Using 998 kg/m3 as the density of the pumped

Static head calculations were made to deter-

water, the actual power head is

tors were derived from empirical data.* The fol-

(A6)

lowing parameters were used:

Flow--113.5 L/s

(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

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

(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,

other factors, such as computed head, available

power, and available suction needed to be exam-

PE)

ined. A shortfall in any of these three parameters

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:

(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 (*h*T) we should see at the dredge should be

psi @ 36.9 ft head)

approximately

The first calculations are for a 12-in. (300-mm)

trim. Adjusting for trim using the head relationship

(A4)

2

=

(A8)

(A5)

where *h*1

=

current outlet head

=

projected outlet head

=

original impeller diameter

* Telephone conversation regarding pump and pipe perfor-

=

impeller diameter of interest,

mance, R. O'Brien, Cornell Pump Co., 1995.

21