4081WWT Flow Measurement

8/2/2019 4081WWT Flow Measurement

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Flow Measurement

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Flow Measurement

ObjectiveTo determine chemical dosage, air supply into the aerationbasins, sludge volume to return into the biological reactors,to provide daily flow records required by regulatoryagencies, and to evaluate infiltration/inflow during wetweather

Locations Within an interceptor or manhole At the head of the plant Downstream of bar screen, grit channel, or primary

sedimentation In the force main of pumping station Before the outfall


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Flow Measurement - continued

Basic types of measurementDifferential pressure producersDirect discharge measurement Positive volume displacement measurement Flow velocity-area measurement

Flow metersVenturi type meter, orifice meter, propeller type meter,magnetic flow meter, ultrasonic flow meter, vortexmeter, rotameter (variable-area meter), flumes, and

weirs

Liquid chemical flowMeasured by positive displacement pumps (orrotameters)

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Flow Measurement - continued

Selection Criteria Type of application: open channel/closed conduits

Proper sizing: range of flow

Fluid composition: compatibility, solids, passage

Accuracy (%) and repeatability

Headloss or hydraulic head available

Installation requirements: straight length,

accessibility, disconnection method

Operating environment: explosion proof, resistance

to moisture and corrosive gases, temp. range

Ease of maintenance: provision for flushing/rodding Cost

Type and accessibility of the conduit


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Flow Metering Devices inWastewater Treatment Facilities

Raw Primary Secondary Primary Return Thickened Mixed ProcessMetering device WW effluent effluent sludge sludge sludge liquor water

For open channelsHead/area

Flume x x x xWeir x x x

OtherMagnetic (insert type) x

For closed conduitsHead/pressure

Flow tube xa xa x xa xa xa,b xxOrifice xPitot tube xRotameter xVenturi xa xa x xa xa xa x

Moving fluid effectsMagnetic (tube type)_ x x x x x x xUltrasonic (doppler) x x x xc

Ultrasonic (transmission) x x xVortex shedding x x xPositive displacement

Propeller xTurbine x x

a Flushing or diaphragm sealed connections recommendedb Use with in-line reciprocating pumps not recommendedc Solids content < 4%

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Venturi Type Flow Meter

Measure differential pressureConsists of a converging section, a throat, and a

diverging recovery sectionThe difference in two heads is analyzed by electrical or

electromechanical instrumentsAccuracy: 1%; range: 4:1 Take considerable space (L/D = 5~20)Cannot be altered for measuring pressure beyond a

maximum velocity


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Flow Nozzle Meter

Measure differential pressureA Venturi meter without the diverging recovery sectionLess expensive than Venturi meter but higher headlossAccuracy: < 1%; range: 4:1

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Orifice Meter

Measure differential pressureEasy to install and fabricateAdvantages: least expensive of all differential pressure

devices and good accuracy (1%)Disadvantages: least efficient, high headloss, easy

clogging, and narrow range of flows (4:1)


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Electromagnetic Meter

Faradays law: a voltageproduced by passing a conductorthrough a magnetic field isproportional to the velocity of theconductor (wastewater)Advantages: good accuracy

(1~2%), capable of measuringlarge range of flows (10:1), noheadloss, and unaffected bytemperature, conductivity,viscosity, turbulance, andsuspended solids

Disadvantages: high initial costand need for trained personnel tohandle routine O&M

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Turbine Meter

Use a rotating element(turbine)A wide range of fluid

applications covering fromwater to oils, solvents toacidsLimited to pipes running

full, under pressure, andliquids low in suspendedsolids

Excellent accuracy(0.25%) and a goodrange of flows (10:1)

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Acoustic Meter

Use sound waves to measurethe flow ratesSonic meter or ultrasonic

meter depending on whetherthe sound waves are in orabove audible frequency rangeDetermine the liquid levels,

area, and actual velocityAdvantages: low headloss,

excellent accuracy (2~3%),usable in any pipe size, no

fouling with solids, and wideflow ranges (10:1)Disadvantages: High initial

cost and need for trainedpersonnel to handle routineO&M


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Parshall Flume

Consists of a convergingsection, a throat, and adiverging section

Self-cleaning and smallheadloss

Converts depth readings todischarge using a calibrationcurve

Less accurate (5~10%) Range: 10:1 ~ 75:1

Parshall Flumes


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Old style mechanical float for measuring depth of wastewater in Parshall flume.

