Diesel: six parameters to check before accepting a cargo

A diesel certificate of quality lists around twenty lines, each with its value, its unit and its test method. All of them matter. But six of them decide, on their own, whether the cargo can be accepted as it stands, has to be renegotiated, or has to be refused.

A certificate is not read from top to bottom. It is read in the order of the risks you are carrying.

1. Density at 15 °C: it decides what you pay

This is the most underestimated parameter, because it is taken for a technical characteristic when it is first and foremost a commercial figure.

The European standard EN 590 sets a range of 820 to 845 kg/m³ at 15 °C. Two methods are authoritative: the hydrometer (ASTM D1298) and the oscillating tube digital density meter (ASTM D4052), which is significantly more precise.

Why this decides the invoice: a cargo is gauged by volume in the tanks, but is most often sold in metric tons. Density is the bridge between the two, applied through volume correction tables to standard temperature. A 0.5 % error on density shifts 0.5 % of the invoiced quantity. On a cargo of several thousand tons, that is no longer a technical detail.

Check: that density is stated at 15 °C and not at tank temperature, and that the method is specified.

2. Cetane, and the calculated index trap

Cetane measures the ability of diesel to ignite under compression. A cetane figure that is too low gives hard starting, knocking and accelerated fouling.

There are two different quantities, and confusing them is the most frequent reading error on a certificate.

• The measured cetane number (ASTM D613, EN ISO 5165) is obtained on a standardised test engine. EN 590 requires a minimum of 51.
• The calculated cetane index (ASTM D4737, EN ISO 4264) is an estimate obtained by formula, from density and the distillation curve. EN 590 requires a minimum of 46.

The second is not the first. Above all, the calculation formula ignores cetane improver additives. A chemically doped product will show a mediocre calculated index and a correct measured number; conversely, a certificate presenting only the calculated index does not prove the real value.

For comparison, the American standard ASTM D975 is considerably less demanding, with a minimum cetane number of 40 for grade 2-D. A product compliant in the United States is therefore not automatically compliant in Europe.

3. Sulphur content: the line that allows or prevents discharge

This is the only parameter in this list whose value is not technical but legal: it depends on the destination market and its regulation on the date of customs clearance.

The subject is covered in our article on sulphur limits by market. The reading rule to remember here: check the unit, check the test method, and require a margin below the legal limit.

4. Water, sediment and total contamination: the leading cause of disputes

When a dispute arises on a diesel cargo, it concerns this family of parameters in most cases.

• Water content: EN 590 sets a maximum of 200 mg/kg (EN ISO 12937).
• Total contamination, that is solid particles retained on a filter: maximum 24 mg/kg (EN 12662).
• The American standard ASTM D975 combines both as "water and sediment", with a maximum of 0.05 % by volume.

Water is almost never present at the refinery gate. It arrives afterwards: tank breathing, condensation in hot and humid climates, washing residues, poor segregation on board. In tropical zones, the day to night temperature swing condenses water in tank ullage space night after night.

The costly point: free water combined with a fatty acid methyl ester content encourages microbial growth at the water and fuel interface. The result is filter plugging that appears weeks after delivery, when liability is already hard to establish.

5. Cold flow behaviour: cloud point and CFPP

A frequent and mistaken reflex: "we are in a hot region, cold flow does not concern us".

Three quantities describe low temperature behaviour:

• Cloud point (ASTM D2500, ISO 3015), the temperature at which the first wax crystals appear.
• Cold filter plugging point, or CFPP (EN 116, ASTM D6371), at which the fuel no longer passes through a standardised filter. This is the operational figure.
• Pour point (ASTM D97), at which the product stops flowing.

EN 590 does not set a single value: it defines seasonal and climatic classes, and the receiving country chooses the applicable class. A European summer grade delivered to a high altitude mining site, or to a desert region where the night falls to a few degrees, can start plugging filters at dawn.

Check: that the class chosen matches the real conditions at the point of consumption, not those at the loading port.

6. Flash point: safety, legality, and a warning sign

EN 590 requires a flash point above 55 °C (EN ISO 2719, ASTM D93). ASTM D975 sets a minimum of 52 °C for grade 2-D.

It is a safety parameter, which governs storage, transport and insurance rules. But for a buyer it is above all a revealer.

An abnormally low flash point is almost never explained by the quality of the distillate itself. It signals contamination by a lighter product, gasoline or kerosene, through a shared transfer line, a poorly purged tank or a multiproduct pipeline. In other words, the low value is not the problem: it is the symptom of a problem that the rest of the certificate may not show.

Two parameters that get forgotten, and are paid for later

Fatty acid methyl ester content. EN 590 allows up to 7 % by volume of biodiesel (EN 14078). In a temperate climate, no consequence. For long storage in a tropical zone, it is a risk: biodiesel is hygroscopic, it retains water and feeds microbial growth. If the product has to sit in a depot for several months, specify a nil content rather than discover the problem in the filters.

Lubricity. Deep desulphurisation removes, along with the sulphur, compounds that naturally lubricated injection pumps. EN 590 therefore imposes a corrected HFRR wear scar below 460 µm at 60 °C (EN ISO 12156-1). A very low sulphur diesel without additive can destroy a high pressure pump in a few hundred hours. The parameter is missing from many summary certificates: it has to be requested.

Key takeaways

• Density decides the invoiced quantity as much as the quality.
• Measured cetane number and calculated cetane index are two different quantities. Check which one is on the certificate.
• Sulphur is a matter of destination market regulation.
• Water and contamination generate most disputes, and deteriorate after loading.
• Cold flow behaviour also concerns hot countries, as soon as altitude or desert is involved.
• A low flash point is a cross-contamination signal, not only a safety matter.

A certificate of quality is only worth as much as the laboratory that issues it and the method it applies. That is why third party inspection, at loading as well as at discharge, is part of the normal file of a serious cargo.

Our diesel specifications are confirmed cargo by cargo, with the corresponding documentation. For a specific requirement, request a quotation.