A method and system for testing the vaporization volume of LNG air temperature vaporizer using liquid nitrogen

Abstract

The present invention discloses a method and a test system for testing the gasification capacity of an LNG air-cooled vaporizer with liquid nitrogen. The method includes Step 1: adjusting the gasification capacity of liquid nitrogen to the marked LNG gasification capacity; Step 2: measuring the inlet and outlet temperatures and pressures of the LNG air-cooled vaporizer; Step 3: determining relevant physical property parameters related to nitrogen, namely Q m , d N , d m , H N , H m , k. Step 4: calculating the LNG gasification capacity under the reference state through the formula #imgabs0#. The present invention provides a test method, a corresponding test system, and a correction coefficient k for the influence of the different physical properties of nitrogen and natural gas on heat transfer. In the calculation of the present invention, not only the state differences between nitrogen and natural gas are considered, but also the influence of the differences in heat exchange performance between nitrogen and natural gas on heat exchange performance is considered. Therefore, the measured value of the gasification capacity of the natural gas air-cooled vaporizer is relatively accurate.

Description

Technical Field

[0001] The present invention relates to a testing method and a testing system. Specifically, the present invention adopts liquid nitrogen instead of liquefied natural gas to test the gasification amount of a liquefied natural gas gasification device. The method can simply and accurately measure the gasification amount of the device. Background Art

[0002] In recent years, natural gas has become increasingly important in energy supply, and the demand has increased significantly. LNG point supply, as an important way of natural gas supply, has also been widely used. Therefore, the standardization of small and medium-sized LNG skid-mounted gas supply devices is also imperative. As an important LNG gasification device, how to test the gasification volume index of the air temperature vaporizer is also one of the standardization contents. The method currently used in the industry is that the outlet temperature of the air temperature vaporizer is not lower than the ambient temperature by 10°C under the marked gasification volume. However, when conducting the gasification volume certification test, liquid nitrogen is generally used instead of LNG for testing. Because of the difference in physical properties between liquid nitrogen and LNG, the measured nitrogen gasification volume needs to be corrected.

[0003] The traditional method for calculating the LNG gasification volume is as follows:

[0004]

[0005] Where:

[0006] Q——nominal flow rate of town gas under reference conditions, in cubic meters per hour (m3 / h);

[0007] Qm——the working flow rate of nitrogen, in cubic meters per hour (m3 / h);

[0008] p——absolute pressure of town gas under reference conditions, which is 0.101325MPa;

[0009] pm——absolute pressure of nitrogen, in megapascals (MPa);

[0010] tm——nitrogen temperature, in degrees Celsius (℃);

[0011] Z——compression factor of town gas under reference conditions;

[0012] Zm——compression factor of nitrogen;

[0013] d——relative density of town gas;

[0014] dm——Relative density of nitrogen.

[0015] The conversion only considers the state difference between nitrogen and natural gas, but does not consider the influence of their physical properties and state on the heat transfer performance. The difference in heat transfer performance between nitrogen and natural gas leads to a large error in the calculation of the gasification amount. Summary of the invention

[0016] The purpose of the present invention is to solve the problem of large gasification amount calculation error caused by the difference in heat exchange performance between nitrogen and natural gas. This purpose is achieved through the following technical solutions:

[0017] A method for testing the gasification amount of an LNG air temperature vaporizer by using liquid nitrogen at a constant flow rate comprises the following steps:

[0018] Step 1: Adjust the liquid nitrogen vaporization volume to the marked LNG vaporization volume.

[0019] Step 2: measuring the inlet and outlet temperature and pressure of the LNG air temperature vaporizer.

[0020] Step 3: Determine the physical property parameters related to nitrogen, i.e. Q m d N d m , H N , H m , k,

[0021] Step 4: Calculate the LNG gasification volume under the baseline state using formula (1):

[0022]

[0023] in:

[0024] Q N ——Nominal flow rate of natural gas under reference conditions;

[0025] Q m ——Operating flow rate of nitrogen;

[0026] d N ——Relative density of natural gas under reference conditions;

[0027] d m ——Relative density of nitrogen under test conditions;

[0028] H N ——The amount of heat absorbed by a unit mass of LNG when the temperature rises from -152°C to a temperature not lower than 10°C below the ambient temperature under the test pressure;

[0029] H m ——The heat absorbed by a unit mass of liquid nitrogen when it is vaporized to the outlet temperature of the LNG air-temperature vaporizer under the test conditions;

[0030] k is the correction factor for the effect of different physical properties of nitrogen and natural gas on heat transfer.

