Hale MG - Manual (Page 14)

Muscle Pumps
1-7
Switching between volume and pressure operation
is generally governed by prevailing fire department
policy. However, here are some general guidelines
if our fire department does not have an established
policy:
1. The pump should be operated so that
engine speed is within its best operating
range.
2. Transfer to volume (parallel) operation if
the pump has to discharge more than 50
percent of its rated capacity. Be certain to
warn everyone involved before switching
between volume and pressure operation.
3. Reduce the pump pressure to 50 to 60 PSI
(3 to 4 BAR) before switching. The engine
speed should especially be reduced when
switching from volume to pressure
operation with hand held hoses in use.
Cavitation
Cavitation occurs when a centrifugal pump is
attempting to discharge more water than it is
receiving. When cavitation occurs, a vacuum is
created near the eye of the impeller. As the vacuum
increases, the boiling point of the water is lowered
until it reaches a point near the impeller eye where
it flashes into vapor and enters the impeller. Once
the vapor pockets, or bubbles, enter the impeller,
the process begins to reverse itself. As the vapor
reaches the discharge side of the pump, it is
subjected to a high positive pressure, which
condenses the vapor back to a liquid. The sudden
change from vapor pockets to a liquid generates a
shock effect which causes a significant noise that is
characteristic of cavitation. This shock damages
the impeller and pump housing. This may happen
while pumping from draft or a hydrant. The
problem in either case is the pump running away
from the supply and this causes a vacuum to occur.
Cavitation is often referred to as “the pump
running away from the water supply”. This means
that the operator is trying to pump more water out
of the pump than is going into the pump. To
eliminate cavitation, the operator must be aware of
the warning signs and correct the situation, or
serious damage 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 the
pump discharge pressure. The operator must not
depend entirely on the vacuum gage to indicate
when a pump is nearing cavitation. This is because
the vacuum gage 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
water being discharged. This is accomplished by
decreasing engine speed or closing discharge
valves. This will allow pressure to increase but
this will result in a reduction of flow.