Seals are divided into two main categories: static seals for relatively stationary joint surfaces, and dynamic seals for relatively moving joint surfaces.
1. Static Seal
A static seal refers to sealing between two stationary surfaces.
It is mainly classified into three types:
- Gasket seal
- Sealant seal
- Direct contact seal
2. Dynamic Seal
A dynamic seal is used for sealing between relatively moving parts of machinery and equipment.
2.1 Packing Seal
Packing seals are widely used for dynamic sealing of moving parts in mechanical process equipment, such as centrifugal pumps, compressors, vacuum pumps, mixers and reaction vessels.
They are also applied to plunger and rod sealing of reciprocating pumps and reciprocating compressors, as well as stem sealing of valves with linear or spiral motion.
Main Types of Packing
1) Asbestos Packing
It features excellent high temperature resistance and corrosion resistance. However, the sealing effect is poor when used alone, so it is usually impregnated or compounded with other materials.
Oil-impregnated asbestos packing has two structures: twisted type and braided type, available in round and square shapes.
2) PTFE Braided Packing
Woven with PTFE thin tapes, it offers outstanding corrosion resistance and can be used for cryogenic media.
3) Rubber O-Ring
It performs well under low pressure, but its working temperature is limited. For example, natural rubber O-rings are only suitable for temperatures below 60℃.
4) Molded Plastic Packing
Mostly designed as three-piece structure or custom shapes. The main material is PTFE; Nylon 66 and Nylon 1010 are also commonly used.
5) Soft Packing Seal
Also known as mud packing, soft packing is made of high-performance synthetic fibers with special barrier agents and lubricants.
It fits any size and shape and can be installed easily by direct injection with a special high-pressure gun, without pre-cutting like traditional packing. It is suitable for stuffing boxes of all sizes.
Application: High-pressure reciprocating pumps, low-speed pumps, high-temperature valve stems and general stuffing box sealing.
Advantages: Simple structure, stable operation and low cost, widely used in general working conditions.
2.2 Expander Ring Seal
The expander ring is a split metal elastic ring, also known as a piston ring.
In free state, the ring opening is spread; after installed into the cylinder, the opening closes tightly. The outer circle closely fits the cylinder inner wall by elastic force.
During operation, medium pressure pushes the ring against one side of the groove. Sealing is achieved by the outer cylindrical surface and the end face of the ring.
Common materials: cast iron and tin bronze.
Typical applications: piston sealing of high-temperature reciprocating compressors and internal combustion engines, as well as shaft sealing of steam turbines.
2.3 Contact Type Dynamic Seal
Gasket Seal
Sealing gaskets are essential spare parts for machinery, equipment and pipelines with fluid media.
Made of metal or non-metal sheet materials by cutting, stamping and trimming, they are used for sealed connection between pipelines and mechanical components.
- Metal gaskets: Copper, stainless steel, iron, aluminum, etc.
- Non-metal gaskets: Asbestos, non-asbestos, paper, rubber, graphite and other new-type gaskets.
Thread Seal
Two common forms:
- Threaded joint with gasket seal
- Threaded connection with filler seal
Both are used for small-diameter threaded connections.
In threaded gasket sealing, the gasket undertakes the sealing function, while the thread only provides compression force.
Sealing performance depends on gasket material, sealing surface roughness and geometric precision of threaded holes.
Non-metallic gaskets bear both pressure and torque during tightening, easy to deform, and are only suitable for low pressure.
Metal gaskets can withstand pressure above 30MPa.
Felt Ring Seal
The wool felt ring is installed in the trapezoidal groove of the bearing cover. It is pressed tightly against the shaft by the groove side to achieve sealing.
It has good flexibility, heat preservation, moisture retention and elasticity.
Welt oil seals are commonly used in gear reducers. A trapezoidal groove is designed at the bearing end cover of input and output shafts to install the felt ring, preventing dust ingress and lubricant leakage.
Oil Seal
Oil seals are used for lubricant sealing, isolating lubricated transmission parts from external parts to prevent oil leakage.
All rotating equipment with internal lubricant and external connection positions need oil seals.
Most are steel-reinforced rubber oil seals, plus pure rubber and metal types.
Typical applications: crankshaft oil seal, gearbox oil seal, half shaft oil seal, final drive oil seal, air compressor crankshaft oil seal, etc.
Fully Enclosed Seal
It completely isolates internal and external leakage channels, or integrates the prime mover and working equipment into one sealed system to eliminate medium leakage.
It is indispensable for production involving highly toxic, radioactive and rare valuable media, as well as precision laboratories.
Diaphragm seal is a typical form.
2.4 Non-Contact Type Dynamic Seal
Floating Ring Seal
Also called oil film seal, it is a high-speed sealing structure for gas and liquid.
For gas sealing, the chamber is filled with lubricating oil for sealing and lubrication.
Sealing is realized by the tiny gap between the floating metal ring and the shaft or housing.
The floating ring can move freely in the housing, less affected by high-speed rotation and vibration, with low friction and wear.
Screw Seal
A type of dynamic seal with spiral grooves processed on the rotating shaft or sleeve. Sealing medium is filled between shaft and sleeve.
The rotating shaft drives the spiral groove to produce a pumping effect to block liquid leakage.
Sealing capacity is affected by spiral angle, pitch, tooth width, tooth height, sealing length and clearance between shaft and sleeve.
No direct friction ensures long service life. Limited by structural space, spiral length is short, so sealing capacity is restricted. The sealing effect will drop obviously under low speed operation.
Centrifugal Seal
It uses centrifugal force generated by rotating components to prevent medium leakage.
Main types: back vane seal and auxiliary impeller seal, widely used in centrifugal pumps.
They allow larger sealing clearance, low wear, long service life and nearly zero leakage.
Suitable for slurry pumps and impurity pumps conveying solid-containing media.
Disadvantages: high power consumption and additional shutdown seal required.
Dry Gas Seal
A new type of shaft end seal using grooved technology for gas sealing.
When the grooved rotating ring runs, hydrodynamic grooves pump high-pressure isolation gas into the seal end face gap.
Gas film pressure changes gradually from outer diameter to inner diameter, forming a thin stable gas film to keep the seal in non-contact state.
The gas film blocks the leakage channel and realizes zero emission.
Advantages:
- Low leakage, environmentally friendly
- Low wear and long service life
- Low power consumption, only about 5% of traditional mechanical seals
- No oil contamination to sealed medium
- Simple auxiliary system and maintenance-free
Labyrinth Seal
Multiple annular sealing teeth are arranged around the rotating shaft, forming a series of throttling gaps and expansion cavities.
Medium is throttled when passing through the tortuous labyrinth channel to reduce leakage.
Advantages: Good high-speed performance, no lubrication needed, no friction, easy maintenance, long service life.
Disadvantages: High machining precision required and difficult assembly.
Magnetic Fluid Seal
Magnetic fluid is filled into the magnetic field gap to form a liquid O-ring.
It can transmit rotational motion into sealed containers and is widely used for vacuum sealing.
2.5 End Face Seal (Mechanical Seal)
End face seal relies on compression fit between perpendicular end faces.
A mechanical seal consists of at least one pair of end faces perpendicular to the rotating shaft.
Under fluid pressure, elastic compensation force and auxiliary seal cooperation, the end faces keep close contact and relative sliding to prevent leakage.
- Metal bellows mechanical seal: uses metal bellows for elastic loading and auxiliary sealing.
- Rubber bellows mechanical seal: used for light-duty working conditions, usually matched with springs for enough compression force.Mechanical seal is generally abbreviated as Mechanical Seal.
