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Old 06-18-2017, 10:32 AM   #1
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Gravity Day-tank - minimum height?

I'm about to install a 35gal day tank, gravity feed.

I have plenty of space, and several location options in the ER. Can you help me with working out minimum height above the engine filter intake the bottom of the tank should be?

Thanks in advance!
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Old 06-18-2017, 12:10 PM   #2
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Anything above zero will work. My Racors tops are about 6' above the bottom of my day tank, but I always have more fuel in the tank. Even if the fuel level is close, I have no problem bleeding the filters. But I have Detroit Diesels and no bleeding issues. I just run an inline electric for a couple minutes and flush any leftover air thru the lines and back to the tank.
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Old 06-18-2017, 02:07 PM   #3
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Thanks Lepke... did you mean your Racors are 6', or 6" above?
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Old 06-18-2017, 02:16 PM   #4
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I would set the Racor below fuel level in tank if possible, with a ball valve right on inlet. That way you can gravity fill it after draining or changing element. But don't go so low that you can't slide a drain bucket underneath.

I think this is what you are asking?
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Old 06-18-2017, 02:27 PM   #5
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Thanks Ski.... I'd like to gravity-feed all the way from day tank to secondary on each engine. I was planning on filtering before the day tank, then run straight from that to secondary, no further primary filter. Rationale: it's a small day tank, deliberately chosen. 35gal gives me 18-20 hours motoring at normal rpm, and say 12 hours if I have to push it. So the fuel in there should be clean.

Sitting in the ER right now, planning to have bottle of day tank 3-4 inches above tops of secondary filters.
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Old 06-18-2017, 02:47 PM   #6
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Does your engine have a lift pump between fuel supply and the secondaries? Which engine(s)?

If bottom of tank is above secondaries, that means tank is pretty high. So can you gravity feed from your bulk tanks to the day tank? I guess my preference is for day tank to be low in bilge.
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Old 06-18-2017, 02:54 PM   #7
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Ski, engines are new Iveco 150s with a lift=-pump on each. I plan to pump (Walbro) through serial Racor 900s, 30Micron and 5Micron into Day Tank. Then gravity feed from there - main reason to make it fail-safe as possible, cutting out possibility of air in system, and lift-pump failure.
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Old 06-18-2017, 03:22 PM   #8
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If running serial racors on inlet to dt, I'd put one on tank supply and the second on tank withdrawl. Tanks get crud from mfr'g and also fuel participates.

Having dt above secondaries does not protect you from lift pump failure. Not only does lift supply fuel, but at a pressure well above head pressure you would get from a few inches of elevation.
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Old 06-18-2017, 05:16 PM   #9
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Thanks Ski. That's the kind of info I was looking for. I'll see how low I can get that tank now!
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Old 06-18-2017, 11:50 PM   #10
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Maybe a bit late in this thread to ask the question, and I don`t want to hijack it, but what`s the advantage of a day tank?
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Old 06-18-2017, 11:59 PM   #11
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As I see it, any boat with old tanks, which can't easily be changed or cleaned, can remove the risk of dirty diesel by using a day tank. Fill it up at the start of day, with filtered fuel, and you're good to go anywhere, without worrying about rough weather stirring things up in the tank.
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Old 06-19-2017, 04:55 AM   #12
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Even a day tank will be helped by installing a sump (like an oil filter housing hanging down) on the bottom that can be drained or bailed.
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Old 06-19-2017, 07:06 AM   #13
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Eamonn

If I were to set up a day tank I'd use the ER set up of a Nordhavn 55/60 as a template. PM Peter on Twisted Tree and possibly he could forward some pictures and elevations of his setup. Nordhavn has hundreds of successful day tank installations.

Also, in my foggy mind I seem to recall that a Racor is better utilized under a vacuum than with a pump feeding it.

Overall, your desires are spot on. Sweating the details, as you are doing, is correct.
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Old 06-19-2017, 07:51 AM   #14
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Thanks Sunchaser & FF.

Here's another variation. Is there any reason why I shouldn't feed both engines from a single pick-up on the day tank? Insert a Tee, and let the lift pumps manage demand. I'm using 3/8th tubing.
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Old 06-19-2017, 08:10 AM   #15
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Use one pickup for each engine and generator. There are failures that could cause air to enter supply line for an engine. If pickup is shared, you lose the other engine too.
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Old 06-19-2017, 08:22 AM   #16
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Good point Ski. Thank you.
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Old 06-19-2017, 10:37 AM   #17
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"Also, in my foggy mind I seem to recall that a Racor is better utilized under a vacuum than with a pump feeding it."

Every filter does better having fuel sucked thru as the pump can chop up and mix the water or goop in the fuel making it harder to filter.

A big chunk could block a single fuel feed , unless its something like 3/4 or more.

With a bottom drain and a valve a glass jar style could warn of gunk/water in the fuel.Not as hood as a real sump, but better than nothing.
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Old 06-19-2017, 03:50 PM   #18
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add a sight glass to the tank so you can see how much fuel is in the tank...

And a sediment bowl down at the bottom, so any sediment gets through the Racor filters will settle into the bowl and let you collect it.
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Old 06-19-2017, 08:51 PM   #19
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If you have room, get the low point you are drawing fuel from at least a few inches above the bottom of the tank so any water won't be sucked in. This is probably what the previous poster is calling a sediment bowl? Have a drain at the very bottom.

When you have the tank made, I would put extra weld-o-lets for level sensors even if you are not going to use them now. You can just put plugs in them, but they would be hard to add later once everything is installed and fuel soaked. have one for full, Low/turn on transfer pump (At level where you would have about 1 Hour if pump doesn't work, and emergency low. The first time you forget to manually fill the tank, and both engines die, you will want to add alarms, and or pump switches.

If the tank is higher than the engine (Gravity feed) even if you have a leak it should leak fuel out, not suck air into the line. This is the big advantage of gravity feed. My preference would be to have dual Racors on a single line from the tank, which could be switched from one to the other if the filter gets plugged, rather than each engine getting it's own, but having to shut the engine down to change the filter. If you want to go crazy and put in dual, dual filters that would be sweet, but seems overkill to me.

If you want to get really fancy, you can plumb the tank so the pickup, and return lines come off the sightless fittings and can be isolated from the day tank with valves. You can figure out the cross sectional area of the inside of the sightless, multiply it by a 12" drop in level, which you mark on the glass. Then you can close the valves, and time how long it takes for the fuel in the sight glass to drop 12", and using the table you made, get a very accurate current fuel burn, in just a couple minutes. My boat was originally set up this way, but a previous owner removed it. I assume it was leaking. I wish they had fixed the leaks, and kept it.
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Old 06-20-2017, 05:34 AM   #20
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It gets chilly but not cold in Vancouver , so if you plan on a diesel heater like a Dickinson range a dedicated gravity feed would allow heat with zero electrical requirements.
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