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TRANSCRIPT
A Comparison of Four Types of Sewage Drop Structures
through Physical Modeling
Tony Margevicius, P.E. (OH) 13-February-2018
WEAT – North Texas Section February Seminar
Agenda Why should we care?
What is a sewage drop structure?
Challenges of deep tunnel sewage drop structures
Hydraulics of sewage drop structures
Pneumatics of sewage drop structures
Comparison of types of sewage drop structures
Summary, questions and answers
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Why Should We Care?
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Why Should We Care?
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Are these recent inventions?
What are the main components of a sewage drop structure?
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What is a Sewage Drop Structure?
Design for wellhole (an extremely deep drop structure), Cleveland, Ohio, circa 1914. Source: Leonard Metcalf and Harrison P. Eddy, American Sewerage Practice, Vol. 1: Design of Sewers, 1st edition (New York: McGraw-Hill, 1914), p. 544.
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What is a Sewage Drop Structure?
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Challenges of Drop Structure Two miscible fluids
Three phase flow regime
As flow falls, it accelerates
Four dimensional flow
Analysis is difficult; two common approaches:
Mathematical models, including Computational Fluid Dynamics (CFD)
Physical models
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Challenges of Drop Structures How to dissipate the energy of falling water / solids?
How to prevent undesirable effects of high velocity?
How to prevent undesirable effects of air flow?
How to maintain a stable, predictable flow regime?
How to fit into site, geology, etc.?
How to handle debris, solids, & grit?
How to analyze?
The biggest challenges are hydraulic and pneumatic.
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Drop Structure Hydraulics “Terminal” drop velocity can exceed 50
fps (15 m/s)
What is terminal velocity of a brick?
At these speeds, several concerns arise:
Noise
Cavitation
Erosion
Vibration
Impact loads
Drop Structure Pneumatics Viscous drag pulls air
with flow
Nearly full tunnels are largest concern
Air is compressible
Air pressure can be up to ten times the water pressure
Vacuum conditions are also possible
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WASTEWATER
Pescod, M.B. and Price, A.C. “Major Factors in Sewer Ventilation”; Journal Water Pollution Control Federation, Vol. 54, Number 4 (April) 1992, pp 385-397
Drop Structure Pneumatics
Problems with air:
Capacity
Surface eruptions
Noise
Effective friction factor
Surge creation
Flow instability
Odor stripping
Corrosion
Pescod, M.B. and Price, A.C. “Major Factors in Sewer Ventilation”; Journal Water Pollution Control Federation, Vol. 54, Number 4 (April) 1992, pp 385-397
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WASTEWATER
AIR VELOCITY CONTOUR CURVE
IN PERCENT OF
WASTEWATER VELOCITY
WASTEWATER SURFACE
DIRECTION
OF FLOW
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Comparisons of Drop Structures Most popular drop structures
Plunge drop
Vortex drop
Helicoidal ramp
Baffle drop
Others (DC WASA, Portland model, drill drop, Chicago “boot,” etc.)
“Apples to apples comparison”
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Plunge Drop Oldest drop
Simple and cheap
Common for small flows and shallow drops
Not usually used for most tunnel systems, due to: Flow instability
Air entrainment
Poor energy dissipater
Releases odors
Noisy
Prone to vibration
Plunge Drop
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Vortex Drop
Structure • Commonly used on CSO
tunnels
• Inlet can be of several types (scroll, circular, tangential, etc.)
• De-aeration chamber is recommended to reduce air entrainment
• Hydraulically efficient
• Proven
• Limited access for people and equipment
Type H-4 Vortex Drop
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Helicoidal Ramp
Drop Structure
• Hydraulically, similar concept to vortex drop
• Can use continuous ramp or intermittent ramp
• Less common for CSO tunnels
• Limited access for personnel and equipment
• Adit design is critical for hydraulics / air entrainment
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Type B Helicoidal Ramp
• Provides access for
people and equipment
• Can introduce several
flows into one drop
• Provides limited ability to
arrest surge and geysers
• With proper design,
provides good de-aeration
• Gaining in popularity
(Cleveland, Toronto area,
London, New Zealand,
China, etc.)
Baffle (Cascade)
Drop Structure
Baffle Drop Structure
The Baffle Drop Manhole has been used for almost a century.
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Baffle Drop Structure
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Comparison of Drop Structures Feature Vortex Plunge Helicoidal Baffle
Total number of sewage installations ++ +++ - +
Commonly used in sewage tunnels +++ + + ++
Hydraulically efficient ++ +++ ++ +
Provides access for equipment/people + + - +++
Surge mitigation built-in + ++ + +++
Costs less to build ++ +++ + ++
Commonly built over top of tunnel + +++ ??? ++
Accommodates multiple inlet sewers - + - +++
Reduces air entrainment +++ - + +++
Reduces odors ++ - ??? +++
Self-cleaning ++ - + +++
Minimizes need for flow conditioning + +++ - +++
Suitable for a wide range of flows + +++ ++ +++
Comparison of Drop Structures
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Comparison of Drop Structures
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Summary Dropping sewage into tunnels presents many
challenges and risks.
There is no such thing as an empty pipe. For sewage drop structures, focus on what you cannot see - air.
There are many types of sewage drop structures. Make sure you know when to use which.
Lots of opportunities to improve the body of knowledge.
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Questions and (Hopefully!) Answers…
Tony Margevicius, P.E. (OH) 1300 East 9th Street, Suite 500 Cleveland, OH 44114 [email protected] +1.216.622.2300