JP2000140508A - Air bubble removing system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air bubble removing system ensuring not only the liquid flow rate of a bypass flow passage but also the liquid flow rate to an air bubble removing device by a protection device provided to the bypass flow passage operated when the viscosity of a liquid rises to a predetermined value and succeedingly operated corresponding to a change in viscosity. SOLUTION: In an air bubble removing system constituted of an air bubble removing circuit wherein the air bubble removing device 100 connected to a tank 1 storing an air bubble-containing liquid is arranged on the upstream side of a flow feed pump 3, the air bubble removing device is a system wherein the revolving stream of the air bubble-containing liquid is generated within a cone by the negative pressure from the flow feed pump 3 to separate air bubbles from the liquid along a revolving axial line by centrifugal separation action, and the liquid is discharged to the outside through a drain 13 and the air bubble removing circuit is further equipped with a bypass flow passage 12 equipped with a protection device 6 and a drain 13 on which control negative pressure acts, and the protection device is equipped with a valve 9 operated against liquid viscosity of a predetermined value or more and also operated so as to increase the bypass flow rate of the liquid in proportion to a rise in liquid viscosity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tank for storing a bubble-containing liquid, and a bubble removal circuit for arranging a bubble removal device connected to the tank upstream of a flow pump and interconnecting them. And a bubble removal system comprising:

[0002]

2. Description of the Related Art Generally, in a liquid circuit, particularly in a hydraulic circuit of a working machine using an actuator such as a hydraulically operated civil construction machine or an agricultural machine, air bubbles are generated upstream of a relatively large capacity flow pump. When a removal device is provided, the bubble removal device is operated by the negative pressure of the flow pump when sucking up the liquid stored in the liquid tank, and the flow path resistance of the bubble removal device in this negative pressure circuit is itself. This can cause a significant decrease in the efficiency of the pump. At the same time, when a liquid having a high viscosity is used, on the other hand, the flow pump becomes susceptible to cavitation due to a shortage of the supplied liquid flow rate, which causes an increase in noise and vibration of the pump, and consequently the pump. It may cause a failure of the device. On the other hand, the bubble removing device itself may be clogged, which may result in a malfunction, and eventually a failure or breakage of the device. Therefore, in consideration of the smooth operation of the equipment, the viscosity of the liquid must be kept as low and stable as possible. However, in the case of civil engineering construction equipment and agricultural machinery used outdoors as described above, in consideration of the use environment, the temperature fluctuations, especially, the situation that the oil viscosity increases due to the decrease in the temperature in winter or cold districts, Inevitable.

On the other hand, in order to solve the original problem of a bubble removing apparatus that achieves a removal rate of bubbles contained in a liquid stably and efficiently over a wide range of viscosity,
The applicant of the present application is Japanese Patent Publication No. 2766604,
Japanese Patent No. 45002 and Japanese Patent Application Laid-Open No. 8-155211 propose an air bubble removing device using an upward swirling flow based on the cyclone principle.

[0004] As a result of actually manufacturing and using the air bubble removing device disclosed in these publications, it has been found that the air bubble removing device exhibits a remarkable performance with respect to the air bubble removal which cannot be considered conventionally. Regarding the flow resistance, which is one of the above-mentioned problems, in particular, in a suction-type use mode in which the inflow pump is operated at a negative pressure arranged on the upstream side of the tank, the flow resistance of the apparatus itself is still small. It has been found that due to the presence of, the degree of dependence on the viscosity of the liquid used is large and not always satisfactory.

[0005]

With respect to the flow path resistance, it is conceivable to improve the shape and size of the flow path and the structure of the bubble removing device itself, but the problem is that the viscosity of the flowing liquid is high or low. Is larger and more decisive. Regarding the level of the viscosity of the liquid, not only the invention described in the above-mentioned publication but also conventional hydraulic equipment using such a type of bubble removing device is not so much omitted, and it is only necessary to deal with a failure of the bubble removing device. In order to solve this problem, Japanese Patent Application Laid-Open No. Hei 8-1 discloses a technique in which a bypass stop valve is provided between a suction port connected to a liquid tank and a suction pump.
It is only found in 55211.

SUMMARY OF THE INVENTION It is an object of the present invention to operate a liquid flow above a predetermined value and, in proportion to an increase in the viscosity, to control the flow rate of a liquid passing through a bubble removing device by operating a bypass flow path and a liquid flowing through the flow path. It is an object of the present invention to provide a bubble removal circuit that can always and stably and efficiently remove bubbles by adjusting the flow rate of the air bubbles, thereby avoiding a failure of the bubble removal device and devices constituting the circuit.

