A method and system for unloading natural gas into an LNG production unit

Abstract

The present application discloses a method and system for unloading natural gas into an LNG production device. A method for unloading natural gas into an LNG production device includes: obtaining the high-low pressure switching value Pm of each unloading unit according to the high-pressure flow value F<subgt;m high< / subgt>; for different unloading units, when the internal pressure value PIm of the unloading unit is greater than or equal to the high-low pressure switching value Pm, controlling the unloading unit to unload the vehicle at the flow rate of the high-pressure flow value F<subgt;m high< / subgt>; when the internal pressure value PIm of the unloading unit is less than the high-low pressure switching value Pm, obtaining the total flow value F<subgt;total low< / subgt> at the unloading outlet on the low-pressure side according to the total unloading flow value F<subgt;total< / subgt> and the actual high-pressure flow values of each unloading unit; and obtaining the low-pressure flow value F<subgt;m low< / subgt> of each unloading unit according to the total flow value F<subgt;total low< / subgt> and the preset low-pressure distribution flow values of all the unloading units, and controlling the unloading unit to unload the vehicle at the flow rate of the low-pressure flow value F<subgt;m low< / subgt>.

Description

Technical Field

[0001] This application relates to the technical field of natural gas processing, and particularly relates to a method and system for unloading natural gas into an LNG production device. Background Art

[0002] Currently, natural gas resources are scarce in the world; in China, due to the vast territory, it is not practical to lay an ultra-long-distance natural gas pipeline network. Therefore, natural gas is often processed and liquefied into LNG at the gas production site and transported by vehicle to other places where natural gas is needed. The raw material natural gas sources for producing LNG are diverse, and among them, a part of the natural gas is high-pressure natural gas (such as special single-well gas, etc.). When the total gas volume of the high-pressure natural gas detected in a certain area is not large, it is not economical to build a new LNG plant. Therefore, it is necessary to unload such high-pressure natural gas to the operating LNG production device through a mobile high-pressure natural gas unloading device to obtain LNG products.

[0003] An LNG production device usually consists of three parts: pressurization, pretreatment, and liquefaction. Producers hope to unload high-pressure natural gas from the high-pressure natural gas device into the LNG production device; the pressure of the high-pressure natural gas is generally between 5 and 30 MPa, while the pressure of the raw material natural gas for producing LNG is generally between 1 and 2 MPa, and the pressure after compression by the raw material gas compressor is generally between 4 and 6 MPa. In this way, when the high-pressure natural gas is unloaded into the LNG production device, due to the too large pressure difference, it may cause an impact on the LNG production device, and the number of unloading times per day is large. The pressures of each high-pressure natural gas device are not equal, and the unloading is an irregular unloading. The unloading of each high-pressure natural gas device may be simultaneous or may not be simultaneous. During the unloading process, the natural gas processing volume of the LNG production device may increase, resulting in problems such as the LNG product temperature rising due to the inability of the liquefaction process to adjust in time and the raw material gas compressor being blocked and tripping. However, in order to avoid the above problems, if the unloading is carried out at a very slow speed, it will cause too long unloading time and the daily unloading plan cannot be completed.

[0004] Based on this, how to achieve fast, efficient, and stable unloading of the high-pressure natural gas device to the operating LNG production device is a technical problem that those skilled in the art need to consider. Summary of the Invention

[0005] To achieve the above object, this application provides a method for unloading natural gas into an LNG production device, which is used to unload the raw material gas in the unloading unit into the LNG production device. The LNG production device includes a raw material gas compressor. The low-pressure side unloading outlets of all unloading units are communicated with the inlet of the raw material gas compressor, and the high-pressure side unloading outlets of all unloading units are communicated with the outlet of the raw material gas compressor; the method includes:

[0006] Determine the total unloading flow value F 总 and the total flow value F at the unloading outlet on the high-pressure side 总高 , and obtain the internal pressure value PIm of each unloading unit; where m ranges from 1 to n, and n is the number of unloading units;

[0007] According to the total flow value F 总高 and the preset high-pressure distribution flow values of all unloading units, obtain the high-pressure flow value F of each unloading unit m高 ;

[0008] According to the high-pressure flow value F m高 obtain the high-low pressure switching value Pm of each unloading unit;

[0009] For different unloading units,

[0010] when the internal pressure value PIm of the unloading unit is greater than or equal to the high-low pressure switching value Pm, control the unloading unit to unload at the flow rate of the high-pressure flow value F m高 ;

[0011] when the internal pressure value PIm of the unloading unit is less than the high-low pressure switching value Pm, according to the total unloading flow value F 总 and the actual high-pressure flow values of each unloading unit, obtain the total flow value F at the unloading outlet on the low-pressure side 总低 ; and according to the total flow value F 总低 and the preset low-pressure distribution flow values of all unloading units, obtain the low-pressure flow value F of each unloading unit m低 , and control the unloading unit to unload at the flow rate of the low-pressure flow value F m低 .

[0012] Compared with the above background technology, the method for unloading natural gas into the LNG production device provided by this application is used to unload the raw material gas in the unloading unit into the LNG production device. The LNG production device includes a raw material gas compressor. The unloading outlets on the low-pressure side of all unloading units are connected to the inlet of the raw material gas compressor, and the unloading outlets on the high-pressure side of all unloading units are connected to the outlet of the raw material gas compressor; the method includes: determining the total unloading flow value F 总 and the total flow value F at the unloading outlet on the high-pressure side 总高 , and obtaining the internal pressure value PIm of each unloading unit; where m ranges from 1 to n, and n is the number of unloading units; according to the total flow value F 总高 and the preset high-pressure distribution flow values of all unloading units, obtain the high-pressure flow value F of each unloading unit m高 ; according to the high-pressure flow value F m高 obtain the high-low pressure switching value Pm of each unloading unit; for different unloading units, when the internal pressure value PIm of the unloading unit is greater than or equal to the high-low pressure switching value Pm, control the unloading unit to unload at the flow rate of the high-pressure flow value F m高Unloading by the flow rate; when the internal pressure value PIm of the unloading unit is less than the high-low pressure switching value Pm, according to the total unloading flow rate value F 总 and the actual high-pressure flow rate values of each unloading unit, obtain the total flow rate value F 总低 at the unloading outlet on the low-pressure side; and according to the total flow rate value F 总低 and the preset low-pressure distribution flow rate values of all unloading units, obtain the low-pressure flow rate values F m低 of each unloading unit, and control the unloading unit to unload at the low-pressure flow rate value F m低 .

