Structural Design and Stress Analysis of a Pressure-Activated Plug Valve

Drill pipe plug valves are a critical valve of well control equipment in major oilfield drilling projects, valued for their flexibility and reliability. However, after a blowout occurs, conventional plug valves become difficult to reopen, hindering well-killing operations and other critical processes. To address the limitations of existing plug valves, this paper introduces a pressure-activated plug valve. The new valve eliminates rotation failure, ensures effective sealing, and enables efficient execution of well-killing operations. Additionally, a theoretical model of the interaction between the ball valve core and the trunnion valve seat was developed. Finite element analysis software (ANSYS) was employed to analyze the contact stress during valve closure.

This paper outlines the advantages of the pressure-activated plug valve, including improved performance and reliability. With minimal structural modifications, it establishes a new flow channel when conventional plug valves fail to rotate, facilitating uninterrupted well-killing operations. Compared to traditional designs, it resolves jamming issues and ensures reliable operation. The valve can consistently open a new flow path under high internal pressure, addressing the failure risks of conventional plug valves in high-pressure environments. This innovative design enhances on-site well control safety and offers significant potential for oilfield operations.

1. Structural and Performance Comparison of Conventional and Newly Designed Plug Valves

1.1 Structure and Characteristics of Conventional Plug Valves

Conventional plug valves used in major oilfields typically feature a self-aligning structure and consist of a valve body, four sealing rings, an elastic retaining ring, fixed and movable valve seats, a ball valve core, a knob block, and a wave spring. Before drilling begins, the knob is turned to actuate the ball valve core. During normal drilling operations, the plug valve remains open, allowing uninterrupted flow of drilling fluid. In the event of a blowout risk, the knob is turned to rotate the ball valve core by 90°. The combined effect of high-pressure blowout fluid and the wave spring’s force compresses the contact surface between the ball valve core and the valve seats, forming a metal-to-metal seal. This effectively blocks the high-pressure fluid and prevents a blowout.

This type of plug valve offers several advantages. First, it enables quick and straightforward operation during normal operations, as the ball valve core can be opened and closed by turning the knob. Second, when closed, it forms a reliable metal-to-metal seal that minimizes leakage. Third, its simple design, lightweight structure, and compact size contribute to lower manufacturing costs.

However, conventional plug valves also have notable drawbacks. During a blowout, once the ball valve core is closed, the combined force of the high-pressure fluid and the wave spring exerts substantial stress on the ball valve core and the valve seats. While this prevents fluid escape, the resulting contact stress can lead to deformation of the ball valve core. More critically, excessive contact stress significantly raises frictional resistance. When the pressure differential exceeds 20 MPa, manual force may be inadequate to turn the knob, resulting in valve rotation failure. Additionally, prolonged use can cause significant wear to the hexagonal hole in the knob block, diminishing torque transmission efficiency and making it even harder to operate the ball valve core under high pressure.

1.2 Structure and Characteristics of the Newly Designed Plug Valve

To address the common issues associated with conventional plug valves, this paper presents a newly designed plug valve that can be pressure-activated, as shown in Figure 1. The valve body, sealing rings, elastic retaining ring, fixed and movable valve seats, ball valve core, wave spring, and knob turn block are similar to those in conventional plug valves. However, the movable valve body and wave spring are housed in a cavity with a radius greater than that of the ball seat and ball valve core. The movable valve body is secured by a fixed limit pin.

1. Valve body 2. Elastic retaining ring 3. Fixed valve seat 4. Four-open ring 5. Knob turn block 6. Ball valve core 7. Wave spring 8. Movable valve seat 9. Fixed limit pin

Figure 1 The structure of a newly designed plug valve with pressure opening

During normal drilling operations, the valve operates similarly to a conventional plug valve, with the ball valve core remaining open to allow the smooth passage of drilling fluid. In the event of a blowout, the ball valve core is rotated 90° by manually adjusting the knob. Under the high-pressure blowout fluid, the upper valve seat and the ball valve core form a tight metal seal, effectively blocking the fluid and preventing the blowout. Once the blowout risk is mitigated and the ball valve core is closed, the fixed limit pin is rotated counterclockwise to release the movable valve seat. The pressure differential causes the high-pressure well-killing fluid to push the ball valve core and movable valve body downward into the cavity, releasing the valve from its closed position in the pipe. This opens a new flow path within the cavity, allowing high-pressure well-killing fluid to pass through the valve and facilitating well-killing operations.