[0007]

According to the present invention, an object of the present invention is to provide an air bubble removing apparatus, wherein a bubble-containing liquid is introduced into a cone provided inside by the suction force of the flow pump, and a tangential line is provided below the cone. Direction and generate a conical rising swirling flow, which separates bubbles from the liquid along the swirl axis by centrifugal action and passes a small amount of liquid through a bubble removal tube provided along the axis of the cone. A bypass passage provided with a protection device that connects the tank side and the flow pump side by-passing the bubble removal device. A drain connected to the bubble removing device for returning a small amount of liquid discharged from the bubble removing tube to the tank together with the bubble, the drain having a control negative pressure acting thereon; and the protective device has a predetermined value. that's all Actuated and the liquid viscosity is solved by the bubble removal system, characterized in that it comprises a valve which operates in proportion to the increase thereof so as to increase the bypass flow rate of the liquid.

[0008] With this configuration, the bubble-containing liquid that has been swirled by the negative pressure of the flow pump is collected by the centrifugal separation action into a gas having a small mass inside the liquid portion having a large mass, and is continuously swirled into the cone. The sucked-up bubble portion flows into the bubble removing tube together with a small amount of liquid while forming a larger aggregate along the swirl axis, and is discharged to the outside by the attached control negative pressure acting drain. Is removed from the bubble removing device by the negative pressure of the flow pump, and then fed to the working section of the device. When the viscosity of the liquid passing through the bubble removing device becomes a certain value or more, the valve of the bypass circuit is activated to start opening the bypass circuit.

When the viscosity of the liquid further increases, the valve operates to increase the flow rate of the bypass flow path in proportion to the increase in the viscosity, so that a part of the liquid is directly discharged from the tank according to the viscosity. Since it is sucked up by the flow pump, both the load on the pump and the load on the bubble removing device are reduced. Except in the case of extremely high viscosity or a failure in the upstream two-way system including the bubble removing device, the amount of liquid corresponding to the flow rate passing through the bubble removing device flows through the bypass flow path, Is simultaneously performed, and the liquid having the increased viscosity at the time of cold or at the time of starting the equipment is safely and smoothly circulated through the air bubble removing circuit without fear of damaging the equipment used such as the bubble removing device and the flow pump. It becomes possible.

According to one embodiment of the present invention, the protection device comprises a check valve with a spring. The negative pressure of the feed pump is constantly acting on the valve element of this check valve with spring through the outlet side of the bypass passage, causing the viscosity of the liquid to increase and causing the cavitation of the feed pump. Then, the valve body is moved in the direction of drawing in against the elasticity of the spring attached to the valve body to increase the valve opening, and the amount of liquid flowing through the bypass passage to the flow pump is increased. This eliminates the cavitation of the flow pump. Therefore, a desired valve opening degree characteristic can be obtained by appropriately selecting a spring having an appropriate spring constant, and such means can be the simplest and cheapest protection means and achieve the intended purpose.

According to another embodiment of the present invention,
The protection device comprises a differential pressure valve which operates with a pressure difference between the liquid inflow side and the liquid outflow side of the bubble removing apparatus. In this case, the differential pressure valve converts the negative pressure obtained by the pressure sensors respectively arranged on the inflow side and the outflow side of the bubble removing device into an electric signal, and according to the value of the difference calculated by the control circuit. It can consist of an actuated electromagnetic valve.

As a different embodiment, a mechanical device which directly introduces the pressure (negative pressure) at the liquid inlet side and the liquid outlet side of the bubble removing device through a pressure conduit and operates by the pressure difference is used. However, it can also be configured. By providing such a differential pressure valve, when a situation occurs in which the viscosity of the liquid increases and the feed pump causes cavitation, the pressure sensors disposed on the inflow side and the outflow side of the bubble removing device are respectively provided. A negative pressure differential appears directly, and the differential pressure valve is operated electrically or mechanically to increase its opening.

Further, according to another embodiment of the present invention, the protection device includes a heat valve that operates in response to a liquid temperature in an upstream flow path of the bubble removing device. In this case, the liquid is used on the assumption that the viscosity of the liquid is inversely proportional to the temperature. For example, it is intended for general oils such as mineral oil such as machine oil or vegetable oil.

