A method for reducing the operating pressure of a closed cycle water system

Abstract

The present application belongs to the technical field of industrial circulating water, and relates to a method for reducing the operation pressure of a closed circulating water system, which comprises a circulating water pump and a heat exchange device connected through a circulating water pipeline to form a closed loop, and is connected to an expansion tank through a connecting pipeline and a connecting valve in a breakable manner; the method comprises the following steps: a system starting step: starting the circulating water pump to form a stable flow of circulating water in the closed loop; a connecting opening step: after the flow of circulating water is stabilized, the connecting valve is opened to form a pressure connection loop between the expansion tank and the closed circulating water system; a pressure regulating step: the operation pressure of the closed circulating water system is controlled within a safe pressure range by regulating the gas pressure in the expansion tank; a shutdown isolation step: when the closed circulating water system needs to be shut down, the connecting valve is closed to cut off the pressure connection between the expansion tank and the closed circulating water system, and then the circulating water pump is closed.

Description

Technical Field

[0001] This invention belongs to the field of industrial circulating water technology and relates to a method for reducing the operating pressure of a closed-loop circulating water system. Background Technology

[0002] With the increasing demands for reliability, energy efficiency, and water cleanliness in modern industries, particularly in chemical, power electronics, marine engineering, and high-end manufacturing, closed-loop circulating water systems have been widely adopted in petrochemical, metallurgical, and natural gas industries in recent years. However, closed systems also present unique technical challenges: since the system medium is water, an incompressible liquid, according to the volumetric thermal expansion formula, as the water temperature rises, the water volume increases, leading to increased pressure within the system. If this pressure exceeds the system's maximum withstand pressure, it can damage pipelines and equipment, reduce equipment lifespan, and cause safety accidents. To address this issue, an expansion tank is required in the system for pressure regulation.

[0003] The minimum operating pressure of traditional expansion tanks must simultaneously meet three conditions: "preventing vaporization at high points in the system", "avoiding water pump cavitation" and "meeting the water supply needs of end users", resulting in a high operating pressure in closed-loop circulating water systems. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method for reducing the operating pressure of a closed-loop circulating water system. By optimizing the control strategy, the minimum pressure of the expansion tank can be determined without using "preventing vaporization at the system high point" as a constraint condition, thereby reducing the operating pressure of the expansion tank and the closed-loop circulating water system.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for reducing the operating pressure of a closed-loop circulating water system, the closed-loop circulating water system including a circulating water pump and a heat exchange device connected by circulating water pipes to form a closed loop, the closed-loop circulating water system being connected to an expansion tank via a connecting pipe and a connecting valve. The method includes the following steps: S1) System startup steps: Start the circulating water pump to make the circulating water flow stably in the closed loop; S2) Connection opening step: After the flow rate of the circulating water stabilizes, open the connection valve to form a pressure connection loop between the expansion tank and the closed circulating water system; S3) Pressure regulation step: By regulating the gas pressure in the expansion tank, the operating pressure of the closed-loop circulating water system is controlled within a safe pressure range; S4) Shutdown and isolation procedure: When the closed-loop circulating water system needs to be shut down, first close the connecting valve to cut off the pressure connection between the expansion tank and the closed-loop circulating water system, and then shut down the circulating water pump.

[0006] Furthermore, in the shutdown isolation step S4, before closing the connecting valve, the circulating water pump is kept running to allow the temperature of the circulating water to drop to ambient temperature before closing the connecting valve and shutting down the circulating water pump.

[0007] Furthermore, the expansion tank is connected to the closed-loop circulating water system via the connecting valve and is located on the return water section of the closed-loop circulating water system.

[0008] Further, in the pressure regulation step S3, the minimum working pressure of the expansion tank is determined as follows: the maximum value between the sum of the heat exchange equipment resistance pressure value, the pipe loss pressure value between the expansion tank and the circulating water pump, and the safety margin pressure value during the operation of the closed-loop circulating water system, and the minimum non-cavitation pressure value allowed by the circulating water pump is taken as the minimum working pressure value of the expansion tank.

