Individual pump stations must regulate pump speed to keep suction
pressures at the next pump station downstream above the minimum. The
normal suction pressure at a pump station is 20 PSI for elevations less
than 5,000 feet and where temperatures are below 100 degrees F (20 PSI is
equivalent to 64 feet of head of Mogas). The minimum suction pressure at
a pump station must be 5 PSI because of pump entrance friction losses and
the possibility of vapor lock in the pump. 5 PSI is equivalent to 16
feet of head of Mogas. To determine flow rate, efficiency, and BHP in
pump station operations the total pressure produced on the discharge side
of the pump station must be determined first.
There are three pumps on line connected in series
operating at 1800 RPM with a discharge pressure of 480 PSI and a
suction pressure of 20 PSI.
They are pumping a product which
has a SP/GR 0.8254.
480 PSI minus 20 PSI = 460 PSI.
In order to use the pump graph, divide 1,287 ft/hd by three because there
are three pumps on line (429 ft/hd). Locate 429 feet of head on the
graph, read to the right of the head curve (1,800 RPM). Read down to 600
GPM. At that flow rate, read up the graph until the line intersects the
brake horsepower curve at 1,800 and read 68 BHP. This represents one
pump and there are three on line. Multiply by three, 204 BHP for the
station. In the same manner, read up to the efficiency curve and read 74
percent. As shown, the graphs can be used for pump stations as well as
800 GPM main line pump graph.
This pump graph (Figure 11-2) looks
different but is constructed and interpreted the same as the other pump
graphs used. It shows total dynamic heads in PSI, feet of head, and flow
rate for water (1.0 sp/gr), DF-2 (0.8254 sp/gr), and Mogas (0.7254 sp/gr)
from 770 RPM to 2,100 RPM.
For example, at a flow rate of 500 GPM and 1,800 RPM pumping DF-2, the
pressure and feet of head can be determined. Locate 500 GPM and read up
to the 1,800 RPM curve. Read to the left 1,359 ft/hd 500 PSI.