Adaptive control system and method for flow and pressure of dynamic true triaxial hydraulic source of rock

Abstract

A flow and pressure adaptive control system and method for a dynamic true triaxial hydraulic source of rock. The system includes an oil tank, a small-flow oil pump, a large-flow oil pump, a dynamic actuator, a high-low pressure automatic control unit, and a dynamic actuator flow adaptive control unit; the small-flow oil pump and the large-flow oil pump are arranged in parallel, the oil inlet of the oil pump is connected to the oil tank, and the oil outlet of the oil pump is connected to the dynamic actuator through the high-low pressure automatic control unit and the dynamic actuator flow adaptive control unit in sequence. The method is divided into high-low pressure automatic control and dynamic actuator flow adaptive control. Through high-low pressure automatic control, no-load startup, low-pressure operation, low-pressure to high-pressure operation conversion, and high-pressure to low-pressure operation conversion can be achieved; the dynamic actuator flow adaptive control can meet the small-flow state and the conversion between small and large flow states. After obtaining small-flow hydraulic oil, the dynamic actuator can maintain the small-flow working state, and then the full-curve stress-strain test can be completed, so that a complete stress-strain curve can be obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of rock mechanics tests, and particularly relates to a flow and pressure adaptive control system and method for a rock dynamic true triaxial hydraulic source. Background Art

[0002] At present, the dynamic true triaxial hydraulic system of a true triaxial testing machine adopts a hydraulic servo loading system. Although the system has large anti-load stiffness, speed stiffness, and position stiffness, and strong anti-interference ability, there are also obvious disadvantages of pressure loss and flow loss. In addition, the pressure of the system is mostly manually selected, which not only has complex operation, but also cannot select the pressure according to the working conditions, resulting in waste of energy. Moreover, to obtain a complete stress-strain curve in a dynamic true triaxial test, the dynamic actuator requires a small flow loading capacity, but the current system cannot keep the dynamic actuator in a small flow working state, so it is difficult to obtain a complete stress-strain curve. Summary of the Invention

[0003] Aiming at the problems existing in the prior art, the present invention provides a flow and pressure adaptive control system and method for a rock dynamic true triaxial hydraulic source, which can adjust the system pressure in real time according to the working conditions, and has the characteristics of simple operation and energy saving; it can keep the dynamic actuator in a small flow working state, so as to obtain a complete stress-strain curve.

[0004] To achieve the above object, the present invention adopts the following technical solutions: A flow-pressure adaptive control system for a dynamic true triaxial hydraulic source of rock, comprising an oil tank, a small-flow oil pump, a large-flow oil pump, a dynamic actuator, a high-low pressure automatic control unit, and a dynamic actuator flow adaptive control unit; the high-low pressure automatic control unit includes a high-pressure filter, a check valve, a high-pressure relief valve, a low-pressure relief valve, a directional control valve, and a pressure relay; the dynamic actuator flow adaptive control unit includes a small-flow servo valve, a first large-flow servo valve, a second large-flow servo valve, a third large-flow servo valve, a first stop valve, a second stop valve, a third stop valve, a first accumulator, and a second accumulator; the inlet of the small-flow oil pump is communicated with the oil tank, and the outlet of the small-flow oil pump is communicated with the inlet of the high-pressure filter; the inlet of the large-flow oil pump is communicated with the oil tank, and the outlet of the large-flow oil pump is communicated with the inlet of the high-pressure filter; the number of the large-flow oil pumps is at least two, and the small-flow oil pump and the multiple large-flow oil pumps are arranged in parallel; the outlet of the high-pressure filter is output in two paths, the first path is communicated with the inlet of the check valve, the second path is communicated with the inlet of the high-pressure relief valve, and the outlet of the high-pressure relief valve is communicated with the oil tank; the outlet of the check valve is output in three paths, the first path is communicated with the pressure relay, the second path is connected to the dynamic actuator flow adaptive control unit, and the third path is communicated with the inlet of the directional control valve, the outlet of the directional control valve is communicated with the inlet of the low-pressure relief valve, and the outlet of the low-pressure relief valve is communicated with the oil tank; the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve all adopt three-position four-way valves and are arranged in parallel; the A port of the small-flow servo valve is communicated with the rod chamber of the dynamic actuator, the A ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are connected together and then communicated with one end port of the first stop valve, and the other end port of the first stop valve is communicated with the rod chamber of the dynamic actuator; the B port of the small-flow servo valve is communicated with the non-rod chamber of the dynamic actuator, the B ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are connected together and then communicated with one end port of the second stop valve, and the other end port of the second stop valve is communicated with the non-rod chamber of the dynamic actuator; the P port of the small-flow servo valve is communicated with the outlet of the check valve, the P ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are connected together and then communicated with one end port of the third stop valve, and the other end port of the third stop valve is communicated with the outlet of the check valve; the T ports of the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are all communicated with the oil tank; the first accumulator is directly connected to the P port of the small-flow servo valve, and the first accumulator is connected to the P ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve through the third stop valve;The second accumulator is directly connected to the T ports of the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve.

