas-liquid multiphase flow into gas and liquid by utilizing the centrifugal force. 
     It is preferable that the guide plate lower section  42  be provided on the outer side surface of the inner pipe  30  at least within a range from the position directly under the guide plate side section  41  to the position directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction, and provided within an angular range of 40 to 180 degrees around the center axis of the body section  13  when viewed from above. In the example illustrated in  FIG. 2 , the guide plate lower section  42  is provided to have a partial circular shape within an angular range of 90 degrees from the position directly under the guide plate side section  41  when viewed from above. If the guide plate lower section  42  is provided within an angular range of 40 degrees or more, a gas-liquid multiphase flow supplied via the inlet section  20  can be easily guided to the desired whirl direction. If the guide plate lower section  42  is provided within an angular range of 180 degrees or less, a situation in which a gas-liquid multiphase flow supplied via the inlet section  20  unnecessarily whirls can be prevented, so that a pressure loss can be suppressed. 
     It is preferable that the guide plate lower section  42  be positioned so that the distance between the guide plate lower section  42  and the lower end of the inlet section  20  is equal to or less than twice the vertical dimension of the inlet section  20 . It is more preferable that the guide plate lower section  42  be positioned so that the distance between the guide plate lower section  42  and the lower end of the inlet section  20  is equal to or less than the vertical dimension of the inlet section  20 . This prevents a situation in which a gas-liquid multiphase flow supplied via the inlet section  20  unnecessarily flows downward, so that a sufficient centrifugal force can be applied to the gas-liquid multiphase flow. 
     As illustrated in  FIGS. 3 and 4 , the guide plate  40  may include a first guide plate  41   a  that is provided on the outer side surface of the inner pipe  30 , and forms the guide plate side section  41 , and a second guide plate  42   a  that is provided on the outer side surface of the inner pipe  30 , and forms the guide plate lower section  42 . In the example illustrated in  FIGS. 3 and 4 , the first guide plate  41   a  and the second guide plate  42   a  adhere to the outer side surface of the inner pipe  30 . 
     In the example illustrated in  FIGS. 3 and 4 , the first guide plate  41   a  is formed in the shape of a plate that extends in the vertical direction. The second guide plate  42   a  is formed in the shape of a plate that extends in the horizontal direction. The first guide plate  41   a  and the second guide plate  42   a  are provided to come in contact with each other. It is preferable that the first guide plate  41   a  and the second guide plate  42   a  be joined by welding or the like so that leakage of a gas-liquid multiphase flow supplied via the inlet section  20  does not occur. Note that the invention is not limited to the above configuration. For example, the first guide plate  41   a  (guide plate side section  41 ) may be inclined with respect to the vertical direction, and the second guide plate  42   a  (guide plate lower section  42 ) may be inclined with respect to the horizontal direction. At least one of the first guide plate  41   a  (guide plate side section  41 ) and the second guide plate  42   a  (guide plate lower section  42 ) may be curved. 
     In the example illustrated in  FIGS. 3 and 4 , the upper side of the second guide plate  42   a  is horizontal (i.e., extends in the horizontal direction). Note that the invention is not limited to the above configuration. For example, the upper side of the second guide plate  42   a  (guide plate lower section  42 ) may be inclined downward to the inner side surface of the container  10 , or may be inclined downward to the outer side surface of the inner pipe  30 . 
       FIG. 5  is a partial enlarged view illustrating another example of the configuration of the inner pipe  30  and the guide plate  40  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. As illustrated in  FIG. 5 , the guide plate side section  41  and the guide plate lower section  42  of the guide plate  40  may be integrated when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. In the example illustrated in  FIG. 5 , the guide plate  40  is formed by a single plate, and adheres to the outer side surface of the inner pipe  30 . 
     As illustrated in  FIG. 3  or  5  and  FIG. 2 , the space  90  is formed in at least part of the area between the guide plate lower section  42  and the body section  13  of the container  10 . In the example illustrated in  FIGS. 2 and 3 , the space  90  is formed between the guide plate lower section  42  (second guide plate  42   a ) and the body section  13  of the container  10 . 
     The space  90  allows a high-density liquid to be separated from a gas-liquid multiphase flow supplied via the inlet section  20 , and flow downward along the inner side surface of the container  10  before the gas-liquid multiphase flow reaches the lower end of the guide plate lower section  42 . Specifically, the space  90  functions as a flow passage that allows liquid separated from a gas-liquid multiphase flow to quickly flow downward inside the container  10 . This makes it possible to enlarge the flow passage for gas separated from a gas-liquid multiphase flow, so that the gas-liquid multiphase flow can be separated into gas and liquid with a high separation efficiency even if there is an increase in the flow rate and the ratio of liquid. Moreover, a pressure loss can be suppressed by providing the space  90 . Note that the space  90  may be provided at a plurality of positions. 
     The gas-liquid separator  100  according to the first configuration example can separate a gas-liquid multiphase flow that changes in flow rate and gas-liquid ratio to a large extent over time into gas and liquid with a high separation efficiency and with a simple configuration. 
     1-2. Second Configuration Example 
       FIG. 6  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100   a  according to a second configuration example, and  FIG. 7  is an exemplary schematic cross-sectional view illustrating the gas-liquid separator  100   a  taken along the line A-A in  FIG. 6 . 
     The guide plate lower section  42  may be formed so that a space is not continuously formed between the guide plate lower section  42  and the inner side surface of the body section  13  at least in an area from a position directly under the guide plate side section  41  to a position directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. Specifically, the guide plate  40  may continuously come in contact with the inner side surface of the body section  13  of the container  10  at least in an area directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. In the example illustrated in  FIGS. 6 and 7 , a space is not continuously formed between the second guide plate  42   a  (guide plate lower section  42 ) and the inner side surface of the body section  13  in an area from a position directly under the guide plate side section  41  to a position directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20 . 
     According to this configuration, a centrifugal force can be applied to fluid at a position around the inlet section  20 , and liquid that has moved to the inner side surface of the container  10  due to a centrifugal force can be efficiently discharged through the space  90  at a position away from the inlet section  20 . 
     As illustrated in  FIG. 7 , the gas-liquid separator  100   a  may be configured so that the width (dimension) of the space  90  increases as the distance from the end of the guide plate lower section  42  decreases. 
     According to this configuration, liquid that has moved to the inner side surface of the container  10  due to a centrifugal force can be efficiently discharged through the space  90 . 
     1-3. Third Configuration Example 
       FIG. 8  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100   b  according to a third configuration example, and  FIG. 9  is an exemplary schematic cross-sectional view illustrating the gas-liquid separator  100   b  taken along the line A-A in  FIG. 8 . 
     As illustrated in  FIGS. 8 and 9 , the gas-liquid separator  100   b  may include a lower-area leakage prevention plate  72  that closes the space formed between the guide plate lower section  42  and the body section  13 . The space formed between the body section  13  and the guide plate lower section  42  at least from a position directly under the guide plate side section  41  to a position directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20  may be closed by the lower-area leakage prevention plate  72  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. In the example illustrated in  FIGS. 8 and 9 , the lower-area leakage prevention plate  72  adheres to the inner side surface of the body section  13 . The lower-area leakage prevention plate  72  may adhere to the guide plate lower section  42  (second guide plate  42   a ) in the area from the position directly under the guide plate side section  41  (first guide plate  41   a ) to an area directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20 . 
     Since the second guide plate  42   a  is provided on the outer side surface of the inner pipe  30 , a space is formed between the second guide plate  42   a  and the inner side surface of the body section  13  of the container  10  during production. Leakage of a gas-liquid multiphase flow through the space formed between the second guide plate  42   a  and the inner side surface of the body section  13  of the container  10  can be prevented by providing the lower-area leakage prevention plate  72  on the inner side surface of the body section  13 . This makes it possible to effectively apply a centrifugal force to the gas-liquid multiphase flow. 
     As illustrated in  FIG. 9 , the gas-liquid separator  100   b  may be configured so that the width (dimension) of the space  90  increases as the distance from the end of the guide plate lower section  42  decreases. 
     According to this configuration, liquid that has moved to the inner side surface of the container  10  due to a centrifugal force can be efficiently discharged through the space  90 . 
     1-4. Modifications of First Configuration Example, Second Configuration Example, and Third Configuration Example 
