Water and Wastewater Engineering

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3-24 WATER AND WASTEWATER ENGINEERING

The design must provide NPSH

 NPSH

. Of the terms in Equations 3-7 and 3-8, the de-

signer can adjust the height of the water surface ( h

) and the friction losses ( h

). Because of the

requirements for piping to the pump, h

i s generally the most easy to manipulate. Example 3-4

illustrates the calculations.

Example 3-4. A pump intake is located 0.5 m below the water surface in a wet well located at

an elevation of 1,500 m above sea level. The water temperature is 5



C. The pump intake friction

headlosses amount to 0.015 m. The selected pump requires a NPSH of 1.0 m. Does the design of

the wet well provide NPSH

Solution:

a . From Table 3-7 , find h

 8.66 m of water at 1,500 m elevation.

b. From Table 3-8 , find h

 0.0889 m of water at 5



c. With h

 0.5 m, the net positive suction head available is

NPSH or m

  866 05 0015 0 889 8 256826... . . .

d. NPSH

is  NPSH

. Therefore, this design is acceptable.

System Head Curves. The system head curve is the TDH curve formed over the range of de-

sign flow rates (that is, the minimum, average, maximum). The TDH will vary with the flow rate

and will be approximately proportional to the square of the flow through the system because of the

change in the velocity head term in Equation 3-3. In addition, the TDH will vary as the static

head

changes because of drawdown in the wet well and changes in the surface elevation of the lake or

Adapted from L. Haar, J. S. Gallagher, and G. S. Kell,

NBS/NRC Steam Tables, Hemisphere Publishing Corp.,

New York, 1984.

TABLE 3-8

Vapor pressure as a function of temperature

Vapor pressure

Temperature, C kPa m of water

0 0.611 0.0623

5 0.872 0.0889

10 1.228 0.1253

15 1.706 0.1740

20 2.339 0.2386

253.169 0.3232

30 4.246 0.4331

35 5.627 0.5740

40 7.381 0.7529

50 12.34 1.259

INTAKE STRUCTURES 3-25

river. Appropriate pumps and pump valve systems must be selected to operate in the range of the

system head curve. Common practice is to plot two curves. One curve is plotted for the maximum

system total head and one for the minimum system total head as shown in Figure 3-16 .

Example 3-5. Develop the system head curves for the low-lift pump

station to the water treat-

ment plant pumping system shown in the sketch below. The minimum flow rate is 3,000 m

/ d.

The average flow rate is 6,000 m

/ d. The maximum flow rate is 12,000 m

/ d.

Minimum lake

elev.

Elev.  175.6 m

Pump

Check valve, swing, fully open

Gate valve, fully open

Note: all elbows = 90º,

flanged, long radius

400 mm Ø DIP

— Elev. = 190.9 m

12 m

2 m 286 m

Wet well

Conduit

Elev.  172.9 m

Flow rate, m

Minimum

total head

Maximum

total head

3,000 6,000 12,000

Total dynamic head, m

FIGURE 3-16

S ystem total head curves.

3-26 WATER AND WASTEWATER ENGINEERING

Solution:

a. Calculate H

stat

for the maximum and minimum drawdown in the wet well.

Using the sketch elevations, the maximum static head is

stat

190 9 172 9 18 0...mmm

and the minimum static head is

stat

190 9 175 6153...mmm

b. Calculate the friction losses using Equation 3-5 for a 400 mm diameter ductile iron pipe

(DIP) at three flow rates.

From Appendix C, select C  100 for DIP. At the maximum flow rate of 12,000 m

/ d

or 0.139 m

/ s, for the 300 m long pipe the total friction headloss is

10 7

0139

100

300

185

m /s m

⎛

⎝

⎜

⎞

⎠

⎟

()

4487

144

⎛

⎝

⎜

⎞

⎠

⎟

 m

Similarly for the average and low flow rates



0 399 0 4

011

or m at average flow

mmat low flow

c. Compared to the values calculated in (a) and (b), the losses for valves and fittings are

negligible and are neglected here. Likewise, the velocity headloss ( v

/2 g ) is very small

and is neglected here.

d. The plotting points for the system curves for the maximum drawdown are

18 0 1 44 19 44 12 000

18 0

.. . ,

mm mat m /d Q

mmm mat m /d

 



0 4 18 4 6 000

18 0 0 11

.. ,

mm at m /d18 11 3 000

.,Q

For the minimum drawdown the plotting points are

153 144 1674 12000

153

.. . ,

mm mat m /d+ Q

mmm mat m /d

04 15 7 6 000

153 011

.. ,

Q

mm at m /d15 4 3 000

.,Q

These points are plotted in Figure 3-16 .

