1. What a thermal oil pump is and why it differs from conventional pumps
A thermal oil pump is a process pump that continuously circulates an organic heat transfer fluid at elevated temperature. For the main circulation of the circuit the usual solution is a centrifugal pump; gear pumps appear in the transfer of products that remain viscous even when heated.
The hydraulic principle is the same as in a process centrifugal pump. The difference lies in the decisions taken for hot service: sealing, pressure limits as a function of temperature, bearings and mounting. In the FBOT manual (MTEC-16/00), for example, the maximum discharge pressure falls with temperature — about 16 bar at 100 °C and about 10 bar at 350 °C in Figure 1 — and sealing combines graphite packing in contact with the hot oil and a mechanical seal.
A pump selected for cold water and applied to hot oil is outside its design basis: sealing, pressure × temperature limits and materials may not have been defined for that service. The hot operating point must therefore be checked against the documentation of the specific pump, not only against the hydraulic curve.
Fluid data must be read at the operating temperature and at the start-up condition. In the table published by Eastman for Therminol 66, density goes from 1,008 kg/m³ at 20 °C to 809 kg/m³ at 300 °C, and kinematic viscosity from 122 mm²/s (cSt) to 0.51 mm²/s. In a centrifugal pump, absorbed power for the same flow and head is proportional to density; at cold start, high viscosity further increases torque and losses.
Power must therefore be checked in both conditions: hot duty and start-up.
Note
With Eastman data for Therminol 66, density drops about 20% between 20 °C and 300 °C. For the same flow and head, hydraulic power at hot duty is lower than cold by the same proportion. The risk of an undersized motor lies in the cold start, not in hot duty: check the motor for the most demanding case and apply the power reserve given in the series manual.
2. Industrial heating systems: where the pump fits in the circuit
A thermal oil heating system is a closed loop with a heater (fired by gas, fuel oil or biomass, or with electric elements), a circulation pump, insulated piping, industrial consumers (heat exchangers, jacketed reactors, presses, dryers, tank coils) and an expansion vessel.
The pump keeps flow through the heater so that the fluid film temperature on heated surfaces stays below the fluid supplier limit. For Therminol 66, for example, Eastman publishes 345 °C as maximum bulk temperature and 375 °C as maximum film temperature. The minimum flow required by the heater is a heater manufacturer figure, not a pump figure.
Stopping circulation with a hot heater is therefore a situation to avoid: fluid standing on heated surfaces may overheat, and Eastman warns that overheating or contamination accelerates decomposition and increases high-boiling compounds and solids. The shutdown procedure (keeping circulation after burner cut-off until the cooling stated by the heater manufacturer) and the response to power failures must be defined in the system design.
Fluid condition also reaches the pump. Eastman recommends keeping Therminol 66 systems under an inert gas blanket to protect the fluid against oxidation, and installing pressure relief devices where required. The FBOT catalog, in turn, requires a fluid free of abrasive particles and of materials that chemically attack pump components: filtration, periodic oil analysis and solids removal are part of circuit maintenance.
Engineering note
The main circulation pump runs continuously during plant campaigns. Define in the design whether an installed standby pump will be used, how switchover between pumps will be done and which spare parts will be kept at the plant; the proposal records the exact scope of supply.
3. Pump types used: centrifugal versus gear
For main thermal oil circulation, the process centrifugal pump is the usual choice: the fluid has low viscosity at operating temperature, flow is continuous and maintenance must happen without dismantling insulated piping, which favours back pull-out construction.
The FBOT series follows this construction: spiral casing cast in one piece, closed radial single-suction impeller, back pull-out construction, a bearing housing with two rolling bearings (one in an oil bath and one shielded, grease-lubricated) and sealing with graphite packing and a mechanical seal (manual MTEC-16/00, items 2.1 to 2.4).
Gear pumps come in when the heated product remains viscous: transfer of asphalt, pitch, heavy fuel oil, resins and other fluids that need heating to flow. The FBE (external gear) offers a heating chamber as an accessory (CA execution) and, in its 2026 catalog, a table of recommended speed per viscosity band, from 30 to 100,000 SSU.
