Critical flow in moisture generation systems for natural gas

Abstract

Moisture generation and analysis device (40) for generating and analyzing moisture of a flowing natural gas stream, the device comprising: a channel for extracting a sample stream of natural gas from the flowing natural gas stream; a moisture analyzer (132) that analyzes the moisture content of the sample stream supplied to it; a bypass flow channel (142) that provides a flow path through the moisture generation and analysis device (40) that extends parallel to a flow path of the moisture analyzer (132) without passing through the moisture analyzer (132); a main opening (120) which is fluidly connected to the moisture generation and analysis device (40) and which is arranged upstream of the moisture analyzer (132), wherein the sample stream supplied to the moisture analyzer (132) is discharged to the main opening (120) and flows through it; and a bypass opening (122) arranged along the bypass flow channel (142); The main opening (120) is dimensioned such that the sample stream supplied to the moisture analyzer (132) via the main opening (120) is provided from the main opening (120) at a pressure such that the sample stream supplied to the moisture analyzer (132) is at a critical flow condition, wherein the absolute pressure prevailing upstream at the main opening (120) is at least 1.98 times the downstream absolute pressure at the main opening (120), and the bypass opening (122) is dimensioned such that the bypass current (F b ) the bypass opening (122) is supplied with a pressure such that the bypass flow (F b ), which flows through the bypass opening (122), is at a critical flow condition, wherein the upstream absolute pressure at the bypass opening (122) is at least 1.98 times the downstream absolute pressure at the bypass opening (122).

Description

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

[0001] The invention relates to the liquefaction of natural gas, and more particularly, the present invention relates to monitoring moisture in natural gas before the natural gas flows to liquefaction. DISCUSSION OF THE STATE OF THE ART

[0002] There are various reasons for liquefaction (i.e., liquid production processes) of gases, and natural gas in particular. For example, one purpose of liquefying raw natural gas is to separate / liquefy it into various hydrocarbon components. Another example of the purpose of liquefying natural gas is that liquefaction reduces the volume of a gas to approximately 1 / 600, which allows the liquefied gas to be stored and transported in containers that are of more cost-effective and practical design. For example, when transporting gas from a source / reservoir via a pipeline to a distant market, it is desirable to operate at an essentially constant load factor. Often, capacity will exceed demand, while at other times demand may exceed the capacity of the pipeline.

[0003] To mitigate peaks where demand would exceed supply, it is desirable to store gas when supply exceeds demand, whereby peaks in demand can be met using stored gas. To this end, it is desirable to enable the storage of gas in a liquefied state and to vaporize the liquid as required by demand. In another embodiment, the liquefaction of natural gas is useful for enabling the transportation of gas from a source of abundant supply to a distant market, especially when the supply source cannot be directly connected to the market via a pipeline. This is particularly true when transportation is to be carried out by ocean-going tanker.The transport of natural gas in the gaseous state would be uneconomical unless the natural gas were highly compressed, in which case the system would be uneconomical because it would be practically impractical to provide containers of suitable strength and capacity.

[0004] Converting natural gas to a liquefied state (i.e., liquefied natural gas or LNG) requires cooling to a temperature of approximately -240°F to -260°F (-151°C to -162°C) at atmospheric pressure. Numerous systems exist for liquefying natural gas or similar products, in which the gas is liquefied by passing it sequentially through several cooling stages to gradually cool the gas to lower temperatures until the liquefaction temperature is reached. For example, cooling can be achieved by indirect heat exchange using one or more expanded refrigerants such as propane, propylene, ethane, ethylene, and methane.

[0005] In general, gas, and especially raw natural gas from a wellhead, must be processed before it can be liquefied. Such processing may involve a process known as "desulfurization," which involves the removal of hydrogen sulfide (H2S) and carbon dioxide (CO2). The removal of mercury (Hg) and moisture (water, H2O) occurs after "desulfurization."

[0006] Dehydration refers to the removal of moisture from a gas (e.g., natural gas). For example, elevated moisture in a natural gas pipeline (e.g., in a liquefied natural gas pipeline) under high pressure can form methane hydrate and block the pipeline. Furthermore, condensation of moisture can corrode equipment, for example. As yet another example, elevated moisture can lead to ice formation, which can significantly damage cryogenic equipment if the amount of ice is large. Generally, the moisture content in natural gas must be reduced to below 1 ppm (part per million) before it can be further cryogenically processed.

[0007] A liquefaction system (e.g., in a gas liquefaction plant) typically incorporates a moisture analyzer to monitor the gas after a gas dehydration process has been performed on the gas, but before a liquefaction process is performed on the gas. The moisture analyzer's purpose is to monitor the moisture content and ensure that it remains below a desired level (e.g., 1 ppm) before the gas flows to the liquefaction process. If the moisture analyzer indicates a moisture content that exceeds the desired level (e.g., an action level value), this can arise from various sources. For example, the action level indication may be a result of a leak in the system or a saturated dryer that is due for regeneration.Before taking any action to eliminate the cause of the action level reading, it is sometimes useful to ensure that the action moisture level indicated by the moisture analyzer is accurate. This means that it may be useful to verify the operation of the moisture analyzer.

[0008] A moisture generation system can be used to impart a known moisture content to a gas in a moisture generation and analysis facility. An example of a moisture generation system may include a permeation tube in which water is provided. In this case, such a gas with a known moisture content can be supplied to the moisture analyzer. If the humidity reading from the moisture analyzer (e.g., the recorded reading) for the gas with known moisture content is within a certain tolerance range of an expected reading, the moisture analyzer is considered reliable. Once the reliability of the moisture analyzer has been confirmed, a logical search can be made for other possible reasons for the action humidity level reading.For example, a technician may need to check for a faulty leak, or a dryer may need to be shut down for regeneration. If the moisture reading from the moisture analyzer for the gas with a known moisture content is not within a certain tolerance range of the expected reading, the moisture analyzer and verification system can be carefully re-examined before taking any further action.

[0009] It is clear that providing the known moisture content to the gas and supplying such a gas with a known moisture content to the moisture analyzer is a selective process. That is, such provision of gas with a known moisture content to the moisture analyzer is performed only at a few selected times during the entire operation of the liquefaction system (e.g., a gas liquefaction plant). Therefore, the moisture generation and analysis device comprises components for selectively imparting moisture to the gas and for selectively supplying the gas with a known moisture content to the moisture analyzer.

