On Sat, 10 Jan 2004 12:53:52 -0700, Keith Hughes
wrote:

Steven Shelikoff wrote:


...filter media, by definition we are ignoring the case. My question is
why does it matter strictly to the performance of the filter media
whether fuel is being pushed through by a pump or pushed through by
atmospheric pressure?

It's often a function of system and pump design. For e.g., when
using a centrifugal pump (or liquid ring, and sometimes vane), the
inlet is typically sized larger than the outlet. The result is
higher fluid velocity on the outlet side versus the 'suction'
side. Higher velocity, higher impact pressure, often resulting in
better particulate retention.

Sounds like we have to limit this further. Ok, I'll limit it to the
typical filter, like a Racor fuel filter, with a typical pump, like the
Walbro, with the pump either before or after the Racor attached to it by
at least a few feet of hose and in both cases, a 3 psi difference
between the outlet and inlet. How would you explain a difference in the
performance of the filter media between the pump pushing or pulling the
fuel?

Additionally, all pump curves I've seen are, to some degree, more
dependent on suction head than discharge head, and cavitation
becomes an issue (i.e. efficiency drops more rapidly for loss of
head on the suction side than for increase in head on the
discharge side). Thus, when the filter begins to clog, you not
only lose flowrate due to loop pressure drop increasing, you lose
pump *efficiency* as well, exacerbating the problem. The result
is, typically, less allowable filter loading before the system
performance is affected, so more frequent filter changes.

Basically, the above is saying that the pump can push better than it cal
pull. That I agree with. But if it can pull well enough to maintain
enough pressure differential across the filter up to the point where you
would want to change the filter anyway, it becomes a non-issue.
Especially if you're not as worried about filter element replacement
costs as you are about other aspects of the system such as polishing
ability and safety.

Whether this is an issue with the Racors or not, I have no idea,
not being familiar with them. But if you want maximum system
efficiency, maximum filter loading capacity, and longest interval
between changeouts, discharge filtration is the way to go.

And if I want maximum life out of the pump (it's always seeing clean
fuel), filtration capability (the pump isn't emulsifying the fuel just
before it gets to the filter) and safety (a leak will shut the system
down rather than pump fuel into the bilge) then I'd go the other way.

You pick the parameters you want to maximize and go with it.

Or...just use more *wind*, and all this diesel stuff is moot :-)

Yup.

Steve