CN114941801B - BOG emptying control system of LNG receiving station - Google Patents
BOG emptying control system of LNG receiving station Download PDFInfo
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- CN114941801B CN114941801B CN202210593035.5A CN202210593035A CN114941801B CN 114941801 B CN114941801 B CN 114941801B CN 202210593035 A CN202210593035 A CN 202210593035A CN 114941801 B CN114941801 B CN 114941801B
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17D—PIPE-LINE SYSTEMS; PIPE-LINES
- F17D1/00—Pipe-line systems
- F17D1/02—Pipe-line systems for gases or vapours
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17D—PIPE-LINE SYSTEMS; PIPE-LINES
- F17D1/00—Pipe-line systems
- F17D1/02—Pipe-line systems for gases or vapours
- F17D1/065—Arrangements for producing propulsion of gases or vapours
- F17D1/07—Arrangements for producing propulsion of gases or vapours by compression
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17D—PIPE-LINE SYSTEMS; PIPE-LINES
- F17D3/00—Arrangements for supervising or controlling working operations
- F17D3/01—Arrangements for supervising or controlling working operations for controlling, signalling, or supervising the conveyance of a product
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F17—STORING OR DISTRIBUTING GASES OR LIQUIDS
- F17D—PIPE-LINE SYSTEMS; PIPE-LINES
- F17D5/00—Protection or supervision of installations
- F17D5/005—Protection or supervision of installations of gas pipelines, e.g. alarm
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- Filling Or Discharging Of Gas Storage Vessels (AREA)
Abstract
The invention discloses a BOG emptying control system of an LNG receiving station, which comprises a BOG main pipe control system and a torch system, wherein an LNG storage tank is connected to a BOG compressor through the BOG main pipe, the BOG compressor outputs BOG gas to a downstream pipe network, the BOG main pipe control system comprises a main control unit, an auxiliary control unit and an air pressure detector I, the air pressure detector I is used for detecting the air pressure of the BOG main pipe, the main control unit comprises a main control valve and a PID controller I, the air pressure detector I is connected with the PID controller I, the PID controller I is connected with the main control valve to open or close the main control valve, the auxiliary control unit comprises an auxiliary control valve and a PID controller II, the air pressure detector I is connected with the PID controller II, and the PID controller II is connected with the auxiliary control valve to open or close the auxiliary control valve. The invention adopts pressure sectional control, the main control valve and the auxiliary control valve are separately controlled, the main control valve adopts PID control, the auxiliary control valve adopts linear control, the abnormal emptying quick switch is ensured, and the stable adjustment of the BOG main pipe pressure under the normal emptying condition is ensured.
Description
Technical Field
The invention relates to the technical field of fuel gas, in particular to a BOG emptying control system of an LNG receiving station.
Background
LNG is a major initiative in countries as a clean energy source, and will meet the rapid growth phase of LNG industry development in the future petrochemical systems. In the initial stage of LNG receiving station operation, because the construction of a downstream gas transmission pipe network is not completed integrally, the gas consumption is relatively small, and the storage tank and the normal evaporation gas consumption of the loading gas phase reflux are temporarily used for supplying gas for downstream users, so that the BOG compressor is temporarily used as an external gas transmission compressor. To control LNG storage tank pressure and BOG header pressure, the gases are vented at high pressure by adjusting BOG compressor load and providing a vent control valve on the BOG header.
At present, the BOG main pipe pressure is regulated by controlling the opening of two control valves simultaneously through a PID regulator, when the two valves are in an automatic mode, the BOG main pipe is easily released excessively, after the excessive release is carried out under the regulation action of the PID, the pressure in front of the valve is rapidly reduced, so that the two control valves are rapidly and excessively closed simultaneously to ensure that the pressure of the LNG storage tank in front of the valve is not too low, and the periodic oscillation is repeatedly presented in a circulating way, so that the pressure oscillation of the LNG storage tank and the BOG main pipe is caused.
Disclosure of Invention
The invention aims to solve the problem that the periodic oscillation is repeatedly caused by quick switching of the main control valve and the auxiliary control valve based on separation control of the main control valve and the auxiliary control valve.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
The BOG emptying control system of the LNG receiving station comprises a BOG main pipe control system and a torch system, an LNG storage tank is connected to a BOG compressor through a BOG main pipe, the BOG compressor outputs BOG gas to a downstream pipe network, the BOG main pipe control system comprises a main control unit, an auxiliary control unit and an air pressure detector I, the air pressure detector I is used for detecting the air pressure of the BOG main pipe, the main control unit comprises a main control valve and a PID controller I, the air pressure detector I is connected with the PID controller I, the PID controller I is connected with the main control valve to open or close the main control valve, the auxiliary control unit comprises an auxiliary control valve and a PID controller II, the air pressure detector I is connected with the PID controller II, and the PID controller II is connected with the auxiliary control valve to open or close the auxiliary control valve; the BOG main pipe branches to form a vent main pipe, a main control valve and an auxiliary control valve which are connected in parallel are arranged on the vent main pipe, and the output end of the vent main pipe is connected to the flare system so as to burn through a ground flare of the flare system.