[0010] Of course, it may not be practical to completely shut off the gas flow through the moisture generation and analysis device if the moisture analyzer is not being selectively operated to detect moisture content. If the gas flow through the moisture generation and analysis device were to completely shut off, moisture could accumulate within the moisture generation and analysis device. Such moisture accumulation could damage components of the moisture generation and analysis device. Furthermore, such moisture accumulation could adversely affect the operation and / or accuracy of the moisture analyzer in the device.For example, such moisture accumulation could occur in one or more dead spaces in / near the moisture generation and analysis device, and when the gas flow toward the moisture generation and analysis device is initiated or occurs, such moisture accumulation could contaminate the gas stream. The contamination would be in the form of added moisture above the level that would otherwise be present in the gas.

[0011] To prevent unwanted moisture accumulation in / near the moisture generation and analysis device, an additional stream / flow is used that flows through the assembly but not through the moisture analyzer. This additional stream is referred to as a bypass stream because the stream bypasses the moisture analyzer in the assembly. One or more switching valves are provided in the moisture generation and analysis device to select the gas flow path, either through the moisture analyzer or through the bypass channel.

[0012] US 4,131,011 A describes a device for determining the end point of drying in a flowing liquid stream, in which a reduction in pressure is used to divert the liquid stream through a moisture sensor with return of the sample to the main liquid stream. The device comprises a tubular element for the first conduit with a curved section to form an opening opposite the liquid stream. A second conduit with a similar shape to the first conduit, but with a larger diameter, is also curved and provided with an opening located downstream of the liquid stream and at a distance from the first opening. The opposite ends of the tubular elements are provided with a measuring device, e.g.a humidity sensor, which is preferably connected to a signal processor that can be used to operate a switch or other control device, e.g. to control a drying oven.

[0013] It has been observed that a flow rate discontinuity may occur during switching of the gas flow path selection (i.e., via the humidity analyzer or via the bypass channel). It has also been observed that the flow rate discontinuity may result in the humidity analyzer no longer being able to operate flawlessly and produce an accurate reading of the sensed humidity. For example, the humidity generation system (e.g., a permeation tube) may provide an incorrect / uneven / fluctuating humidity level during and / or after switching of the gas flow path selection (i.e., via the humidity analyzer or via the bypass channel). In particular, excessive moisture generation may occur, for example. As a result, a longer settling time may be required until the humidity analyzer is again able to produce an accurate / reliable reading.Accordingly, there is a need for improvements in moisture generation and analysis equipment. Such improvements would provide a technical advantage by reducing the duration of the process step, thus enabling improvements in the timeframe for the determination of the flowing gas. BRIEF DESCRIPTION OF THE INVENTION

[0014] The object of the invention is achieved by the moisture generation and analysis device for generating and analyzing moisture of a flowing natural gas stream according to claim 1 and the method for providing and analyzing moisture of a flowing natural gas stream by means of a moisture generation and analysis device according to claim 10.

[0015] The following brief description provides a simplified summary to provide a basic understanding of some aspects of the systems and / or methods discussed herein. This brief description is not an exhaustive overview of the systems and / or methods discussed herein. It is not intended to identify important / crucial elements or limit the scope of protection of such systems and / or methods. Its sole purpose is to present some concepts in a simplified form as an introduction to the more detailed description presented below.

[0016] One aspect of the invention provides a moisture generation and analysis device for generating and analyzing moisture from a flowing natural gas stream. The device includes a conduit for obtaining a natural gas sample stream from the flowing natural gas stream. The device includes a moisture analyzer that analyzes a moisture content of the sample stream supplied to it. The device includes a bypass flow channel that extends in fluid parallel to the moisture analyzer and provides a flow path through the moisture generation and analysis device without traversing the moisture analyzer. The device includes a main opening that is fluidly connected in the moisture generation and analysis device and that is located upstream of the moisture analyzer. The sample stream supplied to the moisture analyzer is output to and flows through the main opening.The arrangement includes a bypass opening arranged along the bypass flow channel. The main opening is suitably sized, and the sample flow delivered to the moisture analyzer via the main opening is provided at a pressure from the main opening such that the sample flow supplied to the moisture analyzer is at a critical flow condition. The bypass opening is suitably sized, and the bypass flow is supplied to the bypass opening at a pressure such that the bypass flow flowing through the bypass opening is at a critical flow condition. Components in the moisture generation and analysis device are suitably selected so that the critical flow rate through the main opening and the critical flow rate through the bypass opening match.

[0017] The above-mentioned moisture generation and analysis device may include: a gas purifier for removing moisture from the natural gas sample stream, a bypass line bypassing the gas purifier, and a first valve for selecting a flow path through the gas purifier or through the bypass line.

[0018] Each moisture generation and analysis device mentioned above may include: a permeation tube for introducing moisture into at least a portion of the natural gas sample stream to produce a moisture-laden gas, a permeation tube bypass line bypassing the permeation tube with respect to a natural gas stream that does not experience the introduction of moisture, and a second and third valve for selectively connecting the permeation tube to either the main port or the bypass port, and for selectively connecting the permeation tube bypass line to the other port, be it the main port or the bypass port.

[0019] The first to third valves of each moisture generation and analysis device mentioned above can be operated to provide a process mode, a dry mode, and a wet mode in the moisture generation and analysis device.

[0020] Each moisture generation and analysis device mentioned above may be designed such that the natural gas sample stream obtained from the flowing natural gas stream is supplied in the process mode by means of the first valve along the bypass line bypassing the gas purifier, by means of the second valve via the bypass line bypassing the permeation tube, to the main port and the moisture analyzer.

[0021] Each moisture generation and analysis device mentioned above may be designed so that the third valve directs gas flowing from the permeation tube to the bypass opening in the process mode.

[0022] Each moisture generation and analysis device mentioned above may be designed such that the natural gas sample stream obtained from the flowing natural gas stream is directed in the dry mode by means of the first valve through the gas purifier, by means of the second valve via the bypass line bypassing the permeation tube, to the main opening and to the moisture analyzer.

[0023] Each moisture generation and analysis device mentioned above may be designed so that the third valve directs gas flowing from the permeation tube to the bypass opening in the dry mode.

[0024] Each moisture generation and analysis device mentioned above may be constructed such that the natural gas sample stream obtained from the flowing natural gas stream is directed in the wet mode by means of the first valve through the gas purifier and by means of the third valve via the permeation tube to the main opening and to the moisture analyzer.

[0025] Each of the above-mentioned moisture generation and analysis devices may be designed such that, in the humid mode, the second valve directs gas flowing from the bypass line bypassing the permeation tube to the bypass port.

[0026] Each moisture generation and analysis device mentioned above can be designed such that the second valve causes a negligible pressure drop on the stream flowing therethrough, and that the third valve causes a negligible pressure drop on the stream flowing therethrough, wherein the permeation tube chamber causes a negligible pressure drop on the stream flowing therethrough, so that a pressure upstream of the main opening and a pressure upstream of the bypass opening are the same.