[0013] The method of unloading natural gas into the LNG production device set in this way can unload at different flow rates for different unloading units. For different unloading units, when the internal pressure value PIm of the unloading unit is greater than or equal to the high-low pressure switching value Pm, control the unloading unit to unload at the high-pressure flow rate value F m高 ; when the internal pressure value PIm of the unloading unit is less than the high-low pressure switching value Pm, according to the total unloading flow rate value F 总 and the actual high-pressure flow rate values of each unloading unit, obtain the total flow rate value F 总低 at the unloading outlet on the low-pressure side; and according to the total flow rate value F 总低 and the preset low-pressure distribution flow rate values of all unloading units, obtain the low-pressure flow rate values F m低 of each unloading unit, and control the unloading unit to unload at the low-pressure flow rate value F m低 . In this way, after each unloading unit finishes unloading, a new unloading unit can be continuously replaced for the next round of unloading. At the same time, if there are spare unloading units, new unloading devices can be added at any time during the unloading of the existing unloading units to unload at any time, so as to realize the rapid, efficient and stable unloading of the unloading unit to the operating LNG production device.

[0014] Among them, the acquisition method of the preset high-pressure distribution flow rate value is as follows:

[0015] Calculate the preset high-pressure distribution flow rate values of each unloading unit through the formula ((PIm - P 系 ) × Vm × 1000 / 8.31 / T) × Kv 额定 / Kv 计算 ,

[0016] where PIm is the internal pressure value of the mth unloading unit, P 系 is the expected pressure value of the LNG production device, Vm is the volume of the mth unloading unit, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, and Kv 计算 is the designed operating point Kv value of the unloading valve.

[0017] Among them, the method for obtaining the high and low pressure switching value Pm is as follows:

[0018] Through the formula ((F m高 × 22.4) 2 × r N × T / (380 2 × Kv 额定 2 ) + P 系 2 × 100) 0.5 / 10, the high and low pressure switching value Pm of each unloading unit is calculated,

[0019] Among them,

[0020] F m高 is the preset high pressure distribution flow value of the mth unloading unit, r N is the density of the raw material gas under standard conditions, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, P 系 is the pressure expectation value of the LNG production device.

[0021] Among them, according to the total unloading flow value F 总 and the actual high pressure flow values of each unloading unit, the steps to obtain the total flow value F 总低 at the low pressure side unloading outlet include:

[0022] By subtracting the actual high pressure flow values of all unloading units from the total unloading flow value F 总 , the total flow value F 总低 at the low pressure side unloading outlet is obtained.

[0023] Among them, the method for obtaining the preset low pressure distribution flow value is as follows:

[0024] Through the formula 3600 × ((PIm - P 系 ) × Vm × 1000 / 8.31 / T) × Kv 额定 / Kv 计算 / (t 总 - tm), the preset low pressure distribution flow value of each unloading unit is calculated,

[0025] Among them, PIm is the internal pressure value of the mth unloading unit, P 系 is the pressure expectation value of the LNG production device, Vm is the volume of the mth unloading unit, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, Kv 计算 is the designed working point Kv value of the unloading valve, t 总 is the estimated total unloading time, tm is the high pressure side unloading time.

[0026] Among them, the LNG production device further includes: a flow regulation loop connected to the inlet of the raw gas compressor;

[0027] The method further includes:

[0028] Controlling the flow value in the flow regulation loop to be the flow value of the LNG production device before unloading minus the sum of the actual flow values of all unloading units, and the sum of the actual flow values is the actual high-pressure flow value during high-pressure side unloading or the actual low-pressure flow value during low-pressure side unloading.

[0029] Among them, the LNG production device further includes a pretreatment device and a liquefaction device. The raw gas compressor, the pretreatment device, and the liquefaction device are connected in sequence. A reflux valve is connected between the inlet and the outlet of the raw gas compressor. A first pressure regulation loop is connected between the raw gas compressor and the pretreatment device. A second pressure regulation loop is connected between the pretreatment device and the liquefaction device;

[0030] The method further includes:

[0031] Controlling the pressure value in the first pressure regulation loop to be the expected pressure value at the outlet of the raw gas compressor plus the pressure deviation value when the reflux valve starts to act;

[0032] Controlling the pressure value in the second pressure regulation loop to be the expected pressure value P of the LNG production device 系 .

[0033] A system for unloading natural gas into an LNG production device is used to unload the raw gas in the unloading unit into the LNG production device according to the above method for unloading natural gas into an LNG production device. The LNG production device includes a raw gas compressor. The low-pressure side unloading outlets of all unloading units are connected to the inlet of the raw gas compressor, and the high-pressure side unloading outlets of all unloading units are connected to the outlet of the raw gas compressor;

[0034] The system for unloading natural gas into an LNG production device is used for:

[0035] Determining the total unloading flow value F 总 and the total flow value F of the high-pressure side unloading outlet 总高 , and obtaining the internal pressure value PIm of each unloading unit; where m ranges from 1 to n, and n is the number of unloading units;

[0036] According to the total flow value F 总高 and the preset high-pressure distribution flow value of all unloading units, obtaining the high-pressure flow value F of each unloading unit m高 ;

[0037] According to the high-pressure flow value F m高 obtaining the high-low pressure switching value Pm of each unloading unit;

[0038] For different unloading units,

[0039] When the internal pressure value PIm of the unloading unit is greater than or equal to the high-low pressure switching value Pm, control the unloading unit to unload at the high-pressure flow rate value F m高 of the flow velocity;

[0040] When the internal pressure value PIm of the unloading unit is less than the high-low pressure switching value Pm, according to the total unloading flow rate value F 总 and the actual high-pressure flow rate values of each unloading unit, obtain the total flow rate value F 总低 at the unloading outlet on the low-pressure side; and according to the total flow rate value F 总低 and the preset low-pressure distribution flow rate values of all unloading units, obtain the low-pressure flow rate values F m低 of each unloading unit, and control the unloading unit to unload at the low-pressure flow rate value F m低 of the flow velocity.

