Flow and Level Monitoring

is screened for treatability as follows. The stream is pre-treated to remove heavy metals and SS, and pH is ad-justed. It is then fed to a batch-activated sludge reactor,and primed with biomass from the plant treatment fa-cility. If the wastewater degrades quickly, as it should,it can be fed into the plants main flow. If it does not,the choices are in-plant pretreatments, PAC addition tothebioreactor,orgranularactuatedcarbon(GAC)treatment of the effluent.The problem associated with combining two streamsthat require different technologies is that the cost of treat-ing the combined stream is almost always more than in-dividual treatment of the separate streams. This is becausethe capital cost of most treatment operations is propor-tional to the total flow of the wastewater, and the oper-ating cost for treatment increases with a decreasing con-centration for a given mass of contaminant.Thus, if two waste streams use the same treatment, com-bining them improves the economics of scale for capitalinvestment and similar operating costs. In contrast, if twotreatmentoperationsarerequired,combiningthetwostreams increases capital costs for both treatment opera-tions. In addition, if the streams are combined before thetreatment, both treatments have lower contaminant con-centrations for the same net contaminant mass, resultingin higher operating costs per lb of contaminant removed(McLaughlin et al. 1992).Negib HarfoucheReferenceMcLaughlin, L.A., H.S. McLaughlin, and K.A. Groff. 1992. Develop aneffective wastewater treatment strategy. Chem. Eng. Prog. (Septem-ber).1999 CRC Press LLC7.6FLOW AND LEVEL MONITORINGThecontrolandmonitoringofflowsandlevelsinthewastewater treatment industry involve the measurement ofwater,biologicalsludge,solidandliquidadditives,andreagent flows. This section discusses methods of flow de-tection followed by a summary of wastewater-related leveldetection techniques.Flow Sensors for the WastewaterIndustryFlow detection applications in the wastewater treatmentindustry include the measurement of large flows in par-tially filled pipes using weirs, flumes, or ultrasonic sensors.Whenwaterisflowinginregularpipelines,magneticflowmeters, venturi tubes, flow nozzles, and pitot tubesare the usual sensors. In smaller pipelines, orifice plates,vortex flowmeters, or variable area flowmeters are used.For sludge services, doppler-type ultrasonic and magneticflowmeter (provided with electrode cleaners), V-cone de-tector,andsegmentalwedge-typedetectorcanbeused.Gas, liquid, or solid additives can be charged by Coriolismass flowmeters (gas or liquid), metering pumps, turbineorpositivedisplacementmeters(liquids),variable-areaflowmeters (gas or liquid), or gravimetric feeders (solids).Table 7.6.1 summarizes flowmeter features and capabili-ties. The following sections provide a brief summary ofthe features and capabilities of the flowmeters used in thewastewater treatment industry.Magnetic FlowmetersDESIGN PRESSUREVaries with pipe size. For a 4 in (100 mm) unit, the maximumpressure is 285 psig (20 bars); special units are available with pres-sure ratings up to 2500 psig (172 bars).DESIGN TEMPERATUREUp to 250F (120C) with Teflon liners and up to 360F (180C)with ceramic liners.MATERIALS OF CONSTRUCTIONLiners: ceramics, fiberglass, neoprene, polyurethene, rubber, Teflon,vitreous enamel, and Kynar; Electrodes: platinum, Alloy 20,Hastelloy C, stainless steel, tantalum, titanium, tungsten carbide,Monel, nickel, and platinum-alumina cermet.Monitoring and Analysis1999 CRC Press LLCTABLE 7.6.1ORIENTATION TABLE FOR FLOW SENSORSType of DesignElbow taps

LL

SR3:1N25/10510*Jet deflection

25:1M20/52*Laminar flowmeters

10:1H15/51/25*Magnetic flowmeters

10:1N5/31/2**2*Mass flowmeters and

SD

SDSD

100:1AN1/2**miscellaneous coriolis

L

SDSD

20:1HN0.15As**Metering pumps

SD

20:1N1/101*Orifice (plate orintegral cell)

LL

SR3:1H20/51/2**2*Pitot tubes

L

SR3:1M30/50.55*Positive displacement10:1gas meters

SD

to 200:1MN1/21***C l e a n L i q u i d sV i s c o u s L i q u i d sS l u r r yG a sS o l i d sD i r e c t M a s s F l o w S e n s o rV o l u m e t r i c F l o w D e t e c t o rF l o w R a t e S e n s o rI n h e r e n t T o t a l i z e rD i r e c t I n d i c a t o rT r a n s m i t t e r A v a i l a b l eL i n e a r O u t p u tR a n g e a b i l i t yP r e s s u r e L o s s T h r u S e n s o rA p p r o x i m a t e S t r a i g h t P i p e - R u n R e q u i r e m e n t1( U p s t r e a m D i a m e t e r / D o w n s t r e a m D i a m e t e r )A c c u r a c y* % F u l l S c a l e* * % R a t e* * * % R e g i s t r a t i o nApplicable to Detectthe Flow of0.11.010102103104kgm/hr0.11.010102103104105106lbm/hr0.050.32.828.3cc/min10-610-510-410-310-20.11.01010210310410-610-510-410-310-20.11.010102103104105.0040.040.43.83837910-610-510-410-310-20.11.010102103104m3/hr10-610-510-410-310-20.11.010102103104105106gpmcc/minm3/hr or Am3/hrscfm or acfmFlow RangeSolidsFlowUnitsGasFlowUnitsLiquidFlowUnitsgpmm3/hrscfmSm3/hrscfmSm3/hrgpmm3/hrscfmSm3/hrgpmm3/hrlbm/hrkgm/hrscfmSm3/hrgpmm3/hrgpmm3/hrscfmSm3/hrgpmm3/hrscfmSm3/hrscfmSm3/hr1999 CRC Press LLCPositive displacementliquid meters

SD

10:1HN0.12**Segmental wedge

SR3:1M15/53**Solids flowmetersSDSD

SDSD

SD

20:15/31/2**4*Target meters

L

SD

SR4:1H20/50.5*5*Thermal meters(mass flow)

