Heavy fuel oil density can vary but maximum values are set which allow separation at temperature.
It can vary either side of the water density from 950 (A10)to 1010kg/m3 (K55) at 15degC.
Now factor in storage temperature....
In fuel flow to engines, centrifuges are used to separate the water, the temperature is typically 98degC at the centrifugre where the oil density range is then 892 to 953kg/m3. Water density at 98degC is 958kg/m3 i.e. just enough difference to be separated.
Somewhere near 40degC, the density of water and the heaviest fuel oils are very similar and below this the water is the least dense.
It is recommended to take samples and check the quality. There are off line test kits which enable some basic properties, including water content, to be tested on site during delivery. (see
but then we come up against the various tricks some suppliers are adept at to defeat such testing.
It is usual to use an inline drip sampler to collect a sample progressively as the fuel is delivered and which is then sent for lab analysis. Spot samples for off line testing are easier to take but easy enough to defeat.
Once the fuel is received, it is not always easy to have it taken away again and there is plenty of opportunity to obscure the issue.... what was in the tank originally? and so on....
One reason for this uncertainty is that HFO is (was) cheap and no one would ever pay for instrumentation.
If online instruments were used and the supplier obliged to provide an actual lab analysis with the fuel (rather than a typical set of values) then most of the problems would be managed.
(see
for examples of how bad fuel oil quality can be ... partly due to a lack of fuel management and partly because, without online instruments, the opportunities for fraud are rife and see
for a method to use density and viscosity measurement as a fuel quality assurance system to validate the suppliers fuel quality certificate.... some engine manufacturers use this approach to checking fuel quality as received in stationary power station applications... the marine industry would rather not just at the moment because the range of fuel management issues that would be revealed by any online instruments would necessitate significant investment ... and who would pay for that?)
JMW