Fundamentals of Urban Drainage
CT-4491
Pressurised systems
Ivo Pothof
CEG room 4.52
[email protected] (www.deltares.nl)
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Content
Pumping stations in systems
Capacity problems in pressurised wastewater systems
• clogged pump
• gas pockets in inverted siphons
Hydraulic design guidelines for pressurised wastewater systems
• Pump pit
• Pumping station
• Pipeline
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Pumping stations in systems
Learning goal
• Understand the interaction between system and pumping station
• Characterise the system, independently of the pumps
> System characteristic
> Static head, suction head, delivery head
> Dynamic head
• Characterise the pump(s), independently of the system
> Pump capacity curve, Q-H curve
> Pump head
• Combine the system characteristic and pump curve(s) to find the
duty point(s)
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Hstat static
head
Hdyn dynamic
head
Suction head
Delivery head
System characteristic
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Hdyn (k=0,5 mm) Hdyn (k=0.1 mm)
Discharge (m3/h)
3,5003,0002,5002,0001,5001,0005000
Head (
m)
55
50
45
40
35
30
25
20
15
10
5
0
Definition:: static + dynamic head as function of discharge Q
System characteristic
2 2
2 5 4
8
2
st
2
st
st st
L vH = H .
D 2g
Q
L AH .
D 2g
LH .Q H C Q
g D D
k=0,5 mm
k=0,1 mm
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Factors affecting the system characteristic
Static head factors (Hstat)
• Suction head
• Delivery head
Dynamic head factors (Hdyn)
• Control valves
• Wall roughness
• Other pumping stations in the same wastewater transportation system
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Pump and system characteristic
• Pump curve is supplied by
manufacturer
• Intersection of pump curve
and system characteristic is
the duty point
pomp at 1450 (rpm) Hdyn (k=0,5 mm)
Hdyn+stat (k=0,5 mm)
Discharge (m3/h)
5,0004,0003,0002,0001,0000
Head (
m)
60
55
50
45
40
35
30
25
20
15
10
5
0
Hdyn+Hstat
Hdyn
Pump Q-H
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Pump and system characteristic
Each pumping station has
to cope with a range of
system curves, due to
• Other pumping stations
• Increased wall friction in
time
• Gas pocket accumulation
0
5
10
15
20
25
30
35
40
45
0 50 100 150 200 250 300 350
Flow rate [l/s]
Pu
mp
head
[m
] Minimum system curve
Maximum system curve
Pump curve
Maximum efficiency
minimum flow
maximum flow
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Suction level variation
Hstat varies
System curve shifts
vertically
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Influence of wall friction / control valve
Hdyn varies
System curve gradient
varies
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Combining pump curves (parallel)
• Sum pump Q at each
pump H
• Flow rate at duty point
does not double with
2nd pump
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Combining pump curves (parallel, different capacity)
• Sum pump Q at
each pump H
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Multistage pump (serial)
• Sum pump H at each
pump Q
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Learning goals
• Capacity problems pressurised wastewater mains
• Understand consequences of insufficient capacity
• Identify possible causes
• Basic understanding of air in pipelines
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Consequences of capacity problems
Increased power consumption
• 30% increase measured, if capacity reduction is acceptable
• 10000 ton CO2 , 3 M€ per year
Financial claims after floods
Increased maintenance costs
• Pigging operations
Extra investments infrastructure
Increased CSO volume
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Causes of reduced capacity
Partially blocked pump impeller
• Pump vibration
Unpredictable capacity reductions (90% of cases)
• gas- and air pockets
> air intake by pumps
> local pipe draining at pump after pump trip
> draining air vessel
> degassing during transport
> biochemical processes
Steadily growing capacity reductions (10%)
• pipe wall deterioration
• biofouling / scaling
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Air entrainment by the pumps
Sewer outflow always above
pump start level (max WL)
Suction pit is (very) small • minimise sedimentation
• minimise floating debris
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Air intake by pump
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Gas pockets cause unpredictable head loss
Rdl Boz
AWPAWP 1
AWP 2
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Detail: inverted siphon
Gas pockets grow in the top of inverted siphons at DWF
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Drilled pipe in urban area (The Hague)
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Detail: inverted siphon
Gas pocket head loss ≈ height of gas pocket
Head loss (appr.)
Large flow rate
Small flow rate
2 2
1 1 2 21 2Bernoulli :
2 2
p v p vz z H
g g g g
Ref
Z 1 Z
2
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Hydraulic gradient with gas pockets
17 March 2010 CT-4491 Fundamentals of Urban Drainage
R&D questions
What are the gas pocket transport modes?
• Dimensionless variables
What is maximum gas pocket length and head loss?