[0041] Among them, any unloading unit includes a low-pressure side sub-unloading outlet and a high-pressure side sub-unloading outlet. The low-pressure side sub-unloading outlet and the high-pressure side sub-unloading outlet are also connected with an unloading valve and a check valve. All the low-pressure side sub-unloading outlets are connected to form a low-pressure side unloading outlet, and all the high-pressure side sub-unloading outlets are connected to form a high-pressure side unloading outlet. A reheater is also provided at the low-pressure side unloading outlet and the high-pressure side unloading outlet. Description of the Drawings

[0042] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0043] Figure 1 It is a schematic diagram of the system for unloading natural gas into the LNG production device provided by the embodiment of the present application;

[0044] Figure 2 For Figure 1 the schematic diagram of the unloading guiding system in;

[0045] Figure 3 It is a flowchart of the method for unloading natural gas into the LNG production device provided by the embodiment of the present application;

[0046] Figure 4 It is a schematic diagram of the distribution process of the high-pressure flow rate value F m高 of the unloading unit;

[0047] Figure 5 It is a schematic diagram of the distribution process of the low-pressure flow rate value F m低 of the unloading unit;

[0048] Figure 6 It is a flow curve graph of the unloading outlet on the high-pressure side of the unloading unit;

[0049] Figure 7 It is a flow curve graph of the unloading outlet on the low-pressure side of the unloading unit;

[0050] Figure 8 It is a total unloading flow curve graph of all unloading units;

[0051] Figure 9 It is a curve graph of the internal pressure value PIm of the unloading unit; [[ID=1⑥]]

[0052] Figure 10 It is a parameter curve graph of the LNG production device during unloading. Specific embodiments

[0053] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0054] In order to enable those skilled in the art of this technology to better understand the solution of the present application, the present application will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0055] [[ID=3⑩]]A method and system for unloading natural gas into an LNG production device provided by an embodiment of the present application. Please first refer to the attached Figure 1 attachment Figure 1 is a schematic diagram of the system for unloading natural gas into an LNG production device. The above method is applied to the system shown in the attached Figure 1 attachment Figure 1 can also be regarded as the overall structure diagram of the LNG production device including this system. Among them, the unloading guidance system, the stable batch control system, and the flow distribution system are the most core improvement points. The functions and setting methods of the raw gas compressor, the pretreatment unit, and the liquefaction unit can refer to the prior art.

[0056] The unloading guiding system is connected to n unloading inlets. That is, the input end of the unloading guiding system is connected to unloading inlet 1, unloading inlet 2,..., unloading inlet n. Here, each unloading inlet is used to receive the raw gas of different unloading units. The output end of the unloading guiding system includes a low-pressure side unloading outlet and a high-pressure side unloading outlet. The low-pressure side unloading outlet is connected to the inlet of the raw gas compressor, and the high-pressure side unloading outlet is connected to the outlet of the raw gas compressor. It can be considered that the low-pressure side unloading outlets of all the unloading units are connected to the inlet of the raw gas compressor, and the high-pressure side unloading outlets of all the unloading units are connected to the outlet of the raw gas compressor.

[0057] Please refer to the appendix Figure 2 , Figure 2 For Figure 1 Figure; it is a schematic diagram of the unloading guiding system in the middle. The unloading guiding system includes 1 to n small units, and each small unit can achieve the same function. Each small unit inlet is connected to a high-pressure natural gas device, that is, the unloading inlet, and their high-pressure outlet sides converge together to form the above-mentioned high-pressure side unloading outlet, and their low-pressure outlet sides converge together to form the above-mentioned low-pressure side unloading outlet. Each small unit contains a pressure transmitter PI1m (where m belongs to 01 to n), a flow transmitter FIC1m (a single flowmeter is shared for the high-pressure side and the low-pressure side), a high-pressure side unloading valve m, a low-pressure side unloading valve m, and two one-way valves. There is a reheater at each of the high-pressure side unloading outlet and the low-pressure side unloading outlet of the unloading guiding system to prevent the temperature from being too low after pressure reduction and throttling. Each small unit has an unloading knob m, and the entire unloading guiding system has an allowable unloading button.

[0058] Please refer to the appendix Figure 3 , Figure 3 is a flowchart of the method for unloading natural gas into the LNG production device provided by the embodiment of the present application. At the same time, Figure 3 can also be regarded as Figure 1 the working flowchart of the stable control system in, where the method for unloading natural gas into the LNG production device includes:

[0059] Determine the total unloading flow value F 总 and the total flow value F 总高 of the high-pressure side unloading outlet, and obtain the internal pressure value PIm of each of the unloading units; where m ranges from 1 to n, and n is the number of the unloading units;

[0060] According to the total flow value F 总高 and the preset high-pressure distribution flow value of all the unloading units, obtain the high-pressure flow value F m高 of each of the unloading units;

[0061] According to the high-pressure flow value F m高Obtain the high-low pressure switching value Pm of each of the said truck unloading units;

[0062] For different ones of the said truck unloading units,

[0063] When the internal pressure value PIm of the said truck unloading unit is greater than or equal to the high-low pressure switching value Pm, control the said truck unloading unit to unload the truck at the flow rate of the said high-pressure flow value F m高 ;

[0064] When the internal pressure value PIm of the said truck unloading unit is less than the high-low pressure switching value Pm, based on the total truck unloading flow value F 总 and the actual high-pressure flow values of each of the said truck unloading units, obtain the total flow value F 总低 at the outlet of the low-pressure side truck unloading; and based on the said total flow value F 总低 and the preset low-pressure distribution flow values of all the said truck unloading units, obtain the low-pressure flow values F m低 of each of the said truck unloading units, and control the said truck unloading unit to unload the truck at the flow rate of the said low-pressure flow value F m低 .

[0065] In the appendix Figure 3 , the subscript m ranges from 1 to n, and there are a total of n truck unloading units. When preparing to unload the m-th truck, turn the truck unloading knob m to "unload the truck", and at this time, the high-low pressure truck unloading flag variable Am corresponds one by one. The truck unloading logics from 1 to n are in a parallel relationship, without affecting each other, and are not timed (that is, they can occur simultaneously, or can occur at different times, or can occur successively with a certain time difference). At any time, when the truck unloading knob m is turned to stop, the high-pressure side truck unloading valve m and the low-pressure side truck unloading valve m will be fully closed and return to the step of preparing to unload the m-th truck.

