A Deep Analysis of Valve Sealing Solutions

In the industrial sector, valves are indispensable components in fluid control systems, and their sealing performance directly affects the safety and efficiency of the system. There are various types of valves, and the structure and motion patterns of each type significantly influence the choice of sealing solutions. This article will delve into the sealing characteristics of different types of valves and explore how to select appropriate sealing elements and surface treatment processes to ensure the reliability and performance of valves.

Rising stem gate valves are a common type of valve, typically featuring a long open-close stroke. This long linear motion can pose significant sealing difficulties, especially during frequent operations. However, in most cases, the operation frequency of gate valves is not high, usually not exceeding once a week, and some valves may only be operated once a year. Despite this, the gap between the packing gland, the stem, and the stuffing box remains a critical factor for sealing. If the gap is too large, the linear motion may cause some sealing elements to be squeezed into the gap or drag impurity particles through the sealing elements, thereby compromising the sealing effect. To avoid this, a cleaning ring can be installed at the bottom of the stuffing box, and in some cases, a cleaning ring can also be installed at the top to reduce the impact of impurities on the sealing elements.

The sealing difficulty of globe valves is relatively high, mainly due to their combined linear and rotational motion patterns. The stem moves in two directions simultaneously, which means that the packing set will gradually come into contact with the entire surface of the stem. Any eccentricity or out-of-roundness of the stem can lead to damage to the packing elements and leakage. Similar to gate valves, linear motion can drag impurity particles through the sealing elements and into the process fluid. Therefore, when selecting sealing elements for globe valves, it is essential to pay special attention to the concentricity and roundness of the stem to ensure that the sealing elements can make even contact with the stem surface, thereby achieving reliable sealing.

Ball valves, butterfly valves, and plug valves are common types of quarter-turn valves. For these valves, the stem rotates ninety degrees relative to the sealing element to complete the entire open-to-close process. This motion pattern makes the sealing of quarter-turn valves relatively easy because their stroke is much shorter than that of other types of valves. Unlike linear motion, quarter-turn motion is less likely to drag impurity particles through the sealing elements, thereby reducing wear on the sealing elements. However, stem eccentricity is still a concern, as some sealing elements are highly sensitive to misalignment of the actuator, which can lead to reduced stem sealing performance. Therefore, when selecting sealing elements, it is necessary to consider the alignment accuracy of the stem to ensure that the sealing elements can accommodate the stem's motion.

The design of quarter-turn valve stuffing boxes is diverse, which often limits the range of sealing element choices. In many cases, the stuffing box is relatively shallow, making it difficult to achieve tight sealing under high-pressure conditions. The depth of the stuffing box is crucial for sealing performance because a shallow stuffing box may not accommodate enough sealing elements, resulting in poor sealing. Therefore, when designing quarter-turn valves, it is necessary to consider the depth of the stuffing box and the sealing requirements comprehensively to select appropriate sealing elements.

Control valves typically face the greatest sealing challenges, mainly due to their frequent operation and the requirement that the stem sealing stress cannot be too high. Control valves play a key role in fluid control systems, with their stems undergoing frequent cyclic operations. For example, a control valve may experience 100,000 stem cycles, while other types of valves often only go through 1,500 cycles. High-frequency cyclic operations can cause wear on the sealing elements, gradually reducing sealing performance over time. Additionally, to optimize fluid control performance, the stem of a control valve cannot withstand excessive frictional forces, so the sealing stress applied to a control valve is significantly lower than that of a manually operated valve. If the sealing elements cause excessive frictional forces on the stem, the valve's movement will lag or exhibit speed deviations, leading to excessive stem movement and reduced fluid control performance.

Linear control valves face greater sealing challenges than rotary control valves. Similar to quarter-turn valves, the stem motion of rotary control valves involves only rotational movement, and the surface area of the stem that needs to be sealed is significantly smaller than that of linear control valves. This gives rotary control valves a certain advantage in sealing because a smaller surface area means less contact area between the sealing elements and the stem, thereby reducing friction and wear. Therefore, when selecting control valves, it is necessary to choose appropriate sealing elements based on their motion patterns and sealing requirements to ensure the reliability and performance of the valves.

The stem material of metallurgical special valves is usually relatively soft, which requires special attention when selecting sealing elements. Ideally, the material of the sealing elements should be softer than that of the stem to minimize stem wear. Additionally, some metallurgical special valves have relatively low yield strength of the gland bolts, and it is necessary to avoid the load on the sealing elements approaching the maximum stress they can withstand. Therefore, when selecting sealing elements, it is necessary to consider the stem material and the strength of the gland bolts comprehensively to ensure that the sealing elements can work under appropriate loads and achieve reliable sealing.

Both small and large valves have their unique characteristics and challenges in sealing. For small valves, the annular cross-section between the stem and the inner wall of the stuffing box is relatively small. In some cases, small size is not necessarily an advantage because it limits the range of sealing element choices. The annular cross-section of small valves is typically only 0.125 inches, making it difficult to install sealing elements that are made of robust materials and have novel designs. Therefore, when designing small valves, special attention must be paid to the size of the stuffing box and the selection of sealing elements to ensure the sealing performance of the valves.

Large valves are not without problems either. The large size can lead to excessive loads on the stem and packing set. When the valve vibrates, the resulting forces may be too great for standard sealing elements. Additionally, the temperature differences between different cross-sectional parts of large valves can be significant, leading to structural deformation. Therefore, when designing large valves, it is necessary to consider the impact of load distribution and temperature changes on the sealing elements to select appropriate sealing elements and structural designs, ensuring the reliability and performance of the valves.

For most types of valves, the ideal proportion of stuffing box dimensions is that the cavity height is approximately three to five times the cross-sectional diameter. This proportion ensures that the sealing elements have enough space to achieve tight sealing while avoiding consolidation of the sealing components, which can lead to loss of sealing stress. For quarter-turn valves with low sealing requirements, even a shallow stuffing box can effectively seal. However, a stuffing box that is too deep means that the sealing components are prone to consolidation, leading to loss of sealing stress and subsequent leakage. Secondly, a deep stuffing box can generate high frictional forces on the stem, which may become an obstacle in some applications. Therefore, when designing the stuffing box, it is necessary to consider the sealing requirements and the motion pattern of the valve comprehensively to select the appropriate stuffing box dimensions.

Depending on the specific circumstances of various sealing systems, the sealing elements and valve body surface treatment processes must be reasonably matched. Taking O-rings as an example, a relatively smooth valve body surface is required so that the O-ring can closely fit and achieve good sealing effects. Other sealing elements may need a rougher surface to seal effectively. In many cases, the stem surface of new valves is too smooth, leading to excessive friction and stick-slip effects with the sealing elements. Low-friction sealing elements, such as polytetrafluoroethylene (PTFE)-based seals, can avoid these undesirable phenomena. However, carbon/graphite-based sealing elements may encounter problems with too smooth surfaces. Additionally, the surface treatment of the stuffing box cavity should also match the sealing elements to ensure that the sealing elements can work under appropriate surface conditions, thereby achieving reliable sealing.

The sealing performance of valves is a crucial factor in fluid control systems. The structure and motion patterns of different types of valves significantly influence the choice of sealing solutions. By gaining a deep understanding of the sealing characteristics of various valves and making rational choices of sealing elements and surface treatment processes, the sealing performance and reliability of valves can be effectively enhanced. In practical applications, it is necessary to consider factors such as the operation frequency, motion pattern, size, material, and sealing requirements of the valves comprehensively to ensure that the selected sealing solutions meet the specific application needs. Only in this way can the safe and efficient operation of fluid control systems be ensured.