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.

2.31*x*460

1,287

0.8254

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

individual pumps.

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.

12-101

QM 5099