[0066] When the flow regulating valve loop FICm is in the high-pressure side truck unloading, mark Am = 2; when it is in the low-pressure side truck unloading, mark Am = 1. When not in the truck unloading state, Am = 0.

[0067] The flow distribution system is an algorithm for distributing the flow control set value in the truck unloading guiding system. Whether it is high-pressure side truck unloading or low-pressure side truck unloading, when the pressure is sufficient, it is desired to control the total truck unloading flow F 总 of the truck unloading guiding system at a fixed value.

[0068] When the truck unloading knob m is turned to "unload the truck", the flow distribution system starts to work. At the beginning, there are m small units in the truck unloading guiding system for flow distribution and unloading the truck on the high-pressure side.

[0069] The objective function F1 + F2 + …… + F m = F 总 ,

[0070] To distinguish between the high-voltage side and the low-voltage side, high and low subscripts are added, so the function is changed to

[0071] F 1高 +F 2高 +……+F m高 =F 总高 . At the beginning, F 总高 =F 总 .

[0072] The objective function F 总高 =F 1高 +F 2高 +……+F m高 is essentially an isopleth, and the value of any point on the line is equal to F 总高 . When the isopleth can intersect with the total constraint interval formed by combining the respective constraint intervals of F m高 , the coordinates of the intersection point correspond to each F m高 , which are assigned to the set value of the flow control. It should be noted that since a space with more than three dimensions cannot be actually drawn, the appendix Figure 4 is only for illustration.

[0073] The straight line F 1高 +F 2高 +……+F m高 =F 总高 is the objective function, and the irregular closed region is the total constraint interval formed by combining the respective constraint intervals of F m高 . When the objective function intersects with the total constraint interval, the corresponding F 1高 , F 2高 , ……F m高 are the assigned set values.

[0074] F m高 is the preset high-voltage distribution flow value of the mth unloading unit, and the calculation method of its constraint interval is as follows:

[0075] F m高 ≤((PIm - P 系 ) × Vm × 1000 / 8.31 / T) × Kv 额定 / Kv 计算 ,

[0076] PIm is the internal pressure value of the mth unloading unit, P 系 is the pressure expectation value of the LNG production device, Vm is the volume of the mth unloading unit, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, and Kv 计算 is the Kv value of the design operating point of the unloading valve.

[0077] When calculating the high and low pressure switching pressure value Pm, the Pm value for this unloading is fixed. Each time the unloading knob is set to "unloading", Pm is calculated and updated.

[0078] The method for obtaining the high and low pressure switching value Pm is as follows:

[0079] Through the formula ((F m高 ×22.4) 2 ×r N ×T / (380 2 ×Kv 额定 2 ) + P 系 2 ×100) 0.5 / 10, the high and low pressure switching values Pm of each of the said unloading units are calculated.

[0080] Among them, F m高 is the preset high - pressure distribution flow value of the m - th said unloading unit, r N is the density of the raw material gas under standard conditions, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, P 系 is the pressure expectation value of the said LNG production device.

[0081] Among them, the step of obtaining the total flow value F 总 of the low - pressure side unloading outlet according to the total unloading flow value F 总低 and the actual high - pressure flow values of each of the said unloading units includes:

[0082] By subtracting the actual high - pressure flow values of all the said unloading units from the total unloading flow value F 总 , the total flow value F 总低 of the low - pressure side unloading outlet is obtained.

[0083] Whenever any one of Am changes from 2 to 1, for example, A k changes from 2 to 1, a flow distribution calculation is performed on the high - pressure side, and the value of F k高 excluding F m高 is redistributed. Whenever any one of Am changes from 0 to 2, for example, A k changes from 0 to 2, a flow distribution calculation is performed on the high - pressure side, and the value of F k including F m高 is redistributed. During the distribution calculation, the objective function F 1高 + F 2高 + …… + F m高 = F 总高 remains unchanged (where F k高 is no longer included or F k高 is newly added). F k高The restricted range is calculated based on the pressure value when A k just changes to 2. F m高 may be the same as or different from the original value each time it is updated. The newly obtained F 1高 +F 2高 +……+F m高 may be equal to F 总高 or may be less than F 总高 . When it is always less than F 总高 , it means that the current maximum unloading capacity on the high-pressure side has reached F 总高 , and even when the unloading valve on the high-pressure side is fully open, it cannot guarantee the original F 总高 . A new high-pressure natural gas unloading device can be added to the unloading guidance system, or the existing unloading device can unload to the low-pressure side through the unloading guidance system.

[0084] Low-pressure side unloading may occur in parallel with high-pressure side unloading.

[0085] Record the time difference between when Am is 1 and when Am is 2, that is, the high-pressure side unloading time tm seconds.

[0086] Allocate a flow rate set value F m低 ,

[0087] Objective function:

[0088] F 1低 +F 2低 +……+F m低 =F 总低 =F 总 -(FIC 101高 +FIC 102高 +……FIC 1m高 )

[0089] Among them, FIC 101高 to FIC 1m高 are flow measurement values.

[0090] When Am changes from 2 to 1 or Am changes from 0 to 1, perform flow distribution calculation.

[0091] Objective function F 总低 =F 1低 +F 2低 +……+F m低 is essentially an isoline, and the value of any point on the line is equal to F 总低 . When the isoline intersects with the total restricted range formed by the restricted ranges of F m低 respectively, the corresponding F m低 values of the intersection coordinates are assigned to the set value of flow control. Among them, a space with more than 3 dimensions cannot actually be drawn. AttachedFigure 5 For illustration only.

[0092] Straight line F 1低 +F 2低 + …… +F m低 =F 总低 is the objective function, and the irregular closed area is F m低 The combined total limit interval of their respective limit intervals. When the objective function intersects with the total limit interval, corresponding to F 1低 , F 2低 , …… F m低 is the assigned set value.

