purpose of motive flow pump
Joa
I'm studying about turbine engines and fuel systems and can't seem to
figure out the purpose of a motive flow fuel pump (jet pump).
Something like used on a Caravan in its header tank.
Why not just use the engine driven fuel pump to supply the fuel
(rather than making it drive the motive pump) and then dump excess
fuel back into the tank?
Thanks!
J o a
GreenClovers
pick up water that collects in the tank sump and deliver it to the engine
for proper disposal.
"Joa" <j...@deschutes.net> wrote in message
news:a860ab2f.0211...@posting.google.com...
Juan Jimenez
help to move the fuel to where you need it?
No Spam
parts, no electrons in the fuel tanks. Worked good, lasts a long time.
On 11/30/02 16:17, in article
a860ab2f.0211...@posting.google.com, "Joa" <j...@deschutes.net>
wrote:
Joa
to the system, it only transfers the energy. The way it appears is
that the pump (venturi) transfers high pressure low volume fuel into
high volume low pressure fuel and maybe that fact in itself is the
reason they use it. But that seems too simple, there's got to be
another reason.
Why not just use the engine driven fuel pump to take fuel out of the
low part of the tank (and any water) and cycle it through the system
and just dump/return any unused fuel back into the tank. Why go
through a venturi/jet-pump at all???
The only other reason (other than converting to HVLP) I could come up
with is that it takes the bubbles/vapor out of the fuel and prevents
cavitation in the engine driven pump.
Any other ideas? Thanks.
Jo a
Dave
the simplest, cheapest, most reliable pump available to an airframer -
that's why they are used.
A motive flow pump however needs a 'motive' source of power, this generally
comes from engine driven pump (referred to as "primary flow").
Turbofan aircraft commonly use jet pumps as the prime source for
pressurizing fuel to the inlet of the main engine fuel pump, the total fuel
into the fuel pump is the sum of the primary flow plus burn flow. As far as
cavitation is concerned, a jet pump is as effective as any other pump for
preventing cavitation, the key is the output pressure.
The minimum inlet pressure required by an engine driven pump is usually
established as a certain level 'above true vapor pressure', in other words,
the fuel inlet pressure must be at least 'X' psi above the point at which
the fuel would boil at all times. The vapor pressure of fuel increases with
fuel temperature, so the actual minimum allowable inlet pressure expressed
in gauge pressure varies with fuel temperature. To cover this, an engine
manufacturer declares in the 'installation manual' that the fuel supply
(from the airframe) must always be maintained at TVP + 'X' psi, the
airframer then makes sure that the jet pump is suitable to meet this
requirement in all corners of the flight envelope.
Note also that aircraft use motive flow pumps as wing transfer pumps,
generally moving fuel from the outboard wing tanks to the inboard or
collector tanks.
Dave
Larry Fransson
j...@deschutes.net (Joa) wrote:
> Why not just use the engine driven fuel pump to take fuel out of the
> low part of the tank (and any water) and cycle it through the system
> and just dump/return any unused fuel back into the tank. Why go
> through a venturi/jet-pump at all???
Because there may well be too much height difference or head loss
between the tank and the engine.
In your basic piston-powered Cessna that burns no more than about 15
gallons per hour, gravity works fine most of the time. In a jet that
has engines mounted above the fuel tanks and can burn more than 300
gallons per hour (at least in the case of the one I fly), there isn't
enough pressure to reliably feed the engine driven fuel pump. The
engine *may* run without some sort of boost, but you can't count on it.
In the Citations I've flown, low fuel pressure causes the electric boost
pumps to turn on automatically (as long as the pilot has left the boost
pump switches in the auto position). In a Learjet (20 and 30 series,
anyway), the first actions taken upon illumination of a low fuel
pressure light are to retard the affected throttle and switch on the
electric standby pump.
One way or another, you need a boost pump of some sort to reliably get
the fuel from the tank to the engine. If you can use something with no
moving parts that generates no heat or sparks and is not prone to
failure, that seems like a good idea to me.
--
Larry Fransson
Seattle, WA
Dave
pump inlet at lower altitudes, as you get higher, the electric boost pumps
are activated.
This happens because the jet pumps in the Citation are fed an 'unregulated'
supply of motive flow, that is the pressure to the jet increases with higher
burn flow and decreases with decreasing burn flow.
At higher altitude, the power of the jet pump decreases (lower burn flow) to
the point where the minimum pressure can no longer be maintained, thus the
electric pump is automatically activated (triggered by a fuel inlet pressure
switch).
The point of the switch over is such that the vapor pressure margin is
maintained, I believe Larry is correct in saying that the system 'may' run
without the boost because a turbofan engine fuel pump can draw a
phenomenonal vacuum, it just depends on what the fuel temperature is at (and
what type of fuel, JP-4 is the worst although not readily available).
Dave
Larry Fransson
"Dave" <dave_kar...@sympatico.ca> wrote:
> On most Citations, the motive flow system provides the pressure to the fuel
> pump inlet at lower altitudes, as you get higher, the electric boost pumps
> are activated.
I've spent significant time in 500 series Citations. I've never once
seen the boost pumps activate in the course of normal operations.
The engine driven fuel pump that provides the motive flow for the jet
pumps (Cessna calls them ejector pumps) puts out significant pressure
(500-700 psi according to the manual) that does not change with
altitude. If the ejector pumps are not providing enough pressure,
something is wrong.
Dave
JT15D and PW500 series engines (Bravo/Encore/Excel) utilize an unregulated
source of motive flow - the 500-700 psi pressure is more of a SL takeoff
type pressure. Cruise at FL390 will generate about 250-300 psi max - there
is no motive flow regulator in the system, just a motive flow on/off
hydraulic switch in some models.
One way to look at it is that the jet pump pressure is tapped off of the
high pressure discharge side of the main fuel pump, this pressure is in turn
created by the backpressure generated from the fuel nozzles (higher flow,
higher back pressure) + a fixed pressure drop across the fuel control. In
addition at altitude the fuel back pressure is also reduced due to
significantly lower compressor discharge pressure which also adds to the
final discharge pressure of fuel pump.
There is a model of Citation that I was familiar with (modified for the Navy
'UNFO' fleet) that used 3 scavenge transfer jet pumps off the inlet (the
commercial versions used 1), this dumped the inlet pressure low enough to
cause the electric boost to be activated as a matter of normal operation - I
may be confusing this aircraft with the rest of the commercial Citation
fleet in which the lower motive flow pressure at altitude generates adequate
inlet pressure to the pump.
The Citations have a LP fuel warning annunciator, do they also have an
annunciator to show if the electric pump is active?
Dave
Larry Fransson
"Dave" <dave_kar...@sympatico.ca> wrote:
> The Citations have a LP fuel warning annunciator, do they also have an
> annunciator to show if the electric pump is active?
Yes. Little green lights next to the boost pump switches.