Created on:2022-04-06 09:11

Analysis and preventive measures of oil leakage of welded pipe joint

Analysis and preventive measures of oil leakage of welded pipe joint

Welded pipe joint has the advantages of simple processing, convenient disassembly and high pressure resistance. It is widely used in the hydraulic pipeline system of heavy machinery such as metallurgy, mining and forging. It can ensure the normal operation of the equipment under the working pressure of 32Mpa and the ambient temperature of - 25 ~ 80 ℃. However, oil leakage at the connecting parts of pipe joints caused by various factors occurs from time to time, which leads to equipment action failure and even accidents. The causes of oil leakage of pipe joint are analyzed from the aspects of machining quality of body and pipe joint connection, pipeline stress, welding and assembly methods, hydraulic system design and so on, and some improvement measures are put forward.

1. Oil leakage caused by large machining error

(1) Oil leakage caused by excessive Perpendicularity Error between threaded hole and end face. The welded pipe joint and the machine body are mainly connected by metric fine thread with good self-locking performance, and the combined sealing washer is pressed by the end face to prevent oil leakage. If the perpendicularity error between the threaded hole and the end face is too large, the installation shield combined sealing washer can only be pressed locally. When the system pressure changes or the oil temperature increases and the viscosity decreases, there will be oil leakage. Therefore, when the metric thread is connected, the threaded bottom hole shall be drilled on the end face of the machine body. While tapping the thread, the drilling depth shall be 1mm and the roughness Ra shall be 3.2um. The diameter of the plane pit perpendicular to the threaded hole shall be greater than the outer diameter of the combined sealing washer used. Only in this way can the perpendicularity of the connecting threaded hole of the machine body and the end face (plane pit) be ensured. After sealing with combined sealing gasket, the part will not leak oil even when the system temperature reaches 60 ℃.

(2) Oil leakage caused by excessive machining error of O-ring groove. When processing the joint body, if the ring groove depth of the 0-shaped seal ring is too deep, the compression deformation of the O-shaped seal ring after assembly is too small to ensure the sealing; The depth of the ring groove is too shallow. During assembly, the end face of the connecting pipe compresses the O-ring to make it seriously deformed and cracked, which destroys the sealing mechanism and will also lead to oil leakage at this part. Therefore, when processing the joint body, the ring groove size for placing the O-ring seal must be processed according to the standard requirements to avoid oil leakage due to poor sealing of the O-ring seal caused by processing error.

2. Oil leakage of pipe joint caused by pipeline stress

(1) Causes of pipeline stress. The usual piping method is to install the joint body with a connecting pipe and screw it on the body or oil circuit block. After aligning the connecting pipe of the steel pipe, spot weld several places around it, remove it and then weld the circumference. This method will produce welding stress. In addition, the stress caused by improper pipeline assembly method will cause pipeline stress to the steel pipe welded with the connecting pipe. Due to the small contact area between the nut end face and the connecting pipe, the pipe stress on the steel pipe can not be offset, resulting in the inclination of the joint surface between the joint body and the connecting pipe, resulting in oil leakage due to the failure of the connecting pipe to press the O-ring, as shown in Figure 79.

(2) Method of eliminating pipeline stress. When assembling the pipeline, the stress caused by the torsion and bending of the steel pipe can be eliminated through the correct assembly process. The welding stress caused by the incorrect welding method in the welding process can not be eliminated by gas welding, red baking pipeline or other heat treatment. The methods to solve the welding stress are as follows.

1) When piping the system, the joint body shall be installed with a combined sealing washer and then tightened on the machine body or oil circuit block.

2) Select two cold drawn seamless steel pipes that are consistent with the pipe diameter and material of the connecting pipe, make its length a little longer than the actual size, weld the connecting pipe directly with one end of the two steel pipes, and preheat the pipe with thick pipe wall before welding.

3) After cooling, tighten the connecting pipe and the joint body at both ends with nuts. This time, do not put in the O-ring seal, and pay attention to holding the steel pipe flat so that the nuts can firmly press the connecting pipe. After the two ends are firmly connected, cut off the long part.