[0031] Furthermore, the reference state is a temperature of 15° C. and a pressure of 0.101325 MPa.

[0032] Furthermore, the correction factor Among them, Φ N ——The heat exchange capacity (kW) when the vaporizer vaporizes LNG with a mass flow rate of q from -152℃ to a temperature not lower than 10℃ below the ambient temperature, Φ m ——The gasification mass flow rate of the gasifier is q m Heat exchange capacity of liquid nitrogen (kW).

[0033] Further, a relationship table between the correction coefficient k and the liquid nitrogen inlet temperature is calculated, and the correction coefficient k is found out according to the relationship table between the K value and the liquid nitrogen inlet temperature.

[0034] A testing system for testing the vaporization amount of an LNG air temperature vaporizer using liquid nitrogen comprises a liquid nitrogen storage tank (01), a temperature transmitter (07), an LNG air temperature vaporizer (11), a temperature and pressure transmitter (08) and a flow transmitter (09) which are connected in sequence; wherein:

[0035] The temperature transmitter (07) is used to collect the temperature parameters at the inlet of the LNG air temperature vaporizer (11);

[0036] The temperature and pressure transmitter (08) is used to collect the temperature and pressure parameters at the outlet of the LNG air temperature vaporizer (11);

[0037] The flow transmitter (09) is used to collect flow parameters at the outlet of the LNG air-temperature vaporizer (11).

[0038] Furthermore, a regulating valve is provided between the liquid nitrogen storage tank (01) and the temperature transmitter (07). Valves are provided between the liquid nitrogen storage tank (01) and the temperature transmitter (07), between the temperature transmitter (07) and the LNG air temperature vaporizer (11), and between the LNG air temperature vaporizer (11) and the temperature and pressure transmitter (08).

[0039] Furthermore, it also comprises a safety relief valve (10), wherein the safety relief valve (10) is used for releasing overpressure of pipeline gas.

[0040] The advantage of the present invention is that: since there was no demand in this regard before, there was no more accurate method in the industry to use nitrogen instead of LNG to test the gasification amount. The present invention provides a test method, a corresponding test system and a correction formula. In the calculation, the present invention not only takes into account the state difference between nitrogen and natural gas, but also takes into account the influence of the difference in heat exchange performance between nitrogen and natural gas on the heat exchange performance, so the measured gasification amount of the natural gas air temperature vaporizer is more accurate. The method of testing the gasification amount of the gasification amount of the liquid nitrogen instead of LNG of the present invention calculates and analyzes the influence of different pipe lengths, flow rates, ambient temperatures, working pressures, liquid nitrogen inlet temperatures, and the differences in physical properties between liquid nitrogen and LNG on heat transfer, and provides a corresponding gasification amount algorithm and related correction coefficients. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] By reading the detailed description of the preferred embodiment below, various other advantages and benefits will become clear to those of ordinary skill in the art. The accompanying drawings are only used for the purpose of illustrating the preferred embodiment and are not considered to be limitations of the present invention. Moreover, the same reference numerals are used to represent the same components throughout the accompanying drawings.

[0042] In the attached picture:

[0043] Figure 1 It is a relationship diagram between the K value of the present invention and the liquid nitrogen inlet temperature;

[0044] Figure 2 It is a relationship diagram between the K value of the present invention and the ambient temperature;

[0045] Figure 3 is a relationship diagram between the K value of the present invention and the gasifier inlet flow rate at the reference state;

[0046] Figure 4 It is the relationship diagram between K value and system pressure of the present invention;

[0047] Figure 5 is a relationship diagram between the K value and the gasifier tube length of the present invention;

[0048] Figure 6 It is a structural schematic diagram of a constant flow rate test system of the present invention;

[0049] Among them, there are liquid nitrogen storage tank (01), valves (02, 03, 04, 05, 06), temperature transmitter (07), temperature and pressure transmitter (08), flow transmitter (09), safety relief valve (10), and LNG vaporizer (11). DETAILED DESCRIPTION

[0050] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0051] It should be understood that the terms used herein are only for the purpose of describing specific example embodiments and are not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms "one", "an" and "said" as used herein may also be meant to include plural forms. The terms "include", "comprise", "contain", and "have" are inclusive, and therefore specify the existence of stated features, steps, operations, elements and / or parts, but do not exclude the existence or addition of one or more other features, steps, operations, elements, parts, and / or combinations thereof. The method steps, processes, and operations described herein are not interpreted as necessarily requiring them to be performed in the specific order described or illustrated, unless the execution order is clearly indicated. It should also be understood that additional or alternative steps may be used.