[0027] Each moisture generation and analysis device mentioned above may be designed such that the moisture analyzer and the bypass opening direct the outlet flow to an exhaust stack having a variable pressure, wherein the critical flow condition through the main opening and the critical flow condition through the bypass opening are not affected by the variable pressure in the exhaust stack.

[0028] Another aspect of the invention provides a method for providing and analyzing moisture in a flowing natural gas stream using a moisture generation and analysis device. The method includes the step of providing a channel for obtaining a natural gas sample stream from the flowing natural gas stream. The method includes the step of providing a moisture analyzer that analyzes a moisture content of the sample stream supplied to it. The method includes the steps of providing a bypass flow channel that extends in fluid parallel to the moisture analyzer and provides a flow path through the moisture generation and analysis device without traversing the moisture analyzer.The method includes the step of providing a main orifice fluidly connected in the moisture generation and analysis device and located upstream of the moisture analyzer. The sample stream delivered to the moisture analyzer is fed to and flows through the main orifice. The method includes the step of providing a bypass orifice disposed along the bypass flow channel. The method includes appropriately sizing the main orifice and delivering the sample stream to the moisture analyzer via the main orifice at a pressure from the main orifice such that the sample stream delivered to the moisture analyzer is at a critical flow condition.The method involves appropriately sizing the bypass opening and delivering the bypass flow to the bypass opening at a pressure such that the bypass flow flowing through the bypass opening is at a critical flow condition. Components in the moisture generation and analysis device are appropriately selected so that the critical flow rate through the main opening and the critical flow rate through the bypass opening match.

[0029] The method may include the steps of: providing a gas purifier for removing moisture from the natural gas sample stream, providing a bypass line bypassing the gas purifier and a first valve for selecting a flow path either through the gas purifier or through the bypass line; and selectively actuating the first valve.

[0030] Any of the above-mentioned methods may include the steps of: providing a permeation tube for introducing moisture into at least a portion of the natural gas sample stream to produce a moisture-laden gas; providing a permeation tube bypass conduit bypassing the permeation tube with respect to a natural gas stream that does not accept introduction of moisture; and providing second and third valves for selectively connecting the permeation tube to either the main port or the bypass port, and for selectively connecting the permeation tube bypass conduit to the other port, be it the main port or the bypass port; and selectively actuating the second and third valves.

[0031] Any of the above-mentioned methods may include the steps of selectively operating the first valve and selectively actuating the second and third valves in the moisture generation and analysis device providing a process mode, a dry mode, and a wet mode.

[0032] Any of the above-mentioned methods may include supplying the natural gas sample stream obtained from the flowing natural gas stream in the process mode by means of the first valve along the bypass line bypassing the gas purifier, by means of the second valve via the bypass line bypassing the permeation tube to the main port and the moisture analyzer, and the third valve directing gas flowing from the permeation tube to the bypass port.

[0033] Any of the above-mentioned methods may include directing the natural gas sample stream obtained from the flowing natural gas stream in the dry mode by means of the first valve through the gas purifier, by means of the second valve via the bypass line bypassing the permeation tube to the main port and to the moisture analyzer, and the third valve directing gas flowing from the permeation tube to the bypass port.

[0034] Any of the above-mentioned methods may include directing the natural gas sample stream obtained from the flowing natural gas stream in the wet mode by means of the first valve through the gas purifier, by means of the third valve via the permeation tube to the main port and to the moisture analyzer, and directing gas flowing from the bypass line bypassing the permeation tube to the bypass port.

[0035] Any of the above-mentioned methods may include the moisture analyzer and the bypass port directing the exhaust flow to an exhaust stack having a variable pressure, wherein the critical flow condition through the main port and the critical flow condition through the bypass port are unaffected by the variable pressure in the exhaust stack. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The above-mentioned and other aspects of the invention will become apparent to those skilled in the art after reading the following description and the accompanying drawings: Fig. Figure 1 schematically illustrates a liquefied natural gas processing system incorporating a moisture generation and analysis device utilizing at least one aspect of the present invention; and Fig.Figure 2 schematically illustrates the moisture generation and analysis device of Fig. 1 and shows details according to at least one aspect of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0037] Embodiments utilizing one or more aspects of the invention are described and illustrated in the drawings. These illustrated examples are not intended to limit the invention. For example, one or more aspects of the invention may be utilized in other embodiments and even in other types of devices. Furthermore, certain terminology is used herein for convenience only and is not intended to limit the invention. Furthermore, like reference numerals are used throughout the figures to refer to like elements.

[0038] An embodiment of a liquefied natural gas (LNG) processing system 10 having at least one feature according to the present invention is shown schematically in Fig. 1. It should be noted that the Fig. 1 is merely an example, and that other examples having at least one feature according to the present invention are possible and contemplated.

[0039] With reference to the example of Fig. 1, untreated or "raw" natural gas 12 is provided by a source 14. Such a source may include one or more natural gas wells, pipelines, storage facilities, or the like. The specific characteristics of the natural gas source are not intended to limit the present invention.

[0040] The untreated natural gas 12 provided via source 14 may contain one or more undesirable components. For example, hydrogen sulfide (H2S), carbon dioxide (CO2), mercury (Hg), and moisture (water H2O) may be present in the untreated natural gas 12 provided by natural gas source 14.

[0041] The natural gas source 14 is in fluid communication 16 with a pretreatment section 18 of the system 10. The pretreatment section 18 may enable the removal of at least some components from the untreated natural gas 12. For example, the pretreatment section 18 (e.g., for dehydrogenation) may include one or more structures for removing moisture from the gas. In addition, the pretreatment section 18 may include structures for removing hydrogen sulfide, carbon dioxide, and mercury.

[0042] In the system 10, a liquefaction section 22 is located downstream of the pretreatment section 18 in fluid communication 24, and pretreated natural gas 26 flows to the liquefaction section. The liquefaction section 22 may include any suitable structure for liquefying the pretreated natural gas 26 to produce liquefied natural gas (LNG) 28. Such a structure may include sections for subjecting the natural gas to refrigeration (i.e., cooling), compressing the natural gas, and the like. Such a structure is, of course, subject to variation and is therefore not intended to specifically limit the present invention. After liquefaction, the liquefied natural gas 28 may be supplied to storage facilities, distribution facilities, or the like. Again, such aspects are not intended to specifically limit the present invention.