     As illustrated in  FIG. 2  and  FIG. 7  or  9 , the gas-liquid separator  100  according to the first configuration example, the gas-liquid separator  100   a  according to the second configuration example, and the gas-liquid separator  100   b  according to the third configuration example may include a side-area leakage prevention plate  70  that closes the space formed between the guide plate side section  41  and the body section  13 . In the example illustrated in  FIGS. 2 ,  7 , and  9 , the side-area leakage prevention plate  70  adheres to the inner side surface of the body section  13 . The side-area leakage prevention plate  70  may adhere to the guide plate side section  41 . 
     In the example illustrated in  FIGS. 2 ,  7 , and  9 , since the guide plate side section  41  (first guide plate  41   a ) is provided on the outer side surface of the inner pipe  30 , a space is formed between the guide plate side section  41  (first guide plate  41   a ) and the inner side surface of the body section  13  of the container  10  during production. Leakage of a gas-liquid multiphase flow through the space formed between the guide plate side section  41  (first guide plate  41   a ) and the inner side surface of the body section  13  of the container  10  can be prevented by providing the side-area leakage prevention plate  70  on the inner side surface of the body section  13 . This makes it possible to effectively apply a centrifugal force to the gas-liquid multiphase flow. 
     Note that the side-area leakage prevention plate  70  and the lower-area leakage prevention plate  72  may be integrally formed (i.e., may be formed by a single member). For example, the side-area leakage prevention plate  70  and the lower-area leakage prevention plate  72  may be formed by a single plate. 
     1-5. Fourth Configuration Example 
       FIG. 10  is an exemplary schematic cross-sectional view illustrating a gas-liquid separator  100   c  according to a fourth configuration example taken along the line A-A in  FIG. 8 . The meridian cross section of the gas-liquid separator  100   c  is the same as that in the example illustrated in  FIG. 8 . 
     As illustrated in  FIGS. 8 and 10 , the guide plate  40  may include a first guide plate  41   b  that is provided on the inner side surface of the body section  13 , and forms the guide plate side section  41 , and a second guide plate  42   a  that is provided on the outer side surface of the inner pipe  30 , and forms the guide plate lower section  42 . 
     Specifically, the first guide plate  41   b  (guide plate side section  41 ) is at least disposed on the inner side surface of the body section  13  at a position on one side of the inlet section  20 , and the second guide plate  42   a  (guide plate lower section  42 ) is at least disposed on the outer side surface of the inner pipe  30  at a position directly under an area opposite to the inlet section  20  along part of the outer side surface of the inner pipe  30  when viewed from above. In the example illustrated in  FIGS. 8 and 10 , the first guide plate  41   b  adheres to the inner side surface of the body section  13 . The second guide plate  42   a  adheres to the outer side surface of the inner pipe  30 . 
     In the example illustrated in  FIGS. 8 and 10 , the first guide plate  41   b  is formed in the shape of a plate that extends in the vertical direction. The second guide plate  42   a  is formed in the shape of a plate that extends in the horizontal direction. The first guide plate  41   b  and the second guide plate  42   a  are provided to come in contact with each other. Note that the invention is not limited to the above configuration. For example, the first guide plate  41   b  (guide plate side section  41 ) may be inclined with respect to the vertical direction, and the second guide plate  42   a  (guide plate lower section  42 ) may be inclined with respect to the horizontal direction. At least one of the first guide plate  41   b  (guide plate side section  41 ) and the second guide plate  42   a  (guide plate lower section  42 ) may be curved. 
     In the example illustrated in  FIGS. 8 and 10 , the upper side of the second guide plate  42   a  is horizontal (i.e., extends in the horizontal direction). Note that the invention is not limited to the above configuration. For example, the upper side of the second guide plate  42   a  (guide plate lower section  42 ) may be inclined downward to the inner side surface of the container  10 , or may be inclined downward to the outer side surface of the inner pipe  30 . 
     As illustrated in  FIG. 10 , the gas-liquid separator  100   c  may include a side-area leakage prevention plate  70   a  that closes the space formed between the guide plate side section  41  (first guide plate  41   b ) and the inner pipe  30 . In the example illustrated in  FIG. 10 , the side-area leakage prevention plate  70   a  adheres to the outer side surface of the inner pipe  30 . The side-area leakage prevention plate  70   a  may adhere to the guide plate side section  41  (first guide plate  41   b ). 
     In the example illustrated in  FIG. 10 , since the guide plate side section  41  (first guide plate  41   b ) is provided on the inner side surface of the body section  13 , a space is formed between the guide plate side section  41  (first guide plate  41   b ) and the outer side surface of the inner pipe  30  during production. Leakage of a gas-liquid multiphase flow through the space formed between the guide plate side section  41  (first guide plate  41   b ) and the outer side surface of the inner pipe  30  can be prevented by providing the side-area leakage prevention plate  70   a  on the outer side surface of the inner pipe  30 . This makes it possible to effectively apply a centrifugal force to the gas-liquid multiphase flow. 
     As illustrated in  FIGS. 8 and 10 , the gas-liquid separator  100   c  may include a lower-area leakage prevention plate  72  that closes the space formed between the guide plate lower section  42  and the body section  13 . The space formed between the body section  13  and the guide plate lower section  42  at least from a position directly under the guide plate side section  41  to a position directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20  may be closed by the lower-area leakage prevention plate  72  when observing the center axis of the body section  13  from the inlet section  20  in the horizontal direction. In the example illustrated in  FIGS. 8 and 10 , the lower-area leakage prevention plate  72  adheres to the inner side surface of the body section  13 . The lower-area leakage prevention plate  72  may adhere to the guide plate lower section  42  (second guide plate  42   a ) in the area from the position directly under the guide plate side section  41  (first guide plate  41   b ) to an area directly under an area of the outer side surface of the inner pipe  30  opposite to the inlet section  20 . 
     Since the second guide plate  42   a  is provided on the outer side surface of the inner pipe  30 , a space is formed between the second guide plate  42   a  and the inner side surface of the body section  13  of the container  10  during production. Leakage of a gas-liquid multiphase flow through the space formed between the second guide plate  42   a  and the inner side surface of the body section  13  of the container  10  can be prevented by providing the lower-area leakage prevention plate  72  on the inner side surface of the body section  13 . This makes it possible to effectively apply a centrifugal force to the gas-liquid multiphase flow. 
     1-6. Fifth Configuration Example 
       FIG. 11  is an exemplary schematic view illustrating the meridian cross section of a gas-liquid separator  100   d  according to a modification. A container  10   a  of the gas-liquid separator  100   d  illustrated in  FIG. 11  includes a lid section that includes a top section  11   a , and a container main body that includes a bottom section  12   a  and a body section  13   a . The lid section and the container main body are connected by a flange coupling. The gas-liquid separator  100   d  is configured in the same manner as the gas-liquid separator  1  illustrated in  FIG. 1  except for the above feature. 
     The gas-liquid separator  100  illustrated in  FIG. 11  has the same effects as those of the gas-liquid separator  1  for the above reasons. 
     1-7. Experimental Examples 
     In the following experimental examples, gas-liquid separation was implemented by using the gas-liquid separator  100  according to the first configuration example. 
     The gas-liquid separator  100  used for the experiments had a configuration in which the diameter of the inner side surface of the body section  13  was 200 mm, the diameter of the outer side surface of the inner pipe  30  was about 165 mm, the distance between the inner side surface of the body section  13  and the outer side surface of the inner pipe  30  was about 17 mm, and the diameter of the inlet section  20  was 50 mm. The guide plate lower section  42  was provided on the outer side surface of the inner pipe  30  within an angular range of 90 degrees from the position directly under the guide plate side section  41  when viewed from above. The width of the space  90  formed between the guide plate lower section  42  and the inner side surface of the body section  13  was about 5 mm. 
     The experiments were performed as follows. Specifically, the liquid outlet section  21  was closed. A water-nitrogen two-phase fluid prepared by mixing water and nitrogen gas at predetermined flow rates was supplied via the inlet section  20 . A volume V 1  of liquid supplied within a time t for which the liquid surface increased to a predetermined position below the lower end of the inner pipe  30  inside the container  10 , and a volume S of liquid discharged via the gas outlet section  22  and separated (captured) by the droplet separator within the time t were measured. The above operation was repeated while changing the flow rate of water and the flow rate of nitrogen gas to measure the volume V 1  and the volume S. 
     The flow rate Vn of nitrogen gas used for the water-nitrogen two-phase fluid was changed within the range of “0 m 3 /h&lt;Vn&lt;300 m 3 /h”. The flow rate Vh of water used for the water-nitrogen two-phase fluid was changed in four stages (i.e., 1 m 3 /h, 5 m 3 /h, 10 m 3 /h, and 15 m 3 /h). 
     The liquid in gas per total liquid was calculated by the following expression using the volume V 1  and the volume S measured by the above operation. 
       Liquid in gas per total liquid (%)=(volume  S /(volume  S +volume  V 1))×100
 