[0052] like Figures 1 to 5 As shown, according to the embodiment of the present invention, a method for testing the vaporization amount of an LNG vaporizer at air temperature with liquid nitrogen is proposed in order to achieve a more accurate vaporization amount test of an LNG vaporizer. The present invention is based on modern heat transfer analysis and measurement technology to achieve a safe and accurate purpose.

[0053] The LNG air-temperature vaporizer described in the present invention is a vaporization device which uses air as a heat source and exchanges heat through natural convection; LNG is the abbreviation of liquefied natural gas.

[0054] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0055] The present invention provides a constant flow test method for testing the gasification amount of an LNG air-temperature vaporizer using liquid nitrogen, and liquid nitrogen is used to replace LNG for testing. First, the gasification amount of liquid nitrogen is adjusted to the marked LNG gasification amount. For example, the base state gasification amount of LNG marked on the equipment is 300m3 / h. During the test, liquid nitrogen is used to replace LNG, and the gasification amount of liquid nitrogen is also adjusted to 300m3 / h. The outlet temperature and pressure of the LNG vaporizer are tested, and the relevant physical properties of nitrogen, such as density and specific enthalpy, are determined based on the temperature and pressure. Then, the physical properties that affect heat transfer are corrected and converted into the base state LNG gasification amount. The correction algorithm is as shown in formula (1):

[0056]

[0057] in:

[0058] Q N ——The nominal flow rate of natural gas under the reference state, in cubic meters per hour (m 3 / h), the reference state is 15℃, 0.101325MPa;

[0059] Q m ——Operating flow rate of nitrogen, in cubic meters per hour (m 3 / h), obtained through testing; the working condition refers to the gas working state;

[0060] d N ——Relative density of natural gas under reference conditions;

[0061] d m ——Relative density of nitrogen under test conditions;

[0062] H N ——The heat absorbed by a unit mass of LNG when the temperature rises from -152°C to not less than 10°C below the ambient temperature under the test pressure, in kilojoules per kilogram (kJ / kg);

[0063] H m ——The heat absorbed by a unit mass of liquid nitrogen when it is vaporized to the vaporizer outlet temperature under the test conditions, in kilojoules per kilogram (kJ / kg);

[0064] k——Correction coefficient for the effect of different physical properties of nitrogen and natural gas on heat transfer, its value is related to the liquid nitrogen inlet temperature, such as Figure 1 As shown, it is summarized after a lot of calculations. The value of the correction coefficient k can be based on Figure 1 find out.

[0065] Formula 1 is derived based on the heat transfer calculation principle of LNG air temperature vaporizer and the similarity principle, which is explained as follows:

[0066] The heat transfer capacity per unit tube length of the gasifier Φ (W / m) can be determined as follows:

[0067] Φ=K(t h -t f )F

[0068] Where F is the area per unit length of the inner wall of the tube, F = πd (m 2 )

[0069] K——Comprehensive heat transfer coefficient of air temperature vaporizer (kW / m 2 .℃);

[0070] t h ——Ambient temperature (℃);

[0071] tf ——Average temperature of the fluid in the pipe (℃).

[0072] The similarity theory is used to analyze the influence of the difference in physical properties of LNG and liquid nitrogen on heat transfer during the gasification process. Because the gasification capacity of the gasifier is equal to its heat transfer capacity, the following relationship can be obtained:

[0073]

[0074] in:

[0075] q N - the mass flow rate that the vaporizer can vaporize if the LNG is heated from -152°C to not less than 10°C below the ambient temperature, in kilojoules per kilogram (kJ / kg);

[0076] H N ——The heat absorbed by a unit mass of LNG when the temperature rises from -152℃ to not less than 10℃ below the ambient temperature under the test pressure, in kilojoules per kilogram (kJ / kg).

