RETURN PIPING
I have received many messages that many boats do not have a positive
means of returning fuel to the same tank from which the engine fuel system
draws it. That situation is dangerous and unacceptable. Don't let the 1999
boating season get to you until you guarantee that all the return fuel goes to
the same tank from which they came. Nothing will ruin your day quicker than
pumping diesel overboard, not to mention a Coast Guard fine.
The integrity of your fuel system MUST NOT depend on you or someone
else remembering what to do after you select a fuel tank. You may forget, or
worse, you may be disabled and unable to cause the proper actions to be taken.
If you have a "Day Tank," think through this problem carefully.
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POLISHING SYSTEM
Don't even think about installing a "Polishing System" until
the "RETURN PIPING" described above has been satisfied.
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TOTAL DIESEL FLOW
Most diesel engines circulate more fuel than they burn. There is no
standard about how much. The designers of different engines choose the total
flow rate by the demands of their engines or what is available in a standard
pump that meets their requirements. No matter how the flow rate is determined,
that information is not generally available to us. The problem is that
the information IS IMPORTANT to us in selecting the other components of our
fuel systems. You may remember that the only way I could determine the rate of
the diesel return to the tank of my ADC Ford was to measure it. You can do the
same thing.
It is generally thought that engines pump more fuel than they actually
do. The magnitude of the misconception is indicated by the general conclusion
that a Lehman‑Ford 120 or 135 circulates 60 gallons per hour. Fuel
filters and other devices are commonly selected using that figure. It turns out
that
this engine circulates only 1.8 gallons per hour more than it burns. If
it burns 3 gallons per hour, it only circulates 4.8 gallons per hour. That's
less than 1/10 of the assumed amount. At maximum output it circulates less than
10 gallons per hour. After my calls to ADC and my measurements, ADC also
measured the return flow rate and got the same results.
Since the total flow rates are important in selecting Racors, pipe
sizes, GCF diesel filters, valves, magnetic elements, etc., I propose that we
develop a database of the RETURN rates of our engines. After they are
collected, they can be posted into the archives and updated, as information becomes
available.
The reason for the return rates is because most injection pumps have
either a spring‑loaded valve or just a restriction in the injection pump
to bypass the fuel that the injection pump doesn't need. The lift pump supplies
fuel to the injection pump and its flow will be essentially constant at
constant RPM. Different engines under different loads will have different burn
rates, but constant lift pump flow rates under varying loads at the same RPM.
Measuring the return flow rate at cruising RPM at no‑load will give us
the maximum return rate under normal operations. The whole thing can be done at
the dock.
Here's how to take the measurement:
1. In almost every case, the return line from the engine will be made
with a flexible hose. Find that hose and determine how to disconnect it from
the return line to the tank.
2. Select and position a container to catch the return fuel. That
container should be a rigid bucket or can, etc. Make sure it will catch all the
fuel as soon as you disconnect the return line.
3. Obtain a large zip‑lock plastic container. You will actually
use this to catch and measure the fuel. The rigid container will just be what
catches your spills.
4. Start the engine and after warm‑up increase the throttle to
normal cruising RPM.
5. Disconnect the return line so that the fuel goes into the rigid
container.
6. You will catch fuel in the zip‑lock bag for a MEASURED amount
of time. Let the fuel flow into the zip‑lock until you have enough for a
good measurement. The longer the time, the more accurate the results.
7. After you have finished gathering the sample, stop the engine,
reconnect the fuel line and measure the amount of fuel in the sample.
With return rates up to 10 gph, the easiest way to get a good measurement
is to use your wife's kitchen 1‑quart measuring cup. The formula for the
conversion from liquid ounces in any number of minutes to gallons per hour is:
Return gph = ounces * 60 /
(minutes of flow * 128)
For example: If you collected 32 ounces of fuel in 10 minutes, you
would have a return rate of 1.5 gph.
At return rates greater than 10 gph, it will be more convenient to
measure the return in quarts or gallons. That formulae are:
Return gph = quarts * 60 / (minutes of flow * 4) or Return gph = gallons
* 60 / minutes of flow
I'd give you a table for the calculation, but the formatting is so
crude in e‑mail that you couldn't read it. If you will make the
measurements and send the results to me by private e‑mail, I'll save them
and post them to the list at one time.
CaptnWil