What is a hydraulic valve

What is a hydraulic valve?

Hydraulic valve is a kind of automatic component operated by pressure oil. It is controlled by pressure oil of distributing valve. It is usually combined with electromagnetic distributing valve. It can be used to remotely control the on-off of oil, gas and water pipeline system of hydropower station. Commonly used in clamping, control, lubrication and other oil circuit. There are direct acting type and pilot type, multi-purpose pilot type.

Classification of hydraulic valves

According to the control method classification: manual, electric control, hydraulic control.
According to the function classification: flow valve (throttle valve, speed control valve, shunt valve), pressure valve (relief valve, pressure reducing valve, sequence valve, unloading valve), directional valve (electromagnetic directional valve, manual directional valve, one-way valve, hydraulic control one-way valve).
According to the installation method: plate valve, pipe valve, superposition valve, screw cartridge valve, cover valve.
According to the operation mode: manual valve, motorized valve, electric valve, hydraulic valve, electro-hydraulic valve, etc.

Direction control

It is divided into one-way valve and directional valve according to the application. One way valve: only allow fluid in the pipeline one-way through, reverse that is cut off. Directional valve: change the on-off relationship between different pipelines. According to the working position of the valve core in the valve body, it can be divided into two and three positions; According to the number of channels controlled, it can be divided into two channels, three channels, four channels and five channels; According to the spool drive mode, it can be divided into pneumatic, motorized, electric, hydraulic, etc. In the late 1960s, the electro-hydraulic proportional control valve was developed on the basis of the above hydraulic control valves. Its output (pressure, flow) can change continuously with the input electrical signal. According to different functions, electro-hydraulic proportional control valve is divided into electro-hydraulic proportional pressure control valve, electro-hydraulic proportional flow control valve and electro-hydraulic proportional direction control valve.

Pressure control 

It can be divided into relief valve, pressure reducing valve and sequence valve according to its application.

• (1) Relief valve: can control the hydraulic system to maintain a constant state when reaching the set pressure. Relief valve for overload protection is called safety valve. When the system fails and the pressure rises to the limit value that may cause damage, the valve port will open and overflow to ensure the safety of the system.
• (2) Pressure reducing valve: it can control the branch circuit to get a stable pressure lower than the main circuit oil pressure. Pressure reducing valve can be divided into constant value pressure reducing valve (output pressure is constant value), constant difference pressure reducing valve (input and output pressure difference is constant value) and constant ratio pressure reducing valve (input and output pressure maintain a certain proportion).
• (3) Sequence valve: it can make one actuator (such as hydraulic cylinder, hydraulic motor, etc.) act, and then make other actuators act in sequence.

Flow control

The throttle area between the valve core and the valve body and the local resistance produced by it are used to adjust the flow, so as to control the movement speed of the actuator. The flow control valve can be divided into five types according to its application.

• (1) Throttle valve: after adjusting the throttle area, the movement speed of the actuator with little load pressure change and low motion uniformity requirement can be basically stable.
• (2) Speed control valve: when the load pressure changes, it can keep the pressure difference between the inlet and outlet of the throttle valve as a fixed value. In this way, after the throttle area is set, no matter how the load pressure changes, the speed control valve can keep the flow through the throttle valve unchanged, so that the movement speed of the actuator is stable.
• (3) Diverter valve: regardless of the load size, can make the same oil source of two actuators get equal flow for the same amount of diverter valve or synchronous valve; The proportional flow distribution valve is obtained.
• (4) Collecting valve: opposite to the diverter valve, it makes the flow into the collecting valve distribute in proportion.
• (5) Diverter and collector valve: it has two functions of diverter valve and collector valve.

Types of hydraulic valves

One way valve

One way valve is a kind of one-way valve, in which the fluid can only flow along the water inlet, but the medium at the water outlet can not flow back. One way valve is also called check valve. It is used to prevent reverse flow of oil in hydraulic system or compressed air in pneumatic system.

When installing the check valve, special attention should be paid to the flow direction of the medium. The normal flow direction of the medium should be consistent with the arrow direction indicated on the valve body, otherwise the normal flow of the medium will be cut off. The bottom valve shall be installed at the bottom of the water pump suction pipe.
When the check valve is closed, it will produce water hammer pressure in the pipeline, and in serious cases, it will cause damage to the valve, pipeline or equipment, especially for the large mouth pipeline or high-pressure pipeline, so the check valve selector should pay attention to it.