Pump Selection. Multiple pumps and/or variable speed pumps are selected to cover the range

of conditions described by the system head curve. It is important to select a pump that will have

its best efficiency within the operating range of the system and preferably at the operating point

(head and dis

charge) where the pump will operate most often.

INTAKE STRUCTURES 3-27

P umps are selected from those commonly available from pump manufacturers. This then

becomes a problem of matching a pump characteristic or head-discharge curve from a pump

manufacturer’s catalog or database to the system head curve.

A typical, constant

speed, vertical turbine curve is shown in Figure 3-17 . Important features

of the curve to be noted are:

• The head delivered (ordinate) is per stage. Thus, for higher heads than noted, multiple

stages are identified in the selection of the pump.

• The maximum number of stages that may be used is specified

• The best efficiency point (BEP) is a function of the flow rate and the head.

• The required net positive suction head (NPSH

) may be specified per stage or as a total for

the maximum number of stages. Typically, it is only critical for the first stage.

• Multiple impeller sizes allow some adjustment over the range of capacity and head.

R a dial loads on the impeller and problems with cavitation are minimized when the pump i

operated at its BEP. Although it is not possible to operate at the BEP all the time, there is a range

of operating conditions that is preferred to optimize the life of the pump. The American National

1.0

2.0

3.0

4.0

100 200 300 400

Discharge, m

NPSH required

at pump inlet, m

Efficiency, % Head per stage, m

NPSH required

Curve no. 3-18

Size 1 Rpm 1750

Single-stage lab head and horse-power with

enameled cast iron bowls and bronze impeller

Effieciency shown for 2 or more stages

No.

stages

Eff.

change

Eff.

change

Material

Eye area  32 cm

Thurst constang - A  2.3

Thurst constang - B  2.3

Thurst c

onstang - C  2.3

Max. no. std. stages  35

Max. operating pressure  3,900 kPa

Std. lateral  0.375

Std. shaft dia. 2.0 cm

Impeller number  P-2397-10

Impeller wt.  25.0 kg

Bowl conn. - flanged

Add 37 cm per additional stage

Enclosed line shaft

Imp.-C.I.

Imp.-C.I. Enn

Bowl-C.I.

Bowl-Brz.

1

35 cm

72 cm

A  32 cm

B  31 cm

FIGURE 3-17

T ypical vertical head-discharge curve.

3-28 WATER AND WASTEWATER ENGINEERING

Standards Institute (ANSI) has provided the following guidance for the Preferred Operating

Range (POR) for vertical pumps (ANSI/HI 9.6.3-1997):

For specific speeds less than 87, the bounded region is between 70 and 120 perc ent of the

BEP flow rate.

For higher specific speeds, the bounded region is between 80 and 115 percent of the BEP

flow rate.

Specific speed is defined as



0 5

075

(3-9)

where n  revolutions per minute

Q  flow rate, m

/ s

 total dynamic head, m

ANSI also establis hed a wider region than the POR called the Acceptable (or Allowable)

Operating Region (AOR). It “is that range of flow recommended by the pump manufacturer over

which the service life of a pump is not seriously compromis ed. Minimum bearing life will be

reduced and noise, vibration, and

component stresses will be increased when a pump is operated

outside its POR. As a result, service life within the AOR may be lower than within the POR.” The

pump may be operated in this region for short, infrequent periods without significant equipment

deterioration (Cooper and Tchobanoglous, 2006).

Example 3-6. Using the pump head-discharge curve shown in Fig

ure 3-17 for the system head

curve shown in Figure 3-16 , and data from Example 3-5, specify the following at the average

flow rate: (1) BEP, (2) number of stages, (3) efficiency at BEP, (4) the operating range of flow

rate, (5) motor power, (6) the depth of the wet well, and (7) the location of the pump intake.

Solution:

a . From Figure 3-16 , the maximum total system head is 18.4 m at the average flow rate

of 6,000 m

/ d. On Figure 3-17, the BEP is found at a flow rate of

6 000

250

, m /d

h/d

m /h

and a head of 3 m/stage.

b. The number of stages required is

No of stages

Total system head

Head per sta

. 

gge

or

18 4

613 7

This is less than the maximum number of s tages allowed of 35 noted in the box in

Figure 3-17 .

c. From Figure 3-17 at 250 m

/h and 3 m head per stage, the efficiency is 78 percent for

impeller A.

d. From Figure 3-17 , the rpm  1,750. The flow rate is

250

3 600

0 0694

m /h

s/h

m /s

.