LL

L20:1A5/312*Turbine flowmeters

LSD

10:1kH15/511/4**V-cone flowmeter

LL

SR3:1M2/51/22**Ultrasonic flowmetersTransit

L

20:1N15/51**2*DopplerL

LL

10:1N15/523*Variable-areaflowmeters

LL

5:1AN1/2*10**Venturi tubes

LL

SR3:1M15/51/2**1*Flow nozzles

L

SR3:1H20/51**2*Vortex shedding

10:1H20/50.51.5**Fluidic

20:1H20/512**Oscillating

10:1H20/50.5*Weirs and flumes

LL

SD100:1MSee Text25*- - - - - Nonstandard RangeLLimitedSDSome DesignsHHighAAverageMMinimalNNoneSRSquare RootThe data in this column is for general guidance only.The inherent rangeability of the primary device is substantially greater than shown. The value used reflects the limitation of the differential pressure sensing device when 1% of theactual flow accuracy is needed. With multiple-range intelligent transmitters, the rangeability can reach 10:1.The pipe size establishes the upper limit.Practically unlimited with the probe-type design.Must be conductive.Can be reranged over 100:1.Varies with upstream disturbance.Can be more at high Re. No. services.Up to 100:1 with high precision design.Commercially available gas flow elements can be 1% of rate.kMore for gas turbine meters.gpmm3/hrgpmm3/hrgpmm3/hrSCFMSm3/hrgpmm3/hrSCFMSm3/hrgpmm3/hrSCFMSm3/hrgpmm3/hrACFMSm3/hrlbm/hr-kgm/hrgpmm3/hr3SCFMSm3/hrgpmm3/hrSCFMSm3/hrgpmm3/hr3SCFMSm3/hrgpmm3/hrACFMSm3/hrgpmm3/hr3TYPE OF FLOW DETECTEDVolumetric flow of conductive liquids, including slurries and corro-sive or abrasive materials.MINIMUM CONDUCTIVITY REQUIREDThe majority of designs require 1 to 5 S/cm. Some probe types re-quire more. Special designs can operate at 0.05 or 0.1 S/cm.FLOW RANGESFrom 0.01 to 100,000 gpm (0.04 to 378,000 liters per minute(lpm)).SIZE RANGESFrom 0.1 to 96 in (2.5 mm to 2.4 m) in diameter.VELOCITY RANGES00.3 to 030 ft/sec (00.1 to 010 m/sec).ERROR (INACCURACY)1% of actual flow with pulsed direct current (dc) units within arange of up to 10:1 if flow velocity exceeds 0.5 ft/sec (0.15 m/sec).1% to 2% full-scale with alternating current (ac) excitation.COSTThe probe designs are least expensive, at a cost of about $1500. A1-in (25-mm) ceramic tube unit can be obtained for under $2000.A 1-in (25-mm) metallic wafer unit can be obtained for under$3000. An 8-in (200-mm) flanged meter that has a Teflon linerand stainless electrodes and is provided with 4 to 20 mA dc out-put, grounding ring, and calibrator costs about $8000. The scan-ning magmeter probe used in open-channel flow scanning costsabout $10,000.PARTIAL LIST OF SUPPLIERSABB Kent-Taylor Inc.; AccuDyne Systems Inc.; Accurate MeteringSystems Inc.; ADE-Applied Digital Electronics; Badger Meter Inc.;Baily Controls Co.; Brooks Instrument Div. of Rosemount;Colorado Engineering Experimental Station; Dantec Electronics;H.R. Dulin Co.; Dynasonics Inc. (probe-type); Edinboro ComputerInstruments Corp.; Electromagnetic Controls Corp.; Endress Hauser Instruments; Engineering Measurements Co.; Fischer &Porter Co.; Foxboro Co.; Harwil Corp.; Honeywell, IndustrialControls Div.; Instrumark International Inc.; Johnson YokogawaCorp.; K & L Research Co. (probe-type); Krone-America Inc.;Marsh-McBirney Inc. (probe-type); Meter Equipment Mfg.; MineSafety Appliances Co.; Monitek Tech. Inc.; Montedoro Whitney;MSR Magmeter Manufacturing Ltd. (probe-type); OmegaEngineering; Rosemount Inc.; Sarasota Measurements & Controls;Schlumberger Industries Inc.; Signet Industrial (probe-type);Sparling Instruments Co.; Toshiba International; Turbo InstrumentsInc.; Vortab Corp.; Wallace & Tiernan Inc.; Wilkerson InstrumentCo.; XO Technologies Inc.; Yokogawa Electric Corp.MagneticflowmetersuseFaradaysLawofelectromag-netic induction for measuring flow. Faradays Law statesthat when a conductor moves through a magnetic field ofgiven strength, a voltage level is produced in the conduc-tor that depends on the relative velocity between the con-ductor and the field. This concept is used in electric gen-erators. Faraday foresaw the practical application of theprinciple to flow measurement because many liquids areadequateelectricalconductors.Infact,heattemptedtomeasure the flow velocity of the Thames River using thisprinciple. He failed because his instrumentation was notadequate, but 150 years later, the principle is successfullyapplied in magnetic flowmeters.THEORYFigure 7.6.1 shows how Faradays Law is applied in theelectromagneticflowmeter.Theliquidistheconductorthat has a length equivalent to the inside diameter of theflowmeter D. The liquid conductor moves with an aver-age velocity V through the magnetic field of strength B.The induced voltage is E. The mathematical relationshipis:E BDV/C 7.6(1)where:C is a constant to take care of the proper unitsWhen the pair of magnetic coils is energized, a mag-netic field is generated in a plane mutually perpendicularto the axis of the liquid conductor and the plane of theelectrodes. The velocity of the liquid is along the longitu-dinal axis of the flowmeter body; therefore, the voltage in-duced within the liquid is mutually perpendicular to thevelocity of the liquid and the magnetic field.The liquid should be considered as an infinite numberof conductors moving through the magnetic field with eachelement contributing to the voltage that is generated. Anincrease in the flow rate of the liquid conductors movingthrough the field increases the instantaneous value of thevoltage generated. Also, each of the individual generatorscontributes to the instantaneously generated voltage.Whether the profile is essentially square (characteristicof a turbulent velocity profile), parabolic (characteristic ofa laminar velocity profile), or distorted (characteristic ofpoor upstream piping), the magnetic flowmeter is excel-lent at averaging the voltage contribution across the me-tering cross section. The sum of the instantaneous voltagesgeneratedrepresentstheaverageliquidvelocitybecauseeach increment of liquid velocity within the plane of theelectrode develops a voltage proportional to its local ve-locity. The signal voltage generated is equal to the aver-age velocity almost regardless of the flow profile. The mag-1999 CRC Press LLCFI G.7 .6 .1 Schematic representation of the magnetic flowme-netic flowmeter detects the volumetric flow rate by sens-ing the linear velocity of the liquid.The equation of continuity (Q VA) is the relation-ship that converts the velocity measurement to volumetricflow rate if the area is constant. The area must be knownand constant and the pipe must be full for a correct mea-surement.DESI GNSANDAPPLI CATI ONSMagneticflowmetersareavailableinconventional(seeFigure 7.6.2), ceramic (see Figure 7.6.3), and probe (seeFigure 7.6.4) constructions.Most liquids or slurries are adequate electrical conduc-tors to be measured by electromagnetic flowmeters. If theliquid conductivity is equal to 20 S per cm or greater,mostconventionalmagneticflowmeterscanbeused.Special