[0043] As mentioned, dehydration is a process for removing moisture content from the gas, and specifically for this discussion, from natural gas. Elevated moisture content in natural gas can cause problems in the system 10. For example, the moisture can block or clog ducts, lines, and the like. Furthermore, such blocking / clogging can be associated with the formation of methane hydrate under high pressure. Furthermore, condensation of moisture in the equipment of the system 10 can cause corrosion. Further, significant ice formation during the liquefaction process can damage equipment (e.g., cryogenic equipment in the liquefaction section 22). In a general example, the moisture content in natural gas is usually reduced to below one part per million before it flows to the liquefaction section 22.Pretreatment is therefore necessary to achieve any necessary moisture removal so that the desired moisture content of, for example, less than one part per million is achieved.

[0044] In the system, a moisture generation and analysis device 40 is fluidly connected to the flow of pretreated gas 26 flowing from the pretreatment section 18 to the liquefaction section 22. The moisture generation and analysis device 40 is suitably connected to allow diverting at least a portion 42 (e.g., a sample amount) of the pretreated natural gas 26 to the moisture generation and analysis device 40. This diverting of the sample amount 42 takes place after the pretreatment has occurred in the pretreatment section 18 and before the pretreated gas reaches the liquefaction section 22.

[0045] One purpose of the moisture generation and analysis device 40 is to monitor the moisture content of the pretreated natural gas to ensure that the moisture content is below the desired level (e.g., one part per million or so) before the gas is sent for liquefaction. In general, the moisture analysis serves to determine if the system 10 may have one or more issues that require attention. For example, there may be a leakage flow that allows unwanted moisture to enter the natural gas stream flowing through the system 10. Additionally, it is possible that a component in the pretreatment section 18 requires attention or maintenance. For example, the pretreatment section may include one or more dryers that may be saturated with the moisture removed from the natural gas.Such dryers may require maintenance to regenerate the dryer for further use in moisture removal. Another purpose of the moisture generation and analysis device 40 is a moisture generation function. The use of moisture allows validation of the moisture analyzer by generating a known (e.g., dry and wet) moisture content in the gas.

[0046] Fig. Figure 2 schematically illustrates an example of the moisture generation and analysis device 40 having at least one feature according to the present invention. Note that only the small portion 42 of the total pretreated natural gas 26 flows to the moisture generation and analysis device 40. The sample amount 42 is selected from the main portion of the flow of pretreated natural gas 26 ( Fig.1) which otherwise flows towards the liquefaction section 22. The sample quantity 42 is for use (e.g., sampling / testing) by the moisture generation and analysis device 40 ( Fig. 2).

[0047] In the exemplary moisture generation and analysis device 40, the gas first flows to a pressure regulator 46, which regulates the pressure of the gas flowing into the moisture generation and analysis device 40. The pressure regulator 46 is in fluid communication 48 with a connection point 50, which in turn is fluidly connected to a gas purifier 52. Furthermore, a fluid connection line 54 extends from the connection point 50 to a bypass point 58, bypassing the gas purifier 52. Thus, the fluid connection line 54 is a bypass line.

[0048] With respect to the gas purifier 52, the gas purifier 52 can receive gas supplied thereto from the pressure regulator 46. The gas purifier 52 serves to effectively remove any moisture from the gas flowing through it. A point 60 forms an exit from the gas purifier 52. Note that the gas output from the gas purifier 52 actually has a zero humidity point for use in the moisture generation and analysis device 40.

[0049] The moisture generation and analysis device 40 includes a valve (V0) 62 located downstream of the gas purifier 52 and the fluid connection (bypass) line 54. Specifically, the bypass point 58 and the outlet point 60 from the gas purifier 52 are two switching points in the valve (V0) 62. Specifically, the valve (V0) 62 has three points. In the valve (V0) 62, point 64 is a common point "1," point 60 is a switching point "2," and bypass point 58 is a switching point "3." The valve (V0) 62 can be switched between two states, wherein a first state forms a connection from the common valve point (point 1) 64 to the switching point (point 2) 60 of the outlet of the gas purifier 52, and wherein a second state forms a connection from the common valve point (point 1) 64 to the switching point (point 3) 58 of the bypass line 54.

[0050] Downstream of the valve (V0) 62, the moisture generation and analysis device 40 contains a proportional valve 68. The entire natural gas flow through the moisture generation and analysis device 40 flows through the proportional valve 68, regardless of whether the gas flows through the gas purifier 52 or whether it flows along the bypass line 54 and thus does not flow through the gas purifier 52. The proportional valve 68 is designed and serves to ensure a natural gas flow through the main opening at a constant pressure P c upstream of the main opening.

[0051] A permeation tube 72, which in turn is arranged in a permeation tube chamber 74, is fluidly connected to the outlet of the proportional valve 68 at a connection point 70. The permeation tube 72 is an encapsulated piece of tubing that can be made of a permeable material, such as TEFLON®. A finite amount of water is provided in the permeation tube 72. This water can combine with the natural gas flowing through the permeation tube 72 by penetrating the permeable material. The permeation rate of water through the material to combine with the natural gas depends on the difference in water vapor pressure between the inside and outside of the tube, the tube surface area, the tube wall thickness, and the permeability of the tube material. Furthermore, the permeability is a function of the temperature of the permeation tube 72.The permeation tube chamber 74 enables temperature regulation of the permeation tube 72.

[0052] It should be noted that all of the gas supplied to the permeation tube 72 may initially have no moisture content (e.g., because it may have been removed by the gas purifier 52). It should also be noted that the gas flowing through the permeation tube 72 may absorb some moisture content. The amount of moisture absorbed may be a known, controlled amount resulting from the control of structural and operational aspects of the permeation tube 72. The outlet of the permeation tube extends to a point 76.

[0053] Returning to the output of the proportional valve 68 and thus to the connection point 70, a second fluid connection line 80 extends from the connection point 70 and outside the permeation tube 72. Thus, the connection line 80 is a bypass line that bypasses the permeation tube 72 and can be referred to as a permeation tube bypass channel. The second bypass line 80 extends to a point 82.

[0054] A flow distributor 90 is provided in the moisture generation and analysis device 40. The flow distributor 90 has three inlets (i.e., a first, second, and third inlet) 92, 94, 96. The first distributor inlet 92 is fluidly connected to a point 102. The second distributor inlet 94 is fluidly connected to a point 104 and a point 106. The third distributor 96 is fluidly connected to a point 108.

