Commission a thermal oil heater in this order: foundation and rigging → piping with a valveless expansion line → hydro test → flushing → filling and cold circulation → deaeration holds at 120–150°C → a staged ramp of 20–50 K per hour → handover at temperature. Almost every commissioning problem traced back to a factory is really a skipped step in that list, and the two most commonly skipped are flushing and deaeration.
This manual is the sequence we hand over with every unit. It covers gas, oil, biomass and electric heaters from 100 kW to 28 MW, because the loop behaves the same way regardless of what heats it.
Before anything arrives on site
| Item | Requirement | Why it matters |
|---|---|---|
| Foundation | Level within 2 mm/m, cured before rigging | Distortion cracks refractory and misaligns the pump |
| Clearance at coil end | Free space equal to coil length | The coil is the part that eventually needs pulling |
| Ventilation / air changes | Per local code, minimum 6–10 air changes per hour for the plant room | Oil vapour and flue gas must not accumulate |
| Fire separation | Heater separated from storage and occupied areas | Oil leaks onto hot insulation are the classic ignition path |
| Expansion tank position | Highest point of the loop, above every consumer | Provides static head and the only vent path |
| Circulation pump position | Lowest point, flooded suction | A pump running dry destroys its seal in seconds |
| Stack height and draft | Per burner datasheet, clear of windows and intakes | Poor draft costs 3–5 points of efficiency and trips flame supervision |
| Safety valve discharge | Piped to a safe location, never to a floor drain | Discharge is hot oil vapour, not water |
Step 1 — Rigging and alignment
Lift only at the marked lifting points; the coil and the shell have different centres of gravity. Set the unit level, grout the base frame, and leave the sliding end free — a 6 m heater body grows 25–30 mm between cold and 300°C. If both ends are anchored, that growth goes into the coil welds instead of the slide plate.
Step 2 — Piping, the part that decides whether the system works
Five rules cause most of the failures we are called out to inspect:
- The expansion line has no valve. The pipe from the heater outlet to the expansion tank must be continuous, unpumped, and free of any shut-off device. A closed valve here turns the loop into a sealed bomb as it heats.
- Slope and vent everything. Air pockets form at high points; every local peak needs a manual vent, and horizontal runs slope back toward the tank.
- Keep the pump suction flooded. Suction piping is short, generously sized, and taken from the lowest point — with a 40–60 mesh strainer ahead of the pump, not ahead of the heater.
- Respect the velocity window. Design for 1.5–2.5 m/s in the coil. Below 1.5 m/s the oil film at the tube wall overheats and cokes; above 3 m/s pressure drop and erosion become the problem.
- No rubber gaskets. Use graphite or spiral-wound metal gaskets throughout. Elastomer gaskets fail at 200°C and the resulting weep lands on insulation, where it smoulders.
A drain tank (low-level tank) below the expansion tank collects overflow and gives you a place to dump the loop for maintenance. Overflow and vent lines from the high tank must reach it, not the floor.
Insulate with 50–100 mm of mineral wool, and leave the flange joints and valve bonnets accessible — you will re-torque them hot.
Step 3 — Electrical and interlocks
| Interlock | Setting | Action |
|---|---|---|
| Low flow / pump failure | Flow switch or differential pressure | Trips burner within seconds |
| Over-temperature | Dual sensors, one independent of the control loop | Trips burner, alarms |
| Flame failure | Per burner management system | Post-purge, lockout |
| High flue-gas temperature | 30–50 K above normal | Alarm: fouling warning |
| Expansion tank low level | Per sight glass minimum | Alarm: fluid loss or air ingress |
| Pump motor protection | Thermal overload, phase failure | Trips pump, then burner |
Wire the low-flow interlock to trip the burner directly, not through the PLC logic only. A software-only interlock is one firmware update away from being bypassed.
Step 4 — Pressure test
Hydro-test the piping at 1.5× design pressure, hold per local code, and record it. Two cautions specific to thermal oil systems: isolate instruments and the pump before testing, and dry the loop thoroughly afterwards — residual water in a hot oil system flashes at commissioning and looks exactly like pump cavitation. If the plant cannot tolerate water, use a pneumatic test at 1.1× with a documented leak survey instead.
Step 5 — Flushing
New pipework arrives with mill scale, weld spatter and cutting oil. All of it ends up on the hot tube wall.
- Flush with a light flushing oil or the system oil itself, through a temporary bypass around the heater and instruments
- Circulate at the highest practical flow, 2–4 hours, with temporary fine screens at the pump suction
- Clean the screens, repeat until a screen shows no grit after an hour
- Never leave a water-based flush in the loop; if you used one, follow with dry air or nitrogen until dew point is stable
Step 6 — Filling and deaeration
Fill from the expansion tank through a fill pump with a filter, to the cold-fill level marked on the sight glass.
- Cold circulation, 2–4 hours. Watch the pump differential pressure; a falling value means the suction strainer is loading and needs cleaning.
- Heat to 120–150°C and hold. Moisture and light ends boil off here. The loop will sound like it is knocking and the tank level will rise — that is the deaeration working. A 2–4 hour hold is normal; a 30-minute hold is how you buy a cavitated pump.
