Flow Measurement

2

Flow Measurement

ObjectiveTo determine chemical dosage, air supply into the aeration basins, sludge volume to return into the biological reactors, to provide daily flow records required by regulatory agencies, and to evaluate infiltration/inflow during wet weather

LocationsWithin an interceptor or manholeAt the head of the plantDownstream of bar screen, grit channel, or primary

sedimentationIn the force main of pumping stationBefore the outfall

3

Flow Measurement - continued

Basic types of measurement· Differential pressure producers· Direct discharge measurement· Positive volume displacement measurement· Flow velocity-area measurement

Flow meters Venturi type meter, orifice meter, propeller type meter,

magnetic flow meter, ultrasonic flow meter, vortex meter, rotameter (variable-area meter), flumes, and weirs

Liquid chemical flowMeasured by positive displacement pumps (or

rotameters)

4

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

Flow Metering Devices in Wastewater Treatment Facilities

Raw Primary SecondaryPrimary Return ThickenedMixedProcess

Metering device WW effluent effluent sludge sludge sludge liquorwater

For open channelsHead/area Flume x x x

x Weir x x

xOther Magnetic (insert type)

xFor closed conduitsHead/pressure Flow tube xa xa x xa xa xa,b x

x Orifice

x Pitot tube

x Rotameter

x Venturi xa xa x xa xa xa xMoving fluid effects Magnetic (tube type)_ x x x x x x

x Ultrasonic (doppler) x x x xc

Ultrasonic (transmission) x xx

Vortex shedding x xx

Positive displacement Propeller

x Turbine x

x

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

5

6

Venturi Type Flow Meter· Measure differential pressure· Consists of a converging section, a throat, and a

diverging recovery section· The difference in two heads is analyzed by electrical or

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

maximum velocity

7

Flow Nozzle Meter· Measure differential pressure· A Venturi meter without the diverging recovery section· Less expensive than Venturi meter but higher headloss· Accuracy: < ±1%; range: 4:1

8

Orifice Meter· Measure differential pressure· Easy to install and fabricate· Advantages: 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)

9

Electromagnetic Meter· Faraday’s law: a voltage

produced by passing a conductor through a magnetic field is proportional to the velocity of the conductor (wastewater)· Advantages: good accuracy

(±1~2%), capable of measuring large range of flows (10:1), no headloss, and unaffected by temperature, conductivity, viscosity, turbulance, and suspended solids· Disadvantages: high initial cost

and need for trained personnel to handle routine O&M

10

11

Turbine Meter· Use a rotating element

(turbine) · A wide range of fluid

applications covering from water to oils, solvents to acids· Limited to pipes running

full, under pressure, and liquids low in suspended solids· Excellent accuracy

(±0.25%) and a good range of flows (10:1)

12

Acoustic Meter· Use sound waves to measure

the flow rates· Sonic meter or ultrasonic

meter depending on whether the sound waves are in or above audible frequency range· Determine the liquid levels,

area, and actual velocity· Advantages: low headloss,

excellent accuracy (2~3%), usable in any pipe size, no fouling with solids, and wide flow ranges (10:1)· Disadvantages: High initial

cost and need for trained personnel to handle routine O&M

13

Parshall Flume· Consists of a converging

section, a throat, and a diverging section· Self-cleaning and small

headloss· Converts depth readings to

discharge using a calibration curve

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

14

Palmer-Bowlus Flume

· Creates a change in the flow pattern by decreasing the width 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 upstream depth, the discharge is read from a calibration curve.· Lower headloss than the Parshall flume· Less accurate (±5~10%)

15

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 channels

Accuracy: ±5%; Range: 500:1· Advantages:

relatively accurate, simple to install, and inexpensive· Disadvantages:

large amounts of headloss and settling of solids upstream of the weir and more maintenance

16

Ultrasonic Meter· Measured based on the time

required for an ultrasonic pulse to diagonally traverse a pipe or channel against the liquid flow.· Clamp-on types measure flow

through the pipe without any wetted parts, ensuring that corrosion and other effects from the fluid will not deteriorate the sensors. · Accuracy: ± 1% for a flow

velocity ranging from 1 to 106 ft/sec. Should be free of particles and air bubbles.

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

17

Vortex Meter

· The frequency at which the vortices are generated is proportional to the velocity of the liquid flow.