[0055] In the moisture generation and analysis device 40, a valve 110 (V1) is provided at points 82, 102, and 104, and a valve 112 (V2) is provided at points 76, 106, and 108. The valve 110 (V1) is a two-position valve, with point 82 being a common point, designated "1" in the drawings. A first position of the valve 110 (V1) connects the common point 82 "1" to a point 104 (designated "2" in the drawings), and a second position of the valve 110 (V1) connects the common point 82 "1" to a point 102 (designated "3" in the drawings). Thus, the valve 110 (V1) in the first position connects the bypass line 80 to the second distributor inlet 94, and the valve 110 (V1) in the second position connects the bypass line 80 to the first distributor inlet 92.

[0056] The valve 112 (V2) is a two-position valve, with point 76 being a common point, designated "1" in the drawings. A first position of the valve 112 (V2) connects the common point 76 "1" to a point 108 (designated "2" in the drawings), and a second position of the valve 112 (V2) connects the common point 76 "1" to a point 106 (designated "3" in the drawings). Thus, in the first position, the valve 112 (V2) connects the permeation tube 72 to the third manifold inlet 96, and in the second position, the valve 112 (V2) connects the permeation tube 72 to the second manifold inlet 94.

[0057] Two openings 120, 122 are formed in the flow distributor 90. Specifically, a first or main opening 120 and a second or bypass opening 122 are formed. The main opening 120 has an inlet connected to the second inlet 94 of the flow distributor. Furthermore, a pressure sensor 126 for detecting a pressure Pc is provided at the inlet to the main opening 120, and thus at the second inlet 94 to the flow distributor 90. The pressure Pc is thus the pressure at the inlet to the main opening 120. The first and third inlets 92 and 96 of the flow distributor 90 are fluidly connected to an inlet of the bypass opening 122 in the flow distributor 90.

[0058] It should be noted that valve 112 (V2) can be operated via the connections of the manifold inlets 92-96 to the main and bypass ports 120 and 122 to connect the outlet of the permeation tube 72 to either the main port 120 or the bypass port 122. Likewise, valve 110 (V1) serves to connect the bypass line 80, which bypasses the permeation tube 72, to either the main port 120 or the bypass port 122.

[0059] The main opening 120 has an outlet that extends via a first outlet 130 of the flow distributor 90 to a moisture analyzer 132 of the moisture generation and analysis device 40. A flow emanating from the main opening 120 can be referred to as a main flow (F c ) and the main current F c has a pressure P dThe moisture analyzer 132 includes structures for determining the moisture content in the gas stream (ie, in the main stream (F c )) is present, which is fed to the moisture analyzer.

[0060] An outlet 136 of the moisture analyzer 132 is fluidly connected to an exhaust stack 138. The exhaust stack 138 carries gases and the like that are used, consumed, or otherwise no longer required. Thus, the gas flowing through the moisture analyzer 132 is possibly carried out of and away from the moisture generation and analysis device 40 along with other gas streams. It should be noted that the gas flowing through the outlet 136 to the exhaust stack 138 represents a very small proportion compared to the other gases flowing in the exhaust stack. The exhaust stack 138 may be connected to further treatment components, e.g., to an environmental pollutant monitoring device 139, to the surrounding atmosphere, or the like.It should be noted that the environmental pollutant monitoring device 139 may include components for combusting constituents flowing in the exhaust stack 138 as flare gas or for otherwise treating the constituents flowing in the exhaust stack. The pressure P S1 in the exhaust stack 138 may be relatively low. In one example, the pressure P S1 in the exhaust stack 138 in the range of 0-12 psig.

[0061] The bypass opening 122 is fluidly connected via a second outlet 140 extending from the flow distributor 90 to a bypass flow line 142 extending out of the moisture generation and analysis device 40. The bypass flow line 142 conveys a bypass flow F b , which has a pressure P eThe bypass flow line 142 is fluidly connected to the exhaust chimney 138, so that the bypass flow F originating from the bypass opening 122 bis directed to the exhaust stack 138. Thus, the gas flowing through the bypass opening 122 is also transported out of and away from the moisture generation and analysis device 40, if necessary, together with other gas flows. It should be noted that gas flowing through the bypass flow line 142 to the exhaust stack 138 represents a very small proportion compared to the other gases flowing in the exhaust stack. It should also be remembered here that the exhaust stack 138 may be connected to further processing components, e.g., to an environmental pollutant monitoring device 139, and that the environmental pollutant monitoring device may include components for combusting constituents flowing in the exhaust stack 138 as flare gas, or for otherwise processing the constituents flowing in the exhaust stack.

[0062] The moisture generation and analysis device includes a pressure sensor 150 for determining an ambient pressure P0 in the moisture generation and analysis device 40. Further, it should be noted that a series of electronic circuits 160 is operatively connected 162 to the proportional valve 68, operatively connected 164 to the permeation tube chamber 74, operatively connected 166 to the flow distributor 90, operatively connected 167 to the pressure sensor 126, and operatively connected 168 to the ambient pressure sensor 150.

[0063] Referring now to the operative connection 162 of the electronic circuit 160 to the proportional valve 68, the connection can provide a proportional, integrated and differentiated (PID) connection and control of the proportional valve 68, so that the pressure P cupstream of the main opening is constant. This ensures that the critical flow rate through the main opening is consistently constant. Furthermore, the control / regulation is generally designed to regulate the pressure of the gas flowing through the moisture generation and analysis device 40.

[0064] With respect to the operative connection 164 of the electronic circuit 160 to the permeation tube chamber 74, the connection may provide a PID connection. The permeation tube chamber 74 is designed to allow the temperature of the permeation tube chamber to be regulated / controlled. This temperature control within the permeation tube chamber 74 thus allows a desired temperature for the permeation tube to be maintained. As mentioned, the permeation of moisture past the permeation tube 72 is a function of temperature. Therefore, the temperature of the permeation tube is controlled such that the transfer of moisture through the permeation tube occurs at a desired rate.

[0065] The operative connection 166 between the electronics 160 and the temperature control of the flow distributor 90 may be provided as a PID connection. As mentioned, the electronics 160 is operatively connected 168 to the ambient pressure sensor 150. The ambient pressure P0 derived from the ambient pressure sensor 150 and the pressure P sensed by the sensor 126 via a connection 167 at the inlet of the main port 120 c are used to determine a critical flow rate through the main orifice.

