A hot oil circulation method and a hot oil circulation system under a low temperature environment

Abstract

The application discloses a hot oil circulation method and system in a low-temperature environment, relates to the field of hydraulic system preheating, and comprises a multi-way valve group composed of multiple reversing valves, each of which is connected in parallel on a main oil circuit; one reversing valve a is selected from the multiple reversing valves, the selection principle of the reversing valve a is a valve piece which does not cause misoperation of an actuator, an A port and a B port on the reversing valve a are connected through a circulation passage, a first one-way valve is arranged on the circulation passage, and the first one-way valve only allows hydraulic oil to flow from the B port to the A port; the method is to utilize the reversing valve a which does not cause misoperation of the actuator, increase the circulation passage, and make the hydraulic oil flow in the circulation passage, so that throttling and temperature rise occur in the flowing process, and then the temperature of the hydraulic oil is increased. The application realizes the purpose of preheating of the hydraulic oil at low cost and without increasing the installation space.

Description

Technical Field

[0001] This invention relates to the field of hydraulic system preheating, and in particular to a hot oil circulation method and hot oil circulation system under low temperature conditions. Background Technology

[0002] The automated tooling system for oil drilling rigs—the iron drill—is primarily used for the safe and efficient connection and disconnection of drill strings. This equipment needs to be compatible with different drill string sizes and precisely control the connection torque for each size. The connection torque is proportionally adjusted via a multi-way valve assembly controlled by an electro-proportional relief valve. Simultaneously, the iron drill, through the coordinated action of a rotary motor and a support arm cylinder, aligns itself with the wellbore center, enabling rapid connection and disconnection of drill strings.

[0003] However, in low-temperature environments, the increased viscosity of the hydraulic oil and insufficient circulation lead to uneven hydraulic oil temperature within the system. This reduces the control accuracy of the electro-proportional control valve, resulting in decreased precision in the output pressure and torque values ​​when the strike cylinder actuates. Consequently, the "strike torque" varies with oil temperature, torque control accuracy is low, and torque value deviation is large. Simultaneously, the electro-control valves controlling the rotary motor and support arm cylinders of the drill bit's main body cannot accurately position themselves when aligned with the wellbore center due to the high viscosity of the hydraulic oil. To ensure the control accuracy and working efficiency of the drill bit, it is necessary to implement preheating measures for the drill bit's hydraulic system.

[0004] The two most common preheating methods for hydraulic systems are as follows: The first method is "pipeline preheating." Specifically, for equipment operating in low-temperature environments, an electric heat tracing and insulation system is installed. This involves preheating the hydraulic oil in the main pipeline from the hydraulic station to the drill bit, as well as the hydraulic oil in the drill bit's main body pipeline. Heat tracing tape and insulation layers are wrapped around the pipelines. Before using the equipment, the electric heat tracing system is activated to preheat the hydraulic oil remaining in the pipelines. This method solves the problem of preheating the hydraulic oil in the external pipelines, but it doesn't create a circulating oil path for the hydraulic oil in the actuators, such as the cylinders and rotary motors, resulting in viscous hydraulic oil. Furthermore, the electric heat tracing system requires complex on-site installation, is costly, and requires explosion-proof measures near the wellhead.

[0005] Therefore, based on the shortcomings of the "pipeline preheating measures," the commonly used method is the second approach: "starting the hydraulic station in advance to achieve circulating preheating of the pipeline system." Specifically, before starting the hydraulic drill, the hydraulic oil supply station is started first, the heater on the hydraulic station's oil tank is turned on, and the main hydraulic oil circuit is circulated by starting the rotary motor of the rotary actuator in advance. Actuators that have not been activated for a long time need to run 2-3 reciprocating strokes under no-load to displace the cold oil in the hoses. Only after the hydraulic oil in the hydraulic station's oil tank, the hydraulic fluid in the hydraulic drill, and the hydraulic oil inside each actuator are circulated can the system's hydraulic oil temperature reach equilibrium. This method solves the problems of preheating the oil supply and displacing cold hydraulic oil inside the actuators. However, due to limitations in equipment installation location and space, as well as the lag in actuator operation in low-temperature environments, there are safety hazards. There may be situations where operation is not allowed before the equipment is officially started. In such cases, the actuators cannot operate, requiring a solution to the problem of hot oil circulation in the control circuit.

