Hale MG - Manual (Page 12)

Muscle Pumps
1-5
Two-Stage Pump Operation
The primary difference between a single -stage and
a two-stage pump is that the former has only one
impeller and no transfer valve to switch between
volume and pressure operation. A transfer valve is
a two-position valve that permits the impellers in a
two-stage pump to be operated in parallel (volume)
or series (pressure). Both types of operation are
explained in the following paragraphs.
Volume (Parallel) Operation
Volume operation (figure 1-5), results in the
pressure at the pump intake being added to the
pressure developed by both impellers, and the
amount of water delivered to the discharge being
the sum of the flows of the two impellers. For
example, if the inlet pressure is 30 PSI (2 BAR)
and the flow of each impeller is 500 GPM (1,892
LPM) at 150 PSI (10 BAR), the pressure and
volume at the discharge is:
Flow/Impeller x # Impellers
1,000 GPM (3,785 LPM) at 180 PSI (12 BAR):
500 GPM per Impeller X 2 Impellers = 1,000
GPM (1,892 LPM per Impeller X 2 Impellers =
3,785 LPM)
30 PSI Inlet Pressure + 150 PSI Pump Pressure =
180 PSI
(2 BAR Inlet Pressure + 10 BAR Pump Pressure =
12 BAR)
Pressure (Series) Operation
Pressure operation (figure 1-6), finds the impellers
connected in series. That is, the output of the
impeller supplied from the pump intake is supplied
to the input of the next impeller. The pressure at
the pump discharge is the sum of the pressure of
the two impellers plus the pressure of the intake.
The amount of water delivered to the discharge is
the same amount that entered the first impeller.
Using the example above when in series operation,
the discharge pressure will be 330 PSI, (22 BAR)
and the discharge volume will be 500 GPM (1,892
LPM).
Volume Versus Pressure Operation
Selection of volume versus pressure operation is
determined by three factors:
q Generally, the pump should be operated so
that the pump gives the desired perform-
ance at the lowest engine speed.
q Transfer to volume (parallel) operation for
higher flows (see figure 1-5).
q Transfer to pressure (series) operation
when higher water pressures are required
(see figure 1-6).