Note that the flume was cast as part of the concrete wall.

Downstream of Parshall flume showing entrance to horizontal

flow grit chamber on right.


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A modern Parshall flume with prefabricated (white) insert to assure correct

dimensional relationships for accurate flow measurement.

Close up view of Parshall flume insert. Elevation of wastewater in

flume is measured ultrasonically.

Ultrasonic source

and transducer

to capture

reflected signal.


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Critical flow downstream of ultrasonic level measurement.

Backup manual wastewater depth scale which is also used to

calibrate the ultrasonic measurement.


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Although this Parshall flume is not being used for wastewater flow measurement, it was selected

to illustrate three features: (1) upstream stilling well and (2) downstream stilling well for depth

measurement and (3) the need to maintain a clear flow path through the flume for accurate

measurements.

Stilling wells

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Palmer-Bowlus Flume

Creates a change in the flow pattern by decreasing thewidth of the channel without changing its slope.Installed in a sewer at a manhole which causes the back-up

of the water in the channel. By measuring the upstreamdepth, the discharge is read from a calibration curve.Lower headloss than the Parshall flumeLess accurate (5~10%)


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Weirs (Rectangular, Cipolletti,Triangular, or V-Notch)

The head over the weir is measured by a float, hook gauge,or level sensor

Measure the flow in open channelsAccuracy: 5%;Range: 500:1

Advantages:relatively accurate,simple to install,and inexpensive

Disadvantages:large amounts ofheadloss and

settling of solidsupstream of theweir and moremaintenance

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Ultrasonic MeterMeasured based on the timerequired for an ultrasonicpulse to diagonally traverse apipe or channel against theliquid flow.

Clamp-on types measure flowthrough the pipe without anywetted parts, ensuring thatcorrosion and other effectsfrom the fluid will notdeteriorate the sensors.Accuracy: 1% for a flow

velocity ranging from 1 to106 ft/sec. Should be free ofparticles and air bubbles.

http://www.sensorsmag.com/articles/1097/flow1097/main.shtml


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Vortex Meter

The frequency at whichthe vortices are generatedis proportional to thevelocity of the liquidflow.Accuracy: 1% for a

flow range of 12 to 1.Headloss: two times the

velocity head

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Rotameters

Consist of glass tubecontaining a freely moving

float.

May be used for both gas andliquid flow measurement

Read or measured visuallyMay be applied for very low

flow rates, 0.1~140 gph forwater and 1~520 scfm for air.


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Selection Guide (1)

FlowMeter

RecommendedService

TurndownTypicalPressure

Loss

TypicalAccuracy

Requiredupstreampipe,

Effectsfrom

changingviscosity?

TurbineClean, viscous

liquids20 to 1 High

+/- 0.25%of rate

5 to 10 High

PositiveDisplacement

Clean, viscousliquids

10 to 1 High+/- 0.5% of

rateNone High

Electromagnetic(Mag-Meter)

Clean, dirty,viscous, conductiveliquids and slurries

40 to 1 None+/- 0.5% of

rate5 None

Variable Area(VA, Rota-meter)

Clean, dirty, viscousliquids

10 to 1 Medium+/- 1 to10% FS

None Medium

Thermal MassFlow (TMF)

Clean dirty viscousliquids some

slurries10 to 1 Low +/- 1% FS None None

Coriolis MassMeter

Clean, dirty. viscousliquids, someslurries

10 to 1 Low +/- 0.5% ofrate

None None

Orifice PlateClean, dirty, liquids

someslurries

4 to 1 Some+/- 2 to 4%

FS10 to 20 High

FS=full scale http://www.buygpi.com/selectionguide.aspx

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Selection Guide (2)

Flow

Meter

Recommended

ServiceTurndown

Typical

Pressure

Loss

Typical

Accuracy

Required

Upstream

pipe,

Effects

from

changing

viscosity?