designs are available to measure the flow of liq-uids with threshold conductivities as low as 0.1 S.Magnetic flowmeters are not affected by viscosity orconsistency (referring to Newtonian and nonNewtonianfluids,respectively).Changesintheflowprofileduetochanges in Reynolds numbers or upstream piping do notgreatly affect the performance of magnetic flowmeters. Thevoltage generated is the sum of the incremental voltagesacross the entire area between the electrodes, resulting ina measure of the average fluid velocity. Nevertheless, themeter should be installed with five diameters of straightpipe before and three diameters of straight pipe followingthe meter.Magnetic flowmeters are bidirectional. Manufacturersoffer converters with output signals for both direct and re-verse flows.The magnetic flowmeter must be full to assume accu-rate measurement. If the pipe is only partially full, the elec-trode voltage, which is proportional to the fluid velocity,is still multiplied with the full cross section, and the read-ing will be high. Similarly, if the liquid contains entrainedgases, the meter measures them as liquid, the reading willbe high.The meters electrodes must remain in electrical contactwith the fluid being measured and should be installed inthe horizontal plane. In applications where a buildup orcoating occurs on the inside wall of the flowmeter, peri-odic flushing or cleaning is recommended.Special meters for measuring sewage sludge flow are de-signed to prevent the buildup and carbonizing of sludgeon the meter electrodes. They use self-heating to elevatethemeteringbodytemperaturetopreventsludgeandgrease accumulation.1999 CRC Press LLCFI G. 7 .6 .2 The short-form magnetic flowmeter.FI G. 7 .6 .3 The ceramic insert-type magnetic flowmeter.FI G. 7 .6 .4 The probe-type magnetic flowmeter.METERING ELECTRODES (SINTERED)SENSORSOLENOIDELECTRODESLINES OF MAGNETIC INDUCTIONADVANTAGESMagnetic flowmeters have the following advantages:1. The magnetic flowmeter has no obstructions or mov-ing parts. Flowmeter pressure loss is no greater thanthatofthesamelengthofpipe.Pumpingcostsarethereby minimized.2. Electricpowerrequirementscanbelow,particularlywith the pulsed dc types. Electric power requirementsas low as 15 or 20 W are common.3. The meters are suitable for most acids, bases, waters,and aqueous solutions because the lining materials arenot only good electrical insulators but are also corro-sion-resistant. Only a small amount of electrode metalis required, and stainless steel, Alloy 20, the Hastelloys,nickel,Monel,titanium,tantalum,tungstencarbide,and even platinum are all available.4. The meters are widely used for slurry services not onlybecause they are obstructionless but also because someof the liners, such as polyurethane, neoprene, and rub-ber, have good abrasion or erosion resistance.5. Themetersarecapableofhandlingextremelylowflows. Their minimum size is less than Ak in (3.175 mm)inside diameter. The meters are also suitable for highvolume flow rates with sizes as large as 10 ft (3.04 m).6. The meters can be used as bidirectional meters.LIMITATIONSMagneticflowmetersdohavesomespecificapplicationlimitations:1. The meters work only with conductive fluids. Pure sub-stances, hydrocarbons, and gases cannot be measured.Most acids, bases, water, and aqueous solutions can bemeasured.2. The conventional meters are relatively heavy, especiallyin larger sizes. Ceramic and probe-type units are lighter.3. Electrical installation care is essential.4. The price of magnetic flowmeters ranges from moder-ate to expensive. Their corrosion resistance, abrasionresistance, and accurate performance over wide turn-down ratios can justify the cost. Ceramic and probe-type units are less expensive.5. Periodicallycheckingthezeroonac-typemagneticflowmeters requires block valves on either side to bringthe flow to zero and keep the meter full. Cycled dc unitsdo not have this requirement.Coriolis Mass FlowmetersSIZESaQh to 6 in (1.5 to 150 mm).FLOW RANGE0 to 25,000 lb/m (0 to 11,340 kg/m).FLUIDSLiquids, slurries, compressed gases, and liquified gases; not gas-liq-uid mixtures or gases at below 150 psig (10.3 bars).OUTPUT SIGNALLinear frequency, analog, digital, scaled pulse, and display.DETECTOR TYPESElectromagnetic, optical, and capacitive.OPERATING PRESSUREDepends upon tube size and flange rating: 1800 psig (124 bars)typical standard; 5000 psig (345 bars) typical high-pressure.PRESSURE DROP REQUIREDFrom under 10 psig (0.7 bars) to over 100 psig (6.9 bars) as afunction of viscosity and design.OPERATING TEMPERATUREDepends on the design: 100 to 400F (73 to 204C) typicalstandard; 32 to 800F (0 to 426C) high-temperature.MATERIALS OF CONSTRUCTIONStainless steel, Hastelloy, titanium, and NiSpan C as standard; tan-talum and Tefzel-lined as special.INACCURACY0.15 to 0.5% of rate0.15% of rate mazsesrofloowffseratte 100%Zero offset depends on flowmeter size and design; for a 1 in (25mm) meter with a typical maximum flow rate of 400 to 1000 lb/m(180 to 450 kg/m), the zero offset typically ranges from 0.03 to 0.1lb/m (0.014 to 0.045 kg/m), which is under 0.01%.REPEATABILITY0.05 to 0.2% of rate.RANGEABILITY20:1 calibration range (typical).COSTDepends on the size and design: aQh in (1.5 mm)$3950; 6 in(150 mm)$21,000; typical 1 in (25 mm) meter, with full-scaleflow rate of 400 to 1000 lb/m (180 to 450 kg/m)$5300.A typical flowmeter comes standard with one pulse or frequencyoutput that represents flow rate; one analog output configurablefor flow rate, density, or temperature; and a display or digital out-put that provides flow rate, density, temperature, and flow total. Inaddition, most devices provide standard alarm outputs. The num-ber and type of outputs vary from one manufacturer to another.Additional analog, frequency, pulse, and digital outputs are oftenprovided as options.PARTIAL LIST OF SUPPLIERSBailey Controls; Danfoss A/S (Denmark); Endress & HauserInstruments; Exac Corp.; Fischer & Porter Co.; The Foxboro Co.;Heinrichs, K-Flow; Krohne, Bopp & Reuther; Micro Motion Inc.;Neptune Measurement Co.; Schlumberger Industries; Smith MeterInc.Coriolis flowmeters are not often used in wastewater ap-plications. They are used on additive charging applicationswhere the chemical is added on a weight basis or where1999 CRC Press LLCtheir capability to detect both the mass flow and densityof slurry streams is an advantage.Since the appearance of the first commercial meters inthe late 1970s, Coriolis flowmeters (see Figure 7.6.5) havebecome widely used. Their ability to measure mass flowdirectly with high accuracy and rangeability and to mea-sure a variety of fluids makes Coriolis flowmeters the pre-ferredflowmeasurementinstrumentformanyapplica-tions. Coriolis flowmeters are also capable of measuringprocess fluid density and temperature. Since Coriolis flowmeasurement is