[0093] Among them, the acquisition method of the preset low-pressure distribution flow value is:

[0094] Through the formula 3600×((PIm - P 系 )×Vm×1000 / 8.31 / T)×Kv 额定 / Kv 计算 / (t 总 -tm), the preset low-pressure distribution flow values of each unloading unit are calculated,

[0095] Among them, PIm is the internal pressure value of the mth unloading unit, P 系 is the pressure expectation value of the LNG production device, Vm is the volume of the mth unloading unit, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, Kv 计算 is the designed operating point Kv value of the unloading valve, t 总 is the estimated total unloading time, and tm is the high-pressure side unloading time.

[0096] Whenever any Am changes from 1 to 0, for example, A k changes from 1 to 0, a flow distribution calculation is performed on the low-pressure side, and F except for F k低 is redistributed. Whenever any Am changes from 2 to 1, for example, A m低 changes from 2 to 1, a flow distribution calculation is performed on the low-pressure side, and F including F k is redistributed. k低 in it. m低 value.

[0097] During the distribution calculation, the objective function F 1低 +F 2低 + …… +F m低 =F 总低 remains unchanged (where F k低 is no longer included or new F k低 is added). F k低 The limit interval is recalculated according to the high and low pressure switching pressure value Pm.m低 The value may be the same as or different from the original after each update.

[0098] The newly obtained F 1低 +F 2低 +……+F m低 may be equal to F 总低 , or may be less than F 总低 . When it is always less than F 总低 , it indicates that the current maximum unloading capacity on the low-pressure side has reached F 总低 , and even when the unloading valve on the low-pressure side is fully open, it cannot guarantee the original F 总低 . At this time, a new high-pressure natural gas unloading device can be added to the unloading guiding system, or the total unloading flow can be reduced until the unloading is completed. Finally, the stable batch control system determines whether the unloading on the low-pressure side is completed.

[0099] Please continue to refer to the appendix Figure 1 , the LNG production device further includes: a flow regulation circuit connected to the inlet of the raw gas compressor;

[0100] The above method further includes:

[0101] Controlling the flow value in the flow regulation circuit to be the flow value of the LNG production device before unloading minus the sum of the actual flow values of all the unloading units, and the sum of the actual flow values is the actual high-pressure flow value during high-pressure side unloading or the actual low-pressure flow value during low-pressure side unloading.

[0102] The LNG production device further includes a pretreatment device and a liquefaction device. The raw gas compressor, the pretreatment device, and the liquefaction device are connected in sequence. A reflux valve is connected between the inlet and the outlet of the raw gas compressor. A first pressure regulation circuit is connected between the raw gas compressor and the pretreatment device. A second pressure regulation circuit is connected between the pretreatment device and the liquefaction device;

[0103] The above method further includes:

[0104] Controlling the pressure value in the first pressure regulation circuit to be the expected pressure value at the outlet of the raw gas compressor plus the pressure deviation value when the reflux valve starts to act;

[0105] Controlling the pressure value in the second pressure regulation circuit to be the pressure expectation value P of the LNG production device 系 .

[0106] Appendix Figure 1 The stable batch control system in

[0107] The flow set value of FIC02 is F = Expected value of FI03 - (FIC101 + FIC102 + …… FIC1n);

[0108] Among them, FIC101 to FIC1n are flow measurement values. The main purpose of FIC02 adjustment is to control the flow fluctuations of the LNG production unit at the beginning of unloading, when there are large errors in the flow distribution system adjustment, and at the end of unloading. The expected value of FI03 is the flow of the LNG production unit before unloading.

[0109] The pressure set value of PIC02 is P = Expected value of PI02 + δP;

[0110] Among them: δP is the pressure deviation value at which the expected reflux valve starts to operate;

[0111] The pressure set value of PIC03;

[0112] P 系 = Expected value of the system pressure;

[0113] The set values Fm of FIC101 to FIC1n are the expected flow values for unloading, which are determined by the flow distribution system.

[0114] The pressure switching value Pm for high and low pressure unloading is determined by the flow distribution system.

[0115] The flow set value F0 at the end of unloading.

[0116] The total expected unloading flow value F 总 , which is set by the production personnel (generally determined by the total expected cumulative amount N of this unloading and the total expected current unloading time t).

[0117] When all of the above are ready, enter the unloading process, that is, execute the control process in Appendix Figure 3 .

[0118] For example, for an LNG production unit that receives unloading from three unloading units, before the unloading is about to start, the stable operating point of the LNG production unit is:

[0119] The inlet pressure PI01 of the raw gas compressor is about 1550 kPa, the measured value of the outlet pressure PIC02 of the compressor is about 5400 kPa, the total natural gas processing volume FI03 is about 1875 kmol / h (i.e., 1 million Nm of natural gas per day 3 of natural gas processing volume), the measured value of the flow FIC02 into the raw gas compressor is about 1875 kmol / h, and the measured value of the system pressure PIC03 is about 5200 kPa.

[0120] Before the unloading is about to start, the operating point of the unloading guiding system is:

[0121] The pressure of Tank 1 (i.e., the pressure PI101 at the unloading inlet 1) is approximately 20000 kPa, the pressure of Tank 2 (i.e., the pressure PI102 at the unloading inlet 2) is approximately 16000 kPa, and the pressure of Tank 3 (i.e., the pressure PI103 at the unloading inlet 3) is approximately 18000 kPa. The unloading flow rates of Tank 1, FIC101, Tank 2, FIC102, and Tank 3, FIC103 are all approximately 0 kmol / h. The volume of the tank on each high-pressure natural gas device is 18 m 3 . The ambient temperature is 40 °C. The rated Kv of the unloading valve 额定 = 8, and the calculated Kv at the time of selection 计算 = 1.5.

[0122] Before unloading is about to start, the set values of the stable batch control system and the flow distribution system are as follows:

[0123] The production personnel expect to unload 15000 Nm 总 within 40 minutes (i.e., t 3 = 2400 s), so F 总 can be set as 15000 / 22.4×60 / 40 ≈ 1004.5 kmol / h.

[0124] The set value of the flow rate of FIC02 at the compressor inlet

[0125] F = 1875 - (FIC101 + FIC102 + FIC103), where FIC101, FIC102, and FIC103 are measured values, F is the flow rate into the raw material gas compressor (compressor inlet flow rate), FIC02 is the set value of the control loop, and 1875 kmol is the total hourly processing volume of natural gas during stable production, that is, the amount entering the liquid chemical process per hour. In the above formula, F is obtained by subtracting the sum of the measured values of all flow meters in the unloading guiding system from 1875.

