Hale 4DK Series - Manual (Page 9)

Manual p/n: 029-0020-79-0 9
4DK Series Pumps
The impeller develops discharge pressure
and directs the fluid to a single cut water and
then to the discharge opening.
The cut water is wedge-shaped and divides
the fluid between the volute and the pump
discharge. Figure 2: "Impeller Operation" on
page 8 shows the flow of fluid through the
Hale 4DK series pump.
Cavitation
Cavitation occurs when a centrifugal pump
is attempting to discharge more fluid than it
is receiving. When cavitation occurs,
bubbles are created under the vacuum,
near the eye of the impeller. Cavitation is
often referred to as “the pump running away
from the fluid supply.”
This means that the operator is trying to
pump more fluid out of the pump than is
going into the pump. The formation of
bubbles in the low pressure regions of the
impeller cause the impeller to "slip" in the
water since the impeller is designed to
move liquid, not the air in the bubbles.
To eliminate cavitation, the operator must
be aware of the warning signs and immedi-
ately correct the situation or serious dam-
age to the pump and impeller will occur.
The most reliable indication that a pump is
approaching cavitation is when an increase
in engine RPM does not cause an increase
in pump discharge pressure. A low barom-
eter, high elevation, or elevated fluid tem-
perature will also contribute to cavitation.
Pumps are rated at standard temperatures
and barometric pressures. When conditions
vary from standard, the maximum capacity
of the pump from draft can be affected.
The operator must not depend entirely on
the vacuum gauge (if one is installed in the
application) to indicate when a pump is
nearing cavitation.
The vacuum gauge is usually tapped into
the intake chamber several inches away
from the leading edge of the impeller eye
where the greatest amount of vacuum
occurs. The most common way to eliminate
cavitation is to decrease the amount of fluid
being discharged. This is accomplished by
decreasing engine speed or closing dis-
charge valves.
This will allow pressure to increase, how-
ever this will result in a reduction of flow.
Cavitation can also be eliminated be in-
creasing the pressure on the pump inlet.
This is accomplished with reduced vertical
lift, reduced inlet losses, or running from
positive pressure supplies.
Refueler pumps are generally gravity fed
from the tank, therefore cavitation is less of
an issue unless design limits are exceeded.
Reduced performance can be expected
when defueling aircraft due to the inlet
restrictions involved.
PUMP COMPONENTS
The basic parts of a Hale 4DK series centrifugal
pump are briefly described as follows:
Pump Body
The pump body and related parts are con-
structed from aluminum.
The Hale 4DK series pump body is a single
piece. Service of the impeller, clearance
rings, and mechanical seal is accomplished
by removing the pump body. This can often
be accomplished without removing the entire
pump and gearbox assembly from the appa-
ratus. Additionally, a minimum of piping need
be removed since the connections use
Victaulic type couplings.