ASTM D3948 and D7224: Water Separation (MSEP) of Jet Fuel and What It Means for Coalescers

Water has to be taken out of aviation fuel before it reaches an aircraft, and coalescers and filter separators do that work. They only work if the fuel will let go of its water. ASTM D3948 and ASTM D7224 rate how readily it does.

What the tests measure

Both are rapid, portable methods for the field or the laboratory. A sample of fuel is mixed with a small, fixed amount of water to make an emulsion, which is then forced through a standard cell of fibreglass coalescing material. The instrument, a Micro-Separometer, measures how clear the fuel is afterwards and gives a rating, known as the MSEP rating, from 50 to 100. The higher the rating, the more readily the fuel gives up its water.

A low rating points to surfactants: surface-active substances that keep water dispersed in the fuel. They can be traces of refinery treating chemicals, substances picked up between the refinery and the airport, or some additives.

ASTM D3948

D3948 rates aviation turbine fuels. It has two modes, which differ in how fast the emulsion is pushed through the coalescer cell: about 45 seconds in Mode A and about 25 seconds in Mode B. Mode A is used for Jet A, Jet A-1 and the military kerosine fuels; Mode B is used for JP-4 containing corrosion and icing inhibitors.

ASTM D7224

D7224 rates kerosine-type aviation turbine fuels, with or without additives: Jet A, Jet A-1, JP-5, JP-7, JP-8 and JP-8+100. With strong surfactants it gives about the same low ratings as D3948. But it is less affected by weak surfactants, and when static dissipator or corrosion inhibitor additives are present it gives somewhat higher ratings than D3948. According to the standard, those higher ratings match how filter separators actually perform with such fuels, whereas D3948 can wrongly suggest that the additised fuel will stop them separating water.

Why the rating matters for coalescers

Coalescer media works by gathering fine water droplets into larger drops that are heavy enough to settle out. Surfactants coat the fibres and the droplets and stop this happening; the media is then said to be disarmed, and water passes downstream. A fuel with a low MSEP rating is a warning that coalescers and filter separators may not remove its water in service. Changing elements does not correct the fuel: the source of the surfactant has to be found and removed upstream.

Vens fuel filters and D3948 and D7224

Depending on the product, Vens fuel filters can meet the requirements of specifications that call for ASTM D3948 or D7224: the fuel leaving the filter has to keep the water separation rating its specification sets. Options include high flow cartridges (PermeaFil) and cartridges in stainless steel housings. Tell us the fuel, the flow and the specification, and we will confirm the element.

More: ASTM D2276 and D5452: particulates in aviation fuel and ASTM standards for filters.

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ASTM is a trademark of ASTM International. Vens Hydroluft is not affiliated with ASTM International. The standards are described in our own words from the scopes ASTM publishes; work to the edition named in your specification. Whether a filter meets a requirement depends on the product, its rating and the conditions of use, so we confirm it for each enquiry. See also our product information disclaimer.