[0066] With regard to the operation of the moisture generation and analysis device 40, the assembly can operate in three different modes. These modes are referred to as (1) process mode, (2) dry mode, and (3) wet mode. The operation of the moisture generation and analysis device 40 depends on the actuated positions of the three valves 62, 110, and 112 (V0-V2). The functional positions of the three valves 62, 110, and 112 (V0-V2) for the three operating modes are shown in Table 1 below. Table 1: Operating modes V0 V1 V2 Process mode 1-3 1-2 1-2 Dry mode 1-2 1-2 1-2 Wet mode 1-2 1-3 1-3

[0067] In the process mode, a gas sample, which would otherwise be taken from the pretreatment section 18 of the system (see Fig. 1) to the condensing section 22 to determine a moisture content. For such a process mode, the valve 62 (V0) is in Fig.2 in a connection between points 64 and 58 (position 1-3), so that the gas does not flow through the gas purifier 52, but instead bypasses the gas purifier via the bypass line 54. In this case, the gas flows through the proportional valve 68 and to the connection point 70 located upstream of the permeation tube 72. The gas can flow through the permeation tube 72. However, in the case of the process mode, the valve 112 (V2) is located in a connection between points 76 and 108 (position 1-2), so that the outlet of the permeation tube 72 is fluidly connected to the inlet of the bypass opening 122 in the flow distributor 90. Accordingly, such gas is directed to the bypass opening 122. In the usual course, such gas flows through the bypass opening 122 and is fed to the exhaust stack 138. It should be noted that moisture may be added to such gas flowing through the permeation tube 72 and through the bypass opening 122.However, such addition of moisture has no effect on operation. The gas is simply discharged to the exhaust stack 138 without monitoring the moisture content.

[0068] Returning to the connection point 70 upstream of the permeation tube 72, it is understood that gas also flows to the valve 110 (V1) via the permeation tube bypass line 80. In the process mode, the valve 110 (V1) is in position 1-2 (connection between point 82 and point 104), so that the bypass line 80 bypassing the permeation tube 72 is fluidly connected to the second inlet 94 of the flow distributor 90 and thus fluidly connected to the inlet of the main opening 120 in the flow distributor. Accordingly, such gas can flow through the main opening 120 and to the moisture analyzer 132. Such gas analyzed by the moisture analyzer is a sample of the main stream of pretreated natural gas 26 supplied from the pretreatment section 18 ( Fig.1) of the system 10 to the liquefaction section 22 of the system. Thus, the actual moisture content of the pretreated natural gas 26 flowing to the liquefaction section 22 is monitored.

[0069] The moisture generation and analysis device 40 ( Fig.2) provides the ability to verify / confirm that the moisture analyzer 132 is properly analyzing / determining the moisture content. To perform such verification, the moisture generation and analysis device 40 will run through the dry mode and then the wet mode. In the dry mode, the valve 62 (V0) is in the 1-2 position (points 64 and 60 are connected). Accordingly, the bypass line 54 bypassing the gas purifier 52 is disconnected, and gas flows through the gas purifier for the removal of moisture and the like. Furthermore, in the dry mode, the valve 110 (V1) is in the 1-2 position (points 82 and 104 are connected), so that gas flowing from the proportional valve 68, which has been dried by the gas purifier 52, can flow via the bypass line 80 past the permeation tube and to the main opening 120.This allows dried gas to flow through the main port 120 and through the moisture analyzer 132. The flow of such dried gas dries this series of lines and the components arranged thereon. The valve 112 (V2) is in position 1-2 (points 76 and 108 are connected), so that gas flowing through the permeation tube 72 is directed to the inlet of the bypass port 122 and subsequently to the exhaust stack 138. Moisture can be added to such gas via the permeation tube 72. However, this is of no consequence during operation in the dry mode.

[0070] As mentioned, verification may require operation of the moisture generation and analysis device 40 in wet mode. In wet mode, valve 62 (V0) is held in position 1-2 (points 64 and 60 are connected) so that the bypass line 54 bypassing the gas purifier 52 is disconnected, and the gas merely flows through the gas purifier to be dried (e.g., freed of moisture) and the like. Here, too, the gas flows through the proportional valve 68 and to the connection point 70 upstream of the permeation tube 72. Valve 112 (V2) is in position 1-3 (points 76 and 106 are connected) so that gas flowing through the permeation tube 72 is connected to the inlet to the main port 120. Note that in the permeation tube 72, moisture may be added to the gas flow therethrough.Thus, the gas flowing from the main port 120 to the moisture analyzer 132 contains moisture (i.e., it is loaded with moisture). The amount of moisture introduced or added to the gas via the permeation tube 72 is controlled. Specifically, the introduction of moisture is based on a known amount so that there is an expectation of the moisture content that the moisture analyzer 132 will detect. Such a procedure can be used as a check to determine the proper operation of the moisture analyzer 132. Specifically, if the moisture analyzer 132 provides a sensed moisture content reading that substantially matches the controlled, known moisture content introduced through the permeation tube, the moisture analyzer 132 and the system 10 as a whole can be considered to be operating properly.Conversely, if the moisture analyzer 132 provides an indication of a perceived moisture content that is substantially inconsistent with the controlled, known moisture content introduced through the permeation tube, this is an indication that a portion or aspect of the moisture generation and analysis device 40 may not be operating properly.

[0071] Furthermore, valve V1 is in wet mode at connection 1-3 (points 82 and 102 are connected). Accordingly, a flow bypassing permeation tube 72 via bypass line 80 is connected to the inlet of bypass opening 122 and subsequently to exhaust stack 138. This sustained flow is advantageous, as explained below.

[0072] It should be noted that both the main port 120 and the bypass port 122 provide some restriction or resistance to a flow of gas flowing therethrough. It should be noted that the requirement of the bypass flow, which bypasses the moisture analyzer 132, in the moisture generation and analysis device 40 is advantageous in maintaining at least some constant flow through the moisture generation and analysis device 40. Such a constant flow can help keep the moisture generation and analysis device 40 dry, thus preventing undesirable moisture accumulation.Without a bypass stream, moisture could gradually accumulate in one or more dead spaces and contaminate a stream in the moisture generation and analysis device 40 when it is desired to perform a moisture analysis via the moisture analyzer 132.

[0073] Turning now to specific characteristics of the operation of the moisture generation and analysis device 40, moisture introduced at the permeation tube 72 produces a moisture content that can be defined by: Hppm=PrMWH2OPrMWH2O+P0⋅FcR0⋅T0 with: P r Water emission rate (ng / minute) at a certain temperature; MWH2O Molecular weight of water (g / mol) P0 Ambient, standard pressure (pa) - usually at atmospheric pressure 1 T0 Standard temperature (K) - usually at 25 °C R0 Universal Gas Constant - 8.31 J / (mol*K) F c Mass flow rate (standard cc per minute) at P0 and T0

[0074] As mentioned, maintaining a constant temperature of the permeation tube 72 helps to reduce the water emission rate (P r ) constant.

