The Sealing Principle of Packing Glands

The Sealing Principle of Packing Glands

Packing glands play a critical role in sealing rotating or reciprocating shafts in industrial equipment, including components such as titanium parallel gate valve manufacturer products. Their sealing principle primarily relies on two key effects: the labyrinth effect and the bearing effect.

1. Sealing Principle of Packing Glands
The effectiveness of packing gland seals depends mainly on the labyrinth effect and the bearing effect.

Labyrinth Effect: At a microscopic level, shaft surfaces are uneven. The packing gland cannot achieve full contact, resulting in tiny gaps between the packing and the shaft. These gaps form a labyrinth-like path. Pressurized media flowing through these gaps undergo repeated throttling, dissipating energy and achieving the sealing function.

Bearing Effect: A thin film of liquid forms between the packing gland and the shaft. This film creates a scenario similar to a sliding bearing, providing essential lubrication. This lubrication prevents excessive wear on both the packing material and the shaft surface.

2. Material Requirements for Packing Glands
The performance of packing glands is subject to demanding conditions, including the sealed medium's temperature, pressure, pH, as well as equipment factors like linear speed, surface roughness, coaxiality, radial run-out, and eccentricity. Consequently, packing materials must possess the following characteristics:

Certain Elasticity and Plasticity: To conform to the shaft and accommodate vibration/eccentricity.

Chemical Stability: Resistance to degradation by the sealed medium.

Impermeability: Minimizing leakage through the material itself.

Self-lubricating Properties: Reducing friction and wear, especially important if external lubrication is limited.

Temperature Resistance: Maintaining integrity and performance across the operating temperature range.

Ease of Installation and Removal: For maintenance and replacement.

Simplicity of Manufacture and Cost-Effectiveness: For practical application.
Finding materials that perfectly meet all these requirements is challenging. Therefore, developing superior sealing materials and enhancing their properties remains a key research focus in the sealing technology field.

3. Forms and Characteristics of Packing Glands
Advancements in production technology have led to diverse braiding methods for packing glands. The chosen braiding structure significantly impacts sealing performance and service life under specific operating conditions and environments. Common braiding techniques include:

Braided (Square Plait): Uses eight spindles on two tracks. Lacks a fluffy core at corners or center. Features a square cross-section. Offers looseness which provides some compensation for shaft vibration and eccentricity. Primarily used for small cross-sections; larger sections suffer from rough appearance, loose structure, and poor density.

Interlocked Braiding: Employs 8, 12, 16, 24, 36, 48, or 60 spindles on two tracks. The number of layers (usually 1-4) depends on packing size. Lacks a core. Offers good density and sealing capability. However, the layered structure without interconnecting fibers makes it prone to delamination. Best suited for static seals or low-speed equipment.

Jacketed (Core-Wrapped) Braiding: Uses a core (rubber or metal) wrapped with layers of fiber braid. Layer count is adjustable. Similar to interlocked braiding, it provides good density, high strength, good flexibility, and effective sealing. Like interlocked braid, the outer layer is susceptible to wear and detachment after surface damage. Commonly used in pumps and valves, rarely in reciprocating equipment.

Die-Formed (Through-Core) Braiding: Uses 8, 12, 16, 24, 36, 48, or 60 spindles on three or four tracks. Features a square cross-section, smooth surface, excellent elasticity and abrasion resistance, high strength, and superior density. Offers a larger, more uniform contact area with the shaft compared to braided packing, with minimal gaps between fibers, resulting in excellent sealing. The structure remains intact even after surface wear, ensuring long service life. This is considered an advanced braiding structure.
Selecting the appropriate braiding form based on the specific equipment's operating conditions is crucial for packing glands to deliver optimal sealing performance.

4. Classification, Composition, and Application of Packing Glands
Packing gland types are highly diverse due to varying operating conditions. To facilitate identification and selection, they are typically classified by the primary sealing base material:

Natural Fiber Packing: Primarily made from cotton, flax, jute, or wool.

Mineral Fiber Packing: Primarily asbestos-based packings (historically common, use now often restricted).

Synthetic Fiber Packing: Includes:

Graphite-based packing

Carbon fiber-based packing

PTFE (Teflon®)-based packing

Aramid (Kevlar®)-based packing

Acrylic fiber with silicone packing

Ceramic and Metallic Fiber Packing: Includes:

Silicon Carbide (SiC) packing

Boron Carbide (B4C) packing

Medium-alkali glass fiber packing
Single fiber materials often possess inherent limitations. Braiding with a single fiber leaves gaps between strands, increasing leakage potential. Furthermore, some fibers lack sufficient self-lubricity and exhibit high friction coefficients. To enhance density and lubricity, packings are impregnated with lubricants, fillers, and special additives. Common impregnants include:

Mineral oils or molybdenum disulfide (MoS2) greases blended with graphite powder.

Talc, mica, glycerin, vegetable oils.

PTFE dispersions, often including surfactants and dispersing agents.
Special additives, such as zinc particles, barrier agents, and molybdate-based corrosion inhibitors, are also used to reduce the packing's corrosiveness to equipment.

Technological Advancement: Nanotechnology
The application of nanotechnology in sealing materials is rapidly expanding. Nanotechnology unlocks inherent, often hidden, properties within materials. Some of these properties mitigate or eliminate material drawbacks while amplifying their superior characteristics.
Within the sealing industry, R&D in sealing materials is a cutting-edge domain. Through long-term collaboration with renowned domestsic and international nanotechnology research institutions, a series of nano-enhanced packing glands has been successfully developed. Patents for these innovations have been filed with the State Intellectual Property Office. This breakthrough opens new application fields for packing technology, representing a significant leap forward in packing gland sealing performance. The application of nano-enhanced packing glands leads to markedly improved equipment sealing integrity and extended service life, delivering substantial economic benefits to users.