A Comprehensive Introduction to Mechanical Seals

I. Definition of Mechanical Seal

A mechanical seal is a device designed to seal the gap between a rotating shaft and its housing. It consists of at least one pair of sealing faces perpendicular to the shaft axis. Under the combined action of fluid pressure and the elastic (or magnetic) force from compensation mechanisms – assisted by secondary sealing elements – these faces maintain contact while sliding relative to each other, thereby preventing fluid leakage. Widely used in rotating fluid machinery such as pumps, compressors, and reactor agitators, mechanical seals also serve as shaft seals in gearboxes, ship stern shafts, and similar applications. As such, it is a universal shaft sealing solution.

Mechanical seals vary in design, with the face seal being the most common type. In a face seal, the stationary ring and rotating ring form a friction pair responsible for preventing media leakage. The stationary ring and rotating ring must exhibit excellent wear resistance. The rotating ring must move freely axially to automatically compensate for seal face wear, ensuring optimal contact with the stationary ring. The stationary ring requires some flexibility to act as a buffer. Consequently, sealing faces demand high machining quality to guarantee effective contact. Key components of a mechanical seal include: Stationary ring, Rotating ring, Gland (seal housing), Drive sleeve (push ring), Spring, Positioning ring, Shaft sleeve, Rotating ring seal, Stationary ring seal, Shaft sleeve seal etc.

Elastic Elements (Springs, Bellows)
Primarily providing preload, compensation, and buffering, these elements must retain sufficient elasticity to overcome friction from secondary seals and drive components, as well as the inertia of the rotating ring. This ensures consistent contact between the sealing faces and allows the rotating ring to track axial movement. Materials must resist corrosion and fatigue.

Secondary Seals (O-rings, V-rings, U-rings, Wedge rings, Custom profiles)
These elements primarily seal the stationary and rotating rings while also enabling floatation and buffering. Seals for the stationary ring must ensure leak-tightness between the ring and the gland, while allowing limited axial movement. Seals for the rotating ring must prevent leakage between the ring and the shaft (or sleeve) and permit axial floatation. Materials must withstand heat and other operational conditions.

Basic Components

1) Face Seal Friction Pair (Stationary ring/Rotating ring interface)

2) Buffer, Compensation, and Preloading Mechanism (Springs/bellows with drive components)

3) Secondary Seals (Flexible Elements) (O-rings/V-rings/etc.)

4) Drive Mechanism (Components transmitting shaft rotation to rotating ring)

Primary Sealing Points

1) Main Sealing Interface (Contact face between stationary and rotating rings)

2) Stationary Ring-to-Gland Sealing Point (Secondary seal at static component)

3) Rotating Ring-to-Shaft (Shaft Sleeve) Sealing Point (Secondary seal at rotating component)

4) Gland-to-Pump Housing Sealing Point (Static gasket connection)

 

II. Types and Applications of Sealing Materials
Sealing materials must fulfill functional sealing requirements. Given variations in sealed media and equipment operating conditions, materials must demonstrate specific adaptability. General performance criteria for sealing materials include:

1) Low Permeability: High density to minimize media leakage.

2) Mechanical Robustness: Adequate strength and hardness.

3) Elastic Recovery: Excellent compressibility and resilience with minimal permanent set.

4) Thermal Stability: No softening/decomposition at high temperatures; no hardening/ embrittlement at low temperatures.

5) Chemical Resistance: Maintain stable volume and hardness during long-term exposure to acids, alkalis, oils, and other media; non-adherent to metal surfaces.

6) Tribological Performance: Low friction coefficient and high wear resistance.

7) Conformability: Flexibility to establish effective sealing contact.

8) Durability: Superior aging resistance and long service life.

9) Manufacturability: Ease of processing, cost-effectiveness, and material availability.

 

III. Working Mechanism of Mechanical Seals

Understanding leakage is fundamental to mastering sealing technology. By analyzing leakage principles, the corresponding sealing mechanisms become clear.

There are three primary leakage types:

Interfacial Leakage: Escape through gaps between sealing faces.

Permeation Leakage: Sealed fluid migration through material capillaries.

Diffusive Leakage: Mass transfer driven by concentration gradients through gaps or capillaries.

Sealing Methodologies
Key sealing approaches include:

Minimize Sealing Points: Reduce potential leakage paths.

Blocking & Isolation: Physically obstruct leakage channels.

Injection/Withdrawal: Introduce barrier fluid or divert leaks.

Leakage Path Resistance: Increase flow resistance in leakage channels.

Energy-Absorbing Elements: Incorporate dynamic components in flow paths.

