Draining a Water Separator

[sfellows]

This may sound like a stupid question, but if the water separator is above the level of the fuel tank, won't opening the water drain mean that the diesel will siphon back into the tank and get air into the system?

I've got to learn how to do this but am worried I'll just make things worse if I try to drain any water....

 

[Moodysailor]

If you don't open the bleed screw at the top of the separator, you might be lucky enough that the syphon effect will draw fuel from the tank. Depending on the height above the tank however, it is more likely that this won't work - sorry.

It sounds like a good time to learn how to bleed the fuel system. This comes under what I consider an essential skill for anyone with a diesel engine in their yacht, so read up, get a cuppa and have some fun learning a new skill

 

[snowbird30ds]

Hook the drain to an oil sucker and apply a bit of vacuum before opening, assuming your drain has a spigot.

 

[Oily Rag]

You are right to be concerned. If you let air in at the top of a siphon, it will deprime. Unless....you have closed valves on either side of your separator. Or if there is a non-return valve integral with the separator.
You may have an upstream valve at the fuel tank and could close that to prevent reverse flow. If you don't have a valve downstream, fuel may not drain that way, but think about an air bubble entering the line. You will have to reprime the engine.
Regardless, when you have drained separator of water, you will have to top it up with fuel or you will leave a bubble.

I have a similar arrangement, but with valves either side of the separator. After draining water - about once every two years - I top up from a can, seal the separator up and then open the valves. I believe my separator has an integral non return valve to prevent back flow, but have never tested it!

Good luck, but do your trials alongside or on a mooring.

 

[Iliade]

If you have a petcock rather than a screw you can put a pipe on the outlet ending below the bottom of the tank.

If not, open the bleed screw on top before opening the drain, then prime the filter before restarting the engine.

 

[ip485]

You may have a check ball in the seperator to stop exactly this?

You may find where the supply tube enters the seperator that it is well below the top of the seperator so some fuel can be drained without the level falling below the tube, and even topped up as the water / dirt drains.

You should have an on / off tap on the tan so you can turn off the supply, drain, top up and turn the supply back on.

 

[sfellows]

Thank you all - good information.

 

[ip485]

sfellows - out of interest did any of the points in my email above apply to you? Good luck with it.