What is the directional valve

What is the 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. By the spool and valve body relative movement direction control valve. There are rotary valve type and slide valve type. According to the number of working positions the valve core stays in the valve body, it can be divided into two positions, three positions, etc; According to the number of oil lines connected with the valve body, it is divided into two-way, three-way, four-way and six way; There are manual, mobile, electric, hydraulic, electro-hydraulic and other modes to operate the spool movement.

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Directional valve, also known as Chris valve, is a kind of valve with multi-directional adjustable channel, which can timely change the flow direction of fluid. It can be divided into manual directional valve, electromagnetic directional valve, electro-hydraulic directional valve, etc.
When working, the drive shaft is rotated by the drive transmission mechanism outside the valve, the crank arm is shaken, and the valve plate is started, so that the working fluid sometimes flows from the left inlet to the lower outlet of the valve, and sometimes from the right inlet to the lower outlet, so as to realize the purpose of periodic change of flow direction.
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.

Working principle of directional valve

The six way directional valve is mainly composed of valve body, seal assembly, cam, stem, handle and valve cover (Fig. 1). The valve is driven by the handle, which drives the stem and cam to rotate. The cam has the function of positioning drive and locking the opening and closing of the seal assembly. When the handle rotates counterclockwise, the two groups of sealing components close the two channels at the lower end under the action of the cam, and the two channels at the upper end are respectively connected with the inlet of the pipeline device. On the contrary, the two channels at the upper end are closed, and the two channels at the lower end are connected with the inlet of the pipeline device to realize the non-stop reversing.

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Fig. 1 six way directional valve

1. Upper valve cover 2. Handle 3. Stem 4. Cam 5. Seal assembly 6. Valve cover 7. Valve body
(1) The valve body of the six way valve is divided into two chambers by the diaphragm, and each chamber has three channels, with oil inlet in the middle and oil outlet at both ends. The valve body is of carbon steel plate welding structure with small volume, light weight and compact structure, which improves the utilization rate of materials, shortens the production cycle and reduces the cost. The sealing surface is overlaid with stainless steel to prevent rust and corrosion. The sealing surface is polished and ground after finishing, and the surface roughness Ra ≤ 0.8 μm.
(2) The six way valve has two sets of seal assemblies. Each group of sealing components (Fig. 2) is composed of valve disc, sealing ring, adjusting block, adjusting screw, clamping plate and bolt. The disc is a carbon steel plate weldment with stiffeners, which not only increases the strength of the disc, but also plays a guiding role to ensure the coaxiality between each group of disc. The disc is inlaid with polyurethane rubber ring, which has the characteristics of oil resistance, wear resistance, stable performance, good sealing and long service life. Under the action of the cam, the spherical surface of the sealing ring contacts with the sealing surface of the valve body to produce extrusion elastic deformation, so as to achieve the sealing effect. The adjusting block and the adjusting screw can play an adjusting role when the two groups of sealing components can not be synchronized in place to ensure that the sealing performance of each channel is synchronized in place.
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Figure 2 seal assembly

1. Splint 2. Bolt 3. Adjust block 4. Disc 5. Seal ring 6. Adjusting screw
(3) O-ring is used for axial seal between valve stem, valve body diaphragm and upper valve cover.
(4) The diaphragm of valve body and the shaft hole of upper valve cover are inlaid with copper sleeve, which can reduce the friction torque with O-ring. The sealing components can be opened and closed flexibly, and the operating torque is small.
(5) The upper valve cover is equipped with indicator board and limit screw, and the valve stem is equipped with pointer to clearly indicate the connection status of each channel, which is easy to operate.

Classification of reversing valve

  • (1) Motor reversing valve, motor-operated valve is also called travel valve.
  • (2) The solenoid valve is a kind of directional control valve which uses the electromagnetic attraction to control the valve core shifting.
  • (3) Electrohydraulic reversing valve is a compound valve composed of solenoid valve and hydraulic reversing valve.
  • (4) Manual reversing valve, manual reversing valve is a direction control valve which uses the hand push lever to control the valve core position change.

