Thermal oil pumps are the heart of high-temperature heat transfer systems, circulating hot oil through heaters, heat exchangers, reactors, and process equipment. These pumps operate at 150-400°C with flammable fluids, requiring careful material selection, sealing, and thermal management. This guide covers thermal oil pump selection, design, and operation for high-temperature heat transfer systems.
Thermal Oil Systems
- Heater: Fired heater or electric heater heats thermal oil
- Pump: Circulates oil through heater and users
- Users: Heat exchangers, reactors, jacketed vessels, distillation reboilers
- Expansion tank: Accommodates thermal expansion, maintains NPSH, nitrogen blanket
- Flow: 10-500 m³/h (typical), head: 30-150 m
- Temp: 150-400°C (mineral up to 320°C, synthetic up to 400°C)
Thermal Oil Types
| Type | Temp Range | Vapor Pressure | Cost | Notes |
|---|---|---|---|---|
| Mineral oil | -10 to 320°C | Moderate | Low | General, coking >340°C film |
| Synthetic (aromatic) | -40 to 400°C | High | High | High temp, some toxic |
| Silicone | -60 to 400°C | Low | Very high | Specialty, low toxicity |
| PAO (synthetic) | -40 to 300°C | Low | Medium | Good stability, non-toxic |
Mineral Thermal Oil
- Most common, low cost, good heat transfer
- Temp: up to 320°C (some up to 340°C)
- Film temp limit: 340°C (above this, coking occurs)
- Flammable, requires nitrogen blanket (oxidation at high temp)
- Pump: high-temperature chemical pump with mechanical seal + Plan 21
Synthetic Thermal Oil
- Diphenyl oxide/biphenyl (Dowtherm A, Therminol VP-1)
- Temp: up to 400°C
- High vapor pressure at 400°C (~11 bar) — system must be pressurized
- Some are toxic (Dowtherm A) — require zero-leakage pumps
- Freeze point: 12°C (VP-1) — freeze protection in cold climates
- Pump: centerline support, double seal or magnetic drive (zero leakage)
Pump Type Selection
Horizontal Centrifugal (OH2)
- Most common for thermal oil circulation
- Flow: 10-500 m³/h, head: 30-150 m
- Centerline support (mandatory for >150°C)
- Material: carbon steel (WCB) up to 350°C, 2.25Cr-1Mo (F22) up to 450°C
- Seal: single mechanical with Plan 21 (recirculation + cooler)
- Advantages: efficient, reliable, easy maintenance, wide range
Vertical Canned Motor (VS6)
- For high-temperature, hazardous, zero-leakage requirements
- Flow: 10-200 m³/h, head: 50-500 m
- No shaft seal — zero leakage (critical for flammable/toxic oil)
- Material: 316, 310S, Inconel, Hastelloy
- Advantages: zero leakage, no seal maintenance, compact
- Limitations: lower efficiency (can losses), high cost, limited power
Magnetic Drive Pump
- Zero leakage, no shaft seal
- Flow: 1-100 m³/h, head: 20-150 m
- Temp limit: typically <350°C (magnet material limit)
- Advantages: zero leakage, no seal maintenance, good for small-medium flow
- Limitations: temp limit, torque limit, eddy current losses
Multistage Centrifugal (BB3/BB4)
- For high-pressure thermal oil systems (e.g., long pipelines, high-rise buildings)
- Flow: 50-500 m³/h, head: 200-1,000 m
- Material: 316/17-4PH wetted, carbon steel or alloy casing
- Key: low NPSH first stage, minimum flow protection
- For high-pressure thermal oil: multistage pump (similar to boiler feed)
Material Selection
Casing
- WCB (carbon steel): up to 350°C, general thermal oil
- 2.25Cr-1Mo (F22): up to 450°C, high-temp, hydrogen resistance
- 316/CF8M: up to 400°C, corrosive or synthetic oil
- 310S: up to 1000°C (oxidation), extreme high temp
- For mineral oil: WCB is sufficient (oil is non-corrosive)
- For synthetic oil (Dowtherm): 316 or WCB (verify compatibility)
Impeller and Shaft
- Impeller: 316, CF8M, or WCB (non-corrosive oil)
- Shaft: 316, 17-4PH, or 2.25Cr-1Mo
- Shaft sleeve: 316, Stellite, tungsten carbide
- Wear rings: bronze, 316, composite (non-galling)