Hybrid Methods: Combine multiple sealing strategies.

Common Seal Types including Gasket Seals, Packing Seals (includes: Soft Packing, Hard Packing, Molded Packing [O-rings, Y-rings, Oil Seals]), Mechanical Seals, Non-Contact Seals (includes: Clearance Seals, Labyrinth Seals, Floating Seals, Fluid Dynamic Seals, Magnetic Fluid Seals, Hermetic Seals), Pressure-Activated Leak Repair Seals.

Performance of Common Gaskets

When using valves, it is often necessary to replace the original gasket based on specific conditions. Common gasket types include: rubber flat gaskets, rubber O-rings, plastic flat gaskets, PTFE-clad gaskets, asbestos rubber gaskets, metal flat gaskets, metal profile gaskets, metal-jacketed gaskets, corrugated gaskets, and spiral wound gaskets.

Rubber Flat Gasket: Easily deformable and requires minimal compression force. However, its pressure and temperature resistance are relatively poor, limiting its use to low-pressure, low-temperature applications. Natural rubber offers moderate acid and alkali resistance, with a maximum service temperature of 60°C. Neoprene (chloroprene rubber) can withstand certain acids and alkalis up to 80°C. Nitrile rubber (NBR) is oil-resistant and usable up to 80°C. Fluorocarbon rubber (FKM) provides excellent chemical resistance and superior temperature resistance compared to general rubbers, suitable for media temperatures up to 150°C.

Rubber O-ring: Features a circular cross-section which provides a degree of self-sealing. Offers better sealing performance than flat gaskets and requires even less compression force.

Plastic Flat Gasket: The primary advantage of plastics is excellent corrosion resistance; however, most plastics have poor temperature resistance. Polytetrafluoroethylene (PTFE) is the preeminent plastic, offering exceptional chemical resistance and a wide usable temperature range from -180°C to +200°C for continuous service.

PTFE-clad Gasket: Designed to leverage PTFE's advantages while compensating for its lower elasticity. Consists of a rubber or asbestos rubber core fully enclosed (clad) by PTFE. This combines the corrosion resistance of a solid PTFE gasket with good elasticity, enhancing sealing effectiveness and reducing the required compression force. Its cross-section is shown in Figure 4-20.

Asbestos Rubber Gasket: Cut from asbestos rubber sheet. Its composition is 60-80% asbestos fiber, 10-20% rubber, plus fillers and vulcanizing agents. It offers good heat resistance, cold resistance, and chemical stability, is readily available, and inexpensive. Requires only moderate compression force. Since it can adhere to metal surfaces, it is advisable to coat the surface with graphite powder to facilitate disassembly.

Asbestos rubber sheets are color-coded:

Gray: For low pressure (Grade XB-200, max. pressure ≤ 16 kg/cm², max. temp. 200°C)

Red: For medium pressure (Grade XB-350, max. pressure 40 kg/cm², max. temp. 350°C)

Purple-red: For high pressure (Grade XB-450, max. pressure 100 kg/cm², max. temp. 450°C)

Green: For oil services, also offers good pressure resistance.

Metal Flat Gasket:

Lead: Max. temp. 100°C (Poor pressure resistance)

Aluminum: Max. temp. 430°C (Pressure resistance ~64 kg/cm²)

Copper: Max. temp. 315°C

Low Carbon Steel: Max. temp. 550°C

Silver: Max. temp. 650°C

Nickel: Max. temp. 810°C

Monel (Ni-Cu alloy): Max. temp. 810°C

Stainless Steel: Max. temp. 870°C

(Except lead and aluminum, materials listed can withstand high pressure).

Metal Profile Gaskets:

Lens Ring Gasket: Self-sealing, used in high-pressure valves.

Oval Ring Gasket: Also a high-pressure, self-sealing type.

Conical Twin Ring Gasket: Used for high-pressure internal self-sealing.

Other profiles (e.g., Square, Diamond, Triangular, Serrated, Dovetail, B-type, C-type) are generally used only in medium and high-pressure valves.

Metal-jacketed Gasket: Combines the excellent temperature and pressure resistance of metal with good elasticity. Jacket materials include aluminum, copper, low-carbon steel, stainless steel, and Monel. Filler materials include asbestos, PTFE, and fiberglass.

Corrugated Gasket: Characterized by low required compression force and effective sealing. Often utilizes a metal-nonmetal composite construction.

