FB Bombas Engineering Team16 min read
A mechanical seal closes the shaft passage through the pump casing with two faces in contact: one stationary and one turning with the shaft. The liquid film between them lubricates and cools; without it, the seal runs dry, a frequent cause of damage. A seal can be single, with one face pair, or double, with two pairs and an external fluid, and any arrangement can come as a cartridge.
Read the complete answer
On the FBCN, FB Bombas Engineering gives 90 °C and 10 bar with a mechanical seal (MTEC-03/00, Table 3) and refers the seal temperature to the manufacturer’s recommendation (Table 1).
At a glance
- A mechanical seal closes the shaft passage with a stationary face and a rotating one, kept in contact by springs or bellows, and a liquid film between them that lubricates and cools.
- A single seal has one face pair and accounts for more than 90% of seals in use; a double seal uses an external fluid above product pressure; a cartridge is any arrangement assembled as a unit.
- For the FBCN, Table 3 (MTEC-03/00) gives 90 °C and 10 bar with a mechanical seal and up to 105 °C with packing; Table 1 refers the seal temperature to the manufacturer’s recommendation.
- Dry running, when the film between the faces breaks down, is a frequent cause of damage; before startup, remove air from the seal area and connect the auxiliary fluids.
- The standard FBE seal is type 21, with ceramic and graphite faces, Viton and an AISI 304 spring; the FBOT uses graphite packing and a single seal submerged in the bearing oil.
Operating principle, types (single, double, cartridge), face materials, difference between seal and packing, and how to specify for centrifugal and gear pumps.
1. What is a mechanical seal?
A mechanical seal is a sealing device installed where the rotating shaft passes through the pump casing. That point, the stuffing box, is a leak path, and the US Department of Energy and Hydraulic Institute (DOE/HI) sourcebook names packing and mechanical seals as the usual ways to seal it. Packing needs some leakage to stay lubricated and cooled; the mechanical seal provides superior sealing.
According to the DOE/HI sourcebook, most pumps sold are supplied with mechanical seals. Superior sealing, however, does not mean zero leakage: the EagleBurgmann BURGMANN ABC lexicon treats leakage control as part of operation, and periodically recording the leakage rate reveals the condition of the seal and of the sealed system.
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 (MTEC-03/00, §2.3), and fluids with suspended solids call for a review by FB Bombas Engineering before selection.
2. How a mechanical seal works
A mechanical seal has a stationary face, held in the housing or gland, and a dynamic face that turns with the shaft; O-rings provide the secondary sealing of these parts. Springs or bellows usually keep the faces in contact. According to the DOE/HI sourcebook, the faces need precise flatness and alignment with tolerances on the order of microns, because they must remain in constant contact as the shaft spins.
A lubrication film forms between the faces, from the product itself or from an external medium, and it lubricates and cools them. According to the BURGMANN ABC lexicon, the thickness of this film varies with the fluid, sealing pressure, temperature, sliding velocity and face material pair.
The film separates, partially or totally, when the product vaporizes in the sealing gap, because frictional heat is not adequately removed or because sealing pressure falls to the vapour pressure or below. That is dry running.
Because pumps often rotate at 1,800 or 3,600 rpm, even slight variations in contact between the faces quickly destroy the seal’s effectiveness, according to the DOE/HI sourcebook. At FBEI startup, the pump never runs without fluid: all air is removed from the seal area and the direction of rotation is checked with the pump uncoupled, to avoid dry running (MAN001-10).
Warning
Absent or inadequate lubrication film is a frequent cause of seal damage. Before startup, confirm the seal chamber is full and free of air and the auxiliary fluids are installed and connected.
3. Types of mechanical seal
Mechanical seals are classified by face arrangement and by mounting method. The types listed in the 2026 FBEI catalogue, single, double and cartridge, and the tandem arrangement work as follows:
- Single seal: it has one sealing gap, that is, one face pair, and the film between the faces comes from the product itself. According to the BURGMANN ABC lexicon, more than 90% of mechanical seals are used as single seals. It can be internally or externally arranged, with springs in contact with the product or facing away from it; for highly corrosive fluids, the DOE/HI sourcebook recommends external springs.
- Double seal (double acting): two face pairs, back-to-back, face-to-face or with a common drive collar, with an external fluid that is clean and compatible with the product, about 2 to 3 bar or 10% above its pressure. It is used when no product leakage may escape the machine, due to explosion risk or environmental protection, and for gases, poorly lubricating media or media that polymerize, bond or form deposits.
- Tandem seal: two single seals in succession. The product-side seal is lubricated by the product itself, and a quench medium, at a lower pressure than the product, can stabilize the film.
- Cartridge seal: any of these arrangements can be built as a cartridge unit, which can be leak-tested before fitting and prevents installation errors.
4. Face materials and elastomers
The face material pair weighs on seal service life. According to the DOE/HI sourcebook, the two primary failure mechanisms are degradation of the face material, usually from debris wedged between the faces, and loss of spring or bellows tension, from fatigue, fouling or corrosion. That is why the combination depends on the fluid, temperature, pressure and the presence of suspended solids.
