Question 1

Why does a fuel changeover take time instead of happening instantly?

Accepted Answer

Switching the supply valve does not empty the fuel system. Service tank residue, the mixing or buffer tank, the supply and circulating lines, filters, heaters and the injection system all still hold the previous fuel, and the new fuel dilutes that inventory gradually. The sulphur content in the engine therefore decays exponentially towards the new fuel's value rather than dropping in a step, which is why the changeover must be started well before the boundary.

Question 2

How is the changeover time calculated?

Accepted Answer

The wetted fuel system is treated as a continuously stirred volume. With a system volume V and a consumption or circulation flow Q, the sulphur fraction after time t is S_new + (S_old - S_new) x e^(-Qt/V). Rearranging for the compliant limit gives the time to reach it. Using circulating pump flow rather than engine consumption gives a much faster and more realistic figure on systems where the return line feeds back to the mixing tank.

Question 3

What cooling rate is safe when changing from HFO to distillate?

Accepted Answer

Engine builders generally limit the rate of fuel temperature change to about 2 degrees Celsius per minute to avoid thermal shock and seizure of fuel pumps and injectors. Some builders specify 1 degree per minute. The calculator uses your entered ramp rate to show the minimum cooling time from heated residual temperature down to the distillate supply temperature, and flags when the flushing time is shorter than the cooling time.

Question 4

What viscosity must be maintained at the engine inlet?

Accepted Answer

Two-stroke and four-stroke builders normally require a minimum viscosity at the injection pumps, commonly 2 cSt and often 3 cSt for distillate operation, to preserve lubrication of the pump elements. Low-viscosity distillate may need a fuel cooler to stay above that limit. Check the specific engine manual: the minimum viscosity, not the temperature, is the controlling parameter.

Question 5

What has to be recorded for a port state control inspection?

Accepted Answer

MARPOL Annex VI regulation 14 requires a written changeover procedure and a log of the date, time and position of the vessel when the changeover to compliant fuel was completed before entry, and when it began after exit, together with the quantity of low sulphur fuel in each tank. Bunker delivery notes and representative samples must be retained. Keeping the entries, the procedure and the BDNs in one evidence trail is what makes an inspection short.

Question 6

Does this replace the ship-specific changeover procedure?

Accepted Answer

No. It is a planning and screening tool. Real systems have dead legs, unequal mixing, separate booster circuits and per-engine differences that a single stirred-volume model cannot capture. Use the result to plan when to start and to sanity-check an existing procedure, and follow the approved ship-specific procedure and engine builder limits for the actual operation.

Fuel changeover calculator.

Fuel system

Sulphur content

Temperature ramp

Boundary planning

Result

Sulphur in the system

Fuel changeover questions.

Every changeover, logged and inspection-ready.