Failure Mechanisms and Torque Control of High-Pressure Slab Gate Valves (Part One)

Slab gate valves are critical components of choke and kill manifolds in oil and gas drilling operations, primarily used for well shut-in and flow-path switching. A systematic analysis was conducted on the service history of 189 choke-manifold assemblies covering three specifications and both domestic and international brands in the Tarim Basin over a three-year period. The investigation identified abnormal torque as the predominant failure mode of high-pressure slab gate valves, accounting for 55 of 69 documented failures (79.7%).

Key factors affecting valve opening and closing torque include:

· Plate–seat interface friction

· Valve stem friction

· Bearing friction

These factors contribute approximately 74%, 20%, and 6% of the total torque, respectively.

The main failure mechanisms involve:

· Plate–seat interface failure

· Valve stem failure

· Bearing failure

To reduce valve torque and prevent operational anomalies, several countermeasures were proposed:

· Optimizing valve plate and seat materials and their manufacturing processes to reduce surface friction

· Establishing replacement criteria when the plate–seat friction coefficient exceeds acceptable limits

· Designing sand-proof valve seat structures to prevent rock-cutting accumulation

· Using high-strength, high-hardness valve stem materials

· Applying rolling processing to improve stem surface smoothness and hardness

These measures can reduce the opening and closing torque of high-pressure slab gate valves, lower failure rates, and improve the safety and reliability of oil and gas drilling operations.

In recent years, major oilfields in China have been advancing ultra-deep drilling projects. The Tarim Oilfield is a notable example. The Takol Well reached a depth of more than 10,000 meters, becoming the first well in Asia to exceed the 10,000-meter vertical depth. Ultra-deep and extra-deep oil and gas exploration and development involve significant well-control risks, placing stringent performance requirements on well-control systems.

Choke and kill manifolds play a critical role in:

· Controlling well overflows

· Preventing blowouts

· Maintaining well-pressure balance

· Ensuring well-control safety

The choke and kill manifolds currently used in the Tarim Oilfield are shown in Figure 1.

(a) Current choke manifold (b) Current kill manifold

Figure 1 Current choke and kill manifolds in Tarim Oilfield

Among the components of choke and kill manifolds, high-pressure slab gate valves are the most numerous. High-pressure slab gate valves regulate fluid flow and isolate different pressure zones, rendering them indispensable components of well-control systems. Owing to the strong winds and harsh environmental conditions in the Tarim Oilfield, non-stick flat gate valves have become the mainstream high-pressure flat gate valves used in the region. The structure of the high-pressure flat gate valve currently in use is shown in Figure 2.

In actual production, these valves are subject to several challenges, including:

· Abnormal opening and closing

· Sealing failure

· Bottom-plate cracking

Effectively mitigating these failure modes is currently a pressing challenge for well-control equipment manufacturers. To address this, scholars have conducted analyses of the chemical composition, microstructure, and mechanical properties of bottom-plate cracking incidents in slab gate valves, identifying insufficient tensile strength and inadequate bottom-wall thickness as the primary causes. They further summarized the primary failure modes of slab gate valves as:

· External leakage

· Inflexible valve operation

· Internal leakage

These failures were mainly attributed to sealing-component failure, bearing and pin damage, and poor sealing caused by loose valve-plate material or contaminants. Overall, high-pressure slab gate valves are susceptible to various failure modes during actual service. However, comprehensive studies of their failure modes, underlying mechanisms, and corresponding countermeasures remain limited, both domestically and internationally.

(a) Product Diagram(b) Structural Diagram

Figure 2 High-Pressure Flat Gate Valve Currently in Service in the Tarim Oilfield

Therefore, this study first identifies the main failure modes of existing high-pressure slab gate valves encountered during actual production and service in the Tarim Oilfield through research and analysis. Subsequently, a theoretical mechanical model is established to comprehensively investigate the failure mechanisms and dominant controlling factors of the non-stick flat gate valve. Finally, based on the analyzed failure mechanisms and controlling factors, countermeasures are proposed to effectively mitigate valve failures.

