The 78-105 specification is currently the most widely used non-rising-stem high-pressure slab gate valve in the Tarim Oilfield. Chongqing Xintai is the primary manufacturer of these gate valves. Therefore, the Chongqing Xintai 78-105 high-pressure slab gate valve is selected as a representative case for calculating and analyzing its opening and closing torque.This non-rising-stem slab gate valve is equipped with GB/T 4663 thrust cylindrical roller bearings, with AA taken as 0.005. The valve stem thread specification is T32 × 6 LH-7e, and the equivalent friction coefficient of the stem thread, ff, is taken as 0.1. The friction coefficient between the gate and valve seat, pp, is taken as 0.1.The gate and valve seat adopt a post-seat sealing configuration, while the valve stem is sealed with a PTFE composite sealing ring. The composite sealing ring can withstand pressures of up to 140 MPa, and its friction coefficient can be as low as 0.01 when lubricated with grease.
By substituting the relevant parameters of this valve model into Equation (10) presented in the previous section, the maximum opening and closing torque distribution of the non-rising-stem high-pressure slab gate valve can be calculated, as shown in Figure 6.The maximum opening and closing torque of the 78-105 non-rising-stem slab gate valve is 166.8 N·m, which is close to the measured opening and closing torque of 175 N·m for a new 78-105 slab gate valve under pressure testing at the site. This agreement verifies the accuracy of the mechanical model established for the high-pressure slab gate valve.
The opening and closing torque of the high-pressure slab gate valve consists of the following components:
- 74% from valve stem thread friction torque
- 20% from bearing friction torque
- 6% from valve stem sealing friction torque
Therefore, valve stem friction torque is the primary contributor to the opening and closing torque of the slab gate valve. Accordingly, the failure mechanisms and key factors governing the failure of non-rising-stem high-pressure slab gate valves should be systematically investigated, with particular emphasis on valve stem friction torque. All subsequent calculations of the opening and closing torque of the high-pressure slab gate valve are based on the data for this 78-105 slab gate valve.
Figure 6. Distribution of the Maximum Opening and Closing Torque of the 78-105 High-Pressure slab gate valve
3.2 Analysis of the Failure Mechanism and Key Controlling Factors of the High-Pressure slab gate valve
The valve plate and valve seat of the non-rising-stem high-pressure slab gate valve employ a metal-to-metal sealing structure to prevent slurry leakage and maintain pressure stability in the pipeline system. However, the metal-to-metal seal between the valve plate and valve seat generates substantial friction torque during the opening and closing of the high-pressure slab gate valve. To enhance wear resistance and reduce friction at the sealing surfaces, valve manufacturers worldwide commonly employ surface treatment processes such as hardfacing and surface coating on the valve plate and valve seat. The non-rising-stem high-pressure slab gate valve is repeatedly opened and closed under pressure, causing the friction coefficient of the valve plate–seat sealing surfaces to gradually increase. When the friction coefficient of the valve plate–seat sealing surfaces increases beyond a certain level, the non-rising-stem high-pressure slab gate valve may experience abnormal opening and closing, potentially resulting in failure of the metal seal between the valve plate and valve seat.
A brand-new valve plate and a used valve plate, both of specification 78-105, were selected at the No. 2 Well Control Center of Tarim Oilfield for microscopic surface observation and friction coefficient testing. As shown in Figure 7(a), the surface of the brand-new 78-105 valve plate is smooth, with uniformly distributed sprayed particles, whereas Figure 7(b) shows that the surface of the used 78-105 valve plate exhibits regular parallel scratches and microcracks. This indicates that the coating on the valve plate and valve seat gradually wears away as the number of pressurized opening and closing cycles of the high-pressure slab gate valve increases.
The surface friction coefficients of the two valve plates were measured using a vertical friction and wear testing machine. The test parameters were as follows:
Steel ball diameter: 6 mm
Normal load: 50 N
Reciprocating stroke: 5 mm
Sliding speed: 10 mm/s
Test duration: 900 s
As shown in Figure 8(a), the surface friction coefficient of the brand-new 78-105 valve plate is 0.11, whereas that of the used 78-105 valve plate is 0.17, as shown in Figure 8(b). The surface friction coefficient of the valve plate therefore increases by 54%. Substituting this value into Equation (10) in Section 2 shows that the opening and closing torque of the high-pressure slab gate valve increases by 56%. In summary, the friction coefficient of the valve plate–seat sealing surfaces is one of the key factors governing abnormal opening and closing of the high-pressure slab gate valve.