[0006] The iron drill has two sets of multi-way valve assemblies for controlling the circuit: one set controls the main body movement of the iron drill to achieve precise engagement; the other set controls the insert and screw-in action to achieve rapid engagement and disengagement. In low-temperature environments, the residual oil in each multi-way valve assembly has high viscosity. If the equipment is started under these conditions, the control accuracy will decrease. Therefore, hot oil circulation measures for the iron drill's multi-way valve assembly are necessary. Based on this, current methods in the field involve adding an additional valve plate with a smaller flow rate to the multi-way valve assembly. The circulation and throttling of this valve plate achieves hot oil circulation for the multi-way valve assembly. However, this method increases cost and requires more installation space due to the added valve plate, resulting in poor economic efficiency and practicality.

[0007] Therefore, it is necessary to study a new technical solution to achieve the purpose of hydraulic oil preheating without increasing installation space and at low cost. Summary of the Invention

[0008] The purpose of this invention is to provide a hot oil circulation method and system for low-temperature environments, which addresses the aforementioned problems and achieves hydraulic oil preheating at low cost without increasing installation space.

[0009] The technical solution adopted in this invention is as follows: A method for hot oil circulation in a low-temperature environment, comprising the following steps: S1: Select a directional valve a from each multi-way valve group. The selection principle for directional valve a is that it has a valve plate that does not cause malfunction of the actuator. Connect port A and port B of directional valve a through a circulation passage. At least a first check valve is provided on the circulation passage. The installation direction of the first check valve only allows hydraulic oil to flow from port B to port A. S2: Switch the oil supply direction of directional valve a, so that directional valve a supplies oil through port B and returns oil through port A; S3: Start the rotary motor connected to the hydraulic station oil tank so that the hydraulic oil in the main oil circuit supply pipe flows out from port B after passing through the reversing valve a, and flows back to the reversing valve a from port A after passing through the circulation passage, and then enters the return oil pipe of the main oil circuit to flow back to the hydraulic station oil tank. S4: In step S3, the hydraulic oil undergoes throttling and heat generation during its flow, forming hot oil. The hot oil flows back to the hydraulic station oil tank through the return oil pipe, raising the temperature of the hydraulic oil in the hydraulic station oil tank, and then enters the supply oil pipe under the action of the rotary motor. Since all the directional valves a in the multi-way valve group share the same main oil circuit, the hot oil circulation of the entire multi-way valve group is achieved through heat transfer.

[0010] Furthermore, in step S1, if the multi-way valve group is a three-way multi-way valve group used to control the movement of the main body of the iron drill, then the selected reversing valve a is the reversing valve a that controls the rise and fall of the column. Or / and in step S1, if the multi-way valve group is a six-way multi-way valve group used to control the insert and the rotary action, then the selected directional valve a is the directional valve a that supplies oil to the standby equipment.

[0011] Furthermore, for the circulation path connected to the directional valve a in the three-way valve group, a throttle valve is also provided on the circulation path. The throttle valve is located downstream of the first check valve, and the hydraulic oil is throttled and heated through the throttle valve.

[0012] Furthermore, for the directional valve a in the three-way multi-port valve group, port B is connected to the rod chamber of the column lifting cylinder, port A is connected to the rodless chamber of the column lifting cylinder, and the throttle valve is not located on the pipeline between the rodless chamber and port A.

[0013] Furthermore, the standby equipment requires a small flow rate, and the hydraulic oil is throttled and heated within the directional valve a.

[0014] Furthermore, for the directional valve a in the six-way multi-port valve group, port A is connected to the oil inlet of the standby equipment, port B is connected to the oil return port of the standby equipment, and a second check valve is provided between port B and the oil return port of the standby equipment. The installation direction of the second check valve only allows hydraulic oil to flow from the oil return port of the standby equipment to port B.

[0015] A hot oil circulation system for low-temperature environments, applied to the aforementioned hot oil circulation method for low-temperature environments, includes a multi-way valve group composed of multiple directional valves, each directional valve being connected in parallel to the main oil line; one directional valve a is selected from the multiple heat exchange valves, the selection principle of which is to have a valve plate that does not cause malfunction of the actuator, the A port and B port of the directional valve a are connected through a circulation path, and a first check valve is provided on the circulation path, the first check valve only allowing hydraulic oil to flow from port B to port A.