[0005] A method for adaptively controlling the flow and pressure of a rock dynamic true triaxial hydraulic source adopts the rock dynamic true triaxial hydraulic source flow and pressure adaptive control system described above. The specific method is as follows:

[0006] I. High and low pressure automatic control

[0007] ① No-load start: When the small-flow oil pump starts without load, the low-pressure overflow valve is energized and connected. The hydraulic oil is first filtered by the high-pressure filter, and then flows back to the oil tank through the check valve, the directional valve, and the low-pressure overflow valve in sequence.

[0008] ② Low-pressure operation: When the small-flow servo valve is working, the low-pressure overflow valve is de-energized and reset. The hydraulic oil is first filtered by the high-pressure filter, and then flows through the check valve. Part of the hydraulic oil flowing out of the check valve flows to the dynamic actuator flow adaptive control unit where the small-flow servo valve is located, and the other part flows back to the oil tank through the directional valve and the low-pressure overflow valve in sequence. At this time, the system works under the low pressure set by the low-pressure overflow valve.

[0009] ③ Low-pressure to high-pressure operation conversion: When the system pressure rises to the upper limit value of the pressure relay, the pressure relay is triggered. After the directional valve is energized, the oil inlet circuit of the low-pressure overflow valve is disconnected, and the system enters the high-pressure working state. At this time, the hydraulic oil is first filtered by the high-pressure filter. Part of the hydraulic oil flowing out of the high-pressure filter flows to the dynamic actuator flow adaptive control unit where the small-flow servo valve is located through the check valve, and the other part flows back to the oil tank through the high-pressure overflow valve. At this time, the system works under the high pressure set by the high-pressure overflow valve.

[0010] ④ High-pressure to low-pressure operation conversion: When the system pressure drops to the lower limit value of the pressure relay, the pressure relay is triggered. The directional valve is de-energized and reset, and the oil inlet circuit of the low-pressure overflow valve is restored to conduction. The hydraulic oil is first filtered by the high-pressure filter, and then flows through the check valve. Part of the hydraulic oil flowing out of the check valve flows to the dynamic actuator flow adaptive control unit where the small-flow servo valve is located, and the other part flows back to the oil tank through the directional valve and the low-pressure overflow valve in sequence. At this time, the system works under the low pressure set by the low-pressure overflow valve.

[0011] II. Dynamic actuator flow adaptive control

[0012] ① Small-flow state: Only the small-flow oil pump is started, and only the small-flow servo valve is working. At the same time, the first shut-off valve, the second shut-off valve, and the third shut-off valve are all adjusted to the closed state. The hydraulic oil enters the dynamic actuator only through the small-flow servo valve.