As an option, the PID controller I adopts PID control to close or open the main control valve, when the real-time air pressure of the BOG main pipe is larger than the normal air pressure preset value, the main control valve is controlled to be opened, and the opening I of the main control valve reaches full load when the air pressure difference is 0.024 Mpa; the PID controller II adopts linear control to enable the auxiliary control valve to be closed or opened, when the real-time air pressure of the BOG main pipe is 0.024Mpa larger than the normal air pressure preset value, the auxiliary control valve is controlled to be opened, and the opening II of the auxiliary control valve reaches full load when the air pressure difference value is 0.027 Mpa. Thus, the two PID controllers realize that the two valves are separated to control the main valve and the auxiliary valve to form the main valve and the auxiliary valve, the main control valve adopts PID control, the auxiliary control valve adopts linear control, only the main control valve is stably emptied under the condition of normal emptying, the auxiliary control valve can be rapidly opened by rapidly rising pressure under emergency, and when the pressure reaches 0.027Mpa, the main control valve and the auxiliary control valve are fully opened, thereby effectively ensuring the rapid emptying requirement of a torch system.
Wherein the opening degrees of the main control valve and the auxiliary control valve meet the following relation,
When the ground torch is one-stage,
When the number of the ground torches is two,
As an option, the BOG header control system further includes a reflux control unit, where the reflux control unit includes a reflux valve, a PID controller III, a gas pressure detector II, and a temperature detector and a gas pressure detector II are respectively configured to detect a temperature of gas at an inlet of the BOG compressor and detect a gas pressure at an outlet of the BOG compressor, the outlet of the BOG compressor branches off to be connected to the LNG tank via the reflux valve, the temperature detector and the gas pressure detector II are respectively connected to the PID controller III, and the PID controller III is connected to the reflux valve, and performs temperature control based on the temperature detector or gas pressure control based on the gas pressure detector II, so that the reflux valve is closed or opened to an opening degree III. Therefore, temperature control is adopted firstly, so that the rapid increase of air pressure caused by temperature abnormality is prevented; when the temperature is normal, air pressure control is started, and the opening of the outlet reflux valve of the BOG compressor is controlled according to the pressure difference change between the high selection value PV and the set value SP of the inlet air pressure of the compressor, so that the inlet air pressure of the compressor is automatically controlled and stabilized at the set normal value according to actual needs.
When the BOG compressor is started, the temperature is controlled by a set value, and when the compressor is started to normally and stably run, the reflux valve is controlled by the inlet air pressure and the temperature of the compressor, the temperature control is controlled in preference to the pressure control, and the temperature control is automatically switched to the temperature control under the abnormal temperature condition, so that the pressure control and the undisturbed switching of the temperature control can be realized. The control process of the PID controller III for controlling the reflux valve is as follows;
Firstly, executing a temperature control program after starting up; then judging whether the acquired real-time temperature is normal, if so, executing an air pressure control program, otherwise, executing a temperature control program; finally, repeating the steps until the machine is stopped, specifically, after the current air pressure control program or the temperature control program is executed for a period of time, judging whether the acquired real-time temperature is normal, if so, executing the air pressure control program, otherwise, executing the temperature control program; in which a switching program is configured to switch between a temperature control program and a barometric pressure control program with each other in a situation operated by an operator.
As an option, the flare system comprises a ground flare and an emptying control unit, the emptying control unit comprises a PID controller IV, an air pressure detector III, an emptying branch pipe and an emptying valve, the emptying main pipe is divided into the emptying branch pipe to be connected to the ground flare respectively, the emptying valve is arranged on the emptying branch pipe, the air pressure detector III is used for detecting the air pressure of the emptying main pipe, the air pressure detector III is connected with the PID controller IV, and the PID controller IV is connected with the emptying valve to control the opening or closing of the emptying valve. Therefore, the air pressure detector III detects the real-time air pressure of the air of the emptying header pipe, the PID controller IV obtains the real-time air pressure value, the real-time air pressure value is compared with a corresponding preset value, then the emptying valve is controlled to be opened, automatic control of the emptying valve is achieved, and air is burned and emptying through ground torches such as a pilot lamp of a torch system.
