Diesel Return Lines

[JohnV8r]

Hey Guys,

When I had the new fuel tank built for Shambala, I included a fitting for the return lines for diesel engines. Now that I am putting diesel engines in, I am curious what the normal protocol is for connecting return lines from both engines to one return fitting. Is it just a "T" fitting and you don't worry about possible backflow into the opposite line?

Additionally, what is typically done when you have a third return line from a generator?

Thanks in advance!

JohnV8r

[mike ohlstein]

I believe that you should have a separate return for each engine.

Your genset might need a fuel pump (or maybe not). Remember not to pump fuel though a filter. Always use suction to pass fuel through a filter. Pumping tends to emulsify fuel and water, making the separator less efficient.

[Snipe]

John I put mine into a tee but I was skeptical. Thinking the same as you about one back flowing to the other I might separate them. In one of Capt Pats post I saw him run them both to a tee like I have done.

[JohnV8r]

I’m relieved to see that! I could not figure out why John Gallo at Speedtanks would have only put one return fitting in if that was going to be an issue. John was recommended to me here and I did reference return lines - plural - in the quote request.

I am going to have a welder/fabricator come out to tack weld the engine mount ramps with the engines either hanging or blocked up in place (I’m doing ramps like Capt. Pat’s). He said he could weld modifications in the fuel tank while he was out, so I may add at least one additional return line fitting if I add the Racor Fuel Polisher.

Interestingly, the welder recommended adding a drain line fitting to the low point on the aft end of the tank to be able to drain water that settles in the bottom of the fuel tank bi-annually. I’m not crazy about a fitting that could potentially fail and let 260 gallons of fuel out in my bilge. However, that issue did cause me to start thinking about a fuel polishing system that could be plumbed into the boat.

Not having any real diesel experience, I want to make sure I don’t set myself up for some boneheaded problem that could have been avoided. Try to separate practical from paranoia/FOMO is causing me to ask a lot of questions.

I really appreciate the feedback!

[Yannis]

John,
Why are you worrying so much about water? Do you fear that they’ll trick you at the pump or that a wave will force itself in the tank?

[JohnV8r]

No, I’m just trying to be careful and protect a $60K investment in new motors. Forcing fuel to go through separate filters or having a way to ensure there is no water in my fuel is cheap insurance.

[Ironworker]

Water and trash in fuel in parts of the Bahamas is a big problem. My home port of Spanish Wells has good fuel but beware of others marinas especially in the southern Bahamas

[Tony Meola]

John

I don't think here in the states you really need to worry about water outside of condensation or some idiot shooting a hose up the outboard vent. Shortly after I repowered, I pulled some water out through the Racors. Not much a couple of ounces out of each filter. Have not had a drop since. I would not worry about it.

[scenarioL113]

The return line is an important aspect of the fuel system and proper running of the engine. If it was clogged or restricted it will affect the engine.

When diesel engines are running under load they are returning more fuel thru the line than you would think. There will be a calculation on how much flow and what size the fitting needs to be.

Personally I would run those return lines "separately" to two separate fittings in the tank. It may be fine the way it is shown "snipes" picture but they are meant to be separate.

@Jason, I would run them separate

[JohnV8r]

Hi Guys,

I had a call with John Gallo this morning at Atlantic Coastal Welding/Speedytanks.com about the fuel tank and the return he built on my custom tank. John was recommended to me in 2017 from this site based on fuel tanks he built for others here. I asked him two questions:

Q: Is there any reason I should be concerned about running a tee on return lines for common rail diesel engines. Should I be considering adding a second return coupling to the tank?

A: No, a tee on a 1/2" return coupling from 5/16" return lines is standard practice on diesel fuel tanks. That is the way I build all of my fuel tanks unless someone specs it differently. I've never had a problem or a complaint using a tee on a return.

Q: Am I able to run on one engine with a tee or will that cause a possible backflow into the other engine that is not running?

A: Not an issue. You can run on one engine all day long.

I just wanted to pass that along. I would certainly consider spec'ing a 2nd return if I were building a new tank today. However, based on the feedback I got from John, I'm going to use a tee. I'll let you know if there are any issues that pop up.