[0126] The set value of the compressor outlet pressure

[0127] The set value of the PIC02 pressure P = 5400 kPa + 50 kPa = 5450 kPa

[0128] The set value of the flow rate at the end of unloading F0 = 10 kmol / h;

[0129] Among them, the 50 kPa of δP is an empirical value. The purpose is that the compressor reflux valve will only open when the pressure has indeed increased. In order not to act frequently, a "non-action range" is required. Of course, 30 kPa, 50 kPa, or 70 kPa, etc. can also be taken, as long as it is not set too large or too small.

[0130] In addition, F0 represents the marked flow rate when the unloading of the truck has reached its limit. There is no fixed value for it, and it can be set to 5, 10, 15, etc. The larger this value is set, the earlier the process ends. The 10 kmol / h in this article is an empirical value.

[0131] After the above data is prepared, click the "Allow Unloading" button, and all the high-pressure side unloading valves 1, 2, 3, and the low-pressure side unloading valves 1, 2, 3 in the truck unloading guiding system are initialized and closed.

[0132] Set the truck unloading knobs 1, 2, and 3 to "Unloading", and the flow rate distribution system starts to calculate.

[0133] F 总高 = F 总 = 1004.5 kmol / h;

[0134] F 1高 + F 2高 + F 3高 = 1004.5 kmol / h;

[0135] F 1高 ≤ 545.6 kmol / h, F 2高 ≤ 398.2 kmol / h, F 3高 ≤ 471.9 kmol / h;

[0136] Among them, as an example, only the calculation formula of F 1高 is given. F 1高 ≤ (20 - 5.2) × 18 × 1000 / 8.31 / 313.15 × 8 / 1.5 ≈ 545.6 kmol / h. The specific calculation process is as recorded in the previous text. Let the objective function F 1高 + F 2高 + F 3高 = 1004.5 kmol / h be an isocline in a space. Let this isocline pass through the limit regions of F 1高 , F 2高 , F 3高 . When there are intersection points, the intersection points are F 1高 , F 2高 , F 3高 . When there are no intersection points, the upper limit values are the values of F 1高 , F 2高 , F 3高 .

[0137] Let the objective function F 1高 + F 2高 + F 3高 = 1004.5 be combined with F 1高 ≤ 545.6, F 2高 ≤ 398.2, F3高 The total limit interval of the sum of the limit intervals of ≤471.9 intersects to obtain:

[0138] F1≈408kmol / h, F2≈262kmol / h, F≈335kmol / h;

[0139] High and low pressure switching pressure values P1 = 6.97 MPa, P2 = 5.99 MPa, P3 = 6.44 MPa.

[0140] As an example, only the calculation formula of P1 is given, P1=((408×22.4) 2 ×0.78×313 / (380 2 ×8 2 )+5.2 2 ×100) 0.5 / 10≈6.97MPa.

[0141] Circuit FIC101 is put into automatic mode. Since the pressure of tank 1 PI101 = 20000 kPa > P1 = 6970 kPa, circuit FIC101 will control the high-pressure unloading valve 1.

[0142] Circuit FIC102 is put into automatic operation. Since the pressure of tank 2 PI102 = 16000 kPa > P2 = 5990 kPa, circuit FIC102 will control the high-pressure unloading valve 2.

[0143] Circuit FIC103 is put into automatic operation. Since the pressure of tank 3 PI103 = 18000 kPa > P3 = 6440 kPa, circuit FIC103 will control the high-pressure unloading valve 3.

[0144] At 1040 seconds, the pressure of tank 1 is lower than P1=6970 kPa, the high-pressure side unloading valve 1 is closed, and FIC101 switches to control the low-pressure side unloading valve 1. Am1 changes from 2 to 1.

[0145] Among them, the pressure of tank 1, namely PI101, is monitored in real time and can be set in the unloading guide coefficient. When it is detected that this pressure is lower than P1, it switches to the low-pressure side to prepare for unloading.

[0146] The high-voltage side continues with F 总高 =1004.5 as the objective function value, and in F 2高 ≤398.2 kmol / h, F 3高 Flow distribution calculation for ≤471.9 kmol / h yields:

[0147] F 1高 =0, F 2高 ≈398.2 kmol / h, F 3高≈471.9 kmol / h.

[0148] Continue with the unloading of the vehicle. The measured values are FIC101 = 0 kmol / h, FIC102 ≈ 327.9 kmol / h, and FIC103 ≈ 407.7 kmol / h;

[0149] F 低总 = 1004.5 - 327.9 - 407.7 = 268.9 kmol / h;

[0150] The objective function F 1低 + F 2低 + F 3低 = F 低总 = 268.9 kmol / h, where F 1低 ≤ 451.2 kmol / h. Since only F 1低 participates in the distribution, we get F 1低 = 268.9 kmol / h, F 2低 = 0, F 3低 = 0;

[0151] When the loop FIC103 switches to the low-pressure side, exclude F 3高 from the high-pressure side flow distribution algorithm, and add the condition of F 3低 to the low-pressure side flow distribution algorithm, then perform the flow distribution calculation for both the high-pressure and low-pressure sides; when the loop FIC102 switches to the low-pressure side, exclude F 2高 from the high-pressure side flow distribution algorithm, and add the condition of F 2低 to the low-pressure side flow distribution algorithm, then perform the flow distribution calculation for both the high-pressure and low-pressure sides.

[0152] ……

[0153] According to the stable batch control system:

[0154] When the measured value of FIC101 < 10 kmol / h and after timing for 1 minute, the unloading of tank 1 is completed, and a low-pressure side flow distribution calculation is performed. Prompt "The current unloading has been completed. Please set the unloading knob 2 to 'Stop'". At this time, set the unloading knob 1 to stop, and this unloading is completed.

[0155] When the measured value of FIC103 < 10 kmol / h and after timing for 1 minute, the unloading of tank 3 is completed, and a low-pressure side flow distribution calculation is performed. Prompt "The current unloading has been completed. Please set the unloading knob 3 to 'Stop'". At this time, set the unloading knob 3 to stop, and this unloading is completed.