- For high-temp: verify material strength at operating temp (derate)
Seal Materials
- Seal faces: SiC vs SiC (high temp), SiC vs carbon (general)
- Springs: Inconel 718 (high temp/corrosion)
- Elastomers: limited to <200°C — use metal bellows or PTFE wedge above
- Metal bellows: Inconel 718 (for >200°C, no elastomer)
- Gaskets: spiral wound (316/graphite), camprofile, RTJ (high pressure)
- For thermal oil: Viton or FFKM elastomers (NBR may swell in oil)
Seal Systems for Thermal Oil
Plan 21 (Recirculation + Cooler)
- Most common for thermal oil pumps
- Oil from discharge through cooler and orifice to seal chamber
- Cools seal from 300°C to 150-200°C (elastomer safe)
- Components: water-cooled or air-cooled heat exchanger, orifice, valves, temp gauge
- Verify cooler capacity (remove enough heat to keep seal <200°C)
Plan 32 (External Flush)
- Cool, clean oil from external source (expansion tank, separate pump)
- Best cooling (seal temp — flush temp)
- For >350°C or where Plan 21 insufficient
- Flush oil: cool thermal oil from expansion tank (50-100°C)
- Limitations: requires external source, flush mixes with system oil
Plan 62 (Quench)
- Steam or air quench on atmospheric side of seal
- Prevents coking of oil at seal face (oil solidifies when cooled)
- Often combined with Plan 21 or 32
- For thermal oil: steam quench (100-150°C) keeps oil fluid at seal face
Double Seal (Plan 53)
- For synthetic oil (toxic) or zero-emission requirements
- Barrier fluid: water, glycol, or compatible oil (cool, clean)
- Plan 53A (gas pressurized), 53B (piston), 53C (accumulator)
- Zero oil leakage to atmosphere
- For hazardous thermal oil: recommended
Metal Bellows Seal
- For >200°C where elastomers fail
- No elastomer secondary seal (metal bellows)
- Bellows material: Inconel 718
- Combined with Plan 21 or 32 for cooling
- For thermal oil >250°C: ideal (no elastomer degradation)
System Design
Expansion Tank
- Accommodates thermal expansion of oil (oil expands ~10% from 20→300°C)
- Maintains NPSH for pumps (nitrogen blanket, elevated above pumps)
- Prevents oxidation (nitrogen blanket for mineral oil)
- Sizing: 10-20% of system volume (depending on temp range)
- Location: above pumps (gravity feed to pump suction)
- Connections: equalizing line to pump suction, nitrogen, vent, drain, level gauge
Minimum Flow Protection
- Thermal oil pumps must not run below minimum flow (overheating, coking)
- Automatic recirculation valve (ARV): self-contained, opens at low flow
- Orifice plate: simple, but continuous bypass (energy waste)
- Control valve: flow meter + valve + controller (precise)
- Bypass returns to expansion tank (not pump suction — avoids heating)
- For thermal oil: ARV is standard (prevents coking at low flow)
Heat Tracing and Freeze Protection
- Mineral oil: pour point -10 to -30°C (generally no freeze protection needed)
- Synthetic oil (VP-1): freeze at 12°C — heat tracing in cold climates
- Methods: electric heat tracing, steam tracing, jacketing
- Pump: jacketed casing or heat tracing on bearing housing
- Preheat pump before startup (if oil has solidified or is very viscous)
Piping Design
- Suction line: large diameter (velocity <1.5 m/s), short, straight
- No strainer on suction (pressure drop) — use large mesh if required
- Discharge line: check valve, isolation valve, pressure gauge
- Expansion joints: accommodate thermal growth (piping and pump)
- Supports: independent of pump (piping weight not on nozzles)
- Slope: for drainability (maintenance, oil change)
- Insulate: maintain temp, energy efficiency, personnel protection
NPSH Considerations
- Thermal oil near vapor pressure at high temp — NPSH critical
- NPSHa = (P_tank - P_vapor)/ρg + elevation - losses
- Expansion tank pressurized with nitrogen → P_tank > P_vapor