Directional valve

Directional valve is a directional control valve with more than two flow forms and more than two oil ports. It is a valve to realize the communication, cut-off and reversing of hydraulic oil flow, as well as pressure unloading and sequential action control.
This kind of shift valve is widely used in petroleum and chemical production, and is most commonly used in synthetic ammonia gasification system. In addition, the directional valve can also be made into a disc type structure, which is mostly used for small flow occasions. When working, just turn the hand wheel to change the flow direction of the working fluid through the valve disc.

Directional valve: two position two way

Two position means that the valve core works in two states. When the coil is not electrified, the valve core is in one position, and when it is electrified, it moves to another position; Two way means that the valve has two interfaces (one in and one out).
The two position two-way valve is actually a stop valve, which has no reversing function for the purpose of turning off / opening the piping system.

Directional valve: two position four way

Two position four-way directional valve is suitable for dry oil or thin oil centralized lubrication system to change oil supply direction or open and close oil supply pipeline.
The reversing valve is driven by high torque DC reduction motor, so the reversing action is very reliable even under bad working conditions (such as low temperature or high viscosity grease). In the process of reversing, the piston runs at a high speed in a very short time to reduce the reciprocating, so as to avoid premature wear.
After receiving the commutation signal in the system, the DC motor rotates and converts the rotary motion into the linear reciprocating motion of the piston rod through the eccentric wheel. When the piston moves from one end to the other end and reaches the required reversing position, the limit switch acts to make the DC motor lose power, and then the motor stops rotating, and the reversing process is completed.

Directional valve: three position four way

P is the oil supply port, O is the oil return port, and a and B are the output ports to the actuator.

• When the valve core is in the middle position, all the oil ports are cut off and the actuator does not move;
• When the valve core moves to the right position, P is connected with a, B is connected with O;
• When the valve core moves to the left position, P and B are connected, and a and o are connected.

In this way, the actuator can move forward and backward.

Manual directional valve

Manual directional valve is a kind of directional control valve which relies on the force of manual lever to drive the spool movement to realize the on-off or switching of oil circuit.
The function of manual directional valve in hydraulic system is the same as that of electromagnetic directional valve. It is easy to operate, reliable and without electricity.

Throttle valve

Throttle valve is a valve that controls fluid flow by changing throttle section or throttle length.

Throttle valve has no flow negative feedback function, which can not compensate the speed instability caused by load change. It is generally only used in the situation of little load change or low speed stability requirements.
There is no difference between the shape structure of throttle valve and globe valve, but the shape of their opening and closing parts is different. Most of the opening and closing parts of the throttle valve are cone-shaped and streamlined, through which the flow and pressure can be adjusted by changing the cross-sectional area of the channel. The throttle valve is used to reduce the medium pressure when the pressure drop is great.
The flow velocity of the medium between the throttle disc and the valve seat is very high, which causes the surface of these parts to be damaged quickly – the so-called cavitation phenomenon. In order to minimize the impact of cavitation, the valve disc is made of cavitation resistant material (alloy steel) and made into streamlined cone with a sharp angle of 140-180, which can also make the valve disc have a larger opening height. Throttling under small gap is generally not recommended.

Diverter and collector valve

Diverter valve, also known as synchronous valve, is an independent hydraulic device integrating the functions of hydraulic diverter valve and collecting valve.

There are many ways to realize synchronous movement, but among them, the synchronous control hydraulic system with shunt and collecting valve and synchronous valve has many advantages, such as simple structure, low cost, easy manufacture and strong reliability, so the synchronous valve has been widely used in the hydraulic system.

Sequence valve

Sequence valve is a kind of valve which controls the action sequence of the actuator according to the pressure of the circuit in the system with more than two branch circuits.

According to the different assembly structure, sequence valve can realize different loop functions, such as relief valve, sequence valve and balance valve.
If the opening and closing characteristics of the sequence valve are too bad, the primary pressure will be too high when the flow is large, and the efficiency of the circuit will be reduced. The opening and closing characteristics have hysteresis loop, the opening pressure is lower than the closing pressure, and attention should be paid when the load flow changes.
When opening the valve with too low pressure, the front leakage occurs when the pressure is lower than the set pressure, which causes the actuator misoperation. When the flow through the valve is far less than the rated flow, vibration or other instability will occur. At this time, measures should be taken on the circuit.

Speed control valve

The speed regulating valve is composed of a constant differential pressure reducing valve and a throttle valve in series. It is a throttle valve with pressure compensation.