     Specifically, a low liquid in gas per total liquid indicates a high gas-liquid separation efficiency. 
       FIG. 12  is a graph illustrating the results for the liquid in gas per total liquid measured by using the gas-liquid separator  100  according to the first configuration example. The horizontal axis indicates the flow rate of nitrogen gas used for the water-nitrogen two-phase fluid, and the vertical axis indicates the liquid in gas per total liquid. A symbol that indicates the measurement point indicates the flow rate of water used for the water-nitrogen two-phase fluid. 
     As illustrated in  FIG. 12 , the liquid in gas per total liquid was 1% or less under the above measurement conditions. 
     The gas-liquid separator  100  according to the first configuration example thus can separate a gas-liquid multiphase flow into gas and liquid with a high separation efficiency and with a simple configuration over a wide flow rate range and a wide gas-liquid ratio range. 
     2. Batch-Type Multiphase Flow Rate Measurement Device 
     2-1. Batch-Type Multiphase Flow Rate Measurement Device According to First Embodiment 
       FIG. 13  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  1  according to a first embodiment. The batch-type multiphase flow rate measurement device  1  includes the gas-liquid separator  100  described in the section entitled “1-1. First configuration example”. Note that the gas-liquid separator  100   a ,  100   b ,  100   c , or  100   d  may be used instead of the gas-liquid separator  100 . 
     The batch-type multiphase flow rate measurement device  1  includes a main pipe  110 . The main pipe  110  includes a gas-liquid multiphase flow inlet section  111  to which a gas-liquid multiphase flow is supplied, a gas-liquid multiphase flow outlet section  112  from which a gas-liquid multiphase flow is discharged, a branch section  113 , and a confluence section  114 , the branch section  113  and the confluence section  114  being provided between the gas-liquid multiphase flow inlet section  111  and the gas-liquid multiphase flow outlet section  112 . In the example illustrated in  FIG. 13 , the confluence section  114  includes a first confluence section  114   a  that is positioned relatively close to the gas-liquid multiphase flow inlet section  111 , and a second confluence section  114   b  that is positioned relatively away from the gas-liquid multiphase flow inlet section  111 . Note that the confluence section  114  may include a first confluence section  114   a  that is positioned relatively away from the gas-liquid multiphase flow inlet section  111 , and a second confluence section  114   b  that is positioned relatively close to the gas-liquid multiphase flow inlet section  111 . 
     The batch-type multiphase flow rate measurement device  1  includes an inlet pipe  120 . The inlet pipe  120  connects the branch section  113  and the inlet section  20  of the gas-liquid separator  100 . Specifically, the inlet pipe  120  communicates with the main pipe  110  via the branch section  113 , and communicates with the inner space of the gas-liquid separator  100  via the inlet section  20 . The inlet pipe  120  extends toward the center axis of the body section  13  of the gas-liquid separator  100  via the inlet section  20  when viewed from above. 
     The batch-type multiphase flow rate measurement device  1  includes a liquid outlet pipe  121 . The liquid outlet pipe  121  connects the confluence section  114  and the liquid outlet section  21 . Specifically, the liquid outlet pipe  121  communicates with the main pipe  110  via the confluence section  114 , and communicates with the inner space of the gas-liquid separator  100  via the liquid outlet section  21 . In the example illustrated in  FIG. 13 , the liquid outlet pipe  121  connects the first confluence section  114   a  and the liquid outlet section  21 . 
     The batch-type multiphase flow rate measurement device  1  includes a gas outlet pipe  122 . The gas outlet pipe  122  connects the confluence section  114  and the gas outlet section  22 . Specifically, the gas outlet pipe  122  communicates with the main pipe  110  via the confluence section  114 , and communicates with the inner space of the gas-liquid separator  100  via the gas outlet section  22 . In the example illustrated in  FIG. 13 , the gas outlet pipe  122  connects the second confluence section  114   b  and the gas outlet section  22 . 
     The batch-type multiphase flow rate measurement device  1  includes a flow passage switch means  130 . The flow passage switch means  130  switches the flow passage of a gas-liquid multiphase flow between a first path that dose not pass through the gas-liquid separator  100  and a second path that passes through the gas-liquid separator  100 . In the example illustrated in  FIG. 13 , the flow passage switch means  130  includes a third opening/closing means  133  that is provided in the main pipe  110 , and opens or closes the path from the branch section  113  to the confluence section  114 , and a fourth opening/closing means  134  that is provided in the inlet pipe  120 , and opens or closes the path from the branch section  113  to the inlet section  20  of the gas-liquid separator  100 . A known valve may be used as the third opening/closing means  133  and the fourth opening/closing means  134 . Note that the flow passage switch means  130  may include a three-way valve that switches the flow passage between the first path and the second path. 
     The batch-type multiphase flow rate measurement device  1  includes a first opening/closing means  131 . The first opening/closing means  131  is provided in the liquid outlet pipe  121 , and opens or closes the path from the liquid outlet section  21  of the gas-liquid separator  100  to the confluence section  114 . A known valve may be used as the first opening/closing means  131 . 
     The first opening/closing means  131  may be provided at a position higher than the height of the confluence section  114 . This makes it possible to easily discharge liquid from the gas-liquid separator  100 . In the example illustrated in  FIG. 13 , the first opening/closing means  131  is provided at a position lower than the height of the gas-liquid separator  100  and higher than the height of the first confluence section  114   a . It is preferable that the liquid outlet pipe  121  provided from the liquid outlet section  21  to the confluence section  114  (first confluence section  114   a ) be a linear pipe. This makes it possible to prevent a situation in which liquid remains in the liquid outlet pipe  121 , so that the time required for discharging liquid from the gas-liquid separator  100  can be reduced. 
     The batch-type multiphase flow rate measurement device  1  includes a second opening/closing means  132 . The second opening/closing means  132  is provided in the gas outlet pipe  122 , and opens or closes the path from the gas outlet section  22  of the gas-liquid separator  100  to second confluence section  114   b . A known valve may be used as the second opening/closing means  132 . 
     The batch-type multiphase flow rate measurement device  1  includes a pressure measurement section  200 . The pressure measurement section  200  measures pressure at two or more measurement points that differ in height in at least one of the gas-liquid separator  100  and the liquid outlet pipe  121  above the first opening/closing means  131 . In the example illustrated in  FIG. 13 , the pressure measurement section  200  includes a pressure measurement connecting pipe  201  that is provided in the liquid outlet pipe  121 , and used as a pressure measurement point, pressure measurement connecting pipes  202  and  203  that are provided in the body section  13  of the gas-liquid separator  100 , and used as pressure measurement points, a differential pressure transmitter  210  that outputs a signal based on the difference between the pressure inside the connecting pipe  201  and the pressure inside the connecting pipe  202 , and a differential pressure transmitter  220  that outputs a signal based on the difference between the pressure inside the connecting pipe  202  and the pressure inside the connecting pipe  203 . In the example illustrated in  FIG. 13 , the connecting pipe  201  is provided at a position lower than the connecting pipe  202 , and the connecting pipe  202  is provided at a position lower than the connecting pipe  203 . It is preferable that the connecting pipe  201  be positioned close to the first opening/closing means  131  from the viewpoint of the liquid flow rate measurement accuracy (described later). It is preferable that the connecting pipe  203  be positioned below the lower end of the inner pipe  30  from the view point of the pressure measurement accuracy. 
     The batch-type multiphase flow rate measurement device  1  may include a controller section  300  that controls the flow passage switch means  130 , the first opening/closing means  131 , the second opening/closing means  132 , and the pressure measurement section  200 . The controller section  300  may be implemented by a dedicated circuit that performs each control process described later, or may be implemented by causing a central processing unit (CPU) (i.e., computer) to execute a control program stored in a storage means (not shown) or the like to perform each control process, for example. A control example of the controller section  300  is described later in the section entitled “2-2. Batch-type multiphase flow rate measurement method using batch-type multiphase flow rate measurement device according to first embodiment”. 
     The batch-type multiphase flow rate measurement device  1  may include a liquid flow rate calculation section  400 . The liquid flow rate calculation section  400  calculates the flow rate (liquid flow rate) of liquid included in a gas-liquid multiphase flow. The liquid flow rate calculation section  400  calculates the liquid flow rate based on pressures measured by the pressure measurement section  200  at two or more measurement points that differ in height, and the elapsed time when the flow passage of a gas-liquid multiphase flow is the second path by the flow passage switch means  130 . A calculation example of the liquid flow rate calculation section  400  is described later in the section entitled “2-3. Liquid flow rate calculation example”. 