High-pressure slab gate valves are widely used in well-control equipment in the Tarim Oilfield. However, these valves are prone to various failures during service. This section summarizes the failure modes encountered by non-stick high-pressure slab gate valves during production and service in the Tarim Oilfield over the past three years, thereby providing a basis for subsequent investigations into their failure mechanisms and dominant controlling factors. The failure and maintenance records of high-pressure slab gate valves in the Xinjiang Tarim Oilfield over the past three years were statistically analyzed, and the failure modes encountered during this period were classified and systematically organized. As shown in Figure 3, the primary failure modes of high-pressure slab gate valves in the Tarim Oilfield over the past three years include:

· Abnormal opening and closing torque

· Valve-stem dynamic seal failure, evidenced by mud seepage from the upper end of the stem

· Failure of the valve plate to achieve tight shutoff

Abnormal valve opening and closing occurred 55 times, accounting for 79.7% of all flat gate valve failures over the past three years, making it the predominant failure mode of non-stick slab gate valves in the Tarim Oilfield during this period. Valve-stem seal failure occurred eight times, accounting for 11% of all flat gate valve failures over the past three years, while failure of the valve plate to achieve tight shutoff occurred six times, representing 9% of the total. Therefore, abnormal valve opening and closing is currently the most prevalent failure mode of high-pressure slab gate valves in the Tarim Oilfield. As shown in Figure 4, these failures can be further classified according to the affected components as:

· Bearing wear

· Valve stem wear and bending

· Damage to the valve plate–seat interface

Among these, bearing wear occurred five times, accounting for 9% of the abnormal opening and closing failures; valve stem wear and bending occurred 10 times, representing 18%; and valve plate–seat surface damage occurred 40 times, accounting for 73%.

Figure 3 Analysis of the failure forms of high-pressure flat gate valves in Tarim in the Past Three Years

Figure 4 Analysis of Abnormal Opening and Closing Failure Modes of High-Pressure Slab Gate Valves in the Tarim Oilfield Over the Past Three Years

Based on three years of failure data from the Tarim Oilfield, abnormal opening and closing was identified as the dominant failure mode of high-pressure slab gate valves, with failures attributable to three distinct causal categories. Drawing on the survey results presented in the previous section, a mechanical model of high-pressure slab gate valve actuation is developed to provide a theoretical basis for subsequent investigations into failure mechanisms and the identification of dominant controlling factors. When a high-pressure slab gate valve is fully closed, the valve plate and valve seat sealing surfaces are in full contact, resulting in maximum sliding friction. At the same time, the medium exerts a continuous radial force on the valve stem, which further hinders valve operation. Consequently, when the non-stick slab gate valve is fully closed:

· The valve stem is subjected to its maximum axial force

· The stem torque simultaneously reaches its peak value

The torque analysis of the valve stem under pressure in the fully closed state is shown in Figure 5.

Figure 5 Analysis of forces acting on the torque source in a high-pressure plate gate valve

Let F1 denote the total frictional resistance between the valve plate and valve seat under pressure, F2 the radial pressure exerted by the slurry on the valve stem, and F3 the bearing reaction force. Then:

With μ₁ denoting the friction coefficient between the valve plate and seat, and N the total pressure on the plate, the friction force is given by:

Let d F be the major diameter of the valve stem thread and q the medium pressure. The upward slurry pressure force F₂ on the valve stem is then given by:

The torque analysis of the valve stem shows that the total stem torque ∑M consists of the thread friction torque M₁, the friction torque between the sealing ring and the stem M₂, and the bearing friction torque M₃:

Denoting the mean diameter of the trapezoidal valve-stem thread as d', its pitch as P, the equivalent friction coefficient as f, the equivalent friction angle as φ v , and the lead angle as φ . Then:

As the specifications of stemless gate valves have increased, so too have the demands on their pressure-bearing capacity. In high-pressure designs, the valve stem seal typically employs a rotary sealing ring in place of the packing seal used in low-pressure versions. Let μ₂ be the friction coefficient of the valve stem rotary sealing ring material, d′F the sealing diameter, and L the effective length of the sealing ring along the valve stem axis. The friction torque M₂ is then given by:

Let the bearing inner diameter be D and the bearing friction coefficient be μ₃. The bearing friction torque M₃ is then given by:

From equations (4) and (8) through (10), it follows that:

Equation (11) shows that the primary components affecting the pressure torque of the stemless gate valve are the valve plate–seat interface, the valve stem, the stem sealing ring, and the bearing. The primary parameters for the valve plate–seat pair are the normal pressure and the surface friction coefficient, whereas those for the valve stem assembly are the thread pitch, major diameter, and equivalent thread friction coefficient; the key parameter for the valve stem seal is the seal friction coefficient, while that for the bearing is the bearing friction coefficient.