Optimizing PTFE Valve Seat Design for High-Pressure Floating Ball Valves

Abstract: Polytetrafluoroethylene (PTFE) is widely regarded as an ideal sealing material due to its exceptional corrosion resistance and near-zero solubility, making it particularly suitable for use in ball valve seats. However, PTFE is susceptible to cold flow under high pressure, leading to plastic deformation and sealing failure—an issue particularly pronounced in large floating ball valves. Furthermore, under conditions of high pressure, high-speed fluid impact, and elevated temperatures, PTFE seats may experience bending fatigue, resulting in shear-induced leakage. This study presents a structural optimization of the PTFE valve seat using finite element analysis. The results indicate that increasing the seat length, eliminating deformation grooves, and enhancing support and anti-tilt features significantly improve seat stiffness, structural stability, and installation reliability. These optimizations make the PTFE seat better suited for high-pressure, large-diameter floating ball valve applications.

Polytetrafluoroethylene (PTFE), commercially known as Teflon, is often called the "King of Plastics" because of its exceptional corrosion resistance. It was first synthesized by DuPont in 1938. With its excellent chemical stability, low friction coefficient, corrosion resistance, and wide operating temperature range, PTFE is widely used in industries such as chemical processing, electronics, machinery, food, and pharmaceuticals. PTFE is resistant to acids, alkalis, and nearly all organic solvents, and it is virtually insoluble in all known chemicals. This makes it an ideal material for corrosion-resistant sealing applications, especially in ball valve seats. Among commonly used soft-sealing materials, PTFE has the lowest friction coefficient, excellent self-lubricating properties, and minimal water absorption, all of which contribute to reducing the operating torque of ball valves. However, PTFE exhibits cold flow behavior—under high stress at ambient or low temperatures, its molecular chains can slip, leading to irreversible plastic deformation. This deformation compromises sealing performance and increases the risk of leakage, particularly in large-diameter floating ball valves. To address these limitations, this study proposes a structural redesign of the PTFE valve seat. By analyzing common failure modes and conducting finite element modeling under stress conditions, effective optimization strategies are identified. These findings offer valuable insights for improving the reliability of PTFE valve seats in large-scale, high-pressure floating ball valve systems.

Floating ball valves equipped with PTFE seats typically perform well in low-pressure tests but are prone to leakage during high-pressure testing. Furthermore, after undergoing high-pressure testing, these valves may begin to leak even during subsequent low-pressure tests—particularly in valves with diameters of DN100 and above (Problem 1). Upon disassembly, it is often observed that the sealing surfaces between the valve seat and the ball no longer make full contact. A localized depression typically appears at the leakage point on the seat, indicating permanent deformation. In more severe instances, the valve seat displays trumpet-shaped deformation, which is attributed to the cold flow behavior of PTFE, as shown in Figure 1. Although high-pressure testing is an optional requirement under standards such as GB/T 13927 (Industrial Valve Pressure Testing) and GB/T 26480 (Inspection and Testing of Valves), it is still recommended to optimize the PTFE seat structure to enhance sealing performance and reliability under high-pressure conditions. 

Another common issue occurs during actual pipeline and system operation, where the PTFE seat may be sheared off during valve actuation—particularly during closing (Problem 2), as shown in Figure 2. This typically occurs under conditions of high pressure, high flow rate, and elevated temperature. Valve opening and closing speed also plays a significant role. The root cause lies in the inherently low strength and poor rigidity of PTFE, which allows the seat to become dislodged from its groove. During valve opening (as the ball rotates 90° from the closed position), the flow channel between the ball and the seat gradually widens, causing the flow rate to first increase and then decrease.

During this transition, one side of the seat begins to separate from the ball surface, reaching maximum separation at approximately 45° of rotation. The changing flow dynamics can cause the seat to detach from its groove, but as the ball continues to rotate, the seat is forced back into place. The same process occurs during closing. As the ball rotates 90° back to the closed position, the flow passage narrows and the flow rate increases. Once again, one side of the seat separates from the ball surface, with maximum separation occurring at approximately 45°. The increased flow velocity raises the drag force on the seat, which may cause it to be dislodged from the groove before being pushed back as the valve fully closes.