Advantages of the directional valve

The operation is accurate, automatic and stable, but the driving and cooling system is needed, which is complex in structure; Valve disc structure is simple, and it is mostly used in the production process with small flow.
In the fields of petroleum, chemical, mining and metallurgy, six way reversing valve is an important fluid reversing equipment. The valve is installed in the pipeline of the thin oil lubrication system to deliver the lubricating oil. Through changing the relative position of the seal assembly in the valve body, the passage of the valve body is connected or disconnected, so as to control the fluid reversing and start and stop.

Common failure causes and treatment measures of directional valve

Leakage

Leakage is a common fault of manual reversing valve, and its main reason is as follows:
(1) Loose coupling screws. The screw is not tightened enough during the coupling or the vibration of the machine during operation may cause the coupling screw to loosen. At this time, just tighten with a suitable tool.
(2) The seal ring is missing or the sealing ring fails. There are several performances of sealing ring failure:

  • ① Appearance rupture;
  • ② Local flattening;
  • ③ Corrosion damage;
  • ④ The elasticity is poor.

The exclusion is to replace the new sealing ring.
(3) The oil pressure is too high. Excessive oil pressure can also cause external leakage. At this time, the pressure can be read out from the pressure gauge to determine whether the pressure is too high. If the pressure is higher than the system pressure, it means that the pressure is too high. Elimination method: adjust the system pressure through pressure valve.
(4) Rough mounting surface. The surface roughness of installation does not meet the requirements, which may cause external leakage. Removal method: grind the mounting surface to make its roughness meet the product requirements. Generally, the roughness value of the valve mounting surface is not greater than 0.8 μ m。
For the above four reasons, the order of inspection is: first check the pressure gauge, then check whether the end cover is loose, finally open the valve, check the sealing ring, and measure the roughness of the installation surface. This order embodies the principle of “easy to difficult” and “from outside to inside”.

Valve core cannot move

The main failure of manual reversing valve is that the valve core cannot move, and the reasons are as follows:
(1) The valve core and the valve body bore fit the gap is not proper. When the diameter of valve core is less than 20mm, the normal fit clearance is within 0.008mm to 0.015mm; When the diameter of valve core is greater than 20mm, the normal fit clearance is within the range of 0.015mm to 0.025mm. The clearance is too large, the valve core is skewed in the valve body, which makes the valve core stuck. At this time, the valve core needs to be re matched, and electroplating process can be adopted. The clearance is too small, friction resistance increases, and the valve core cannot move. At this time, the valve core needs to be ground.
(2) The valve core is deformed. There are three main reasons for valve core deformation:

  • ① The oil temperature is too high, which makes the valve core heat deformation stuck. At this time, find out the reason for the oil temperature rise and reduce the oil temperature;
  • ② The valve core is stuck by bending deformation due to external force, and it needs to be corrected or replaced;
  • ③ The valve core surface is scratched and the internal hole of the valve body is scratched. At this time, open the reversing valve, clean carefully and grind and repair the valve body.

(3) Other reasons. For example, the manual link is worn or failed, the oil pollution and the choke valve core, the spring elasticity does not match, the oil viscosity is too large, etc. When the manual connecting rod is worn or fails, it needs to be replaced or repaired; When the oil pollution is stuck or the viscosity of the oil is too large, the valve core should be cleaned carefully or the oil should be replaced; When spring elasticity does not match, replace spring.

The actuator has no action or slow action

The actuator has no action or slow action, which is a fault phenomenon and is caused by the fault of the reversing valve. The causes of this failure are:

  • (1) The connecting pin of handle and valve core is loose. The coupling pin needs to be refitted.
  • (2) The valve core is worn seriously and the internal leakage increases. The increase of internal leakage makes a large amount of oil flow directly to the return chamber from the high pressure area inside the valve, so that the actuator does not act or acts slowly. At this time, the valve core needs to be replaced.
  • (3) Oil pollution is clogging the oil port. Oil contamination clogging the oil port may also cause the actuator to be inactive or slow. At this time, there is usually too high oil pressure. The way to remove is to clean the valve and the pipeline.

Source: China Directional Valve Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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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