Well, the U-Boats (contrary to the US subs) were not fitted with fuel oil purifiers. The fuel delivery systems on U-Boats contained only simple strainers and gravity tanks. The US subs were fitted in the centrifugal purifiers.

Here you have diagram of the fuel oil delivery system on type VIIC U-Boat:
[uboatarchive.net]

Here you have description of the fuel oil system on US fleet submarine:
[maritime.org]

--
Regards
Maciek


[www.tvre.org]