INTAKE STRUCTURES 3-29

e. From Figure 3-16 , the maximum TDH at 250 m

/h is 18.4 m.

The specific speed is



()( )

()

1750 0 0694

18 4

3 0 5

075

rpmm/s

4461 0

888

5192



f. For specific speeds less than 87, the POR is from 70 to 120 percent of the BEP flow rate or

()( )070 250175

. m /h m /h

()( )12 250 300

. m /h m /h

g. The pump curve is plotted on the system curve in Figure 3-18 .

h . Using Equation 3-3, the motor brake power is

P 



()()()9 807 0 0694 18

078

15 7

kN/mm/s m

i. The depth of the wet well is determined by the hydraulics of the conduit, the depth of the

conduit carrying water to the wet well, and the minimum submergence required by

the pump. The following sketch shows the relationships using the results of calculations

in Example 3-3, the NPSH

for the selected pump, and the NPSH

a ssuming an altitude

of 500 m and a temperature of 5



Maximum lake

elevation

Pump motor

Operating deck

Pump room floor

Elev. = 175.6 m

Maximum NPSH

= 1.6 m

NPSH

 9.6 m

Minimum lake

surface evevation

Intake

conduit

Maximum drawdown

Elev. = 172.9

Minimum pump

submergence

0.5D

Comment. The selected pump is not “ideal” for the system curves. A pump with fewer stages

and a flatter head-discharge curve would be a better choice.

3-30 WATER AND WASTEWATER ENGINEERING

Infiltration Gallery or Ranney Wells

When the geological c onditions are favorable, an infiltration gallery or collector wells m ay be

placed onshore or offshore.

Direction. The infiltration gallery consists of perforated pipe laid parallel to the shore for

lakes. For rivers, the pipe gallery may be either at a right angle to the river flow or parallel to

shore depending on the groundwater flow pattern (Kawamura, 2000). As shown in Figure 3-6 ,

the Ranney well system will have well screens radiating from a

collector caisson.

Depth. The actual depth should be determined by hydrogeological studies that identify the pi-

ezometric surface and its variability. Common depths are generally in the range of 4 to 5 m below

the bottom of the river or lake.

Pipe Material. Because of the high potential for corrosion, perforated polyvinyl chloride

(PVC) or con

crete pipe is used for the collection gallery. Ranney well screens are generally made

of stainless steel.

Orifice Size. The diameter of each orifice is generally 1 to 2 cm. The number of orifices is 20

to 30 per square meter of the collector surface (Kawamura, 2000).

Length. The gallery must be long enough to meet the design capa

city. The velocity into each

orifice at the design capacity is limited to 0.03 m/s to minimize entrainment of sand.

Preferred

slope of

pump curve

Pump

curve

Maximum

total

head

Minimum

total

head

BEP

Lower

operating

point

POR

Upper operating poin

3,000

6,000

Flow rate, m

⁄d

12,000

Total dynamic head, m

∗

FIGURE 3-18

P ump and system curves.

INTAKE STRUCTURES 3-31

Slope. The gallery can be horizontal, but a slight slope of 500:1 will minimize air binding/

blockage (Kawamura, 2000).

Pipe Velocity. At design capacity, the velocity is limited to 1 m/s at the outlet of the gallery.

Backfilling. Because the Ranney well screens are bored into the natural aquifer material, there

is no backfilling required. The pipe laid in the gallery is backfilled with gravel and

sand similar

to that used in filter bed gradation (see Chapter 11 for details).

Junction Wells. At junction points in the gallery where the pipe changes direction and at the

end of the gallery, a well is provided. The recommended minimum diameter is 1 m (Kawamura,

2000).

Regulatory Considerations. In addition to the withdrawal restrictions that may be impos

ed,

the water removed from the infiltration gallery may be considered under the influence of the sur-

face water and thus subject to water quality regulations that apply to surface water. Nonetheless,

the filtering action of the ground improves the quality of the water and makes it easier to treat.

3-4 OPERATIONAL CONSIDERATIONS

Grit

Abrasive materials such as silt, sand, and shells is called grit. This material is very abrasive and

will result in excessive wear to mechanical equipment as well as potentially settling out during

low-demand flows.

When the source water bottom is soft and

subject to scour or when flood conditions result in

a high sediment load and turbidity, grit chambers are provided. The grit chamber is a horizontal

flow settling tank ( Figu re 3-19 ). The theoretical design basis of the grit chamber is Stokes’ law

which is discussed in Chapter 10.