[0009] Furthermore, when the design volume of the expansion tank is greater than the maximum volume of a single tank, multiple sets of expansion tanks are connected in series. The multiple sets of expansion tanks are connected in sequence through pipelines to form a series loop. The expansion tank at the first end of the series loop is connected to the closed-loop circulating water system through the connecting valve.

[0010] Furthermore, in the series circuit, the first set of expansion tanks directly connected to the connecting valve is mainly filled with liquid medium, while the remaining expansion tanks connected in series after the first set of expansion tanks are mainly filled with gaseous medium, and adjacent sets of expansion tanks are connected by pipes arranged on the upper part of the expansion tanks.

[0011] Furthermore, each expansion tank is equipped with a gas replenishment valve, a gas exhaust valve, and a safety valve on its top. The gas replenishment valve is used to replenish gas when the gas pressure inside the expansion tank is lower than a preset lower limit. The gas exhaust valve is used to discharge gas when the gas pressure inside the expansion tank is higher than a preset upper limit. The safety valve is used to forcibly release pressure when the pressure inside the expansion tank exceeds a safety threshold. The expansion tank is equipped with a level gauge and a pressure gauge for real-time detection of the liquid level and pressure inside the tank.

[0012] Furthermore, the gaseous medium in the expansion tank is nitrogen or an inert gas, and the gaseous medium is insoluble in the circulating water.

[0013] Furthermore, the connecting valve is an electric valve, which can completely block the flow of media between its expansion tank side and the circulating water pipeline side when closed.

[0014] The beneficial effects of this invention are as follows: 1. The method of this invention, by performing orderly opening and closing operations on the connecting valve during system start-up and shutdown, ensures that the expansion tank is connected to the circulating water system only during normal system operation and isolated from the system during system shutdown. This overcomes the limitation on the minimum operating pressure of the expansion tank imposed by the constraint of "preventing vaporization at the highest point of the system" in traditional connection methods. The minimum operating pressure of the expansion tank only needs to meet the requirements of cavitation prevention for the circulating water pump and system pipeline blockage, significantly reducing the operating pressure of the expansion tank and the entire circulating water system. This reduces the pressure rating required for system pipelines, valves, and equipment, thereby lowering equipment procurement and construction / installation costs.

[0015] 2. The reduction in system operating pressure decreases the stress on pressure-bearing components, helping to extend the service life of pipelines, valves, and equipment, and improving the long-term safety and reliability of the system. The method of this invention is simple to operate, requiring only the addition of a connecting valve and adjustment of the start-stop sequence to the traditional system. It does not require large-scale modifications to the existing pipeline system, facilitating upgrades and widespread adoption on existing systems. The expansion tank continues to function normally as a pressure buffer and volume compensation tank during system operation, without affecting the system's heat exchange performance and operational stability.

[0016] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a schematic flowchart of a method for reducing the operating pressure of a closed-loop circulating water system according to the present invention. Figure 2 This is a plan view of the expansion tank and connecting pipe in this invention; Figure 3 This is a schematic flowchart of a method for reducing the operating pressure of a closed-loop circulating water system (with the relative elevation of the closed-loop circulating water system) in an embodiment.

[0018] Attached reference numerals: 1-Expansion tank; 2-Connecting valve; 3-Connecting pipe; 1-Expansion tank; 11-Air inlet; 12-Exhaust outlet; 13-Pressure gauge port; 14-Safety valve port. Detailed Implementation

[0019] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0020] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0021] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0022] To facilitate understanding of the technical solution of this invention, the basic technical principles involved in this invention will first be explained.