[0075] In the wet mode, F c Gas flowing through the main opening is called the main flow (F c ). Gas flowing through the bypass opening is called bypass flow (F c ). As mentioned, the main current (F c ) is analyzed by the moisture analyzer 132 before being discharged into the exhaust stack 138. The bypass stream (Fb) is a gas stream that flows through the rest of the verification system to keep it dry. Without the bypass stream, moisture would gradually accumulate in dead spaces and contaminate the main stream.

[0076] According to one aspect of the present invention, it has been discovered that it is advantageous to increase the rate of the bypass flow F b to maintain or regulate the rate of the main flow F c as close as possible. If F b essentially unequal to (ie ≠) F c the rate of the main flow F c otherwise experience a discontinuity when changing the verification operation mode. As a result, the main stream F c require an additional period of time to settle before the moisture analyzer 132 can produce a reliable reading. Secondly, when changing the operating mode from dry mode to wet mode, if F b ≠ F c , excessive moisture may be observed from the moisture analyzer 132 due to different flow rates in the permeation tube during dry mode and wet mode. If Fb ≠ F c Thirdly, the introduction of moisture through the permeation tube 72 may become unstable when the operating mode changes from dry mode to wet mode. The temperature of the permeation tube 72 and the temperature of the flow distributor 90 only reach constant values ​​when the gas flows F b and F c are stable. This is due to the fact that the ambient temperature can experience a change (from, for example, -20 °C to 50 °C), and that the gas temperature is affected by this change in temperature. If the gas flow in the permeation tube 72 and in the flow distributor 90 is continuously stable and constant, thermal equilibrium is reached after an initial, relatively short settling time. If the gas flow F b ≠ F cHowever, if the current becomes unstable when switching the operating mode from dry mode to wet mode, the temperature of the permeation tube and the temperature of the flow distributor become unstable. Excessive moisture introduction through the permeation tube 72 may occur, and additional time is required for the moisture to stabilize, which affects response time.

[0077] One aspect of the present invention is based on using a critical flow concept to determine the main flow F c and the bypass current F b consistent and constant. It should be noted that a critical flow occurs at an orifice if the upstream absolute pressure is at least 1.98 times the downstream absolute pressure. The following applies to the main flow if: PcPd>(k+12)kk−1, the flow through the main orifice becomes critical and its flow rate is given by: Fc=Crc⋅A⋅TrefPref⋅PC+PDT⋅k⋅RDMW⋅(2k+1)k+1k−1 with: Cr c Flow coefficient for the main stream A c Cross-sectional area for the main flow opening T ref Reference temperature in K, usually 273.15 K P ref Reference pressure in Pascal, usually 1.013*10 5 Pa P c pressure prevailing upstream of the main flow opening (P a ) P0 ambient pressure (Pa) T Temperature of the flow distributor opening (K) k specific heat of the gas (1.4 for most gases) R0 Universal gas constant (8.31 J / mol*K) MW molecular weight of the gas

[0078] When the main flow becomes critical, the flow rate F cindependent of the downstream pressure condition. The back pressure of the exhaust stack can vary between 0 and approximately 10 psig (pound-force per square inch). If the critical flow condition is not met, the rate of the main flow will vary depending on the back pressure fluctuations of the exhaust stack. However, if the critical flow condition is met, the rate of the main flow (F c ) constant as long as the upstream pressure P c Such stability is achieved via the bypass current F b shown becomes critical if: Pc'Pe>(k+12)kk−1 Fb=Crb⋅Ab⋅TrefPref⋅PC'+PDT⋅k⋅RDMW⋅(2k+1)k+1k−1 with: Cr b Flow coefficient for the bypass flow A b Cross-sectional area for the bypass channel flow opening P c ' pressure prevailing upstream of the bypass channel flow opening (Pa)

[0079] The pressure drop across V1 is known as DP V1 The pressure drop across V2 is referred to as DP V2 The pressure drop across the permeation tube is referred to as DP PT P c and P c ' are linked by the following relationship: Pc=Pup−DPPT−DPV2 Pc'=Pup−DPV1

[0080] For example, V1 and V2 can be electromagnetic 3-way valves designed for general purpose applications. The openings in V1 and V2 can be very large so that their pressure drops are minimal, i.e., DP V1 = DP V2 0. This means that the valves V1 and V2 cause negligible pressure losses in a flowing stream. Furthermore, the permeation tube chamber 74 is based, for example, on a stainless steel housing in which the permeation tube 72 is housed. In such an example, there is no flow restriction in the chamber 74, so that its pressure drop DP PTcan be considered negligible. Thus, the pressure upstream of the main orifice and the pressure upstream of the bypass orifice are equal. Consequently, Pc=Pc'

[0081] The main opening 120 and the bypass opening 122 are identical in design and dimensions within a tolerance, so that: Crc=Crb Ac=Ab

[0082] This results in (see equations 2-8): Fc=Fb

[0083] If F c and F b become critical, and F c = F b , they become stable even if the pressures downstream of their openings fluctuate, provided that the pressure P c and the flow distributor temperature T are stable. P cis electronically controlled by the upstream proportional valve. The flow distributor temperature T is electronically controlled. F c and F b may vary due to changes in the molecular weight of the gas when the gas compositions change.

[0084] Accordingly, one aspect of the present invention is based on applying the critical flow concept discussed above in conjunction with the use of two critical orifices to allow a consistent amount of gas flow to flow simultaneously through the permeation tube 72 and through the remainder of the moisture generation and analysis device 40. In one exemplary aspect, the main orifice 120 is appropriately sized, and the sample stream 132 delivered to the moisture analyzer is provided via the main orifice at a pressure from the main orifice such that the sample stream delivered to the moisture analyzer is at a critical flow condition, and the bypass orifice 140 is appropriately sized, and the bypass stream is delivered to the bypass orifice at a pressure such that the bypass stream flowing through the bypass orifice is at a critical flow condition.A possible advantage according to one aspect of the present invention is the ability to discharge process gas, dry gas, and wet gas at a constant, stable flow rate, unaffected by downstream back pressure fluctuations. The critical flow condition through the main port and the critical flow condition through the bypass port are unaffected by the fluctuating pressure in the exhaust stack. It may be considered an advantage that the gas composition of natural gas is predominantly based on methane, making the present invention particularly useful for natural gas.

[0085] The invention has been described with reference to the embodiments set forth above. Upon careful reading of this description, modifications and variations will occur to others. Embodiments utilizing one or more aspects of the invention are intended to include all such modifications and variations, provided they fall within the scope of the appended claims.

