Mechanical seals types selection
Mechanical seals are devices that create a sealed barrier between surfaces in relative motion, preventing fluid leakage in rotating equipment such as centrifugal pumps, compressors, and agitators. Their operation is based on the sliding contact between two flat faces perpendicular to the shaft, one rotating and one stationary, subjected to hydraulic and spring load. Selection of the appropriate type depends on operating pressure, temperature, speed, fluid nature, and environmental requirements, and can be classified into single, double, balanced, and unbalanced seals, each with specific capacity ranges and applications.
Operating principle of mechanical seals
Section titled “Operating principle of mechanical seals”A mechanical seal operates through intimate contact between two extremely flat and polished annular surfaces, one fixed to the rotating shaft and the other to the stationary housing. The closing load, generated by fluid pressure and a spring (single or multiple), maintains a lubricating film of micrometric thickness between the faces, reducing friction and wear. This film is formed by the controlled entry of the process fluid or a barrier fluid in double configurations. Sealing is achieved by balancing the hydrodynamic opening force and the closing force, ensuring typical leakage below 1 cm³/h / 0.061 in³/h, well below conventional packings.
Types of mechanical seals
Section titled “Types of mechanical seals”Mechanical seals are classified according to their construction configuration:
- Single seal: Composed of one pair of contact faces. Suitable for clean, non-toxic, non-flammable fluids. Working pressures up to 20 bar / 290 psi for unbalanced versions.
- Double seal: Incorporates two pairs of faces arranged in series (“face-to-face” or “back-to-back”), with a barrier fluid between them at a pressure higher than the process. Used for hazardous, toxic, or contaminating fluids. Reaches barrier pressures up to 30 bar / 435 psi.
- Tandem seal: Two single seals mounted in opposite orientation; the secondary one acts as a backup without barrier pressure higher than the process. For low-pressure fluids or where contamination is not allowed.
- Balanced seal: Designed to reduce the hydraulic load on the faces by means of a step on the shaft or sleeve, allowing operation at pressures above 20 bar / 290 psi, up to 50 bar / 725 psi in standard versions.
- Unbalanced seal: The full hydraulic load acts on the faces. Limited to pressures below 20 bar / 290 psi and moderate speeds.
- Bellows seal: Replaces the helical spring with a metallic or elastomeric bellows that provides the closing load and eliminates the need for a secondary dynamic element. Ideal for fluids with solids or crystallizing fluids.
- Cartridge seal: Pre-assembled and factory-adjusted seal that simplifies installation and reduces assembly errors.
Selection criteria by application
Section titled “Selection criteria by application”The selection of the mechanical seal is determined by the following process parameters:
| Parameter | Typical application range | Recommended seal type |
|---|---|---|
| Operating pressure | 0 – 20 bar / 0 – 290 psi | Unbalanced single |
| 20 – 50 bar / 290 – 725 psi | Balanced single or double | |
| > 50 bar / 725 psi | Balanced with external pressure support | |
| Operating temperature | -50 °C – 200 °C / -58 °F – 392 °F | Single with standard elastomers (NBR, EPDM) |
| 200 °C – 400 °C / 392 °F – 752 °F | Metal bellows, PTFE or perfluoroelastomer (FFKM) | |
| Peripheral speed | 0 – 20 m/s / 0 – 65.6 ft/s | Balanced single, carbon-ceramic faces |
| 20 – 35 m/s / 65.6 – 115 ft/s | Double with forced cooling | |
| Process fluid | Clean, non-toxic, low viscosity | Unbalanced single |
| With abrasive or crystallizing solids | Bellows, double with clean barrier | |
| Toxic, flammable, hazardous | Pressurized double or tandem with containment | |
| High viscosity (> 500 cP / 500 mPa·s) | Single with silicon carbide faces | |
| Face material | Water, light oils, solvents | Carbon vs. ceramic (Al₂O₃, 96 %) |
| Acids, slurries, corrosive fluids | Silicon carbide (SiC) vs. silicon carbide | |
| Shaft | 12 – 100 mm / 0.472 – 3.937 in (diameter) | Single or cartridge per balance and pressure |
Standard dimensions of mechanical seals (Type 21, unbalanced single seal)
Section titled “Standard dimensions of mechanical seals (Type 21, unbalanced single seal)”| Shaft diameter | Seal diameter (OD) | Total length | Free height (spring load) | Fixed/rotating face material |
|---|---|---|---|---|
| 12 mm / 0.472 in | 23.5 mm / 0.925 in | 20.5 mm / 0.807 in | 14.7 mm / 0.579 in | Carbon – Ceramic |
| 16 mm / 0.630 in | 27.0 mm / 1.063 in | 22.3 mm / 0.878 in | 17.1 mm / 0.673 in | Carbon – Ceramic |
| 25.4 mm / 1.000 in | 39.4 mm / 1.552 in | 25.4 mm / 1.000 in | 26.2 mm / 1.031 in | Carbon – Ceramic |
| 31.8 mm / 1.250 in | 47.7 mm / 1.878 in | 27.0 mm / 1.062 in | 32.8 mm / 1.291 in | Carbon – Ceramic |
| 38.1 mm / 1.500 in | 53.9 mm / 2.124 in | 28.6 mm / 1.125 in | 39.3 mm / 1.546 in | Carbon – Ceramic |
| 44.5 mm / 1.750 in | 62.1 mm / 2.446 in | 34.9 mm / 1.375 in | 45.6 mm / 1.796 in | Carbon – Ceramic |
| 50.8 mm / 2.000 in | 69.2 mm / 2.724 in | 38.1 mm / 1.500 in | 52.2 mm / 2.055 in | Carbon – Ceramic |
Typical dimensions of Type 21 seal (John Crane) available in commercial catalogs. Seat tolerances follow ISO 286-2 H7/f7 for housing and shaft. The maximum permissible wear on the carbon face is 0.5 mm / 0.020 in before replacement.