4) The length of seamless steel pipes on both sides has been appropriate, and the nuts at both ends should be tightened to ensure that the two ends face correctly when the hand leaves the steel pipe. Trim the end faces of the two steel pipes to leave a gap of 0.5mm in the middle, and then weld the butt joint circumference of the two steel pipes firmly, that is, there is one more weld in the middle of an oil pipeline, and then remove it after cooling.

5) After the pipeline is welded, the slag and spatter at and around the weld shall be cleaned, and the pressure test shall be carried out (the test pressure is 1.5 ~ 2 times of the maximum working pressure). If it is unqualified, repair welding shall be carried out, and the repair times of the same part shall not exceed three times.

6) According to the purification process regulations, all pipelines shall be thoroughly cleaned before the second installation. At this time, all kinds of sealing rings shall be installed and the connecting parts shall be tightened.

The welding stress can be completely eliminated only by configuring the system pipeline according to the above steps. Even if the system pressure reaches 32Mpa and the oil temperature rises to 60 ℃, the connecting part of the pipe joint will not leak oil.

3. Oil leakage caused by unreasonable structure of welded pipe joint

In order to avoid oil leakage caused by pipeline stress or processing error of O-ring groove on the joint body and damage of O-ring and nut, the welded pipe joint with improved structure shown in Figure 80 can be adopted if the installation of system pipeline allows. The connecting thread between the pipe joint and the machine body can be metric fine thread or tapered pipe thread. When the taper pipe thread connection is adopted, because the taper pipe thread and the combined sealing washer play the sealing role at the same time, the leak proof effect is better. The following points should be paid attention to when adopting and designing the pipe joint with improved structure.

(1)       Reduce the distance from the hexagon to the thread as much as possible to improve the stress condition of the thread.

(2)       Correctly design and process the D and e values to ensure that the combined sealing gasket can be installed smoothly. When the pipe joint has sufficient strength, the combined sealing gasket has appropriate compression when tightening the pipe joint.

4. Oil leakage of pipe joint caused by improper pipeline assembly procedure

All configured pipelines can be installed only after they are thoroughly cleaned. When installing the system pipeline, the forced torsion and bending of the steel pipe and the excessive force when installing the joint body or tightening the nut connecting the joint body and the connecting pipe will cause the pipeline stress caused by its deformation, thread damage, nut crack and excessive deformation, which will lead to oil leakage at the connecting part. Therefore, pay attention to the following points during the final installation of the system pipeline.

(l) Ensure that the groove edges and corners of the joints, nuts and sealing rings used are free of scars, burrs or broken threads.

(2)       Select the sealing ring that meets the standard requirements and is compatible with the medium. Do not use the aging, deformed or defective sealing ring. The used sealing ring cannot be reused.

(3)       When tightening the joint body and nut, the tightening torque shall be uniform and comply with the regulations of the component factory. Do not knock or pull with a hammer.

(4)       After the O-ring seal is installed on the joint body, the nut pre installed on the connecting pipe can be screwed in. When screwing in and tightening the nut, both ends shall be carried out at the same time, and the steel pipe shall not be rotated to prevent the nut at one end from being screwed too tightly to damage the thread, nut and O-shaped seal, while the clearance at the other end is too large to ensure the seal.

(5) Keep the axes of pipe joints and steel pipes in the same straight line to avoid forced torsion and irregular bending of pipes. In order to reduce the pipeline vibration, the long pipeline shall be fixed with damping support or pipe clamp.

5. Oil leakage of pipe joint caused by system pressure impact

When the liquid flow in the pipeline system changes direction or stagnates rapidly, the pressure in the system will change sharply, forming an instantaneous pressure peak several times larger than the normal pressure, resulting in the hydraulic impact of the system. The hydraulic shock will also cause the vibration of the system. The pressure shock and vibration of the system often loosen the pipe joints and nuts. In serious cases, it will lead to cracks or even fractures in the weld and heat affected zone of the welding part, which will not only leak oil, but also easily cause accidents. Therefore, reducing pressure impact is also an important link to prevent oil leakage of welded pipe joints, which can be avoided by taking the following measures.

(1)       Avoid the mechanical vibration of the equipment and the emergency braking under the moving condition of the actuator.

(2)       When designing the hydraulic system, pay attention to the design of anti impact circuit and overload protection circuit to avoid the forward impact phenomenon and impact load of the actuator.

(3) Select hydraulic components with small pressure pulsation and minimize the use of pipe joints.


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