An increase in the friction coefficient of the valve plate–seat sealing surfaces leads to a rapid increase in valve stem thread friction torque. The underlying failure mechanism is the continuous wear of the valve plate and valve seat surfaces during repeated pressurized opening and closing cycles of the slab gate valve. The surface coating of the valve plate and valve seat gradually wears away and peels off, resulting in smooth scratches on the sealing surfaces. Under severe wear conditions, surface cracks may also develop. Surface wear of the valve plate and valve seat significantly increases the friction coefficient and may eventually cause failure of the metal seal, resulting in abnormal opening and closing of the non-rising-stem slab gate valve.
(a) Microscopic view of the surface of a brand-new valve plate (b) Microscopic view of the surface of a valve plate after use
Figure 7 Microscopic view of the surface of a valve plate in Tarim Oilfield 78-105
(a) Curve of friction coefficient of the surface of a brand new valve plate (b) Curve of friction coefficient of the surface of a valve plate after use
Figure 8 Curve of friction coefficient of the valve plate in friction test of Tarim Oilfield 78-105
As shown by Equation (8) in Section 2, when the non-rising-stem high-pressure slab gate valve is under pressure, an increase in the normal force acting on the valve plate leads to an increase in the friction torque of the valve stem thread. When the high-pressure slab gate valve is under pressure, the medium pressure q is relatively constant, while the normal force acting on the valve plate depends on the sealing mechanism between the valve plate and valve seat.
The metal sealing configuration between the valve plate and valve seat of a high-pressure slab gate valve can be classified as downstream sealing or upstream sealing. Downstream sealing is typically used in single-seat configurations, whereas upstream sealing is commonly used in double-seat configurations. Typical single-seat and double-seat configurations currently used in the Tarim Oilfield are shown in Figures 9 and 10, respectively.
In the single-seat configuration, when the slab gate valve is closed under pressure, the mud medium pushes the valve plate against the downstream seat, creating a self-sealing effect at the contact interface between the valve plate and downstream seat. Consequently, the valve plate is subjected to pressure on only one side. In the double-seat configuration, when the slab gate valve is closed under pressure, both the upstream and downstream seats seal simultaneously. As the mud medium enters under pressure, the inner and outer seats are forced slightly apart. The inner seat remains tightly pressed against the valve plate, while the outer seat remains tightly pressed against the valve body, causing the valve plate to be subjected to pressure on both sides. A sealing ring between the inner and outer seats prevents internal leakage through the gate valve.
Figure 9. Typical Structure of the Single-Seat Valve Currently Used in the Tarim Oilfield
Figure 10. Typical Structure of the Double-Seat Valve Currently Used in the Tarim Oilfield
The single-seat and double-seat non-rising-stem high-pressure slab gate valves currently used in the Tarim Oilfield were selected as the research subjects, and their maximum opening and closing torques were calculated using Equation (10) in Section 2. The maximum opening and closing torque of the single-seat slab gate valve is 166.4 N·m, whereas that of the double-seat slab gate valve is 235.2 N·m, indicating that the opening and closing torque of the double-seat valve is 41.3% higher than that of the single-seat valve. Therefore, the sealing configuration between the valve plate and valve seat is one of the key factors governing the opening and closing torque of the high-pressure slab gate valve.
Although the downstream sealing configuration of the single-seat valve results in a lower opening and closing torque than the upstream sealing configuration of the double-seat valve, its resistance to sand ingress is inferior. When the single-seat high-pressure slab gate valve is closed, the inlet-side valve seat is in a floating state, allowing mud to gradually penetrate and accumulate at the interface between the inlet-side valve seat and the valve body. When a sufficient amount of mud accumulates at this interface, the buildup can force the inlet-side valve seat against the valve plate, causing the valve seat to jam.
As shown in Figure 11, the single-seat non-rising-stem slab gate valves currently used in the Tarim Oilfield are all susceptible to mud and sand accumulation. The current mitigation measure employed in the Tarim Oilfield is to grind and polish the mating surfaces of the valve body and valve seat. Therefore, the currently used non-rising-stem high-pressure slab gate valve has relatively poor resistance to mud and sand ingress, indicating that its sand-prevention performance needs to be further improved.