- Top up as the level settles, then continue.
For systems running above 300°C, add nitrogen blanketing to the expansion tank to keep the fluid from oxidising.
Step 7 — The commissioning ramp
| Stage | Target outlet | Hold | What to check |
|---|---|---|---|
| 1 | 100–120°C | 1–2 h | Pump differential, tank level, leaks at flanges |
| 2 | 150–180°C | 1 h | Re-torque flanges accessible at temperature |
| 3 | 220–250°C | 1 h | Burner modulation, flue-gas temperature |
| 4 | Design temperature | 2 h | Full-load stability, all alarms proved |
Ramp at 20–50 K per hour. Faster ramps crack refractory, distort flanges and shock the coil; the holds above are not optional formalities, they are where bedding-in happens.
Step 8 — Normal operation
Start sequence: start the circulation pump → confirm flow and differential pressure → confirm expansion tank level → start the burner → raise temperature at the design ramp.
Stop sequence: stop the burner → keep the pump running until outlet temperature is below 80°C → stop the pump.
That second rule is the one people break. Static oil in a 300°C coil cracks into carbon within minutes; the deposits then insulate the tube, raise film temperature, and start a cycle that ends in a coil replacement.
Log these readings every shift:
| Reading | Typical normal band |
|---|---|
| Outlet / return temperature | Design ±5 K |
| Pump differential pressure | Steady; a slow fall suggests strainer loading |
| Expansion tank level | Between min and normal, stable at temperature |
| Flue-gas temperature | Within 30–50 K of commissioning baseline |
| Pump motor current | Steady; a rise suggests fouling or valve misposition |
Step 9 — Maintenance schedule
| Interval | Task |
|---|---|
| Daily | Level, differential pressure, flue-gas temperature, visible leaks |
| Weekly | Pump seal weep, valve packing, alarm lamp test |
| Monthly | Clean suction strainer, check safety valve for seizure, inspect insulation for oil staining |
| Quarterly | Burner combustion analysis, flame scanner clean, refractory inspection through the peep door |
| Annually | Full mechanical inspection, interlock function test, coil external cleaning, pump coupling alignment |
| Every 500 h then 6–12 months | Oil sampling: viscosity, total acid number, flash point, insolubles |
| Every 3–5 years | Fluid replacement (sooner if analysis shows cracking) |
Troubleshooting
| Symptom | Most likely cause | Action |
|---|---|---|
| Outlet temperature will not reach setpoint | Coked coil, fouled fireside, burner underfiring, low flow | Check flue-gas temperature first: high means fouling, normal means flow or firing |
| Pump noisy, pressure fluctuating | Cavitation from air ingress or low tank level | Check tank level and pump suction; re-deaerate at 120–150°C |
| Expansion tank overflowing on heat-up | Closed expansion line valve, blocked vent, overfilled cold level | Verify the valveless expansion line, then the vent |
| Tank level falling steadily | Leak in the loop or the drain line | Pressure-test sections; look for oil staining on insulation |
| Flue-gas temperature climbing over months | Fireside fouling or scale | Dry clean the gas passes, inspect burner settings |
| Oil analysis shows rising viscosity | Film temperature too high, flow too low | Recheck flow velocity and heat flux before adding oil |
| Safety valve weeping | Thermal expansion locked in by a closed valve | Find the closed valve; do not just replace the valve |
Safety points worth repeating
- Oil-soaked insulation ignites. Fix every weep immediately; do not paint over staining.
- Never open the expansion tank while hot. It holds oil vapour above 150°C.
- On power failure, the coil stays hot: provide a backup or emergency pump feed, or a documented drain-down sequence.
- Fire response: isolate the burner, keep circulating if the pump still runs, use foam, dry powder or CO₂ — never water.
Handover checklist
- [ ] Expansion line verified valveless and clear
- [ ] Hydro test recorded, loop dried
- [ ] Flushing screens clean
- [ ] Cold circulation completed, strainers cleaned
- [ ] Deaeration holds at 120–150°C completed
- [ ] Ramp to design temperature completed, flanges re-torqued
- [ ] All six interlocks proved by test, not by inspection
- [ ] Baseline log recorded: temperatures, pressures, flue gas, motor current
- [ ] Oil batch certificate and fill date filed
- [ ] Operators trained on the pump-after-flame rule
Where to go next
- Gas and oil fired thermal oil heater — 100–28,000 kW, ≥92% efficiency
- Coal and biomass fired thermal oil heater — ≥86% efficiency
- Skid-mounted thermal oil heater — pre-piped and pre-wired, cutting site work to days
- Chemical industry and food processing solutions — typical loop configurations for these duties
If you would rather have the commissioning sequence written for your specific duty, send the loop drawing, fluid volume and target temperature, and we will mark up the ramp schedule.
Key takeaways
- Sequence is the whole game: rig → pipe → test → flush → fill → deaerate → ramp → handover.
- The expansion line must be valveless, and the tank sized at 25–30% of loop volume.
- Deaeration at 120–150°C is not optional; it is the step that protects the pump and the fluid.
- Ramp at 20–50 K per hour with holds at 120, 180, 250 and design temperature.
- Burner off first, pump off last — below 80°C, never before.
