ASHRAE Journal - June 2024 - 23

COLUMN ENGINEER'S NOTEBOOK
FIGURE 2 Impeller pressure drop.
Relative Pressure Through a Pump Impeller
Plus
O
Suction
Connection
Minus
A
B
C
D
E
The chart above illustrates how the pressure drops as water fl ows through the impeller on the
path with the blue arrow on the cutaway to the right.
1. The pressure drop between A and B is primarily due to friction between the water, pipe and
pump casing but also includes an entrance loss where the pipe fl ange connects to the pump
fl ange.
2. The pressure drop from B to C is primarily due to friction between the water and the pump
casing.
3. The pressure drop from C to D is due to friction between the water and impeller, turbulence
and the entrance loss at the vane tips.
4. Between D and E, there continues to be friction between the water and impeller, turbulence,
and an exit loss where the water leaves the impeller. But those losses are offset by the
energy added to the fl uid stream by the impeller rotation, and there is a net increase from the
point of lowest pressure at D.
accelerate it needs to come from the fl uid's static
pressure (potential energy). Thus, as a fl uid moves from
the pipe into the impeller's eye and through the reduced
cross-sectional area (relative to the suction line) in the
impeller, there is a signifi cant drop in the static pressure
(commensurate with an increase in the velocity pressure
associated with the faster moving fl uid). See Figure 2.
If the static pressure at any point in the process drops
below the vapor pressure of the fl uid in the pipe, the
fl uid will boil.
Most of us think of water as boiling at 212°F (100°C),
which is the very specifi c temperature at which it will
boil when it is at a very specifi c pressure, that being sea
level pressure. If we travel to Denver, Colo. (the Mile
High City), we would discover that water would boil
at about 203°F - 204°F (95°C - 95.6°C) due to the lower
atmospheric pressure there.
A relationship exists between the pressure and
temperature at which water (or most fl uids) will
boil or change phase. For water, this relationship is
documented in " steam tables, " which are the result of
very meticulous, dedicated efforts on the part of our
predecessors to document these things and subsequent
efforts by very smart people to develop empirical
relationships that will let us predict these things.
In the context of our current discussion, if you
Discharge
Connection
Adaptor and Mechanical Seal
Bearing Housing and Bearings
Impeller
Pump Shaft
Support
Pump Casing
If cavitation occurs, it will begin at the point of lowest pressure, point D on the path through the
impeller.
This image is based on information published in the Durco Pump Engineering Manual and end
suction pump cross-sections published by Thelco Pumps on their website at https://www.
thelco.com/pump-technical-drawings/.
extrapolate the Denver boiling point concept, you might
realize that for a given temperature and fl uid, if the
pressure drop that occurred as the fl uid passed through
the impeller was large enough, the fl uid might change
phase at the low pressure point in the impeller.
For this column, I'm going to constrain the fl uid to
water, but the concepts apply to other fl uids. If water
changes phase from a liquid to a vapor, there is a
signifi cant change in volume. At atmospheric pressure,
a cubic inch of liquid water becomes about a cubic foot
of water vapor.
This sudden expansion-somewhat of an explosion-
occurring in the confi ned space of the impeller
generates a force, which, if repeated, will damage it.
The impeller, of course, was designed to move a liquid,
not a vapor. So, when the phase change happens, the
fl ow of fl uid suddenly drops off.
However, it was the fl ow of fl uid and the associated
pressure drop that created the low pressure required for
the fl uid to change phase in the fi rst place. Thus, with
the pressure drop due to fl ow reduced via the reduction
in fl ow associated with trying to pump vapor vs. a liquid,
the static pressure in the eye of the impeller increases to
the point where the vapor condenses.
The cubic foot of vapor once again becomes a cubic
inch of liquid. This implosion generates another force,
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23
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