Spiral Wound Gasket: Formed by spirally winding thin, alternating layers of metal strip and non-metallic filler tape into a multi-layered ring. The cross-section is wave-like, providing excellent elasticity and sealing. Metal strip materials include 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel, etc. Non-metallic filler materials include asbestos and PTFE.

* Important Note Regarding Performance Data:
The numerical values (pressure/temperature ratings) mentioned above for gasket performance are closely related to flange design, media properties, and installation/repair techniques. Actual performance may sometimes exceed or fall short of these values. Furthermore, pressure and temperature capabilities are interdependent; for instance, pressure resistance typically decreases as temperature increases. These nuanced factors can only be fully understood through operational experience.

New Materials and Technologies

The gaskets introduced previously are by no means exhaustive, especially considering the rapid development in sealing technology. Below are examples of several new materials and techniques:

Liquid Sealing: With the rapid advancement of high-molecular-weight organic synthetic industries, liquid sealants have emerged for static sealing applications. This new technology is commonly referred to as liquid sealing. The principle relies on the adhesive properties, fluidity, and monomolecular film effect (where thinner films exhibit a greater tendency to self-recover) of the liquid sealant. Under appropriate pressure, it functions like a gasket. Hence, the applied sealant is also called a liquid gasket.

Unsintered PTFE (Tape) Sealing: Polytetrafluoroethylene (PTFE) is also a high-molecular-weight organic compound. Before being sintered into finished products, it exists as a soft, unsintered material (raw stock) which also exhibits the monomolecular film effect. Tape made from this unsintered material is called PTFE tape (or thread seal tape), which can be stored long-term on spools. During use, it is highly formable, allows for easy splicing, and forms a uniform, sealing annular film under pressure. To use it as a gasket between the valve body and bonnet, simply pry open a small gap (without removing the disc or gate) and insert the PTFE tape. It requires low compression force, is non-tacky to hands or flange faces, and is very easy to replace. It is particularly well-suited for tongue-and-groove flanges. Unsintered PTFE can also be formed into tubes or rods for sealing purposes.

Metal Hollow O-Ring: Offers excellent elasticity, requires low compression force, and provides a self-sealing action. Various metal materials can be selected, making it suitable for use with cryogenic temperatures, high temperatures, and highly corrosive media.

Graphite Sheet Gasket: While graphite is typically perceived as brittle with limited elasticity and toughness, specially processed graphite can be soft and highly elastic. This allows graphite's superior heat resistance and chemical stability to be utilized effectively in gasket materials. Such gaskets require minimal compression force and deliver exceptionally effective sealing. This processed graphite can also be made into tape and combined with metal strips to form high-performance spiral wound gaskets. The advent of graphite sheet gaskets and graphite-metal spiral wound gaskets represents a significant breakthrough in high-temperature, corrosion-resistant sealing. 

IV. Seal Selection

Mechanical seals are categorized by operating conditions and media properties, including: High/Low-Temperature Resistant Seals, High-Pressure/Corrosion-Resistant Seals, Abrasive Media Handling Seals, Seals for Volatile Light Hydrocarbon Media. Selection requires matching seal design and materials to specific applications. Mechanical seals are universally deployed across industries including: Pump Manufacturers, Textile Machinery Plants, Paper Mills, Power Generation Facilities, Chemical/Petrochemical Plants, Pharmaceutical & Shipbuilding Industries, Wastewater Treatment Plants, Commercial Applications (Medical, Agricultural, Equipment Cooling, Leather Processing, Heavy Machinery)

Key Selection Parameters:

Seal Chamber Pressure (MPa)

Fluid Temperature (°C)

Rotational Speed (m/s)

Fluid Characteristics

Available Installation Space

Selection Principles:

Pressure-Based Configuration: determine balanced/unbalanced design, select single/double seal arrangement (Based on seal chamber pressure);

Velocity-Driven Design: choose rotating or stationary orientation, specify hydrodynamic or non-contact type (According to operational speed);

Material & System Specification: define friction pair and secondary seal materials, configure supporting systems: Lubrication/Flushing, Heat Tracing/Cooling (Driven by temperature/fluid properties);

Space-Optimized Components: select spring type: multiple/single/wave springs, choose internally/externally mounted configuration (Based on spatial constraints).

 

V. Installation

1. Dimensional Tolerances for Seal Installation (Pump Application Example)
(1) Radial runout at seal mounting location (shaft/sleeve): ≤ 0.04–0.06 mm TIR
(2) Rotor axial float: ≤ 0.3 mm
(3) Face runout of gland locating surface relative to shaft/sleeve: ≤ 0.04–0.06 mm TIR

2. Seal Verification
(1) Confirm seal model matches specifications.
(2) Cross-check components against assembly drawing for completeness.
(3) For spring-rotated seals: Match spring coil direction (left/right-hand) to shaft rotation.