[0086] A moisture generation and analysis device includes a channel for obtaining a natural gas sample stream, a moisture analyzer, and a bypass flow channel. A main port is located in the moisture generation and analysis device and is fluidly connected to a stream flowing to the moisture analyzer. A bypass port is arranged along the bypass flow channel. The main port is suitably sized, and the flow provided to the moisture analyzer via the main port has a pressure such that the flow provided to the moisture analyzer is at a critical flow condition. The bypass port is suitably sized, and the bypass flow is supplied at a pressure such that the bypass flow is at a critical flow condition.Components in the moisture generation and analysis device are suitably selected so that the critical flow rate through the main opening and the critical flow rate through the bypass opening match.

Claims

[1] A moisture generation and analysis device (40) for generating and analyzing moisture from a flowing natural gas stream, the device comprising: a conduit for obtaining a natural gas sample stream from the flowing natural gas stream; a moisture analyzer (132) that analyzes a moisture content of the sample stream supplied thereto; a bypass flow channel (142) providing a flow path through the moisture generating and analyzing device (40) extending parallel to a flow path of the moisture analyzer (132) without passing through the moisture analyzer (132); a main opening (120) fluidly connected to the moisture generation and analysis device (40) and arranged upstream of the moisture analyzer (132), wherein the sample stream supplied to the moisture analyzer (132) is output to and flows through the main opening (120); and a bypass opening (122) arranged along the bypass flow channel (142); the main opening (120) is dimensioned such that the sample stream supplied to the moisture analyzer (132) via the main opening (120) is provided from the main opening (120) at a pressure such that the sample stream supplied to the moisture analyzer (132) is at a critical flow condition, the absolute pressure prevailing upstream at the main opening (120) being at least 1.98 times the downstream absolute pressure at the main opening (120), and the bypass opening (122) is dimensioned such that the bypass flow (F b ) is supplied to the bypass opening (122) at a pressure such that the bypass flow (F b ) flowing through the bypass opening (122) is at a critical flow condition, wherein the absolute pressure prevailing upstream at the bypass opening (122) is at least 1.98 times the downstream absolute pressure at the bypass opening (122). [2] The moisture generation and analysis device (40) of claim 1, comprising: a gas purifier (52) for removing moisture from the natural gas sample stream, a bypass line (54) bypassing the gas purifier (52), and a first valve (62) for selecting a flow path either through the gas purifier (52) or through the bypass line (54). [3] A moisture generation and analysis device (40) according to claim 1 or 2, comprising: a permeation tube (72) for introducing moisture into at least a portion of the natural gas sample stream to produce a moisture-laden gas, a permeation tube bypass line (80) bypassing the permeation tube (72) with respect to a natural gas stream that does not experience the introduction of moisture, and second and third valves (110, 112) for selectively connecting the permeation tube (72) to either the main port (120) or the bypass port (122), and for selectively connecting the permeation tube bypass line (80) to the remaining port, be it the main port (120) or the bypass port (122); and wherein the first to third valves (62, 110, 112) are configured to provide a process mode, a dry mode, and a wet mode in the moisture generation and analysis device (40). [4] Moisture generation and analysis device (40) as discussed in claim 3, wherein the natural gas sample stream obtained from the flowing natural gas stream is supplied to the main port (120) and the moisture analyzer (132) in the process mode by means of the first valve (62) along the bypass line (54) bypassing the gas purifier (52), by means of the second valve (110) via the bypass line (80) bypassing the permeation tube (72); and / or wherein the third valve (112) directs gas flowing from the permeation tube (72) to the bypass port (122) in the process mode. [5] Moisture generation and analysis device (40) as discussed in claim 3 or 4, wherein in the dry mode, the natural gas sample stream obtained from the flowing natural gas stream is directed by means of the first valve (62) through the gas purifier (52), by means of the second valve (110) via the bypass line (80) bypassing the permeation tube (72), to the main opening (120) and to the moisture analyzer (132); and / or wherein in the dry mode, the third valve (112) directs gas flowing from the permeation tube (72) to the bypass opening (122). [6] Moisture generation and analysis device (40) as discussed in any one of claims 3-5, wherein in the wet mode, the natural gas sample stream obtained from the flowing natural gas stream is directed by means of the first valve (62) through the gas purifier (52), by means of the third valve (112) via the permeation tube (72) to the main opening (120) and to the moisture analyzer (132). [7] A moisture generation and analysis device (40) as discussed in any one of claims 3-6, wherein in the moist mode, the second valve (110) directs gas flowing from the bypass line (80) bypassing the permeation tube (72) to the bypass opening (122). [8] A moisture generation and analysis device (40) as discussed in any one of claims 3-7, wherein the second valve (110) causes a negligible pressure drop on the stream flowing therethrough, and wherein the third valve (112) causes a negligible pressure drop on the stream flowing therethrough, wherein the permeation tube chamber causes a negligible pressure drop on the stream flowing therethrough such that a pressure upstream of the main opening (120) and a pressure upstream of the bypass opening (122) are the same. [9] A moisture generation and analysis device (40) as discussed in claim 1, wherein the moisture analyzer (132) and the bypass opening (122) direct the exhaust flow to an exhaust stack having a variable pressure, wherein the critical flow condition through the main opening (120) and the critical flow condition through the bypass opening (122) are unaffected by the variable pressure in the exhaust stack. [10] A method for providing and analyzing moisture in a flowing natural gas stream by means of a moisture generating and analyzing device (40), the method comprising the steps of: Providing a conduit for obtaining a natural gas sample stream from the flowing natural gas stream; Providing a moisture analyzer (132) that analyzes a moisture content of the sample stream supplied thereto; Providing a bypass flow channel (142) providing a flow path through the moisture generation and analysis device (40) that extends parallel to a flow path of the moisture analyzer (132) without passing through the moisture analyzer (132); Providing a main opening (120) fluidly connected to the moisture generation and analysis device (40) and arranged upstream of the moisture analyzer (132), wherein the sample stream supplied to the moisture analyzer (132) is output to and flows through the main opening (120); Providing a bypass opening (122) arranged along the bypass flow channel (142); Sizing the main orifice (120) and providing the sample stream to the moisture analyzer (132) via the main orifice (120) at a pressure from the main orifice (120) such that the sample stream supplied to the moisture analyzer (132) is at a critical flow condition, wherein the absolute pressure upstream of the main orifice (120) is at least 1.98 times the downstream absolute pressure at the main orifice (120); and Sizing the bypass opening (122) and providing the bypass flow (F b ) at a pressure to the bypass opening (122), so that the bypass flow (F b ) flowing through the bypass opening (122) is at a critical flow condition, wherein the absolute pressure prevailing upstream at the bypass opening (122) is at least 1.98 times the downstream absolute pressure at the bypass opening (122).

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