How to prevent coking
- Coking occurs because the temperature and the oil residue time are higher than the oil stability limitations.
- Coke formation increases dramatically as local metal contact temperatures exceed 300°C.
- Operational factors can influence coke formation, such as hot shutdowns, which promote coke build up.
- Obstructions to flow or directional changes cause reduction in flow rate and increases in oil residence time.
- High post-shutdown temperatures due to conducted or convected heat increase deposition in low-drainage areas.
- Low-clearance scavenge ports increase the likelihood of blockage due to shedding.
- Prolonged aircraft inactivity enables moisture absorption of coke deposits, which typically shed after startup
- Unprotected reactive metals in the oil system, such as lead, cadmium or magnesium, can increase deposits (reaction between oil and metals).
- Low-alloy iron and copper can stimulate coke formation by catalysis.
- Positive washing of system surfaces by high liquid oil flow reduces residence time.
- Increased airflow in high-temperature metal contact zones reduces the amount of coke deposits formed by promoting oil volatilization.
- Thermal insulation of supply, scavenge or vent lines in the gas path can significantly reduce deposits.
- While deposits are not desirable, if they form, staying where they form is preferred.
- Coke shedding can cause blockage of filters and engine oil system passageways.
- Shedding can occur by absoprtion of moisture during prolonged shutdown periods and engine thermal cycles.
- Cracking, crazing and lifting of deposits from formation surfaces causes more oil to be trapped, increasing coke formation.
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