Thermal Oil Pumps up to 350°C: Selection Criteria and Common Pitfalls

1. What is an industrial thermal oil system

A thermal oil system is a closed loop where an organic thermal fluid (typically a synthetic hydrocarbon or special mineral oil) is heated by a boiler, circulated to industrial consumers (heat exchangers, presses, dryers, reactors) and returns to the boiler in a closed circuit.

The advantage of thermal oil over steam is that it allows high temperatures (up to 350°C in typical applications) at much lower pressures — a thermal oil boiler typically operates at 6-10 bar, while steam saturation pressure at 350°C would exceed 160 bar.

This dramatically simplifies system engineering — thinner piping, cheaper valves, lower regulatory requirements. But it shifts the technical challenge to the circulation pump, which must work continuously with a hot, lubricating and slightly oxidizing fluid. Industries using thermal oil systems include: pharmaceutical, chemical, food, textile, plastics, pulp and paper, wood presses and asphalt.

2. The three unique technical challenges of thermal oil pumping

Compared to a conventional process centrifugal pump for water or moderate chemicals, a thermal oil pump faces three distinct and cumulative physical problems. Ignoring any of them causes premature failure and unplanned downtime — the kind of problem a thermal oil boiler should never have, because it feeds the entire production process.

The first challenge is shaft sealing. At temperatures above 200°C, most standard mechanical seals for chemical process degrade quickly: elastomer o-ring gaskets lose elasticity, carbide faces may crack from thermal shock, and residual lubrication between faces evaporates. High-temperature seals require specific materials (graphite, metal bellows) and, more importantly, need to be cooled — which in a conventional centrifugal pump is done with external sealing water, creating an additional failure point.

The second challenge is differential thermal expansion. A pump operating at 350°C has its components expanded relative to cold assembly — the impeller grows, the shaft grows, the casing grows. If the design does not compensate for this expansion, internal clearances go out of spec or the pump itself misaligns with the motor and coupling. This causes vibration, accelerated bearing wear and seal rupture.

The third challenge is bearing lubrication. In a conventional centrifugal pump, bearings are grease or oil lubricated in a sealed chamber. At 350°C, grease melts and drains, and common mineral oil oxidizes and darkens within weeks. Thermal oil pumps use high-temperature synthetic oil and bearing chambers with passive cooling — external fins that dissipate heat via natural convection — or in more critical applications, forced circulation of refrigerated oil.

3. How the FBOT solves the three challenges

FB Bombas' FBOT line was designed from the drawing board for continuous operation in organic thermal oil up to 350°C. It is not an adaptation of a standard centrifugal — it is a specific machine, and that specificity is reflected in every construction detail.

Shaft sealing is done by a special mechanical seal immersed in its own oil reservoir inside the sealing chamber. This sealing oil is isolated from the main thermal fluid and cooled by fins integrated into the chamber casing — eliminating external water and making installation simpler and less prone to failure.

The result: the FBOT's mechanical seal operates at a much lower internal temperature than the process 350°C, extending service life to tens of thousands of hours even in continuous operation.

Thermal expansion compensation is done by construction clearances calculated for hot operating conditions, not for cold assembly conditions. This means that when assembling the pump at 25°C, some clearances appear "tight" — this is intentional and desired, because when the pump reaches 300-350°C, the parts assume the design size. Assembly and commissioning follow specific procedures that take this behavior into account.

Bearing lubrication uses high-temperature synthetic oil dispensed via sight gauge or optional automatic refill cup. The bearing chamber is thermally isolated from the pump casing and has external fins for convection dissipation. In very severe installations, the design allows forced lubrication with external cooling — an option FB Bombas engineering evaluates case by case during the technical briefing phase.

4. Selection by thermal fluid

The universe of organic thermal fluids is more diverse than many realize. The most common brands in the Brazilian industrial market include families of mineral and synthetic hydrocarbons with different maximum operating temperature ranges, viscosities, densities and chemical compatibilities. FBOT pump selection must consider which specific fluid will be used, because each family has characteristics that affect pumping.