During operation, the ball valve cycles between open and closed positions. Throughout this process, the valve seat is repeatedly subjected to drag forces from the fluid and compression from the ball. These repeated stresses cause the seat to bend and fatigue, resulting in deformation that prevents it from returning to its proper installation position, as shown in Figure 2(a). Eventually, the uneven stress distribution during compression causes the valve seat to shear. This shearing typically occurs during the closing stroke, as illustrated in Figure 2(b), with the shear point usually located along the edge of the valve seat, as shown in Figure 2(c). Shearing of the valve seat is especially common in large-diameter floating ball valves. Once sheared, the valve seat’s sealing performance is severely compromised, resulting in significant leakage. Therefore, when floating ball valves with PTFE seats operate under such conditions, the seat structure must be optimized to prevent shearing caused by bending fatigue and deformation.

Figure 1 Valve seat leakage observed during high-pressure testing

Using the NPS 4 floating ball valve seat as an example, the seat has an inner diameter of 106 mm, an outer diameter of 137 mm, a length of 14 mm, and a sealing surface width of 4.3 mm. This seat design is compact, with a clearance fit between the outer diameter of the seat and the valve body. A deformation groove is located at the back of the seat, while the inner-diameter support section is relatively short, measuring only 1.2 mm, as shown in Figure 3. SolidWorks software was used to create a 3D model of the valve seat and its associated components. Since the primary focus of this analysis is the valve seat, it was modeled in full detail, while the valve body and ball were simplified to reduce computational complexity. The complete model is shown in Figure 4, and the material properties of the valve seat are listed in Table 1. The boundary conditions are shown in Figure 5. To simulate installation and pressure-loading conditions, the end face and outer circumference of the valve body were defined as fixed supports. The ball was constrained to prevent rotation but allowed radial movement, while axial displacement remained unrestricted, as indicated by the green arrows in Figure 5. The contacts between the valve body, valve seat, and ball were defined using global contact conditions with no penetration. During the simulation, a uniform medium pressure of 4 MPa was applied perpendicular to the ball surface, as shown by the red arrows in Figure 5.

(a) Valve seat misalignment (b) Shearing location (c) Valve seat after disassembly

Figure 2 Schematic diagram of valve seat shearing

Figure 3 Valve seat structure before optimization

Figure 4 3D model of valve seat and mating components

Table 1 PIFE parameters

Figure 5 Boundary conditions and external loads

Since the valve seat is the primary focus of the analysis, mesh control was applied to increase mesh density specifically in the valve seat area, thereby enhancing the accuracy of the simulation results. For the valve body and ball, a Jacobian quality parameter of 4 points was used, with a global element size of 10 mm and a mesh tolerance of 0.5 mm. The resulting mesh division is shown in Figure 6.

(a) Valve seat (b) Overall model

Figure 6 Mesh division results
 

Finite element analysis was conducted using SolidWorks Simulation. The stress distribution and deformation results for the valve seat are shown in Figures 7 and 8, respectively. The stress distribution results reveal a highly uneven stress field, with significant stress concentration extending from the valve seat’s sealing surface toward its back end. In this region, the stress exceeds the material’s yield strength, making the valve seat prone to cold flow deformation. The deformation results indicate that the maximum displacement occurs at the valve seat’s sealing surface. For visualization, the deformation is magnified tenfold, clearly revealing a flared (or trumpet-shaped) deformation.

(a) Overall view (b) Cross-sectional view

Figure 7 Stress distribution of valve seat

(a) Overall deformation diagram (b) Cross-sectional deformation

Figure 8 Valve seat deformation
 

Analysis of Problem 1 shows that the primary cause of valve seat leakage under high pressure is permanent plastic deformation of the sealing surface due to cold flow of PTFE. This deformation occurs when the stress on the valve seat exceeds the material’s yield strength. Therefore, enhancing the structural strength of the valve seat is crucial to resolving this issue. Besides improving the material properties of PTFE, optimizing the valve seat’s structure can significantly reduce stress and improve support. As shown in Figure 3, the original PTFE valve seat is prone to permanent plastic deformation under high pressure due to insufficient support within the valve seat groove and excessive stress concentration. Therefore, the valve seat structure should be optimized in two key areas: minimizing stress concentration under high pressure and enhancing the mechanical support of the valve seat.