     The batch-type multiphase flow rate measurement device  1  includes a gas flowmeter  500 . The gas flowmeter  500  is provided in the gas outlet pipe  122 , and measures the flow rate of gas discharged via the gas outlet section  22  of the gas-liquid separator  100 . A volumetric flowmeter, a mass flowmeter, or the like may be used as the gas flowmeter  500 . The gas flowmeter  500  includes a thermometer and a pressure gauge necessary for calculating the gas flow rate in a normal state, or includes functions of measuring the temperature and the pressure. The gas flowmeter  500  may be provided on the upstream side or the downstream side of the second opening/closing means  132 . When the gas flowmeter  500  is provided on the upstream side of the second opening/closing means  132 , the temperature T and the pressure P measured when the pressure measurement section  200  measures the pressure the liquid flow rate are desirably used to calculate the oil density, the water density, and the liquid density (described later). When the gas flowmeter  500  is provided on the downstream side of the second opening/closing means  132 , the average temperature T and the average pressure P when gas stably flows immediately before the second opening/closing means  132  is in a closed state are desirably used to calculate the oil density, the water density, and the liquid density. 
     2-2. Batch-Type Multiphase Flow Rate Measurement Method Using Batch-Type Multiphase Flow Rate Measurement Device According to First Embodiment 
     A batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  1  according to the first embodiment is described below.  FIG. 14  is a flowchart illustrating an example of the batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  1  according to the first embodiment. 
     The batch-type multiphase flow rate measurement method illustrated in  FIG. 14  includes a first step that includes switching the flow passage of a gas-liquid multiphase flow from the first path to the second path by using the flow passage switch means  130 , and opening the path from the gas outlet section  22  to the confluence section  114  by using the second opening/closing means  132  (step S 100 ), a second step that includes switching the flow passage of a gas-liquid multiphase flow from the second path to the first path by using the flow passage switch means  130 , and closing the path from the gas outlet section  22  to the confluence section  114  by using the second opening/closing means  132  (step S 102 ), a third step that includes measuring the pressure at two or more measurement points that differ in height in at least one of the gas-liquid separator  100  and the liquid outlet pipe  121  by using the pressure measurement section  200  (step S 104 ), a fourth step that includes opening the path from the liquid outlet section  21  to the confluence section  114  by using the first opening/closing means  131  (step S 106 ), and a fifth step that includes closing the path from the liquid outlet section  21  to the confluence section  114  by using the first opening/closing means  131  (step S 108 ). 
     An example in which the batch-type multiphase flow rate measurement method illustrated in  FIG. 14  is mainly implemented under control of the controller section  300  is described below. Note that the batch-type multiphase flow rate measurement method illustrated in  FIG. 14  may be implemented by a manual operation or the like. 
     In the first step (step S 100 ), the controller section  300  performs a first process that causes the flow passage switch means  130  to switch the flow passage of a gas-liquid multiphase flow from the first path to the second path, and causes the second opening/closing means  132  to open the path from the gas outlet section  22  to the confluence section  114 . For example, the controller section  300  may perform the first process that causes the fourth opening/closing means  134  to open the path from the branch section  113  to the inlet section  20  of the gas-liquid separator  100 , causes the second opening/closing means  132  to open the path from the gas outlet section  22  to the confluence section  114 , and then causes the third opening/closing means  133  to close the path from the branch section  113  to the confluence section  114 . This makes it possible to prevent a situation in which the flow of a gas-liquid multiphase flow is blocked, so that the flow rate can be safely measured. 
     In the second step (step S 102 ), the controller section  300  performs a second process that causes the flow passage switch means  130  to switch the flow passage of a gas-liquid multiphase flow from the second path to the first path, and causes the second opening/closing means  132  to close the path from the gas outlet section  22  to the confluence section  114 . For example, the controller section  300  may perform the second process that causes the third opening/closing means  133  to open the path from the branch section  113  to the confluence section  114 , and then causes the fourth opening/closing means  134  to close the path from the branch section  113  to the inlet section  20  of the gas-liquid separator  100 . This makes it possible to prevent a situation in which the flow of a gas-liquid multiphase flow is blocked, so that the flow rate can be safely measured. 
     The controller section  300  may perform the first step (step S 100 ) when a predetermined time td 1  has elapsed after the measurement results started to be recorded, for example. The controller section  300  may perform the second step (step S 102 ) when a predetermined pressure is measured by the pressure measurement section  200 , for example. In the example illustrated in  FIG. 13 , the controller section  300  may perform the second step (step S 102 ) when a difference in pressure equal to or greater than a predetermined value dP 1  has been measured by the differential pressure transmitter  220 . The predetermined value dP 1  may be a difference in pressure that corresponds to a state in which the liquid surface is positioned between the connecting pipe  202  and the connecting pipe  203 , for example. 
     In the third step (step S 104 ), the controller section  300  performs a third process that causes the pressure measurement section  200  to measure the pressure at two or more measurement points that differ in height in at least one of the gas-liquid separator  100  and the liquid outlet pipe  121 . In the example illustrated in  FIG. 13 , the pressure measurement section  200  measures the pressure at three measurement points at which the connecting pipes  201 ,  202 , and  203  are respectively provided. The liquid flow rate can be calculated based on the pressure measured by the pressure measurement section  200 . The liquid flow rate calculation section  400  may calculate the liquid flow rate. A liquid flow rate calculation example is described later. 
     The controller section  300  may perform the third step (step S 104 ) when a predetermined time td 2  has elapsed after completion of the second step (step S 102 ), for example. The predetermined time td 2  may be a time required for the liquid surface to become stable within the required measurement accuracy range, for example. The predetermined time td 2  may be experimentally determined depending on the specification of the batch-type multiphase flow rate measurement device  1 . 
     Note that the controller section  300  may also perform the third process in a period other than the third step (step S 104 ). For example, the controller section  300  may successively perform the third process in a period from a time before the first step (step S 100 ) is started to a time after completion of the fifth step (step S 108 ). 
     In the fourth step (step S 106 ), the controller section  300  performs a fourth process that causes the first opening/closing means  131  to open the path from the liquid outlet section  21  to the confluence section  114 . The controller section  300  may perform the fourth step (step S 106 ) after completion of the third step (step S 104 ), for example. 
     In the fifth step (step S 108 ), the controller section  300  performs a fifth process that causes the first opening/closing means  131  to close the path from the liquid outlet section  21  to the confluence section  114 . In the example illustrated in  FIG. 13 , the controller section  300  may perform the fifth step (step S 108 ) when a difference in pressure equal to or less than a predetermined value dP 2  has been measured by the differential pressure transmitter  210 . The predetermined value dP 2  may be a difference in pressure that corresponds to a state in which the liquid surface is lower than the connecting pipe  201 , for example. 
     2-3. Liquid Flow Rate Calculation Example when Using Batch-Type Multiphase Flow Rate Measurement Method Using Batch-Type Multiphase Flow Rate Measurement Device According to First Embodiment 
     A liquid flow rate calculation example when using the batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  1  according to the first embodiment is described below taking a gas-liquid multiphase flow that consists of oil, water, and gas as an example. The following description is given on the assumption that oil and water are separated (i.e., an emulsion is not formed). 
       FIG. 15  is a schematic view illustrating a liquid flow rate calculation example.  FIG. 15  schematically illustrates the meridian cross section of the main part of the batch-type multiphase flow rate measurement device  1 .  FIG. 15  illustrates a state in which the liquid surface is positioned between the connecting pipe  202  and the connecting pipe  203 , and the water-oil interface is positioned between the connecting pipe  201  and the connecting pipe  202  in the third step (step S 104 ). 
     The height of the water-oil interface from the height of the connecting pipe  201  is referred to as h W , the height of the connecting pipe  202  from the height of the connecting pipe  201  is referred to as h 1 , and the height of the liquid surface from the height of the connecting pipe  201  is referred to as h L . The oil density calculated under the pressure P at the temperature T measured by the gas flowmeter  500  is referred to as ρ O , the water density calculated under the pressure P at the temperature T measured by the gas flowmeter  500  is referred to as ρ W , the gravitational acceleration is referred to as g, the pressure difference output by the differential pressure transmitter  210  is referred to as dP W-1 , the pressure difference output by the differential pressure transmitter  220  is referred to as dP L , and the elapsed time from the start of the first step (step S 100 ) to completion of the second step (step S 102 ) is referred to as dt L . 
     When the oil density ρ O  is lower than the water density ρ W , the height h L  of the liquid surface from the height of the connecting pipe  201  is calculated by the following expression (1). 
     