3. Installation Procedure
(Methods vary by seal type and equipment, but core principles apply universally)

Steps & Precautions:
(1) Dimensional Compliance: maintain installation dimensions per manufacturer's manual or datasheet.
(2) Pre-Installation Preparation: deburr shaft/sleeve and gland surfaces; verify bearing condition; clean all components (seals, shaft, cavity, gland); apply thin oil film to shaft contact area to reuce friction, soapy water may serve as alternatives considering material compatibility of Rubber O-rings; And floating stationary rings without anti-rotation pins should also Install dry.
(3) First, mount stationary ring with gland onto shaft (avoid shaft contact), then install rotating ring assembly, after gradually tighten set screws on spring retainer/drive collar in crisscross pattern. Before gland fixation, manually compress compensation ring axially and verify smooth return without binding, finally uniformly tighten gland bolts.

4. Commissioning
(1) Implement auxiliary systems for:
- High/Low fluid temperatures → Cooling/Heating
- Abrasive media → Filtration
- Flammable/Toxic fluids → Barrier systems
(2) Pre-operation check:
- Rotate manually to assess torque
- Verify absence of scraping/abnormal noise

 

VI. Critical Considerations

1. Installation Precautions
a. Prevent Installation Misalignment
(1) Tighten gland bolts after shaft alignment. Use crisscross pattern for uniform compression. Verify face parallelism with feeler gauge: ≤ 0.05 mm deviation at any point.
(2) Check gland-to-shaft/sleeve concentricity: Maximum 0.01 mm radial clearance (uniform around circumference, verified with feeler gauge).

b. Spring Compression Compliance

Maintain specified compression tolerance: ±2.00 mm; over-compression will accelerate face wear due to excessive unit load; under-compression may lead to sealing failure due to insufficient closing force.

c. Rotating Ring Functionality

Verify free axial movement on shaft and confirm automatic return when compressed against spring.

2. Decommissioning Precautions
a. Safe Removal Procedure

Prohibit hammer/chisel use to avoid the risk of component damage; recommended tool: custom wire hooks engaged in drive notches. For seized seals: clean before disassembly.

b. Dual Seal Handling

When both ends equipped: Coordinate removal/installation to prevent oversight.

c. Post-Operation Seal Assessment

Rotating/stationary rings must be replaced if there is any gland loosening caused seal movement during service

Rationale: Once relocated faces lose track alignment, sealing integrity will be compromised

 

VII. Normal Operation and Maintenance of Mechanical Seals

1. Pre-Start Preparations and Precautions
a. Conduct a thorough inspection of the mechanical seal, ensuring all auxiliary devices and pipelines are correctly installed, complete, and meet technical requirements.
b. Perform a hydrostatic test on the mechanical seal before startup to check for leaks. If significant leakage occurs, identify and eliminate the cause. If leakage persists, disassemble the seal for inspection and reinstall. The standard hydrostatic test pressure is 2–3 kg/cm² (0.2–0.3 MPa).
c. Rotate the pump manually (barring over) in its designated direction to verify smooth and even rotation. If resistance is encountered or rotation is impossible, check for incorrect assembly dimensions or improper installation.

2. Startup and Shutdown Procedures
a. Ensure the seal chamber is completely filled with liquid before starting. For pumps handling solidifying media, use steam to heat the seal chamber and liquefy the medium. Always rotate the pump manually before starting to prevent sudden startup and fracture of soft seal rings.
b. For mechanical seals utilizing an external seal oil system, start the seal oil system before pump startup. Shut down the seal oil system last after stopping the pump.
c. After stopping a hot oil pump, do not immediately cut off the cooling water to the seal oil cavity or seal faces. Allow the oil temperature at the seal faces to drop below 80°C before stopping cooling water to prevent seal component damage.

3. Operation Procedures
a. Minor leakage may occur immediately after startup. Observe for a period; if leakage persists without reduction after 4 hours of continuous operation, stop the pump for inspection.
b. Maintain steady pump operating pressure. Pressure fluctuations should not exceed 1 kg/cm² (0.1 MPa).
c. Avoid pump cavitation during operation to prevent dry running of seal faces and seal failure.
d. Monitor seal performance regularly. If operational leakage exceeds the allowable standard (≤ 5 drops/min for heavy oils; ≤ 10 drops/min for light oils) and shows no sign of improvement within 2–3 days, stop the pump to inspect the seal assembly.