[0014] As with the differential pressure valve, there are two types that are configured electrically and mechanically. The electrically configured one is located upstream of a branch point of the first flow path with the third flow path, which is a bypass flow path, for example, in a filter normally disposed between the liquid tank and the branch point. In this embodiment, a temperature sensor is provided, and an electromagnetic signal is operated via a control circuit by an output signal of the temperature sensor. Also, the mechanically configured one is to directly operate the valve by sensing the liquid temperature in the bypass flow path, and the sensing of the liquid temperature and the operation of the valve are, for example, bimetal and shape memory. A temperature-sensitive metal element such as an alloy is used.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A bubble removing circuit according to the present invention will be described below in detail with reference to the embodiments shown in the accompanying drawings.

FIG. 1 is a schematic diagram of an air bubble removing system according to the present invention, and FIG. 2 is a schematic diagram of a system similar to FIG. 1, in which an electrically operated differential pressure valve is applied as a protection device. FIG. 3 is a system schematic diagram similar to FIG. 2 and shows an embodiment in which a heat valve that operates electrically as a protection device is applied. FIG.
Is a longitudinal sectional view of the bubble removing device constituting the present invention,
FIG. 5 is a cross-sectional view taken along a broken line XX in FIG.

For convenience of explanation, the bubble removing apparatus according to the present invention indicated by reference numeral 100 will be described first with reference to FIGS. 4 and 5. The cone in which the swirling flow chamber 102 is formed by the conical cone 101 whose both ends are closed is vertically arranged with the largest diameter portion downward. An annular preliminary swirl channel 112 surrounding the maximum diameter portion at the lower end of the cone is integrally formed. A liquid supply port 104 is provided at one end of the annular preliminary swirl channel with a peripheral wall surface of the flow channel 112. Are connected in a tangential direction.

As shown in FIG. 5, the annular preliminary swirl flow path 112 is formed so as to make substantially one round of the outer circumference of the maximum diameter portion of the cone 101. The liquid supply port 104 is connected to one end of the annular preliminary swirling flow channel 112, and a guide 113 is provided between the liquid supply port 104 and the other end close to the connection point. , A single opening 114 is formed in the peripheral wall surface of the cone 101 so as to communicate the other end of the annular preliminary swirl flow channel 112 with the swirl flow chamber 2.
The end face of the guide 113 is substantially tangential to the inner peripheral surface of the cone via the opening 114.

A large number of small holes 10 penetrating inside and outside the cone are provided on the peripheral wall surface from the center to the upper end of the cone 101.
6 are drilled. Cone 101 through these small holes
A cylindrical housing 10 surrounding the cone 101 so as to collect the liquid flowing out from the inside and guide the liquid to the liquid outlet 107.
8 is provided integrally with the cone 101 and the annular preliminary swirl channel 112.

Further, the swirling flow chamber 102 in the cone 101
Is provided with a bubble removing tube 109 along the central axis. A large number of small holes 110 penetrating the inside and outside of the tube are formed in the cylindrical peripheral wall of the bubble removing tube 109. The lower end of the bubble removing tube is connected to a bubble outlet 111 penetrating through the center of the bottom of the cone 101, and the bubble portion that enters the bubble removing tube 109 from the swirl flow chamber 102 through the small hole 110 together with a small amount of liquid is returned to the original state. In order to return the liquid storage tank through a drain on which a negative pressure for control acts, the liquid storage tank is led to the bubble discharge port 111.

Next, the operation of the bubble removing apparatus 100 having the above-described configuration will be described. When a liquid containing fine bubbles is sucked into the liquid supply port 104 by suction means such as a pump, the liquid flows from the supply port 104 into the annular preliminary swirl channel 112 along the tangential direction, and the swirl flow And flows through the preliminary swirl channel 112 from one end to the other end,
The outer circumference of the cone 101 is substantially circled.

Due to the centrifugal force generated by the swirling motion, the fine bubbles in the liquid grow and gradually grow into larger bubbles by repeating coalescence and coalescence while gathering in the inner peripheral portion of the annular preliminary swirling flow channel 112. In addition, the high-density liquid containing almost no air bubbles is distributed on the outer peripheral portion of the annular preliminary swirl channel 112. Then, while flowing to the other end of the annular preliminary swirling flow path, the liquid is in a laminar flow state, and flows in the cone 101 from the position where the guide 113 and the opening 114 are formed along the tangential direction while maintaining this laminar state. This causes a strong swirling flow without reducing the turning acceleration.

