Fully Welded Ball Valves in Natural Gas Pipelines: Design Principles and Typical Fault Analysis

Abstract: Fully welded ball valves are widely used in natural gas distribution stations, performing essential functions such as connecting or isolating the flow and changing its direction. The performance of these valves directly affects the safe and stable operation of the pipeline network. This article examines the structural principles of fully welded ball valves and summarizes common failure modes, along with essential considerations for troubleshooting and maintenance. Fully welded ball valves offer advantages such as fast actuation, excellent sealing performance, and high reliability. These features make them well-suited for the safety requirements of natural gas transmission, particularly at key locations like distribution stations and line block valve chambers. However, factors such as transportation, installation, operational practices, maintenance conditions, pipeline cleanliness, and erosion caused by high-velocity gas flow can lead to internal leakage and other malfunctions. Such faults may compromise the valve’s intended isolation performance, and in severe cases, may result in downstream gas supply disruptions or even hazardous incidents like gas leaks or explosions.

1. Structural Principle of Fully Welded Ball Valves

A fully welded ball valve is a type of valve in which the opening and closing component—the ball—is driven by the valve stem to rotate around a fixed axis. The main components include the valve body, valve seat, valve stem, ball, and actuation device. The valve operates by rotating the ball 90°, enabling opening or closing of the flow path. The valve is fully open when the ball bore aligns with the inlet and outlet, and fully closed when the ball blocks the flow path. Unlike floating ball valves and split-body fixed ball valves, fully welded ball valves feature a one-piece welded body that eliminates potential leakage at flange connections. The valve body is constructed from materials with good weldability, and its design requires high assembly precision and strict manufacturing process control. Based on the valve seat sealing principle, fully welded ball valves can be equipped with various types of seat structures, including single-piston effect seats, double-piston effect seats, and a combination of both.

(1) Single-Piston Effect Valve Seats

This type of valve seat provides sealing in only one direction, allowing pressure relief from the opposite side. If the upstream seat loses its sealing capability, leakage may occur. Single-piston effect seats are primarily used in pipelines carrying liquid media or unstable gases.

(2) Double-Piston Effect Valve Seats

This design enables both the valve seat and the valve cavity to independently withstand the full pressure differential. As a result, the valve can achieve simultaneous sealing on both the upstream and downstream sides. Double-piston effect seats are commonly used in natural gas transmission pipelines and urban gas distribution networks due to their enhanced sealing reliability.

(3) Combined Single and Double-Piston Effect Valve Seats

This configuration incorporates one single-piston effect seat and one double-piston effect seat. It is commonly employed in liquid media systems or in environments prone to condensation. Proper flow direction must be clearly defined and strictly observed during installation.

2. Typical Faults and Maintenance Considerations

Based on observable failure symptoms, the common issues with fully welded ball valves can be categorized into four main types: internal leakage, external leakage, operational difficulties (opening/closing), and grease injection system failures.

Internal Leakage:
Internal leakage is a common and somewhat complex issue during the operation of fully welded ball valves. True internal leakage occurs when the ball surface is scratched, the soft sealing element is damaged or defective, or other mechanical damage compromises the seal. In contrast, fluctuations in pressure or temperature can cause temporary internal leakage, while incorrect valve positioning or adhesion between the soft seal and the ball may result in false leakage.

Therefore, accurately identifying the type of leakage is essential.
– For false leakage caused by valve position deviation, the fully closed position can be repeatedly fine-tuned until the sound of airflow disappears or is minimized, thus determining the optimal shut-off point.

- For false leakage caused by adhesion between the soft seal and the ball, injecting a suitable cleaning fluid into the valve and allowing it to operate for 24 hours under observation often eliminates or significantly reduces the leakage.

-For true internal leakage, it is typically necessary to inject grease or sealing compound into the valve cavity to seal the leakage path.

External Leakage: External leakage typically occurs at two locations: the valve stem and the grease fitting (grease nipple).

Valve stem leakage is typically caused by damage to the valve stem’s sealing ring, which allows high-pressure natural gas from the valve cavity to escape along the stem. In such cases, sealing grease can be injected through the stem’s grease port to restore the seal.

Grease fitting leakage can be caused by a damaged spring inside the fitting, a failed sealing ball, or contaminants that prevent the spring from returning to its sealing position. To resolve this issue, the grease fitting should be disassembled for cleaning and inspection, with any damaged components replaced. Before disassembly, professional testing tools should be used to verify the functionality of the built-in check valve.

Difficulty in Opening and Closing: Operational difficulties with fully welded ball valves usually manifest as high operating torque or complete inability to operate. Common causes include rust on the ball surface, aging or hardening of the sealing grease, foreign debris inside the valve cavity, corrosion at the connection between the valve stem and actuator, or actuator failure.

Recommended corrective measures include:
– Injecting cleaning fluid into the valve and fully operating it to remove contaminants;
– Removing the actuator cover to clean rusted components;
– Inspecting the gearbox or disassembling the valve to check for problems such as improper engagement between the worm and gear, severe corrosion on the valve stem, or bearing damage. Any issues found should be addressed based on the specific conditions identified during the inspection.

Grease Injection System Failure: This failure typically manifests as excessively high injection pressure, inability to inject grease, or leakage from the grease fitting. Common causes include:

A grease check valve with an undersized diameter, which restricts grease flow;

Ingress of hard foreign materials, such as PTFE tape or debris, causing blockage in the injection channel;

Solidification or drying of grease inside the valve.

In most cases, normal operation can be restored by injecting an appropriate valve cleaning fluid. When injecting cleaning fluid into the ball valve using an extension rod, the grease injection tool should maintain high pressure to compress the fluid, allowing it to soften and flush out any solidified or dried grease deposits.

3. Conclusion

Issues such as internal leakage, external leakage, operational difficulties, and grease injection system failures in fully welded ball valves directly affect the safe operation of natural gas pipeline networks. This is especially critical because these valves function as shut-off devices at key locations—such as receiving and sending ball barrels, as well as inlet and outlet stations—where maintaining valve integrity is essential. Therefore, natural gas pipeline operators should enhance the operational management of process valves, strengthen equipment inspection and preventive maintenance programs, and rigorously carry out periodic maintenance and overhauls in accordance with established standards, operating procedures, and system documentation. By doing so, they can continuously enhance the safety management of equipment and facilities, ensuring the reliable performance of fully welded ball valves.