Part empty tanks will always draw in water--because of temperature differences and the latent heat of evaporation of water, which combines to pump water into the tank over a long period of time--just as it pumps water into PVC aerial cables
which run uphill. The water collects in the topmost terminal. Day/night temperature fluctuations reduce volumes and allow the ingress of evening moisture laden air--which then condenses on the metal tank walls and sinks to the bottom of the main fuel tank
, where the diesel fuel covers it and makes it less likely to evaporate again come morning. Enough of the why fore--what to do about it?
Firstly--make sure you purchase
only clean fuel.
My tanks are circulated continuously when the engine
is running, and a header tank with constant flow pump through a pair of large filters does this bit--keeping the header tank full and putting the filtered fuel back into the top of the main tank where it is slightly warmer than the rest of the fuel--and the pump draws from the bottom of the main tank where the previously filtered fuel is colder. The next ploy is to fill all main tanks before leaving the vessel, and to make sure the tanks air-inlet system is separate from the filler cap and properly water proof. Mine goes through a stainless steel
wool filter (easily made by my good self which stops wildlife getting in and also serves as a flame excluder as well. behind this is a carbon canister filter--as used in any automotive fume collecting device.
The next ploy is to use a biocide if you are not using the vessel over winter or for any extended period of time. There are several brand products available which seem to work OK. Cleaning
out a badly contaminated tank is a bastard of a job, using heat or strong disinfectants and steam cleaning--followed by a thorough drying using hot dry air which usually means the tanks have to come out. Much better to prevent problems in the first place.