Under normal operating conditions, the contact interface between the valve plate and valve seat of a slab gate valve is relatively flat and uniform. As shown in Figure 11, mud and sand may accumulate inside the valve body, and the current mitigation measure involves grinding and polishing the mating surfaces of the valve body and valve seat.
Figure 11. Mud and Sand Deposition in a Single-Seat Non-Rising-Stem slab gate valve: (a) Mud and Sand Deposition Inside the Valve Body; (b) Grinding and Polishing of the Valve Body
If the valve plate of a high-pressure slab gate valve undergoes elastic deformation during pressurized opening and closing, the contact interface between the valve plate and valve seat may become uneven, increasing friction between the two components during valve operation and ultimately resulting in abnormal opening and closing of the slab gate valve. To determine whether valve plate deformation significantly affects the pressurized opening and closing torque of the slab gate valve, a contact-friction simulation model of the valve plate–seat pair was established to analyze the contact friction between the rigid valve plate and valve seat during opening and closing. The contact stress distribution between the rigid valve plate and valve seat is shown in Figure 12, while the final variation curve of the contact friction force between the rigid valve plate and valve seat is shown in Figure 13.
The contact friction force of the rigid valve plate differs from that of the elastically deformable valve plate by only approximately 1.9% during opening and closing. This indicates that, under the specified strength conditions, elastic deformation of the valve plate has a relatively minor effect on the opening and closing torque of the slab gate valve.
Figure 12. Contact Stress Distribution of Rigid and Elastic Valve Plates During Opening and Closing: (a) Rigid Valve Plate; (b) Elastic Valve Plate
Figure 13. Opening and Closing Friction Force Curves for Rigid and Elastic Valve Plates
Based on an investigation into failures of existing non-rising-stem high-pressure slab gate valves in the Tarim Oilfield and the development of a mechanical model for the slab gate valve, the model indicates that an increase in the equivalent friction coefficient of the valve stem thread caused by thread wear is one of the key factors governing abnormal opening and closing and eventual failure of the slab gate valve. However, the friction coefficient of the valve stem thread cannot be measured directly because the friction occurs between the metal surfaces of the valve stem thread and valve stem nut. Therefore, the initial equivalent friction coefficient of the valve stem thread is assumed to be 0.1. Assuming that the valve stem thread undergoes continuous wear and that its equivalent friction coefficient increases to 0.3, the resulting opening and closing torque curve of the high-pressure slab gate valve is shown in Figure 14. When the equivalent friction coefficient of the valve stem thread increases from 0.1 to 0.3, the opening and closing torque increases from 166.8 N·m to 321.8 N·m, representing an increase of approximately 93%.
Figure 14. Effect of the Equivalent Friction Coefficient of the Valve Stem Thread on the Opening and Closing Torque of the slab gate valve
The failure mechanism underlying the abnormal opening and closing of the slab gate valve caused by an increase in the equivalent friction coefficient of the valve stem is as follows. The non-rising-stem high-pressure slab gate valves currently used in the Tarim Oilfield primarily employ downstream sealing, which cannot effectively prevent mud from entering the valve cavity and valve stem nut through clearances. As mud continuously infiltrates the valve stem nut, it causes progressive wear of the valve stem thread. As a result, the equivalent friction coefficient of the valve stem thread increases significantly, reducing the thread's transmission efficiency and ultimately causing abnormal opening and closing of the high-pressure slab gate valve. If the valve stem has already experienced wear during actual operation and the slab gate valve continues to be forcibly opened and closed, the valve stem thread may deform or bend, preventing the valve from closing promptly and potentially leading to well-control incidents. Valve stem thread wear and bending have been observed during actual operation in the Tarim Oilfield, as shown in Figure 15.
