Optimization of On-Off Ball Valve Packing Sealing

In numerous industries such as petrochemical, coal chemical, and fine chemical industries, on-off ball valves play a crucial role. As terminal control elements, they are especially indispensable when handling hazardous media such as toxic, corrosive, flammable, and explosive substances. The sealing performance of on-off ball valves directly affects the safe operation of equipment, and packing sealing is the key to their sealing. This article will delve into the optimization of packing sealing for on-off ball valves to address the issue of medium leakage and ensure the safe operation of the equipment.

The sealing performance of on-off ball valves mainly relies on the mutual sealing between the packing, the valve stem, and the packing gland. In practical applications, the quality of packing sealing plays a vital role in the safe operation of on-off ball valves. Once the packing sealing fails, it may lead to medium leakage, causing safety accidents and even significant economic losses. Therefore, in-depth research on packing sealing technology and optimization of the packing sealing structure are of great significance for improving the reliability and safety of on-off ball valves.

During the startup of the equipment, on-site technicians discovered a slight leakage at the junction of the packing and the valve stem, as well as the packing and the packing gland of the on-off ball valve. After an initial inspection, it was found that most of the leakage at the packing gland was related to the surface roughness (smoothness) of the valve stem and the inner hole of the packing gland. Technicians promptly applied pre-tightening treatment to the packing, but the effect was not significant. This indicates that the packing sealing problem may involve more complex factors.

To accurately identify the cause of the leakage at the packing gland, technicians carefully examined the packing, valve stem, packing gland, and other components of the on-off ball valve and had detailed communication with on-site technicians. After analysis, it is believed that the following factors may cause leakage at the valve packing gland: compatibility between the medium and the packing, surface finish of the valve stem, surface finish of the packing gland inner wall, and the sealing specific pressure of the packing. The following sections will provide a detailed analysis of these four aspects.

In the chemical industry, the compatibility between the medium and the packing is one of the important factors affecting sealing performance. After communication and exchange with on-site process technicians, it was learned that the medium flowing through this type of valve is chlorosilane (a mixture of trichlorosilane and silicon tetrachloride), without any other components. This medium is not corrosive to the selected polytetrafluoroethylene (PTFE) packing seal. Except for oxygen, molten metals, high-temperature chlorine trifluoride, and fluorine elements, as well as nuclear power plant conditions, PTFE packing seals can be used for other medium conditions. Therefore, from the perspective of compatibility between the medium and the packing, the selected PTFE packing is appropriate, and the possibility of leakage due to incompatibility between the medium and the packing can be ruled out.

The surface finish of the valve stem has a direct impact on the packing sealing performance. When using PTFE sealing packing, if the surface finish of the valve stem is insufficient, the sealing performance will be reduced, leading to increased leakage. Technicians inspected the valve stem after disassembling the on-site valve. After cleaning, the valve stem surface was smooth and had a high surface finish. Through testing, the surface finish of the valve stem was higher than grade 12 (with a roughness of Ra=0.04), which fully meets the sealing requirements. Therefore, the possibility of leakage due to insufficient surface finish of the valve stem can be ruled out.

The surface finish of the packing gland inner wall is also crucial for sealing performance. Technicians further tested the surface finish of the packing gland inner wall, and the results showed that it reached grade 7 (Ra=1.6). During the rotation of the ball valve, the packing and the packing gland belong to static sealing. PTFE packing is a soft packing that is easy to deform and is not sensitive to surface roughness. After technicians confirmed the surface finish, the current surface finish of the packing gland meets the sealing requirements. Therefore, the surface finish of the packing gland inner wall is not the cause of leakage.

