New England Water Environment Association Annual ...
Transcript of New England Water Environment Association Annual ...
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Hydraulics of Pressure Sewer Systems
New England Water Environment Association Annual Conference and Exhibit January 21st – 24th 2018 Boston, MA
Keith J McHale, P.E. Environment One Corporation
Flow Characterization of Downhill Pumping
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Pressure Sewer System
• Wastewater collection systems that use individual residential pumps to convey the flow to a central treatment system, lift station, gravity sewer, or force main
• System consists of – Grinder pump – Small diameter
pressure pipe
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The Heart of Pressure Sewers
• Pump basin • Pumps
– Progressing cavity (semi-positive displacement)
– Centrifugal • Liquid level sensors • Pump control • Pump removal guidance
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Are Advantages Advantages?
• Key advantage compared to gravity – Buried just below the frost line – Follows the contours of the land
• Segments of downhill, pumped flow are not uncommon
• Is downhill pumping a problem?
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Pump Wet End
• Progressing Cavity – Semi-positive displacement – Stainless steel helical rotor
and stationary elastomer stator
– The rotation of the rotor within the stator creates a series of cavities that moves the fluid to the discharge
– Produces a constant flow rate, only marginally effected by system pressure
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SPD Pump Characteristics
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System Design Methodology
• The Probability Method (aka Sim Ops) – Is the preferred design methodology
used for systems with near-vertical pressure head – discharge curves (semi-positive displacement pumps)
– is based on the assumption that each pump that is running will produce a near identical flow rate
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Peak Flow Design Basis
• Simultaneous Pump Operation
• Predicts the maximum number of pumps expected to be running simultaneously
Pump Cores Connected
Pumps Operating
Simultaneously
1 1 2 – 3 2 4 – 9 3
10 – 18 4 19– 30 5 31– 50 6 51 – 80 7 81 – 113 8
114 – 146 9 147 – 179 10 312 – 344 15 477 – 509 20
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Flow Velocity and Friction Loss
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N5 = 19 to 30 connections N10 = 147 to 179 connections
N20 = 477 to 509 connections
Flow Velocity and Friction Loss
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Transitional Pressure Sewers
Bowne, W.C. (1983) Two Phase Flow in Pressure Sewers and Small Diameter Sewers
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Evaluation of Pressure Sewers
• Hydraulic evaluation completed by E/One and Modern Energy LLC
• Evaluation consisted of – a comprehensive theoretical
evaluation (table top evaluation) – a physical, field demonstration
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Phase 1: Table-Top Evaluation
• Evaluation Objectives – Characteristics of SPD pump – Nature of pipe flow: full, partially full – Energy of fluid flow – Pipe friction and its relation to energy – Interaction of pump, pipe, and friction – Effect of pipe size, elevation changes,
and pipe slope
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Pipe Capacity vs. Down Slope
0 0.02 0.04 0.06 0.08 0.15
10
15
20
25
30
S [-] drop/run
Q [
gpm
]
Capacity of 1 1/4-in PVC pipe vs downward slope
E-ONE single pump average discharge rate
rang
e to
pum
p do
wn
slop
e
range to flow by gravity down slope
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Natural Flow Capacity of Pipe
0 0.02 0.04 0.06 0.08 0.10
100200300400500600700800900
10001100120013001400
Slope [-] drop/run
Q [
gpm
]
6-inch6-inch4-inch4-inch
2-inch2-inch1 1/4 -inch1 1/4 -inch
Natural flow capacity of SDR21 pipes vs. downward slope
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Natural Flow Capacity: 4-inch
0 0.02 0.04 0.06 0.08 0.10
50
100
150
200
250
300
350
400
450
500
S [-] Slope=drop/run
gpm
[gp
m]
Full pipeFull pipe
0.50 full depth0.50 full depth
0.75 full depth0.75 full depth
0.25 full depth0.25 full depth
4-inch SDR21 pipe capacity vs depth and slope
N5 = 19 to 30 connections N10 = 147 to 179 connections
N20 = 477 to 509 connections
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Example: 3-inch @ 5% slope
0 0.2 0.4 0.6 0.8 10
40
80
120
160
200
n [-] ratio of depth of fluid/pipe diameter
gpm
[gp
m]
3-inch SDR21 pipe flow at 5% slope
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Conclusions Part I
• Properly vented descending pipe sections allow for free flow
• Design guideline: Pipe diameter on downward pipe sized for 3/4 full depth flow
• Downward pipe with full depth flow yields pressured flow
• In no case should valve throttling or restrictions be used to “make hydraulics work”
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Field Apparatus Tests
• Characteristics of the pipe when vented and partially filled with water at various rates
• Pressure versus flow for a range of flows with the vent valve operating and vent valve blocked
• Hill and valley pipe configuration
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Field Apparatus Test Set-Up
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Field Apparatus Test Set-Up
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Field Apparatus Test Set-Up
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Field Apparatus Test Set-Up
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Field Apparatus Test Results
0 2 4 6 8 100
0.2
0.4
0.6
0.8
1
Flow Rate- [gpm]
Nor
mal
ized
dep
th in
pip
e - m
/m Colebrook-White EquationColebrook-White EquationTest data Aug 28, 2012Test data Aug 28, 2012
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Field Apparatus Test Results
• Wave formation in partially filled pipe – Instability of fluid hydraulics? – Irregularity in field apparatus?
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Air Venting Tests
• Test were conducted to demonstrate behavior of fluid under different conditions – Vary flow rate in the system with vent
shut-off valve open – Vary the flow rate in the system with
the vent shut-off valve closed • Flow and pressure at vent connection
were recorded
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Air Venting Test Results
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0
Pressureattop
ofslope
-feeto
fwater
FlowRate- GPM
Pressurevs.Flowratewithairreleasevalveopen.
Flow
rate
whe
reventvalvecloses
Pumpingdownhillinthisflowzone;ventclosed.
Ventopenno significantpressure.
Vent open.Flowispropelledbygravityalone.
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Air Venting Test Results
-0.60
-0.40
-0.20
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0
Pressureatv
entcon
nection-feeto
fwater
FlowRate- GPM
PressurevsFlow-Ventclosedoff1.25in.tube,2%slope
Below9gpmpipeisnaturallyvented fromopenendbecauseitisnotfull.
Zoneofnegativepressure.Gravityovercomespipefriction.l
Regionwherepipeisfullandpumpisovercomingpipefriction.Gravityaloneisinsufficient.
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Conclusions Part II
• Flow behavior predicted by the “table-top” evaluation is validated
• Even with an SPD pump, trapped air is not pushed forward through the downward slope
• Air pocket causes a flow resistance – The pump curve characteristics favor a
SPD pump versus a centrifugal pump – Higher friction lost is likely
unanticipated • Air pockets in valleys will behave
differently
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Acknowledgements
• Modern Energy LLC – Ron Amberger,
• Environment One – Clark Henry – Michael Crowley – Skip Murrell
• Paper available on request