[0077] q m ——In order to have similar flow conditions, the mass flow rate (kg / h) of liquid nitrogen is converted to the LNG design volume vaporization amount (reference state) equivalent to the vaporizer mark;

[0078] H m ——The heat absorbed by a unit mass of liquid nitrogen when it is vaporized to the outlet temperature of the vaporizer under the test conditions, in kilojoules per kilogram (kJ / kg);

[0079] Φ N ——The heat exchange capacity (kW) when the vaporizer vaporizes LNG with a mass flow rate of q from -152°C to a temperature not lower than 10°C below the ambient temperature;

[0080] Φ m ——The gasification mass flow rate of the gasifier is q m Heat exchange capacity of liquid nitrogen (kW).

[0081] Φ N With Φ m The difference should be mainly due to the difference in physical properties. Assume:

[0082]

[0083] Since the heat transfer area F remains unchanged, the above formula can also be expressed as

[0084] k=[K n / K m ]*[(t hn -t fn ) / (thm- t fm )] (3-1)

[0085] Also because

[0086] q N =Q N d N (4)

[0087] q m =Q m d m (5)

[0088] Substituting equations (3), (4), and (5) into equation (2), we can obtain the aforementioned modified equation (1):

[0089]

[0090] In order to examine the effects other than physical properties, the heat transfer correction coefficient k is calculated by changing the heat exchange tube structure (cross-sectional dimensions and tube length), gasified medium flow rate, ambient temperature, and working pressure. For the convenience of calculation, the following settings are made: the physical properties of pure methane are used instead of LNG; referring to the experience in actual operation, the temperature of LNG entering the heat exchanger (hereinafter referred to as the inlet temperature) is taken as -152℃ as the design condition; the liquid nitrogen inlet temperature is taken as -185℃. The results are as follows Figure 2-5 shown.

[0091] analyze Figure 2-5 As a result, the k value is basically maintained at around 0.855, and the variation range is less than 1%, so the influence of the heat exchange tube structure (cross-sectional dimensions and tube length), the flow rate of the gasified medium, the ambient temperature, the working pressure, etc. can be ignored.

[0092] However, since the inlet temperature of the vaporized medium also affects the thermal properties of nitrogen, such as specific enthalpy and specific gravity, which are related to temperature, different inlet temperatures will affect its physical properties to a certain extent, and thus affect the heat transfer correction coefficient k. Since LNG and liquid nitrogen are both stored in a supercooled state, the medium temperature at the inlet of the heat exchanger will vary with changes in the storage environment and time. If the LNG inlet temperature is only converted to the design conditions, the present invention defines it as -152°C, and the impact of changes in the LNG inlet temperature can be ignored. The liquid nitrogen inlet temperature is actually tested and will change, and formula (3-1) is not convenient for calculations in on-site operations. Therefore, the impact of changes in the liquid nitrogen inlet temperature was calculated. Through a large number of calculations, the correction table given in this method was obtained. The results are as follows. Figure 1 Therefore, in the correction calculation, the liquid nitrogen inlet temperature can be used to check Figure 1 Get the k value.

[0093] The present invention also relates to a liquid nitrogen constant flow test system for testing the vaporization amount of an LNG air temperature vaporizer, such as Figure 6As shown, the system includes: a liquid nitrogen storage tank (01), valves (02, 03, 04, 05, 06), a temperature transmitter (07), a temperature and pressure transmitter (08), a flow transmitter (09), and a safety relief valve (10). Valves (02, 04, 05) are respectively arranged between the liquid nitrogen storage tank (01) and the temperature transmitter (07), between the temperature transmitter (07) and the LNG air temperature vaporizer (11), and between the LNG air temperature vaporizer (11) and the temperature and pressure transmitter (08). Valve (04) is a regulating valve used to adjust the amount of liquid nitrogen vaporization; valve (02) is a shut-off valve used to cut off the connection between the liquid nitrogen storage tank and the subsequent system; valve (03) is a shut-off valve for the spare liquid nitrogen storage tank, and has the same function as valve (02). It is in a closed state when there is no spare liquid nitrogen storage tank; valve (05) is a shut-off valve used to cut off the connection between the air temperature vaporizer (11) and other systems; valve (06) is a shut-off valve, which is arranged at the end of the pipeline and is used to cut off the discharge of nitrogen in the test system to the outside. The valve is usually closed when the test is stopped.