Figure 15. Valve Stem Thread Wear and Valve Stem Thread Bending: (a) Valve Stem Thread Wear; (b) Valve Stem Thread Bending
As indicated by Equation (8), the diameter and pitch of the valve stem thread are among the key factors governing the opening and closing torque of the slab gate valve. At the same time, the valve stem thread diameter and pitch directly determine the shear strength and other mechanical properties of the thread. The relationships between valve stem thread diameter and opening and closing torque and between thread pitch and opening and closing torque are shown in Figure 16. As shown in Figure 16(a), when the valve stem thread diameter increases from 26 mm to 46 mm, the opening and closing torque of the high-pressure slab gate valve increases by 32%. As shown in Figure 16(b), when the valve stem thread pitch increases from 4 mm to 8 mm, the opening and closing torque of the high-pressure slab gate valve increases by 32%. Compared with the other factors investigated, the valve stem thread diameter and pitch have a relatively minor effect on the opening and closing torque of the high-pressure slab gate valve.
Figure 16. Effects of Valve Stem Thread Parameters on the Opening and Closing Torque of the High-Pressure slab gate valve: (a) Opening and Closing Torque as a Function of Valve Stem Thread Diameter; (b) Opening and Closing Torque as a Function of Valve Stem Thread Pitch
The high-pressure slab gate valve primarily uses thrust cylindrical roller bearings conforming to GB/T 4663. Under lubricated conditions, the friction coefficient of these bearings can be as low as 0.05. However, the bearings of the slab gate valves in the Tarim Oilfield are not routinely maintained, which can result in problems such as corrosion of the bearing raceways and consequently increase the opening and closing torque of the high-pressure slab gate valve. As shown in Figure 17, when the bearing friction coefficient increases from 0.005 to 0.015, representing a 200% increase, the opening and closing torque of the high-pressure slab gate valve increases by 27.8%. Therefore, the bearing friction coefficient has a relatively minor effect on the opening and closing torque of the high-pressure slab gate valve.
When designing a high-pressure slab gate valve with fixed specifications, parameters such as the valve stem thread pitch and diameter, bearing inner diameter, and valve seat dimensions are determined primarily based on requirements such as the valve opening and closing time and the strength of the valve stem thread, after which the valve plate–seat sealing configuration is selected accordingly. The valve stem thread pitch and diameter have relatively minor effects on the opening and closing torque of high-pressure slab gate valves, while increasing the frequency of bearing maintenance can effectively prevent an increase in the bearing friction coefficient. However, the friction coefficients of the valve plate–seat sealing surfaces and the valve stem thread may increase uncontrollably during the pressurized opening and closing of high-pressure slab gate valves. Both friction coefficients increase continuously with repeated pressurized opening and closing of the high-pressure slab gate valve, and the curves showing the variation in the opening and closing torque of the high-pressure slab gate valve with these two friction coefficients are presented in Figure 18.
Based on field surveys and tests of existing high-pressure slab gate valves in the Tarim Oilfield, together with a review of the relevant literature, the surface friction coefficient of the valve plate–seat pair was found to range from 0.10 to 0.17, while the equivalent friction coefficient of the valve stem thread ranges from 0.10 to 0.20. When both friction coefficients reach their maximum values, the opening and closing torque of the high-pressure slab gate valve reaches 486.3 N·m, representing a 191% increase compared with that at the minimum friction coefficients. This substantial increase in torque can result in abnormal opening and closing of the slab gate valve.
Figure 17. Effect of Bearing Friction Coefficient on the Opening and Closing Torque of the High-Pressure slab gate valve
Figure 18. Effects of Valve Plate–Seat and Valve Stem Friction Coefficients on the Opening and Closing Torque of the High-Pressure slab gate valve
For existing high-pressure slab gate valves, preventing failures such as abnormal opening and closing primarily depends on addressing two critical components: the valve seat and the valve stem. Selecting appropriate materials and surface treatment processes, such as surface coatings, for the valve plate and valve seat can reduce the friction coefficient of their sealing surfaces. Designing an effective sand-resistant structure for the valve seat can prevent mud and sand from accumulating at the mating interface between the valve body and valve seat, thereby reducing the risk of valve seat jamming. Selecting appropriate valve stem materials and thread surface treatment processes can improve the shear strength of the valve stem threads while reducing their equivalent friction coefficient. Appropriate machining methods can further improve the dimensional accuracy, surface quality, and wear resistance of the valve stem threads.