The emptying branch pipe comprises a first branch pipe, a second branch pipe and a third branch pipe, the first branch pipe is directly discharged, the second branch pipe is provided with a second-stage emptying valve, the third branch pipe is provided with a third-stage emptying valve, a PID controller IV is respectively connected with the second-stage emptying valve and the third-stage emptying valve, when the air pressure of the emptying main pipe is 0-2Kpa, the second-stage emptying valve and the third-stage emptying valve are controlled to be opened when the air pressure of the emptying main pipe is 2-6Kpa, when the air pressure of the emptying main pipe is 4-8Kpa, the second-stage emptying valve and the third-stage emptying valve are controlled to be opened, the second-stage emptying valve corresponds to the valve by 0-100% in the air pressure of the emptying main pipe, and the third-stage emptying valve corresponds to the valve by 0-100% in the air pressure of the emptying main pipe by 4-8 Kpa. Therefore, three-section type emptying control is formed, and meanwhile, the air pressure sections are overlapped, so that the emptying valve can be opened in time when the excessive emptying is realized.
The control process of the PID controller IV for controlling the emptying valve is as follows:
When the air pressure of the emptying main pipe is 0-2Kpa, the air is directly discharged through the first branch pipe; when the air pressure of the air vent main pipe is more than 2Kpa, the PID controller IV further controls to open the secondary air vent valve, and the secondary air vent valve is kept in an open state when the air pressure of the air vent main pipe is 2-6 Kpa; when the air pressure of the air manifold is greater than 4Kpa, the PID controller IV further controls to open the three-stage air valve, and the two-stage air valve and the three-stage air valve are kept in an open state when the air pressure of the air manifold is 4-8 Kpa. Thus, three-section type emptying control is formed, and the three-section type emptying control is orderly opened, so that the valve opening is ensured to be stable; meanwhile, the air pressure section is overlapped to ensure that the air release valve can be opened for air release in time when the air release valve is excessively released.
By adopting the technical scheme, the invention has the following beneficial effects:
1. in the valve for the BOG main pipe emptying, pressure sectional control is adopted, a main control valve and an auxiliary control valve are adopted for separation control, main and auxiliary collocations are formed, the main control valve adopts PID control, the auxiliary control valve adopts linear control, abnormal emptying quick switching can be ensured, and stable regulation of the BOG main pipe pressure can be ensured under the normal emptying condition.
2. The override control principle is utilized in BOG compressor optimization, so that control logic is controlled according to priority, the requirement of the BOG compressor on effective load lifting under normal working conditions is met, automatic switching can be realized on temperature alarm set according to the compressor, and the compressor is ensured to be in a safe state all the time.
3. The ground torch system adopts a control mode of dividing the pressure into three sections and having cross overlapping, so that stable emptying is effectively realized, the orderly opening and closing of primary, secondary and tertiary emptying are ensured, and the ground torch system is opened and emptied in time.
Drawings
Fig. 1 is a block diagram of a control system of the present invention.
Fig. 2 is an example of a compressor control flow of the present invention.
Fig. 3 is a graph of LNG tank pressure day for the present invention.
Fig. 4 is a graph comparing pressure month curves of LNG tanks before and after optimization of the present invention.
FIG. 5 is a graph comparing month curves of opening of the output control valve before and after the optimization of the BOG compressor of the invention.
FIG. 6 is a graph of 18 days after optimization of the BOG manifold vent valve of the present invention.
FIG. 7 is a graph comparing BOG manifold pressure profiles of the present invention.
FIG. 8 is a graph comparing the 10 day fluctuation curves of the pressure of the pre-and post-blowdown header of the present invention.
FIG. 9 is a graph of the opening month of the optimized secondary and tertiary blow-down valves of the present invention.
Detailed Description
The following is a further description of the specific embodiments of the invention with reference to the accompanying drawings.
Examples
As shown in fig. 1, the BOG emptying control system of the LNG receiving station comprises a BOG main pipe control system and a flare system, wherein an LNG storage tank is connected to a BOG compressor through a BOG main pipe, the BOG compressor outputs BOG gas to a downstream pipe network, the BOG main pipe control system comprises a main control unit, an auxiliary control unit and a gas pressure detector I, the gas pressure detector I is used for detecting gas pressure at a P1 position of the BOG main pipe, the main control unit comprises a main control valve and a PID controller I, the gas pressure detector I is connected with the PID controller I, the PID controller I is connected with the main control valve to open or close the main control valve, the auxiliary control unit comprises an auxiliary control valve and a PID controller II, the gas pressure detector I is connected with the PID controller II, and the PID controller II is connected with the auxiliary control valve to open or close the auxiliary control valve; the BOG main pipe branches to form a vent main pipe F1, a main control valve and an auxiliary control valve which are connected in parallel are arranged on the vent main pipe, and the output end of the vent main pipe is connected to the flare system so as to burn through a ground flare of the flare system.