a relatively new technology, many of thesubtleties of its operation are still being investigated.ADVANTAGESCoriolis flowmeters have the following advantages:1. They are capable of measuring a range of fluids thatare often incompatible with other flow measurementdevices. The operation of the flowmeter is independentof the Reynolds number; therefore, extremely viscousfluids can also be measured. A Coriolis flowmeter canmeasure the flow rate of Newtonian fluids, all types ofnonNewtonian fluids, and slurries. Compressed gasesand cryogenic liquids can also be measured by somedesigns.2. Coriolisflowmetersprovideadirectmass-flowmea-surement without the addition of external measurementinstruments. While the volumetric flow rate of the fluidvaries with changes in density, the mass-flow rate ofthe fluid is independent of density changes.3. Coriolisflowmetershaveoutstandingaccuracy.Thebaseinaccuracyiscommonly0.2%.Inaddition,theflowmeters are extremely linear over their entire flowrange.4. The rangeability of the flowmeters is usually 20:1 orgreater. Coriolis flowmeters have been successfully ap-plied at flow rates 100 times lower than their rated full-scale flow rate.5. A Coriolis flowmeter is capable of measuring mass-flowrate, volumetric flow rate, fluid density, and tempera-tureall from one instrument.6. The operation of the flowmeter is independent of flowcharacteristicssuchasturbulenceandflowprofile.Therefore, upstream and downstream straight run re-quirementsandflowconditioningarenotnecessary.They can also be used in installations that have pul-sating flow.7. Coriolis flowmeters do not have internal obstructionsthat can be damaged or plugged by slurries or othertypes of particulate matter in the flow stream. Entrainedgasorslugsofgasintheliquiddonotdamagetheflowmeter.Theflowmeterhasnomovingpartsthatwearoutandrequirereplacement.Thesedesignfea-tures reduce the need for routine maintenance.8. The flowmeter can be configured to measure flow ineither the forward or reverse direction. In reverse flow,a time or phase difference occurs between the flow de-tector signals, but the relative difference between thetwo detector signals is reversed.9. Coriolis flowmeter designs are available for use in san-itary applications and for the measurement of shear sen-sitive fluids. Materials are available that permit the mea-surement of corrosive fluids.LIMITATIONSCoriolis flowmeters have the following limitations:1. They are not available for large pipelines. The largestCoriolisflowmeterhasamaximumflowratingof25,000 lb/min (11,340 kg/min) and is equipped with 6-in. (15-cm) flanges. Larger flow rates require more thanone flowmeter mounted in parallel.2. Some flowmeter designs require high fluid velocities toachieve significant time or phase difference between theflow detector signals. These high velocities can result inhigh pressure drops across the flowmeter.3. Coriolis flowmeters are expensive. However, the costof a Coriolis meter is often comparable to (or below)the cost of a volumetric meter plus a densitometer usedtogether to determine the mass-flow rate.4. Coriolis flowmeters have difficulty measuring the flowrateoflow-pressuregas.Applicationswithpressures1999 CRC Press LLCFI G. 7 .6 .5 Coriolis mass flowmeter.less than 150 psig are marginal with the flowmeter de-signs currently available. Low-pressure gases have lowdensity,andtheirmass-flowrateisusuallylow.Generating sufficient Coriolis force requires a high gasvelocity.Thishighvelocitycanleadtoprohibitivelyhigh pressure drops across the meter.Metering PumpsTYPESA. PeristalticB. Piston or plunger types (provided with packing glands)C. Diaphragm or glandless types (mechanical, hydraulic, double-di-aphragm, and pulsator designs)CAPACITYA. 0.0005 cc/min to 20 gpm (90 lpm)B. 0.001 gph to 280 gpm (0.005 lph to 1250 lpm)C. Mechanical diaphragms: from 0.01 to 50 gallons per hour (gph)(0.05 to 3.7 lpm); mechanical bellows: from 0.01 to 250 gph (0.05to 18 lpm); and others: from 0.01 to 800 gph (0.05 liters per hour(lph) to 60 lpm); pulsator pumps: from 30 to 1800 gph (2 to 130lpm)ERROR (INACCURACY)A. 0.1 to 0.5% of full scale over a 10:1 rangeB & C. 0.25 to 1% of full scale over a 10:1 range; can be asgood as 0.1% full scale at 100% stroke and tends to drop asstroke is reducedMAXIMUM DISCHARGE PRESSUREA. 50 psig (3.5 bars)B. 50,000 psig (3450 bars)C. Mechanical bellows: up to 75 psig (5 bars); mechanical di-aphragm: up to 125 psig (8.5 bars); hydraulic Teflon diaphragm:1500 psig (104 bars); pulsator pumps: up to 5000 psig (345 bars);and hydraulic metallic diaphragms: up to 40,000 psig (2750 bars)MAXIMUM OPERATING TEMPERATUREA. 70 to 600F (57 to 315C)B. Jacketed designs: up to about 500F (260C)C. Units containing hydraulic fluids can handle from 95 to 360F(71 to 182C), Teflon and Viton diaphrams are limited to 300F(150C), and neoprene and Buna N are limited to 200F (92C).The metal bellows and the remote head designs can operate fromcryogenic to 1600F (870C).MATERIALS OF CONSTRUCTIONA. Neoprene, Tygon, Viton, and siliconeB. Cast iron, steel, stainless steel, Hastelloy C, Alloy 20, Carpenter20, Monel, nickel, titanium, glass, ceramics, Teflon, polyvinyl chlo-ride (PVC), Kel-F, Penton, polyethylene, and other plasticsC. Polyethylene, Teflon, PVC, Kel-F, Penton, steel, stainless steel,Carpenter 20, Monel, Hastelloy B & CCOSTA. $200 to $800B. $1000 to $6000C. $1000 to $12,000PARTIAL LIST OF SUPPLIERSAmerican LEWA Inc. (A,B,C); Barnant Co. (A); Blue WhiteIndustries; Bran & Luebbe Inc.; Clark-Cooper Corp. (B,C); Cole-Parmer Instrument Co.; Flo-Tron Inc. (B); Fluorocarbon Co.;Gerber Industries; Hydroflow Corporation; LDC Analytical; Leeds& Northrup, Unit of General Signal; Liquid Metronics Inc. MiltonRoy Div. (B); Plast-O-Matic Valves Inc.; Ruska Instrument Corp.;S J Controls Inc.; Valcor Scientific; Wallace & Tiernan Inc. (B,C)In the wastewater treatment industry, metering pumps areoften used to charge reagents, coagulants, or other addi-tives. While they require periodic recalibration, their ad-vantages include high accuracy (similar to turbine or pos-itivedisplacementflowmeters),highrangeability,suitability for slurry service, and the ability to both pumpand meter the fluid.OrificesDESIGN PRESSUREFor plates, limited by the readout device only; integral orifice trans-mitter to 1500 psig (10.3 MPa)DESIGN TEMPERATUREFunction of the associated readout system when the differentialpressure unit must operate at the elevated temperature. For the in-tegral orifice transmitter, the standard range is 20 to 250F (29to 121C).SIZESMaximum size is the pipe size.FLUIDSLiquids, vapors, and gasesFLOW RANGEFrom a few cc/min using integral orifice transmitters to any maxi-mum flow; limited only by pipe sizeMATERIALS OF CONSTRUCTIONNo limitation on plate materials. Integral orifice transmitter wettedparts can be obtained in