[0013] ②. Small flow converts to large flow state: At any moment during loading, start the large flow oil pump, and the first large flow servo valve, the second large flow servo valve, and the third large flow servo valve work. At the same time, all the first cut-off valve, the second cut-off valve, and the third cut-off valve are adjusted to the open state. The hydraulic oil enters the dynamic actuator through the small flow servo valve, the first large flow servo valve, the second large flow servo valve, and the third large flow servo valve;

[0014] ③. Large flow converts to small flow state: Turn off the large flow oil pump, and the first large flow servo valve, the second large flow servo valve, and the third large flow servo valve stop working. At the same time, all the first cut-off valve, the second cut-off valve, and the third cut-off valve are adjusted to the closed state. The hydraulic oil only enters the dynamic actuator through the small flow servo valve.

[0015] Advantages of the present invention:

[0016] The flow-pressure adaptive control system and method of the rock dynamic true triaxial hydraulic source of the present invention can adjust the system pressure in real time according to the working conditions, and has the characteristics of simple operation and energy saving; it can keep the dynamic actuator in a small flow working state, so that a complete stress-strain curve can be obtained. Description of the drawings

[0017] Figure 1 is the hydraulic schematic diagram of the flow-pressure adaptive control system of the rock dynamic true triaxial hydraulic source of the present invention;

[0018] In the figure, 1 - oil tank, 2 - small flow oil pump, 3 - large flow oil pump, 4 - dynamic actuator, 5 - high and low pressure automatic control unit, 6 - dynamic actuator flow adaptive control unit, 7 - high pressure filter, 8 - check valve, 9 - high pressure relief valve, 10 - low pressure relief valve, 11 - reversing valve, 12 - pressure relay, 13 - small flow servo valve, 14 - first large flow servo valve, 15 - second large flow servo valve, 16 - third large flow servo valve, 17 - first cut-off valve, 18 - second cut-off valve, 19 - third cut-off valve, 20 - first accumulator, 21 - second accumulator. Specific embodiments

[0019] The following further describes the present invention in detail with reference to the drawings and specific embodiments.

[0020] As Figure 1As shown in the figure, a self-adaptive control system for the flow and pressure of a dynamic true triaxial hydraulic source for rocks includes an oil tank 1, a small-flow oil pump 2, a large-flow oil pump 3, a dynamic actuator 4, a high-low pressure automatic control unit 5, and a dynamic actuator flow self-adaptive control unit 6; the high-low pressure automatic control unit 5 includes a high-pressure filter 7, a one-way valve 8, a high-pressure relief valve 9, a low-pressure relief valve 10, a reversing valve 11, and a pressure relay 12; the dynamic actuator flow self-adaptive control unit 6 includes a small-flow servo valve 13, a first large-flow servo valve 14, a second large-flow servo valve 15, a third large-flow servo valve 16, a first stop valve 17, a second stop valve 18, a third stop valve 19, a first accumulator 20, and a second accumulator 21; the inlet of the small-flow oil pump 2 is connected to the oil tank 1, and the outlet of the small-flow oil pump 2 is connected to the inlet of the high-pressure filter 7; the inlet of the large-flow oil pump 3 is connected to the oil tank 1, and the outlet of the large-flow oil pump 3 is connected to the inlet of the high-pressure filter 7; the number of the large-flow oil pumps 3 is at least two, and the small-flow oil pump 2 and multiple large-flow oil pumps 3 are arranged in parallel; the outlet of the high-pressure filter 7 outputs in two paths, the first path is connected to the inlet of the one-way valve 8, the second path is connected to the inlet of the high-pressure relief valve 9, and the outlet of the high-pressure relief valve 9 is connected to the oil tank 1; the outlet of the one-way valve 8 outputs in three paths, the first path is connected to the pressure relay 12, the second path is connected to the dynamic actuator flow self-adaptive control unit 6, the third path is connected to the inlet of the reversing valve 11, the outlet of the reversing valve 11 is connected to the inlet of the low-pressure relief valve 10, and the outlet of the low-pressure relief valve 10 is connected to the oil tank 1; the small-flow servo valve 13, the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 all adopt three-position four-way valves and are arranged in parallel; the A port of the small-flow servo valve 13 is connected to the rod chamber of the dynamic actuator 4, the A ports of the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 are joined together and then connected to one end port of the first stop valve 17, and the other end port of the first stop valve 17 is connected to the rod chamber of the dynamic actuator 4; the B port of the small-flow servo valve 13 is connected to the non-rod chamber of the dynamic actuator 4, the B ports of the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 are joined together and then connected to one end port of the second stop valve 18, and the other end port of the second stop valve 18 is connected to the non-rod chamber of the dynamic actuator 4; the P port of the small-flow servo valve 13 is connected to the outlet of the one-way valve 8, the P ports of the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 are joined together and then connected to one end port of the third stop valve 19, and the other end port of the third stop valve 19 is connected to the outlet of the one-way valve 8;The T ports of the small-flow servo valve 13, the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 are all connected to the oil tank 1; the first accumulator 20 is directly connected to the P port of the small-flow servo valve 13, and the first accumulator 20 is connected to the P ports of the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 through the third stop valve 19; the second accumulator 21 is directly connected to the T ports of the small-flow servo valve 13, the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16.