Pitot tube Clean liquids 3 to 1 Very low+/- 3 to 5%

FS20 to 30 Low

Ultrasonic

(Doppler)

Dirty, viscous, liquids

and slurries10 to 1 None +/- 5% FS 5 to 30 None

Ultrasonic

(Transit Time)

Clean, viscous, liquids

some dirty liquids

(depending on brand)

40 to 1 None+/- 1 to 3%

FS10 None

Venturi

Some slurries but

clean, dirty liquids

with high viscosity

4 to 1 A little +/- 1% FS 5 to 18 High

Vortex Clean, dirty liquids 10 to 1 Medium+/- 1% of

rate10 to 20 Medium


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Flow SensorsSensor Range Accuracy Advantages DisadvantagesOrifice 3.5:1 2-4% of full span

Low cost

Extensive industrial practice

High pressure loss

Plugging with slurries

Venturi 3.5:1 1% of full span

Lower pressure loss than

orifice

Slurries do not plug

High cost

Line under 15 cm

Flow nozzle 3.5:1 2% full spanGood for slurry service

Intermediate pressure loss

Higher cost than orifice plate

Limited pipe sizes

Elbow meter 3:15-10% of full

spanLow pressure loss Very poor accuracy

Annubar

(Pitot tube)3:1

0.5-1.5% of full

span

Low pressure loss

Large pipe diameters

Poor performance with dirty or

sticky fluids

Turbine 20:10.25% of

measurement

Wide rangeability

Good accuracy

High cost

Strainer needed, especially for

slurries

Vortex

shedding10:1

1% of

measurement

Wide rangeability

Insensitive to variations in

density, temperature,

pressure, and viscosity

Expensive

Positive

displacement

10:1 or

greater

0.5% of

measurement

High reangeability

Good accuracy

High pressure drop

Damaged by flow surge or

solids

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Checklist for Design of

Flow-Measuring DeviceCharacteristics of the liquid (SS, density, temp., pressure, etc.)Expected flow range (max. and min.)Accuracy desiredAny constraints imposed by regulatory agenciesLocation of flow measurement device and piping system

(force main, sewer, manhole, channel, or treatment unit)

Atmosphere of installation (indoors, outdoors, corrosive, hot,cold, wet, dry, etc.)

Headloss constraintsType of secondary elements (level sensors, pressure sensors,

transmitters, and recorders) Space limitations and size of deviceCompatibility with other flow measurement devices if already

in operation at the existing portion of the treatment facilityEquipment manufacturers and equipment selection guide


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Design Example

Conditions 92-cm (36-inch) force main Max. flow: 1.321; min. flow: 0.152 m3/sec Measurement error: < 0.75% at all flows Headloss: < 15% of the meter readings at all flows Capable of measuring flows of solids bearing liquid Reasonable cost

Select a Venturi meter

Design equation

Use Bernoulli energy equation for two sections of pipewith the assumption that the headloss is negligible andthe elevations of the pipe centerline are the same.

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Governing Equations

Bernoullis equation

[Pressure head]+[Elevation head]+[Velocity head]

whereP = pressure, m; = density, kg/m3; z =

elevation, m; v = velocity (m/sec), and g = 9.8

m/sec2.

Continuity equationQ = v1 A1 = v2 A2

where A = Cross-sectional area.

0 0


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Design Example - continued

where Q = pipe flow, m3/sec;C1 = velocity, friction, or discharge coefficient

h = piezometric head difference, m;A1 = force main cross-sectional area, m

2;A2 = throat cross-sectional area, m

2; andD1 and D2 = diameter of the pipe and the throat, m.

Standard Venturi meterTube beta ratio (throat /force main ): 1/3~1/2K = 1.0062 (1/3 beta ratio), 1.0328 (1/2 beta ratio)C1 = 0.97~0.99; normally provided by the manufacturer

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Design Example - continued

Develop calibration equation:Assume C1 = 0.985

= 0.7489 h m3/sech = (Q/0.7489)2

At Qmax, h = 3.111 m; at Qmin, h = 0.041 m

Headloss calculations

K = 0.14 for angles of divergence of 5

hL/h = 0.147 < 0.15; thus acceptable


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Level Measurement

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Level Measurement Essential item in plant operations

Levels of all chemical storage tanks and silos, andthe pressure of water or compressed air lines - thatis, the water level in the distribution mains and theutility lines.

Liquid levels: a float, pressure elements, bubblersystems, or ultrasonic systems

Dry, powdery materials: ultrasonic systems,photocell systems, rotary paddle switches,diaphragm units, wire strain gauge systems, andload cells (measure the total weight).


