APPENDIX A: ANALYSIS OF DREDGE PUMP OPTIONS
Using 998 kg/m3 as the density of the pumped
Static head calculations were made to deter-
water, the actual power head is
mine the theoretical head at full flow. Friction fac-
hT = Pout/ ρ
tors were derived from empirical data.* The fol-
(A6)
lowing parameters were used:
hT = (103 103) (10.2 102)/998
Flow--113.5 L/s
hT = 10.5 m.
(A7)
Friction factor--25-cm-diam. polypipe: 1.69
m/100 m
The total head is about 30% less than what the
system output should be if the equipment were
Friction factor--20-cm-diam. rubber hose: 3.7
operating properly. A call to the equipment manu-
m/100 m
facturer resulted in the discovery that the pump
Height of top of berm from water level: 6.52
trim and pressure relief settings were not correct.
m
The system was set up with a 356-mm (14-in.) im-
Drop from berm to spoils line outlet: 0.81 m
peller operating at 1250 rpm. It was designed to run
Total head for the system:
with a 300-mm (12-in.) trim operating at an 1800-
rpm impeller speed (Fig. A1).
Static head: 6.52 0.81 = 5.71 m
Given this situation, there were three options
Friction head, polypipe (335 m): 335 1.69/
available. The first was to leave the impeller as is
100 = 5.7 m
and increase the hydraulic pressure to run the
Friction head, hose (76 m): 76 3.7/100 = 2.81
pump at a sufficient speed to attain the flow rates in
Total head will be the sum of all three compo-
the specifications. As can be seen below (Fig. A2),
nents above, or:
that speed is 1470 rpm. Note that the power
requirements are the same due to increased pump
h = hs + Σhf = 5.71 + 5.70 + 2.81 = 14.22 m. (A1)
efficiency. The second option was to trim the impel-
The friction head for the PE pipe was also cal-
ler to 330 mm (13 in.) and increase the pressure to
culated using the HazenWilliams formula to
attain the correct flow. The third option was to trim
verify the empirical data:
the impeller to 300 mm (12 in.) and increase the im-
peller speed to make the pump operate as originally
hf = 10.44 (L)(Q)1.85/[(C)1.85 (D)4.8655]
(A2)
planned. The first option was the quickest method
of obtaining the desired result, so that option was
where L = length of pipe in feet (1000 ft)
favored. However, before a decision could be made,
Q = flow through the pipe (1800 gpm)
other factors, such as computed head, available
C = HazenWilliams constant ≈1.40 for
power, and available suction needed to be exam-
PE)
ined. A shortfall in any of these three parameters
D = pipe inside diameter in inches (≈10
would dictate the consideration of an alternative
in.).
strategy.
Substituting the values and constants above
A system analysis was performed using the
for PE pipe into eq A2 and converting, we get
pump affinity laws. For the system as delivered we
have the following parameters:
hf = 1.61 m/100 m
(A3)
Trim: 356 mm (14 in.)
thus validating the value used in eq 1. Design
Max. shaft speed: 1280 rpm
pump flow is actually 106.3 L/s, thus the total
Max. outlet pressure: 110 kPa @ 11.25 m head (16
head (hT) we should see at the dredge should be
psi @ 36.9 ft head)
approximately
The first calculations are for a 12-in. (300-mm)
hT = hs+(V1/V2)2 (Σ hf)
trim. Adjusting for trim using the head relationship
(A4)
2
hT = 5.71 + (106.3/113.5)2 (5.7 + 2.81)
h2 d2
=
(A8)
h1 d1
hT = 13.2 m
(A5)
where h1
=
current outlet head
h2
=
projected outlet head
d1
=
original impeller diameter
* Telephone conversation regarding pump and pipe perfor-
d2
=
impeller diameter of interest,
mance, R. O'Brien, Cornell Pump Co., 1995.
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