[0021] A flow-pressure adaptive control method for a rock dynamic true triaxial hydraulic source adopts the rock dynamic true triaxial hydraulic source flow-pressure adaptive control system described above. The specific method is as follows:

[0022] I. High and low pressure automatic control

[0023] ①. No-load start: When the small-flow oil pump 2 starts without load, the low-pressure overflow valve 10 is energized and connected. The hydraulic oil is first filtered by the high-pressure filter 7, and then flows back to the oil tank 1 through the one-way valve 8, the reversing valve 11, and the low-pressure overflow valve 10 in sequence, featuring low starting noise and low system heat generation.

[0024] ②. Low-pressure operation: When the small-flow servo valve 13 operates, the low-pressure overflow valve 10 is de-energized and reset. The hydraulic oil is first filtered by the high-pressure filter 7, and then flows through the one-way valve 8. Part of the hydraulic oil flowing out of the one-way valve 8 flows to the dynamic actuator flow adaptive control unit 6 where the small-flow servo valve 13 is located, and the other part flows back to the oil tank 1 through the reversing valve 11 and the low-pressure overflow valve 10 in sequence. At this time, the system operates under the low pressure set by the low-pressure overflow valve 10.

[0025] ③. Low-pressure to high-pressure operation conversion: When the system pressure rises to the upper limit value of the pressure relay 12, the pressure relay 12 is triggered, and the reversing valve 11 is energized to disconnect the oil inlet circuit of the low-pressure overflow valve 10. The system enters the high-pressure operation state. At this time, the hydraulic oil is first filtered by the high-pressure filter 7. Part of the hydraulic oil flowing out of the high-pressure filter 7 flows through the one-way valve 8 to the dynamic actuator flow adaptive control unit 6 where the small-flow servo valve 13 is located, and the other part flows back to the oil tank 1 through the high-pressure overflow valve 9. At this time, the system operates under the high pressure set by the high-pressure overflow valve 9. When the system pressure exceeds the limit value of the high-pressure overflow valve 9, the hydraulic oil flows back to the oil tank 1 through the high-pressure overflow valve 9, and the system pressure does not rise further, ensuring the safety of the system.

[0026] ④. High-pressure to low-pressure operation: When the system pressure drops below the lower limit value of the pressure relay 12, the pressure relay 12 is triggered, the reversing valve 11 loses power and resets, and the oil inlet passage of the low-pressure overflow valve 10 is restored to conduction. The hydraulic oil is first filtered by the high-pressure filter 7, then flows through the check valve 8. Part of the hydraulic oil flowing out of the check valve 8 flows to the dynamic actuator flow adaptive control unit 6 where the small-flow servo valve 13 is located, and the other part flows back to the oil tank 1 through the reversing valve 11 and the low-pressure overflow valve 10 in sequence. At this time, the system operates under the low pressure set by the low-pressure overflow valve 10;