[0016] Furthermore, if the multi-way valve group is a three-way multi-way valve group used to control the main body movement of the iron drill, then the selected directional valve a is the directional valve a that controls the rise and fall of the column; a throttle valve is also provided on the circulation path, the throttle valve is located downstream of the first check valve, and the hydraulic oil is throttled and heated through the throttle valve. If the multi-way valve assembly is a six-way multi-way valve assembly used to control the insert and the rotary action, then the selected directional valve a is the directional valve a that supplies oil to the standby equipment.

[0017] Furthermore, for the directional valve a in the three-way multi-port valve group, port B is connected to the rod chamber of the column lifting cylinder, port A is connected to the rodless chamber of the column lifting cylinder, and the throttle valve is not located on the pipeline between the rodless chamber and port A.

[0018] Furthermore, for the directional valve a in the six-way multi-port valve group, port A is connected to the oil inlet of the standby equipment, port B is connected to the oil return port of the standby equipment, and a second check valve is provided between port B and the oil return port of the standby equipment. The installation direction of the second check valve only allows hydraulic oil to flow from the oil return port of the standby equipment to port B.

[0019] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: This invention improves upon the existing system without adding new valve plates. It utilizes a directional valve a that does not cause malfunctions in the actuator, increasing the circulation path to allow hydraulic oil to flow. During this flow, throttling and heating occur, raising the hydraulic oil temperature. The oil then flows back to the hydraulic station tank via the return pipe, raising the temperature of the hydraulic oil in the tank. Under the action of the rotary motor, it enters the supply pipe. Since all directional valves a in the multi-way valve group share the same main oil circuit, heat transfer achieves hot oil circulation throughout the multi-way valve group. This achieves hydraulic oil preheating at low cost and without increasing installation space. Attached Figure Description

[0020] The present invention will be described by way of example and with reference to the accompanying drawings, wherein: Figure 1 This is a schematic diagram illustrating the hot oil circulation principle of a three-way multi-port valve assembly. Figure 2 This is a schematic diagram illustrating the hot oil circulation principle of a six-way multi-port valve assembly. The markings in the diagram are: 1-Hydraulic station oil tank; 2-Main oil circuit; 21-Oil supply pipe; 22-Oil return pipe; 31-Three-way valve assembly; 32-Six-way valve assembly; 4-Directional control valve a; 5-Circulation passage; 51-First check valve; 52-Throttle valve; 6-Column lifting cylinder; 7-Standby equipment; 8-Second check valve. Detailed Implementation

[0021] In the description of this specification, it should be noted that if terms such as "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," or "outer" appear to indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product is in use, they are only for the convenience of describing this specification and simplifying the description, and do not indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this specification.

[0022] Furthermore, the use of terms such as "horizontal" or "vertical" in this specification does not imply that the component must be absolutely horizontal or suspended, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0023] In the description of this specification, it should also be noted that, unless otherwise expressly specified and limited, the terms “set up,” “install,” “connect,” and “link” should be interpreted broadly. For example, a link can be a fixed link, a detachable link, or an integral link; it can be a mechanical link or an electrical link; it can be a direct link or an indirect link through an intermediate medium; it can be a connection within two components.

[0024] Example 1 like Figures 1-2 As shown, a hot oil circulation method under low-temperature conditions includes the following steps: S1: Select a directional valve a4 from each multi-way valve group. The selection principle of the directional valve a4 is that the valve plate does not cause malfunction of the actuator. Connect the A port and B port of the directional valve a4 through the circulation passage 5. At least a first check valve 51 is provided on the circulation passage. The installation direction of the first check valve 51 only allows hydraulic oil to flow from port B to port A. S2: Switch the oil supply direction of the directional valve a4, so that the directional valve a4 supplies oil through port B and returns oil through port A; S3: Start the rotary motor connected to the hydraulic station oil tank 1 so that the hydraulic oil in the main oil circuit 2 supply pipe 21 flows out from port B after passing through the reversing valve a4, and flows back to the reversing valve a4 from port A after passing through the circulation passage 5. Then it enters the return oil pipe 22 of the main oil circuit 2 and flows back to the hydraulic station oil tank 1. S4: In step S3, the hydraulic oil undergoes throttling and heat generation during its flow, forming hot oil. The hot oil flows back to the hydraulic station oil tank 1 through the return oil pipe 22, raising the temperature of the hydraulic oil in the hydraulic station oil tank 1. Under the action of the rotary motor, it enters the supply oil pipe 21. Since all the directional valves a4 in the multi-way valve group share the same main oil circuit 2, the hot oil circulation of the entire multi-way valve group is achieved through heat transfer.