[0094] During the test, the liquid nitrogen storage tank (01), the temperature transmitter (07), the LNG air temperature vaporizer (11), the temperature and pressure transmitter (08), the flow transmitter (09) and the safety relief valve (10) are connected in sequence, and then the valves (02, 04, 05, 06) are opened in sequence, and then the valve (04) is adjusted to make the volume vaporization amount of the liquid nitrogen reach the volume vaporization amount indicated on the equipment. At the same time, the data of the flow transmitter (09) is observed to make the volume vaporization amount reach the volume vaporization amount indicated on the equipment. At this time, the output value of the flow transmitter (09) is the working flow rate Q of the nitrogen. m ; Observe the temperature data of the temperature transmitter (07), the temperature and pressure data of the temperature and pressure transmitter (08), read the temperature, pressure and other related parameters after the temperature and pressure are stable, and then close the valves (02, 04, 05, 06) in sequence, and the test is over. Finally, based on the measured temperature and pressure, use tools such as the "Liquid Natural Gas Technical Manual" (Edited by Gu Anzhong) to find out the relative density d and dm of nitrogen under the test conditions, and the heat H and Hm absorbed by the unit mass of liquid nitrogen when it is vaporized to the vaporizer outlet temperature under the test conditions, and then use Figure 1 By finding the value of the correction coefficient k, the LNG vaporization volume of the vaporizer can be calculated according to formula (01). When the gas in the pipeline is over-pressured, the safety relief valve (10) can discharge a certain amount of gas to prevent the liquid nitrogen from being over-pressured and causing an explosion.

[0095] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A method for testing the gasification amount of an LNG air temperature vaporizer using liquid nitrogen at a constant flow rate, characterized in that: The following steps are involved: Step 1: Adjust the liquid nitrogen vaporization volume to the marked LNG vaporization volume. Step 2: measuring the inlet and outlet temperature and pressure of the LNG air temperature vaporizer. Step 3: Determine the physical property parameters related to nitrogen, i.e. Q m ,d N ,d m , H N , H m , k, Step 4: Calculate the LNG gasification volume under the baseline state using formula (1): in: Q N ——Nominal flow rate of natural gas under reference conditions; Q m ——Operating flow rate of nitrogen; d N ——Relative density of natural gas under reference conditions; d m ——Relative density of nitrogen under test conditions; H N ——The amount of heat absorbed by a unit mass of LNG when the temperature rises from -152°C to a temperature not lower than 10°C below the ambient temperature under the test pressure; H m ——The heat absorbed by a unit mass of liquid nitrogen when it is vaporized to the outlet temperature of the LNG air-temperature vaporizer under the test conditions; k——Correction coefficient for the effect of different physical properties of nitrogen and natural gas on heat transfer. Among them, Φ N ——The heat exchange capacity (kW) when the vaporizer vaporizes LNG with a mass flow rate of q from -152℃ to a temperature not lower than 10℃ below the ambient temperature, Φ m ——The gasification mass flow rate of the gasifier is q m Heat exchange capacity of liquid nitrogen (kW).

2. The testing method according to claim 1, characterized in that: The reference state is a temperature of 15° C. and a pressure of 0.101325 MPa.

3. The testing method according to claim 1, characterized in that: The relationship table between the correction coefficient k and the liquid nitrogen inlet temperature is calculated, and the correction coefficient k is found out according to the relationship table between the k value and the liquid nitrogen inlet temperature.

4. A liquid nitrogen test system for testing the vaporization volume of an LNG air temperature vaporizer, characterized in that: It comprises a liquid nitrogen storage tank (01), a temperature transmitter (07), an LNG air temperature vaporizer (11), a temperature and pressure transmitter (08) and a flow transmitter (09) which are connected in sequence; wherein: The temperature transmitter (07) is used to collect the temperature parameters at the inlet of the LNG air temperature vaporizer (11); The temperature and pressure transmitter (08) is used to collect the temperature and pressure parameters at the outlet of the LNG air temperature vaporizer (11); The flow transmitter (09) is used to collect flow parameters at the outlet of the LNG air temperature vaporizer (11); The test system is used to execute a constant flow test method for testing the vaporization volume of an LNG air-temperature vaporizer using liquid nitrogen as described in any one of claims 1 to 3.

5. The test system according to claim 4, characterized in that: Valves are respectively provided between the liquid nitrogen storage tank (01) and the temperature transmitter (07), between the temperature transmitter (07) and the LNG air temperature vaporizer (11), and between the LNG air temperature vaporizer (11) and the temperature and pressure transmitter (08).

6. The test system according to claim 4, characterized in that: It also comprises a safety relief valve (10), wherein the safety relief valve (10) is used for releasing overpressure of pipeline gas.

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