Du Ke, Cao Jia, Zhang Yushu, CNPC Drilling Engineering Technology Research Institute
Abstract: A small number of Style 10 and Style 80 rotary unions produced by a company in Jiangsu were penetrated during service. The main cause was an oversized chamfer located exactly at a 90-degree bend. At this point the flow direction changes sharply, the chamfer step creates flow resistance, and large quartz-sand particles erode the step area until washout penetration occurs. After the root cause was identified, the chamfer step was changed to 10 degrees and the chamfer was started 10 mm from the inlet of the WN elbow, avoiding placement at the 90-degree bend as much as possible. After the improvement, washout penetration no longer occurred in the swivel unions.
Keywords: rotary union; swivel joint; washout penetration; chamfer step; improvement
The Style 10 and Style 80 rotary swivel joints, also called movable elbows, were manufactured according to API Spec 16C. During use, a small portion of the products experienced penetration. A combined analysis of the penetrated samples showed three features: the penetration locations were basically the same and were all on the outer curved passage of the WN elbow; penetration occurred at the outside arc of the third raceway; and the penetrated location was not the weakest or thinnest section.
1. Analysis of Penetrated Samples
1.1 Raw Material
The rotary swivel joint was made from 30CrMo round steel. The round steel was produced through rolling, piercing, bending and related processes. Sampling analysis of the penetrated part showed that the chemical composition was within the normal range and met the requirements of GB/T 3077.
1.2 Quenched-and-Tempered Microstructure
After quenching and tempering, the normal microstructure of 30CrMo steel used for the swivel joint is tempered sorbite with a small amount of lower bainite. This is a normal microstructure for the material.
1.3 Mechanical Properties
After heat treatment, the average hardness values measured on three impact specimens were 239, 245 and 253 HB. The hardness and mechanical properties fully met the requirements for 75 K material in API Spec 16C. The mechanical properties of the 30CrMo steel are shown in Table 1.
Table 1. Mechanical properties of 30CrMo steel
| Tensile strength / MPa | Yield strength / MPa | Elongation / % | Reduction of area / % | Impact toughness, longitudinal / J (API Spec 16C: -29 C) |
|---|---|---|---|---|
| 855 | 717 | 19 | 45 | 45, 51, 39 (average 45) |
1.4 Penetration Location
The geological environment of Changqing Oilfield is complex, with hard underground rock. The fluid inside the line contains hard quartz sand, so the scouring effect is obvious. The penetration locations of the Style 10 and Style 80 swivel joints were all on the outer arc of the first bend at the circulating-fluid inlet. This shows that quartz-sand erosion was one cause of penetration, but not the main cause, because not every swivel joint experienced penetration.
2. Causes of Penetration
The swivel joint was penetrated at a thicker wall section rather than at a thinner wall section. Careful analysis of the assembly drawing showed that penetration occurred at the outer arc of the third raceway. This point is at the fluid inlet. Along the flow direction there was a 30-degree chamfer, and this chamfer step created resistance to the fluid. When large quartz-sand particles reached the chamfer step, they paused briefly and then scoured along the step, forming an erosion pit. As the pit gradually enlarged, penetration occurred.
Therefore, the main cause of penetration was that the chamfer was too large and was located exactly at the 90-degree bend. At this position the flow direction changes sharply, the chamfer step resists the flow, and large quartz-sand particles erode the chamfer-step area. In addition, the inner bore at this location was irregular after forming, and the machined step varied in size. This explains why some swivel joints were penetrated while others were not.
3. Improvement Measures
After the root cause was found, the chamfer step was changed to 10 degrees. The chamfer was started 10 mm from the inlet of the WN elbow and was kept away from the 90-degree bend as much as possible. After these measures were adopted, the rotary swivel joints no longer showed washout penetration.
4. Recommendations
1. When designing high-pressure fluid-control components, avoid chamfer steps wherever possible.
2. During routine oilfield operations, high-pressure fluid lines should be arranged with as few bends as practical.
3. If a chamfer step must be used in a high-pressure fluid-control component, it should be designed with a small chamfer angle.
