Water separator installation

[JimShalliker]

I’m fitting the fuel lines for a new diesel engine but I’m uncertain about the location of the pre-filter/water separator. The most convenient place is near to the tank with the top of the water separator higher than the tank.

I’ve been told that it’s OK to install a water separator for a diesel inboard higher than the level of fuel in the tank, but this seems odd because:
1) When I prime the fuel delivery system, isn’t the first think I need to bleed the water separator? But wouldn’t this let air in, allowing fuel to drain back into the tank, rather than push the air out
2) Won’t undoing the tap to drain water/muck from the bottom allow air in from the bottom to replace the fluid that has come out? (Like water glugging out from an upturned narrow-neck bottle being replaced by air coming in)

Grateful for any comments

 

[Drifter]

You really need to position the primary filter lower than the tank or bleeding the system will be forever problematic. The diesel lift pump will of course draw the fuel (and air) through the filter from the tank and but it just makes good sense to position the primary filter at a point lower or at least level with the bottom of the fuel tank.

 

[enterprise]

The trick is to close the fuel shut-off on the inlet pipe before you drain the separator, when reassembling top up the filter/separator housing with fresh fuel before sealing and opening the fuel shut-off, simple, and no need in many cases to bleed using the lift pump.

 

[PaulGooch]

This should definitely not cause you any problems. Mine is fitted a good foot above the tank. When you route the pipes, run the fuel supply line a little higher than the separator, then down to it. When you undo the drain on the bottom of the separator the escaping diesel draws fresh fuel from the tank. When changing the pre-filter element, close the fuel tap, change the element then fill the housing with diesel before you put the top back on. If the secondary filter has a prime pump on the top, pump that until it goes hard.

 

[Drifter]

The previous post is not wrong because the lift pump will pull diesel from the tank through the filter element even if the primary filter is above the tank. However, in terms of pumping systems / fluid dynamics and the associated longevity of pumps, it is always desirable to have a "drowned suction" or net positive suction head (NPSH) and in fact many pumps require a minimum amount of positive suction head to prevent cavitation (NPSHR). If the filter is below the tank level it will initiate a syphon action once a flow is established thereby creating a drowned suction which assists the lift pump and in turn extends it's life expectancy.