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Miscellaneous FlowMeasurement Devices

Depth Measurement Need to measure the flow depth and sewer slope

and use Manning equation for flow estimation. Frequently used for interceptor flow estimation

Open Flow Nozzle Crude devices used to measure flow at the end of

freely discharging pipes.

Must have a section of pipe that has a length of atleast six times the diameter with a flat slopepreceding the discharge.

Examples: Kennison nozzle and the California pipe

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Level Measurement DevicesMagnetostrictive RF Transmitter Radar

UltrasonicMagnetic LevelGauge Magnetic

Switch

FloatSwitch

RFSwitch

Vibrating

ForkThermalDispersion

Seal Pothttp://www.sensorsmag.com/sensors/article/articleDetail.jsp?id=360729


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Float System

The float-operated transmitter- simple and reasonablyaccurate system

The installation is very timeconsuming and expensive dueto the need for a stilling welland a collection of wires,wheels, and tackles.

Requires a periodicmaintenance to assurefriction-free motion of thefloat and cable assembly.

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Pressure ElementsVery commonly used in watertreatment plantsA pressure transducer connected to

the pressure elements measures thewater pressure at the base of the tankand directly reads the liquid level.

Pressure element type levelmeasurers: the bourdon tube (hashelical and spiral units; suited for

high pressure measurement), bellowelement (for intermediate pressures),diaphragm element (for small rangein the low-pressure zone), andmanometer (limited to pilot studiesor temporary use).


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Bubble Tube System

Has a tube placed inside a tankwhich runs from the top and opens3 in. from the bottom. During theoperation, compressed air issupplied to the tube via a regulatoror a purge rotameter.Measure the back pressure of the

hydrostatic head. Widely used for open tanks Advantages: simple design, easy

accessibility and little concern overthe corrosion of the pressuresensing device, and the ability to beinstalled at the bottom of the tank

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Ultrasonic Level Detector

Used to monitor either the waterlevel in a tank or dry material storedin a storage bin open to theatmosphere.

Measured by means of an acousticpulse; the ultrasonic transmitter andreceiver units are located above themaximum level of the object.

The time elapsed between pulsegeneration and the detection of thereflected pulse energy is a function ofthe speed of sound in air. Needs atemperature correction factor.


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Valves

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Valve Selection Purpose: Regulate the flow of water from

reservoirs, tanks, or channels.

Primary functions: shut-off, throttle, preventionof backflow, or a combination of thesefunctions

Considerations: type of fluid or gas to beregulated, temperature, flow range, pressure of

the system, valve function, valve location, typeof valve operator, and reliability and cost of thevalve.


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Type of Fluid or Gas

Type 18-8 stainless steel: for corrosive liquid orgas

Type 316 stainless steel and Teflon seats: for ozonegas lines

No internal recess in the valve: for a chemical slurry

If abrasive matter is present in the liquid, the fluidpassage must be composed of materials that are

resistant to this type of erosion.

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TemperatureImportant when valves are used in conjunction with

auxiliary equipment such as heating boilers andcertain types of chemical feed system - that handleexothermic chemicals such as caustic soda andsulfuric acid.

Ordinary valves used in the water treatment processshould not be used at operating temperatures above150F due to thermal distortion, unless specialmetal parts are specified.


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Flow Range

Important when selecting throttling valves.

Most throttling valves have a limited range.

Not important for simple shut-off.

If the water velocity exceeds 35 ft/sec based onthe valve port area, most valves are unsuitable

for such service and the engineer must thereforespecify special instructions for valveconstruction.

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Pressure Should know the max. differential pressure across

the valve, and normal and extreme line pressure.

Valve Function Isolation of a line, drainage or a tank, prevention

of backflow, reduction in pressure, or flowmodulation.

Valve Location In a valve vault, a pipe gallery, in the wall at the

entrance of a tank, at the exit of a pipeline, buried inthe ground, or submerged in the water.


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Valve Operator Manual or power For manual valve, the type of

operator (i.e., a wheel or a square nut with key) andthe orientation of both the operator and systemsupport must be specified.

Power operators are energized by means ofelectricity, compressed air, water or oil.

Reliability and Cost Compare the relative costs of the various sizes

and types of valve for each application. List valve cost, projected maintenance costs and

the cost of replacing equipment when necessary.

