T&T: Fuel piping arrangements

[Ken Bloomfield]

Hi Rich,

and thanks for the reply.  The air ingestion point is well taken, and I 
realize that some positive pressure provided by a tank that has fuel level 
higher than the lift pump helps to prevent this.  I mainly made the point as 
the originator of the thread was in the UK and to import a vessel must 
follow some reasonably strict regulations.  I will bet a dollar to a donut 
that the top of tank extraction will be one of them.

My old MT-50 takes the fuel out of the top of the tank, and I have never had 
a problem.  After I replace Racor filters, I just crack the valve for the 
respective engine feed and the siphon principle works just fine to fill the 
Racor bowl/body.  It does not seem very hard to me to keep a leak free run 
from the tank to the Racor, especially with flexible fuel-line.  Perhaps 
others have different experience.  I know that the older copper tube runs 
were problematic with too many fittings that each were a potential source of 
leak.

We had two cases of leaks this past year in our marina.  In both cases, it 
was copper pipe and it had cracked right at a fitting, probably due to 
improper support of runs.  The bummer was that it leaked slowly for a long 
time, being winter and the owners never came down to their boats, and 
finally the combination of bilge-water and diesel turned on the bilge pump 
and next thing was the whole marina had a lovely rainbow pattern on the 
water.

My MT also apparently had the feed from the bottom originally, but was 
modified to top feed by a PO.  It was in charter service at one point, which 
may have driven the modification to ABYC recommend.

It is interesting that your air leaks have all been benign in the sense that 
they inleaked but apparently never outleaked.  I am not always that lucky! 
I do know, however, that this is not uncommon, and makes finding an air leak 
a real bear in some cases.

As regards suction, granted if there is air leaks, then the siphon effect is 
broken, and your lift pump may have more work on its hands.  However, if the 
fuel line is intact/leak-free, then the work for the pump is equal 
regardless of how is is taken off (top or bottom).  In other words, if your 
pump is above the level of the fuel in the tank then the pump must have some 
active suction (i.e. there must be some negative pressure on the suction 
side of the pump.  If the pump is below the level, the pump may never feel a 
negative pressure if the line is big enough diameter that the gravity 
induced pressure is enough to satisfy the flow requirements versus the line 
restriction (i.e. the CV).

Last but not least, with a drain from the bottom, it is not totally uncommon 
for fuel that sits around a lot to build up asphaltines to the point that 
the line will not flow any more, a problem that can be just as annoying as 
water.

Last but in support of your point about holding back the flow, it is fairly 
common for people to have a vacuum cleaner pull on the tank fill and the 
air-leakage in will pretty much preclude fuel flowing out of a temporarily 
disconnected fitting.

Here is a really nice article on fuel tank design:
www.tek-tanks.com/downloads/Custom%20Fuel%20Tank%20Design%20Guide.pdf

All the best,
Ken.