The throttle valve is used to regulate the flow through, and the constant differential pressure reducing valve automatically compensates the influence of load change, so that the pressure difference before and after the throttle valve is a constant value, which eliminates the influence of load change on the flow.
When the load pressure increases, the hydraulic pressure acting on the left end of the pressure reducing valve increases, the valve core moves to the right, the pressure reducing port increases, and the pressure drop decreases, so that the pressure difference of the throttle valve remains unchanged; vice versa. In this way, the flow of speed control valve is constant (not affected by load).
Speed control valve can also be designed to throttle first and then depressurize.

Relief valve

Overflow valve is a kind of hydraulic pressure control valve, which mainly plays the role of constant pressure overflow, stabilization, system unloading and safety protection in hydraulic equipment.

Constant pressure overflow function: in the throttling regulation system of the quantitative pump, the quantitative pump provides constant flow. When the system pressure increases, the flow demand will decrease. At this time, the relief valve is opened to make the excess flow overflow back to the oil tank and ensure the inlet pressure of the relief valve, that is, the pump outlet pressure is constant (the valve port is often opened with pressure fluctuation).
Pressure stabilizing function: the relief valve is connected in series on the oil return road, the relief valve produces back pressure, and the stability of the moving parts increases.
Unloading function of the system: a small flow overflow solenoid valve is connected in series with the remote control port of the overflow valve. When the solenoid is energized, the remote control port of the overflow valve is connected to the oil tank, and the hydraulic pump is unloaded. The relief valve is now used as an unloading valve.
Safety protection function: when the system works normally, the valve is closed. Only when the load exceeds the specified limit (the system pressure exceeds the set pressure), the overflow will be opened for overload protection, so that the system pressure will not increase (usually the set pressure of the overflow valve is 10% – 20% higher than the maximum working pressure of the system).
The main requirements for relief valve are: large pressure regulating range, small pressure regulating deviation, small pressure swing, sensitive action, large overload capacity and low noise.

Pilot operated relief valve

The pilot relief valve acts on the pressure measuring surface of the main valve core and the pilot valve core, and is composed of the pilot valve and the main valve.
When working, the hydraulic pressure acts on the pressure measuring surface of the main valve core and the pilot valve core at the same time.

Pressure relief valve

The pressure reducing valve is a kind of valve that reduces the inlet pressure to a certain required outlet pressure by regulation and keeps the outlet pressure stable automatically by relying on the energy of the medium itself.
From the point of view of fluid mechanics, the pressure reducing valve is a throttling element whose local resistance can be changed, that is, by changing the throttling area, the flow velocity and the kinetic energy of the fluid can be changed, resulting in different pressure loss, so as to achieve the purpose of reducing pressure. Then, the fluctuation of the pressure behind the valve is balanced with the spring force by the regulation of the control and regulation system, so that the pressure behind the valve remains constant within a certain error range.

Cartridge valve

The cartridge valve is different from the common hydraulic control valve. Its flow rate can reach 1000L / min and its diameter can reach 200-250mm.

The valve core has simple structure, sensitive action and good sealing performance. Its function is relatively single, mainly to realize the on or off of the hydraulic circuit. When it is combined with the common hydraulic control valve, it can realize the control of the system oil direction, pressure and flow.

Advantages of hydraulic valve

• 1) Flexible action, reliable function, small impact and vibration, low noise and long service life.
• 2) When the fluid passes through the hydraulic valve, the pressure loss is small; When the valve port is closed, it has good sealing performance, small internal leakage and no external leakage.
• 3) The controlled parameters (pressure or flow) are stable, and the variation is small under external interference.
• 4) Compact structure, easy installation, debugging, use and maintenance, good versatility.

Application and maintenance of hydraulic valve

When designing hydraulic valves, we should first consider whether the hydraulic valves can meet the requirements of the whole hydraulic valve group, and whether each functional requirement of each hydraulic machinery can be realized. Finally, we have to judge whether the design is carried out according to the principle of the whole hydraulic system. Although the hydraulic valve group is composed of a certain number of hydraulic valves, there are still many components in it, and the number of these components can neither be too much nor reduced. If the number of these components is too much, it will increase the burden of the hydraulic valve group, that is, the design is unreasonable. Once the number of components is too small, the oil circuit integration will lose part of its function, fail to achieve the expected effect, and waste materials.