[0027] II. Dynamic actuator flow adaptive control

[0028] ①. Small-flow state: Only the small-flow oil pump 2 is started, and only the small-flow servo valve 13 works. At the same time, the first shut-off valve 17, the second shut-off valve 18, and the third shut-off valve 19 are all adjusted to the closed state. The hydraulic oil enters the dynamic actuator 4 only through the small-flow servo valve 13. After obtaining the small-flow hydraulic oil, the dynamic actuator 4 at this time can maintain the small-flow working state, and then can complete the full-curve stress-strain test, so as to obtain a complete stress-strain curve;

[0029] ②. Small-flow to large-flow state: At any moment during loading, the large-flow oil pump 3 is started, and the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 work. At the same time, the first shut-off valve 17, the second shut-off valve 18, and the third shut-off valve 19 are all adjusted to the open state. The hydraulic oil enters the dynamic actuator 4 through the small-flow servo valve 13, the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16. At this time, the dynamic actuator 4 can apply dynamic loading at any moment during the full-curve stress-strain test, and during the dynamic loading process, the first accumulator 20 and the second accumulator 21 absorb the impact caused by the frequent commutation of each servo valve to ensure the stability of the system;

[0030] ③. Large-flow to small-flow state: The large-flow oil pump 3 is closed, and the first large-flow servo valve 14, the second large-flow servo valve 15, and the third large-flow servo valve 16 stop working. At the same time, the first shut-off valve 17, the second shut-off valve 18, and the third shut-off valve 19 are all adjusted to the closed state. The hydraulic oil enters the dynamic actuator 4 only through the small-flow servo valve 13.

[0031] The solutions in the embodiments are not intended to limit the patent protection scope of the present invention. Any equivalent implementation or modification without departing from the present invention is included in the patent scope of this case.

