1. What is a mechanical seal?
A mechanical seal is a sealing device that prevents fluid leakage in the region where the rotating shaft passes through the pump casing. Unlike packing (which allows small controlled dripping), the mechanical seal provides virtually total sealing.
In the FB Bombas FBCN series, the mechanical seal is one of two sealing options — the other is packing. The shaft is mounted with a protective sleeve in the sealing region, as specified in technical manual MTEC-03/00.
2. How a mechanical seal works
A mechanical seal consists of two extremely polished flat faces: a stationary face (fixed to the pump casing) and a rotating face (attached to the shaft). A spring keeps the faces in contact, and a micrometric lubricating film of the pumped fluid itself forms between them.
This film is essential: thin enough to seal, but present to lubricate and prevent dry wear. If the seal operates without fluid (dry running), the faces are damaged within seconds. This is why the centrifugal pump must never run without priming.
3. Types of mechanical seal
Industrial mechanical seals are classified by configuration and mounting method. The three main types applied to centrifugal and gear pumps are:
- Single seal — One sealing face. Suitable for water, solvents, light chemicals and general applications where minimal leakage is acceptable. Most common type in the FBCN series.
- Double seal — Two sealing faces with barrier fluid between them. Required when the fluid is toxic, flammable, volatile or cannot leak to the environment. Requires auxiliary barrier fluid system.
- Cartridge seal — Pre-assembled and pre-adjusted unit installed as a single assembly on the shaft. Eliminates mounting errors and reduces maintenance time. Ideal for pumps with frequent maintenance.
4. Face materials and elastomers
Face material selection is critical for seal service life. The combination depends on the pumped fluid, temperature and presence of suspended solids.
| Combination | Typical application | Temperature limit |
|---|---|---|
| Carbon / SiC | Water, solvents, light chemicals | up to 200°C |
| SiC / SiC | Fluids with solids, abrasives | up to 250°C |
| TC / TC (tungsten carbide) | High temperature, thermal oil | up to 300°C+ |
| Carbon / Ceramic | Clean water, low cost | up to 120°C |
5. Mechanical seal vs. packing — when to use each
Packing is the most traditional sealing, composed of braided material rings compressed against the shaft. It is cheaper, easy to replace and accepts controlled dripping that lubricates and cools. The mechanical seal is more expensive but provides virtually total sealing and longer life when correctly specified.
In the FB Bombas FBCN series, both options are available. Manual MTEC-03/00 specifies: with packing, the pump operates up to 105°C and 12–16 bar (depending on flange class); with mechanical seal, up to 90°C and 10 bar. For fluids with suspended solids, packing is often preferred — the manual recommends consulting FB Bombas.
6. Mechanical seal in centrifugal pump (FBCN)
In the FBCN normalized centrifugal pump, the mechanical seal is installed in the region between the casing (volute) and the bearing housing, over the shaft protective sleeve. Pressure in the seal chamber is defined by the pump discharge pressure, which directly influences seal selection.
Data from FBCN manual (MTEC-03/00): with mechanical seal, the combined limit is 90°C and 10 bar. For applications above these limits, FB Bombas offers special solutions with double seals, seal plans (API Plan 11, 13, 21, 32, 52, 54) and special materials upon consultation.
7. Mechanical seal in gear pump (FBE/FBEI)
In FBE and FBEI gear pumps, sealing faces different challenges: fluids are typically viscous (oils, resins, asphalt), which naturally aids sealing. However, temperature can be very high — FBE operates with fluids up to 350°C when used as a thermal oil pump (FBOT series).
The FBE manual (MTEC-01/01) offers both packing and mechanical seal. For cold viscous fluids (asphalt, molasses), packing is adequate. For hot or volatile fluids, mechanical seal is necessary. FBEI, with internal gears, has lower pulsation and vibration, which favors seal service life.
8. Common problems and mechanical seal maintenance
The most frequent causes of mechanical seal failure are: dry running (no fluid in the seal chamber), shaft misalignment, excessive vibration from operation outside BEP, temperature above the elastomer limit and contamination by abrasive solids.
- Always verify fluid is present in the seal chamber before startup
- Keep pump-motor alignment within coupling tolerances
- Operate pump near BEP — operation at very low or very high flow increases vibrations
- Monitor leakage: constant dripping indicates face wear
- Replace the complete seal — never reuse worn faces or springs
9. How to specify a mechanical seal for your pump
To correctly specify a mechanical seal, provide FB Bombas engineering with the following process data:
- Pumped fluid — name, composition, viscosity and density at operating temperature
- Temperature — operating and maximum (defines elastomer material: Viton up to 200°C, EPDM up to 150°C, Buna-N up to 120°C)
- Pressure — in the seal chamber (depends on discharge pressure and pump geometry)
- Solids presence — type, concentration and particle size (determines seal or packing)
- Shaft diameter — defines seal size (standardized by norm)
- Environmental or safety requirements — zero leakage (double seal), API standards, ATEX