       
         
           
             
               
                 
                   
                     h 
                     L 
                   
                   = 
                   
                     
                       h 
                       1 
                     
                     + 
                     
                       
                         dP 
                         L 
                       
                       
                         
                           ρ 
                           o 
                         
                          
                         g 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The height h W  of the water-oil interface from the height of the connecting pipe  201  is calculated by the following expression (2). 
     
       
         
           
             
               
                 
                   
                     h 
                     W 
                   
                   = 
                   
                     
                       
                         dP 
                         
                           W 
                           - 
                           1 
                         
                       
                       - 
                       
                         
                           ρ 
                           o 
                         
                          
                         
                           gh 
                           1 
                         
                       
                     
                     
                       
                         ( 
                         
                           
                             ρ 
                             W 
                           
                           - 
                           
                             ρ 
                             o 
                           
                         
                         ) 
                       
                        
                       g 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     When the vertical height from the connecting pipe  201  is referred to as h, the volume V of the liquid outlet pipe  121  and the gas-liquid separator  100  from the upper part of the first opening/closing means  131  to the vertical height h is expressed by a function of the vertical height h corresponding to the shape of the liquid outlet pipe  121  and the gas-liquid separator  100 . Specifically, the volume V is expressed by V=f v (h).  FIG. 16  is a graph illustrating an example of the relationship between the volume V and the height h. The liquid flow rate (liquid volume flow rate) Q L  is calculated by the following expression (3) using the above function, and the water cut WC is calculated by the following expression (4) using the above function. 
     