[0023] Relationship between thermal expansion and system pressure: In a closed-loop circulating water system, the circulating water flows within a closed circuit consisting of a circulating pump, heat exchange equipment, and associated piping, with no free liquid surface contact between the water and the external atmosphere. As an incompressible liquid, water expands in volume due to its thermal expansion properties when its temperature rises. Because the system is closed, this expansion is constrained by the pipe and equipment walls, leaving excess water with nowhere to go, thus increasing the internal pressure. Conversely, when the water temperature decreases, its volume contracts, and the internal pressure decreases.

[0024] The function of an expansion tank is precisely to absorb the volume increase or decrease caused by temperature changes. The expansion tank is typically divided into two zones: the lower zone is the liquid phase, connected to the water in the circulating water system; the upper zone is the gas phase, pre-filled with compressed gas at a certain pressure. When the water temperature in the system rises and its volume expands, excess water enters the liquid phase zone of the expansion tank through the connecting pipe, causing the liquid level to rise and compressing the gas in the gas phase zone. This reduces the gas volume and increases its pressure, thus absorbing the excess volume increase in the system and preventing excessive pressure rise. When the water temperature decreases, the compressed gas in the gas phase zone expands, pushing the water in the liquid phase zone back into the system pipeline, maintaining stable system pressure.

[0025] Determining the minimum working pressure of the expansion tank under traditional connection methods: In traditional closed-loop water systems, expansion tanks are typically permanently connected to the system via piping, meaning that the expansion tank and the circulating water system maintain a constant pressure connection throughout the entire system's operation and shutdown.

[0026] When the system stops running, the circulating water pump is not working, and the water in the pipeline is stagnant. At this time, the pressure at each point in the pipeline is determined by the static pressure distribution—that is, the absolute pressure at each point is equal to the absolute pressure at the expansion tank surface plus (or minus) the difference in water column height between that point and the expansion tank surface. For pipeline points above the expansion tank surface, the absolute pressure is lower than the absolute pressure at the expansion tank surface; the higher the location, the lower the pressure.

[0027] The highest point in the system is the highest point in the pipeline, where the absolute pressure is lowest. To prevent the water at this point from vaporizing due to excessively low absolute pressure (i.e., water will vaporize when its absolute pressure is lower than the saturated vapor pressure of water at that temperature), the absolute pressure at the expansion tank surface must be sufficiently high to overcome the entire hydrostatic head difference between the expansion tank surface and the highest point of the system, while maintaining a certain safety margin. Mathematically, this can be expressed as: P 表压低 ≥ P1 + P2 + P3 Wherein, P 表压低 P1 is the minimum working pressure (gauge pressure) of the expansion tank, P2 is the maximum pressure of the closed-loop circulating water system, P3 is the saturated steam pressure, and P4 is the safety margin pressure.

[0028] When the elevation difference between the highest point of the system and the expansion tank is large (e.g., tens of meters), the constraint condition will impose a very high minimum working pressure requirement on the expansion tank. This is the fundamental reason why the minimum working pressure of the expansion tank is too high under traditional connection methods.

[0029] In addition, the minimum operating pressure of the expansion tank must also meet the following two conditions: (1) Prevent cavitation of the circulating water pump during system operation. During operation, the relative pressure at the inlet of the circulating water pump must not be lower than the minimum allowable inlet pressure of the circulating water pump (this value is determined by the cavitation performance of the pump and is usually provided by the pump manufacturer; it is generally greater than the 0 m, 80°C water column value). If the pressure at the liquid level in the expansion tank is too low, the pump inlet pressure may be lower than this limit after pipeline losses, leading to cavitation at the pump impeller inlet.

[0030] (2) Meet the water supply pressure requirements of end users. When the system supplies water to end equipment such as heat exchange equipment, it must ensure that the water supply pressure meets the minimum inlet pressure required for normal operation of the equipment.

[0031] In traditional solutions, the three constraints mentioned above act simultaneously, and the minimum operating pressure of the expansion tank is the maximum value among the three. In many practical engineering projects, especially when the system's highest point is high and the water temperature is high, the pressure required by the constraint of "preventing vaporization at the system's highest point" is often much higher than the other two conditions, becoming the decisive factor in controlling the minimum operating pressure of the expansion tank.