[0025] In this embodiment, there are two ways of heat transfer in step S4. First, the hydraulic oil is mixed to achieve heat integration and raise the temperature of the low-temperature oil. Second, the valve plates in the multi-way valve group are relatively close to each other, and heat is transferred through heat conduction.

[0026] It should be noted that the selection of the directional valve a4 in step S1 is the basis for the implementation of the scheme disclosed in this embodiment. The selection principle is "having a valve plate that does not cause malfunction of the actuator". Specifically, the purpose of hot oil circulation is to heat the hydraulic oil to reduce its viscosity. During the heating process, the action of the actuator should not be affected by the flow of hot oil circulation to avoid malfunction of the actuator and subsequent misoperation. Therefore, the selection principle of the directional valve a4 is "having a valve plate that does not cause malfunction of the actuator", which effectively ensures the stability of the hydraulic oil in the entire system during heating.

[0027] In practice, the multi-way valve group in step S1 can be a three-way multi-way valve group 31 used to control the main body movement of the iron drill, or a six-way multi-way valve group 32 used to control the insert and screw-locking movements. Of course, in practice, both the three-way multi-way valve group 31 and the six-way multi-way valve group 32 exist, so the hot oil circulation method is used for hydraulic systems controlled by both the three-way multi-way valve group 31 and the six-way multi-way valve group 32.

[0028] In this embodiment, for the multi-way valve group 31, which is a three-way multi-way valve group used to control the main body movement of the iron drill, the selected reversing valve a4 is the reversing valve a4 that controls the rise and fall of the column. This reversing valve a4 will not have any effect on the actuator. Specifically, for the reversing valve a4 in the three-way multi-way valve group 31, the B port is connected to the rod chamber of the column lifting cylinder 6, and the A port is connected to the rodless chamber of the column lifting cylinder 6.

[0029] When oil is supplied through port B, oil is continuously supplied to the rodless chamber of the column lifting cylinder 6, thus keeping the column lifting cylinder 6 in a retracted state, i.e., the column is always in a low position. Maintaining this state is equivalent to locking the column, thus preventing malfunctions in other actuators. When the hydraulic oil is preheated to the target temperature, oil supply through ports A and B can be switched to control the column lifting.

[0030] For the circulation passage 5 connected to the reversing valve a4 in the triple multi-way valve group 31, a throttle valve 52 is also provided on the circulation passage 5. The throttle valve 52 is located downstream of the first check valve 51. The hydraulic oil is throttled and heated through the throttle valve 52. Since the flow rate provided by the reversing valve a4 is large, the throttle valve 52 is added to provide a throttle effect. As a result, the hydraulic oil experiences a throttle heating phenomenon when passing through the throttle valve 52.

[0031] It should be noted that the throttle valve 52 is not located on the pipeline between the rodless chamber and port A, so as to avoid the throttle valve 52 affecting the flow rate of the return and inlet oil of the rodless chamber.

[0032] In this embodiment, for the directional valve a4 in the six-way multi-port valve group 32, port A is connected to the oil inlet of the standby device 7, port B is connected to the oil return port of the standby device 7, and a second check valve 8 is provided between port B and the oil return port of the standby device 7. The installation direction of the second check valve 8 only allows hydraulic oil to flow from the oil return port of the standby device 7 to port B.

[0033] When oil is supplied through port B, the presence of the second check valve 8 prevents hydraulic oil from entering the standby device 7. After the hydraulic oil is preheated to the target temperature, when it is necessary to start the standby device 7, the oil supply can be switched to port A, and the oil will return through port B after passing through the second check valve 8.