steel, stainless steel, Monel, nickel, andHastelloy.INACCURACYThe orifice plate, if the bore diameter is correctly calculated andprepared, can be accurate to 0.25 to 0.5% of the actual flow.When a conventional d/p cell is used to detect the orifice differen-tial, that adds a 0.1 to 0.3% of the full-scale error. The errorcontribution of smart d/p cells is only 0.1% of the actual span.INTELLIGENT D/P CELLSInaccuracy of 0.1%, rangeability of 40:1, the built-in propor-tional integral and derivative (PID) algorithmRANGEABILITYIf rangeability is defined as the flow range within which the com-bined flow measurement error does not exceed 1% of the actualflow, then the rangeability of conventional orifice installations is3:1. When intelligent transmitters with automatic switching capa-bility between the high and low spans are used, the rangeabilitycan approach 10:1.COSTA plate only is $50 to $300, depending on size and materials. Forsteel orifice flanges from 2 to 12 in (50 to 300 mm), the costranges from $200 to $1000. For flanged meter runs in the samesize range, the cost ranges from $400 to $3000. The cost of elec-tronic or pneumatic integral orifice transmitters is between $15001999 CRC Press LLCand $2000. The cost of d/p transmitters ranges from $900 to$2000, depending on type and intelligence.PARTIAL LIST OF SUPPLIERSABB Kent-Taylor Inc. (includes integral orifices); CraneManufacturing Inc.; Daniel Flow Products Inc. (orifice plates andplate changers); Fischer & Porter Co. (includes integral orifices);Fluidic Techniques, a Div. of FTI Industries; Foxboro Co. (includesintegral orifices); Honeywell Industrial Div.; Lambda Square Inc.;Meriam Instrument, Div. Scott & Fetzer (orifice plates);Rosemount Inc.; Vickery-Simms, a Div. of FTI Industries. In addi-tion, orifice plates, flanges, and accessories can be obtained frommost major instrument manufacturers.The orifice plate, when installed in a pipeline, causes anincrease in flow velocity and a corresponding decrease inpressure. The flow pattern shows an effective decrease inthe cross-section beyond the orifice plate, with a maximumvelocity and minimum pressure at the vena contracta (seeFigure 7.6.6). This location can be from .35 to .85 pipediameters downstream from the orifice plate depending onthe ratio and the Reynolds number.This flow pattern and the sharp leading edge of the ori-fice plate (see Figure 7.6.6) that produces it are important.The sharp edge results in an almost pure line contact be-tween the plate and the effective flow, with negligible fluid-to-metal friction drag at this boundary. Any nicks, burrs,orroundingofthesharpedgecanresultinlargemea-surement errors.Whendifferentialpressureismeasuredatalocationclose to the orifice plate, friction effects between the fluidand the pipe wall upstream and downstream from the ori-fice are minimized so that pipe roughness has a minimumeffect. Fluid viscosity, as reflected in the Reynolds num-ber,hasaconsiderableinfluence,particularlyatlowReynolds numbers. Since the formation of the vena con-tracta is an inertial effect, a decrease in the ratio of iner-tial to frictional forces (decrease in Reynolds number), andthecorrespondingchangeinflowprofile,resultsinlessconstriction of flow at the vena contracta and an increaseof the flow coefficient. In general, the sharp edge orificeplate should not be used at pipe Reynolds numbers under10,000.TheminimumrecommendedReynoldsnumbervaries from 10,000 to 15,000 for 2-in (50-mm) through4-in (102-mm) pipe sizes for ratios up to 0.5 and from20,000 to 45,000 for higher ratios. The Reynolds num-ber requirement increases with pipe size and ratio andcan range up to 200,000 for pipes 14 in (355 mm) andlarger. Maximum Reynolds numbers can be 106for 4-in(102-mm) pipe and 107for larger sizes.WASTEWATERAPPLI CATI ONSIf the water is dirty, containing solids or sludge, the pres-sure taps must be protected by clean water purging or byuse of chemical seals and the orifice plates should be thesegmentaloreccentricorificetype(seeFigure7.6.7).Annular orifices and V-cone meters are also applicable todirty services. Because the pressure recovery of orifices islow, they are not recommended to measure larger flows1999 CRC Press LLCFI G. 7 .6 .6 Pressure profile through an orifice plate and the different methods of de-tecting the pressure drop.due to the excessive pumping costs. In these applications,venturi-type, high-recovery flow elements should be used.The main advantages of orifices are their familiarity,simplicity, and the fact that they do not need calibration.The disadvantages include their low rangeability, low ac-curacy, high pressure drop, and potential plugging.Pitot TubesTYPESA. Standard, single-portB. Multiple-opening, averagingC. Area averaging for ductsAPPLICATIONSLiquids, gases, and steamOPERATING PRESSUREPermanently installed carbon or stainless steel units can operate atup to 1400 psig (97 bars) at 100F (38C) or 800 psig (55 bars) atapproximately 700F (371C). The pressure rating of retractableunits is a function of the isolating valve.OPERATING TEMPERATUREUp to 750F (399C) in steel and 850F (454C) in stainless steelconstruction when permanently installedFLOW RANGESGenerally 2-in (50-mm) pipes or larger; no upper limitMATERIALS OF CONSTRUCTIONBrass, steel, and stainless steelMINIMUM REYNOLDS NUMBERRange from 20,000 to 50,000RANGEABILITYSame as orifice platesSTRAIGHT-RUN REQUIREMENTSDownstream of valve or two elbows in different planes, 2530pipe diameters upstream and 5 downstream; if straightening vanesare provided, 10 pipe diameters upstream and 5 downstreamINACCURACYFor standard industrial units: 0.5 to 5% of full scale. Full-travers-ing Pitot Venturis under National-Bureau-of-Standards-type labora-tory conditions can give 0.5% of the actual flow error. IndustrialPitot Venturis must be individually calibrated to obtain 1% ofrange performance. Inaccuracy of individually calibrated multiple-opening averaging pitot tubes is claimed to be 2% of the rangewhen the Reynolds numbers exceed 50,000. Area-averaging ductunits are claimed to be between 0.5 and 2% of the span. The errorof the d/p cell is additional to the errors listed.COSTSA 1-in-diameter averaging pitot tube in stainless steel costs $750 iffixed and $1400 if retractable for hot-tap installation. The costusually doubles if the pitot tube is calibrated. Hastelloy units forsmokestack applications can cost $2000 or more. A local pitot in-dicator costs $400; a d/p transmitter suited for pitot applicationswith 4 to 20 mA dc output costs about $1000.PARTIAL LIST OF SUPPLIERSABB Kent-Taylor Inc. (A); Air Monitor Corp. (C); AlnorInstrument Co. (A); Andersen Instruments Inc. (A); Blue WhiteIndustries (A); Brandt Instruments (C); Davis Instrument Mfg. Co.