·Accuracy: ± 1% for a flow range of 12 to 1.·Headloss: two times the

velocity head

18

Rotameters· Consist of glass tube

containing a freely moving float.·May be used for both gas and

liquid flow measurement

· Read or measured visually

·May be applied for very low flow rates, 0.1~140 gph for water and 1~520 scfm for air.

19

Selection Guide (1)

Flow Meter

RecommendedService Turndown

TypicalPressure

Loss

TypicalAccuracy

Required upstreampipe, Ф

Effects from

changing viscosity?

Turbine Clean, viscous liquids 20 to 1 High +/- 0.25%

of rate 5 to 10 High

PositiveDisplacement

Clean, viscous liquids 10 to 1 High +/- 0.5% of

rate None High

Electromagnetic(Mag-Meter)

Clean, dirty, viscous, conductive liquids and slurries

40 to 1 None +/- 0.5% of rate 5 None

Variable Area (VA, Rota-

meter)

Clean, dirty, viscous liquids 10 to 1 Medium +/- 1 to

10% FS None Medium

Thermal Mass Flow (TMF)

Clean dirty viscous liquids some

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

Coriolis Mass Meter

Clean, dirty. viscous liquids, some

slurries10 to 1 Low +/- 0.5% of

rate None None

Orifice PlateClean, dirty, liquids

someslurries

4 to 1 Some +/- 2 to 4% FS 10 to 20 High

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

20

Selection Guide (2)

Flow Meter

RecommendedService Turndown

TypicalPressure

Loss

TypicalAccuracy

Required Upstreampipe, Ф

Effects from

changing viscosity?

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

FS 20 to 30 Low

Ultrasonic(Doppler)

Dirty, viscous, liquids and slurries 10 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

rate 10 to 20 Medium

21

Flow SensorsSensor Range Accuracy Advantages Disadvantages

Orifice 3.5:1 2-4% of full spanLow costExtensive industrial practice

High pressure lossPlugging with slurries

Venturi 3.5:1 1% of full spanLower pressure loss than orifice

Slurries do not plug

High costLine under 15 cm

Flow nozzle 3.5:1 2% full spanGood for slurry serviceIntermediate pressure loss

Higher cost than orifice plateLimited pipe sizes

Elbow meter 3:15-10% of full

spanLow pressure loss Very poor accuracy

Annubar(Pitot tube)

3:10.5-1.5% of full

spanLow pressure lossLarge pipe diameters

Poor performance with dirty or sticky fluids

Turbine 20:10.25% of

measurementWide rangeabilityGood accuracy

High costStrainer needed, especially for slurries

Vortex shedding

10:11% of

measurement

Wide rangeabilityInsensitive to variations in density, temperature, pressure, and viscosity

Expensive

Positive displacement

10:1 or greater

0.5% of measurement

High reangeabilityGood accuracy

High pressure dropDamaged by flow surge or solids

22

Checklist for Design ofFlow-Measuring Device

· Characteristics of the liquid (SS, density, temp., pressure, etc.)· Expected flow range (max. and min.)· Accuracy desired· Any constraints imposed by regulatory agencies· Location 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 constraints· Type of secondary elements (level sensors, pressure sensors,

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

in operation at the existing portion of the treatment facility· Equipment manufacturers and equipment selection guide

23

Design ExampleConditions 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 costSelect a Venturi meterDesign equation

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

24

Governing Equations Bernoulli’s equation

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

where P = pressure, m; ρ = density, kg/m3; z = elevation, m; v = velocity (m/sec), and g = 9.8 m/sec2.

Continuity equation

Q = v1 A1 = v2 A2

where A = Cross-sectional area.

0 0

25

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, m2;A2 = throat cross-sectional area, m2; 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

26

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 calculationsK = 0.14 for angles of divergence of 5°

hL/h = 0.147 < 0.15; thus acceptable

Level Measurement

28

Level Measurement· Essential item in plant operations· Levels of all chemical storage tanks and silos, and

the pressure of water or compressed air lines - that is, the water level in the distribution mains and the utility lines.

· Liquid levels: a float, pressure elements, bubbler systems, or ultrasonic systems

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

29

Miscellaneous Flow Measurement 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 at

least six times the diameter with a flat slope preceding the discharge.