Design principle of hydraulic valve

The design of the hydraulic valve is mainly for the design of the hydraulic valve group. Before the design of the hydraulic valve group, the oil circuit must be considered. It is necessary to determine which part of the oil circuit can be integrated in advance. The design of the oil circuit must be simple and unnecessary steps should be omitted. After determining the oil circuit, the main thing is inclined hole and process hole. These things on the oil circuit should be reduced, so long as they are enough, it is unnecessary to have too many. In the design of inclined hole and process hole, attention should be paid to the collocation of aperture and flow, the direction and position must be appropriate, and the overall situation should be considered to ensure that the requirements are met. If the direction or position is not suitable and the components need to be adjusted, it is necessary to ensure simple and convenient operation and maintenance.
The design of hydraulic valve is discussed from the design dimension of hydraulic valve. The length, width and height of the hydraulic valve group are generally determined by referring to the size of the parts that make up the hydraulic valve group. For example, the height of the hydraulic valve group should be consistent with the height of the components as far as possible without affecting the actual effect. The length of the hydraulic valve group is determined by the diameter or length of the screw hole without affecting the layout and structure. The width and length of the hydraulic valve group are determined by roughly the same factors. Secondly, we introduce dimensioning. This step is mainly used in design and drawing. We need to mark the port, aperture, size of each group and so on of some components. Finally, the design of the channel, in which the most important is the layout, the overall arrangement of the system, the classification of the oil, first of all must make the main oil conduction, and then, for small oil or other oil gradually conduction. The passage of these hydraulic valves, in the design, must ensure that the length is appropriate, the angle is less, the hole of the oil passage is appropriate, so as to effectively control the weight, size and volume of the hydraulic valve group, and make rational use of it.

Application of hydraulic valve group

In the present situation, according to different situations and different uses of hydraulic valves, hydraulic valves can be divided into many kinds. This article focuses on one of them. This kind of hydraulic valve group includes four parts, which is a two-way cartridge valve composed of cartridge, control cover plate, pilot control valve and manifold block.
The main contents and functions of these four parts are introduced below. The structure of the insert can be considered as a slide valve or poppet valve, which plays an important role in controlling the flow direction, flow speed and pressure of the oil in the channel or other places. The control cover plate can effectively control the real-time working condition of the cartridge valve, mainly because there are many pilot control components in its composition, which can adjust or control the working condition of the cartridge valve. The control cover plate is basically a bridge connecting the control valve and components, and the pilot control valve should also be installed on it. The two-way cartridge valve is relatively common in China. Considering the economy, because its components and necessary pipeline connections are relatively few, it is easy to integrate, so it is very convenient, suitable for mass production, which can greatly reduce the production cost. From the perspective of use, its structure determines that its volume will be very small, so it is very convenient, and its control switch speed is very high, so it is very efficient and reliable in use. In addition, it can also be controlled with high power, which has the advantage of reducing the loss of pressure and heat. Finally, the two-way cartridge valve is not affected by the commutation. The main function of two-way cartridge valve is to connect or control the oil line to be disconnected.

Maintenance of hydraulic valve

With the growth of time, the hydraulic valve will inevitably have some faults, which involves the maintenance of the hydraulic valve. In the actual production, the maintenance of the hydraulic valve is mainly timely cleaning, the combination and matching of some parts, and the repair of the size. There is oil in the channel of the hydraulic valve, it is easy to have oil precipitation, which will make the hydraulic valve have some unnecessary faults, so we must clean up in time, dismantle and clean the hydraulic valve, so as to restore the function of the hydraulic valve in time. Pay attention to safety when cleaning. A lot of sediments are not easy to clean, and some cleaning fluids are needed. These cleaning fluids are generally corrosive or toxic, and even some are flammable, so be careful. The parts after cleaning must be well kept to prevent corrosion. After cleaning, the installed hydraulic valve can continue to operate after testing. If part of the hydraulic valve parts are damaged, and more serious, in this case, the general choice of re combination matching. If the hydraulic valve is maintained by special experienced personnel, its service life will be greatly increased, and its reliability and working performance will be maintained at the original level. Even so, we should be vigilant in daily production to prevent accidents.