Claims

1. A method for adaptively controlling the flow and pressure of a dynamic true triaxial hydraulic source for rocks, which adopts a dynamic true triaxial hydraulic source flow and pressure adaptive control system for rocks, is characterized in that The specific method is as follows: I. High and low pressure automatic control ① No-load start: When the small-flow oil pump starts without load, the low-pressure relief valve is energized and connected. The hydraulic oil is first filtered through the high-pressure filter, and then flows back to the oil tank through the one-way valve, the directional valve and the low-pressure relief valve in sequence; ② Low-pressure operation: When the small-flow servo valve is working, the low-pressure relief valve is de-energized and reset. The hydraulic oil is first filtered through the high-pressure filter, and then flows through the one-way valve. Part of the hydraulic oil flowing out of the one-way valve flows to the dynamic actuator flow adaptive control unit where the small-flow servo valve is located, and the other part flows back to the oil tank through the directional valve and the low-pressure relief valve in sequence. At this time, the system works under the low-pressure state set by the low-pressure relief valve; ③ Low-pressure to high-pressure operation: When the system pressure rises to the upper limit value of the pressure relay, the pressure relay is triggered. After the directional valve is energized, the oil inlet circuit of the low-pressure relief valve is disconnected, and the system enters the high-pressure working state. At this time, the hydraulic oil is first filtered through the high-pressure filter. Part of the hydraulic oil flowing out of the high-pressure filter flows through the one-way valve to the dynamic actuator flow adaptive control unit where the small-flow servo valve is located, and the other part flows back to the oil tank through the high-pressure relief valve. At this time, the system works under the high-pressure state set by the high-pressure relief valve; ④ High-pressure to low-pressure operation: When the system pressure drops to the lower limit value of the pressure relay, the pressure relay is triggered. The directional valve is de-energized and reset, and the oil inlet circuit of the low-pressure relief valve is restored to conduction. The hydraulic oil is first filtered through the high-pressure filter, and then flows through the one-way valve. Part of the hydraulic oil flowing out of the one-way valve flows to the dynamic actuator flow adaptive control unit where the small-flow servo valve is located, and the other part flows back to the oil tank through the directional valve and the low-pressure relief valve in sequence. At this time, the system works under the low-pressure state set by the low-pressure relief valve; II. Dynamic actuator flow adaptive control ① Small-flow state: Only the small-flow oil pump is started, and only the small-flow servo valve is working. At the same time, the first shut-off valve, the second shut-off valve and the third shut-off valve are all adjusted to the closed state. The hydraulic oil enters the dynamic actuator only through the small-flow servo valve; ② Small-flow to large-flow state: At any moment during loading, the large-flow oil pump is started, and the first large-flow servo valve, the second large-flow servo valve and the third large-flow servo valve are working. At the same time, the first shut-off valve, the second shut-off valve and the third shut-off valve are all adjusted to the open state. The hydraulic oil enters the dynamic actuator through the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve and the third large-flow servo valve; ③ Large-flow to small-flow state: The large-flow oil pump is shut down, and the first large-flow servo valve, the second large-flow servo valve and the third large-flow servo valve stop working. At the same time, the first shut-off valve, the second shut-off valve and the third shut-off valve are all adjusted to the closed state. The hydraulic oil enters the dynamic actuator only through the small-flow servo valve; The flow and pressure adaptive control system of the dynamic true triaxial hydraulic source for rocks includes an oil tank, a small-flow oil pump, a large-flow oil pump, a dynamic actuator, a high-low pressure automatic control unit, and a dynamic actuator flow adaptive control unit; the high-low pressure automatic control unit includes a high-pressure filter, a check valve, a high-pressure relief valve, a low-pressure relief valve, a reversing valve, and a pressure relay; the dynamic actuator flow adaptive control unit includes a small-flow servo valve, a first large-flow servo valve, a second large-flow servo valve, a third large-flow servo valve, a first stop valve, a second stop valve, a third stop valve, a first accumulator, and a second accumulator; the inlet of the small-flow oil pump is connected to the oil tank, and the outlet of the small-flow oil pump is connected to the inlet of the high-pressure filter; the inlet of the large-flow oil pump is connected to the oil tank, and the outlet of the large-flow oil pump is connected to the inlet of the high-pressure filter; the number of the large-flow oil pumps is at least two, and the small-flow oil pump and the multiple large-flow oil pumps are arranged in parallel; the outlet of the high-pressure filter is output in two paths, the first path is connected to the inlet of the check valve, the second path is connected to the inlet of the high-pressure relief valve, and the outlet of the high-pressure relief valve is connected to the oil tank; the outlet of the check valve is output in three paths, the first path is connected to the pressure relay, the second path is connected to the dynamic actuator flow adaptive control unit, the third path is connected to the inlet of the reversing valve, the outlet of the reversing valve is connected to the inlet of the low-pressure relief valve, and the outlet of the low-pressure relief valve is connected to the oil tank; the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve all adopt three-position four-way valves and are arranged in parallel; the A port of the small-flow servo valve is connected to the rod chamber of the dynamic actuator, the A ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are connected together and then connected to one end port of the first stop valve, and the other end port of the first stop valve is connected to the rod chamber of the dynamic actuator; the B port of the small-flow servo valve is connected to the non-rod chamber of the dynamic actuator, the B ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are connected together and then connected to one end port of the second stop valve, and the other end port of the second stop valve is connected to the non-rod chamber of the dynamic actuator; the P port of the small-flow servo valve is connected to the outlet of the check valve, the P ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are connected together and then connected to one end port of the third stop valve, and the other end port of the third stop valve is connected to the outlet of the check valve; the T ports of the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve are all connected to the oil tank; the first accumulator is directly connected to the P port of the small-flow servo valve, and the first accumulator is connected to the P ports of the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve through the third stop valve; the second accumulator is directly connected to the T ports of the small-flow servo valve, the first large-flow servo valve, the second large-flow servo valve, and the third large-flow servo valve.

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