• Water discharge
• Pipe angle
• Air discharge
What is influence on gas discharge of
• Length of downward slope
• Water quality
• Pipe diameter
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Experimental research – test set-up side view
stroming
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Experimental facility at treatment plant
• In operation from April 2008 – April 2009
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Hydraulic jumps in large scale facility
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Visual observations in 150 mm pipe
Experiment in D=150 mm, slope 10 degrees
velocity 0,25 – 1,0 m/s
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Visual observations in large scale facility
See also
www.youtube.com/capwat
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Conclusions from visual observations (1)
Gas transport mechanism
• hydraulic jump downstream of gas pocket
• turbulence extracts gas bubbles from the pocket
• drag force > buoyant force gas bubble transported
• drag force < buoyant force gas bubble rises
•
•
Both mechanisms occur simultaneously
• small bubbles flow downward
• large bubbles flow upward
Net air-water discharge ratio is ~ 0.001 only
2 22 20.5 0.5D b w b D b w b bDrag C A v v C R v v R
3 34
3l g b l g b bBuoyancy g V g R R
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Dimensionless parameters
Head loss is related to height of
air pocket
• Elevation difference of downward
sloping reach L*sinq
Water velocity is related to drag –
buoyancy ratio
2 2 3 sind b bC v R gR
F v gD
q
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Air pocket head loss measurements
0
0.25
0.5
0.75
0 0.3 0.6 0.9 1.2Liquid flow number, Fl [-]
Me
asure
d d
iffe
ren
tial pre
ssure
,
p [b
ar]
Fg=0Fg*1000=0.4Fg*1000=0.8 Fg*1000=2Fg*1000=4Fg*1000=6
17 March 2010 CT-4491 Fundamentals of Urban Drainage
0
1
2
3
4
5
6
7
0 10 20 30 40 50 60 70 80
Surface tension (mN/m)
H
ga
s (
mw
c)
Results – Wastewater
F=0.63 and Qg=7.1 l/min
Wastewater (DWF)
Clean water
Gas pocket resistance is equal
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Results – Increased pressure
Low gas flow (0.71 l/min)
Increasing pressure reduces gas pocket
0
1
2
3
4
5
6
7
1.5 1.7 1.9 2.1 2.3 2.5 2.7
P2(bar)
dH
ga
s (
mw
c)
F
0.27
0.45 0.63
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Gas transport mechanisms
0
0.2
0.4
0.6
0.8
1
0 30 60 90
q
F w
Clearing flow criterion
Multiple gas pockets
dHgas > 0.9*dz
1 hydraulic jump
dissolving gas
0.05*dz < dHgas < 0.9*dz
multiple hydraulic jumps
turbulent bubble transport
dHgas < 0.05*dz
no gas accumulation
gas volume transport
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Solutions to maintain design capacity
Pump pit
• Deflection plate reduces air entrainment with factor 1000
Pumping station
• Most air valves on pumps can be closed without adverse effects
• Evaluate appropriate switch-off level and switch-off procedure
Pipeline
• Downward sloping reach
> the steeper, the better
> Maximum air transport capacity in vertical pipe
• If air admission in pumping station is minimised, additional measures
in pipeline not necessary
17 March 2010 CT-4491 Fundamentals of Urban Drainage
CAPWAT main project results
• New design guidelines available on the hydraulic design and
operation of pressurised wastewater mains
• PhD report Lubbers, 2007
• Focus on lab experiments
• PhD report Pothof, 2011
• Air transport model and
• validation experiments in long downward sloping pipe with clean water and
wastewater
• Many scientific questions still unanswered (MSc/Phd project?)
• How does turbulent mixing drive the air flow?
• Is velocity of rising air pockets correctly predicted by model?
• When does surface entrainment start to enhance the air transport in closed
conduits?
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Exercise (old exam)
See hard copy
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Answer to Exercise
Part 1, no air in system
a. H = Hstat + C* Q2= 5 + 80*Q2 , Q in m3/s
a. C is derived from Darcy-Weisbach: C = *L / (2*g*D*A2)
b. See graph
c. Duty point is Q = 1400 m3/h at 17 m
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Answers part 2, with air
Part 2, with air
a. Water and pump inertia cause level drop in pump pit after
stop
b. 1.68 m * Area (0.2 m2) * 6 cycles/hr = 2 m3/hr;
c. Fg = Q/(A*sqrt(g*D)) = 0.0008
d. Rescale gaspocket head loss data using
a. L*sin11 = 7.6 m
b. Translate Flow number to discharge in m3/h. Fw = 0.6 Q = 1500
m3/h. See result in graph
e. New duty point at Q = 1200 m3/h at H = 20 m
17 March 2010 CT-4491 Fundamentals of Urban Drainage
Answer 1b) and 2d)
Pump curve
0
5
10
15
20
25
30
0 300 600 900 1200 1500 1800
Discharge [m3/h]
Pum
p h
ead
[m
]
H [m]
Sys.char (no air)
Sys.char (air)