[0032] The core idea of ​​this invention is to add an operable connecting valve between the expansion tank and the circulating water system, and optimize the start-up and shutdown sequence of the system so that the expansion tank is connected to the system only during normal system operation (circulating water pump is running and water is flowing in the pipeline); during system shutdown (circulating water pump stops and water is still in the pipeline), the expansion tank is isolated from the system by closing the connecting valve.

[0033] The technical effect of this isolation measure is that when the system is shut down, there is no pressure connection between the expansion tank and the circulating water system because the connecting valve is closed. The pressure inside the expansion tank only acts on the expansion tank itself and the short pipe section between it and the connecting valve, and does not need to resist the static pressure of the entire water column from the expansion tank to the highest point of the system. In other words, the minimum operating pressure of the expansion tank is no longer constrained by the constraint of "preventing vaporization at the highest point of the system".

[0034] Because the circulating water pump is working during normal system operation, the water in the pipeline is in a flowing state. The pump provides the system with a dynamic head, and the pressure at each point in the pipeline is determined by the pump head and pipeline resistance loss. The pressure at the highest point of the system is guaranteed during pump operation. Therefore, when connecting the expansion tank during system operation, the expansion tank only needs to meet the requirements of pump cavitation prevention and system pipeline resistance loss, without needing to consider the static pressure head at the highest point of the system.

[0035] In this way, the minimum operating pressure of the expansion tank only needs to be the larger of the following two conditions: (1) the sum of the pressure loss of the heat exchange equipment, the pressure loss of the pipeline between the expansion tank and the circulating water pump, and the safety margin pressure during system operation; (2) the minimum non-cavitation pressure allowed by the circulating water pump. This pressure value is much lower than the value required to simultaneously meet the constraint of "preventing vaporization at the highest point of the system" in the traditional scheme, thus significantly reducing the operating pressure of the expansion tank and the entire circulating water system.

[0036] Example 1 like Figures 1-2 As shown, the closed-loop circulating water system of the present invention mainly includes the following components: A closed-loop circulating water system consists of a circulating water pump, heat exchange equipment (such as plate heat exchangers, shell-and-tube heat exchangers, etc.), a pipeline filter, and circulating water pipes connecting these devices. Circulating water flows within this closed loop, powered by the circulating water pump. After heat exchange is completed by the heat exchange equipment, the water returns to the pump inlet, forming a closed loop.

[0037] Expansion Tank 1: Serving as a pressure buffer and volume compensation device for the system, the expansion tank has a gas phase zone (filled with compressed gas) at the top and a liquid phase zone at the bottom. The gas medium in the gas phase zone is preferably nitrogen or an inert gas, which is insoluble in circulating water, to avoid the gas dissolving in the water and reducing the buffering function. Each expansion tank is equipped with a gas inlet 11 (for a gas inlet valve), a gas outlet 12 (for a gas outlet valve), a pressure gauge port 13 (for a pressure gauge), and a safety valve port 14 (for a safety valve) on its top. The gas inlet valve replenishes gas into the tank when the internal gas pressure is below a preset lower limit to maintain a minimum buffer pressure; the gas outlet valve discharges excess gas when the internal gas pressure is above a preset upper limit to prevent excessive pressure; the safety valve is an overpressure protection device that automatically opens to force pressure relief when the internal pressure exceeds a safety threshold. A level gauge is also installed inside the expansion tank to monitor the internal liquid level in real time, providing a basis for system operation monitoring.