The FBEI (internal gear) is published with independent maximums of 136 m³/h, 14 bar differential, 140,000 cP and 340 °C; its heating jacket has its own limits in manual MAN001-10: steam up to 185 °C and thermal fluid up to 232 °C, both up to 10 bar.
The choice between centrifugal and gear pump depends on the real temperature, the fluid viscosity at that temperature, flow, pressure and duty (continuous circulation or batch transfer). With these data, engineering confirms the series, the model and the limit that applies to each configuration.
| Service | Technology | FB Bombas series and documentary reference |
|---|---|---|
| Circulation of organic thermal oil in the heating circuit | Dedicated back pull-out centrifugal | FBOT — up to 350 °C (series); discharge limited by Figure 1 of MTEC-16/00 |
| Transfer of heated viscous product (asphalt, pitch, heavy oil) | External gear, with heating chamber when needed | FBE — recommended speed per viscosity (2026 catalog, p.5) |
| Very high-viscosity product, constant flow | Internal gear with gear reducer | FBEI — up to 140,000 cP and 340 °C (independent maximums); jacket: steam 185 °C, thermal fluid 232 °C, 10 bar |
| Utility water and clean or turbid liquids | Standardized centrifugal | FBCN — up to 260 °C in the general data of MTEC-03/00; actual limit depends on sealing and cooling chamber |
4. Technical selection criteria: flow, pressure, temperature and NPSH
Specifying a thermal oil pump starts with the hot operating point — flow and head at working temperature — and with the start-up condition. The criteria below must be checked against the data of the chosen fluid and the curve of the selected model.
The main criteria are: NPSH available versus NPSH required at operating temperature; the model minimum continuous flow compared with the lowest real flow at partial load; discharge pressure within the pump pressure × temperature limit; compatibility of the fluid with materials and sealing; and margin between operating temperature and the limits of the fluid and the pump.
NPSH available in a hot circuit depends on expansion vessel pressure, liquid column, suction losses and the fluid vapor pressure at maximum temperature. Different fluids behave very differently: Therminol 66 has a normal boiling point of 359 °C and a vapor pressure of 30.7 kPa at 300 °C (Eastman), whereas Dow states vapor-phase use of Dowtherm A from 257 °C — operated as a liquid above that temperature, the circuit must be pressurized above the vapor pressure.
NPSH required comes from the curve of the selected model; the FBOT manual does not publish NPSHr per model, so this figure is confirmed during selection.
The second criterion is minimum continuous flow. Below it, internal recirculation heats the liquid inside the casing and increases loads on impeller, bearings and seal. The value is specific to each model and must be part of the selection; do not use BEP percentages transferred from another series or another manufacturer. If the system runs for long periods at partial load, provide a recirculation (bypass) line or speed control, sized for the hot condition.
The third criterion is pressure. In manual MTEC-16/00, maximum suction pressure is 10 bar and maximum discharge pressure varies with temperature (Figure 1: about 16 bar at 100 °C and about 10 bar at 350 °C). Add the expansion vessel pressure to the pressure generated by the pump and check the result at the maximum operating temperature, not at installation temperature.
Key point
For a detailed calculation of NPSH available with hot viscous fluids, see the technical article "NPSH in gear pumps for viscous fluids" in Resources. The NPSH available part (pressures, losses and vapor pressure at operating temperature) follows the same principle for thermal oil centrifugal pumps; NPSH required comes from the model curve.
5. Material selection, sealing and thermal expansion
For materials, the starting point is the series documentation and compatibility with the fluid. The FBOT catalog requires a fluid free of abrasive particles and of materials that chemically attack components. Manual MTEC-16/00 limits peripheral speed per impeller material — cast iron up to 40 m/s, ductile iron and carbon steel up to 60 m/s, stainless steels up to 80 m/s — and specifies ANSI B16.42 150 lb or DIN EN 1092-2 PN16 flanges.
The material combination of each supply is confirmed in the proposal.