The des

ign objective of the grit chamber is to remove sand or silt particles greater than 0.1 mm

in diameter. Generally, particles smaller than this do not pose a hazard to pumps and pipelines.

The design objectives for raw water grit chambers are significantly different from those used for

wastewater treatment. Thus, wastewater treatment plant design

s should not be used for raw water

grit removal. Typical design criteria are listed in Table 3-9 .

Ice

In cold regions, ice in its various forms is of concern. Surface ice and ice floes create struc-

tural hazards to exposed intakes. Selection of submerged intake alternatives alleviates this

problem.

Frazil ice i s small, disk-shaped ice crystals

(Foellmi, 2005). It has also been described as

slush. Frazil ice adheres to surfaces such as intake screens and pipe walls. The reduction in area

lowers the flow rate into the intake structure and ultimately will plug it. Frazil ice forms when

turbulent water is supercooled.

3-32 WATER AND WASTEWATER ENGINEERING

0.3 L

0.08 L

0.25 L

Diffuser

wall

Screen

0.1 L

Baffle

wall

Walkway

70°

Plan

Profile

FIGURE 3-19

Grit chamber.

Parameter Range of values

Location Downstream of the fine screen

and upstream of the raw water pumps

Number of chambers 2

Water depth

Mechanical grit removal 3 to 4 m

Manual grit removal 4 to 5 m

Length to width 4:1

Length to depth 6:1

Velocity 0.05 to 0.08 m/s

Detention time 6 to 15 min

Overflow rate

10 to 25 m/h.

TABLE 3-9

Water intake grit chamber design criteria

Overflow rate  Q / surface area of water in tank.

A dapted from Kawamura, 2000.

E xperience on the Great Lakes and elsewhere indicates that location and design features of

submerged intakes can significantly reduce the buildup of frazil ice but probably cannot elimi-

nate it. Submerging intakes in deep water (10 m appears to be the minimum with 12 to 14 m

preferred) and sizing inlet ports for a maximum velocity of 10

cm/s minimizes frazil ice buildup.

INTAKE STRUCTURES 3-33

Heating the water at the intake port to 0.1



C also appears to work, but the energy requirements

are extremely large. Using smooth materials su ch as solid plastic and plastic-coated metal also

helps (Foellmi, 2005, Bosserrman et al., 2006). Wood is commonly used because of its low ther-

mal conductivit

Anchor ice i s differentiated from frazil ice in that it forms in sheets and grows by attachment

of frazil ice. The methods of control are the same as those for frazil ice.

Zebra Mussels

The zebra mussel ( Dreissena polymorpha ) is a small bivalve mollusk that has alternating dark

and light bands on its shell and averages about 2.5 cm in length. The mussel grows filamentous

threads that allow it to attach to hard surfaces. Zebra mussels reproduce prolifically and as a con-

sequence clog intakes and pumps

. They are well established in the Great Lakes and are forecast

to infest all freshwater in two-thirds of the United States and all of southern Canada.

The control techniques currently in use are listed in Table 3-10 . In the Great Lakes area,

chlorine and potassium permanganate feed systems piped to the inlet port have been successful

in mitigating the problem. Intermittent chlorination is

used to kill the juveniles (velligers) before

shells develop and attachment becomes virtually permanent. This dosing scheme minimizes the

use of chlorine. Periodic cleaning of the screens is required to remove adult and dead mussels as

the chemical treatment d

oes not remove them. Care should be used in the application of chlorine

as the presence of naturally occurring organic matter may lead to the formation of trihalometh-

anes, a group of compounds that are regulated (see Chapter 2 for details).

Fish Protection

The need to provide a means to prevent fish from entering the intake structure is addressed by

one of three approaches:

• Physical barrier screens,

• Behavioral guidance systems, and

• Capture and release systems.

The highest standard of protection is for juvenile, endangered species. The National Oceani

c and

Atmospheric Administration (NOAA) has developed guidelines for inlets to protect these fish.

Treatment techniqueRemarks

Thermal 35C for 2 h

100% effective

Repeat 2 to 3 times per year

Chemical Oxidizing chemicals such as chlorine, potassium permanganate, and ozone

Continue for 2 or 3 weeks; continuous application may be required

Mechanical Shovel or scrape, high-pressure hose, sandbla

sting

Other Ultrasound, electrocution, oxygen depletion, UV light

TABLE 3-10

Zebra mussel control