[0038] Connecting pipe 3 and connecting valve 2: Connecting pipe 3 is a section of piping, one end of which is connected to the bottom or lower side liquid phase zone interface of expansion tank 1, and the other end is connected to the pipeline of the closed-loop circulating water circuit. Connecting valve 2 is installed on connecting pipe 3 and serves as a control valve for pressure connection between expansion tank 1 and the circulating water system. When connecting valve 2 is closed, it can completely block the flow of medium between the expansion tank side and the circulating water pipeline side, ensuring that no leakage or crossflow occurs between the two. Preferably, connecting valve 2 is an electric valve with excellent sealing performance to achieve remote control and automated operation. The connection node between the expansion tank and the closed-loop circulating water system through the connecting valve and connecting pipe is preferably set on the return water section of the system (i.e., the section where water flows from the heat exchange equipment back to the inlet of the circulating water pump), because the water temperature in the return water section is usually lower than that in the supply water section, which helps to reduce the thermal expansion effect and extend the service life of the connecting valve.

[0039] When the design volume of the expansion tank is large, exceeding the maximum manufacturing volume of a single expansion tank, multiple expansion tanks can be connected in series. Multiple expansion tanks are connected sequentially via pipelines to form a series loop. The expansion tank at the beginning of the series loop is connected to the closed-loop circulating water system via a connecting valve. In the series configuration, the first expansion tank (closest to the circulating water system side) directly connected to the connecting valve primarily contains a liquid medium, serving as a direct connection to the system water. The remaining expansion tanks in the subsequent series primarily contain a gaseous medium, mainly serving as gas buffers and pressure regulators. Adjacent expansion tanks are connected via pipelines located at the top of the expansion tanks; this top connection facilitates gas transfer and equalization between the tanks.

[0040] Reference Figure 1 The method for reducing the operating pressure of a closed-loop circulating water system according to the present invention includes the following four steps: S1) System startup steps Before starting the system, a pretreatment check should be performed on the closed-loop circulating water system to confirm that there are no leaks in the pipelines, all equipment (circulating water pumps, heat exchangers, filters, etc.) are in normal condition, and all valves are in the correct switch positions. At this time, the connecting valve 2 should be in the closed position, and the expansion tank 1 should not be connected to the circulating water system.

[0041] After confirming everything is correct, start the circulating water pump. The pump draws in circulating water from its inlet, pressurizes it, and sends it to the heat exchange equipment. After heat exchange, the water returns to the pump inlet through the return pipe, forming a closed-loop circulation. During the initial startup phase, the water in the pipeline gradually accelerates from rest to the design flow rate, requiring a certain stabilization time.

[0042] The core purpose of this step is to establish a stable flow state of the circulating water in the pipeline before the expansion tank is connected to the system. When the circulating water is in a flowing state, the pressure at each point in the system pipeline is determined by the pump head and pipeline resistance loss. The pressure at the highest point of the system is guaranteed by the dynamic head provided by the pump, rather than relying on the pressure of the expansion tank to counteract the static pressure head.

[0043] S2) Steps to activate China Unicom Once the flow rate of the circulating water has stabilized (this can be confirmed by a flow meter installed on the pipeline to ensure that the flow rate reaches the design value and the fluctuation is within the allowable range), the operator or the automatic control system issues a command to open the connecting valve 2.

[0044] After the connecting valve 2 is opened, the liquid phase zone of expansion tank 1 is pressure-connected to the return water pipeline of the circulating water system through the connecting pipe 3. At this time, the pressure inside the expansion tank reaches equilibrium with the pressure in the system pipeline, and the compressed gas in the gas phase zone of the expansion tank begins to act as a pressure buffer for the system.

[0045] Since the water in the system pipeline is already in a stable flow state before the connecting valve is opened, and the water pump has already provided the operating head for the system, the internal pressure of the expansion tank automatically matches the operating pressure of the system after it is connected, and there is no need for the expansion tank to provide additional pressure to counteract the static pressure head of the system.

[0046] S3) Pressure Regulation Steps During normal system operation, expansion tank 1 remains connected to the circulating water system, serving as a pressure buffer and volume compensation unit.