       
         
           
             
               
                 
                   
                     Q 
                     L 
                   
                   = 
                   
                     
                       
                         f 
                         V 
                       
                        
                       
                         ( 
                         
                           h 
                           L 
                         
                         ) 
                       
                     
                     
                       dt 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   WC 
                   = 
                   
                     
                       
                         f 
                         V 
                       
                        
                       
                         ( 
                         
                           h 
                           W 
                         
                         ) 
                       
                     
                     
                       
                         f 
                         V 
                       
                        
                       
                         ( 
                         
                           h 
                           L 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     2-4. Gas Flow Rate Calculation Example when Using Batch-Type Multiphase Flow Rate Measurement Method Using Batch-Type Multiphase Flow Rate Measurement Device According to First Embodiment 
     A gas flow rate calculation example when using the batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  1  according to the first embodiment is described below taking a gas-liquid multiphase flow that consists of oil, water, and gas as an example. 
     When the start time of the first step (step S 100 ) is referred to as t 1 , the start time of the second step (step S 102 ) is referred to as t 2 , and the time required for the gas flow rate to become stable after the time t 1  is referred to as dt, the average gas volume flow rate is indicated by the average value of the gas volume flow rate Q G  measured by the gas flowmeter  500  from the time t 1 +dt to the time t 2 . Specifically, the average gas volume flow rate is given by the following expression (5). 
       Average gas volume flow rate    Q     G =average value of gas volume flow rate  Q   G   (5)
 
     Since gas present in the gas-liquid separator  100  is discharged to the gas outlet pipe  122  as liquid separated by the gas-liquid separator  100  is accumulated in the gas-liquid separator  100 , a more accurate average gas volume flow rate is given by the following expression (6). 
       More accurate average gas volume flow rate=   Q     GT   =  Q     G   −Q   L   (6)
 
     Note that the gas volume flow rate can be converted into the gas volume flow rate in a normal state by using the average temperature and the average pressure from the time t 1 +dt to the time t 2 . 
     2-5. Batch-Type Multiphase Flow Rate Measurement Device According to Second Embodiment 
       FIG. 17  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  2  according to a second embodiment. Note that the elements identical to those of the batch-type multiphase flow rate measurement device  1  according to the first embodiment are indicated by identical symbols (reference numerals), and detailed description thereof is omitted. 
     The batch-type multiphase flow rate measurement device  2  includes a connecting pipe  204  that is provided in the body section  13  of the gas-liquid separator  100 , and used as a pressure measurement point. In the example illustrated in  FIG. 17 , the connecting pipe  201  is provided at a position lower than the connecting pipe  204 , the connecting pipe  204  is provided at a position lower than the connecting pipe  202 , and the connecting pipe  202  is provided at a position lower than the connecting pipe  203 . The horizontal cross-sectional area in the gas-liquid separator  100  is identical in an area between the connecting pipe  204  and the connecting pipe  203 . It is preferable that the connecting pipe  204  be disposed at a position close to the lowest part of the range in which the horizontal cross-sectional area in the gas-liquid separator  100  is identical within the allowable design range. 
     The pressure measurement section  200  of the batch-type multiphase flow rate measurement device  2  includes a differential pressure transmitter  211  that outputs a signal based on the difference between the pressure inside the connecting pipe  201  and the pressure inside the connecting pipe  204 , and a differential pressure transmitter  212  that outputs a signal based on the difference between the pressure inside the connecting pipe  204  and the pressure inside the connecting pipe  202 , instead of the differential pressure transmitter  210 . 
     The batch-type multiphase flow rate measurement device  2  can measure the flow rate of liquid included in a gas-liquid multiphase flow in the same manner as described in the section entitled “2-2. Batch-type multiphase flow rate measurement method using batch-type multiphase flow rate measurement device according to first embodiment”. 
     2-6. Liquid Flow Rate Calculation Example when Using Batch-Type Multiphase Flow Rate Measurement Method Using Batch-Type Multiphase Flow Rate Measurement Device According to Second Embodiment 
     A liquid flow rate calculation example when using the batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  2  according to the second embodiment is described below taking a gas-liquid multiphase flow that consists of oil, water, and gas as an example. The following description is given on the assumption that oil and water are not separated (i.e., an emulsion is present) in the lower area. 
       FIG. 18  is a schematic view illustrating a liquid flow rate calculation example.  FIG. 18  schematically illustrates the meridian cross section of the main part of the batch-type multiphase flow rate measurement device  2 .  FIG. 18  illustrates a state in which the liquid surface is positioned between the connecting pipe  202  and the connecting pipe  203  in the third step (step S 104 ). 
     The height of the connecting pipe  202  from the height of the connecting pipe  201  is referred to as h 1 , the height of the connecting pipe  204  from the height of the connecting pipe  201  is referred to as h 2 , and the height of the liquid surface from the height of the connecting pipe  201  is referred to as h L . The oil density calculated under the pressure P at the temperature T measured by the gas flowmeter  500  is referred to as ρ O , the water density calculated under the pressure P at the temperature T measured by the gas flowmeter  500  is referred to as ρ W , the gravitational acceleration is referred to as g, the pressure difference output by the differential pressure transmitter  211  is referred to as dP W-2 , and the pressure difference output by the differential pressure transmitter  212  is referred to as dP W-3 . 
     When the oil density ρ O  is lower than the water density ρ W , and it is considered that only oil is present at a position higher than the height of the connecting pipe  202 , the height h L  of the liquid surface from the height of the connecting pipe  201  is calculated by the above expression (1). 
     When the vertical height from the connecting pipe  201  is referred to as h, the volume V of the liquid outlet pipe  121  and the gas-liquid separator  100  from the upper part of the first opening/closing means  131  to the vertical height h is expressed as a function of the vertical height h corresponding to the shape of the liquid outlet pipe  121  and the gas-liquid separator  100 . Specifically, the volume V is expressed by V=f v (h). Specifically, the volume V W  of water is given by the following expression (7) using the above function. 
     
       
         
           
             
               
                 
                   
                     V 
                     W 
                   
                   = 
                   
                     
                       
                         
                           f 
                           V 
                         
                          
                         
                           ( 
                           
                             h 
                             2 
                           
                           ) 
                         
                       
                        
                       
                         
                           
                             dP 
                             
                               W 
                               - 
                               2 
                             
                           
                           - 
                           
                             g 
                              
                             
                                 
                             
                              
                             
                               ρ 
                               o 
                             
                              
                             
                               h 
                               2 
                             
                           
                         
                         
                           
                             ( 
                             
                               
                                 ρ 
                                 W 
                               
                               - 
                               
                                 ρ 
                                 o 
                               
                             
                             ) 
                           
                            
                           
                             gh 
                             2 
                           
                         
                       
                     
                     + 
                     
                       
                         ( 
                         
                           
                             
                               f 
                               V 
                             
                              
                             
                               ( 
                               
                                 h 
                                 1 
                               
                               ) 
                             
                           
                           - 
                           
                             
                               f 
                               V 
                             
                              
                             
                               ( 
                               
                                 h 
                                 2 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                        
                       
                         
                           
                             dP 
                             
                               W 
                               - 
                               3 
                             
                           
                           - 
                           
                             g 
                              
                             
                                 
                             
                              
                             
                               
                                 ρ 
                                 o 
                               
                                
                               
                                 ( 
                                 
                                   
                                     h 
                                      
                                     
                                         
                                     
                                      
                                     1 
                                   
                                   - 
                                   
                                     h 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             ( 
                             
                               
                                 ρ 
                                 W 
                               
                               - 
                               
                                 ρ 
                                 o 
                               
                             
                             ) 
                           
                            
                           
                             g 
                              
                             
                               ( 
                               
                                 
                                   h 
                                    
                                   
                                       
                                   
                                    
                                   1 
                                 
                                 - 
                                 
                                   h 
                                   2 
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     The water cut WC is given by the following expression (8) using the volume V W  of water. 
     
