Flow Measurement 1

2 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 RawPrimarySecondaryPrimaryReturnThickenedMixedProcess Metering deviceWWeffluenteffluentsludgesludgesludgeliquorwater For open channels Head/area Flumexxxx Weirxxx Other Magnetic (insert type)x For closed conduits Head/pressure Flow tubex a x a xx a x a x a,b x Orificex Pitot tubex Rotameterx Venturix a x a xx a x a x a x Moving fluid effects Magnetic (tube type)_xxxxxxx Ultrasonic (doppler)xxxx c Ultrasonic (transmission)xxx Vortex sheddingxxx Positive displacement Propellerx Turbinexx 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  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

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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. 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 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 1High +/- 0.25% of rate 5 to 10High Positive Displacement Clean, viscous liquids 10 to 1High +/- 0.5% of rate NoneHigh Electromagnetic (Mag-Meter) Clean, dirty, viscous, conductive liquids and slurries 40 to 1None +/- 0.5% of rate 5None Variable Area (VA, Rota-meter) Clean, dirty, viscous liquids 10 to 1Medium +/- 1 to 10% FS NoneMedium Thermal Mass Flow (TMF) Clean dirty viscous liquids some slurries 10 to 1Low+/- 1% FSNone Coriolis Mass Meter Clean, dirty. viscous liquids, some slurries 10 to 1Low +/- 0.5% of rate None Orifice Plate Clean, dirty, liquids some slurries 4 to 1Some +/- 2 to 4% FS 10 to 20High 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 tubeClean liquids3 to 1Very low +/- 3 to 5% FS 20 to 30Low Ultrasonic (Doppler) Dirty, viscous, liquids and slurries 10 to 1None+/- 5% FS5 to 30None Ultrasonic (Transit Time) Clean, viscous, liquids some dirty liquids (depending on brand) 40 to 1None +/- 1 to 3% FS 10None Venturi Some slurries but clean, dirty liquids with high viscosity 4 to 1A little+/- 1% FS5 to 18High VortexClean, dirty liquids10 to 1Medium +/- 1% of rate 10 to 20Medium

21 Flow Sensors SensorRangeAccuracyAdvantagesDisadvantages Orifice3.5:12-4% of full span Low cost Extensive industrial practice High pressure loss Plugging with slurries Venturi3.5:11% of full span Lower pressure loss than orifice Slurries do not plug High cost Line under 15 cm Flow nozzle3.5:12% full span Good for slurry service Intermediate pressure loss Higher cost than orifice plate Limited pipe sizes Elbow meter3:1 5-10% of full span Low pressure lossVery poor accuracy Annubar (Pitot tube) 3: % of full span Low pressure loss Large pipe diameters Poor performance with dirty or sticky fluids Turbine20:1 0.25% of measurement Wide rangeability Good accuracy High cost Strainer needed, especially for slurries Vortex shedding 10:1 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

22 Checklist for Design of Flow-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 Example Conditions  92-cm (36-inch) force main  Max. flow: 1.321; min. flow: m 3 /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 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] whereP = pressure, m; ρ = density, kg/m 3 ; z = elevation, m; v = velocity (m/sec), and g = 9.8 m/sec 2. Continuity equation Q = v 1 A 1 = v 2 A 2 where A = cross-sectional area. 0 0

25 Design Example - continued whereQ = pipe flow, m 3 /sec; C 1 = velocity, friction, or discharge coefficient h = piezometric head difference, m; A 1 = force main cross-sectional area, m 2 ; A 2 = throat cross-sectional area, m 2 ; and D 1 and D 2 = diameter of the pipe and the throat, m. Standard Venturi meter Tube beta ratio (throat  /force main  ): 1/3~1/2 K = (1/3 beta ratio), (1/2 beta ratio) C 1 = 0.97~0.99; normally provided by the manufacturer

26 Design Example - continued Develop calibration equation: Assume C 1 = =  h m 3 /sec h = (Q/0.7489) 2 At Q max, h = m; at Q min, h = m Headloss calculations K = 0.14 for angles of divergence of 5° h L /h = < 0.15; thus acceptable