Wastewater Engineering Design (253414)

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Wastewater Engineering Design(253414)

• ?.??.?????? ????????
• B.Eng. Chiang Mai University
• M.Eng. Chulalongkorn University
• PhD University of Newcastle upon Tyne, UK

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????? Applied Hydraulics(Tchobanoglous, G. 1981
Wastewater Engineering Collection and Pumping of
Wastewater, Chapter 2)

• Principal factors affecting the flow of
  wastewater in sewers are
• 1. The slope
• 2. The conduits cross-sectional area and shape
• 3. The roughness of the interior pipe surface
• 4. The conditions of flow, e.g. flowing full or
  partly full and if the flow is steady or varied
• 5. The existing of obstructions, bends, etc.
• 6. The character, specific gravity, and viscosity
  of the liquid

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Three basic equations of fluid mechanics used in
the analysis of both flow in pipes and open
channels

• The equation of continuity
• The energy equation
• The momentum equation

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Terms

• Laminar flow Reynolds number (NR) lt 1,500 to
  2,000
• Turbulent flow NR gt 6,000 to 10,000
• Pipe and open-channel flow (Fig 2-1)
• Head loss (hL) Loss of energy occurring when
  liquids flow in pipes and open channels

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Pipe flow and open-channel flow
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Terms (continued)

• Hydraulic grade line A measure of the pressure
  head available at these various points, In open
  channels, HGL profile of water surface
• Energy grade line Elevation head (z)Pressure
  head (y)Velocity head (v2/2g)

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Equation of Continuity

• The conservation of mass from section to section
  in a streamtube control volume (Fig 2-4)
• If the fluid is incompressible (like water), then
  ?1 ?2 and
• A1v1 A2v2 Q1 Q2

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Flow through a streamtube control volume
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Energy Equation

• -Pressure energy (Ep)
• -Velocity energy (Ev)
• -Potential energy (Eq)
• -Thermal or Internal energy (Ei)
• -Mechanical energy (Em)
• transferred to () e.g. pump
• from (-) e.g. turbine
• -Heat energy (Eh)
• transferred to () e.g. heat exchanger
• from (-) e.g. heat exchanger

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Law of conservation of energy (Fig 2-5)

• (EpEvEqEi)1EmEh (EpEvEqEi)2losses
• General expression for an incompressible liquid
• For ideal fluid (frictionless) and no mechanical
  or hear energy is transferred (Bernoullis
  equation)
• hL includes the frictional head loss (hf) and
  those occurred at discontinuities in flow
  geometry and called minor losses

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Energy equation and application to a pipeline
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Momentum Equation

• Vector relationship, in which both magnitude and
  direction of forces and velocities are important
  (Fig 2-8)
• The forces Fx and Fy needed to maintain
  equilibrium are applied through the pipe wall by
  supporting structures, hangers, tie rods, thrust
  blocks, etc.

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Application of momentum equation
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Momentum Equation (continued)

• The principles of momentum and continuity are
  also used in the equation for a hydraulic jump in
  rectangular open channels (Fig 2-9)

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Flow Equations

• To design facilities for the transmission of
  fluids, one must know
• Relationship between hL or the slope of the EGL
  and the flowrate
• The fluid characteristics
• The conduit or channel roughness and
  configuration

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Equations used

• Poiseuille equation laminar flow
• Darcy-Weisbach equation for pipe friction
  determination
• Manning equation recommended in the design of
  sewer systems (gravity flow) (??????????????
  2546)
• Hazen-Williams equation recommended in the
  design of sewer systems under pressure (with
  pump) (?????????????? 2546)

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Darcy-Weisbach equation

• The value of f varies with the NR, pipe
  roughness, pipe size, and other factors.
• Relationships shown in Moody diagrams (Fig 2-10
  and 2-11)
• To determine f, NR needs to be checked to see
  the type of flow (laminar, transition, or
  turbulent) (Ex 2-1)

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Manning equation

• S slope of EGL but can be equivalent to the
  slope of channel bottom under the uniform flow
• R hydraulic radius cross-sectional area of
  flow / wetted perimeter D/4 (flowing full)
• n values see in Table 2-1

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Hazen-Williams equation

• Ex 2-2

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Pipe size

• Table 2-3 pipe size using in the US
• Better check with the availability of pipe size
  in Thailand

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Sewer pipe size (USA)
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Design Charts and Tables

• Flowing full Nomographs in Fig 2-12 to Fig 2-15
  can be used (n 0.013 and 0.015) but it might be
  quicker and more accurate using the common spread
  sheet (Microsoft Excel)
• Partly full Most of the time flow in the sewers
  is partly full. Both v and Q can be determined
  using Fig 2-16 and Table 2-5 (Ex 2-4 and 2-5)

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Hydraulic elements for circular sewers
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Table for circular channels in terms of diameter
in the equation
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Minor Losses

• In a closed-conduit system, most minor losses
  can be expressed as
• See Appendix C for extensive listing to
  determine the minor losses
• Ex2-6 Practice minor loss determination

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Example 2-6