Basic Knowledge of Packing Seals

In the industrial field, whether in large-scale chemical equipment or precise mechanical devices, sealing performance is one of the key factors ensuring the normal operation of equipment. Packing seals, as an indispensable part of sealing technology, silently guard the sealing defense line of equipment with their unique sealing principles and diverse material properties. Today, let us delve deeply into the mysteries of packing seals and unveil their enigmatic veil.

The sealing principle of packing seals mainly relies on two core effects: the labyrinth effect and the bearing effect. These two effects cooperate to achieve efficient sealing.

At the microscopic level, the shaft surface is not as smooth and flat as it appears to the naked eye, but is full of tiny irregularities. When the packing seal contacts the shaft, due to these microscopic unevenness, the packing only partially fits tightly against the shaft, while small gaps remain in other parts. These gaps form a complex labyrinth. When pressurized media flow through this maze, they undergo multiple throttling stages. Each throttling reduces the media pressure, effectively preventing leakage. This labyrinth effect acts like a series of insurmountable obstacles, causing the media to lose direction within the labyrinth between the packing and shaft, ultimately achieving the sealing purpose.

In addition to the labyrinth effect, there exists a thin liquid film between the packing and the shaft. This liquid film makes the movement between packing and shaft resemble a sliding bearing. It serves not only as lubrication, reducing friction between packing and shaft, but also effectively prevents excessive wear. This lubrication effect is like laying down a smooth sliding track between packing and shaft, allowing them to slide smoothly relative to each other rather than rubbing or colliding. As a result, the service life of the packing is extended, and the stable operation of the equipment is ensured.

Packing seals work in complex and variable environments. Factors such as the sealing media's temperature, pressure, acidity/alkalinity (pH value), as well as equipment's linear speed, surface roughness, coaxiality, radial runout, and eccentricity pose severe challenges to the performance of packing seals. To meet these demanding conditions, packing materials must possess a series of excellent properties.

Elastic-plasticity refers to the material's ability to undergo both elastic and plastic deformation under external force. For packing seals, this property is crucial. During operation, when vibrations or eccentricities occur, packing seals need to flexibly adapt to these changes by elastic deformation to fill gaps and maintain sealing. Over long-term use, packing seals also undergo some plastic deformation to better conform to the shaft surface, further improving sealing performance. This elastic-plasticity is like a "transformer," flexibly adjusting its shape and state according to different working conditions to ensure sealing reliability.

In various complex chemical environments, packing seals must have good chemical stability. Whether the medium is acidic, alkaline, or neutral, the packing material must not chemically react to avoid sealing failure caused by corrosion. Chemical stability is like a "steel guardian," resisting various chemical media corrosion, ensuring stable sealing performance under harsh chemical conditions.

Impermeability directly reflects the sealing performance of packing seals. It requires that no microscopic channels exist within the packing material that allow media penetration. Even under high pressure and high temperature, media must not leak through the internal structure of the packing. This impermeability is like a “water-tight guardian,” firmly protecting the equipment's sealing defense line and ensuring no leakage into the external environment.

Self-lubrication means the material itself possesses lubricating properties, able to reduce friction with other materials without external lubricants. Good self-lubrication in packing seals effectively lowers the friction coefficient between the packing and shaft, reducing wear. This not only prolongs the service life of both packing and shaft but also reduces energy consumption during operation. Self-lubrication acts as a “lubrication messenger” between packing and shaft, laying down a smooth lubricating layer to enable smoother relative motion.

Industrial equipment often operates in high-temperature environments, demanding packing seals with good temperature resistance to maintain stable performance within specified temperature ranges. Whether direct contact with high-temperature media or heat generated during operation, the packing material must not degrade due to rising temperatures. Temperature resistance acts as the “thermal stability guardian,” ensuring reliable sealing even under high-temperature conditions.

Besides the above requirements, packing seals also need to satisfy practical conditions such as easy installation/removal, simple manufacturing, and low cost. During equipment maintenance and inspection, being able to quickly and conveniently disassemble and install packing seals is very important. Simple manufacturing processes ensure stable quality, and low cost helps reduce maintenance expenses. These three act like the “three driving forces” of practicality, jointly promoting the widespread use of packing seals in industry.

With advancing industrial technology, braiding forms of packing seals have diversified. Each type offers unique structural and performance benefits to meet specific sealing requirements.

This form uses eight bobbins operating on two tracks, resulting in a square cross-section. It is relatively loose, compensating for vibration and eccentricity, making it suitable for small cross-section packing. However, for larger cross-sections, it may suffer from rough patterns, structural looseness, and poor compactness, which degrade sealing and shorten lifespan.

Commonly used, this structure involves 8, 12, 16, up to 60 bobbins, with 1–4 braided layers. It offers high density and strong sealing performance but lacks fiber interconnections between layers, leading to possible delamination. Thus, it's best for static seals or low-speed equipment under stable conditions.

This special braid incorporates a rubber or metal core wrapped in successive fiber layers. It offers good compactness, strength, flexibility, and sealing. It disperses stress under pressure and friction, extending lifespan. However, surface layers may peel off after wear, limiting its use to pumps and valves (rarely in reciprocating systems).

