Flow Control

Monroe L. Weber-Shirk

School of Civil and

Environmental Engineering

Flow ControlFlow Control

Creativity without a trip

Variations on a drip

Giving head loss the slip

Overview

Why is constant flow desirable? If you had electricityHypochlorinators in Honduras – Hole in a BucketConstant head devices

Overflow tanksMarriot bottleFloatsFloat valve

Orifices and surface tension Flow Measurement

Applications of Constant FlowApplications of Constant Flow

POU treatment devices (Point of Use)UV disinfection clay pot filtersSSF (slow sand filters) arsenic removal devices

Reagent addition for community treatment processesAlum for ____________Calcium or sodium hypochlorite for ____________Sodium carbonate for _____________

Could you make a flow control device that increased the dose in proportion to the main flow?

coagulationdisinfection

pH control

Why is constant flow desirable for POU treatment devices?

Why is constant flow desirable for POU treatment devices?

Slow constant treatment can use a smaller reactor than intermittent treatment

It isn’t reasonable to expect to treat on demand in a householdFlow variations are huge (max/average=_____)System would be idle most of the time

Use a mini clearwell so that a ready supply of treated water is always available

40

If you had electricity…If you had electricity…

Metering pumps (positive displacement)PistonsGearsPeristaltic

Valves with feedback from flow sensors So an alternative would be to raise the per capita

income and provide electrical service to everyone…

But a simpler solution would be better!

Constant Head: Floats (variation on hypochlorinator)

Constant Head: Floats (variation on hypochlorinator)

orifice

VERY Flexible hose

Head can be varied by changing buoyancy of float

Supercritical open channel flow!

Unaffected by downstream conditions!

2orifice orificeQ K A g h

h

Floating BowlFloating Bowl

Adjust the flow by changing the rocksNeed to make

adjustments (INSIDE) the chemical tank

Rocks are submerged in the chemical

Safety issues

Chemical Metering (Hypochlorinator)

Chemical Metering (Hypochlorinator)

Transparent flexible tube

(0.5”)

1.0 m

1.05 m1.78 m

1.5” PVC overflow tube

Float

PVC needle valve 0.5” PVC tube

Water in the distribution tank

What is the simplest representation that captures the fluid mechanics of this system?

Hole in a BucketHole in a Bucket

Vena contracta

0.6vc orificeA A

Orifice

2orifice orificeQ K A g h

h

0.6orificeK

Transparent flexible tube

(0.5”)

1.0 m

1.05 m1.78 m

1.5” PVC overflow tube

Float

PVC needle valve 0.5” PVC tube

Water in the distribution tank

Transparent flexible tube

(0.5”)

1.0 m

1.05 m1.78 m

1.5” PVC overflow tube

Float

PVC needle valve 0.5” PVC tube

Water in the distribution tank

Use Control Volume Equation: Conservation of Mass

h0or

cv

Q dVt

2or or orQ K A gh

2 0res or or

dhA K A gh

dt

resor

A dhdVQ

dt dt

ˆcs cv

dA dVt

r r¶

× =-¶ò òV n

Orifice in the PVC valve

Integrate to get h as f(t)

volume

2V gh

Finding the chlorine depth as f(t)

0 02

h tres

hor or

A dhdt

K A g h

1/ 2 1/ 202

2res

or or

Ah h t

K A g

0 22

oror

res

Ah h tK g

A

Integrate

Solve for height

Separate variables

Finding Q as f(t)Finding Q as f(t)

2or orQ K A gh

02 22

or oror or

res

tK AQ K A g h g

A

0

02or

or

QA

K gh

Find Aor as function of initial target flow rate

Set the valve to get desired dose initially

0 22

oror

res

Ah h tK g

A

Surprise… Q and chlorine dose decrease linearly with time!

Surprise… Q and chlorine dose decrease linearly with time!

