Type 10 and Type 80 swivel joints are less likely to be broken during use. The main cause of breakdown is that the chamber is too large, and the chamfered portion is at the 90° bend where the fluid flow changes sharply, the chamfered step creates resistance to the fluid, and the large grained quartz sand is at the chamfered step. Erosion produces a breakdown phenomenon. After finding out the root cause of the breakdown, make the following improvements: change the chamfering step to 10°, and start chamfering at 10 mm from the WN bend joint inlet, and try not to place it at the 90° elbow. After the improvement, the short radius swivel joint no longer produces breakdown.
Breakdown sample analysis
1.1 Raw materials
The material used for short radius swivel joint is 30CrMo round steel, which is made by rolling, perforating and bending. Sampling and analysis of the breakdown sample showed that the chemical composition was within the normal range and met the requirements of GB/T 3077.
1.2 tempering tissue inspection
Short radius swivel joint 30CrMo steel after quenching and tempering normal structure is tempered sorbate, tempered sorbite plus a small amount of lower bainite, is a normal tissue.
1.3 Mechanical properties
After the materials were heat treated and tempered, the average hardness of the three impact samples was 239, 245, and 253 HB, respectively. The hardness and mechanical properties of the materials were in full compliance with the requirements of 75 K in API Spec 16C.
1.4 breakdown site
The geological environment of Changqing Oilfield is complex, the underground rock is relatively hard, and the fluid in the pipeline contains hard quartz sand, which has an obvious scouring effect. The breakdown of the short radius swivel joint of type 10 and type 80 is in the outer arc of the first bend of the circulating fluid inlet. This indicates that the flushing action of quartz sand is one of the causes of breakdown, but it is not the main reason, because it is not All short radius swivel joints have a breakdown.
2 breakdown reasons
Short radius swivel joint is not at the wall thickness but at the wall thickness. After careful analysis of the assembly drawing, it was found that the breakdown was located at the outer arc of the third ballistic. Here at the fluid inlet, there is a 30° chamfer along the direction of the fluid, which creates a resistance to the fluid. When the large-grained quartz sand reaches the chamfering step, it pauses a little and then rushes along the chamfered step to form an etch pit, and the crater gradually expands to cause a breakdown. Therefore, the main cause of breakdown is that the chamber is too large, and the chamfering portion is at the 90° bend where the fluid flow direction changes drastically, the chamfering step creates resistance to the fluid, and the large grain quartz sand is chamfered. Erosion occurs at the place where the breakdown occurs. In addition, the inner hole is irregular during molding, and the step is large and small after processing. This is the reason why the short radius swivel joint is broken down without being broken down.
3 Improvement measures
After finding out the root cause of the breakdown, the following improvements were made: the chamfered step was changed to 10°, and the chamfering was started at 10 mm from the WN bent joint inlet, and was not placed at the 90° elbow. After taking the above improvements, the short radius swivel joint no longer produces breakdown.
(1) When designing high-pressure fluid control components, try not to have chamfered steps.
(2) In the daily construction process of the oil field, when the high-pressure fluid pipeline is arranged, the fewer the corners, the better.
(3) When designing high-pressure fluid control components, if the chamfering steps must exist, try to design a small-angle chamfering step.