[0034] The backup equipment 7 requires a small flow rate, so the directional valve a4 is a small flow valve plate, which actually plays a throttling role, that is, the hydraulic oil is throttled and heated in the directional valve a4.

[0035] It should be noted that, due to the presence of the first check valve 51, oil supplied from port A will not directly flow back to port B from the circulation passage 5. When the column lifting cylinder 6 controls the column to rise, the oil supplied from port A can only enter the rodless chamber. That is, the hydraulic oil flowing back from the rod chamber to port B will not enter port A through the first check valve 51, thus not interfering with the normal oil supply to port A and enabling the column to rise at a normal speed. Similarly, when the column lifting cylinder 6 controls the column to descend, the oil supplied from port B will divert some hydraulic oil to flow into port A through the first check valve 51. Therefore, under normal oil supply conditions, the oil flow rate in the rod chamber will be slightly reduced. However, a throttle valve 52 is installed downstream of the first check valve 51, which can establish the working pressure for normal descent and enable the column to descend slowly. The first check valve 51 is more in line with the working conditions required for the column. As for the backup equipment 7, only port A is allowed to supply oil. Therefore, setting the first check valve 51 will not affect the normal operation of the backup equipment 7. Moreover, the presence of the second check valve 8 can further prevent the adverse effects caused by the incorrect switching of the reversing valve a4.

[0036] Example 2 like Figures 1-2As shown, a hot oil circulation system for low-temperature environments, applied to the aforementioned hot oil circulation method for low-temperature environments, includes a multi-way valve group composed of multiple directional valves, each directional valve being connected in parallel on the main oil circuit 2; a directional valve a4 is selected from the multiple heat exchange valves, the selection principle of which is that the valve plate does not cause malfunction of the actuator; the A port and B port of the directional valve a4 are connected through a circulation passage 5, and a first check valve 51 is provided on the circulation passage 5, the first check valve 51 only allows hydraulic oil to flow from port B to port A.

[0037] Furthermore, if the multi-way valve group is a three-way multi-way valve group 31 used to control the main body movement of the iron drill, then the selected reversing valve a4 is a reversing valve a4 that controls the rise and fall of the column; a throttle valve 52 is also provided on the circulation passage 5. The throttle valve 52 is located downstream of the first check valve 51, and the hydraulic oil is throttled and heated through the throttle valve 52. If the multi-way valve assembly is a six-way multi-way valve assembly 32 used to control the insert and the rotary action, then the selected directional valve a4 is the directional valve a4 that supplies oil to the standby equipment 7.

[0038] Furthermore, in the three-way valve assembly 31, the reversing valve a4 is connected to the rod chamber of the column lifting cylinder 6, the A port is connected to the rodless chamber of the column lifting cylinder 6, and the throttle valve 52 is not located on the pipeline between the rodless chamber and the A port.

[0039] Furthermore, for the directional valve a4 in the six-way multi-port valve group 32, port A is connected to the oil inlet of the standby device 7, port B is connected to the oil return port of the standby device 7, and a second check valve 8 is provided between port B and the oil return port of the standby device 7. The installation direction of the second check valve 8 only allows hydraulic oil to flow from the oil return port of the standby device 7 to port B.

[0040] This invention is not limited to the specific embodiments described above. The invention extends to any new feature or combination disclosed in this specification, as well as any new method or process step or combination disclosed herein.