After ruling out the compatibility between the medium and the packing, as well as the surface finish of the valve stem and the packing gland inner wall, technicians focused on the sealing specific pressure of the packing. Sealing specific pressure refers to the average normal pressure acting on a unit sealing surface, which is an important factor affecting the sealing performance of gaskets or packing. Usually, during assembly, a pre-tightening force is applied to the packing gland to generate a certain specific pressure on the packing sealing surface, causing the packing to deform. The packing fills the tiny gaps between the contact surfaces with the valve stem through deformation. The longer the path of the fluid through these gaps, the greater the flow resistance loss, and the better the sealing performance.

In practice, when leakage occurred in the packing, technicians applied pre-tightening treatment to the packing, but the problem was not resolved. Considering that the on-off ball valve was newly purchased and there was no issue of packing failure, it is speculated that the insufficient sealing specific pressure between the packing and the valve stem, as well as between the packing and the packing gland inner wall, is the cause of medium leakage.

To improve the sealing specific pressure between the packing and the valve stem, as well as between the packing and the packing gland, technicians decided to adjust the packing structure. The valves of this type with packing problems on-site all used PTFE packing, which has a low friction coefficient and excellent sealing performance and applicability for various chemical media, acids, and alkalis.

According to the pressure distribution theory of packing sealing, only the top 1-2 layers of packing truly exert sealing force when the packing gland applies pre-tightening force to the packing. The first layer of packing undergoes axial deformation under the pre-tightening force of the gland. Under the pre-tightening force, the packing then undergoes radial deformation, and the packing is squeezed and deformed against the valve stem and the inner wall of the packing gland, ultimately forming a seal between the packing and the valve stem and the packing gland. The pre-tightening force is consumed layer by layer during the packing sealing process, continuously decreasing. The lower the packing layer, the less force it receives, the less it deforms, and the worse the sealing performance. Therefore, only the top 1-3 layers of packing in the packing gland truly play a sealing role.

In response to this situation, technicians adopted measures to adjust the packing structure, modifying the bottom of the original V-shaped packing to a U-shaped packing. Under the pre-tightening force of the packing gland, the U-shaped packing can further release its deformation capability, increasing the radial sealing pressure of the packing. Based on the pressure distribution theory of packing sealing and performance tests of U-shaped packing, a calculation formula for the radial pressure of the packing, that is, the main sealing pressure, was derived. By adjusting the packing structure, all the packing of the on-off ball valves with leakage problems were replaced with U-shaped PTFE packing.

In addition, to address the issue of decreased sealing specific pressure of the packing due to wear or stress relaxation during the use of on-off ball valves, technicians added a pre-tightening measure with a Belleville spring below the packing gland. This measure not only ensures sufficient pre-tightening force for the packing but also effectively increases the sealing specific pressure, reducing the maintenance workload of on-site instrument personnel on the packing position and ensuring reliable packing gland sealing.

After adopting U-shaped packing and adding Belleville spring pre-tightening measures, the on-off ball valves have operated safely for over a year without any leakage problems at the packing gland. The implementation of this plan effectively solved the problem of medium leakage from on-off ball valves and met the requirements of actual working conditions. By analyzing and deducing the valve stem structure, the surface finish of the packing gland, and the sealing specific pressure between the packing and the valve stem, as well as between the packing and the inner wall of the packing gland, technicians identified the cause of medium leakage at the packing part of the on-off ball valves and adjusted the packing structure of the on-off ball valves through the calculation and adjustment of packing sealing pressure. After adding Belleville springs and other measures under the packing gland, good sealing was formed between the packing and the valve stem and between the packing and the inner wall of the packing gland, solving the leakage problem that occurred during the operation of the on-off ball valves and providing strong support for the long-term safe operation of the equipment.

The optimization of packing sealing technology for on-off ball valves is a systematic project that requires comprehensive consideration of multiple factors, including the compatibility between the medium and the packing, the surface finish of the valve stem and the packing gland inner wall, and the sealing specific pressure of the packing. Through the analysis and practice in this article, we successfully solved the problem of medium leakage from on-off ball valves, providing a valuable reference for the safe operation of equipment in the chemical industry.