1. Flow Measurement 1

2. Flow Measurement Objective
To 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
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

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, weirs, ect.
Liquid chemical flow
Measured 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

5. Flow Metering Devices in Wastewater Treatment Facilities
Raw Primary Secondary Primary Return Thickened Mixed Process Metering device WW effluent effluent sludge sludge sludge liquor water For open channels Head/area Flume x x x x Weir x x x Other Magnetic (insert type) x For closed conduits Head/pressure Flow tube xa xa x xa xa xa,b x Orifice x Pitot tube x Rotameter x Venturi xa xa x xa xa xa x Moving fluid effects Magnetic (tube type)_ x x x x x x x Ultrasonic (doppler) x x x xc Ultrasonic (transmission) x x x Vortex shedding x x x Positive displacement Propeller x Turbine x x a Flushing or diaphragm sealed connections recommended b Use with in-line reciprocating pumps not recommended c Solids content < 4% 5

6. Venturi Type Flow Meter
Measures 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
Takes 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. 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.

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 only
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 Recommended Service Turndown
Typical Pressure Loss
Typical Accuracy
Required upstream pipe, Ф
Effects from changing viscosity?
Turbine
Clean, viscous liquids
20 to 1
High
+/- 0.25% of rate
5 to 10
Positive Displacement
10 to 1
+/- 0.5% of rate
None
Electromagnetic (Mag-Meter)
Clean, dirty, viscous, conductive liquids and slurries
40 to 1 5
Variable Area (VA, Rota-meter)
Clean, dirty, viscous liquids
Medium
+/- 1 to 10% FS
Thermal Mass Flow (TMF)
Clean dirty viscous liquids some slurries
Low
+/- 1% FS
Coriolis Mass Meter
Clean, dirty. viscous liquids, some slurries
Orifice Plate
Clean, dirty, liquids some slurries
4 to 1
Some
+/- 2 to 4% FS
10 to 20
FS=full scale

20. Selection Guide (2) Flow Meter Recommended Service Turndown
Typical Pressure Loss
Typical Accuracy
Required Upstream pipe, 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
Ultrasonic (Transit Time)
Clean, viscous, liquids some dirty liquids (depending on brand)
40  to 1
+/- 1 to 3% FS 10
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
Medium
+/- 1% of rate
10 to 20

21. Positive displacement
Flow Sensors
Sensor
Range
Accuracy
Advantages
Disadvantages
Orifice
3.5:1
2-4% of full span
Low cost
Extensive industrial practice
High pressure loss
Plugging with slurries
Venturi
1% of full span
Lower pressure loss than orifice
Slurries do not plug
High cost
Line under 15 cm
Flow nozzle
2% full span
Good for slurry service
Intermediate pressure loss
Higher cost than orifice plate
Limited pipe sizes
Elbow meter
3:1
5-10% of full span
Low pressure loss
Very poor accuracy
Annubar
(Pitot tube)
% of full span
Large pipe diameters
Poor performance with dirty or sticky fluids
Turbine
20:1
0.25% of measurement
Wide rangeability
Good accuracy
Strainer needed, especially for slurries
Vortex shedding
10:1
1% of measurement
Insensitive to variations in density, temperature, pressure, and viscosity
Expensive
Positive displacement
10:1 or greater
0.5% of measurement
High reangeability
High pressure drop
Damaged by flow surge or solids