With the advancement of ultra-deep drilling technology in oilfields worldwide, the high pressure of drilling mud in ultra-deep wells places increasingly stringent demands on the performance of valve plate and seat materials used in high-pressure slab gate valves. According to standards such as API 6A and API 17D, the selection of valve plate and seat materials for high-pressure slab gate valves primarily depends on the operating pressure and temperature range of the drilling mud. Currently, high-pressure slab gate valves manufactured both domestically and internationally predominantly use FF, HH, and EEO material classes, with stainless steel and nitrided steel being the most commonly used materials for valve plates and seats. According to existing research, the base materials commonly used for the valve plates and seats of domestically manufactured high-pressure slab gate valves are predominantly corrosion-resistant stainless steels, such as 12Cr13 and 35CrMo.
Surface coating technology involves applying a coating material to the surface of the valve plate, valve seat, or valve body substrate to improve wear resistance and corrosion resistance while reducing the friction coefficient of the valve plate–seat sealing surfaces. The application of surface coating technology to the valve plate–seat contact surfaces of slab gate valves has been widely adopted in the petrochemical industry. A review of domestic and international research on coatings for slab gate valve plates and seats indicates that three main types of coating materials are currently used: ceramic coatings, Ni60-based coatings, and tungsten carbide coatings. The relevant parameters of ceramic, Ni60, and tungsten carbide coatings are presented in Table 1.
Ceramic coatings are characterized by high hardness, excellent wear resistance, and superior corrosion resistance. However, the high cost of zirconia and silicon carbide ceramic materials, together with their relatively poor adhesion and susceptibility to delamination, limits their application in high-pressure slab gate valves. Ni60 coatings can be deposited by flame spraying, plasma spraying, and other processes. However, these coatings are prone to pore formation during deposition, which can adversely affect their performance, while their bonding strength with the substrate is relatively limited. Tungsten carbide coatings are primarily deposited using high-velocity oxy-fuel (HVOF) spraying. These coatings exhibit a low coefficient of friction, high bond strength, high hardness, good corrosion resistance, and excellent impact resistance. Therefore, domestic slab gate valve manufacturers predominantly use imported equipment to apply tungsten carbide coatings to the surfaces of valve plates and seats.
The high-pressure slab gate valves presently in service at the Tarim Oilfield are predominantly of a single-seat configuration, which is adopted to minimize the operating torque resulting from differential pressure during opening and closing. However, the single-seat configuration is prone to mud and sand accumulation. To mitigate this problem, an extended valve seat structure can be designed to effectively prevent debris from settling. Figure 19 illustrates the extended valve seat structure, with a 0.5 mm clearance designed between the valve seat and the gate plate.
Multiphase flow and particle-phase behavior were simulated using the k–ε turbulence model and the Discrete Phase Model (DPM) in CFD software to compare and validate the sand-prevention performance of the conventional single-seat and extended valve seat configurations. Flow-field simulation models were developed for the conventional single-seat and extended-seat configurations, and the corresponding drilling fluid parameters were specified as presented in Table 2. The boundary conditions were specified with a pressure of 105 MPa at the inlet valve seat and 0 MPa at the outlet, and the corresponding simulation results are shown in Figure 20. In the flow-field simulations, the extended valve seat produced denser mud velocity streamlines, indicating that, compared with the conventional single-seat configuration, it effectively retards mud deposition and prevents significant drilling fluid accumulation in the lower valve chamber.
Table 1 Surface Coating Parameters
Coating Type | Hardness / HV | Bond Strength / MPa | Porosity / % |
Ceramic Coating | 1000 | 70 | 1.0 |
Ni60 Coating | 800 | 60 | 1.05 |
Tungsten Carbide Coating | 1400 | 90 | 0.5 |
Figure 19 Extended Valve Seat Structure
Table 2 Drilling Fluid (Mud) Parameter Settings
Parameter | Value |
Particle Shape | Circular |
Particle Diameter / (×10⁻⁶ m) | 0–2 |
Volume Fraction / % | 8 |
Mass Flow Rate / (kg·s⁻¹) |
|
(a) Flow Field of Ordinary Single Valve Seat (b) Flow Field of Extended Valve Seat
Figure 20 Simulation Results of Flow Field of Ordinary Single Valve Seat and Extended Valve Seat
The valve stem is a critical component of a high-pressure slab gate valve, serving as the actuating element for opening and closing the valve. Consequently, the selection of an appropriate valve stem material is of paramount importance. Studies show that the majority of high-pressure slab gate valves currently on the Chinese market are made of 318 stainless steel, a high-alloy grade renowned for its outstanding corrosion resistance. The most commonly employed material, 318 stainless steel, exhibits a tensile strength of ≥785 MPa and a hardness of ≤40 HRC. Nevertheless, in practical use, valve stems of this grade are often found to experience thread wear and bending. This thread wear and bending indicate that the shear strength of the valve stem thread is insufficient to meet the strength requirements when the opening and closing torque of the slab gate valve increases.