Claims

1. A method for hot oil circulation under low-temperature conditions, characterized in that: Includes the following steps: S1: Select a directional valve a (4) in each multi-way valve group. The selection principle of the directional valve a (4) is that it has a valve plate that does not cause malfunction of the actuator. Connect the A port and B port of the directional valve a (4) through a circulation passage (5). At least a first check valve (51) is provided on the circulation passage (5). The installation direction of the first check valve (51) only allows hydraulic oil to flow from port B to port A. If the multi-way valve group is a three-way multi-way valve group (31) used to control the main body movement of the iron drill, then the selected reversing valve a (4) is the reversing valve a (4) used to control the rise and fall of the column. Or / and in step S1, if the multi-way valve group is a six-way multi-way valve group (32) used to control the insert and the turn-lock action, then the selected directional valve a (4) is the directional valve a (4) that supplies oil to the standby equipment (7). S2: Switch the oil supply direction of the directional valve a (4) so ​​that the directional valve a (4) supplies oil through port B and returns oil through port A; S3: Start the rotary motor connected to the hydraulic station oil tank (1) so that the hydraulic oil is in the oil supply pipe (21) of the main oil circuit (2). The hydraulic oil in the oil supply pipe (21) flows out from port B after passing through the reversing valve a (4), and flows back to the reversing valve a (4) from port A after passing through the circulation passage (5). Then it enters the return oil pipe (22) of the main oil circuit (2) and flows back to the hydraulic station oil tank (1). S4: In step S3, the hydraulic oil undergoes throttling and heat generation during flow, forming hot oil. The hot oil flows back to the hydraulic station oil tank (1) through the return oil pipe (22), raising the temperature of the hydraulic oil in the hydraulic station oil tank (1), and enters the supply oil pipe (21) under the action of the rotary motor. Since all the directional valves a (4) in the multi-way valve group share the same main oil circuit (2), the hot oil circulation of the entire multi-way valve group is realized through heat transfer.

2. The hot oil circulation method according to claim 1, characterized in that: For the circulation passage (5) connected to the reversing valve a (4) in the triple multi-way valve group (31), a throttle valve (52) is also provided on the circulation passage (5). The throttle valve (52) is located downstream of the first check valve (51), and the hydraulic oil is throttled and heated through the throttle valve (52).

3. The hot oil circulation method according to claim 2, characterized in that: For the directional valve a (4) in the three-way multi-way valve group (31), the B port is connected to the rod chamber of the column lifting cylinder (6), the A port is connected to the rodless chamber of the column lifting cylinder (6), and the throttle valve (52) is not located on the pipeline between the rodless chamber and the A port.

4. The hot oil circulation method according to claim 1, characterized in that: The backup equipment (7) requires a small flow rate, and the hydraulic oil is throttled and heated within the reversing valve a (4).

5. The hot oil circulation method according to claim 1, characterized in that: For the directional valve a (4) in the six-way multi-way valve group (32), the A port is connected to the oil inlet of the standby device (7), the B port is connected to the oil return port of the standby device (7), and a second check valve (8) is provided between the B port and the oil return port of the standby device (7). The installation direction of the second check valve (8) only allows hydraulic oil to flow from the oil return port of the standby device (7) to the B port.

6. A hot oil circulation system for low-temperature environments, applied to the hot oil circulation method for low-temperature environments as described in any one of claims 1-5, characterized in that: It includes a multi-way valve group consisting of multiple directional valves, each of which is connected in parallel on the main oil circuit (2); a directional valve a (4) is selected from multiple heat exchange valves, and the selection principle of the directional valve a (4) is that it has a valve plate that does not cause malfunction of the actuator. The A port and B port on the directional valve a (4) are connected through a circulation passage (5). A first check valve (51) is set on the circulation passage (5), and the first check valve (51) only allows hydraulic oil to flow from port B to port A; If the multi-way valve group is a three-way multi-way valve group (31) used to control the main body movement of the iron drill, then the selected reversing valve a (4) is the reversing valve a (4) used to control the rise and fall of the column; a throttle valve (52) is also provided on the circulation passage (5). The throttle valve (52) is located downstream of the first check valve (51). The hydraulic oil is throttled and heated through the throttle valve (52); If the multi-way valve group is a six-way multi-way valve group (32) used to control the insert and the turn-lock action, then the selected directional valve a (4) is the directional valve a (4) that supplies oil to the standby equipment (7).

7. The hot oil circulation system according to claim 6, characterized in that: For the directional valve a (4) in the three-way multi-way valve group (31), the B port is connected to the rod chamber of the column lifting cylinder (6), the A port is connected to the rodless chamber of the column lifting cylinder (6), and the throttle valve (52) is not located on the pipeline between the rodless chamber and the A port.

8. The hot oil circulation system according to claim 6, characterized in that: For the directional valve a (4) in the six-way multi-way valve group (32), the A port is connected to the oil inlet of the standby device (7), the B port is connected to the oil return port of the standby device (7), and a second check valve (8) is provided between the B port and the oil return port of the standby device (7). The installation direction of the second check valve (8) only allows hydraulic oil to flow from the oil return port of the standby device (7) to the B port.

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