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Types of Valve (1)

Slide valve: a sliding disktravelling perpendicular to theflow direction - e.g., gate valve


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Gate Valve

Best Suited Control: Quick Opening

Recommended Uses:

1. Fully open/closed, non-throttling

2. Infrequent operation

3. Minimal fluid trapping in line

Applications: Oil, gas, air, slurries, heavy liquids, steam,

noncondensing gases, and corrosive liquids

Advantages: Disadvantages:

1. High capacity 1. Poor control2. Tight shutoff 2. Cavitate at low pressure drops

3. Low cost 3. Cannot be used for throttling

4. Little resistance to flow

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Types of Valve (2)

Rotary valve: a plug or disk moving in a rotaryfashion - e.g., butterfly, ball, plug, and conevalves


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Butterfly Valve

Best Suited Control: Linear, Equal percentage

Recommended Uses:1. Fully open/closed or throttling services2. Frequent operation3. Minimal fluid trapping in line

Applications: Liquids, gases, slurries, liquids withsuspended solids

Advantages: Disadvantages:1. Low cost and maint. 1. High torque required for2. High capacity control3. Good flow control 2. Prone to cavitation at lower4. Low pressure drop flows

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Ball Valve

Best Suited Control: Quick opening, linear

Recommended Uses:

1. Fully open/closed, limited-throttling

2. Higher temperature fluids

Applications: Most liquids, high temperatures, slurries


1. Low cost 1. Poor throttling characteristics

2. High capacity 2. Prone to cavitation3. Low leakage and maintenance

4. Tight sealing with low torque


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Types of Valve (3)

Swing valve: a swingcheck valvepreventing reverseflow - a combinationof rotary and glovevalves

Globe valve: a plug or diskmoving parallel to the flowdirection - e.g., home plumbingfixtures.

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Glove Valve

Best Suited Control: Linear and equal percentage

Recommended Uses:

1. Throttling service/flow regulation

2. Frequent operation

Applications: Liquids, vapors, gases, corrosive

substances, slurries


1. Efficient throttling 1. High pressure drop2. Accurate flow control 2. More expensive than

3. Available in multiple other valves

ports


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Types of Valve (4)

Multijet (sleeve) valve:inner and outer pipescovered with a multitudeof small orifices - usedexclusively to reducehigh pressure and tocontrol flow rate withoutcausing cavitation.

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Valve Selection Select the proper type of valve, followed by sizing

Evaluate the pressure drop characteristics and theworking range of the valves

Selection Criteria

Rangeability: the ratio between the max. andmin. controllable flow rates.

Turn-down: a ratio of the normal max. flow ratevs. the min. controllable flow rate.

For water pressure control, the ball and butterflyvalves should be selected for ordinary caseswhere there is a normal pressure drop of at least15% but less than 30%.


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Valve Selection - continued

If a higher pressure drop such as 50% isexpected, a valve with linear characteristics(plug or multijet valve) should be specified.

For the control of liquid level, a valve with linearcharacteristics such as a plug valve, is mostappropriate.

Equal percentage valves are most appropriate for

a fast acting process, in situations requiring highrangeability, if the dynamics of the system arenot well known, and in the case of heatexchangers.

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Valve Sizing (1)

STEP #1: Define the system

The system is pumping water from one tank toanother through a piping system with a totalpressure drop of 150 psi. The fluid is water at70F. Design (maximum) flowrate of 150 gpm,operating flowrate of 110 gpm, and a minimumflowrate of 25 gpm. The pipe diameter is 3inches. At 70F, water has a specific gravity of 1.0.

Key Variables: Total pressure drop, design flow,operating flow, minimum flow, pipe diameter, andspecific gravity

http://www.cheresources.com/valvezz.shtml


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Valve Sizing (2)

STEP #2: Define a maximum allowable pressuredrop for the valve

Note the trade off: larger pressure drops increasethe pumping cost (operating) and smaller pressuredrops increase the valve cost because a larger valveis required (capital cost).

The usual rule of thumb is that a valve should bedesigned to use 10~15% of the total pressure dropor 10 psi, whichever is greater. For the system,

10% of the total pressure drop is 15 psi which isused as our allowable pressure drop when the valveis wide open.

