Separate fuel return fittings in tank?

[sainted]

I'm installing a diesel generator on my Stevens 47. It does not currently have one. I need to install a new fuel pickup in the fuel tank, but I'm considering using the same return fitting used by the engine. I know they are usually separate, but it doesn't seem to me that the volume of return fuel from either engine is sufficient to cause problems with the other.

Thoughts?

[DougR]

Don't see that it would be a problem..

I've seen lots of power boats that were repowered from gas to diesel where the fuel tank didn't have a fuel return fitting. The engine installer put a "T" into the fuel tank fill line and routed the return fuel into that. Not the prettiest setup known to man, but functional.

Your idea should be functional also.

DougR

[a64pilot]

A large use of the excess fuel is as a coolant of the fuel pump, by recirculating the fuel you will allow it to be heated to a much higher temp of course.
Is that bad? Depends I guess on how hot is gets, my Duramax Diesel even had a fuel cooler for the return fuel, but it was common rail, so likely due to very high pressures it heated the fuel higher than an old style Diesel.
I would if possible fit a return fitting

[nhschneider]

I think using a single return fitting for both the main engine and genset would be okay but I think I would avoid running both engines at the same time. However, I have never seen an original installation plumbed that way and that’s probably for a good reason.

Fair winds and calm seas.

[sainted]

Originally Posted by a64pilot

A large use of the excess fuel is as a coolant of the fuel pump, by recirculating the fuel you will allow it to be heated to a much higher temp of course.
Is that bad? Depends I guess on how hot is gets, my Duramax Diesel even had a fuel cooler for the return fuel, but it was common rail, so likely due to very high pressures it heated the fuel higher than an old style Diesel.
I would if possible fit a return fitting

Thanks for your reply, however, I am thinking about combining the propulsion and generator RETURNS, so I don't think heating the fuel will be a problem. If there is an issue it will be with back pressure when both engines are running.

[Terra Nova]

Originally Posted by sainted

...it doesn't seem to me that the volume of return fuel from either engine is sufficient to cause problems with the other.

Thoughts?

Really bad guess.

Much more fuel is returned to the tank than is used by the engine. Don't let parsimony be your guide in decisions like this. It is a simple matter to install separate fuel return (and pickup, for that matter) in a new clean-out plate.

[sainted]

Originally Posted by Terra Nova

Really bad guess.

Much more fuel is returned to the tank than is used by the engine. Don't let parsimony be your guide in decisions like this. It is a simple matter to install separate fuel return (and pickup, for that matter) in a new clean-out plate.

Thanks for your reply. I know that installing pickups and returns in the plate is a straightforward matter. The issue is room. The plate already has the pickup and return from the engine, plus a dip stick and the fuel level sender. The tank is under the cabin sole and there is no other access to the tank other than the current clean out plate, which has all of the aforementioned stuff on it. It's tight.

[dkenny64]

is the fuel line larger enough to handle the flow of fuel needed to run both engines? if so


I'd use a 'T' on both supply and return lines. then a pump to push the fuel into the primary filter. each engine should have its own primary filter and pump.
ideally each pump on its own switch. it'll be easier to bled the system when you change filters


if not then you need 2 supply and 2 returns.



-dkenny64

[flyingfin]

Do you suppose putting back pressure on the main engine return line when the main engine is not running would cause problems? Is there a check valve on the return line between the main engine fuel pump and the fuel tank? Or will you now be delivering fuel to the main engine injectors?

[ggray]

Our catamaran with two 3GM30s came with the two return lines Tee'd together right at the tank.
When I installed a genset, I replaced the Tee with a 4 way cross, so all three engines return through the same fitting. I have had no problems running multiple engines, 'though I doubt I have run three at one time.

[dkenny64]

Originally Posted by flyingfin

Do you suppose putting back pressure on the main engine return line when the main engine is not running would cause problems? Is there a check valve on the return line between the main engine fuel pump and the fuel tank? Or will you now be delivering fuel to the main engine injectors?

only if there are multiple failures..
the return is plugged going into the tank.
the other engine injectors have to leak or the burst pressure of the fuel line is higher than the injector pop-off level( not likely since returns are rubber, not metal)


usually the return line is separate from the supply. better setups have them separate


-dkenny64

[a64pilot]

Originally Posted by sainted

Thanks for your reply, however, I am thinking about combining the propulsion and generator RETURNS, so I don't think heating the fuel will be a problem. If there is an issue it will be with back pressure when both engines are running.

I doubt there would be an issue either, ought to be fine.
A plugged return on some pumps can cause a run away, but connecting them together still shouldn’t cause much if any pressure in an open line, the flow rate would have to be enormous to build pressure, and I’m sure it’s not that high.

[billknny]

Just because some people have "never seen it" doesn't mean it isn't done.

My boat has a factory install of generator and main engine with the return tee'd together and a single return point to the tank. It has worked perfectly for 23 years. And YES you can run them both at the same time.

In fact, both engines share a common feed line and primary filter. As long as it is big enough, and rated for the full fuel flow to both engines, why not?

There is no reason you need a separate return for each engine. None at all. Go with what is convenient. If you are really paranoid, make the flow area of the return line as big as the combined area of the two feed lines, and you can be assured of never having a problem.

[Terra Nova]

Originally Posted by billknny

...My boat has a factory install of generator and main engine with the return tee'd together and a single return point to the tank...
In fact, both engines share a common feed line and primary filter...

Curious what boat you have. Some entry level builders take silly shortcuts to save every penny they can. Some do it because they don't know any better.

[steve77]

Originally Posted by Terra Nova

Curious what boat you have. Some entry level builders take silly shortcuts to save every penny they can. Some do it because they don't know any better.

My "entry level boat builder" also took this "silly shortcut". We have an Amazon 49.

I agree with Billkinny. Just because your boat is not built this way doesn't mean it's a bad idea.

I'm surprised no one has asked the OP for the size of his fuel lines. Ours are 5/16" and we have no problem running the 4JH3-HTE and the genset with a 3GM driver simultaneously.

Steve