Notes on Hydraulics of Sedimentation Tanks

A Step by Step Procedure

Think of Sedimentation as Part of Water Treatment Plant Processes

1. Draw a Schematic of the System

From Previous Treatment Process

To Next Treatment Process

Q

Q

Q/2 Q/2

Q/2 Q/2

Q

Influent

Effluent

Tank Wall

Louder

Distribution Well

Gate Valve

Influent Channel

2. Choose Weir Type and Number of Weirs per Foot Length

• For the type see additional slides bellow

• For the number per foot usually we choose two

3. Estimate total number of weirs

Assume that the flow go through the lateral spillway channel along the width of the rectangular tank or the circumference of the cylindrical tank

# = Tank width in ft (rectangular) divided by the # of weirs per foot

# = Tank circumference in ft (cylindrical) divided by the # of weirs per foot

4. Compute the flow through a single weir

• divide the flow through the tank versus the # of weirs you get the flow rate though a single weir

5. Weir height

• Use one of the equations listed in the additional slide bellow such as:

H = (Q/2.5)0.4

6. Elevation of Weirs and inlet and outlet channels

• In addition to account for loses (minor and major-frictional) you should allow an additional one foot head for flow assurance (see next slide).

7. Estimation of minor and Major loses

• Follow the flow from previous treatment process to the following treatment process and pin point

• Each pipe material, diameter, length, friction, roughness, and flows

• Each open channel material, width, depth, friction, roughness, and flows

• Each valve type, diameter, K, and flow• Each elbow type, diameter, K, and flow• Any pipe enlargements or reductions• Any open channel gates• Any head difference along the flow such as the

difference in head due to bottom tank slope

• Estimate the head loss for each minor or major loss as indicated in the following additional slides

Total head loss to be considered to set the elevation of the tank would be:

H = (minor loses + major loses)

8. Hydraulic and Energy Grade Lines (HGL and EGL)

• After estimating the head losses for each treatment process make a table for the head loses from the start to the end of the processes.

• Indicate in the Table total elevation difference corresponding to HGL and EGL (see the additional slides bellow)

9. Check water Hammer Occurrence in the System

Additional Useful and More Detailed Information

HYDRAULIC CONSIDERATIONS

Fluid Flow Rate

                    discharge or flow rate

                    friction coefficient

                     pipe diameter

                     hydraulic grade line slope

Mean Fluid Velocity

                   mean flow velocity

                   friction coefficient

                    

hydraulic radius

                    

hydraulic grade line slope

• Where

• v = mean flow velocity• C = Hazen Williams friction coefficient • r = hydraulic radius • s = hydraulic grade line slope • Q = flow rate or discharge• D = pipe diameter

Head loss in Pipes Due to FrictionDarcy-Weisbach

L

D V

Minor Loses due to flow though valves, bends, gates, pipe reductions or enlargements and other minors

• For one minor loss cause, head loss equal to:

hLm = Km * (v2/2g)

For a pipe section with several causes (valves plus bends plus reductions, etc) minor head loss for the pipe equal to:

hLm = Km * (v2/2g)

• Bend loss (Hb) loss due to change in direction of flow. Use change in angle of primary flow line.

Hb = K (Vi2/2g)

• 90 degrees K = 0.70   40 degrees K = 0.38

• 80 degrees K = 0.66   30 degrees K = 0.28

• 70 degrees K = 0.61   25 degrees K = 0.22

• 60 degrees K = 0.55   20 degrees K = 0.16

• 50 degrees K = 0.47   15 degrees K = 0.10

Hydraulic and Energy Grade Line

• Hydraulic Grade Line (HGL) — A line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a given fluid flow path, such as a pipeline or a treatment plant or ground-water streamline or other.

• Energy Gradient or Energy Grade Line (EGL)— A line whose plotted ordinate position represents the sum of the change in energy per unit length in the direction of flow or motion

Hydraulic Grade Line

Weirs

Or

H = (Q/2.5)0.4

Table 3. Discharge rates for 90-degree V-Notch Weirs.

Head FLOW RATES Head FLOW RATES

(Ft) GPM CFS (Ft) GPM CFS

0.05 1 .001 0.80 648 1.43

0.10 4 .008 0.85 752 1.66

0.15 10 .023 0.90 870 1.92

0.20 21 .046 0.95 992 2.19

0.25 36 .080 1.00 1128 2.49

0.30 57 .125 1.05 1273 2.81

0.35 83 .184 1.10 1427 3.15

0.40 116 .256 1.15 1595 3.52

0.45 155 .343 1.20 1771 3.91

0.50 202 .445 1.25 1961 4.33

0.55 255 .564 1.30 2162 4.77

0.60 317 .700 1.35 2374 5.24

0.65 387 .854 1.40 2598 5.74

0.70 467 1.03 1.45 2835 6.26

0.75 553 1.22 1.50 3083 6.81