(A); Dietrich Standard, a Dover Industries Company (AnnubarB); Dwyer Instruments Inc. (B); Fischer & Porter Inc. (A); FoxboroCo. (Pitot VenturiA); Land Combustion Inc. (A); MeriamInstrument, a Scott Fetzer Company (B); Mid-West Instrument(Delta TubeB); Preso Industries (EllipticalB); Sirco IndustriesLtd. (A); Ultratech Industries Inc. (A); United Electric Controls Co.(A)While pitot sensors are low-accuracy and low-rangeabil-itydetectors,theydohaveaplaceinwastewatertreat-ment-related flow measurement. Pitot tubes should be usedwhen the measurement is not critical, the water is reason-ably clean, and a low cost measurement is needed. Thesesensors can be inserted in the pipe without shutdown andcan also be removed for periodic cleaning while the pipeis in use. Multiple-opening pitot tubes (see Figure 7.6.8)are less sensitive to flow velocity profile variations thansingle-opening (see Figure 7.6.9) tubes. In some dirtier ap-plications, purged pitot tubes are also used.1999 CRC Press LLCFI G. 7 .6 .7 Segmental and eccentric orifice plates.FI G. 7 .6 .8 The design of an averaging pitot tube. (Reprinted,withpermission,fromDietrichStandard,aDoverIndustriesCompany.)Segmental Wedge FlowmetersAPPLICATIONSClean, viscous liquids or slurries and fluids with solidsSIZES1- to 12-in (25.4- to 305-mm) diameter pipesDESIGNSFor smaller sizes (1 and 1.5 in), the wedge can be integral; forlarger pipes, remote seal wedges are used with calibrated elements.WEDGE OPENING HEIGHTFrom 0.2 to 0.5 of the inside pipe diameterPRESSURE DROPS25 to 200 in H2O (6.2 to 49.8 kPa)MATERIALS OF CONSTRUCTIONCarbon or stainless steel element; stainless or Hastelloy C seal; spe-cial wedge materials like tungsten carbide are available.DESIGN PRESSURE300 to 1500 psig (20.7 to 103 bars) with remote sealsDESIGN TEMPERATURE40 to 700F (40 to 370C) but also used in high-temperatureprocesses up to 850F (454C)INACCURACYThe elements are individually calibrated; the d/p cell error contribu-tion to the total measurement inaccuracy is 0.25% of full scale.The error over a 3:1 flow range is usually not more than 3% ofthe actual flow.COSTA 3-in (75-mm) calibrated stainless steel element with two stainlesssteel chemical tees and an electronic d/p transmitter provided withremote seals is about $3500.PARTIAL LIST OF SUPPLIERSABB Kent-Taylor Inc.The segmental wedge flow element provides a flow open-ingsimilartothatofasegmentalorifice,butflowob-struction is less abrupt (more gradual), and its sloping en-trance makes the design similar to the flow tube family. Itis primarily used on slurries. Its main advantage is its abil-ity to operate at low Reynolds numbers. While the squareroot relationship between the flow and pressure drop insharp-edged orifices, venturis, or flow nozzles requires aReynolds number above 10,000, segmental wedge flowme-ters require a Reynolds number of only 500 or 1000. Forthis reason the segmental wedge flowmeter can measureflows at low flow velocities and when process fluids areviscous. In that respect, it is similar to conical or quadrantedge orifices.For pipe sizes under 2 in (50 mm), the segmental wedgeflow element is made by a V-notch cut into the pipe andasolidwedgeweldedaccuratelyinplace(seeFigure7.6.10). In sizes over 2 in, the wedge is fabricated fromtwo flat plates that are welded together before insertioninto the spool piece. On clean services, regular pressuretaps are located equidistant from the wedge (see Figure7.6.10), while on applications where the process fluid con-tainssolidsinsuspension,chemicalteesareaddedup-stream and downstream of the wedge flow element. Thechemical seal element is flush with the pipe, eliminatingpockets and making the assembly self-cleaning. The sealsaremadeofcorrosion-resistantmaterialsandarealsosuited for high-temperature services. Some users have re-ported applications on processes at 3000 psig (210 bars)and 850F (454C).Variable-Area FlowmetersVariable-area flowmeters are used to regulate purge flowand as flow indicators or transmitters.PURGEFLOWMETEROnevarietyofvariable-areaflowmetersisthepurgeflowmeter (see Figure 7.6.11). The features and charac-teristics of these instruments are summarized next.1999 CRC Press LLCFI G. 7 .6 .9 Schematic diagram of an industrial device for sens-ing static and dynamic pressures in a flowing fluid.FI G. 7 .6 .1 0 The segmental wedge flowmeter designed for cleanfluid service.APPLICATIONSLow flow regulation for air bubblers, for purge protection of in-struments, for purging electrical housings in explosion-proof areas,and for purging the optical windows of smokestack analyzersPURGE FLUIDSAir, nitrogen, and liquidsOPERATING PRESSUREUp to 450 psig (3 MPa)OPERATING TEMPERATUREFor glass tubes up to 200F (93C)RANGESFrom 0.01 cc/min for liquids and from 0.5 cc/min and higher forgases. A Af-in (6-mm) glass tube rotameter can handle 0.05 to 0.5gpm (0.2 to 2 lpm) of water or 0.2 to 2 scfm (0.3 to 3 cmph) ofairINACCURACYGenerally 2 to 5% of the range (laboratory units are more accu-rate)COSTSA 150-mm glass-tube unit with Ak-in (3-mm) threaded connection,316 stainless steel frame, and 16-turn high-precision valve is $260;the same with aluminum frame and standard valve is $100.Adding a differential pressure regulator of brass or aluminum con-struction costs about $150 (of stainless steel, about $500). Forhighly corrosive services, all-Teflon, all-PTFE, all-PFA, and all-CTFA units are available which, when provided with valves, cost$550 with Af-in (6-mm) and $1300 with Df-in (19-mm) connec-tions.PARTIAL LIST OF SUPPLIERSAaborg Instruments & Controls Inc.; Blue White Industries; BrooksInstrument, Div. of Rosemount; Fischer & Porter Co.; FisherScientific; Flowmetrics Inc.; ICC Federated Inc.; Ketema Inc.Schutte and Koerting Div.; Key Instruments; King Instrument Co.;Krone America Inc.; Matheson Gas Products Inc.; OmegaEngineering Inc.; Porter Instrument Co. Inc.; Scott Specialty;Wallace & Tiernan Inc.VARI ABLE-AREAFLOWMETERSInthewastewatertreatmentindustry,variable-areaflowmeters are also used as flow indicators or transmit-ters if the process fluid is clean. Figure 7.6.12 shows theiroperating principles, and their features and capabilities arelisted next.TYPESA. Rotameter (float in tapered tube)B. Orifice/rotameter combinationC. Open-channel variable gateD. Spring and vane or pistonSTANDARD DESIGN PRESSUREA. 350 psig (2.4 MPa) average maximum for glass metering tubes,dependent on size. Up to 720 psig (5 MPa) for metal tubes andspecial designs to 6000 psig (41 MPa)STANDARD DESIGN TEMPERATUREA. Up to 400F (204C) for glass tubes and up to 1000F (538C)for some models of metal tube metersEND CONNECTIONSFemale pipe thread or flangedFLUIDSLiquids, gases, and vaporsFLOW RANGEA. 0.01 cc/min to 4000 gpm (920 m3/hr) of liquid0.3 cc/min to 1300 scfm (2210 m3/hr) of gasINACCURACYA. Laboratory rotameters can be accurate to As% of actual flow;most industrial rotameters perform within 1 to 2% of full scaleover a 10:1 range, and purge or bypass meters perform within5to 10% of full range.B and D. 2 to 10% of full rangeC. 