A method and system for detecting and controlling the net positive pressure head of an LNG submersible pump

Abstract

The present invention proposes a method and system for detecting and controlling the net positive pressure head of an LNG submersible pump. The method comprises the following steps: after the liquid source components at the gas filling station are stabilized, measuring the LNG saturation temperature-pressure curve; obtaining the net positive pressure head P required for the normal operation of the submersible pump; N After the LNG submersible pump is pre-cooled, before starting the pump, collect the real-time temperature at the pump inlet and the real-time pressure P in the submersible pump pool. 实 , according to the saturation temperature-pressure curve, obtain the saturation pressure P at the temperature 饱 ; Get the real-time net positive pressure head of LNG: P N实 =P 实 ‑P 饱 ; Judge P N and P N实 The size relationship: If P N实＞ P N When the LNG submersible pump is running, the net positive pressure head required is sufficient; if P N实≤ P N The present invention detects and adjusts the net positive pressure head of the LNG submersible pump so that the net positive pressure head always accurately meets the working requirements, prevents cavitation, protects the submersible pump, extends the life of the submersible pump, and improves the filling efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of filling devices for marine and vehicle LNG filling stations, and particularly relates to a method and system for detecting and controlling the net positive pressure head of an LNG submersible pump. Background Art

[0002] LNG (Liquefied Natural Gas) filling stations use LNG submersible pumps as pressurizing equipment to pressurize the 0-1.2MPa LNG in the LNG storage tank to a maximum of 1.6MPa, and then fill it into LNG fuel vehicles after metering by LNG filling machines.

[0003] When LNG is stored in a sealed, insulated container, it often exists as a boiling liquid and is in a state of temperature-pressure equilibrium. When the LNG pressure drops below its boiling point or when the LNG absorbs heat from the outside, a certain amount of liquid will evaporate into gas (LNG flash), and the liquid temperature will also drop to its boiling point at that pressure until a new equilibrium is reached. When the LNG pressure rises above its boiling point or when LNG at a lower temperature is added from the outside, the evaporation of LNG is suppressed and the liquefaction of BOG (Boil Off Gas, LNG volatile gas) is accelerated until a new equilibrium is reached.

[0004] LNG filling stations use submersible pumps as LNG delivery and pressurization equipment. Submersible pumps are centrifugal pumps. When a centrifugal pump is operating, the pump's suction creates a certain pressure drop at the impeller inlet, causing local evaporation of the LNG and resulting in cavitation. To prevent cavitation, the LNG at the pump's intake must have a certain net positive pressure head when the pump is operating to prevent the pressure from dropping below the boiling point, generating flash vapor and causing cavitation.

[0005] Currently, when submersible pumps are operating at LNG filling stations, control systems determine whether cavitation is occurring based on ongoing phenomena. If cavitation occurs, the pump is shut down to protect the pump, but by then the pump has already suffered some damage. Furthermore, due to the varying composition of LNG at filling stations and the varying gas sources, it is difficult to determine whether the required net positive head pressure is sufficient before operation. This determination can only be based on the operator's long-term experience, which can result in significant errors. Summary of the Invention

[0006] The present invention aims to solve the technical problems existing in the prior art and provides a method and system for detecting and controlling the net positive pressure head of an LNG submersible pump.

[0007] According to a first aspect of the present invention, the present invention provides a method for detecting and controlling the net positive head pressure of an LNG submersible pump, which comprises the following steps:

[0008] S1. After the liquid source components at the gas filling station are stable and the LNG in the storage tank reaches 50% - 90%, measure the LNG saturation temperature - pressure curve.

[0009] S2. Obtain the net positive suction head P required for the submersible pump to operate normally. N ;

[0010] S3. After the pre - cooling of the LNG submersible pump is completed and before starting the pump, collect the real - time temperature at the pump inlet and the real - time pressure P in the submersible pump sump. 实 , and according to the saturation temperature - pressure curve obtained in step S1, obtain the saturation pressure P at this real - time temperature. 饱 ;

[0011] S4. Obtain the real - time net positive suction head of LNG: P N实 = P 实 - P 饱 ;

[0012] S5. Judge the magnitude relationship between P N and P N实 :

[0013] If P N实＞ P N When, the net positive suction head required for the LNG submersible pump operation is sufficient, the submersible pump can operate, and exit.

[0014] If P N实≤ P N When, the net positive suction head required for the LNG submersible pump operation is insufficient, the submersible pump cannot operate, then the system starts the pressurization program and returns to step S3 until the net positive suction head requirement is met.