In high-temperature closed circuits, sealing is a design decision. A conventional mechanical seal in direct contact with oil at 300–350 °C needs some form of thermal protection. Known approaches include recirculating the fluid itself through a cooler back to the seal chamber (API 682 Plan 23), injecting clean liquid from an external source (Plan 32) and separating the hot fluid from the seal.
The FBOT uses two-stage sealing: graphite packing in the first stage, in contact with the thermal oil, and a mechanical seal in the second stage, protected from the hot fluid and with no external cooling (FBOT 2026 catalog, p.2; MTEC-16/00, p.1 and item 2.3). Confirm the arrangement on the data sheet of the quoted configuration. Without a seal-water line, the installation does away with an auxiliary circuit that would need its own maintenance and inspection.
The FBOT manual also lists the auxiliary connections of the series: mechanical seal leakage indicator tube (D2), oil feed (FF), vent (V) and bracket drain (D1), all ¼″ BSP (Table 4). Include inspection of these points in the operating routine.
Thermal expansion is the third concern. With an expansion coefficient in the order of 12 × 10⁻⁶ /°C, typical of steels, each metre of component grows about 3.6 mm between 20 °C and 320 °C. Hot piping must not transfer loads to the nozzles, and pump-motor alignment must be checked after thermal stabilization, using the coupling manufacturer tolerance and the pump manual procedure.
| Item | Documented value | Use in selection |
|---|---|---|
| Maximum temperature | 350 °C (series limit) | Margin to the maximum operating temperature and to the fluid limit |
| Maximum suction pressure | 10 bar | Check against expansion vessel pressure and static column |
| Maximum discharge pressure | Falls with temperature (Figure 1: ~16 bar at 100 °C; ~10 bar at 350 °C) | Check total pressure at maximum temperature |
| Hydrostatic test | 1.5 × operating pressure | Value and certificate defined in the proposal |
| Impeller | Closed radial, single suction; max. peripheral speed: cast iron 40 m/s, ductile iron and carbon steel 60 m/s, stainless 80 m/s | Impeller material compatible with speed and diameter |
| Flanges | ANSI B16.42 150 lb or DIN EN 1092-2 PN16 | Match the existing piping |
6. Typical operations and commercial thermal fluids
A thermal oil plant rarely has a single pump. There is main circulation (sometimes with an installed standby), transfer for loading and unloading fluid storage tanks and, in systems with heavy-oil burners, burner feed. Each has different requirements: circulation runs continuously, transfer runs in batches and burner feed needs stable flow and pressure.
In the FB Bombas range, the FBOT is the series dedicated to thermal oil circulation, and the FBE lists oil burners and lubrication systems among the applications in its 2026 catalog (p.3).
Fluid choice belongs to the system designer. For the pump, what matters is the data of the chosen fluid, taken from the fluid supplier data sheet. The table below shows which data to request, using the values published by Eastman for Therminol 66 as an example; for another fluid, use its own data sheet.
| Fluid data | Published example (Therminol 66) | Use in pump selection |
|---|---|---|
| Use range and film temperature | −3 to 345 °C (bulk); film up to 375 °C | Margin to the fluid and pump limits (FBOT: 350 °C) |
| Density | 1,008 kg/m³ at 20 °C; 809 kg/m³ at 300 °C | Power at hot duty and at start-up |
| Kinematic viscosity | 122 mm²/s at 20 °C; 3.77 mm²/s at 100 °C; 0.51 mm²/s at 300 °C | Starting torque, losses and performance |
| Vapor pressure | 30.7 kPa at 300 °C; normal boiling point 359 °C | NPSH available and expansion vessel pressurization |
| Cold pumpability | 300 mm²/s at 11 °C; 2,000 mm²/s at −3 °C | Pre-heating and motor check at start-up |
| Composition and compatibility | Modified terphenyl; non-corrosive to metals commonly used in heat transfer systems, according to Eastman | Pump materials, packing and elastomers |
7. Five common specification errors and how to avoid them
Transitions — cold start, shutdown, power failure and fluid change — need special attention in the design, because they change temperature, viscosity and pressure at the same time.