- Elevate expansion tank above pumps → positive NPSH
- Use low-NPSH pump design (large eye, inducer if needed)
- For synthetic oil (high vapor pressure): verify NPSH at max temp
Operation
Startup
- Verify expansion tank level and nitrogen pressure
- Preheat pump to >pour point (if cold, use heat tracing or warm-up line)
- Open suction valve, vent pump (remove air)
- Close discharge valve (or throttle to minimum)
- Start pump, verify rotation
- Gradually open discharge valve (or increase speed)
- Gradually increase system temp (50-100°C per hour, avoid thermal shock)
- Monitor: suction/discharge pressure, flow, vibration, bearing temp, seal leakage
- Verify seal cooling (Plan 21/32 flow and temp)
Normal Operation
- Maintain expansion tank level and nitrogen pressure
- Operate within AOR (70-120% BEP)
- Maintain minimum flow (ARV opens if needed)
- Monitor: vibration (<4.5 mm/s RMS), bearing temp (<90°C), seal leakage (<5 drops/min)
- Monitor oil condition: annual oil analysis (viscosity, acid number, insolubles)
- For synthetic oil: monitor for degradation (acid number, viscosity change)
Shutdown
- Gradually reduce system temp (50-100°C per hour)
- Once temp <100°C, reduce pump flow
- Stop pump
- Close discharge valve
- Keep suction valve open (pump remains flooded)
- For synthetic oil in cold climate: maintain heat tracing (prevent freeze)
Common Issues
- Thermal oil coking: Film temp >340°C (mineral) or low flow. Maintain minimum flow, use synthetic for >320°C, avoid hot spots.
- Seal leakage: Elastomer degradation, no cooling. Use Plan 21/32, metal bellows for >200°C, double seal for hazardous.
- Cavitation: Low NPSH (high vapor pressure, low tank level). Elevate tank, nitrogen blanket, low-NPSH pump.
- Thermal growth misalignment: Centerline support, flexible coupling, align cold with target offset.
- Bearing overheating: Heat conduction from casing. Finned/water-cooled housing, oil circulation.
- Oil oxidation (mineral): Air in system, high temp. Nitrogen blanket, maintain tank level, oil analysis.
- Fire hazard: Flammable oil, high temp. Zero-leakage seal, leak detection, proper insulation, emergency shutdown.
- Synthetic oil freeze: Cold ambient, no heat tracing. Heat tracing, preheat, maintain temp >freeze point.
Procurement Recommendations
- Provide: oil type, temp range, flow, head, system layout, hazardous classification
- For mineral oil (<320°C): specify high-temp chemical pump, WCB casing, Plan 21 seal
- For synthetic oil (320-400°C): specify centerline support, 2.25Cr-1Mo or 316, metal bellows seal + Plan 21/32
- For hazardous/toxic oil: specify double seal (Plan 53) or magnetic drive / canned motor
- For high pressure: specify multistage (BB3/BB4/BB5) with minimum flow protection
- Specify centerline support for >150°C
- Include expansion tank sizing and NPSH calculation
- Specify minimum flow protection (ARV)
- For synthetic oil: include heat tracing/freeze protection
- Specify SiC seal faces, Inconel 718 springs/bellows
- For >200°C: no elastomers (metal bellows or PTFE wedge)
- Include seal cooling system (Plan 21/32 cooler)
- Require performance test per API 610 / ASME PTC 8.2
- Include spare parts: seal cartridge, bearings, wear rings, impeller, gaskets
- Verify supplier has thermal oil system references
Related Industries
Thermal oil pump principles apply to chemical processing (reactor heating, distillation), oil and gas (refinery, upstream), food and beverage (process heating, frying), pulp and paper (dryer, calender), pharmaceuticals (reactor, sterilization), textile (drying, finishing), and asphalt (storage, processing). The core principle — reliable circulation of high-temperature heat transfer fluid with proper sealing, cooling, and thermal management — is consistent across thermal oil applications.