The following will be described with reference to specific examples.
The safe operating pressure (air pressure) of the LNG storage tank is controlled to be 0.5KPa-40 KPa according to the design requirement. The BOG main pipe main control valve and the auxiliary control valve are used for controlling the pressure of the BOG main pipe and the pressure of the storage tank, the set value of the BOG main pipe main control valve and the set value is higher than the vacuum pressure of the LNG storage tank by 0.5KPa and lower than the opening pressure of the safety valve by 40KPa, and the normal control range is 18KPa-26KPa.
The BOG treatment system is designed to consist of 2 low-temperature reciprocating compressors with the capacity of 8.5t/h, if the flow rate of the BOG is higher than the treatment capacity of the compressors, the pressure of a storage tank and a main pipe is increased, and when the pressure is increased to 0.026MPa, a pressure control valve on the main pipe of the BOG is automatically opened, and part of evaporated gas exceeding the process requirement is discharged to a flare system.
The torch system is designed to be composed of two sets of relatively independent closed ground torch systems, the total processing capacity of the torch system is designed to be 90t/h, the torch system is used for safely discharging high-pressure emptying gas under accident working conditions, and the processing capacity of each set of ground torch is designed to be 45t/h. Under the normal operation condition, 2 sets of ground torches are operated according to 2 simultaneous operation modes, and 1-use 1-standby operation is not adopted; when the torch needs to be run according to 1 and 1 under special conditions, the opening angles of the main control valve and the auxiliary control valve are controlled to be 17 percent.
The BOG header control system will be described below.
As an option, the PID controller I adopts PID control to close or open the main control valve, when the real-time air pressure of the BOG main pipe is larger than the normal air pressure preset value, the main control valve is controlled to be opened, and the opening I of the main control valve reaches full load when the air pressure difference is 0.024 Mpa; the PID controller II adopts linear control to enable the auxiliary control valve to be closed or opened, when the real-time air pressure of the BOG main pipe is 0.024Mpa larger than the normal air pressure preset value, the auxiliary control valve is controlled to be opened, and the opening II of the auxiliary control valve reaches full load when the air pressure difference value is 0.027 Mpa.
On the basis of the original design, a PID controller is added, and the controller sets a pressure relief safety range of 0.0240MPA-0.027Mpa according to the process safety pressure requirement, so that the opening of the auxiliary control valve is in a proportional linear relation with the pressure in the pressure range. The two emptying valves are separated and controlled, and meanwhile, the newly added controller adopts linear control to ensure that the two emptying valves can be emptied rapidly and timely under the condition that emergency emptying is sudden or the single valve cannot be completely emptied, so that the pressure stability of the BOG main pipe is ensured. The two controllers form main and auxiliary collocations, the main control valve adopts PID control, the auxiliary control valve adopts linear control, only the main control valve is stably emptied under the condition of normal emptying, the pressure is rapidly increased under emergency conditions, the auxiliary control valve can be rapidly opened, and when the pressure reaches 0.027Mpa, the two valves are fully opened, thereby effectively ensuring the rapid emptying requirement of a torch system. The valve position control table of the main control valve and the auxiliary control valve is shown in the following table 1
Table 1 below, master and auxiliary valve position control tables
| Action | Valve action | Single torch (total valve limit) | Two torches (total valve limit) | |
| Main control valve | Maintaining stable can pressure | Smooth adjustment | 17％ | 30％ |
| Auxiliary control valve | Emergency emptying | Quick opening | 17％ | 30％ |
The opening conditions of the auxiliary control valve are as follows:
(1) When the BOG main pipe pressure rises rapidly under abnormal working conditions, the main control valve controller cannot adjust the pressure in time to exceed the normal pressure range by 0.024Mpa, and the auxiliary control valve is opened and reaches full load to be emptied when the pressure reaches 0.027 Mpa.
(2) Under normal working conditions, when the pressure exceeds 0.024Mpa due to the fact that the air discharge quantity exceeds the full-load air discharge quantity of the main control valve, the auxiliary control valve is opened.
(3) Under normal working conditions, the main control valve is blocked, so that the auxiliary control valve is opened when the pressure exceeds 0.024 Mpa.
The opening degrees of the main control valve and the auxiliary control valve satisfy the following relationship,
When the ground torch is one-stage,
When the number of the ground torches is two,
Based on the foregoing example, in a preferred example, PID control programs and linear control programs are configured in both controllers of the main control valve and the auxiliary control valve, and mutual linkage can be formed or manual switching can be performed, so that when the main control valve is stopped, the auxiliary control valve can be used as a main control valve, so as to realize stable adjustment of the valve, and maintain stable pressure of the BOG main pipe and the storage tank.