Bubbles that have become larger due to coalescence in the annular preliminary swirl channel 112 are concentrated in the cone 102 along the rotation axis of the swirling flow, and grow into larger bubbles while repeating coalescence.

Further, the outer peripheral portion of the swirling flow becomes a liquid flow containing almost no air bubbles, rises while swirling along the inner surface of the cone 101, and has many small holes 10 formed in the cone.
From 6, it flows out of the cone and is guided to the liquid discharge hole 107 connected to the feed pump. On the other hand, the bubbles concentrated along the rotation axis of the swirling flow are sucked by the control negative pressure acting on the drain through the small holes 110 into the bubble removing pipe 109, and guided to the bubble discharge port 111.

By the way, the bubble removal rate of the bubble removal device 100 is controlled by a negative pressure acting in a drain disposed downstream. For example, when the negative pressure is increased, a liquid portion discharged together with the bubbles also increases. Although the bubble removal rate per unit time at a constant liquid volume is necessarily reduced, the liquid throughput per unit time is increased. Contrary to this, if the negative pressure is reduced, the bubble removal rate is improved, but the liquid processing amount is decreased. It is determined by suitability.

Next, the air bubble removing system according to the present invention shown in FIG. 1 will be described. Reference numeral 1 denotes a tank storing a bubble-containing liquid, and the above-described bubble removing device 100 is connected to the tank 1 by a first flow path 10. However, a filter 2 for removing unnecessary contaminants is provided upstream of the tank 1 according to a general example.
It is inserted into the channel. Then, the bubble removing device 100
Is connected to the hydraulic pump 3 by a second flow path 11.
Further, the hydraulic pressure regulating valve 4 supplies a pressurized liquid to the working section 5 via the pressure conduit 14b.
The liquid used in the action section 5 is returned to the tank 1 through the return channel 15. At this time, it is also possible to insert the pressure regulating valve 8 into the return flow path 15 according to a general example.

As is apparent from the arrangement of the flow paths and the components, the feed pump 3 is the only liquid pressurizing means, and the first and second flow paths 10 and 11 on the upstream side thereof are provided in the first and second flow paths 10 and 11. The negative pressure acts, and the air bubble removing device 100 and the filter 2
The liquid flow from the tank 1 passing therethrough is sucked up by the negative pressure of the feed pump 3.

The bubble removing circuit composed of the tank 1, the bubble removing device 100, the first flow path 10, the second flow path 11 and the feed pump 3 further bypasses the bubble removing device 100 and the bubble removing device. The protection device 6 is inserted between one branch point a in the first flow path 10 connecting the removal apparatus 100 and the other branch point b in the second flow path 11 connecting the bubble removal apparatus 100 and the feed pump 3. Bypass flow path 12
And a drain 13 on which a control negative pressure acts to return a small amount of liquid discharged from the bubble discharging hole 111 of the bubble removing tube 109 constituting the bubble removing device 100 to the tank together with the bubbles. are doing. The control negative pressure controls the operation of the bubble removing device 100 as described above. The negative pressure generating means 7 includes a hydraulic pressure adjusting valve inserted into the drain 13 and passing through the relief flow path 16. An ejector, an aspirator, a small-capacity pump, or the like that generates a negative pressure by using the excess positive-pressure liquid discharged from 4 is used.

Further, the protection device 6 is provided with a valve which operates for a liquid viscosity equal to or higher than a predetermined value and which operates so as to increase the bypass flow rate of the liquid in proportion to the rise of the liquid viscosity. When the viscosity rises above a predetermined viscosity, the valve starts operating to form a bypass flow path 12 that bypasses the bubble removing device 100 that connects the first flow path 10 and the second flow path 11 at each of the branch points a and b. Then, when the viscosity further increases, the valve opening degree is increased so as to further increase the flow rate of the liquid passing through the bypass channel 12,
For example, a check valve 9 with a spring having a spring constant appropriately selected.
For example, the method can be performed by a simple method without using a special device.