0 0

11

2res

design

hQ t

Q t h

0

02or

or

QA

K gh

02 22

or oror or

res

tK AQ K A g h g

A

Relationship between Q0 and Ares?Assume flow at Q0 for time (tdesign) would empty reservoir

0 design res resQ t A h 0 res

res design

Q h

A t

2

200

11

2Cl res

Cl design

C ht

C t h

0

0 0

12 res

tQQ

Q A h Linear decrease in flow

Effect of tank height above valveEffect of tank height above valve

2

020

Qh h

Q

0 2 4 6 80

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

Qratio t tdesign hres h0 h t tdesign hres h0 h0 hres

hres

t

day

Depth in reservoir

Case 1, h0=50 m, hres = 1 m, tdesign=4 days

0 2 4 6 80

0.2

0.4

0.6

0.8

0

0.2

0.4

0.6

0.8

Qratio t tdesign hres h0 h t tdesign hres h0 h0 hres

hres

t

day

Case 1, h0=1 m, hres = 1 m, tdesign=4 days

Constant Head: Overflow TanksConstant Head: Overflow Tanks

Surface tension effects here

What controls the flow?

2orifice orificeQ K A g h

h

orificeA

Constant Head: Marriot bottle

Constant Head: Marriot bottle

A simple constant head device

Why is pressure at the top of the filter independent of water level in the Marriot bottle?

What is the head loss for this filter?

Disadvantage? ___________

2 2

2 2in in out out

in in P out out T L

p V p Vz h z h h

g g

Lh

batch system

Constant Head: Float ValveConstant Head: Float Valve

Float adjusts opening to maintain relatively constant water level in lower tank (independent of upper tank level)NOT Flow Control!

?

Describe sequence of events after filling

Flow Control Valve (FCV)

Limits the ____ ___ through the valve to a specified value, in a specified direction

Calculate the sizes of the openings and the corresponding pressures for the flows of interest

flow rate • Expensive• Work best with large

Q and large head loss

Raw water reservoir and SSF

Flow control device

Clean water reservoir

Small diameter tubing

Float valve and small tube

Float valve and small tube

Floating Ball ValveFloating Ball Valve

Float valve

Small diameter tube

Float valve with IV dripFloat valve with IV drip

8.3 cm11.0 cm

0.5 cm

4.4 cm

6.5 cm

2 mm

2.3 cm

9.1 cm

2 mm

5.6 cm

1.5 cm

2 cm

5.2 cm Housing Dimensions:ID = 7.85 cmOD = 8.8 cm

Float mass:6 grams

IV roller clamp

Rubber tip

Barb tubing adapter

PVCstem

IV tubing (~10 drops/mL)

8.3 cm11.0 cm

0.5 cm

4.4 cm

6.5 cm

2 mm

2.3 cm

9.1 cm

2 mm

5.6 cm

1.5 cm

2 cm

5.2 cm Housing Dimensions:ID = 7.85 cmOD = 8.8 cm

Float mass:6 grams

IV roller clamp

Rubber tip

Barb tubing adapter

PVCstem

IV tubing (~10 drops/mL)

Floating Bowl with OrificeFloating Bowl with Orifice

Sand colum

nHJR

Holding container (bucket or glass column)

Pong pipe

Sealing pipe

Driving pressure for sand column

Upflow prevents trapped air

(keyword: “prevent”)!

Flow Control Competition Results from CEE 454 in 2004

Flow Control Competition Results from CEE 454 in 2004

What are the two essential elements of gravity powered flow control?Constant head (float valve wins!)Head loss elements

____________________________________________________________________________________________

Can use flexible tube to facilitate adjusting the head

Orifice i.e.. small hole or restrictionLong small diameter tubePorous media

Flow control device

Small diameter tubing

Float valve and small tube (Gravity dosing system)

Float valve and small tube (Gravity dosing system)

hlf 2 4

32 128LV LQh

gD g D

4l

128

h g DQ

L

chemical stock tank

If laminar flow!