The BURGMANN ABC lexicon’s combination matrix, summarized in the table, relates common pairs to four requirements: conditional emergency running, corrosion resistance, wear resistance and resistance to maximum pressure and temperature. Temperature limits rest with the seal supplier.
FB Bombas Engineering fits the FBE with a standard type 21 seal: ceramic and graphite faces, Viton elastomer and an AISI 304 stainless steel spring (MTEC-01/01, §2.5). The elastomer is part of the selection too: the lexicon lists O-ring swelling among the chemical causes of damage and O-ring embrittlement among the thermal causes.
| Face pair | Requirements associated in the matrix |
|---|---|
| Resin-impregnated carbon graphite × silicon carbide (pressureless or reaction sintered) | Conditional emergency running; corrosion resistance |
| Resin-impregnated carbon graphite × nickel-bonded tungsten carbide | Conditional emergency running; corrosion resistance |
| Resin-impregnated carbon graphite × aluminium oxide (ceramic) | Conditional emergency running; corrosion resistance |
| Silicon carbide (pressureless or reaction sintered) × nickel-bonded tungsten carbide | Corrosion and wear resistance |
| Reaction-sintered silicon carbide × reaction-sintered silicon carbide | Corrosion and wear resistance |
| Nickel-bonded tungsten carbide × nickel-bonded tungsten carbide | Corrosion and wear resistance |
| Antimony-impregnated carbon graphite × tungsten carbide or reaction-sintered silicon carbide | Resistance to maximum pressure and temperature |
5. Mechanical seal vs. packing — when to use each
In packing, sealing rings are compressed against the shaft by the packing gland. According to the DOE/HI sourcebook, it is much less expensive and needs less specialized maintenance, but it requires some leakage to stay lubricated and cooled; overtightening generates heat that destroys the packing and can damage the shaft, and wear calls for periodic retightening. The mechanical seal seals better, but it is typically more expensive and harder to repair or replace.
For the FBCN, Table 3 (MTEC-03/00) gives the pressure limit by temperature for each sealing option. With packing and ANSI B16.1 125 lbs flanges: 12 bar from −28 to 65 °C, 11 bar at 93 °C and 10 bar at 105 °C; with 250 lbs flanges, 16 bar over the same range. With a mechanical seal: 90 °C and 10 bar. Table 1 gives the temperature with a seal as “per the manufacturer’s recommendation”.
The seal chamber varies by application (§10). In the packing executions, S1, S2 and S3 use liquid from an external source and go up to 105 °C; with an external source, a cooling chamber cannot be used. Table 1 gives about 1 l/min of liquid for sealing and 3 to 5 l/min for flushing. Each execution serves one case:
- ST, standard execution: lubrication by the pumped fluid itself, for clean, non-aggressive fluids.
- S1: sealing with clean liquid from an external source, for toxic, aggressive or foul-smelling fluids and for suction from a tank under vacuum or below atmospheric pressure.
- S2: lubrication and sealing with liquid from an external source, for fluids with solid particles or small fibres in suspension.
- S3: flushing with clean liquid from an external source, for fluids with abrasive particles in suspension or a tendency to crystallize.
- S4: thermal oil service, with no connections in the region wetted by the pumped fluid and with pure graphite or carbon-fibre graphite packing.
Engineering note
DOE/HI sourcebook criterion: packing where fluid leakage is not a significant concern, for its lower cost and less specialized maintenance; a mechanical seal where the application calls for superior sealing, as with a toxic fluid, where leakage contaminates, or an expensive one, where leakage is a direct cost. If no product leakage may escape the machine, the BURGMANN ABC lexicon points to the double seal.
6. Mechanical seal in centrifugal pump (FBCN and FBOT)
The FBCN has a radial single-suction impeller and FB Bombas Engineering designs its mechanical construction to ASME B73.1 (MTEC-03/00, p.1), the specification for horizontal end-suction centrifugal pumps for chemical process. The sealing runs on the shaft protective sleeve, and the back-pull-out construction lets the bearing section come out of the volute without disconnecting or misaligning the piping, which eases assembly, disassembly and maintenance.
Pressure in the seal chamber is not necessarily the discharge pressure and can fall below atmospheric, as with suction from a tank under vacuum, a case for which the FBCN provides the S1 execution (MTEC-03/00, §10). The DOE/HI sourcebook warns that, in this condition, badly installed packing can let air in, which can degrade performance by 3% or more and keep self-priming pumps from repriming on startup.
On the FBCN, the mechanical seal execution is defined “according to the application’s needs and request” (MTEC-03/00, §10).
The FBOT, a centrifugal pump for thermal oil, has sealing with a dual safety system: graphite packing in the first stage, in the region wetted by the thermal oil, and a mechanical seal inside the bearing housing, submerged in the bearing oil (MTEC-16/00, p.1 and §2.3). Table 1 gives 5.5 mm graphite-fibre packing and a single mechanical seal of Ø32 or Ø42 mm, and connection D2 is a seal leakage indicator tube.