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Valve Sizing (3)

STEP #3: Calculate the valve characteristic

For the system,

Dont go to the valve charts or characteristic curves

and select a valve yet. Proceed to Step #4!

where Q = design flowrate (gpm);G = specific gravity; andP = allowable pressure drop

across wide open valve.


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Valve Sizing (4)

STEP #4: Preliminary valve selection

Don't make the mistake of trying to match a valvewith your calculated Cv value. The Cv valueshould be used as a guide in the valve selection, nota hard and fast rule.

Some other considerations are: Never use a valve that is less than half the pipe size

Avoid using the lower 10% and upper 20% of the valve

stroke. The valve is much easier to control in the 10-

80% stroke range.

Before a valve can be selected, decide what type ofvalve will be used. For the case, an equalpercentage, globe valve will be used.

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Valve Sizing (5)

STEP #4: Preliminary valve selection - continued

The valve chart supplied by the manufacturer.


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Valve Sizing (6)

STEP #4: Preliminary valve selection continued

The 2 inch valve appears to work well for the Cvvalue at about 80~85% of the stroke range.

If 1 inch valve is used, two consequences wouldbe experienced: the pressure drop would be a littlehigher than 15 psi at the design (max) flow and thevalve would be difficult to control at maximumflow. Also, there would be no room for error with

this valve, but the valve chosen will allow for flowsurges beyond the 150 gpm range with severeheadaches!

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Valve Sizing (7)

STEP #5: Check the Cv and stroke percentage at theminimum flow

Judgments plays role in many cases.

Select the valve for the range that the valve is operated mostoften.

A Cv of 6.5 that corresponds to a stroke percentage ofaround 35-40% is certainly acceptable.

Although the pressure drop across the valve will be lower atsmaller flowrates, using the maximum value gives us a"worst case" scenario.

If the Cv at the minimum flow would have been around 1.5,there would not really be a problem because the valve has aCv of 1.66 at 10% stroke and since the maximum pressuredrop is used, the estimate is conservative. Essentially, atlower pressure drops, Cv would only increase which in thiscase would be advantageous.


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Valve Sizing (8)

STEP #6: Check the gain across applicable flowrates

Gain is defined as:

The difference between these values should be less than 50%

of the higher value.

0.5 (3.3) = 1.65 > 3.3-2.2 = 1.1 No problem in controlling

the valve.

The gain should never be less than 0.50.

Flow (gpm) Cv Stroke (%) flow (gpm) Stroke (%) Gain

25 6.5 35 110-25 = 85

150-110 = 40

73-35 = 38

85-73 = 12

2.2

3.3

110 28 73

150 39 85

Gain =flow

stroke or travel

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Valve Control

Equal Percentage

Equal increments of valve travel produce

an equal percentage in flow change

Linear

Valve travel is directly proportional to the

valve stoke

Quick Opening

Large increase in flow with a small

change in valve stroke


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Equal Percentagea. Used in processes where large changes

in pressure drop are expectedb. Used in processes where a small percentageof the total pressure drop is permitted by the valve

c. Used in temperature and pressure control loops

Lineara. Used in liquid level or flow loopsb. Used in systems where the pressure drop across the

valve is expected to remain fairly constant(i.e., steady state systems)

Quick Openinga. Used for frequent on-off service

b. Used for processes where "instantly" large flow is

needed (i.e., safety systems or cooling water systems)

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Control Valve Flow Characteristics


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Control Valve Flow Characteristics

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Inherent Flow Characteristics

Linear - flow capacity increases linearly withvalve travel.

Equal percentage - flow capacity increasesexponentially with valve trim travel. Equalincrements of valve travel produce equalpercentage changes in the existing Cv.

A modified parabolic characteristic isapproximately midway between linear and equal-percentage characteristics. It provides finethrottling at low flow capacity and approximatelylinear characteristics at higher flow capacity.

Quick opening provides large changes in flow forvery small changes in lift. It usually has too high avalve gain for use in modulating control. So it islimited to on-off service, such as sequentialoperation in either batch or semi-continuousprocesses.


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Other Valves

Check Valves

Restrict the flow to one direction.

Relief Valves

Regulate the operating pressure ofincompressible flow

Safety Valves

Release excess pressure in gasesor compressible fluids

4081WWT Flow Measurement

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