Maintenance method of hydraulic valve

In the actual use of hydraulic equipment, due to various factors, there may be some faults in the hydraulic valve, which has a negative impact on the normal use of hydraulic equipment. Therefore, it is an important means to maintain the fault of hydraulic valve through scientific and reasonable methods and better solve it. As the hydraulic equipment management and maintenance personnel, they should have a detailed understanding of the hydraulic valve maintenance method, and master it skillfully, so as to effectively deal with the hydraulic valve fault and ensure the good operation of the hydraulic equipment.
1. Cleaning hydraulic valve
In the process of hydraulic valve maintenance, the disassembly and cleaning of hydraulic valve belongs to the first process. In the actual use of hydraulic equipment, due to the hydraulic oil pollution will lead to oil precipitation, or some granular impurities in the hydraulic oil, which will lead to the failure of the hydraulic valve. Usually, these failures can be eliminated after disassembly and cleaning, so that the hydraulic valve can restore its function. For the disassembly and cleaning of hydraulic valve, it mainly includes the following five aspects:

• (1) Remove. For the hydraulic valve, although most of its parts are connected by bolts, the hydraulic valve is not disassembled when it is designed. If it is forcibly disassembled due to the lack of special equipment or professional technology, the result may be damage to the hydraulic valve. Therefore, before disassembly, the maintenance personnel should master the structure of the hydraulic valve, and should master the connection mode between various parts. During the disassembly process, the position relationship between different parts should be recorded.
• (2) Check and clean. Check the valve body, valve core and other parts to observe the dirt deposition. On the basis of no damage to the working surface, use brush, cotton yarn and non-metallic scraper to remove the concentrated dirt.
• (3) Rough wash. The valve core and valve body are placed on the tray of the cleaning box, heated and soaked, compressed air is filled at the bottom of the cleaning tank, and the residual dirt is cleaned by using the stirring effect of bubbles. Ultrasonic cleaning is feasible if conditions permit.
• (4) Fine. Firstly, high pressure positioning cleaning was carried out with cleaning solution, and then hot air drying was carried out. If the enterprise has the conditions, it can choose the existing fresheners. In some special occasions, it can also use organic cleaning agents, such as gasoline and diesel.
• (5) Assembly. Assemble according to the schematic diagram of hydraulic valve or the assembly relationship of parts recorded during disassembly. Pay attention to careful operation during assembly to prevent parts from being damaged. For some original sealing materials, they are easy to be damaged in the actual disassembly process, so they should be replaced during assembly.

2. Optional assembly parts maintenance
In the process of manufacturing hydraulic valves, in order to effectively improve the assembly accuracy, most of them choose the matching method. That is to say, for a batch of processed parts, such as valve core and valve body, the most reasonable matching clearance should be selected based on the actual size, and then the assembly should be carried out, So that the valve core can ensure good sliding performance and good sealing performance. In other words, for the same type of hydraulic valve, there may be some differences in the matching size between the valve body and the valve core. In the actual use process, when there are a large number of failed hydraulic valves in the enterprise, all the hydraulic valves can be disassembled and cleaned, and the parts can be inspected and measured, and the parts can be classified according to the test results, Then, a certain method is used to select the combination again. After the inspection, if it is found that the valve body and valve core are worn, but there is no serious scratch on the working surface or there is no serious wear in some parts, the valve body and valve core can be reassembled with appropriate clearance; If the clearance between the valve body and the valve is larger than that specified in the product drawing, and the increase degree is between 20% and 25%, a certain method should be taken to increase the size of the valve core, and then research and repair. As for the seat and spool in the cone valve group, if the contact surface of the cone seat valve is not well sealed, because the cone valve can automatically compensate the clearance under the action of the spring, it can only be ground. If the wear of the valve body and the valve is not uniform or there is scratch on the working surface, and the function of the hydraulic valve can not be recovered well by using the above methods, the interference in the machining allowance requirements should be met when selecting the valve body and the valve core. For the valve body hole, it should be reamed properly, and for the valve core, it should be ground properly, So that the shape accuracy and assembly clearance can be reasonable.
3. Dimension Recovery repair
For repair and maintenance process, it includes many kinds. Brush plating maintenance method is more suitable and widely used in hydraulic valve maintenance, which is also called electric coating maintenance. For the repair and repair method of electro plating, the reasonable repair thickness is within 0.12mm, which can basically meet the maintenance requirements of uniform wear hydraulic valve, and it still needs further processing after repair. In the repair and maintenance of electro coating, the common process is chemical composite electro coating, which is based on the mature electro coating process. Its advantages are convenient operation, simple equipment, low cost, and easy to control its reaction, The composite coating with more components can be precipitated on the surface of valve hole or valve core by this process. The coating can be firmly combined with parent metal, and has higher mechanical strength, better thermal conductivity, lower coefficient of thermal expansion, lower coefficient of friction and stronger self repairing ability, Therefore, this method can get ideal effect in the maintenance of hydraulic valve.

Source: China Valve Supplier: www.epowermetals.com

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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