2 2

2 2in in out out

in in P out out T L

p V p Vz h z h h

g g

L in outh z z Neglecting minor losses

Long small tube head lossLong small tube head loss

Laminar flow

Turbulent Flow

f 2 4

32 128LV LQh

gD g D

2

f 2 5

8f

LQh

g D2

0.9

0.25f

5.74log

3.7 ReD

D

Q4Re

Flow proportional to hf

Orifice flowOrifice flow

2

42

8

v

QD K

g h

2orQ K A gh2

2 2 4

1 8

or

Qh

K g D

2

1

or

KK

Solve for h and substitute area of a circle to obtain same form as minor loss equation

Kor = 0.63 therefore K=2.5

2.5 d 8 d

d

h

D

Porous Media Head Loss: Kozeny equation

Porous Media Head Loss: Kozeny equation

f 2

32 pore

pore

LVh

gd

apore

VV

Velocity of fluid above the porous media

Laminar flow assumption

2

f3 2

136 a

sand

Vhk

L gd

k = Kozeny constantApproximately 5 for most filtration conditions

Tube vs. OrificeTube vs. Orifice

CloggingAdjustability

0 50 100 150 2000

1

2

3

Dtube Q 20cm 1m ( )

mm

Dorifice 2.5 Q 20cm( )

mm

Q

mL

min

Dtube Q hf L 128 L Qg hf

1

4

Dorifice K Q he K8 Q

2

g 2 he

1

4

Minor losses Major losses

Surface TensionSurface Tension

hIs the force of gravity stronger than surface tension?

343 2g

rF g

343 2g

rF g

2rF=

Fp= 3

242 r

3 2r

g g h r

3

242 r

3 2r

g g h r 2g h r

Will the droplet drop?

Surface Tension can prevent flow!Surface Tension can prevent flow!

0.0500.0550.0600.0650.0700.0750.080

0 20 40 60 80 100

Temperature (C)

Sur

face

tens

ion

(N/m

)

3

242 r

3 2r

g g h r

3

242 r

3 2r

g g h r

3

2

42 r

3 2r

gh

g r

Solve for height of water required to form droplet

2 23r

hgr

Design constraint for flow control devices: Surface Tension

Design constraint for flow control devices: Surface Tension

0.1 1 101

10

100

h r( )

mm

r

mm

2 23r

hgr

Delineates the boundary between stable and unstable

No droplets form to left of line

Flow control devices need to be designed to operate to the right of the red line!

Hypochorinator FixHypochorinator Fixhttp://web.mit.edu/d-lab/honduras.htm

What is good?How could you improve this system?What might fail?Safety hazards?

Modular Flow Control

Modular Flow Control

Identify the Flow Controller Failure Modes

Identify the Flow Controller Failure Modes

Moving partsWearCorrosion (especially with corrosive chemicals) Precipitation (e.g. calcium carbonate) Incompatible materialsDon’t forget sunlight has UV rays!CloggingDesign errors…

Flow Measurement DevicesFlow Measurement Devices

Orifice in the side of a pipe Pipe vented through water

surface Jet of water must free fall

inside the pipeKorifice is due to the vena

contracta and has a value of approximately 0.6.

hgAKQ orificeorifice 2

h

Free Surface with Orifice limitationsFree Surface with Orifice limitations

The head loss from making the measurement is wasted (likely on the order of 20 cm)

Ability to include this type of flow measurement depends on availability of excess potential energy

The useable measurement range doesn’t include the range where the orifice is only partially submerged

Thus large diameter orifices aren’t ideal because they limit the measurement range

For reasonably small head loss the flow per orifice can’t be much greater than 100 Lpm

Use multiple orifices for larger flow rates

Qplant d h( ) Korifice d

24

2 g h

40 60 80 100 120 1400

5

10

15

20

25

h

cm

Qplant d h( )

L

min

Alternative Flow MeasurementsAlternative Flow Measurements

Block the effluent port from a small tank and measure the rate of depth increaseThe grit chamber at the head of a water treatment plant

could be used for this purposeBut this causes a major flow disturbance for the plant

open channel weirs for very large flow rate measurements

Orifice plates in a pipe (use manometer to measure pressure drop)

If you have access to electricity, then there are a large number of measurement techniques available