Additionally, this plug valve minimizes deformation of the ball valve core. As the ball valve core and movable valve body move into the cavity, the high-pressure fluid inside the cavity balances the external pressure on the ball valve core, significantly reducing deformation. Although the newly designed plug valve retains a similar structure to the conventional design, its key difference lies in the movable valve body’s ability to achieve axial movement. This movement is guided by the fixed limit pin, which restricts displacement. This design minimizes structural changes, thereby reducing manufacturing costs. By incorporating an enlarged cavity and limit mechanism, the design creates a straightforward flow path that prevents high-pressure valve failure, enabling efficient well control operations such as well-killing.

2. Theoretical Analysis of Contact Stress Between Main Sealing Surfaces

This paper presents an analysis of the mechanical model of contact stress between the fixed valve seat and the ball valve core in scenarios where the plug valve cannot rotate until the fixed limit pin is removed. In the sealed state, the contact surface of the spherical valve core tightly engages the fixed valve seat under the pressure of the blowout fluid. The theoretical model of contact stress is developed using the finite element method. Figure 2 illustrates the forces acting on the spherical valve core. The contact surface between the fixed valve seat and the spherical valve core forms an annular sealing area on the ball.

Figure 2 Force analysis of the spherical valve core

The contact stress calculation is based on the following formula:

Derived from the formula：

In this formula, σ represents the contact pressure between the spherical valve core and the fixed valve seat, R is the radius of the spherical valve core, and P denotes the pressure of the blowout fluid within the pipe. The variables α1 and α2 represent the angles formed between the line connecting the center of the spherical valve core to the upper and lower boundary lines of the contact surface and the central axis of the plug valve.

The formula shows that when the blowout fluid pressure (P) is fixed, the contact pressure (σ) between the spherical valve core and the fixed valve seat depends solely on α1 and α2. The parameters of the plug valve in this study are as follows: α1 = 38°, α2 = 46°, and R = 37.5 mm. Assuming the blowout fluid pressure (P) in the pipe is 70 MPa, calculations using Formula (3-1) yield a contact stress (σ) of 261 MPa between the spherical valve core and the fixed valve seat.

3. Numerical Analysis of Contact Stress in Plug Valve Sealing Surfaces

This study analyzes the contact stress between the ball valve core and the fixed valve seat using ANSYS finite element analysis software. The ball valve core and fixed valve seat are fabricated from 42CrMnMo, a commonly used material in plug valves. The material properties are as follows: elastic modulus of 206 GPa, Poisson's ratio of 0.3, and density of 7850 kg/m³. Using these parameters, the assembly was modeled and analyzed in ANSYS.

Given the symmetrical structure and loading of the plug valve, only one-quarter of the model was used for finite element stress analysis. Following the principle of selecting primary and secondary contact surfaces, the fixed valve seat was designated as the primary sealing surface, and the spherical valve core as the secondary surface, with a friction coefficient of 0.2. A three-dimensional solid element mesh was applied to the critical stress-concentration areas in the models of the spherical valve core and the fixed valve seat. The fixed valve seat consists of 860 elements, while the spherical valve core consists of 7014 elements.

As only one-quarter of the model was used, symmetric boundary conditions were applied along the longitudinal axis, and fixed constraints were imposed on the bottom surface of the fixed valve seat to simulate typical working conditions. A stress load of 70 MPa was applied using the amplitude curve loading method. The resulting distribution of contact stress is shown in Figure 3.

Figure 3 Contact stress cloud diagram of ball valve core and fixed valve seat

The figure indicates that maximum strain occurs at the upper center of the ball valve core. The maximum contact stress on the interface between the ball valve core and the valve seat is 290.1 MPa, closely matching the theoretical calculation of 261 MPa. This confirms the viability of using finite element analysis to evaluate the stress on the sealing surface between the ball valve core and fixed valve seat.

4. Conclusion

(1) The newly designed plug valve presented in this paper addresses the issue of ball valve core rotation failure under high pressure through targeted structural modifications. By introducing a new flow channel, the design reduces the stress on the ball valve core caused by the interaction between well-killing fluid and high-pressure blowout fluid. This design minimizes deformation of the ball valve core, significantly extending the service life of the plug valve.

(2) This paper develops a detailed theoretical model for the ball valve core and the fixed valve seat. The contact stress at the interface between the ball valve core and the fixed valve seat under high pressure is calculated using a formula developed from the theoretical model. Additionally, a finite element model of the newly designed plug valve is created in ANSYS software based on its design parameters, enabling the evaluation of contact stress on the main sealing surfaces. The theoretical results are closely aligned with simulation data from ANSYS, confirming the validity of the model. This method provides a proven approach for analyzing contact stress on metal sealing surfaces in plug valves operating under high-pressure conditions.