[0015] The present invention detects and adjusts the net positive suction head of the LNG submersible pump, making the net positive suction head always accurately meet the working requirements, preventing cavitation, protecting the submersible pump, prolonging the life of the submersible pump, and improving the filling efficiency.

[0016] In the prior art, the actual net positive suction head cannot be measured according to experience judgment. While the present invention detects the relationship between the temperature and saturation pressure of natural gas with unknown components and tests to obtain the real - time net positive suction head, and the result is more accurate.

[0017] According to a preferred embodiment of the present invention, the method for measuring the LNG saturation temperature - pressure curve is as follows:

[0018] Transfer the LNG in the LNG storage tank through the submersible pump and send it into the liquid inlet at the top of the LNG storage tank. After operating for t1 time and then standing for t2 time, where t1 < t2, collect the LNG temperature and pressure in the storage tank through the temperature transmitter and pressure transmitter.

[0019] The saturation pressure of the storage tank is adjusted, and a submersible pump is operated at a low speed with a frequency of no more than 30 Hz. The LNG in the storage tank is transported to the LNG vaporizer for heating, and then the LNG is heated by inlet from the bottom of the storage tank into the LNG storage tank. During the heating process, the tank pressure is collected every time the temperature rises by 1°C until the tank pressure reaches 0.8 MPa.

[0020] The collected tank temperature and pressure data are placed into the controller for submersible pump control, and other parameters not collected are calculated using the interpolation method.

[0021] The present invention detects the relationship between the temperature and saturation pressure of natural gas of unknown components and obtains the real-time net positive pressure head through testing, resulting in more accurate results.

[0022] According to the second aspect of the present invention, the present invention provides a net positive pressure head detection and control system for an LNG submersible pump, wherein a temperature transmitter and a pressure transmitter are arranged in the LNG storage tank, and an LNG submersible pump is arranged on the external pipeline of the LNG storage tank. The temperature transmitter and the pressure transmitter collect the temperature signal and the pressure signal in the LNG storage tank and transmit them to the controller, the controller performs LNG saturation temperature-pressure curve measurement, and uses the net positive pressure head detection and control method described in claim 1 to detect and control the net positive pressure head of the LNG submersible pump.

[0023] The present invention detects and adjusts the net positive pressure head of the LNG submersible pump, so that the net positive pressure head always accurately meets the working requirements, prevents cavitation, protects the submersible pump, extends the life of the submersible pump, and improves the filling efficiency.

[0024] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. DETAILED DESCRIPTION

[0025] The following describes embodiments of the present invention in detail. Examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended only to explain the present invention and are not to be construed as limiting the present invention.

[0026] In the description of the present invention, it should be understood that the terms "longitudinal", "transverse", "vertical", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0027] In the description of the present invention, unless otherwise specified and limited, it should be noted that the terms "installed", "connected" and "connected" should be understood in a broad sense. For example, it can be a mechanical connection or an electrical connection, or it can be the internal communication between two components. It can be a direct connection or an indirect connection through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to the specific circumstances.

[0028] The present invention provides a method for detecting and controlling the net positive pressure head of an LNG submersible pump, which comprises the following steps:

[0029] S1: After the liquid source composition at the gas filling station is stable, unload a truck of liquid with a saturation pressure close to 0 (gauge pressure). When the LNG in the storage tank reaches 50% to 90% (preferably 80% to 90%), measure the LNG saturation temperature-pressure curve. If the specific gas source comes from the same factory, it is assumed to be stable.

[0030] S2, obtain the net positive pressure head P required for the normal operation of the submersible pump N ;

[0031] S3, after the LNG submersible pump is pre-cooled and before starting the pump, the real-time temperature of the pump inlet and the real-time pressure P in the submersible pump pool are collected. 实 According to the saturation temperature-pressure curve obtained in step S1, the saturation pressure P at the real-time temperature is obtained. 饱 ;

[0032] S4, get the real-time net positive pressure head of LNG: P N实 =P 实 -P 饱 ;

[0033] S5, judge P N and P N实 Size relationship:

[0034] If P N实＞ P N When the net positive pressure head required for the operation of the LNG submersible pump is sufficient, the submersible pump can operate and exit;

[0035] If P N实≤ P N When the net positive pressure head required for the operation of the LNG submersible pump is insufficient and the submersible pump cannot operate, the system starts the pressurization program and returns to step S3 until the net positive pressure head requirement is met.

