What Determines the Sealing Performance of Ball Valves?

In the modern industrial field, ball valves are a widely used type of valve, and their sealing performance is one of the key indicators to measure their quality. The design of the sealing structure, the choice of materials, and the machining accuracy all play a role in determining how well a ball valve seals, which in turn affects its reliability and lifespan under various working conditions. This article will dive into the elements that affect the sealing performance of ball valves and explore how to enhance their sealing capabilities through better design and material selection.

The sealing structure of a ball valve mainly consists of three parts: the seal between the main and secondary valve bodies; the seal between the main valve body and the valve stem; and the seal between the ball and the main and secondary valve bodies. Each part has a different sealing function, and together they ensure the overall sealing performance of the ball valve.

• The seal between the main and secondary valve bodies is usually achieved through a sealing adjustment gasket. The material and thickness of this gasket are crucial. It needs to be able to maintain a seal while also accommodating changes in pressure and temperature. The right gasket material can form an effective barrier in the tiny gaps between the valve bodies, preventing fluid leakage.
• The seal between the main valve body and the valve stem is typically done using a standard V-shaped packing or an O-ring. The purpose of this external sealing structure is to prevent fluid from leaking through the gap between the valve stem and the valve body. The material and design of the packing or O-ring should be chosen based on the working fluid and conditions of the ball valve to ensure its reliability and durability over the long term.
• The seal between the ball and the main and secondary valve bodies is the main sealing structure of the ball valve, which is achieved through a sealing seat. The sealing performance of this part directly affects how well the ball valve works and is a critical aspect of its design and manufacturing. Factors like the material, design, and machining accuracy of the sealing seat all have a significant impact on the effectiveness of the main sealing structure.

Sealing performance is a core indicator of a ball valve's quality, and there are many factors that influence it. Before we look into how to optimize ball valve sealing, we need to understand these key factors. They not only determine how well the ball valve seals in practical applications but also directly affect its lifespan and reliability.

The surface roughness of the sealing faces is one of the important factors affecting the sealing performance of ball valves. No matter how advanced the machining technology is, it's impossible to completely eliminate microscopic defects on the sealing faces. Therefore, determining the appropriate surface roughness value is crucial. Generally, the maximum allowed roughness Ra value for the ball's sealing face is 0.4 µm; and when the sealing seat uses polytetrafluoroethylene (PTFE) as the sealing material, its roughness Ra value should not exceed 0.8 µm. The right roughness value can ensure good sealing while reducing machining difficulty and cost.

The sphericity error of the ball is also a key factor affecting sealing performance. If the ball is not perfectly round, it can lead to uneven contact between the sealing faces, which affects the sealing effect. Although it's not possible to completely eliminate the sphericity error in actual machining, precise machining processes and quality control can keep it within a reasonable range to ensure the sealing performance of the ball valve.

Sealing materials are a key part of the ball valve's sealing structure, and their performance directly affects the valve's sealing effect and lifespan. With the continuous development of industrial technology, sealing materials have evolved from traditional metal materials to modern engineering plastics, which has greatly improved the sealing performance and adaptability of ball valves. Here's a look at the development of sealing materials and their application in ball valves.

In the early days, most ball valves used metal materials for their sealing structures. This hard sealing method could meet sealing requirements to some extent, but it had its limitations. Metal sealing structures have poor elastic and plastic deformation capabilities, making them prone to sealing failure under conditions of temperature changes and pressure fluctuations. Moreover, the high machining accuracy required for metal sealing structures increases manufacturing costs and difficulty.

With the development of engineering plastics, especially the advent of PTFE, ball valves have gained a more ideal material choice for their sealing structures. PTFE has excellent chemical stability, corrosion resistance, and a low friction coefficient, and it can maintain stable performance over a wide temperature range. Common soft sealing forms for the main sealing structure of ball valves include rigid sealing seats, sealing seats with elastic elements, PTFE lip seals, and combined-type sealing seats made up of elastic expansion rings and PTFE lip seals.

Lip seals, thanks to their lip-shaped structure, have good elasticity. They can ensure a pre-compression amount on the sealing face and compensate for it, which is better than rigid structures. However, in actual use, it's found that PTFE's elastic and plastic deformation capabilities and stability under temperature changes are not ideal, and it has a significant tendency to cold flow. Additionally, since the working pressure of ball valves can vary greatly, lip seals made of PTFE as the sealing material can still leak under low-pressure conditions or after long-term use.

To address these issues, a combined-type sealing seat made up of an elastic expansion ring and a PTFE lip seal has become an effective solution. This combined-type sealing seat can rely on the good elastic and plastic deformation capabilities of the metal elastic expansion ring to enhance its overall deformation and compensation abilities, thereby improving the sealing capability of the ball valve. By properly designing the structure of the elastic expansion ring and the lip seal, good sealing performance can be maintained under different working conditions, extending the lifespan of the ball valve.