Examples: Kennison nozzle and the California pipe

30

Level Measurement DevicesMagnetostrictive

RF Transmitter RadarUltrasonic

Magnetic LevelGauge Magnetic

Switch

FloatSwitch

RFSwitch

VibratingForkThermalDispersion

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

31

Float System· The float-operated transmitter

- simple and reasonably accurate system· The installation is very time

consuming and expensive due to the need for a stilling well and a collection of wires, wheels, and tackles.· Requires a periodic

maintenance to assure friction-free motion of the float and cable assembly.

32

Pressure Elements· Very commonly used in water

treatment plants· A pressure transducer connected to

the pressure elements measures the water pressure at the base of the tank and directly reads the liquid level.

· Pressure element type level measurers: the bourdon tube (has helical and spiral units; suited for high pressure measurement), bellow element (for intermediate pressures), diaphragm element (for small range in the low-pressure zone), and manometer (limited to pilot studies or temporary use).

33

Bubble Tube System· Has a tube placed inside a tank

which runs from the top and opens 3 in. from the bottom. During the operation, compressed air is supplied to the tube via a regulator or a purge rotameter.· Measure the back pressure of the

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

accessibility and little concern over the corrosion of the pressure sensing device, and the ability to be installed at the bottom of the tank

34

Ultrasonic Level Detector· Used to monitor either the water level

in a tank or dry material stored in a storage bin open to the atmosphere.· Measured by means of an acoustic

pulse; the ultrasonic transmitter and receiver units are located above the maximum level of the object. The time elapsed between pulse generation and the detection of the reflected pulse energy is a function of the speed of sound in air. Needs a temperature correction factor.

Valves

36

Valve Selection· Purpose: Regulate the flow of water from

reservoirs, tanks, or channels.· Primary functions: shut-off, throttle, prevention

of backflow, or a combination of these functions

· Considerations: type of fluid or gas to be regulated, temperature, flow range, pressure of the system, valve function, valve location, type of valve operator, and reliability and cost of the valve.

37

Type of Fluid or Gas

· Type 18-8 stainless steel: for corrosive liquid or gas

· Type 316 stainless steel and Teflon seats: for ozone gas lines

· No internal recess in the valve: for a chemical slurry

· If abrasive matter is present in the liquid, the fluid passage must be composed of materials that are resistant to this type of erosion.

38

Temperature· Important when valves are used in conjunction with

auxiliary equipment such as heating boilers and certain types of chemical feed system - that handle exothermic chemicals such as caustic soda and sulfuric acid.

· Ordinary valves used in the water treatment process should not be used at operating temperatures above 150°F due to thermal distortion, unless special metal parts are specified.

39

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 on the valve port area, most valves are unsuitable for such service and the engineer must therefore

specify special instructions for valve construction.

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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 flow modulation.

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 in the ground, or submerged in the water.

41

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

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

Power operators are energized by means of electricity, 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 disk travelling perpendicular to the flow direction - e.g., gate valve

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Gate Valve

Best Suited Control:  Quick Opening

Recommended Uses:1.  Fully open/closed, non-throttling2.  Infrequent operation3.  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 drops3.  Low cost    3.   Cannot be used for throttling4.  Little resistance to flow

44

Types of Valve (2)

· Rotary valve: a plug or disk moving in a rotary fashion - e.g., butterfly, ball, plug, and cone valves

45

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 with suspended solids

Advantages:   Disadvantages:1.  Low cost and maint.  1.  High torque required for 2.  High capacity control 3.  Good flow control 2. Prone to cavitation at lower 4.  Low pressure drop flows

46

Ball Valve

Best Suited Control: Quick opening, linear

Recommended Uses:1.  Fully open/closed, limited-throttling2.  Higher temperature fluids

Applications: Most liquids, high temperatures, slurries

Advantages:     Disadvantages:1.  Low cost 1.  Poor throttling characteristics2.  High capacity 2.   Prone to cavitation3.  Low leakage and maintenance4.  Tight sealing with low torque 

47

Types of Valve (3)

· Swing valve: a swing check valve preventing reverse flow - a combination of rotary and glove valves

· Globe valve: a plug or disk moving parallel to the flow direction - e.g., home plumbing fixtures.

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Glove Valve

Best Suited Control:  Linear and equal percentage

Recommended Uses:1.  Throttling service/flow regulation2.  Frequent operation

Applications:  Liquids, vapors, gases, corrosive substances, slurries

Advantages:             Disadvantages:1.  Efficient throttling       1.  High pressure drop2.  Accurate flow control  2.  More expensive than 3.  Available in multiple other valves ports

49

Types of Valve (4)

· Multijet (sleeve) valve: inner and outer pipes covered with a multitude of small orifices - used exclusively to reduce high pressure and to control flow rate without causing cavitation.

