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KNOWLEDGE · K9

Thermal oil heater protection: flow, level and temperature

In oil systems the expensive failure is degraded oil and cleaning downtime, not the heater. Protection settings exist to cap sheath film temperature; control accuracy comes second.

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  • Trip heat, keep pump running
  • Low level must cut power
  • Loss of flow by meter or ΔP
  • Write start-stop order into logic

Thermal oil heaterPublished 2026-10-08中文版 ↗

01What each guard covers

High outlet temperature

Probe on the straight run after the heated section. Action: cut heaters first, keep the circulation pump running to carry residual heat away, stop the pump only after the oil falls back.

Low level

Switch on the expansion vessel or the lowest sensible point; a trip opens the main contactor. Low level can expose the heated section — that is dry firing.

Loss of circulation

Flow switch or inlet-outlet differential pressure. Below design velocity the heat is not carried away and film temperature far exceeds bulk oil.

Independent sheath guard

A separate measurement near the shell or terminals, independent of process control, as a second line of defence.

02Why film temperature is the constraint

Oil data sheets quote a maximum bulk temperature, while the film touching the sheath always runs hotter, and ageing is judged on that film. In practice this means keeping heated-section watt density at the low end for static oil (1.5~3 W/cm²) and energising only under flow — both more effective at protecting the oil than any extra alarm.

Check the ceiling against three numbers: the oil manufacturer maximum service temperature, the heated-section watt density, and minimum flow while circulating. If any one is short, reduce the duty (group switching) rather than raising the alarm threshold.

03Sequence and typical missettings

Energise before the pump

Heating starts with no circulation; the first minutes carry the highest film temperature and produce most of the coking.

Stop pump and power together

Residual heat keeps cooking stationary oil — effectively a local dry-fire. Delay the pump stop.

Alarm set on the control value

If the alarm equals the setpoint it can never act first. Separate them by 5~10 °C.

Probe on the return line

Return oil is cooler than outlet, so high-temperature protection lags badly.

04What to confirm during commissioning

Cold commissioning first proves the interlocks: simulate low level, loss of circulation and high temperature one by one and confirm the main contactor opens while the pump stays running, then compare measured phase current with the nameplate. During hot commissioning log one cold-to-temperature ramp; a spike past setpoint followed by fallback means the control band and group switching need adjustment.

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Medium, flow, temperatures, pressure. With those four, duty and architecture are a same-day answer.

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FAQ

Common questions

01Can an oil system be controlled like a water system?+

No. Water tolerates brief dry-fire; oil starts degrading once film temperature goes over, so circulation and level come first, process accuracy second.

02Should alarms annunciate audibly?+

Yes. Loss of circulation and low level damage the oil immediately, so operators need to know on the spot.

03How often to sample the oil?+

By running hours or seasonally, watching acid number, carbon residue and viscosity. If they drift, investigate watt density and circulation before simply replacing the charge.

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