Figure 3-2.
Operating characteristics of standard pipeline pumping
stations.
Design Gradient/True Gradient: The difference between design gradient and
true gradient is important for you to understand because of the suction
pressures required. The normal suction pressure required is 20 psi or 64
ft/hd and the minimum is 5 psi or 16 ft/hd. Knowing this when we placed the
pump stations using the true gradient our suction pressure is zero at pump
stations 2, 3, and 4. The true gradient is represented by a solid line.
The design gradient we will show as a dotted line 64 ft/hd below the solid
line, insuring that each station will receive 20 pounds of suction pressure.
NOTE: For temperatures over 100 F suction pressure required is 30 psi.
Atmospheric pressure is comparable to static pressure in liquids. So,
elevations above 3,000 ft design loads are decreased by 4 percent for each
1,000 ft of elevation. Hydraulic gradient is the rate of head loss due to
friction. Pressure reducing stations are installed on long downhill slopes
or grades.
PART B PUMP STATION SUCTION AND DISCHARGE PRESSURE
Determining Pump Station Suction and Discharge Pressure:
Since the design gradient was used to place the pump stations we can now use
the operating gradient to determine suction and discharge pressures. Our
flow rate is 600 GPM, MOGAS 60, F. Our gradient is based on a loss of 97
ft/mi times 10 miles or 970 ft/hd loss over 10 miles. Starting at pump
station number 1 (using our gradient) it will take 1398 ft/hd (439 PSI)
discharge pressure to reach pump station 2 at 20 PSI (64 ft/hd).
If the fuel is DF with a gravity of .8448 this changes the head from 64
ft to 55 ft in order to maintain 20 PSI suction. At pump station 2 the
parallel line will give you the suction and discharge head required. At
pump station 4 we have a little different situation, assuming we must enter
the terminal at 20 PSI or 55 ft/hd, using the hydraulic gradient and
parallel lines our discharge head must be 100 ft/hd (37 PSI). This
procedure can be accomplished by starting at the end of the pipeline on the
right hand side of the graph or at pump station 1 on the left hand side of
the graph. If we start at the end of the pipeline and work backwards the
procedure is called back chaining; either way the system will work and
becomes a matter of preference.
Using graph number 3 the procedures are the same except we will consider
whether we can bypass any of the pump stations in case of an emergency or
for efficiency. To accomplish this lets look at pump station 3. We must
create enough head to get over the peak at mile 35. If we create 960 ft/hd
(351 PSI) we can bypass pump station number 4 and enter the terminal at 510
ft/hd (187 PSI). Caution should be used in doing this as you run the risk
of creating an over pressure on the pipeline on the long down hill slope at
mile 28. Counting down from the gradient at this point we have 1,060 ft/hd
12-32
QM 5099
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