In industrial production, valves are indispensable components in fluid control systems, and sealing performance is one of the key factors determining whether a valve can operate normally. Different valve types, due to their structures and modes of motion, have a significant impact on the choice of sealing solutions. This article will provide a detailed introduction to the sealing characteristics of various common valve types and how to select appropriate sealing components according to specific needs to ensure system safety and reliability.
Valves come in a wide variety of types in industrial applications, and each type has unique structures and functions. These characteristics directly affect the choice of sealing solutions. Sealing components must not only adapt to the valve's motion but also withstand pressure, temperature, and media corrosion under different working conditions. Therefore, selecting the right sealing components requires comprehensive consideration of valve type, operating frequency, working environment, and other factors.
Rising-stem gate valves are a common type of valve, usually featuring a relatively long stroke. This long linear motion can pose significant sealing challenges under frequent operation. However, in most cases, gate valves are not operated frequently, typically no more than once per week, and some valves are only operated once per year. In this case, choosing sealing components requires special attention to the gaps between the packing gland, stem, and stuffing box. If the gaps are too large, the linear motion may cause part of the sealing component to be pressed into the gaps or drag particulate impurities through the sealing component, leading to leakage. To address this issue, a cleaning ring can be installed at the bottom of the packing, and in some cases, a cleaning ring can also be installed at the top to reduce wear from impurities on the sealing components.
Globe valves are relatively more difficult to seal because they use a rising-stem plus rotary motion. The stem moves in two directions simultaneously, and the packing gradually contacts the entire surface of the stem. Any eccentricity or out-of-roundness of the stem may cause damage to the packing and result in leakage. Like gate valves, linear motion can also drag particulate impurities through the sealing components and into the process fluid. Therefore, when selecting sealing components, special attention must be paid to stem surface quality and the wear resistance of the sealing components.
Ball valves, butterfly valves, and plug valves are common types of rotary valves. The stem rotates only ninety degrees relative to the sealing component to complete the full open-to-close process. This motion makes sealing relatively easier because the stroke is much shorter than that of other valve types. Unlike linear motion, right-angle rotary motion does not easily drag particulate impurities through the sealing components. However, attention must be paid to stem eccentricity, as some sealing components are highly sensitive to actuator misalignment, which may reduce stem sealing performance. In addition, the stuffing box designs of right-angle rotary valves vary widely, which often limits the selection of sealing components. Under high-pressure conditions, valves with shallow stuffing boxes may struggle to achieve tight sealing.
Control valves generally have the most challenging stem sealing requirements, mainly because they operate frequently and the stem sealing stress cannot be too high. For example, a control valve may experience 100,000 stem cycles, whereas other types of valves in the system often only experience 1,500 cycles. High-frequency operation causes wear on sealing components, gradually reducing sealing performance over time. To optimize fluid control performance, the stem of a control valve cannot withstand excessive friction. Therefore, the sealing stress acting on control valves is significantly lower than that on manual valves. If the sealing components cause excessive friction on the stem, valve action may lag or show speed deviation, resulting in excessive stem movement and reduced fluid control performance. Linear control valves are more difficult to seal than rotary control valves because rotary control valves only involve circumferential motion, and the stem surface area requiring sealing is significantly smaller than that of linear control valves.
Valve size is an important factor affecting the choice of sealing components. Whether small or large, the different sizes present different sealing challenges and solutions.
For small valves, the annular cross-section between the stem and the stuffing box inner wall is relatively small. In some cases, small size is not necessarily advantageous, as it limits the range of sealing components. The annular cross-section of small valves is usually only 0.125 inches, making it difficult to install sealing components with robust material and innovative design. Therefore, when selecting sealing components, special attention must be paid to their size and material compatibility.
Large valves are not without challenges either. Their size may result in excessive loads applied to the stem and packing. When the valve vibrates, the resulting forces may be too high for standard sealing components. In addition, the temperature difference across different cross-sectional areas of large valves may cause structural deformation. Therefore, when selecting sealing components, their performance under high load and temperature differences must be considered.
For most valve types, the ideal stuffing box height is approximately three to five times the cross-sectional diameter. For right-angle rotary valves with low sealing requirements, even a shallow stuffing box can effectively seal. However, excessively deep stuffing boxes have two main issues: first, the sealing assembly is prone to solidification, causing a loss of sealing stress and resulting in leakage; second, the friction on the stem is higher, which may hinder operation in some applications.
Packing is a traditional gland-type seal that can be made from various materials. When stuffing packing, it can be structurally loose (usually mixed with lubricants); formed into a specific cross-section (generally rectangular, but sometimes round); cut to appropriate length for spiral winding or molded installation on the gland. Regardless of the structure, sealing is achieved by compressing the packing against the sealing surface through gland pressure.
The compressive force from the gland generates radial pressure that produces the sealing effect. Radial pressure is distributed exponentially along the entire packing length. To keep the packing "dry," the radial pressure on the inner rings must at least equal the system's internal pressure, which means the radial pressure on the outer rings is much higher, often excessive in most applications (causing excessive friction, stem wear, and failure of pneumatic seals). Therefore, in most uses, the compressive force is adjusted so that a small amount of leakage occurs at the last packing ring, where radial pressure is slightly below system pressure. However, if the gland is adjusted to the minimum compression that produces no leakage, most packing rings may still show some leakage.
Another factor complicating optimal gland compression is that some packing expands during operation, for example, when the temperature rises. In such cases, applying a small preloading force on the gland may be necessary. In addition, to compensate for packing wear and relaxation and maintain satisfactory sealing, periodic re-tightening of the gland is required.
For conventional packing materials, the radial pressure generated is approximately 0.6–0.7 times the axial pressure applied during gland compression. Packing remains a primary choice for many applications, especially in large stuffing boxes and heavy-duty conditions, such as process pumps, steam supply, and gravity water treatment. Packing also has the advantage of being suitable for both reciprocating and rotary operations. For many reciprocating applications, particularly in large, heavy-duty conditions, flexible sealing components or individual seals may replace packing, except where minimal leakage is required, in which case mechanical seals may be more suitable. However, with the widespread use of mechanical seals, there is no apparent reduction in the need for gland packing seals.
In areas requiring lubrication or some degree of cooling in the stuffing box, additional lubricant or coolant can be supplied to the stuffing box center. This method provides limited cooling, and in high-temperature conditions, it may be necessary to cool the entire stuffing box to keep its operating temperature within the packing's allowable limit.
Fibrous packing materials require high-pressure compression, and inadequate lubrication can cause excessive friction and overheating, resulting in various issues. These problems can be addressed by using recently developed packing made from aramid fibers coated with PTFE. These new materials offer better lubrication and wear resistance, maintaining good sealing performance under high temperature and pressure conditions.
Selecting appropriate sealing components is crucial to ensuring valve sealing performance. Different valve types have distinct requirements for sealing components due to differences in structure and motion. When choosing sealing components, factors such as valve type, size, operating frequency, working conditions, and stem surface treatment must be comprehensively considered.
Although packing as a traditional sealing component has some limitations, it remains the primary choice in many applications. In practice, suitable sealing components should be selected according to specific needs, with regular maintenance and adjustment to ensure system safety and reliability.
Proper selection and use of sealing components can effectively improve valve sealing performance, extend valve service life, reduce maintenance costs, and enhance the overall operational efficiency and safety of industrial systems.