T&T: Diesel Fuel system design

[Candy Chapman and Gary Bell]

Mark stated:

I'm rebuilding an engine room from scratch on a 56' steel trawler (circa 
1955). I've spec'd out Racor filters and the ESI polishing system and 
will have several manifolds sending fuel from one of assorted fuel tanks 
to the various fuel 'users' (main engine, gen set, furnace), and again 
with their 'return' lines.

Mr. Science comments:

As you scheme out your plumbing system try to make it so idiot proof 
that the fuel return from every engine returns to the same tank it is 
drawing from. Otherwise you could overfill a tank with the prospect of 
fuel in your bilge or overboard, with attendant cleanup costs, the 
possibility of a stunning fine and a small but real risk of fire.

Mark's question:

What would be the ideal fittings and hose types I should use? Should I 
<>hard pipe the fuel lines, except for the last little length to the 
engines?  <>If so, should this be in copper, stainless?

<>

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Mr. Science replies:

I prefer top quality rubber fuel hose with swaged on flare fittings and 
generous 'chafing pads' of big tygon tube at EVERY spot where the hose 
passes through or could possibly rub against any bulkhead, brace, etc.. 
These can be custom made in any length, and for some applications a 
generous loop of extra hose, properly supported, can provide both 
vibration isolation and the possibility of moving things around for 
maintenance access. This is the way my beloved PDQ is plumbed, and 
although I don't have ready access to the AYBC standards, I believe that 
this is their current standard.  Cars (and some boats) are plumbed in 
rigid steel pipe except where a lot of movement needs to be accomodated 
of course. Note the flexible pads they use in clamping these pipes to 
the chassis, essential to prevent vibration from making the tubing 
brittle and cracking. Automotive pipe appears to be mild steel, which is 
a good deal better at resisting hardening in a vibrating environment 
than stainless steel would be.  Stainless could offer better corrosion 
resistance only if 'crevice corrosion' can be avoided by not having some 
spot where oxygen is excluded by caulking or the like.  Copper pipe is 
commonly used in home plumbing because it is very easy to work with and 
vibration is 'never' a problem in houses, but because it becomes work 
hardened and vulnerable to cracking I very strongly discourage it's use 
in boats. You should actually try this: bend a short chunk of quarter 
inch copper tube a few times, and note how even the first bend makes it 
very hard.  You will not be able to completely straighten even the first 
bend, and after just a few flexes a kink and a gaping break will form. A 
copper fuel line subjected to the vibration found in boat engine rooms 
can be quickly work hardened and an invitation to trouble.  You could 
use rigid mild steel pipe (or even 'black iron pipe') for long runs in 
your steel hull where vibration is not a problem, where corrosion can be 
prevented by adequate painting and if you are very careful about threads 
matching, paying particular attention to the tapered threads on common 
household iron water pipe and fittings vs. the straight threads 
elsewhere.  Thermal expansion is another issue for long runs of pipe, 
however if your hull and pipe are similar alloys you should have few 
problems (note that the returning fuel may well be a good deal warmer 
than the hull your pipe is attached to). 

Mark asked:

For fittings, what type do you recommend? Any 'makes' preferred over
others? Is 37 SIC preferred over 45 SAE.or something else completely?
Are these even the preferred terms?

Mr. Science replies:

Use whichever your hose fabricator and valve manufacturer uses. Just be 
certain to not mix them (look up Capt. Maurice's comments in the 
archives for a detailed discussion on the flare fittings issue -- I have 
a long harangue on the copper pipe subject back in the archives as well.)