[0156] When the measured value of FIC102 < 10 kmol / h and after timing for 1 minute, the unloading of Tank 2 from the vehicle is completed, and a low-pressure side flow distribution calculation is performed. A prompt "The current vehicle unloading has been completed. Please set the vehicle unloading knob 2 to 'Stop'" is given. At this time, set the vehicle unloading knob 2 to Stop, and this vehicle unloading ends.

[0157] Among them, the timing of 1 minute can be set in the stable batch control system. According to actual experience, it can also be 2 minutes, etc. If it has been below 10 for several minutes, it is considered that it cannot be unloaded anymore. If you insist on unloading to 0, then the time may be extended for a long time, wasting time.

[0158] For the measured values of each part of the entire vehicle unloading process, please refer to Appendix Figure 6 to Appendix Figure 9 , during the total time of 1900 seconds (about 32 minutes) of the entire vehicle unloading process, the requirement of unloading the vehicle within the previously expected 40 minutes is met. In addition, during the production process, the main impacts on the LNG production device depend on three parameters: the compressor outlet pressure, the total natural gas processing volume, and the system pressure. As Figure 10 shown.

[0159] It can be seen from this that this application can complete the unloading of high-pressure natural gas in three high-pressure natural gas devices (each with a tank of 18 m in the case) in a short time, and meets the requirements of being fast, efficient, and stable. 3 of the tank) and realizes the requirements of being fast, efficient, and stable.

[0160] After the unloading of the tank of each natural gas vehicle unloading device is completed, a new natural gas vehicle unloading device can be continued to be replaced for the next round of vehicle unloading. If there are spare vehicle unloading units in the vehicle unloading guidance system, new natural gas vehicle unloading devices can be added at any time while the existing vehicle unloading units are unloading, and unloading can be carried out at any time.

[0161] This application also provides a system for unloading natural gas into an LNG production device, which is used to unload the raw material gas in the vehicle unloading unit into the LNG production device according to the method of unloading natural gas into the LNG production device described above. The LNG production device includes a raw material gas compressor. The low-pressure side vehicle unloading outlets of all the vehicle unloading units are connected to the inlet of the raw material gas compressor, and the high-pressure side vehicle unloading outlets of all the vehicle unloading units are connected to the outlet of the raw material gas compressor;

[0162] The system for unloading natural gas into the LNG production device is used for:

[0163] Determine the total vehicle unloading flow value F 总 and the total flow value F 总高 of the high-pressure side vehicle unloading outlet, and obtain the internal pressure value PIm of each vehicle unloading unit; where m ranges from 1 to n, and n is the number of vehicle unloading units;

[0164] According to the total flow value F 总高 and the preset high-pressure distribution flow values of all the unloaders, the high-pressure flow values F of each unloader are obtained m高 ;

[0165] According to the high-pressure flow value F m高 the high-low pressure switching values Pm of each unloader are obtained;

[0166] For different unloaders

[0167] when the internal pressure value PIm of the unloader is greater than or equal to the high-low pressure switching value Pm, control the unloader to unload at the flow rate of the high-pressure flow value F m高 ;

[0168] when the internal pressure value PIm of the unloader is less than the high-low pressure switching value Pm, according to the total unloading flow value F 总 and the actual high-pressure flow values of each unloader, the total flow value F of the low-pressure side unloading outlet is obtained 总低 ; and according to the total flow value F 总低 and the preset low-pressure distribution flow values of all the unloaders, the low-pressure flow values F of each unloader are obtained m低 , and control the unloader to unload at the flow rate of the low-pressure flow value F m低 .

[0169] As described above, any one of the unloaders includes a low-pressure side sub-unloading outlet and a high-pressure side sub-unloading outlet. The low-pressure side sub-unloading outlet and the high-pressure side sub-unloading outlet are also connected with an unloading valve and a check valve. All the low-pressure side sub-unloading outlets are connected to form the low-pressure side unloading outlet, and all the high-pressure side sub-unloading outlets are connected to form the high-pressure side unloading outlet. A reheater is also provided at the low-pressure side unloading outlet and the high-pressure side unloading outlet.

[0170] It should be noted that in this specification, relational terms such as first and second are only used to distinguish one entity from several other entities, and do not necessarily require or imply any actual relationship or order between these entities.

[0171] Specific examples are used in this article to elaborate on the principles and implementation manners of the present application. The description of the above embodiments is only used to help understand the method and its core idea of the present application. It should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present application, several improvements and modifications can be made to the present application, and these improvements and modifications also fall within the protection scope of the claims of the present application.

Claims

1. A method for unloading natural gas into an LNG production unit, characterized in that, For unloading the raw gas in the unloading unit into the LNG production device, the LNG production device includes a raw gas compressor and an unloading guiding system. The low-pressure side unloading outlets of all the unloading units are communicated with the inlet of the raw gas compressor, and the high-pressure side unloading outlets of all the unloading units are communicated with the outlet of the raw gas compressor. The method includes: Determine the total unloading flow value F 总 and the total flow value F of the high-pressure side unloading outlet 总高 , and obtain the internal pressure value PI of each of the unloading units m ; where m ranges from 1 to n, and n is the number of the unloading units According to the total flow value F 总高 and the preset high-pressure distribution flow values of all the said unloader units, the high-pressure flow value F of each said unloader unit is obtained m高 ; According to the high-pressure flow value F m高 obtain the high-low pressure switching value P of each of the unloading units m ; For different ones of the unloading units, When the internal pressure value PIm of the unloader unit is greater than or equal to the high-low pressure switching value P m control the unloader unit to unload the vehicle at the flow rate of the high-pressure flow value F m高 ; When the internal pressure value PI of the unloader unit m is less than the high-low pressure switching value P m , based on the total unloading flow value F 总 and the actual high-pressure flow values of each unloader unit, obtain the total flow value F 总低 at the unloading outlet on the low-pressure side; and based on the total flow value F 总低 and the preset low-pressure distribution flow values of all the unloader units, obtain the low-pressure flow values F m低 of each unloader unit, and control the unloader unit to unload at the flow rate of the low-pressure flow value F m低 ; The obtaining method of the preset high-pressure distribution flow value is: Through the formula ((PI m -P 系 ) × V m × 1000 / 8.31 / T) × Kv 额定 / Kv 计算 The preset high - pressure distribution flow values of each of the said unloading units are calculated, Among them, PI m is the internal pressure value of the m-th unloading unit, P 系 is the pressure expectation value of the LNG production device, V m is the volume of the m-th unloading unit, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, Kv 计算 is the Kv value at the designed operating point of the unloading valve; The high-low voltage switching value P m is obtained as follows: Through the formula ((F m高 ×22.4) 2 ×r N ×T / (380 2 ×Kv 额定 2 ) + P 系 2 ×100) 0.5 / 10, the high and low pressure switching value P of each of the said unloader units is calculated m , Among them, F m高 is the preset high-pressure distribution flow value of the m-th unloading unit, r N is the density of the raw material gas under standard conditions, T is the ambient temperature, and Kv 额定 is the rated Kv value of the unloading valve, and P 系 is the pressure expectation value of the LNG production device; The unloading guiding system includes 1 - n small units, and each small unit has an unloading knob. When preparing to unload the m-th vehicle, the corresponding unloading knob is placed in the "unloading" position. At this time, the high - and - low - pressure unloading flag variable A m corresponds one by one. Among them, when unloading on the high - pressure side, the marker A m = 2; when unloading on the low - pressure side, the marker A m = 1; when not in the unloading state, A m = 0; When A k changes from 2 to 1, a flow distribution calculation is performed on the high-pressure side, and the F k高 values other than F m高 are redistributed; when A k changes from 0 to 2, a flow distribution calculation is performed on the high-pressure side, and the F k高 values including F m高 are redistributed; Wherein, k is any one of m, and m is any one of the first to nth.