To ensure that a ball valve meets the required sealing standards, the pressure of the fluid has to push the ball (in floating ball valves) or the sealing seat (in fixed ball valves) to create a small displacement. This causes the sealing seat surface to deform elastically and plastically, forming the necessary sealing specific pressure. Controlling the sealing specific pressure is crucial because both too high and too low pressures can negatively affect the sealing and operating performance of the ball valve.

If the sealing specific pressure is too high, it can damage the sealing seat by exceeding the material's allowable pressure, leading to sealing failure. Moreover, excessive sealing specific pressure can also increase the operating torque, affecting the ball valve's operation. In practice, the range of sealing specific pressure should be reasonably determined based on the working fluid, pressure, and temperature of the ball valve to ensure its normal operation.

On the other hand, if the sealing specific pressure is too low, the elastic and plastic deformation of the sealing seat surface won't be enough to compensate for the machining errors of the ball. This means the main seal won't meet the required sealing standards. Therefore, during the design and manufacturing of ball valves, the appropriate sealing specific pressure value should be determined through precise calculations and experimental verification to ensure good sealing performance under various working conditions.

Ball valves can be divided into two main types based on their structural design: floating and fixed. These two types have different sealing characteristics and are suitable for different working conditions. Understanding their sealing characteristics is crucial for correctly selecting and using ball valves.

In floating ball valves, when the ball is closed, it moves (displaces) under the pressure of the fluid, leaving the inlet sealing seat and pressing against the outlet sealing seat. This blocks the fluid at the outlet sealing seat. So, in floating ball valves, only the outlet sealing seat actually performs the sealing function, while a small floating gap is maintained between the inlet sealing seat and the ball. This structural design gives floating ball valves good sealing performance under low-pressure conditions, but their sealing performance may be somewhat limited under high-pressure or large-diameter conditions.

Fixed ball valves are often used for high-pressure or ball valves with a nominal diameter of DN ≥ 200 mm. In fixed ball valves, the ball doesn't move, which ensures bidirectional sealing and reduces the operating torque of the valve. The load fluctuations on the sealing seat are also smaller. Fixed ball valves can be divided into ball front sealing (inlet sealing) and ball rear sealing (outlet sealing) types. To ensure sealing when the working pressure of the fluid is low, a certain pre-tightening specific pressure must be formed between the ball and the sealing seat. In fixed ball valves, properly designing and adjusting the pre-tightening specific pressure is the key to ensuring good sealing performance.

Pre-tightening specific pressure is one of the key parameters for the sealing performance of ball valves. It directly affects the sealing effect and operating performance of the ball valve. A reasonable pre-tightening specific pressure can ensure good sealing performance under various working conditions while avoiding damage to the sealing seat and increased operating torque caused by excessive specific pressure. Therefore, adjusting and optimizing the pre-tightening specific pressure is an important aspect of ball valve design and use.

In rigid sealing seats, the reliability and lifespan of the ball valve depend on correctly choosing the pre-compression amount of the sealing seat. If the pre-compression amount is insufficient, it can't ensure low-pressure sealing of the ball valve. If it's too large, it can increase the friction torque between the ball and the sealing seat, affecting the valve's operation and possibly causing plastic deformation of the sealing seat, leading to sealing failure. For PTFE sealing seats, the pre-tightening specific pressure should generally be 0.1 PN and no less than 1.02 MPa. The adjustment of the pre-compression amount in rigid sealing seats is done by changing the thickness of the sealing adjustment gasket. Errors in the machining of the gasket can affect the adjustment result. Therefore, proper assembly and adjustment are key to achieving good sealing.

To solve the problems of rigid sealing seats, using sealing seats with elastic elements has become an effective way. In this case, the pre-compression amount and its adjustment no longer rely on the sealing adjustment gasket but are achieved through the elastic element. Sealing seats with elastic elements can not only obtain the necessary pre-compression amount but also compensate for the pre-tightening specific pressure within the elastic deformation range of the elastic element. As a result, the lifespan of the ball valve is relatively longer. The design and material selection of the elastic element need to be optimized based on the specific working conditions of the ball valve to ensure its reliability and stability over the long term.

The sealing performance of ball valves is a key indicator of their quality, and it is determined by the design of the sealing structure, the choice of materials, and the machining accuracy. By optimizing the design of the sealing structure, selecting suitable sealing materials, and strictly controlling machining accuracy and assembly adjustment, the sealing performance and lifespan of ball valves can be effectively improved. In practical applications, it's necessary to choose the type and sealing structure of the ball valve based on specific working conditions and to carry out regular maintenance and inspection to ensure the safe and reliable operation of the ball valve.