Based on the material grade requirements of API standards, Inconel 718 emerges as a viable option for high-pressure slab gate valves. As a high-grade nickel-based superalloy, it is distinguished by its excellent shear resistance, corrosion resistance, and high-temperature stability. Inconel 718 valve stems are priced at 140 RMB/kg in bulk, compared with 80 RMB/kg for 318 stainless steel stems, reflecting a higher material cost for Inconel 718. However, compared with 318 stainless steel, Inconel 718 offers a 13% improvement in shear strength and a 33% increase in hardness. This alloy is typically subjected to solution annealing and age hardening treatments, and its microstructure is shown in Figure 21.
Figure 21. Microscopic Image of the 718 Sample
The microstructure of Inconel 718 is primarily composed of an austenitic matrix, together with α′ (Ni₃(Al,Ti)), γ″ (Ni₃Nb), and δ (Ni₃Nb) precipitate phases. These precipitates contribute to precipitation strengthening, thereby increasing the strength and hardness of the alloy. The friction coefficients of the valve stem surfaces of both materials were determined using a vertical friction and wear tester. The friction tests were conducted under the following conditions: a 6 mm-diameter steel ball, an applied load of 50 N, a reciprocating stroke of 5 mm, a sliding speed of 10 mm/s, and a testing duration of 100 s.
Friction tests were performed on newly manufactured valve stem samples made of 318 stainless steel and Inconel 718, and their surface friction coefficients were recorded. The corresponding friction coefficient curves are presented in Figures 22 and 23. The test results indicate that Inconel 718 exhibits a lower friction coefficient than 318 stainless steel, demonstrating the superior wear resistance of this nickel-based superalloy compared with 318 stainless steel.
Figure 22. Friction Coefficient Curve of the New 318 Stainless Steel Valve Stem Material
Figure 23. Friction Coefficient Curve of the New Inconel 718 Valve Stem Material
Unlike valve plates and seats, valve stem threads are not amenable to spray coating techniques. Therefore, the standard practice both in China and abroad is to apply thermosetting anti-friction coatings, most commonly MOLYKOTE 106, to valve stem threads. These coatings provide both high load-bearing capacity and a low coefficient of friction.
Trapezoidal threads on valve stems are primarily machined by turning, milling, or roller burnishing, with turning being the conventional approach. In turning, an appropriate cutting tool is mounted, and the thread is generated by rotating the valve stem while feeding the tool at the prescribed pitch and lead angle. Milling employs a rotary cutter with axial valve stem feed to produce the trapezoidal thread in multiple passes, whereas roller burnishing is a chipless forming method that uses a profiling roller die to plastically deform the stem material into the required thread profile.
An analytical model for thread rolling was developed using slip-line field (SLF) theory, and experimental rolling tests were carried out. The findings indicate that trapezoidal thread rolling offers superior surface quality, a lower friction coefficient, and improved material efficiency compared with turning and milling. Therefore, for the future development of higher-specification high-pressure slab gate valves, the large-scale production of valve stems with rolled trapezoidal threads will be a key strategy for reducing the opening and closing torque.
(1) An investigation of the maintenance records of high-pressure slab gate valves in the Tarim Oilfield over the past three years revealed the predominant failure modes and failure-prone components. Based on these findings, a mechanical model of the valve stem was developed.
(2) Using the identified failure-prone components and the established mechanical model of the valve stem, the failure mechanisms of the high-pressure slab gate valve were elucidated through computational analysis. A comprehensive analysis of the primary factors governing valve failure identified two uncontrollable parameters: the friction coefficient of the valve plate–seat pair and the equivalent friction coefficient of the valve stem thread.
(3) Based on the failure mechanism analysis and identification of the primary controlling factors, targeted countermeasures were proposed for the key components of the high-pressure slab gate valve to effectively prevent valve failure. These findings provide a foundation for further research and development of ultra-high-pressure slab gate valves.
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