Thanks for all the feedback. If nothing else it raised an issue that compelled me to do some due diligence.

Thanks,

JohnV8r

[Carl]

The mechanic I had helping me install my motors was emphatic about the return lines not flowing in from the top of tank. He wanted a tube to go to the bottom of tank like a pickup. The reason he gave was if you let the fuel flow in from the top, the dropping fuel can create bubbles in the fuel. Bubbles could lead to air in your fuel which diesels do not like.

I have little opinion of knowledge if that is true, needed or the least bit beneficial...but I added a tube to the inside of each fitting that brought fuel to the bottom of the tank. At worst it can't hurt and I had everything anyway.
I know my dad doesn't have and has been fine for many many years.

Just tossing it out there.

[Raybo Marine NY]

Regarding a drain at the bottom of the tank ( only legal on diesel tanks by the way )
what are the plans to use this drain? a shut of valve and a threaded fitting to install a hose I would think?
Plan this out now don't wait until you have 150+ gallons of diesel you want to drain and no way to connect a hose to it

The original glass tanks Rendered obsolete by ethanol fuel but with tank requirements what they have been for a while the original tank in bertram is not legal for gasoline use. There cannot be clean out or removable ports on gasoline fuel tank.

I would probably agree that I wouldnt put too much thought into adding additional return line(s) to a perfectly good tank, but when having one made whats the big deal to just let the guy add them in?
Replacing an elbow with a draw tube is also not a difficult thing to do and would fall onto the lap of the mechanic who likes to do things his way

[scenarioL113]

I will reiterate and say that it "probably" will be "NO" problem to run the return line to a "TEE" that is "directly" connected to the "TOP" of the tank.

I just would not do it that way... I am nobody and far from an expert...

[Tony Meola]

Carl

I don't think bubbles from fuel dropping in from the top of the tank would be a problem until you were about suck the tank dry. I might get concerned if I was getting down to fumes. I ran down until I had about 40 gallons left and did not have a problem. If I started hitting 20 gallons left and wasn't a within a mile of fuel or home, I would be worried about more than bubbles in the fuel.

[Carl]

Tony,

I kinda agree with you in that I didn't really understand the problem with fuel just flowing in from the top.

I know others have done without an issue.

But when you have someone helping who is the mechanic and I the flunky helper...when he says he wants the return elbows to have an extension tube going to the bottom of the tank....I solder an extension tube on, that goes to the bottom of the tank. While it may not help, I cannot see any reason it would hurt.
Well, not exactly true...my 1st attempt to solder the tube to the fittings was 50/50. One perfect, the other I could not blow through. Seems I may have gone a tad heavy as I clogged the elbow with solder. So had to cut off, open passageway and resolder. That would have been a problem.

Anyway, I mention here wondering if I was alone in running the outlet to the bottom. Seeing if the theory holds any water...(diesel fuel)

[Yannis]

I am by no means a connoisseur, but I wonder...if the tank is full or almost full, can the tube that goes all the way down diffuse into the tank all the returned diesel quantity without becoming overwhelmed and sending fuel back to the engine or the other engine?

[Carl]

I installed two separate lines for the returns, he wanted two separate over a single T.

[Yannis]

Carl,
The diesel level INSIDE the tube which is inside the tank is the same as the level of the overall diesel in the tank surrounding it.
So, when the tank is full or almost full, the tube inside the tank is equally full or almost full.
So, the question is, is the rate of diesel return from the engine or engines more important than what can be withheld inside the top empty space inside the tube UNTIL it has the time to level with the overall diesel in the tank?
If not, then it starts filling the tube that leads to the engine. Or escape through the T to the other engine if it can.

Alternatively, if you let the return just fall with gravity inside the tank, it may create bubbles, but in no way can it be blocked as there will always be some immediate space in the tank for it to fall.

[Tony Meola]

Carl

Yannis has a point, but I would think with the amount of fuel being pulled out by the engines it will all balance out. I think no harm no fowl either way.

Besides the vent would keep it from building up back pressure in the tank. You may have had an issue if it the lines were going into a T. Then it may have backed up on you.

The good point of the tubes going to the bottom is that it keeps the bottom stirred up so you get lest junk staying at the bottom of the tank.