[0047] When the circulating water temperature rises and its volume expands due to the heat exchange process, the excess water enters the liquid phase zone of the expansion tank 1 through the connecting pipe 3, causing the liquid level to rise. The compressed gas in the gas phase zone is further compressed, increasing the gas pressure and thus absorbing the excess volume increase in the system, keeping the system pressure within a safe range. Conversely, when the circulating water temperature decreases and its volume shrinks, the compressed gas in the gas phase zone of the expansion tank expands, pushing the water in the liquid phase zone back into the system pipeline, maintaining stable system pressure.

[0048] The minimum working pressure of the expansion tank is determined as follows: the maximum value between the sum of the pressure loss of the heat exchange equipment during system operation, the pressure loss of the pipeline between the expansion tank and the circulating water pump, and the safety margin pressure, and the minimum non-cavitation pressure allowed by the circulating water pump, is taken as the minimum working pressure of the expansion tank.

[0049] The maximum working pressure of the expansion tank is determined by the ratio of the minimum working pressure to the working pressure (which is determined by the design parameters of the expansion tank). By controlling the air supply valve and the air release valve, the gas pressure in the expansion tank can be maintained between the minimum and maximum working pressures, thereby stabilizing the operating pressure of the entire circulating water system within a safe pressure range.

[0050] The expansion tank is equipped with level gauges and pressure gauges that monitor the liquid level and pressure data in real time and feed the data back to the system control center. When the pressure is lower than the preset lower limit, the gas replenishment valve automatically opens to replenish gas into the tank; when the pressure is higher than the preset upper limit, the exhaust valve automatically opens to discharge excess gas; when the pressure exceeds the safety threshold, the safety valve forcibly opens to release pressure, ensuring system safety.

[0051] S4) Shutdown and Isolation Procedures When the system needs to be shut down for planned maintenance, upkeep, or temporary shutdown, follow these steps: Step 1: Cooling Operation. Keep the circulating water pump running to maintain the flow of circulating water in the pipeline, allowing the water in the system to gradually dissipate heat and cool down through continuous circulation until the water temperature drops to near ambient temperature. The purpose of this step is: if the pump is stopped directly when the water temperature is high, the water in the pipeline will expand due to the high temperature. If the connecting valve is closed directly at this time to isolate the expansion tank, the excess water that has entered the liquid phase region in the expansion tank cannot flow back to the system pipeline before isolation. If the pump is stopped directly without cooling and the valve is not closed, the system will become a static pressure state after the pump stops, and the expansion tank will have to resist the full static pressure of the system, violating the isolation principle of this invention. Therefore, cooling first, then isolating, and then stopping the pump is the key sequence to ensure operational safety.

[0052] Step 2: Close the connecting valve. After the circulating water temperature drops to ambient temperature, close connecting valve 2 to cut off the pressure connection between expansion tank 1 and the closed-loop circulating water system. After closing the connecting valve, there is no longer a pressure transmission relationship between the expansion tank and the circulating water system pipeline. The pressure inside the expansion tank only acts on the expansion tank itself and the short pipe section between it and the connecting valve. At this time, even if the water in the system pipeline becomes static after the circulating water pump stops operating, the pressure distribution at each point in the pipeline will change according to the static pressure law, but the expansion tank no longer participates in this static pressure balance process, and the minimum working pressure of the expansion tank does not need to take into account the static pressure head at the highest point of the system.

[0053] Step 3: Shut down the circulating water pump. After the connecting valve is completely closed, shut down the circulating water pump to complete the system shutdown.

[0054] By optimizing the start-up and shutdown sequence, the expansion tank is completely isolated from the circulating water system during system shutdown. Its minimum operating pressure is no longer constrained by the most stringent constraint of "preventing vaporization at the highest point of the system," thereby significantly reducing the operating pressure of the expansion tank and the entire circulating water system.