For example, as seen in the case where the liquid is a mineral oil, when the oil temperature becomes lower than a predetermined value, the viscosity increases remarkably. When the amount of oil drops sharply due to the increase in flow resistance, the amount of oil supplied to the feed pump 3 naturally drops, and as a result, cavitation occurs. Since a large negative pressure acts on the valve body, the valve body is moved in a direction in which the valve body is opened against the elastic force of the spring biased in the closing direction, and the bypass passage 12 is opened without opening. Become. If the oil viscosity continues to increase, the valve body is moved so as to further open the bypass passage 12, and the bubble removing device 10
If an unforeseen situation occurs in the downstream of the pump such as clogging of zero, the negative pressure generated by the feed pump becomes abnormally high. A situation occurs in which the bypass channel 12 is completely replaced with respect to the removing device 100.

It should be noted that when selecting the spring constant of the check valve 9, it is necessary to consider that
It is dependent on the flow path resistance of 00. That is, the flow resistance of the check valve 9 exceeds the flow path resistance of the bubble removing device 100, and the difference determines the starting pressure of the check valve.

Further, in a situation where the liquid from the first flow path 10 is partially flowing through the bypass flow path 12, the liquid to the bubble removing device 100 is still flowing, and the original purpose of removing the bubble is Is also continued to some extent, which also contributes to maintaining safe and reliable operation performance of the circulation pump 3 and the fluid devices arranged upstream thereof. Of course, by providing the bypass flow path provided with such a protection device between the first flow path 10 and the second flow path, safe and reliable operation of the bubble removing device 100 and the flow pump 3 is also guaranteed. Will be done.

Further, another embodiment of the air bubble removing system according to the present invention will be described with reference to FIGS. 2 and 4. FIG. The difference from the embodiment of FIG. 1 is that an electrically operated differential pressure valve is applied as a protection device. Therefore, detailed description other than this point is omitted here.

The protection device 6 is constituted by a differential pressure valve 61 which operates by a pressure difference between the liquid inflow side 104 and the liquid outflow side 107 of the bubble removing apparatus 100. In this case, the differential pressure valve 61 converts the negative pressure obtained by the pressure sensors 62 and 63 disposed on the inflow side 104 and the outflow side 107 of the bubble removing device 100 into an electric signal, and the control circuit 64 Is calculated, and the differential pressure valve 61 constituted by an electromagnetic valve can be operated based on the value of the difference.

Although not shown, the liquid inflow side 10 of the bubble removing apparatus 100 is different from this.
The pressure (negative pressure) at the outlet 4 and the outlet side 107 can be introduced directly through a pressure conduit and can also be constituted by a mechanical differential pressure valve which operates with the pressure difference.

By providing such a differential pressure valve 61, if a situation occurs in which the viscosity of the liquid increases and cavitation occurs in the feed pump 3, the bubble removing device 10
A negative pressure differential appears at the pressure sensors 62 and 63 respectively located on the inflow side 104 and the outflow side 107 of the zero, or in the case of a mechanical valve, and
1 is electrically or mechanically operated so as to increase its opening.

Further, another embodiment of the air bubble removing system according to the present invention will be described with reference to FIG.
The difference from the embodiment of FIG. 1 is that a heat valve that operates electrically is applied as a protection device.
Therefore, detailed description other than this point is omitted here.

In this embodiment, the protection device 6 is constituted by a heat valve 71 which operates in response to the temperature of the liquid flowing through the first flow passage 10 which is the upstream flow passage of the bubble removing device 100. In this case, the liquid is used on the assumption that the viscosity of the liquid is inversely proportional to the temperature. For example, it is intended for general oils such as mineral oil such as machine oil or vegetable oil.

The differential pressure valve 61 in the above-described embodiment
Similarly, two types that are configured electrically and mechanically are included. Here, description will be made with reference to FIG.

The filter 2 normally disposed between the liquid tank 1 and the branch point a, for example, upstream of the branch point a of the first flow path 10 with respect to the third flow path 12 which is a bypass flow path. A temperature sensor 72 is provided therein, and a heat valve 71 substantially constituted by an electromagnetic valve is operated via a control circuit 73 by an output signal of the temperature sensor 72. Therefore, the point of using the control circuit and the electromagnetic valve is common to the embodiment shown in FIG. 2, and is basically different in that it operates only by a change in liquid temperature.

Although the structure of the flow path is not shown here because it is the same as that shown in FIG. 1, a mechanically configured one senses the temperature of the liquid in the bypass flow path. In the heat valve 71 having a simple structure in which the valve is directly operated, for sensing the temperature of the liquid and operating the valve, for example, a metal element having temperature sensitivity such as a bimetal or a shape memory alloy is used.

