Fundamentals of Urban Drainage CT-4491 Pressurised systemsocw.tudelft.nl/wp-content/uploads/3.2_Pressurised_systems_and... · CT-4491 Fundamentals of Urban Drainage 17 March 2010

Fundamentals of Urban Drainage

CT-4491

Pressurised systems

Ivo Pothof

CEG room 4.52

[email protected] (www.deltares.nl)

mailto:[email protected]

http://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


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)


Hstat static

head

Hdyn dynamic

head

Suction head

Delivery head

System characteristic


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


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


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


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


Suction level variation

Hstat varies

System curve shifts

vertically


Influence of wall friction / control valve

Hdyn varies

System curve gradient

varies


Combining pump curves (parallel)

• Sum pump Q at each

pump H

• Flow rate at duty point

does not double with

2nd pump


Combining pump curves (parallel, different capacity)

• Sum pump Q at

each pump H


Multistage pump (serial)

• Sum pump H at each

pump Q


Learning goals

• Capacity problems pressurised wastewater mains

• Understand consequences of insufficient capacity

• Identify possible causes

• Basic understanding of air in pipelines


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


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


Air entrainment by the pumps

Sewer outflow always above

pump start level (max WL)

Suction pit is (very) small • minimise sedimentation

• minimise floating debris


Air intake by pump


Gas pockets cause unpredictable head loss

Rdl Boz

AWPAWP 1

AWP 2


Detail: inverted siphon

Gas pockets grow in the top of inverted siphons at DWF


Drilled pipe in urban area (The Hague)


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


Hydraulic gradient with gas pockets


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


Experimental research – test set-up side view

stroming


Experimental facility at treatment plant

• In operation from April 2008 – April 2009


Hydraulic jumps in large scale facility


Visual observations in 150 mm pipe

Experiment in D=150 mm, slope 10 degrees

velocity 0,25 – 1,0 m/s


Visual observations in large scale facility

See also

www.youtube.com/capwat


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


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


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


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


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


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


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


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?


Exercise (old exam)

See hard copy


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


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


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)

Documents

Fundamentals of Urban Drainage CT-4491 Pressurised systemsocw.tudelft.nl/wp-content/uploads/3.2_Pressurised_systems_and... · CT-4491 Fundamentals of Urban Drainage 17 March 2010