7. Mechanical seal in gear pump (FBE/FBEI)
The FBE was developed for viscous fluids, with sealing selected for the fluid and housed at the front of the pump (MTEC-01/01, p.1). FB Bombas Engineering provides for packing, mechanical seal, lip seal or magnetic coupling, and the shaft has no sleeve in the sealing region; suspended solids are reviewed before selection (§2.3). The 2026 FBE catalogue lists packing, mechanical seal or lip seal.
On the FBE, the packing is pressed by a packing gland adjusted with a hex nut (§2.4), and the mechanical seal, fitted and pressed by the gland, is type 21 as standard (§2.5). The magnetic coupling is a sealing option mounted in a specially built cover or casing, for applications without leakage, such as corrosive, toxic or explosive fluids (§2.7).
Table 5 (MTEC-01/01) refers the temperature with a seal to the manufacturer’s recommendation. For the series, the 2026 FBE catalogue gives 10 to 500 °C (p.3); the execution, materials and sealing define the applicable limit.
Table 17 (MTEC-01/01) provides seal flush (F1) and seal quench (Q1) connections, 1/4″ NPT, and a gland drain (D3), made only according to the application and the customer’s request. According to the BURGMANN ABC lexicon, a quench applies an unpressurized external medium to the faces on the atmosphere side and, with steam, keeps leakage of products with a high solidifying point from solidifying in the seal’s critical area.
The 2026 FBEI catalogue lists packing and single, double or cartridge mechanical seals. Before switching the pump on, the FBEI calls for the seal to be correctly adjusted (MAN001-10), with all accessories and auxiliary fluids installed and connected.
8. Common problems and mechanical seal maintenance
According to the DOE/HI sourcebook, common causes of seal problems are contamination of the faces, overheating from inadequate lubrication and improper installation. The BURGMANN ABC lexicon groups causes of damage into chemical ones, such as corrosion and O-ring swelling; mechanical ones, such as deposits, wear and erosion; thermal ones, such as dry running, thermal shock and thermal stress cracks; and material, production and assembly errors. The analysis starts with an exact damage record.
- Against dry running: before startup, confirm there is liquid in the seal chamber and that all air has been removed from the seal area, as the FBEI requires (MAN001-10).
- Keep pump-motor alignment: according to the DOE/HI sourcebook, the seal must be precisely aligned so that spring movement is minimal on each shaft revolution, and misaligned machines cause vibration and premature component wear (MAN001-10).
- On a centrifugal pump, avoid running far from the BEP: according to the DOE/HI sourcebook, shaft deflection is most problematic at low flow and, when severe, causes shaft seal leakage, and cavitation accelerates bearing and seal wear. On a gear pump, keep operation within the datasheet conditions, as the FBEI requires (MAN001-10).
- Record the leakage rate: according to the BURGMANN ABC lexicon, a rise that is gradual at first and then accelerating, after thousands of hours of near constancy, shows the seal is approaching the end of its life; violent fluctuations come from frequent changes of operating conditions or indicate malfunctions.
- With packing, dripping is expected: FB Bombas Engineering expects all packing to drip in operation (MAN001-10) and calls for adjusting the packing gland by about 1/6 of a turn, watching the first 5 to 8 hours, tightening another 1/6 of a turn if leakage is excessive and, after bedding in, checking it weekly.
- Compare the time to failure with the MTBF estimated by the seal manufacturer; if it is much shorter, investigate the cause (DOE/HI sourcebook). The FBEI sealing set in the 2-year spare parts includes packing or a mechanical seal, and stored seals are cleaned only with dry air, without liquid (MAN001-10).
9. How to specify a mechanical seal for your pump
Enquiring about a part for an installed FB pump? Send the full model, serial number and part reference from the equipment manual. Describe the observed problem and operating conditions. For a sealing specification enquiry, also include the following process data:
- Pumped fluid — name, composition, viscosity and density at operating temperature
- Operating, minimum and maximum temperatures: they guide the choice of face and elastomer materials.
- Suction and discharge pressures and seal chamber pressure, which can fall below atmospheric with suction from a tank under vacuum.
- Solids: type, concentration and particle size. According to the DOE/HI sourcebook, debris wedged between the faces is the usual cause of face degradation, and flushing lines with filters help catch it; on the FBCN, suspended solids call for consulting FB.
- Dimensions: on the FBCN, Table 1 gives a 35 to 90 mm sleeve and a 54 to 122 mm seal chamber, depending on the bearing housing; on the FBE, Table 5 gives the seal diameter by size.
- Containment and safety: if no product leakage may escape the machine, due to explosion risk or environmental protection, the BURGMANN ABC lexicon points to a double seal.
Note
FB Bombas supplies pumps with mechanical seals already specified for your application. No need to purchase the seal separately — send your process data and our engineering team sizes the complete assembly.