An advanced form braided on three or four tracks using 8–60 bobbins, producing square sections. It features a smooth surface, excellent elasticity, high wear resistance, and compactness. It has greater and more uniform shaft contact area than braid over braid, with minimal fiber gaps, offering superior sealing. Even after surface wear, it remains intact and durable—ideal for modern complex applications.

Due to differing working conditions, packing seals come in many types. To better distinguish and select packing seals, they are generally classified by the main sealing base material:

Mainly made from natural cotton, hemp, wool, etc. These natural fibers have good elasticity and some wear resistance, suitable for ordinary sealing conditions. However, their temperature resistance and chemical stability are relatively poor, with performance degradation in high temperature, strong acid, or alkali environments. Thus, natural fiber packing is mostly used in lower temperature and less corrosive media.

Includes asbestos packing and similar. Asbestos fibers have excellent temperature resistance and chemical stability, maintaining stability in high temperature, strong acid, and strong alkali environments. However, asbestos is harmful to human health, and long-term exposure can cause serious health issues. Hence, asbestos packing is increasingly restricted in applications demanding high environmental and health standards.

A recently developed category with high-performance materials, including graphite packing, carbon fiber packing, PTFE (polytetrafluoroethylene) packing, Kevlar packing, acrylic-silicone fiber packing, etc. These synthetic fibers offer excellent overall properties like good temperature resistance, chemical stability, wear resistance, and self-lubrication. Graphite packing has good self-lubrication and chemical stability, suitable for high temperature, high pressure, and strongly corrosive media; carbon fiber packing features high strength and wear resistance, suitable for high speed and high pressure sealing; PTFE packing has outstanding chemical stability and good temperature resistance, applicable for various corrosive media. Synthetic fiber packing brings new options to industrial sealing, meeting diverse complex conditions.

Includes silicon carbide packing, boron carbide packing, alkali-resistant glass fiber packing, etc. These materials boast extremely high temperature resistance and good chemical stability, suitable for extreme high temperature and strongly corrosive environments. For example, silicon carbide packing remains stable up to 1600℃, ideal for high-temperature furnaces; boron carbide packing excels in wear resistance and corrosion resistance, suitable for high speed, high pressure, and strongly corrosive media. However, ceramic and metal fiber packing have higher manufacturing costs and complex processing, thus mostly used in special applications.

Material properties of packing seals directly affect sealing performance and service life. To better evaluate and select packing seals, several key performance indicators are considered:

The ratio of material thickness compressed under load to its original thickness. It reflects packing's compressibility. A higher compression ratio means packing can achieve greater deformation under less force, better conforming to the shaft and improving sealing. But too high compression may cause over-compression, affecting elasticity and rebound.

The ratio of material rebound after unloading to its compression during loading. It reflects the packing's ability to restore shape after stress removal. Good rebound means packing can quickly recover after vibration or impact, maintaining seal integrity. Higher rebound implies better elasticity and fatigue resistance, adapting to dynamic operation.

Degree of material loss caused by relative motion between two surfaces. Packing experiences wear against the shaft during operation. Wear-resistant packing maintains sealing longer, reducing leakage risk. Wear resistance depends on hardness, toughness, and surface treatment.

Material's inherent lubrication property. Good self-lubrication lowers friction between packing and shaft, reducing wear and enhancing efficiency and reliability. Self-lubricating packing needs no external lubricant, avoiding contamination and corrosion.

Percentage decline in stress under constant strain over time. Long-term compression causes internal stress relaxation. Lower stress relaxation means packing maintains compression and sealing better during service.

Percentage of mass lost after burning at specified temperature/time, reflecting high-temperature stability. Lower thermal loss means packing retains physical and chemical properties under heat, ensuring sealing at high temperatures.

Percentage mass loss after acid or alkali treatment, reflecting chemical stability in acidic and alkaline environments. Lower losses mean better resistance to chemical corrosion, ensuring sealing in various chemical media.

In practical applications, selecting suitable packing seals is crucial for ensuring equipment sealing performance. Selection must consider specific working conditions such as media properties, temperature, pressure, operating speed, shaft surface roughness, etc., alongside packing material characteristics, weaving form, and performance indicators.

For example, for sealing high temperature, high pressure, and strongly corrosive media, graphite or PTFE packing with through-core weaving ensures good sealing and long service life; for high-speed, high-pressure equipment, carbon fiber packing with proper weaving form is ideal; for ordinary conditions, natural fiber or synthetic fiber packing may suffice, but wear resistance and self-lubrication must be considered.

In summary, packing seal selection requires comprehensive analysis and balance based on specific conditions, adapting the solution to the “work” to maximize sealing performance and ensure equipment stability.

Though packing seals are small components in industrial equipment, they bear the important mission of ensuring sealing performance. By thoroughly understanding their sealing principles, material properties, weaving forms, types, and performance indicators, we can better select appropriate packing seals, thereby improving equipment sealing and operational reliability. With their unique properties and advantages, packing seals safeguard the stable operation of various equipment and have become indispensable products in the sealing field.