As an option, based on the foregoing example, in a preferred example, the BOG header control system further includes a reflux control unit including a reflux valve, a PID controller III, a gas pressure detector II for detecting the temperature of the gas at the inlet Q of the BOG compressor and for detecting the gas pressure at the outlet of the BOG compressor, respectively, and a temperature detector II for detecting the gas pressure at the outlet of the BOG compressor, which branches off to be connected to the LNG tank via the reflux valve, and a gas pressure detector II connected to the PID controller III, respectively, the PID controller III being connected to the reflux valve, and performing temperature control based on the temperature detector or gas pressure control based on the gas pressure detector II to close or open the reflux valve to the opening degree III.
In order to realize automatic control of the outlet pressure of the compressor in a DCS system, BOG compressor control logic needs to be optimized, a PID control module is added in a DCS program, and the PID module controls the opening of a reflux valve at the outlet of the compressor A according to the pressure difference change between a high selection value PV of the inlet pressure of the compressor A and a pressure set value SP, so that the outlet pressure of the compressor is automatically controlled and stabilized at a set pressure value according to actual needs, and the problem of low output efficiency of the compressor caused by gas extrusion of a downstream pipe network is effectively solved by utilizing the regulating function of the reflux valve. When the compressor A is started, the temperature is controlled by a set value, and when the compressor A is started to normally and stably run, the reflux valve is controlled by the pressure regulation and the temperature regulation of the outlet of the compressor A, the temperature control is controlled in preference to the pressure control, the temperature control is automatically switched to the temperature control under the abnormal temperature condition, and the non-disturbance switching of the pressure control and the temperature control can be realized.
As shown in fig. 2, an example of a compressor control flow is given, the compressor control process is as follows:
(1) The compressor is preferably controlled by a temperature set point during start-up.
(2) A switching button for temperature control and pressure control is added on a DCS interface, and a pressure control program is added on the basis of the temperature control program, so that an operator can select whether to apply the pressure control according to the actual running condition of the compressor.
(3) In order to realize the undisturbed switching of the temperature control mode and the pressure control mode, a switching button is arranged in a program, and an operator can switch the pressure control mode back to the temperature control mode at any time according to actual needs.
Of course, automatic switching control may be adopted, and the control process of the reflux valve controlled by the PID controller III is as follows;
Firstly, executing a temperature control program after starting up; then judging whether the acquired real-time temperature is normal, if so, executing an air pressure control program, otherwise, executing a temperature control program; finally, repeating the steps until the machine is stopped, specifically, after the current air pressure control program or the temperature control program is executed for a period of time, judging whether the acquired real-time temperature is normal, if so, executing the air pressure control program, otherwise, executing the temperature control program; in which a switching program is configured to switch between a temperature control program and a barometric pressure control program with each other in a situation operated by an operator.
Referring to fig. 3-7, pre-and post-operation condition tests are optimized for the BOG manifold control system, as follows.
As shown in fig. 3, the pressure day curve of the LNG storage tank after optimization is shown. After the control optimization of the main control valve, the auxiliary control valve and the compressor is carried out, the pressure oscillation of the LNG storage tank is obviously eliminated, the pressure fluctuation range is basically controlled within 0.5KPa, moreover, the evaporation amount is increased and the change of the BOG (boil off gas) amount for loading is increased when the sunshine is strong in noon along with the change of time, the valve adjustment is relatively frequent, the curve fluctuation is relatively frequent, the conditions of normal evaporation of LNG and weather reflux for loading are met, and the optimization effect of the pressure control system of the LNG storage tank is good.
As shown in fig. 4, the pressure month curve comparison of the LNG storage tanks before and after optimization; the upper graph is that before optimization, the vibration amplitude is large and the frequency is high; the lower graph shows that after optimization, the amplitude is obviously reduced, and the curve tends to be smooth. Therefore, the pressure control of the LNG storage tank is obviously improved on the daily line and the month period, the large-amplitude oscillation condition is basically eliminated, the single-day fluctuation range is basically controlled within 2KPa, and the optimization requirement is met.
As shown in FIG. 5, the open month curves of the output control valves before and after the optimization of the BOG compressor are compared, wherein the upper graph is the temperature control before the optimization and the lower graph is the combined control after the optimization. The compressor frequently adjusts the load before optimization, so that the opening fluctuation of the output control valve is obvious, and the opening of the output control valve is mostly about 60%; after optimization, the compressor stably operates under high load, the opening of the control valve is over 99.6 percent, and the compressor stably and efficiently outputs; the effective load is stably improved, the output efficiency of the compressor is high, the output air quantity can achieve the expected purpose of stable output, the influence of BOG compressor load fluctuation on BOG header pipe pressure fluctuation is effectively eliminated, and the optimization effect is met.
