How to Improve Casting & Sealing of Oil Wellhead Gate Valves

The gate valve used in oil wellhead equipment, commonly known as the "Christmas tree", is essential in oil field operations. These valves are widely used in the oil extraction process to control the flow of oil by opening and closing as needed. However, the current wellhead gate valves have a relatively low qualification rate in production, especially during hydrostatic strength and sealing performance tests, with rates ranging from just 30% to 70%. These issues not only impact the lifespan of the equipment but also reduce operational efficiency. This article analyzes the causes of the low qualification rate and offers systematic improvements to significantly enhance the quality and performance of these products.

Casting Issues and Solutions for Gate Valve Bodies

The body of a wellhead gate valve is typically cast using a sand mold, which is cost-effective but prone to certain unavoidable defects. During valve body processing, defects such as pores and shrinkage cavities often appear around the internal threads that connect with the valve seat. A macro analysis of valve body sections reveals varying degrees of shrinkage in hot spot areas. To address these issues, several casting improvements were implemented.

1. Optimizing the Gating System

In the original design, the gating system was positioned on the cylindrical sides of the valve body, causing uneven metal flow and increasing the risk of shrinkage and porosity. The improvement involved moving the gating system to the bottom of the valve body and using a horizontal gating system, allowing the metal to fill from the bottom up for even flow and reduced defects.

2. Switching from Sand Casting to Investment Casting

While sand casting is affordable, it lacks adequate permeability and uniform cooling, which often results in porosity and shrinkage. By switching to investment casting, metal cools more evenly, enhancing the density and eliminating common casting defects. This method produces a more solid valve body structure, significantly boosting overall quality.

3. Improving the Pouring Method

Traditional side pouring can cause uneven cooling in specific areas. Switching to top pouring with the flange facing downward allows the metal to flow directly from the riser into hot spots, achieving sequential solidification. Investment casting also offers better permeability and faster cooling, which improves structural integrity and reduces the likelihood of shrinkage cavities.
These improvements effectively resolved casting issues, resulting in a denser valve body structure. The qualification rate increased from the original 30%-70% to over 99%.

Enhancing Sealing Performance

Sealing performance is critical to gate valve quality, directly affecting the equipment's safety and lifespan. However, achieving a strong seal has traditionally been a challenge in machining. To improve the sealing effect, several manufacturing process upgrades were introduced.

1. Ensuring Precise Machining of the Valve Body's Inclined Surface

The structure of the gate valve requires the sealing surface to have an inclined angle of 30° to 60° with the central symmetry plane. A horizontal lathe combined with a custom angled fixture was used to achieve this, ensuring the fixture's tilt matches the design requirements. The center height of the machine and the fixture was kept consistent, with a tolerance range of 0.02mm to 0.04mm, ensuring machining precision.

2. Improving Positioning and Clamping Reliability in Machining

To achieve precise positioning and clamping in a single setup, the lathe's vertical spindle surface was used as a reference, with a wedge groove and wedge block for precise alignment. This allows for accurate positioning of one end of the valve flange, ensuring the concentricity of the sealing surfaces.

3. High-Precision Rotary Machining Control

The inner cavity of the valve body needs to be machined by rotating 180° to process both ends. To ensure rotational precision, two symmetrically placed conical positioning pinholes were used, controlling the rotational error within 0.01mm to 0.02mm.

4. Optimizing Fixture Balance Structure

The fixture's balance affects machining stability. By optimizing the counterweight structure of the lathe fixture, reducing the rotational radius, and using an outer-round, inner-square layout, dynamic balance was improved, enhancing machine speed and machining precision.

5. Coordinated Machining of Sealing Components

To ensure high coordination accuracy between sealing components, tighter control over the dimensions and surface finish of each part was implemented, ensuring that each sealing surface fit precisely, thus improving the overall sealing performance.

Results and Benefits of the Improvements

Through these process and manufacturing improvements, the quality of the oil wellhead gate valve was significantly enhanced. The casting improvements produced a denser valve body, minimizing pores and shrinkage and increasing the qualification rate. Furthermore, the enhanced sealing performance extended the valve's lifespan and pressure resistance, meeting oil field demands for high reliability and strong seals. These technical improvements not only boosted production efficiency but also ensured long-term stability for gate valves in oil field applications.

Conclusion

The production quality of oil wellhead gate valves has a direct impact on oil field operational efficiency and safety. The improvements made in casting methods, machining precision, and sealing performance have significantly enhanced the overall performance of the gate valve, raising the qualification rate to over 99%. These advancements not only meet the oil industry's demand for high-quality gate valves but also provide valuable insights for the future manufacture of other wellhead equipment.