       
         
           
             
               
                 
                   WC 
                   = 
                   
                     
                       V 
                       W 
                     
                     
                       
                         f 
                         V 
                       
                        
                       
                         ( 
                         
                           h 
                           L 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     2-7. Batch-Type Multiphase Flow Rate Measurement Device According to Third Embodiment 
       FIG. 19  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  3  according to a third embodiment. Note that the elements identical to those of the batch-type multiphase flow rate measurement device  2  according to the second embodiment are indicated by identical symbols (reference numerals), and detailed description thereof is omitted. 
     The batch-type multiphase flow rate measurement device  3  includes a connecting pipe  205  that is provided in the body section  13  of the gas-liquid separator  100 , and used as a pressure measurement point. In the example illustrated in  FIG. 19 , the connecting pipe  201  is provided at a position lower than the connecting pipe  204 , the connecting pipe  204  is provided at a position lower than the connecting pipe  205 , the connecting pipe  205  is provided at a position lower than the connecting pipe  202 , and the connecting pipe  202  is provided at a position lower than the connecting pipe  203 . 
     The pressure measurement section  200  of the batch-type multiphase flow rate measurement device  3  includes a differential pressure transmitter  213  that outputs a signal based on the difference between the pressure inside the connecting pipe  204  and the pressure inside the connecting pipe  205 , and a differential pressure transmitter  214  that outputs a signal based on the difference between the pressure inside the connecting pipe  205  and the pressure inside the connecting pipe  202 , instead of the differential pressure transmitter  212 . 
     The batch-type multiphase flow rate measurement device  3  can measure the flow rate of liquid included in a gas-liquid multiphase flow in the same manner as described in the section entitled “2-2. Batch-type multiphase flow rate measurement method using batch-type multiphase flow rate measurement device according to first embodiment”. 
     2-8. Oil Density/Water Density Calculation Example when Using Batch-Type Multiphase Flow Rate Measurement Device According to Third Embodiment 
     An oil density/water density calculation example when using the batch-type multiphase flow rate measurement device  3  according to the third embodiment is described below taking a gas-liquid multiphase flow that consists of oil, water, and gas as an example. The following description is given on the assumption that oil and water are separated (i.e., an emulsion is not formed). 
       FIG. 20  is a schematic view illustrating an oil density/water density calculation example.  FIG. 20  schematically illustrates the meridian cross section of the main part of the batch-type multiphase flow rate measurement device  3 .  FIG. 20  illustrates a state in which the liquid surface is positioned between the connecting pipe  202  and the connecting pipe  203 , and the water-oil interface is positioned between the connecting pipe  204  and the connecting pipe  205  in the third step (step S 104 ). 
     The height of the connecting pipe  202  from the height of the connecting pipe  201  is referred to as h 1 , the height of the connecting pipe  204  from the height of the connecting pipe  201  is referred to as h 2 , and the height of the connecting pipe  205  from the height of the connecting pipe  201  is referred to as h 3 . The gravitational acceleration is referred to as g, the pressure difference output by the differential pressure transmitter  211  is referred to as dP W , and the pressure difference output by the differential pressure transmitter  214  is referred to as dP O . 
     In this case, the oil density ρ O  under pressure is calculated by the following expression (9), and the water density ρ W  is calculated by the following expression (10). 
     
       
         
           
             
               
                 
                   
                     ρ 
                     o 
                   
                   = 
                   
                     
                       dP 
                       o 
                     
                     
                       g 
                        
                       
                         ( 
                         
                           
                             h 
                             1 
                           
                           - 
                           
                             h 
                             3 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     ρ 
                     W 
                   
                   = 
                   
                     
                       dP 
                       W 
                     
                     
                       gh 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
       
     
     When the liquid surface is oil forming, the liquid surface may not between the connecting pipe  202  and the connecting pipe  203  but below the connecting pipe  202  after the completion of the second step (S 102 ). In such case, when the liquid surface is between the connecting pipe  205  and the connecting pipe  202 , the batch-type multiphase flow rate measurement device  3  according to the third embodiment can calculate the liquid flow rate and WC by the same method of the batch-type multiphase flow rate measurement device  1  according to the first embodiment. 
     2-9. Batch-Type Multiphase Flow Rate Measurement Device According to Fourth Embodiment 
       FIG. 21  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  4  according to a fourth embodiment. Note that the elements identical to those of the batch-type multiphase flow rate measurement device  1  according to the first embodiment are indicated by identical symbols (reference numerals), and detailed description thereof is omitted. 
     The batch-type multiphase flow rate measurement device  4  is configured in that same manner as the batch-type multiphase flow rate measurement device  1 , except that the connecting pipe  202  and the differential pressure transmitter  220  are omitted. In the example illustrated in  FIG. 21 , the connecting pipe  201  is provided at a position lower than the connecting pipe  203 . 
     The pressure measurement section  200  of the batch-type multiphase flow rate measurement device  4  includes a differential pressure transmitter  215  that outputs a signal based on the difference between the pressure inside the connecting pipe  201  and the pressure inside the connecting pipe  203 , instead of the differential pressure transmitter  210 . 
     The batch-type multiphase flow rate measurement device  4  can measure the flow rate of liquid included in a gas-liquid multiphase flow in the same manner as described in the section entitled “2-2. Batch-type multiphase flow rate measurement method using batch-type multiphase flow rate measurement device according to first embodiment”. 
     2-10. Liquid Flow Rate Calculation Example when Using Batch-Type Multiphase Flow Rate Measurement Method Using Batch-Type Multiphase Flow Rate Measurement Device According to Fourth Embodiment 
     A liquid flow rate calculation example when using the batch-type multiphase flow rate measurement method using the batch-type multiphase flow rate measurement device  4  according to the fourth embodiment is described below taking a gas-liquid multiphase flow that consists of a single-phase liquid (i.e., liquid having a uniform liquid density) and gas as an example. 
       FIG. 22  is a schematic view illustrating a liquid flow rate calculation example.  FIG. 22  schematically illustrates the meridian cross section of the main part of the batch-type multiphase flow rate measurement device  4 .  FIG. 22  illustrates a state in which the liquid surface is positioned between the connecting pipe  201  and the connecting pipe  203  in the third step (step S 104 ). 
     The height of the liquid surface from the height of the connecting pipe  201  is referred to as h L . The liquid density calculated under the pressure P at the temperature T measured by the gas flowmeter  500  is referred to as ρ L , the gravitational acceleration is referred to as g, the pressure difference output by the differential pressure transmitter  215  is referred to as dP L , and the elapsed time from the start of the first step (step S 100 ) to completion of the second step (step S 102 ) is referred to as dt L . 
     In this case, the height h L  of the liquid surface is calculated by the following expression (11), and the liquid flow rate Q L  is calculated by the following expression (12). 
     