[0042]

The present invention comprises a tank storing a bubble-containing liquid, and a bubble removing circuit arranged with a bubble removing device connected to the tank upstream of the flow pump and interconnecting them. In the bubble removing system, the bubble removing device introduces the bubble-containing liquid tangentially below the cone into a cone provided therein by the suction force of the flow pump and generates a conical rising swirling flow. Generating, separating air bubbles from the liquid along the axis of rotation by centrifugal action and discharging with a small amount of liquid to the outside through an air bubble removal pipe provided along the axis of the cone, A bypass flow path provided with a protection device for connecting the tank side and the feed pump side bypassing the bubble removal device, wherein the bubble removal circuit; and a small amount of liquid discharged from the bubble removal pipe. And a drain connected to the air bubble removing device for returning the air to the tank together with the air bubbles.The control device has a drain on which a negative pressure acts, and the protection device operates for a liquid viscosity equal to or higher than a predetermined value, and It has a structure that has a valve that operates to increase the bypass flow rate of liquid in proportion to the rise, so that when liquid viscosity rises normally, the state where the liquid is passing through the bubble removal device is maintained. While flowing through the bypass flow passage at the same time, a certain amount of air bubbles are removed at the same time, and the liquid with the increased viscosity at the time of cold or at the start of the equipment can be safely and smoothly used in the equipment such as the air bubble removing device and the flow pump. A remarkable effect not found in the prior art, such as being able to flow through the air bubble removal circuit without fear of damaging the device, is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a bubble removal system according to the present invention.

FIG. 2 is a schematic diagram of the air bubble removal system similar to FIG. 1, illustrating an example in which an electrically operated differential pressure valve is applied as a protection device.

3 is a schematic diagram of the air bubble removal system similar to FIG. 2, showing an example in which an electrically differential heat valve is applied as a protection device.

FIG. 4 is a longitudinal sectional view of the air bubble removing device constituting the present invention.

FIG. 5 is a transverse sectional view taken along a broken line XX in FIG. 4;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Tank 2 ... Filter 3 ... Flow pump 4 ... Hydraulic pressure regulating valve 5 ... Working part 6 ... Protective device 7 ... Negative pressure generating means 8 ... Pressure regulating valve 9 ... Check valve with spring 10 ... 1st flow path 11 ... second flow path 12 ... third flow path (bypass flow path) 13 ... drain 14a ... pressure conduit 14b ... pressure conduit 15 ... return flow path 16 ... relief flow path 61 ... differential pressure valve 62 ... pressure sensor 63 ... pressure sensor 64 control circuit 71 heat valve 72 temperature sensor 73 control circuit 100 air bubble removing device 101 cone 102 swirling flow chamber 104 liquid supply port 106 small hole 107 liquid discharge hole 108 housing 109 bubble removal Pipe 110: Small hole 111: Bubble discharge port 112: Annular preliminary swirling channel 113: Guide 114: Opening

Claims (4)

[Claims] 1. A bubble removing device comprising a tank storing a bubble-containing liquid, and a bubble removing circuit disposed upstream of a flow pump and connected to the tank, and a bubble removing circuit connecting these components to each other. In the system, the bubble removing device introduces the bubble-containing liquid tangentially to a lower portion of the cone in a cone provided therein by a suction force of the flow pump and generates a conical rising swirling flow, The bubble removing circuit is configured to separate bubbles from the liquid along the axis of rotation by a separating action and discharge the bubbles together with a small amount of liquid to the outside through a bubble removing pipe provided along the axis of the cone. However, a bypass flow path provided with a protection device that connects the tank side and the feed pump side bypassing the bubble removal device, and a small amount of liquid discharged from the bubble removal tube as bubbles. And a drain connected to the air bubble removing device for returning to the tank and on which a control negative pressure acts. Further, the protection device operates for a liquid viscosity equal to or higher than a predetermined value and increases the liquid viscosity. A valve operative to increase the bypass flow rate of the liquid in proportion to: 2. The air bubble removal system according to claim 1, wherein said protection device comprises a check valve with a spring. 3. The air bubble removal system according to claim 1, wherein the protection device comprises a differential pressure valve that operates by a pressure difference between a liquid inflow side and an outflow side of the air bubble removal apparatus. 4. The air bubble removal system according to claim 1, wherein the protection device comprises a heat valve which operates in response to a liquid temperature in an upstream flow path of the air bubble removal device.