[0055] Example 2 Please see Figure 3 The technical solution and beneficial effects of the present invention will be described in detail below through a specific embodiment. This embodiment is based on a closed-loop circulating water system of an industrial project. The basic parameters of the project are as follows: the highest point of the system pipeline has a relative elevation of approximately 22 meters; the system uses a plate heat exchanger as the heat exchange equipment, and the resistance loss of the plate heat exchanger is 6 meters (i.e., 6 meters of water column, approximately 0.06 MPa); the pipeline loss between the expansion tank inlet and the circulating water pump inlet is approximately 2 meters (i.e., 2 meters of water column, approximately 0.02 MPa); the system fluid medium temperature is 80℃, at which point the saturated vapor pressure of water is 4.74 meters of water column; the minimum allowable non-cavitation pressure of the circulating water pump is 10 meters of water column; the user has no special requirements for the water supply pressure. The system design operating pressure and operating temperature data are determined by the engineering design.

[0056] The initial design concept was: when the system is stopped, the minimum working pressure of the expansion tank should be able to prevent vaporization at the highest point of the system (22m) (the fluid medium temperature in this project is 80℃, and the saturated vapor pressure of water is 4.74m), while ensuring that the circulating water pump does not cavitation when the system is running.

[0057] Based on this principle, the critical vaporization pressure at the highest point of the system is calculated to be 18.74 m. This value is higher than the minimum non-cavitation pressure allowed by the circulating water pump. Therefore, the operating pressure range of the expansion tank is calculated as follows: Table 1. Calculation Table of Working Pressure of Expansion Tank in Original Design of Closed-loop Circulating Water System for a Certain Project

[0058] As shown in the table above, the working pressure range of the expansion tank is approximately 0.19-0.35 MPa. Since the expansion tank is connected to the circulating water system, the circulating water system also needs to increase its working pressure by the same amount, resulting in an overall higher system pressure.

[0059] An optimization strategy for reducing the operating pressure of a closed-loop circulating water system, provided by the present invention, is as follows: The specific optimization measures are as follows: Modify the connection logic between expansion tank 1 and the circulating water pipeline according to a method for reducing the operating pressure of a closed-loop circulating water system. Before the system pump is stopped, after the heat load stabilizes, first close the connecting valve between the expansion tank and the circulating water pipeline, and then stop the pump; after starting the pump, open the connecting valve again. With the method of this invention, the expansion tank is isolated from the system during system shutdown, and its minimum operating pressure is no longer constrained by "preventing vaporization at the highest point of the system," but only needs to meet the requirement of preventing cavitation of the circulating water pump during system operation. The specific calculations are as follows: Table 2. Calculation of working pressure of expansion tank after optimization of closed-loop circulating water system in a certain project.

[0060] As shown in the table above, the working pressure range of the expansion tank after optimization is approximately 0.10~0.21MPa, which is about 0.09~0.13MPa lower than the working pressure before optimization. This optimization not only reduces the overall operating pressure of the system, but also simplifies equipment selection and pipeline design, saving costs.

[0061] As can be seen from the above embodiments, the method of the present invention achieves the following beneficial effects by optimizing the connection logic between the expansion tank and the system and the start-stop control timing: (1) The operating pressure of the expansion tank and circulating water system was significantly reduced. In this embodiment, the operating pressure range of the expansion tank was reduced from 0.19 to 0.34 MPa in the traditional scheme to 0.10 to 0.21 MPa, which is a significant reduction. The reduction in the overall operating pressure of the system allows for a corresponding reduction in the pressure rating of pipelines, valves and equipment, saving on equipment procurement costs and construction and installation costs.