[0036] In this embodiment, the method for measuring the LNG saturation temperature-pressure curve is:

[0037] The LNG in the LNG storage tank is transported by a submersible pump and then sent into the liquid inlet at the top of the LNG storage tank. After running for a time t1 and then standing for a time t2, where t1 < t2, preferably t1 is 2 minutes and t2 is 30 minutes. The temperature and pressure of the LNG in the storage tank are collected by a temperature transmitter and a pressure transmitter.

[0038] The saturation pressure of the storage tank is adjusted. The submersible pump runs at a low speed, and the frequency of the low-speed operation does not exceed 30 Hz. The LNG in the storage tank is transported to the LNG vaporizer for heating and then enters the LNG storage tank through the liquid inlet at the bottom of the storage tank to heat the LNG. During the heating process, the storage tank pressure is collected once every 1°C increase in temperature until the storage tank pressure reaches the storage tank operation pressure threshold (for example, 0.8 MPa). Since the actual storage tank pressure is in the range of 0 - 1.2 MPa and generally remains between 0.1 - 0.8 MPa during operation, if it exceeds 0.1 - 0.8 MPa, an approximate value can be obtained by curve fitting.

[0039] The collected storage tank temperature and pressure data are placed into the controller for submersible pump control, and other uncollected parameters are calculated using the interpolation method.

[0040] The present invention also provides a net positive suction head detection and regulation system for an LNG submersible pump. A temperature transmitter and a pressure transmitter are provided in the LNG storage tank, and an LNG submersible pump is provided on the external pipeline of the LNG storage tank. The temperature transmitter and the pressure transmitter collect the temperature signal and the pressure signal in the LNG storage tank and transmit them to the controller. The controller measures the LNG saturation temperature - pressure curve and detects and regulates the net positive suction head of the LNG submersible pump using the net positive suction head detection and regulation method of the present invention.

[0041] In the description of this specification, the description referring to terms such as "preferred embodiments", "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" means that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0042] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and purposes of the present invention. The scope of the present invention is defined by the claims and their equivalents.

Claims

1. A method for detecting and controlling the net positive pressure head of an LNG submersible pump, characterized in that: It includes the following steps: S1. After the liquid source components at the gas filling station are stable, when the LNG in the storage tank reaches 50% - 90%, measure the LNG saturation temperature - pressure curve; S2, obtain the net positive pressure head P required for the normal operation of the submersible pump N ; S3, after the LNG submersible pump is pre-cooled and before starting the pump, the real-time temperature of the pump inlet and the real-time pressure P in the submersible pump pool are collected. 实 According to the saturation temperature-pressure curve obtained in step S1, the saturation pressure P at the real-time temperature is obtained. 饱 ; S4, get the real-time net positive pressure head of LNG: P N实 =P 实 -P 饱 ; S5, judge P N and P N实 Size relationship: If P N实＞ P N When the net positive pressure head required for the operation of the LNG submersible pump is sufficient, the submersible pump can operate and exit; If P N实≤ P N When the net positive pressure head required for the operation of the LNG submersible pump is insufficient and the submersible pump cannot operate, the system starts the pressurization program and returns to step S3 until the net positive pressure head requirement is met; The method for measuring the LNG saturation temperature - pressure curve is as follows: Transport the LNG in the LNG storage tank through a submersible pump and send it into the liquid inlet at the top of the LNG storage tank. After running for t1 time, let it stand for t2 time, where t1 < t2. Collect the temperature and pressure of the LNG in the storage tank through a temperature transmitter and a pressure transmitter; Adjust the saturation pressure of the storage tank. Run the submersible pump at low speed, and the frequency of the low - speed operation does not exceed 30Hz. Transport the LNG in the storage tank to the LNG vaporizer for heating and then enter the LNG storage tank through the liquid inlet at the bottom of the storage tank to heat the LNG. During the heating process, collect the storage tank pressure once every 1°C increase in temperature until the storage tank pressure reaches the storage tank operation pressure threshold; Place the collected storage tank temperature and pressure data into the controller for submersible pump control, and calculate other uncollected parameters using the interpolation method.

2. The method for detecting and controlling the net positive pressure head of an LNG submersible pump according to claim 1, characterized in that: The t1 is 2 minutes, and the t2 is 30 minutes.

3. A net positive pressure head detection and control system for an LNG submersible pump, characterized in that: A temperature transmitter and a pressure transmitter are installed in the LNG storage tank, and an LNG submersible pump is installed on the external pipeline of the LNG storage tank. The temperature transmitter and the pressure transmitter collect the temperature signal and pressure signal in the LNG storage tank and transmit them to the controller. The controller measures the LNG saturation temperature - pressure curve and detects and regulates the net positive suction head of the LNG submersible pump using the net positive suction head detection and regulation method described in claim 1.