2. The method for unloading natural gas into an LNG production device according to claim 1, wherein According to the total flow value F of the truck unloading 总 and the actual high-pressure flow values of each of the truck unloading units, obtaining the total flow value F 总低 at the unloading outlet on the low-pressure side, the steps include: By means of the total flow value F of the truck unloading 总 subtracting the actual high-pressure flow values of all the truck unloading units, the total flow value F at the unloading outlet on the low-pressure side is obtained 总低 .

3. The method for unloading natural gas into an LNG production device according to claim 1, characterized in that, The obtaining method of the preset low-pressure distribution flow value is: Obtained by the formula 3600×((PI m -P 系 )×V m ×1000 / 8.31 / T)×Kv 额定 / Kv 计算 / (t 总 -t m ), the preset low-pressure distribution flow values of each of the said vehicle unloading units are calculated. Among them, PI m is the internal pressure value of the m-th unloading unit, P 系 is the pressure expectation value of the LNG production device, V m is the volume of the m-th unloading unit, T is the ambient temperature, Kv 额定 is the rated Kv value of the unloading valve, Kv 计算 is the Kv value at the designed operating point of the unloading valve, t 总 is the total expected unloading time, t m is the unloading time on the high-pressure side.

4. The method for unloading natural gas into an LNG production device according to any one of claims 1-3, characterized in that The LNG production device further includes a flow regulation loop communicated at the inlet of the raw gas compressor; The method further includes: Controlling the flow value in the flow regulation loop to be the flow value of the LNG production device before unloading minus the sum of the actual flow values of all the unloading units. The sum of the actual flow values is the actual high-pressure flow value during high-pressure side unloading or the actual low-pressure flow value during low-pressure side unloading.

5. The method for unloading natural gas into an LNG production device according to any one of claims 1-3, characterized in that, The LNG production device further includes a pretreatment device and a liquefaction device. The raw gas compressor, the pretreatment device and the liquefaction device are connected in sequence. A reflux valve is communicated between the inlet and the outlet of the raw gas compressor. A first pressure regulation loop is communicated between the raw gas compressor and the pretreatment device. A second pressure regulation loop is communicated between the pretreatment device and the liquefaction device; The method further includes: Controlling the pressure value in the first pressure regulation loop to be the expected pressure value at the outlet of the raw gas compressor plus the pressure deviation value when the reflux valve starts to act; Control the pressure value in the second pressure regulating circuit to the expected pressure value P of the LNG production device 系 .

6. A system for unloading natural gas into an LNG production device, characterized in that, For unloading the raw gas in the unloading unit into the LNG production device according to the method for unloading natural gas into the LNG production device according to any one of claims 1-5 above. The LNG production device includes a raw gas compressor. The low-pressure side unloading outlets of all the unloading units are communicated with the inlet of the raw gas compressor, and the high-pressure side unloading outlets of all the unloading units are communicated with the outlet of the raw gas compressor; The system for unloading natural gas into the LNG production device is used for: Determine the total unloading flow value F 总 and the total flow value F at the unloading outlet on the high-pressure side 总高 , and obtain the internal pressure value PI of each of the unloading units m ; where m ranges from 1 to n, and n is the number of the unloading units Based on the total flow value F 总高 and the preset high-pressure distribution flow values of all the said unloading units, the high-pressure flow value F of each said unloading unit is obtained m高 ; According to the high-pressure flow value F m高 obtain the high-low pressure switching value P of each of the car unloading units m ; For different ones of the unloading units, When the internal pressure value PI of the unloader unit m is greater than or equal to the high-low pressure switching value P m , control the unloader unit to unload the vehicle at the flow rate of the high-pressure flow value F m高 . When the internal pressure value PI of the truck unloading unit m is less than the high-low pressure switching value P m , according to the total truck unloading flow value F 总 and the actual high-pressure flow values of each truck unloading unit, obtain the total flow value F 总低 at the unloading outlet on the low-pressure side; and according to the total flow value F 总低 and the preset low-pressure distribution flow values of all the truck unloading units, obtain the low-pressure flow values F m低 of each truck unloading unit, and control the truck unloading unit to unload at the flow rate of the low-pressure flow value F m低 .

7. The system for unloading natural gas into an LNG production device according to claim 6, characterized in that, Any one of the unloading units includes a low-pressure side sub-unloading outlet and a high-pressure side sub-unloading outlet. The low-pressure side sub-unloading outlet and the high-pressure side sub-unloading outlet are further communicated with an unloading valve and a check valve. All the low-pressure side sub-unloading outlets are communicated to form the low-pressure side unloading outlet, and all the high-pressure side sub-unloading outlets are communicated to form the high-pressure side unloading outlet. A reheater is further provided at the low-pressure side unloading outlet and the high-pressure side unloading outlet.

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