Figure 6 shows an 18 day curve of the BOG header vent valve after optimization. As shown in fig. 7, BOG manifold pressure cycle curve comparison is shown. In the BOG total daily curve, the sum of time periods when the main control valve is at about zero opening is higher than six times, the highest opening is less than 8%, and the auxiliary control valve basically maintains zero opening. The fluctuation of the pressure cycle curve of the BOG main pipe is obviously reduced, the pressure cycle curve is effectively controlled within 3KPa, the occurrence of excessive emptying is effectively eliminated, the optimization effect is good, and the optimization purpose of reducing emptying loss is achieved.
The flare system will be described below.
As an option, the flare system comprises a ground flare and a flare control unit, the flare control unit comprises a PID controller IV, an air pressure detector III, a flare branch pipe and a flare valve, the flare main pipe F is split into flare branch pipes to be connected to the ground flare respectively, the flare valve is arranged on the flare branch pipe, the air pressure detector III is used for detecting the air pressure at the P2 position of the flare main pipe, the air pressure detector III is connected with the PID controller IV, and the PID controller IV is connected with the flare valve to control opening or closing of the flare valve. Therefore, the air pressure detector III detects the real-time air pressure of the air of the emptying header pipe, the PID controller IV obtains the real-time air pressure value, the real-time air pressure value is compared with a corresponding preset value, then the emptying valve is controlled to be opened, automatic control of the emptying valve is achieved, and air is burned and emptying through ground torches such as a pilot lamp of a torch system.
The emptying branch pipe comprises a first branch pipe F1, a second branch pipe and a third branch pipe, wherein the first branch pipe F1 is directly emptied, the second branch pipe is provided with a second-stage emptying valve, the third branch pipe is provided with a third-stage emptying valve, a PID controller IV is respectively connected with the second-stage emptying valve and the third-stage emptying valve, the second-stage emptying valve and the third-stage emptying valve are controlled to be opened when the air pressure of the emptying main pipe is 0-2Kpa, the second-stage emptying valve and the third-stage emptying valve are controlled to be opened when the air pressure of the emptying main pipe is 2-6Kpa, the second-stage emptying valve and the third-stage emptying valve are controlled to be opened when the air pressure of the emptying main pipe is 4-8Kpa, the second-stage emptying valve corresponds to the valve by 0-100 percent of opening, and the third-stage emptying valve corresponds to the valve by 0-100 percent of opening when the air pressure of the emptying main pipe is 4-8 Kpa.
Therefore, the first-stage, second-stage and third-stage emptying control modes are all based on the emptying main pipe pressure control, and the situation that pressure change occurs after the valve of the previous stage is required to be opened in the valve-based pressure control mode is avoided, so that switching untimely is effectively eliminated. Setting the primary control pressure to be 0-2Kpa, and directly discharging the primary control pressure through a first branch pipe; the secondary emptying pressure is set to be 2-6kpa, and the secondary emptying valve and the tertiary emptying valve are controlled to be opened; the three-stage emptying pressure is set to 4-8Kpa, the two-stage emptying valve and the three-stage emptying valve are controlled to be opened, and meanwhile, the pressure section is overlapped to ensure that the three-stage emptying valve can be opened in time when the excessive emptying is carried out.
The control process of the PID controller IV for controlling the emptying valve is as follows:
When the air pressure of the emptying main pipe is 0-2Kpa, the air is directly discharged through the first branch pipe; when the air pressure of the air vent main pipe is more than 2Kpa, the PID controller IV further controls to open the secondary air vent valve, and the secondary air vent valve is kept in an open state when the air pressure of the air vent main pipe is 2-6 Kpa; when the air pressure of the air manifold is greater than 4Kpa, the PID controller IV further controls to open the three-stage air valve, and the two-stage air valve and the three-stage air valve are kept in an open state when the air pressure of the air manifold is 4-8 Kpa. Thus, three-section type emptying control is formed, and the three-section type emptying control is orderly opened, so that the valve opening is ensured to be stable; meanwhile, the air pressure section is overlapped to ensure that the air release valve can be opened for air release in time when the air release valve is excessively released.
Referring to fig. 8-9, the pressure oscillations of the front and back flare blow down manifolds and the flare system deflagration condition test are optimized for the flare system, as follows.
