1. Why viscosity determines pump selection
In external gear pumps, fluid viscosity is the most important parameter for correct selection. It defines three critical variables: maximum allowed speed, maximum differential pressure and drive type (direct, with pulley or gearbox).
More viscous fluids require lower speed to preserve volumetric efficiency and the service life of internal components (gears, bushings and bearings). The FB Bombas FBE series operates with viscosities from 30 to 250,000 SSU, covering everything from light oils to CAP asphalt.
2. Selection table: viscosity × speed × drive type
The table below is reproduced from the FBE Technical Manual (Table 2, MTEC-01/01). It defines the speed and transmission type for each viscosity range, as well as the available FBE models.
| Viscosity (SSU) | Speed (rpm) | Transmission | Available models |
|---|---|---|---|
| 30 to 250 | 1750 | Direct | 1/8" to 1" and 1"D |
| 250 to 2,500 | 1150 | Direct | 1/8" to 1"D, 1.1/2" to 5" |
| 2,500 to 7,500 | 850 | Direct | 1/2" to 1"D, 1.1/2" to 6" |
| 7,500 to 10,000 | 700 to 500 | Pulley or gearbox | 1.1/2" to 6" |
| 10,000 to 50,000 | 500 to 300 | Pulley or gearbox | 1.1/2" to 6" |
| 50,000 to 100,000 | 300 to 150 | Pulley or gearbox | 3" to 6" |
3. Maximum working pressure by viscosity
The FBE pump maximum differential pressure decreases as viscosity increases. This happens because more viscous fluids generate higher loads on bearings and gears. The table below reproduces the values from the FBE Technical Manual (Table 3, MTEC-01/01).
| Pressure (kgf/cm²) | 1,000 | 3,000 | 5,000 | 7,000 | 9,000 | 10,000 | 30,000 | 50,000 | 70,000 | 90,000 | 100,000 | 250,000 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 9 | 26 | 20 | 18 | 16 | 15.5 | 15 | 11 | 10 | 9.5 | 9 | 9 | 6 |
| 7 | 40 | 32 | 28 | 26 | 25.5 | 25 | 20 | 17 | 16 | 15 | 14 | 11 |
| 11 | 43 | 38 | 35 | 34 | 32 | 30 | 25 | 22 | 21 | 20.5 | 20 | 17 |
| 16 | 45 | 42 | 40 | 37 | 38 | 35 | 30 | 28 | 26 | 25 | 25 | 22 |
4. Motor power reserve
The FBE technical manual recommends specific power reserves for electric motor selection. This reserve protects the motor against load peaks during startup, viscosity variations or off-design conditions.
| Required power | Power reserve |
|---|---|
| Up to 2 hp | 50% |
| 3 to 5 hp | 30% |
| 6 to 10 hp | 25% |
| 11 to 25 hp | 15% |
| Above 25 hp | 10% |
5. FBE model nomenclature
The FBE series has variants that extend the flow and pressure range for specific applications. The letters after the diameter indicate the construction configuration per the technical manual:
- A — Larger diameter gear (higher flow in the same body)
- CA — With heating jacket (for fluids that solidify at ambient temperature, such as asphalt)
- D — Longer gear (higher flow per revolution)
- DA — Longer gear with larger module (maximum flow)
- M6 — Module 6 gear
- M8 — Module 8 gear
- M9 — Module 9 gear
- M12 — Module 11 gear
6. Available construction materials
The FBE series is manufactured in five material options per Table 6 of the technical manual. The choice depends on chemical compatibility with the fluid, operating temperature and corrosion resistance requirements.
- Cast iron — Casing ASTM A48 CL30. Standard for mineral oils and derivatives.
- Ductile iron — Casing ASTM A536 60-40-18. Higher mechanical strength for elevated pressures.
- Carbon steel — Casing ASTM A216 WCB. For fluids above 250°C.
- Stainless steel — Casing ASTM A743 CF8M. For corrosive, food-grade and pharmaceutical fluids.
- Bronze — Casing ASTM B584 C93700 (SAE J462 64). For marine and water-based fluid applications.
7. FBE series operating limits
Per Table 5 of the technical manual (Construction Data), the general operating limits of the FBE series are:
- Sizes: DN 1/8" to 6"
- Flow rates: up to 390 m³/h
- Head: up to 220 m
- Temperature: -50°C to 350°C (with mechanical seal)
- Speed: up to 1750 rpm (direct drive)
- Maximum suction pressure: 14 kgf/cm²
- Helical gears (1/8" to 1") and bi-helical (1.1/2" to 6")
- BSP connections (1/8" to 1") or ASME B16.1 125lbs flanges (1.1/2" to 6")
8. How to size: step by step
To request correct sizing of an FBE pump, provide FB Bombas engineering with the following process data:
- Fluid to be pumped (trade name and composition)
- Viscosity at operating temperature (SSU, cSt or cP)
- Operating temperature (°C)
- Desired flow rate (m³/h or L/min)
- Differential pressure (kgf/cm² or bar)
- Available NPSH at the installation
- Presence of suspended solids (yes/no)
