In the industrial sector, the normal operation of various mechanical equipment relies heavily on effective sealing technologies. Packing seals, one of the oldest forms of sealing, continue to play a vital role in numerous applications to this day. This article will delve into the working principles, advantages, leakage mechanisms, and ways to enhance the performance of packing seals through modern technology, helping readers gain a comprehensive understanding of the modern value of this traditional sealing technique.
Packing seals are one of the oldest forms of shaft seals, with a history that dates back a long time. This type of seal is structurally simple and extremely convenient to use and maintain. It is suitable for sealing rotating and reciprocating shafts and rods, as well as low-speed spiral motions (such as valve stems). Although many centrifugal pumps and other rotary machinery shaft seals have been replaced by mechanical seals in modern industry, packing seals still find widespread application due to their unique advantages, especially in high-temperature, highly corrosive, and solid particulate-laden media conditions.
Before we dive deeper into packing seals, let's explore how they work. Only by understanding their working principles can we better grasp their strengths and limitations, and thus maximize their efficiency in practical applications. So, how exactly do packing seals work? Let's gradually uncover the mystery.
The core function of sealing is to prevent leakage. When a sealing device fails to control the normal allowable leakage rate, it means the seal has failed. The failure of packing seals is different from that of mechanical seals; it usually does not result in catastrophic leakage without warning, but rather a gradual increase in leakage. People always hope that once installed, packing seals can operate stably over a long period.
The failure of packing seals is mainly related to the volume changes of the packing and the gland. During operation, the volume of the packing may decrease due to the loss of lubricant, fiber shrinkage, wear, chemical erosion, thermal decomposition, extrusion, and other reasons. Meanwhile, wear on the shaft can cause the gland volume to increase. Therefore, the goal of research on soft packing seals is to extend the cycle of this change process.
There are two fundamental reasons for seal leakage: First, there is a pressure difference (or concentration difference) between the two sides of the seal, or relative motion along the leakage direction. Flow caused by a pressure difference is called pressure-driven flow, leakage caused by a concentration difference is called diffusion leakage, and flow caused by relative motion is called shear flow. Second, there exists a leakage channel, meaning the fluid flow resistance is not infinitely large. Eliminating or reducing either of these factors can prevent or reduce leakage, with the leakage channel factor having the most essential impact on sealing performance.
Why does this ancient sealing technology still hold an important position in modern industry? The answer lies in its unique advantages. These advantages enable packing seals to perform exceptionally well in many applications, and even outperform other sealing technologies under certain specific conditions.
The good lubrication performance of packing seals is a necessary condition for their long-term operation. It ensures low friction power consumption and wear rate. To maintain good lubrication conditions, packing seals typically allow a small amount of leakage. For general packing (excluding self-lubricating packing), they only throttle the fluid flow rather than completely stopping or sealing it. By impregnating the packing with lubricants or enhancing the packing's self-lubricating ability, good lubrication performance can be ensured.
The essence of soft packing seals is to utilize the flexibility of the packing material. Under axial compressive force, the packing expands radially to block potential fluid leakage channels. The design theory of packing seals suggests that the force applied to the soft packing must generate a contact stress between the packing and the sealed surface that can block the pressure of the sealed fluid. The flexibility of the packing allows it to easily fill the micro-leakage channels of the sealing interface after deformation, with low friction power consumption. Good packing resilience can compensate for stress relaxation caused by volume loss and reduce the adverse effects of shaft out-of-roundness and wobble on sealing performance.
To prevent or reduce leakage caused by adhesion, it is necessary to minimize the depth of micro-grooves and reduce the surface roughness of the sealed shaft. An effective method to reduce and prevent dynamic leakage is to avoid residual spiral marks on the rotating shaft surface or control the direction of the spiral marks so that they are opposite to the fluid leakage direction.
Despite the many advantages of packing seals, leakage remains a critical issue that needs to be closely monitored and addressed in practical applications. To better understand and control leakage, we need to delve into the leakage mechanisms of packing seals. This not only helps us optimize seal design but also enhances the reliability and safety of equipment. So, what exactly are the leakage mechanisms of packing seals?
There are two common views on how packing seals ensure durable and stable sealing: the "bearing effect" and the "labyrinth effect." After the packing is installed in the sealing chamber and compressed axially by the gland, it generates a radial force to maintain tight contact with the shaft, establishing a sealed state. At the same time, the lubricant impregnated in the packing is squeezed out, forming a liquid film between the contact surfaces, creating a boundary lubrication state similar to a sliding bearing, hence the term "bearing effect."
The labyrinth effect refers to the formation of small grooves with a thick liquid film in the non-contact cavities after the packing is compressed. When there is relative motion between the shaft and the packing, the contact and non-contact parts form an irregular labyrinth that prevents fluid leakage. Good sealing relies on maintaining this labyrinth effect.
Fluid leakage through packing seals occurs through three pathways: leakage through the interface between the packing and the stationary component; leakage through the packing itself; and leakage through the interface between the packing and the moving component. The leakage through the interface between the packing and the stationary component is similar to leakage through a static seal. The leakage through the packing itself depends on the permeability of the sealed fluid and the internal structure of the packing; braided packing is prone to this type of leakage. When the medium has strong permeability or is a gas, permeation leakage through the packing is almost inevitable, although the specific leakage mechanism requires further exploration. For the vast majority of liquid media, leakage mainly occurs through the interface between the packing and the moving component.
The leakage mechanisms through which the sealed fluid medium leaks through the interface between the packing and the moving component take various forms. Common ones include clearance leakage mechanism, porous leakage mechanism, adhesion leakage mechanism, and dynamic leakage mechanism. The clearance leakage mechanism refers to leakage through macroscopic gaps, with the flow of the leaking fluid following the fluid mechanics principles of fluid flow through narrow gaps. The porous leakage mechanism posits that the surface of the sealing component cannot be an ideal smooth surface; its microscopic surface shape is uneven. Many peaks and valleys often form irregular, interconnected leakage channels. Under the influence of fluid pressure difference or capillary action, fluid leaks through these channels. Adhesion leakage is a unique leakage form of liquids in reciprocating motion mating parts, while the dynamic leakage mechanism is a form of axial leakage of liquids in rotary motion mating surfaces.
By conducting a comprehensive analysis of packing seals, we have not only gained an in-depth understanding of their working principles and significant advantages but also explored their leakage mechanisms and countermeasures. As a sealing technology that has stood the test of time, packing seals still hold an irreplaceable position in modern industry. With their simple structure, strong adaptability, and convenient maintenance, they demonstrate excellent performance in many complex conditions. However, the existence of leakage issues also reminds us that continuous technological innovation and optimization are the keys to ensuring the efficient operation of packing seals.