[0062] (2) Simplified equipment selection. The lower operating pressure range allows for more flexible equipment selection, enabling the use of lower pressure-rated pipes, valves, and heat exchange equipment, thus reducing the initial investment in system construction and improving system operational safety. The reduced system operating pressure means less stress on the pressure-bearing components of pipes and equipment, which helps extend equipment lifespan and reduce safety risks caused by material fatigue during long-term operation; it is also simple to operate and easy to implement. The method of this invention only requires adding a connecting valve to the traditional system and adjusting the start-up and shutdown sequence, without requiring large-scale modifications to the existing pipeline system. The connecting valve is preferably an electric valve, which can be incorporated into the system's automatic control program to achieve automated operation of the start-up and shutdown process, reducing manual intervention; it does not affect the normal operation function of the system. The expansion tank remains connected to the system during normal system operation, normally performing its pressure buffering and volume compensation functions, without affecting the system's heat exchange performance and operational stability.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for reducing the operating pressure of a closed-loop circulating water system, characterized in that, The closed-loop circulating water system includes a circulating water pump and a heat exchange device that are connected to form a closed loop through circulating water pipes. The closed-loop circulating water system is connected to an expansion tank in a slewable manner through a connecting pipe and a connecting valve. The method includes the following steps: S1) System startup steps: Start the circulating water pump to make the circulating water flow stably in the closed loop; S2) Connection opening step: After the flow rate of the circulating water stabilizes, open the connection valve to form a pressure connection loop between the expansion tank and the closed circulating water system; S3) Pressure regulation step: By regulating the gas pressure in the expansion tank, the operating pressure of the closed-loop circulating water system is controlled within a safe pressure range; S4) Shutdown and isolation procedure: When the closed-loop circulating water system needs to be shut down, first close the connecting valve to cut off the pressure connection between the expansion tank and the closed-loop circulating water system, and then shut down the circulating water pump.

2. The method according to claim 1, characterized in that, In the shutdown isolation step S4, before closing the connecting valve, the circulating water pump is kept running to allow the temperature of the circulating water to drop to ambient temperature before closing the connecting valve and shutting down the circulating water pump.

3. The method according to claim 1, characterized in that, The expansion tank is connected to the closed-loop circulating water system via the connecting valve and is located on the return water section of the closed-loop circulating water system.

4. The method according to claim 1, characterized in that, In the pressure regulation step S3, the minimum working pressure of the expansion tank is determined as follows: the maximum value between the sum of the heat exchange equipment resistance pressure value, the pipe loss pressure value between the expansion tank and the circulating water pump, and the safe margin pressure value during the operation of the closed-loop circulating water system, and the minimum non-cavitation pressure value allowed by the circulating water pump is taken as the minimum working pressure value of the expansion tank.

5. The method according to claim 1, characterized in that, When the design volume of the expansion tank is greater than the maximum volume of a single tank, multiple sets of expansion tanks are connected in series. The multiple sets of expansion tanks are connected in sequence through pipelines to form a series loop. The expansion tank at the first end of the series loop is connected to the closed-loop circulating water system through the connecting valve.

6. The method according to claim 5, characterized in that, In the series circuit, the first set of expansion tanks directly connected to the connecting valve is mainly filled with liquid medium, while the remaining expansion tanks connected in series after the first set of expansion tanks are mainly filled with gaseous medium. The two adjacent sets of expansion tanks are connected by pipes arranged on the upper part of the expansion tanks.

7. The method according to claim 1, characterized in that, Each expansion tank is equipped with a gas replenishment valve, a gas exhaust valve, and a safety valve on its top. The gas replenishment valve is used to replenish gas when the gas pressure inside the expansion tank is lower than a preset lower limit. The gas exhaust valve is used to discharge gas when the gas pressure inside the expansion tank is higher than a preset upper limit. The safety valve is used to forcibly release pressure when the pressure inside the expansion tank exceeds a safety threshold. The expansion tank is equipped with a level gauge and a pressure gauge for real-time monitoring of the liquid level and pressure inside the tank.

8. The method according to claim 1, characterized in that, The gaseous medium in the expansion tank is nitrogen or an inert gas, and the gaseous medium is insoluble in the circulating water.

9. The method according to claim 1, characterized in that, The connecting valve is an electric valve, which can completely block the flow of medium between the expansion tank side and the circulating water pipeline side when closed.

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