Assembly and precautions
Section titled “Assembly and precautions”- Verify that the shaft has a surface finish of 0.8 – 1.2 µm Ra / 32 – 48 µin Ra and hardness ≥ 30 HRC in the contact zone with the bellows or secondary seal.
- Thoroughly clean the housing and shaft; any solid particle will cause premature face failure.
- Lubricate the faces and elastomers with the process fluid or a compatible lubricant before assembly; never install dry.
- Apply uniform axial pressure during installation; do not hit the seal with a hammer. Use the installation sleeve supplied with cartridge seals.
- Adjust the spring compression according to the indicated free height (typically 1.5 – 2.5 mm / 0.059 – 0.098 in initial compression).
- After installation, manually rotate the shaft to verify there is no metallic rubbing or jamming.
- During startup, check visual leaks and temperature at the seal housing; minimal initial leaks are normal during bedding-in (up to 24 h).
Common failures and diagnosis
Section titled “Common failures and diagnosis”| Failure | Symptom | Probable cause | Solution |
|---|---|---|---|
| Excessive leakage from startup | Drip ≥ 10 cm³/min / 0.61 in³/min | Damaged faces, incorrect assembly, stuck spring | Replace faces; verify compression height |
| Leakage after short operation | Sudden increase in leakage | Face breakage due to thermal shock or reverse pressure | Install balanced seal; control start and stop |
| Accelerated wear of stationary face | Life ≤ 2000 h | Abrasives in fluid, unsuitable face | Change to SiC; install double seal with barrier |
| Secondary elastomer deformation | Swelling, loss of elasticity | Chemical incompatibility or excessive temperature | Change to FFKM or PTFE per fluid and temperature |
| Dry seal (operation without liquid) | Smoke, discoloration, high temperature | Lack of priming, cavitation, blocked cooling line | Improve suction conditions; install API cooling plan |
Comparison of mechanical seal types
Section titled “Comparison of mechanical seal types”| Seal type | Max. working pressure | Max. temp. | Solids handling capacity | Relative cost |
|---|---|---|---|---|
| Unbalanced single | 20 bar / 290 psi | 200 °C / 392 °F | Low | 1x |
| Balanced single | 50 bar / 725 psi | 260 °C / 500 °F | Low | 1.5x |
| Pressurized double | 30 bar / 435 psi (barrier) | 300 °C / 572 °F (with cooling) | High (clean barrier) | 3x |
| Tandem | 10 – 20 bar / 145 – 290 psi | 200 °C / 392 °F | Medium | 2.5x |
| Metal bellows | 40 bar / 580 psi | 400 °C / 752 °F | High (self-cleaning) | 2x |
| Single cartridge | 25 bar / 363 psi | 200 °C / 392 °F | Low | 1.8x |
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the maximum pressure that a single mechanical seal can withstand?
Section titled “What is the maximum pressure that a single mechanical seal can withstand?”The maximum pressure in an unbalanced single seal is 20 bar / 290 psi, while a balanced seal reaches 50 bar / 725 psi without external support. For pressures above 70 bar / 1015 psi, a double configuration with barrier pressure control is required.
What maximum temperature can a standard mechanical seal withstand?
Section titled “What maximum temperature can a standard mechanical seal withstand?”Seals with standard elastomers operate up to 200 °C / 392 °F. Versions with metal bellows and PTFE or FFKM gaskets can reach 400 °C / 752 °F while maintaining a leakage rate below 5 cm³/h / 0.305 in³/h.
How long does a mechanical seal last under normal conditions?
Section titled “How long does a mechanical seal last under normal conditions?”The average service life of a well-selected and installed mechanical seal is 15,000 to 25,000 hours of continuous operation (approximately 2 to 3 years in continuous service), and can be reduced to 5,000 hours if the fluid contains abrasive solids above 5% by weight or the temperature exceeds 300 °C / 572 °F.
When is a double seal needed instead of a single one?
Section titled “When is a double seal needed instead of a single one?”A double seal is used when the fluid is hazardous, toxic, flammable, with a flash point below 60 °C / 140 °F, or when the vapor pressure of the fluid at pumping temperature exceeds atmospheric pressure. It is mandatory per API 682 standards in services with H2S greater than 50 ppm.
What clearance should the seal have with respect to the shaft?
Section titled “What clearance should the seal have with respect to the shaft?”The diametral clearance between the shaft and the seal should be 0.1 to 0.2 mm / 0.004 – 0.008 in for shafts of 12 to 50 mm / 0.472 – 1.969 in, and 0.2 to 0.3 mm / 0.008 – 0.012 in for larger diameters, in order to allow free axial movement and avoid interference.
How is wear on a mechanical seal measured?
Section titled “How is wear on a mechanical seal measured?”Wear is evaluated by inspecting the depth of the track on the stationary face; wear greater than 0.5 mm / 0.020 in or loss of flatness greater than 0.9 µm / 35 µin indicates need for replacement. Flatness must be maintained within 0.3 µm / 12 µin per 25 mm / 1 in of diameter.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/ductwork-seal-classes-d_2152.html
- mcmaster.com: https://www.mcmaster.com/products/mechanical-seals