Page 124 - Know and Understand Centrifugal Pumps

The System Curve

P

t
~
46.2'
1

69.3'

t
~
23.1'
I

Fiaure 8-8
Here's a quick review of the Bachus & Custodio Formula:
System Wand Hv = [(APDr - APDo) + (APSr - APSO) x 2.311 + sp. gr.
Let's take our readings with water as the test liquid just to keep the
conversions simple. With the system and pump off, note that gauge 5
should be reading 20 psi. This is because it is 46.2 feet below the
surface level in the discharge tank. Confirm that gauge 4 is reading 50
psi. It is 115.5 feet deep into the column. The difference between
gauges 4 and 5 is 30 psi. The APDo = 30 psi.
In the suction line, note that gauge 3 is also reading 50 psi. It also has
11 5.5 feet of liquid elevation on it. Pressure gauge 2 should read 60 psi
because it is 138.6 feet deep into the column. This indicates that the
APSO = 10 psi.

Gauge 1, on the other side of the closed valve, is reading the elevation
in the suction tank. This gauge should be reading 25 psi because it is
58.6 feet deep into its column.

Now, open the gate valve between gauges 1 and 2. Start the pump
motor, and relieve the check valve if it is being mechanically held open.
Permit the pump to run a few minutes to stabilize, relieving any
surging. We'll continue to note pressure gauge readings with the system
functioning.
Because all valves are now open, gauge 1 becomes our upstream gauge
on the suction line. With the pump running all activity on the suction

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