       
         
           
             
               
                 
                   
                     h 
                     L 
                   
                   = 
                   
                     
                       dP 
                       L 
                     
                     
                       
                         ρ 
                         L 
                       
                        
                       g 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
             
               
                 
                   
                     Q 
                     L 
                   
                   = 
                   
                     
                       
                         f 
                         V 
                       
                        
                       
                         ( 
                         
                           h 
                           L 
                         
                         ) 
                       
                     
                     
                       dt 
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
           
         
       
     
     2-11. Batch-Type Multiphase Flow Rate Measurement Device According to Fifth Embodiment 
       FIG. 23  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  5  according to a fifth embodiment. Note that the elements identical to those of the batch-type multiphase flow rate measurement device  1  according to the first embodiment are indicated by identical symbols (reference numerals), and detailed description thereof is omitted. 
     The batch-type multiphase flow rate measurement device  5  includes a droplet separator  600  that is provided in the gas outlet pipe  122 , and separates droplets from gas that is discharged from the gas outlet section  22  of the gas-liquid separator  100 . 
     The droplet separator  600  that separates droplets from gas may be a T-shaped pipe, a Y-shaped pipe, an inverted triangular pipe, or a funnel pipe that communicates with the lower side of the gas outlet pipe  122  via an opening having a cross-sectional area larger than that of the gas outlet pipe  122  so that the fluid velocity (flow rate) is reduced. It is preferable that the gas outlet pipe  122  disposed on the upstream side of the droplet separator  600  have a sufficient horizontal dimension so that separation of droplets from gas is promoted. It is preferable that the gas outlet pipe  122  disposed on the upstream side of the droplet separator  600  be a horizontal pipe or a downwardly inclined pipe. In the example illustrated in  FIG. 23 , the droplet separator  600  is a funnel pipe that communicates with the lower side of the gas outlet pipe  122  via an opening having a cross-sectional area larger than that of the gas outlet pipe  122 . The droplet separator  600  guides liquid mist and a liquid phase that are mixed in gas that passes through the gas outlet pipe  122  to a pipe  610  positioned below the gas outlet pipe  122 , so that a situation in which liquid flows into the gas outlet pipe  122  that is positioned on the downstream side of the droplet separator  600  can be prevented. This makes it possible to more accurately measure the gas flow rate. 
     Moreover, the amount (state) of droplets introduced into the gas outlet pipe  122  can be easily determined by providing the droplet separator  600 . This makes it possible to determine the liquid in gas per total liquid and the measurement accuracy. For example, the measurement accuracy of the liquid flow rate and the gas flow rate is determined to be low when the liquid in gas per total liquid is high, and is determined to be high when the liquid in gas per total liquid is low. 
     The pipe  610  may communicate with the inner space of the gas-liquid separator  100  via a connecting pipe  620 . The liquid flow rate can be more accurately measured by returning liquid separated by the droplet separator  600  to the gas-liquid separator  100 . 
     The batch-type multiphase flow rate measurement device  5  may include a fifth opening/closing means  135  that is provided in the pipe  610 , and opens or closes the path from the droplet separator  600  to the connecting pipe  620 . A known valve may be used as the fifth opening/closing means  135 . In the flowchart illustrated in  FIG. 14 , the fifth opening/closing means  135  is closed in the steps S 100 , S 102 , and S 104 , and is opened after completion of the step S 104 . After returning liquid separated by the droplet separator  600  to the gas-liquid separator  100 , the step S 104  is performed. The step S 106  is then performed after (or at the same time as) closing the fifth opening/closing means  135 . This makes it possible to return liquid separated by the droplet separator  600  to the gas-liquid separator  100 , and determine the liquid in gas per total liquid. It is preferable that the connecting pipe  620  be disposed at a position equal to or higher than the connecting pipe  203 . It is preferable that the pipe  610  does not extend through an area lower than the connecting pipe  620 . This makes it possible to smoothly discharge liquid accumulated in the droplet separator  600  to the gas-liquid separator  100 . 
     2-12. Batch-Type Multiphase Flow Rate Measurement Device According to Sixth Embodiment 
       FIG. 24  is an exemplary schematic view illustrating the meridian cross section of a batch-type multiphase flow rate measurement device  6  according to a sixth embodiment, and  FIG. 25  is an exemplary schematic view illustrating an area of the batch-type multiphase flow rate measurement device  6  according to the sixth embodiment around the flow passage switch means  130 . Note that the elements identical to those of the batch-type multiphase flow rate measurement device  1  according to the first embodiment are indicated by identical symbols (reference numerals), and detailed description thereof is omitted. The flow passage switch means  130  of the batch-type multiphase flow rate measurement device  6  according to the sixth embodiment is provided at a position that differs in height from the inlet section  20  of the gas-liquid separator  100  to only a small extent. 
       FIG. 26  is a schematic view illustrating a slug flow.  FIG. 26  illustrates the vertical section of a pipe. Liquid in the pipe is indicated by diagonal lines, and gas is indicated in white. In the example illustrated in  FIG. 26 , a gas-liquid multiphase flow supplied in the direction indicated by a white arrow flows through a horizontal pipe, a vertical pipe, and a horizontal pipe in this order. As illustrated in  FIG. 26 , formation of slug (i.e., the vertical pipe is filled with liquid) starts as indicated by (1), and is completed as indicated by (2). As illustrated in  FIG. 26 , gas enters the vertical pipe as indicated by (3), and passes through the vertical pipe as indicated by (4). 
     When the slug illustrated in  FIG. 26  has occurred, the liquid flow rate measurement accuracy decreases. Since the batch-type multiphase flow rate measurement device  6  according to the sixth embodiment is configured so that the flow passage switch means  130  is provided at a position that differs in height from the inlet section  20  of the gas-liquid separator  100  to only a small extent, occurrence of a slug flow can be suppressed. This makes it possible to implement a batch-type multiphase flow rate measurement device that achieves high measurement accuracy. 
     Note that the above embodiments and the modifications thereof are merely examples, and the invention is not limited to the above embodiments and the modifications thereof. For example, a plurality of embodiments and/or a plurality of modifications may be appropriately combined. 
     The invention is not limited to the above embodiments. Various modifications and variations may be made without departing from the scope of the invention. For example, the invention includes various other configurations that are substantially the same as the configurations described in connection with the above embodiments (e.g., a configuration having the same function, method, and results, or a configuration having the same objective and results). The invention also includes a configuration in which an unsubstantial section (element) described in connection with the above embodiments is replaced with another section (element). The invention also includes a configuration having the same effects as those of the configurations described in connection with the above embodiments, or a configuration capable of achieving the same objective as that of the configurations described in connection with the above embodiments. The invention also includes a configuration in which a known technique is added to the configurations described in connection with the above embodiments.