flush the solids from the line. Force mains serving small pump stations, which are designed to

operate on an intermittent basis, will be sized to provide a minimum velocity of 1 m/s (3.5 fps) at

the peak discharge rate. For small stations having flows too low to warrant a minimum velocity

of 1 m/s (3.5 fps) with one pump operating, the design may call for both pumps to be operated

manually once a week for a sufficient period of time to flush out the line. Larger stations having

three or more pumping units, which operate a major portion of the time, will require minimum

force main velocities ranging from 0.6 m/s (2.0 fps) with one pump operating, to 1.5 m/s (5.0 fps)

with several pumps operating. In these cases, it is only required that a minimum velocity of 0.75

to 1.00 m/s (2.5 to 3.5 fps) be provided once or twice daily. Large pumping stations which serve

the entire installation or major portions thereof, and which are designed to pump continuously,

will usually have a greater number of pumps operating over a wider range of flowrates. Since the

pumping range may vary from 7 or 8 to 1, it will generally be sufficient to design for velocities of

0.15 up to 2.00 or 2.50 m/s (0.5 up to 7.0 or 8.0 fps). Maximum velocity is set at 3 m/s (10.0

fps).

(3) Slope. The value of S in the formula is equivalent to the kinetic energy loss due to pipe

friction divided by the length of conduit, or S = H f/L. Minor energy losses from fittings and valves

will be converted to equivalent lengths of conduit for use in the formula. Conversion tables for

fittings and valves can be found in standard hydraulics textbooks. The total kinetic energy loss in

a force main will be computed by multiplying the slope of the energy grade line by the total length

of conduit including equivalent lengths, or H f = S x L.

7-2. PUMP ANALYSIS AND SELECTION.

a. Total dynamic head. The head in feet against which a pump must work when wastewater

is being discharged is termed the total dynamic head (TDH). The two primary components of

TDH in wastewater applications are the static discharge head and the kinetic losses due to pipe

friction. Velocity and pressure heads are also present, but are usually insignificant. The TDH will

be calculated with the use of the Bernoulli energy equation which can be written as follows:

(Pd/W + V2d/2g + Zd) - (Ps/W + V2s/2g +Zs) + Hf

TDH

=

where

Pd, Ps

=

gage pressures in kiloPascals (pounds per square foot)

Vd, Vs

=

velocities in meters per second (feet per second)

Zd, Zs

=

static elevations in meters (feet)

Hf

=

kinetic energy loss in meters (feet)from pipe friction, fittings, and valves, as

calculated in paragraph 7-1b(3)

w

=

specific weight of fluid in kilonewtons per cubic meter (pounds per cubic foot)

g

=

All head terms are in meters (feet). Subscripts d and s represent force main discharge and pump

suction, respectively. In order to determine hydraulic conditions at the pump suction, it will be

necessary to write an energy equation from the liquid level in the wet well to the pump suction

nozzle.

b. System head-capacity curve. To determine the head required of a pump, or group of

pumps that would discharge at various flowrates into a force main system, a head-capacity curve

must be prepared. This curve is a graphic representation of the total dynamic head, and will be

7-2

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