• Simple mineral oils (paraffinic): maximum temperature typically 280-300°C, density ~0.85, viscosity at 40°C ~30 cSt. Inexpensive but oxidize in contact with air.
• Synthetic aromatics (diphenyl/diphenyl oxide): maximum temperature 400°C in liquid phase, density ~1.06, significant vapor pressure above 250°C. Require extra attention to NPSH.
• Synthetic alkylbenzenes: maximum temperature 315°C, good chemical stability, density ~0.88. Most used in Brazilian chemical and food industries.
• Liquid silicones: maximum temperature up to 400°C, variable density, excellent oxidative stability. More expensive but with extended service life in well-designed systems.

5. Five common sizing pitfalls

FB Bombas' engineering experience in retrofit and new projects of thermal oil systems in Brazil identifies five recurring mistakes that compromise installation durability and safety. They are listed from most common to most severe.

• Using viscosity at 40°C to size NPSH and friction loss, ignoring that the system starts cold. At cold start with mineral oil, viscosity can be 50-100× the hot operating viscosity — a pump that runs great in regime may not even start on the first start.
• Underestimating fluid vapor pressure at maximum operating temperature. Aromatic fluids (diphenyl) have significant vapor pressure above 250°C, and ignoring this in NPSHa calculation is the recipe for cavitation.
• Sizing suction piping equal to discharge. In thermal oil systems, suction line should be one size larger than discharge to reduce losses and avoid local vaporization at low-pressure points.
• Installing the pump above the expansion tank level. Thermal oil requires flooded suction for safety — in case of line leak, negative pressure can suction air and create unstable bubble, with risk of sudden vaporization and flash fire.
• Using standard mechanical seal with process oil as barrier fluid, without cooling chamber. This was the mistake that led us to design the FBOT as a specific line in the 1970s — there is no way to improvise it.

6. Casing material: cast iron or carbon steel?

The FBOT is available in cast iron and carbon steel. The choice between them depends essentially on maximum operating temperature and working pressure. For systems operating up to 260°C with discharge pressure up to 10 bar, cast iron is adequate, more economical and completely safe. For temperatures above 260°C or pressures above 10 bar, A216 WCB carbon steel is mandatory — cast iron in this range presents thermal embrittlement risk in cyclic start-stops.

Stainless steel (A743 CF8M) is not common in pumps for organic thermal oil — the oil is not corrosive, and stainless offers no significant advantage in this application. The exception are installations with special thermal oils or moisture contamination risk (systems with open exchangers), where stainless corrosion resistance justifies the higher cost.

7. Specification checklist: what to send FB Bombas engineering

When you contact FB Bombas engineering to size an FBOT pump, the information below allows a precise technical response within 48 hours. The more data, the better the sizing — but even with partial information, our team can make preliminary recommendations.

• Commercial brand and family of thermal fluid (e.g. Therminol 66, Mobiltherm 603, Dowtherm A). If unknown, specify type: mineral or synthetic, aromatic or alkylbenzene.
• Maximum process operating temperature and minimum start temperature (cold start regime).
• Desired flow (m³/h) and circuit head (can be estimated by summing consumer losses).
• Installation layout: relative elevation between pump and expansion tank, suction and discharge piping diameter, number of parallel consumers.
• Operating regime: continuous 24/7, shifts, or intermittent with frequent stops (important for material selection).
• Circuit working pressure and maximum allowable discharge flange pressure.

8. Conclusion

Thermal oil systems up to 350°C do not accept adaptation. The pump must be designed from the ground up for this application — cooled mechanical seal without external water, thermal expansion compensated by design, oversized bearings and materials compatible with the temperature + fluid combination. The FB Bombas FBOT line fully meets these requirements and accumulates decades of installations in pharmaceutical, chemical, food, textile, plastics and paper industries in Brazil and LATAM.

If your project involves industrial heating with organic thermal oil, our engineering can evaluate your specific installation — from retrofit of a problem pump to sizing of a completely new system.