As shown in fig. 8, the comparison of the 10 day fluctuation curves of the pressure of the front and rear vent manifolds is shown, wherein the upper graph is the graph after optimization under the control of the pressure according to the front stage valve before optimization. As can be seen, the regional curve of one week of the pre-optimization period fluctuates drastically, and the maximum pressure will be approximately 6pa; the pressure fluctuation range of the emptying main pipe after optimization is effectively controlled within 3KPa, and the vibration frequency is obviously reduced, so that the pressure oscillation problem of the emptying main pipe is eliminated.
As shown in fig. 9, the opening month curve of the optimized secondary and tertiary blow-off valve is shown. The opening degree of the secondary (two-stage) blow-down valve is less than 60%, and the duration time of each opening is short, so that the blow-down requirement can be met; the three-stage (three-stage) emptying valve is basically not opened and is in a standby state; the optimized venting quantity is less, the second section can quickly react and timely vent and burn, and the explosion conditions such as burning of a high-temperature probe, burning of a monitoring camera and the like caused by too high backlog temperature of the second section flame due to excessive venting can not occur.
As described above, the control system is based on the separation control of the main control valve and the auxiliary control valve, and can effectively solve the problem that the periodic oscillation is repeatedly presented due to the rapid switching of the main control valve and the auxiliary control valve; the problem of low output efficiency of the compressor caused by gas extrusion of a downstream pipe network can be effectively solved by utilizing the regulating function of the reflux valve; the three-stage sectional control is adopted to solve the pressure oscillation problem of the emptying main pipe. In particular as follows,
1. In the valve for the BOG main pipe emptying, pressure sectional control is adopted, a main control valve and an auxiliary control valve are adopted for separation control, main and auxiliary collocations are formed, the main control valve adopts PID control, the auxiliary control valve adopts linear control, abnormal emptying quick switching can be ensured, and stable regulation of the BOG main pipe pressure can be ensured under the normal emptying condition.
2. The override control principle is utilized in BOG compressor optimization, so that control logic is controlled according to priority, the requirement of the BOG compressor on effective load lifting under normal working conditions is met, automatic switching can be realized on temperature alarm set according to the compressor, and the compressor is ensured to be in a safe state all the time.
3. The ground torch system adopts a control mode of dividing the pressure into three sections and having cross overlapping, so that stable emptying is effectively realized, the orderly opening and closing of primary, secondary and tertiary emptying are ensured, and the ground torch system is opened and emptied in time.
4. Economic benefit
The main control valve, the PID controller, the air pressure detector and the like are all existing technologies, and are connected and debugged according to the above description. The existing equipment is used as an application example, except for the complex addition of a very small amount of low-cost equipment, the logic optimization process can be realized through background optimization and control logic modification of the existing Siemens control system, and related technical problems of manufacturer personnel can be consulted through a telephone if necessary, so that the investment aspect can be ignored.
According to statistical data of a metering test center, average reduction emptying amount of a torch per hour before and after modification is 3000Nm < 3>, LNG density of liquefied natural gas is 0.42-0.46 ton/cubic meter, average density is 0.44 ton/cubic meter, LNG gasification ratio is 1495Nm < 3 >/ton, and LNG price is calculated according to 4000 yuan/ton of market price, so that the liquefied natural gas is obtained:
5. Social benefits
1) Measuring and calculating carbon dioxide emission according to molecular weight, wherein more than 99.9% of LNG is methane, the molecular weight is estimated according to the methane, the average reduction air discharge per hour is 3000Nm < 3 >, and the method can obtain the following steps:
2) The optimal control effectively eliminates the periodic deflagration problem caused by excessive emptying due to system defects, and eliminates the harm of resonance to the artificial island and the affiliated building. The ground torch system is used as a novel safe torch system, and is also the main stream of the future torch due to the characteristics of small safe distance, low noise, low radiation, sufficient combustion and the like.
The foregoing description is directed to the details and illustrations of the preferred embodiments of the invention, but these descriptions are not intended to limit the scope of the invention claimed, and all equivalent changes or modifications that may be accomplished under the teachings of the invention should be construed to fall within the scope of the invention as defined by the appended claims.