50

Valve Selection Select the proper type of valve, followed by sizing Evaluate the pressure drop characteristics and the

working range of the valvesSelection Criteria· Rangeability: the ratio between the max. and

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

vs. the min. controllable flow rate.· For water pressure control, the ball and butterfly

valves should be selected for ordinary cases where there is a normal pressure drop of at

least 15% but less than 30%.

51

Valve Selection - continued

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

· For the control of liquid level, a valve with linear characteristics such as a plug valve, is most appropriate.

· Equal percentage valves are most appropriate for a fast acting process, in situations requiring high rangeability, if the dynamics of the system are not well known, and in the case of heat exchangers.

52

Valve Sizing (1)

STEP #1: Define the systemThe system is pumping water from one tank to another through a piping system with a total pressure drop of 150 psi.  The fluid is water at 70ºF.   Design (maximum) flowrate of 150 gpm, operating flowrate of 110 gpm, and a minimum flowrate of 25 gpm.  The pipe diameter is 3 inches.  At 70ºF, water has a specific gravity of 1.0.Key Variables:  Total pressure drop, design flow, operating flow, minimum flow, pipe diameter, and specific gravity

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

53

Valve Sizing (2)

STEP #2:  Define a maximum allowable pressure drop for the valve

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

The usual rule of thumb is that a valve should be designed to use 10~15% of the total pressure drop or 10 psi, whichever is greater.  For the system, 10% of the total pressure drop is 15 psi which is used as our allowable pressure drop when the valve is wide open.

54

Valve Sizing (3)

STEP #3:  Calculate the valve characteristic For the system,

Don’t 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; and ΔP = allowable pressure drop across wide open valve.

55

Valve Sizing (4)

STEP #4:  Preliminary valve selection Don't make the mistake of trying to match a valve

with your calculated Cv value.   The Cv value should be used as a guide in the valve selection, not a 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 of valve will be used. For the case, an equal percentage, globe valve will be used. 

56

Valve Sizing (5)

STEP #4:  Preliminary valve selection - continued

The valve chart supplied by the manufacturer.

57

Valve Sizing (6)

STEP #4:  Preliminary valve selection – continued

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

If 1½ inch valve is used, two consequences would be experienced:  the pressure drop would be a little higher than 15 psi at the design (max) flow and the valve would be difficult to control at maximum flow.  Also, there would be no room for error with this valve, but the valve chosen will allow for flow surges beyond the 150 gpm range with severe headaches!   

58

Valve Sizing (7)STEP #5:  Check the Cv and stroke percentage at the

minimum flow Judgments plays role in many cases.  Select the valve for the range that the valve is operated most

often.   A Cv of 6.5 that corresponds to a stroke percentage of

around 35-40% is certainly acceptable.  Although the pressure drop across the valve will be lower at

smaller 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 a Cv of 1.66 at 10% stroke and since the maximum pressure drop is used, the estimate is conservative.   Essentially, at lower pressure drops, Cv would only increase which in this case would be advantageous.

59

Valve Sizing (8)

STEP #6:  Check the gain across applicable flowratesGain 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 (%) Δflow (gpm) ΔStroke (%) Gain

25 6.5 35 110-25 = 85

150-110 = 40

73-35 = 38

85-73 = 12

2.2

3.3110 28 73150 39 85

Gain =Δflow

Δstroke or travel

60

Valve Control

Equal PercentageEqual increments of valve travel produce an equal percentage in flow change

LinearValve travel is directly proportional to the valve stoke

Quick OpeningLarge increase in flow with a small change in valve stroke

61

Equal Percentagea. Used in processes where large changes

in pressure drop are expectedb. Used in processes where a small percentage

of the total pressure drop is permitted by the valvec. 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 serviceb.  Used for processes where "instantly" large flow is needed (i.e., safety systems or cooling water systems)

62

Control Valve Flow Characteristics

63

Control Valve Flow Characteristics

64

Inherent Flow Characteristics Linear - flow capacity increases linearly with

valve travel. Equal percentage - flow capacity increases

exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv.

A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.

Quick opening provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.

65

Other Valves

Check ValvesRestrict the flow to one direction.

Relief ValvesRegulate the operating pressure of

incompressible flowSafety Valves

Release excess pressure in gasesor compressible fluids