Thermal oil systems: choosing the right components

Thermal oil (diathermic oil) systems operate at temperatures up to 350°C at low pressure, creating unique challenges for component selection that differ significantly from steam or water systems.

The challenges

Thermal oil degrades standard elastomers (EPDM, NBR) and attacks standard gasket materials. It becomes very fluid at high temperature (low viscosity) making sealing difficult. Carbon residues from oil degradation can foul valve seats and pump impellers.

Valves

Globe and ball valves in carbon steel with graphite packing and metal-to-metal or graphite-filled PTFE seats. Never use standard PTFE or rubber-seated valves. Control valves must have graphite gland packing rated for the full operating temperature.

Gaskets and seals

Spiral-wound gaskets with graphite filler are the standard for flanged joints. Expanded graphite sheet gaskets for lower-pressure connections. Avoid fibre gaskets — they carbonise and leak. FKM (Viton) o-rings only up to 200°C; above that, use graphite or metal seals.

Pumps and piping

Centrifugal pumps with mechanical seal rated for the operating temperature. Carbon steel piping (ASTM A106 Gr.B seamless) with butt-weld fittings and weld-neck flanges. Expansion joints are essential to accommodate thermal growth.