The five errors below originate in system design and operating decisions. The pump alone does not correct them, so the corresponding information must be part of the enquiry.
- 1. Sizing the motor for hot duty only. At cold start, density and viscosity are higher (for Therminol 66, 1,008 kg/m³ and 122 mm²/s at 20 °C, against 809 kg/m³ and 0.51 mm²/s at 300 °C). Check power and torque at the lowest expected start-up temperature and apply the FBOT manual power reserve: Table 2 of MTEC-16/00 establishes the reserve by power band; confirm the selected motor in the proposal.
- 2. Calculating NPSH available with cold-water data. Use the fluid vapor pressure at the maximum operating temperature, the real expansion vessel pressure and suction line losses in the hot condition, and compare with the NPSH required from the model curve, applying the margin defined in the design criteria.
- 3. Oversizing the pump "for margin". A centrifugal pump running far from BEP recirculates internally and increases loading on bearings and seal. Size for the normal operating point, use speed control or recirculation for partial load and respect the minimum flow stated for the model.
- 4. Ignoring the limits of sealing materials. Seal faces and secondary elastomers have temperature and chemical compatibility limits stated by the seal manufacturer. Check these limits at the real temperature where the seal operates, which depends on the pump sealing arrangement.
- 5. Not planning the shutdown procedure. Stopping circulation with a hot heater may overheat the fluid standing on heated surfaces and accelerate its degradation. The system needs a written cool-down procedure, defined with the heater manufacturer, and a strategy for power failures.
Warning
API 610 is the centrifugal pump standard for the petroleum, petrochemical and natural gas industries. The FBOT series is not declared API 610 compliant in its documentation. If the buyer specification requires API 610, state it in the enquiry for engineering assessment. In industrial heating circuits, the specification usually relies on the fluid data, the pressure × temperature limits and the sealing arrangement described in this guide.
8. FB Bombas FBOT line: direct application of the criteria above
The FBOT is the FB Bombas series developed to pump organic thermal oils at high temperature without external cooling (FBOT 2026 catalog, p.2). The catalog lists the pharmaceutical, chemical, textile and plastics industries and asphalt plants — in this case for circulating the thermal oil that heats the process, not for pumping the asphalt.
The documented design decisions match the criteria of this guide: back pull-out, which allows the bearing assembly to be removed without disconnecting the casing from the piping; two-stage sealing with graphite packing and a mechanical seal; a two-bearing housing; 10 bar maximum suction pressure and discharge pressure limited by temperature (Figure 1); and hydrostatic testing at 1.5 times the operating pressure (MTEC-16/00, Table 1).
The general data — DN 25 to 300, up to 2,200 m³/h, 135 m, 350 °C and 3,500 rpm — are not reached simultaneously by a single model or operating point.
FB Bombas reports starting its activities in 1944; its current plant is in Cabreúva-SP, with in-house CNC machining and assembly. The range also includes the FBE and FBEI (gear) series for viscous fluids and the FBCN (standardized centrifugal) for water and clean or turbid liquids. For the ten large FBOT sizes (150-500 to 300-340), the manual identifies the models, but curve, NPSH, power and impeller are confirmed for the fluid and the operating point at quotation.
Key point
For spare parts, provide the model and serial number from the nameplate and use the parts list in the manual (numbering per DIN EN 24250, Table 3 of MTEC-16/00). Availability, lead time and price are confirmed at quotation.
Prepare the thermal oil circuit data
FBOT combines organic thermal oil service with back-pull-out construction: the bearing assembly can be withdrawn without disconnecting the casing from the piping. To assess the installation, provide the oil, duty point and maintenance space. Use the selection chart as a starting point and check the model curve and drawing before finalizing the configuration.
- Thermal oil name and data sheet; start-up and operating temperatures.
- Required flow and head at the same duty point; speed and suction conditions.
- Seal arrangement, connections, drive and installation drawing to be identified in the offer.