Flow Measurement 1

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

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)

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 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

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

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

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)

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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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)

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

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

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%)

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

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

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

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.

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 http://www.buygpi.com/selectionguide.aspx

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

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) 0.5-1.5% 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