Page 126 - Know and Understand Centrifugal Pumps

The System Curve

With gauge 4 on the pump discharge reading 53 psi, the 5 gauge
should be reading 30 psi less, or 23 psi. This is because the 5 gauge is
69.3 feet above gauge 4. However gauge 5, by observation, is only
reading 18 psi. Therefore APDr = 35psi (53 - 18). We have all the
information we need to insert into the Bachus & Custodio Formula:

Hf & Hv = (APDr - APDo) + (APSr - APSO) x 2.31 i sp. gr.
= (35 - 30) + (12 - 10) x 2.31 i SP. g.
= 5 + 2 x 2.31 + ~p.gr.
= 7~ 2.31 i 1.0
Hf&Hv = 16.17feet

The Bachus & Custodio Formula does not make mistakes. It is not
based on models, or experiments developed 150 years ago. It doesn’t
depend on valves being completely open. It doesn’t depend on the
specific instructions regarding equipment assembly. It doesn’t depend
on new piping. It is not based on municipal water. It depends on the
actual piping and other system fittings, as they are now, and the next
shift, and tomorrow, and next month. If a resistance load changes, it
will be registered on the gauges. If the pipe diameter changes, it is
recorded on the gauges. If new equipment is added, it is visible on the
gauges. The pressure gauges and other instrumentation are the pump’s
control panel. You wouldn’t drive a car without a dashboard. Who is
responsible for spccifjring, selling and buying pumps without adequate
instrumentation? Regarding pump failure, problematic seals and
bearings that need emergency maintenance: in about 80% of all cases,
the pump is telling the operators what the problem is, hours, days and
weeks before the failure event occurs. What’s really happening is that
no one is interpreting the information on the gauges.
Regarding the TDH, isn’t it interesting that the Hs and the Hp are
determined by simple observation? This detailed discussion on the Hf
and Hv probably has the reader ready to throw this book into the
garbage. With the Rachus & Custodio Formula, the differential
pressure gauge readings on the system with the pump turned off, will
cancel any elevation changes (Hs) existing in the system. Exposing both
sides of the system to atmospheric pressure cancels the pressure changes
(Hp). And then with the system operating and the pump turned on,
the further differential gauge readings will record the Hf and Hv that
are being lost in the system. Remember too, that the other mentioned
resistance approximations, Hazen & Williams, and Darcymeisbach,
are only valid in the first few hours or days of service. The system begins
to change once the pump is turned on and production begins.
Operators open and close valves to meter the flow through the pipes.
Filters and strainers begin to clog. Inside pipe diameters form scale.

t 1 1 0 9