7.5% of actual flowMATERIALS OF CONSTRUCTIONA. TUBE: Borosilicate glass, stainless steel, Hastelloy, Monel, andAlloy 20. FLOAT: Conventional typebrass,stainlesssteel,Hastelloy,Monel,Alloy20,nickel,titanium,ortantalum,andspe-cialplasticfloats. Balltypeglass,stainlesssteel,tungstencarbide,1999CRCPressLLCFI G.7.6.11 A purge flow regulator consisting of a glass tuberotameter,aninletneedlevalve,andadifferentialpressurereg-ulator.(Reprinted,withpermission,fromKroneAmericaInc.)FI G.7.6.12 Variable-area flowmeters. The area open to flowis changed by the flow itself in a variable-area flowmeter. Eithergravityorspringactioncanbeusedtoreturnthefloatorvaneasflowdrops.sapphire,ortantalum.END FITTINGS: Brass,stainlesssteel,oral-loysforcorrosivefluids.PACKING: Thegenerallyavailableelas-tomersareusedandO-ringsofcommerciallyavailablematerials;Teflonisalsoavailable.COSTA Af-in (6-mm) glass tube purge meter starts at $100. A Af-in stain-less steel meter is about $300. Transmitting rotameters start atabout $1000; with 0.5% of rate accuracy, their costs are over$2000. A 3-in (75-mm) standard bypass rotameter is about $500; a3-in stainless steel tube standard rotameter is about $2000. A 3-intapered-plug variable-area meter in aluminum construction is about$1000; the same unit in spring and vane design is around $750.PARTIALLISTOFSUPPLIERSAaborgInstruments&ControlsInc.(A);AquamaticInc.(B);BlueWhiteIndustries(A);BrooksInstrumentDiv.ofRosemount(A);DwyerInstrumentsInc.(A);ERDCOEngineeringCorp.(D);ESKOIndustriesLtd.(A);Fischer&PorterCo.(A);FlowmetricsInc.(A);GilfloMetering&InstrumentationInc.(D);GilmontInstrumentsDiv.ofBarnantCo.(B);HeadlandDiv.ofRacineFederatedInc.(D);ICCFederatedInc.(A);ISCOEnvironmentalDiv.(C);KetemaInc.SchutteandKoertingDiv.(A);KeyInstruments(A);KingInstrumentCo.(A);KoboldInstrumentsInc.;KroneAmericaInc.(A);LakeMonitorsInc.;MathesonGasProductsInc.(A);McMillanCo.;MeterEquipmentMfg.Inc.(D);MetronTechnology(A);OmegaEngineeringInc.(A);G.A.PlantonLtd.(D);PorterInstrumentCo.Inc.(A);TurboInstrumentsInc.(D);UniversalFlowMonitorsInc.(D);Wallace&TiernanInc.(A);WebsterInstruments(D)Venturi and Flow TubesDESIGNTYPESA.Venturitubes;B.Flowtubes;C.FlownozzlesDESIGNPRESSUREUsuallylimitedonlybythereadoutdeviceorpipepressureratingsDESIGNTEMPERATURELimitedonlybythereadoutdeviceiftheoperationisatveryloworhightemperatureSIZESA.1in(25mm)upto120in(3000mm)B.4in(100mm)upto48in(1200mm)C.1in(25mm)upto60in(1500mm)FLUIDSLiquids,gases,andsteamFLOWRANGELimitedonlybyminimumandmaximumbeta()ratioandavail-ablepipesizerangeINACCURACYValuesgivenareforflowelementsonly;d/pcellandreadouterrorsareadditional.A.0.25%ofrateifcalibratedinaflowlaboratoryand0.75%ofrateifuncalibratedB.Canrangefrom 0.5to3%ofratedependinguponthede-signandvariationsinfluidoperatingconditionsC.1%ofratewhenuncalibratedto 0.25%whencalibratedMATERIALSOFCONSTRUCTIONVirtuallyunlimited.Castventuritubesareusuallycastiron,butfabricatedventuritubescanbemadefromcarbonsteel,stainlesssteel,mostavailablealloys,andfiberglassplasticcomposites.Flownozzlesarecommonlymadefromalloysteelandstainlesssteel.PRESSURERECOVERY90%ofthepressurelossisrecoveredbyalow-lossventuriwhenthebeta()ratiois0.3,whileanorificeplaterecoversonly12%.(Thecorrespondingenergysavingsina24-in(600-mm)waterlineisabout20hp.)REYNOLDSNUMBERSVenturiandflowtubedischargecoefficientsareconstantatRe 100,000.Flownozzlesareusedathighpipelinevelocities(100ft/secor30.5m/sec),usuallycorrespondingtoRe 5million.Critical-flowventurinozzlesoperateunderchokedconditionsatsonicvelocity.COSTSFlownozzlesarelessexpensivethanventuriorflowtubesbutcostmorethanorifices.AmericanSocietyofMechanicalEngineers(ASME)gasflownozzlesinaluminumfor3-to8-in(75-to200-mm)linescostfrom$200to$750.Epoxyfiberglassnozzlesfor12-to32-in(300-to812-mm)linescostfrom$750to$2500.TherelativecostsofHerschelventurisandflowtubesindifferentsizesandmaterialsareasfollows:6-in 8-inStainlessSteel CastIron 12-inSteelHerschelventuri $8000 $5500 $6000Flowtube $3600 $2100 $2900PARTIALLISTOFSUPPLIERSABBKentTaylor(B);BadgerMeterInc.(A,B);BethlehemCorp.(B);BIFProductsofLeeds&Northrup(A,B,C);DanielFlowProductsInc.(A,C);Delta-TCo.(C);DigitalValveCo.(critical-flowventurinozzles);FieldingCrossmanDiv.ofLisleMetrixLtd.(A,C);Fischer&PorterCo.(B);FlowSystemsInc.(B);FluidicTechniquesInc.(A);FoxValveDevelopmentCorp.(A);F.B.LeopoldCo.(A,B);PermutitCo.Inc.(A,C);PerryEquipmentCorp.(B);HenryPrattCo.(A,B);PresoIndustries(A,B);PrimaryFlowSignalInc.(A,C);STIManufacturingInc.;Tri-FlowInc.(A);Vickery-SimmsDiv.ofFTIIndustries(A);WestCoastResearchCorp.In applications where the flows of large volumes of wateraremeasured,considerationsofthemeasurementpump-ingcostsoftenoutweightheinitialcostofthesensor.Because the venturi flowmeters (see Figure 7.6.13) requirelesspressuredropthananyotherd/p-typeflowsensor,their designs (see Figure 7.6.14) are frequently used in thewastewatertreatmentindustry.LIMITATIONSThemainlimitationofventuritubesiscost,bothforthetubeitselfandoftenforthelongpipingrequiredforthelarger sizes. However, the energy cost savings attributableto their higher pressure recovery and reduced pressure lossusuallyjustifytheuseofventuritubesinlargerpipes.Another limitation is the high minimum Reynolds num-berrequiredtomaintainaccuracy.Forventurisandflowtubes,thisminimumisaround100,000;whileforflow1999CRCPressLLCnozzles,itisover1million.Correctionfactorsareavail-ableforReynoldsnumbersbelowtheselimits,andmea-surementperformancealsosuffers.Cavitationcanalsobeaproblem.Athighflowveloc-ities(correspondingtotherequiredhighReynoldsnum-bers) at the vena-contracta, static pressure is low, and whenit drops below the vapor pressure of the flowing fluid, cav-itation occurs. Cavitation destroys the throat section of thetube since no material can stand up to cavitation. Possiblewaysofeliminatingcavitationincluderelocatingtheme-tertoapointintheprocesswherethepressureishigherandthetemperatureislower,reducingthepressuredropacrossthesensor,orreplacingthesensorwithonethathaslesspressurerecovery.Due to their construction, venturis, flow tubes, and flownozzles are difficult to inspect. Providing an inspection portontheoutletconenearthethroatsectioncansolvethisproblem. An inspection port is important when dirty (ero-sive)gases,slurries,orcorrosivefluidsaremetered.Ondirtyserviceswherethepressureportsarelikelytoplug,the pressure taps on the flow tube can be filled with chem-ical seals that have stainless steel diaphragms installed flushwiththetubeinterior.ADVANTAGESThemainadvantagesofthesesensorsincludetheirhighaccuracy, good rangeability (on high Reynolds number ap-plications), and energy-conserving high-pressure recovery.Forthesereasons,inhighervelocityflowsandlargerpipelines (and ducts), many users still favor venturis in spiteof their high costs. Their hydraulic shape also