Claims (6)
1. LNG receiving station BOG atmospheric control system, LNG storage tank are connected to the BOG compressor through the BOG house steward, and the BOG compressor exports BOG gas to low reaches pipe network, its characterized in that: the system comprises a BOG main pipe control system and a torch system, wherein the BOG main pipe control system comprises a main control unit, an auxiliary control unit and an air pressure detector I, the air pressure detector I is used for detecting the air pressure of the BOG main pipe, the main control unit comprises a main control valve and a PID controller I, the air pressure detector I is connected with the PID controller I, the PID controller I is connected with the main control valve to open or close the main control valve, the auxiliary control unit comprises an auxiliary control valve and a PID controller II, the air pressure detector I is connected with the PID controller II, and the PID controller II is connected with the auxiliary control valve to open or close the auxiliary control valve; the BOG main pipe branches to form a vent main pipe, a main control valve and an auxiliary control valve which are connected in parallel are arranged on the vent main pipe, and the output end of the vent main pipe is connected to the flare system so as to burn through a ground flare of the flare system;
the PID controller I adopts PID control to close or open the main control valve, when the real-time air pressure of the BOG main pipe is larger than the normal air pressure preset value, the main control valve is controlled to be opened, and the opening I of the main control valve reaches full load when the air pressure difference value is 0.024 Mpa; the PID controller II adopts linear control to enable the auxiliary control valve to be closed or opened, when the real-time air pressure of the BOG main pipe is 0.024Mpa larger than the normal air pressure preset value, the auxiliary control valve is controlled to be opened, and the opening II of the auxiliary control valve reaches full load when the air pressure difference value is 0.027 Mpa;
the opening degrees of the main control valve and the auxiliary control valve meet the following relation,
When the ground torch is one-stage,
When the number of the ground torches is two,
2. The BOG emptying control system of an LNG receiving station according to claim 1, wherein: the BOG header control system further comprises a reflux control unit, the reflux control unit comprises a reflux valve, a PID controller III, an air pressure detector II and a temperature detector, the temperature detector and the air pressure detector II are respectively used for detecting the temperature of air at the inlet of the BOG compressor and detecting the air pressure of air at the outlet of the BOG compressor, the outlet of the BOG compressor is branched and separated to be connected to the LNG storage tank through the reflux valve, the temperature detector and the air pressure detector II are respectively connected with the PID controller III, the PID controller III is connected with the reflux valve, and the temperature control is performed based on the temperature detector or the air pressure control is performed based on the air pressure detector II, so that the reflux valve is closed or opened to the opening degree III.
3. The BOG emptying control system of an LNG receiving station according to claim 2, wherein: the control process of the control reflux valve of the PID controller III is as follows;
Firstly, executing a temperature control program after starting up; then judging whether the acquired real-time temperature is normal, if so, executing an air pressure control program, otherwise, executing a temperature control program; finally, repeating the steps until the machine is stopped, specifically, after the current air pressure control program or the temperature control program is executed for a period of time, judging whether the acquired real-time temperature is normal, if so, executing the air pressure control program, otherwise, executing the temperature control program; in which a switching program is configured to switch between a temperature control program and a barometric pressure control program with each other in a situation operated by an operator.
4. The BOG emptying control system of an LNG receiving station according to claim 1, wherein: the torch system comprises a ground torch and an emptying control unit, wherein the emptying control unit comprises a PID controller IV, an air pressure detector III, an emptying branch pipe and an emptying valve, the emptying branch pipe is divided into an emptying branch pipe by the emptying main pipe so as to be connected to the ground torch respectively, the emptying valve is arranged on the emptying branch pipe, the air pressure detector III is used for detecting the air pressure of the emptying main pipe, the air pressure detector III is connected with the PID controller IV, and the PID controller IV is connected with the emptying valve so as to control the opening or closing of the emptying valve.
5. The BOG emptying control system of an LNG receiving station according to claim 4, wherein: the emptying branch pipe comprises a first branch pipe, a second branch pipe and a third branch pipe, the first branch pipe is directly discharged and is provided with a second-stage emptying valve, the third branch pipe is provided with a third-stage emptying valve, the PID controller IV is respectively connected with the second-stage emptying valve and the third-stage emptying valve, when the air pressure of the emptying main pipe is 0-2Kpa, the first branch pipe is directly discharged, the second-stage emptying valve corresponds to the opening of the valve by 0-100% at the air pressure of the emptying main pipe by 2-6Kpa, and the third-stage emptying valve corresponds to the opening of the valve by 0-100% at the air pressure of the emptying main pipe by 4-8 Kpa.
6. The BOG emptying control system of an LNG receiving station according to claim 5, wherein the control process of the PID controller IV for controlling the emptying valve is as follows:
When the air pressure of the emptying main pipe is 0-2Kpa, the air is directly discharged through the first branch pipe; when the air pressure of the air vent main pipe is more than 2Kpa, the PID controller IV further controls to open the secondary air vent valve, and the secondary air vent valve is kept in an open state when the air pressure of the air vent main pipe is 2-6 Kpa; when the air pressure of the air manifold is greater than 4Kpa, the PID controller IV further controls to open the three-stage air valve, and the two-stage air valve and the three-stage air valve are kept in an open state when the air pressure of the air manifold is 4-8 Kpa.
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