By Capt. Neeraj Bharti, Founder, Master Mariner · Published · 2 min read
Fuel oil to gasoil or ULSD: the sequence, and where sulphur comes back to bite
Gasoil and diesel are often the first clean cargoes a ship carries after fuel oil, because they are more tolerant than jet or naphtha. But "more tolerant" does not mean tolerant of everything. Ultra-low sulphur diesel (ULSD) has a sulphur limit measured in parts per million, and fuel oil residue carries far more sulphur than that. A small amount left in the system can put a whole parcel off specification.
Why sulphur is the main risk
Heavy fuel oil contains much more sulphur than ULSD's specification allows. When residue from a fuel oil cargo mixes into diesel, it raises the sulphur content in proportion. Because the ULSD limit is so low, it only takes a small amount of residue in tanks, lines, pumps or coils to make a difference across a large parcel.
The cleaning sequence
- Hot wash the tanks after fuel oil, enough cycles to remove the heavy residue.
- Cold wash or rinse to remove what remains.
- Flush lines, pumps and coils thoroughly, hot where possible. Residue in the cargo system is the most common source of sulphur pickup. See lines, pumps and heating coils.
- Manual finishing: spot clean shadow areas and coils, strip and mop. See mopping, spot cleaning and drying.
- Drain and dry everything, including line low points.
- Pre-inspect before the surveyor.
Where sulphur hides
- Heating coils and their supports
- Pump casings and columns
- Stripping lines and low points
- Suction wells and behind structure
- Old coating that has absorbed fuel oil
Protecting the cargo
- First-foot samples taken after a small initial load can catch contamination before the whole parcel is loaded, and are worth agreeing with the terminal and surveyor.
- Record the preparation: what was washed, how, and when. If sulphur is high later, this is the evidence.
- Consider the coating. Old coating that holds fuel oil can release sulphur-bearing residue into the cargo.
Gasoil vs ULSD
Higher-sulphur gasoil grades are more forgiving than ULSD. The tighter the sulphur limit, the more thorough the cleaning of the whole system needs to be. Check the specification of the nominated grade before planning.
See the DPP to CPP changeover guide.
How Cleanship helps
We plan and carry out fuel oil to gasoil and ULSD changeovers, with particular attention to lines, pumps and coils, from our bases at Fujairah and in India. See our DPP and CPP tank cleaning service or contact us.
Sources & further reading
Frequently asked questions
Why is sulphur the main risk after fuel oil?
Fuel oil residue contains far more sulphur than the ULSD specification allows, so even a small amount in tanks, lines, pumps or coils can raise the sulphur content of a whole diesel parcel.
Where does sulphur contamination usually come from?
Most often from residue in heating coils, pump casings, stripping lines, low points, suction wells and old coating, rather than from the open tank surfaces.
Is gasoil easier than ULSD after fuel oil?
Generally yes, because higher-sulphur gasoil grades have more tolerance. ULSD's very low sulphur limit needs the most thorough cleaning of the whole system.
What are first-foot samples?
Samples taken after a small initial quantity has been loaded, to check for contamination before the whole parcel goes in.
- fuel oil to gasoil changeover
- ULSD tank cleaning
- sulphur contamination tanker
- fuel oil to diesel tanker
- ULSD sulphur pickup
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