Problem 2 stems from the low rigidity of the PTFE valve seat and its excessive freedom within the valve seat groove. During valve operation, one side of the seat is subjected to repeated drag and compression, causing fatigue deformation that can ultimately lead to shearing. Therefore, increasing the seat’s rigidity and controlling its movement (or “swing”) during operation are essential to prevent this type of failure.

Based on the root causes of Problems 1 and 2, the valve seat structure was optimized as follows:

• Removal of the deformation groove at the back of the original valve seat to enhance rear support;
• Extension of the insertion depth into the valve seat groove by 10 mm to increase overall support;
• Lengthening of the inner and outer diameter support sections by 10 mm to improve rigidity;
• Addition of a bell-mouth structure at the outer edge of the valve seat groove to limit lateral movement.

As shown in Figure 9, the outer section of the valve seat groove has been redesigned with a reduced-diameter bell-mouth structure that creates an interference fit with the outer diameter of the valve seat. Note that the interference fit should be moderate to prevent installation difficulties. Once installed, the bell-mouth structure firmly secures the valve seat within the groove, minimizing swing and improving operational stability.

After optimization, the inner diameter of the NPS4 floating ball valve seat remains 106 mm, the outer diameter is 137 mm, and the length has been increased to 24 mm. The sealing surface width remains 4.3 mm. The outer diameter of the valve seat now forms an interference fit with the valve body. The deformation groove on the back of the valve seat has been removed, and the inner diameter support section has been extended to 11.2 mm. The parameter settings and simulation methods remained the same as before. Finite element analysis of the valve seat was performed using SolidWorks Simulation software. The stress distribution and deformation results are shown in Figures 10 and 11, respectively. The stress distribution results show that the maximum stress still occurs at the sealing surface of the valve seat. However, compared to the pre-optimization condition, the overall stress distribution is significantly more uniform, exhibiting smaller stress gradients and no abrupt changes. The high stress at the sealing surface is now more effectively transferred and dissipated throughout the valve seat structure. This improvement is due to the increased distance between the sealing surface and the valve seat groove in the optimized design, which creates a longer rigid contact path and a larger stress transition zone.

As shown in the deformation results, the maximum displacement still occurs at the sealing surface. When magnified tenfold, the valve seat exhibits a uniform expansion, and the previously observed trumpet-shaped deformation is no longer present. This improvement results from the removal of the deformation groove and the increased insertion depth of the valve seat into the groove, which effectively lengthens the overall seat. Consequently, the valve seat’s resistance to axial deformation is significantly enhanced. Additionally, extending the inner and outer support sections of the groove enhances the mechanical support, thereby indirectly improving the overall strength and stability of the valve seat.

Figure 9 Optimized valve seat structure

(a) Overall stress distribution of the valve seat (b) Cross-sectional stress distribution of the valve seat
Figure 10 Stress distribution of the valve seat

(a) Overall deformation distribution of the valve seat (b) Cross-sectional deformation distribution of the valve seat

Figure 11 Deformation of the valve seat

Based on a comprehensive analysis of two common issues in floating ball valves with polytetrafluoroethylene (PTFE) valve seats, along with stress analysis following structural optimization, the following conclusions can be drawn:

• Before optimization, the PTFE valve seat had a compact structure but lacked sufficient rigidity. Due to its high freedom of movement within the valve seat groove and inadequate support, the valve seat experienced uneven internal stress distribution under high-pressure conditions, leading to significant deformation, localized stress concentration, and trumpet-shaped distortion.
• Under identical pressure conditions, the optimized valve seat demonstrated a more uniform internal stress distribution and more consistent overall deformation. No obvious trumpet-shaped deformation was observed, indicating a significant improvement in performance under high-pressure and high-impact conditions.
• By incorporating a limiter to restrict the valve seat’s movement within the groove, fluid drag—caused by one side of the valve seat lifting during valve operation—was effectively mitigated. This design also reduced the risk of deformation and shearing due to bending fatigue.
• The optimized valve seat structure significantly improves rigidity, support strength, and installation stability, providing robust technical support for the use of PTFE valve seats in large-diameter, high-pressure floating ball valves. This optimization holds considerable practical engineering significance