contributestogreaterdimensionalreliabilityandthereforetobetterflow-coefficientstabilitythanthatoforifice-typesensors,whichdependonthesharpedgeoftheorificefortheirflowcoefficient.Theaccuracyofaflowsensorisdefinedastheuncer-tainty tolerance of the flow coefficient. Calibration can im-proveaccuracy.Table7.6.2givesaccuracydatainper-centageofactualflow,asreportedbyvariousmanu-facturers. These values are likely to hold true only for thestatedrangesofbetaratiosandReynoldsnumbers,andthey do not include the added error of the readout deviceord/ptransmitter.Vortex FlowmetersTYPESA.VortexB.FluidicsheddingcoandaeffectC.OscillatingvaneinorificebypassSERVICESA.Gas,steam,andcleanliquidsBandC.CleanliquidsSIZERANGESAVAILABLEA.0.5to12in(13to300mm),alsoprobesB.1to4in(25to100mm)C.1to4in(25to100mm)DETECTABLEFLOWSA.Water2to10,000gpm(8lpmto40m3/hr)Air3to12,000scfm(0.3to1100scmm)Steam(D&Sat150psig[10.4bars])25to250,000lbm/hr(11to113,600kg/hr)B.Water1to1000gpm(4to4000lpm)C.Water5to800gpm(20to3024lpm)FLOWVELOCITYRANGEA.Liquids1to33ft/sec(0.3to10m/sec)Gasandsteam20to262ft/sec(6to80m/sec)1999CRCPressLLCFI G.7.6.14 Proprietaryflowtubes.FI G.7.6.13 Pressurelosscurves.MINIMUMREYNOLDSNUMBERSA.UnderReynoldsnumberof8000to10,000,metersdonotfunctionatall;forbestperformance,Reynoldsnumbershouldex-ceed20,000insizesunder4 in(100 mm)and40,000insizesabove4 in.B.Reynoldsnumber 3000OUTPUTSIGNALSA,B,andC.LinearpulsesoranalogDESIGNPRESSUREA.2000psig(138bars)B.600psig(41bars)below2 in(50 mm);150psig(10.3bars)above2 inC.300psig(30.6bars)DESIGNTEMPERATUREA.330to750F(201to400C)B.0to250F(18to120C)C.14to212F(25to100C)MATERIALSOFCONSTRUCTIONA.Mostlystainlesssteel;someinplasticB.316stainlesssteelwithVitonAO-ringsC.WettedbodyisKynar,sensorisHastelloyCRANGEABILITYA.ReynoldsnumberatmaximumflowdividedbyminimumReynoldsnumberof20,000ormoreB.ReynoldsnumberatmaximumflowdividedbyminimumReynoldsnumberof3000C.10:1forReynoldsnumberatmaximumflowdividedbymini-mumReynoldsnumbersof14,000for1 in,28,000for2 in,33,000for3 in,and56,000for4 inINACCURACYA.0.5to1%ofrateforliquidsand1to1.5%ofrateforgasesandsteamwithpulseoutputs;foranalogoutputs,add0.1%offullscale.B.1to2%ofactualflowC.0.5%offullscaleover10:1rangeCOSTA.Plasticandprobeunitscostabout$1500;stainlesssteelunitsinsmallsizescostabout$2500;insertion-typescostabout$3000.C.Thesensorwithonlyunscaledpulseoutputin1-,2-,3-,and4-insizescosts$535,$625,$875,and$1295,respectively.Theaddi-tionalcostofascaleris$250andofa420mAtransmitteris$350.PARTIALLISTOFSUPPLIERSABBKent(A);AlphasonicsInc.(A);BadgerMeterInc.(Cprox-imityswitchsensor);BrooksDiv.ofRosemount(Aultrasonic);EMCCo.(Adualpiezoelectricsensor);Endress HauserInstruments(Acapacitancesensor);Fischer&PorterCo.(Ain-ternalstraingaugesensor);FisherControls(Adualpiezoelectricsensor);FlowtecAGofSwitzerland(A);FoxboroCo.(Apiezo-electricsensor);JohnsonYokogawaCorp.(Adualpiezoelectricsensor);J-TecAssociatesInc.(Aretractabledesignavailable,ul-trasonicsensor);MCO/Eastech(Aincludinginsertion-type,me-chanical,thermal,orpiezoelectricsensors);MooreProductsCo.(B);NiceInstrumentationInc.(Adualpiezoelectricsensor);Oilgear/BallProducts(Avortexvelocity);SarasotaAutomationInc.(A);SchlumbergerIndustriesInc.(Adualpiezometricsensor);TurboInstrumentsInc.(A);UniversalFlowMonitorsInc.(Aplasticbody,piezoelectricsensor);UniversalVortex(Apiezoelec-tricsensor)Weirs and FlumesTYPESThesedevicesmeasureopen-channelflowbycausinglevelvaria-tionsinfrontofprimaries.Bubblers,capacitance,floatandhydro-staticandultrasonicdevicesareusedaslevelsensors.Thesedevicescanalsomeasureopen-channelflowswithoutprimariesbycalcu-latingtheflowfromdepthandvelocitydataobtainedfromultra-sonicandmagneticsensors.OPERATINGCONDITIONSAtmosphericAPPLICATIONSWasteorirrigationwaterflowsinopenchannelsFLOWRANGEFrom1gpm(3.78lpm)noupperlimitRANGEABILITYMostdevicesprovide75:1,V-notchweirscanreach500:1.1999CRCPressLLCTABLE7.6.2 VENTURI,FLOWTUBE,ANDFLOWNOZZLEINACCURACIES(ERRORS)INPERCENTOFACTUALFLOWFORVARIOUSRANGESOFBETARATIOSANDREYNOLDSNUMBERSLineSize PipeReynolds InaccuracyininInches NumberRange %ofActualFlowSensor (1in 25.4mm) BetaRatio forStatedAccuracy FlowHerschelstandardCast(1)432 .30.75 2 105to1 1060.75%Proprietarytrueventuri Welded 848 .40.70 2 105to2 1061.5%ProprietarytrueventuriCast(2)296 .30.75 8 104to8 1060.5%Proprietarytrueventuri Welded 1120 .25.80 8 104to8 1061.0%ProprietaryflowtubeCast(3)348 .35.85 8 104to1 1061.0%ASMEflownozzles(4)148 .20.80 7 106to4 1071.0%1Nolongermanufacturedbecauseoflonglayinglengthandhighcost.2BadgerMeterInc.;BIFProductsofLeeds&Northrup;FluidicTechniques,Inc.;F.B.LeopoldCo.;PermutitCo.,Inc.;HenryPrattCo.;PrimaryFlowSignal,Inc.;Tri-FlowInc.3BadgerMeterInc.;BethlehemCorp.;BIFProductsofLeeds&Northrup;Fischer&PorterCo.;F.B.LeopoldCo.;HenryPrattCo.; PresoIndustries.4BIFProductsofLeeds&Northrup;DanielFlowProducts,Inc.;PermutitCo.,Inc.;PrimaryFlowSignal,Inc.INACCURACY2to5%COSTSPrimariesusedaspipeinsertscostunder$1000.A6-in(150-mm)Parshallflumecostsabout$1500,anda48-in(1.22-m)Parshallflumecostsabout$5000.Primariesforirrigationapplicationsareusuallyfield-fabricated.Manualdepthsensorscanbeobtainedfor$200;localbubblerorfloatindicatorsfor$750to$1500;andprogrammabletransmittingcapacitance,ultrasonic,orbubblerunitsfrom$1800to$3000.Open-channelflowmeters,whencalcu-latingflowbasedondepthandvelocity,rangefrom$5000toover$10,000.PARTIALLISTOFSUPPLIERSABBKentTaylorInc.(primaries);AmericanSigmaInc.(bubbler);BadgerMeterInc.(Parshallormanholeflume,ultrasonicandopen-channelcomputing);BernharInc.(ultrasonicforpartiallyfilledpipes);Bestobell/Mobrey(ultrasonic);BIFUnitofLeeds&Northrup(primaryanddetector);DrexelbrookEngineeringCo.(capacitanceforflumes);Endress HauserInc.(ultrasonicandca-pacitance);Fischer&PorterCo.(ultrasonic);FreeFlowInc.(pri-maries);GreylineInstrumentsInc.(ultrasonic);InventronInc.(ul-trasonic);ISCOInc.(bubbler,hydrostatic,andultrasonic);Key-Ray/SensallInc.(ultrasonic);Leeds&NorthrupBIF(flownoz-zles);Leupold&StevensInc.(float);ManningEnvironmentalCorp.(primaries);Marsh-McbirneyInc.(electromagnetic);MeadInstrumentsCorp.(velocityprobe);MilltronicsInc.(ultrasonic);MinitekTechnologiesInc.(open-channelmagmeterandultrasonic);Montedoro-WhitneyCorp.(open-channelflowbyultrasonics);MSRMagmeterMfg.Ltd.(roboticmagmeterprobeforopenchan-nel);N.B.InstrumentsInc.(computermonitoringofsewers);Plasti-FabInc.(primaries);PrincoInstrumentsInc.(capacitance);J.L.RochesterCo.(manualdepthsensor);SparlingInstrumentsCo.(primaries);TNTechnologiesInc.(ultrasonic)Inthewastewatertreatmentindustry,theflowinlarge,openpipesorchannelsmustbemeasured.Theweirandflumedesigns,particularlytheParshallflume(seeFigure7.6.15),makesuchmeasurements.Thecommonfeatureof all these flow sensors is that they detect the level rise infrontofarestrictionintheflowchannel.DETECTORSFOROPEN-CHANNELSENSORSThelevelrisegeneratedbyflumesorweirscanbemea-suredbyanyleveldetectorincludingsimpledevicessuchasairbubblers.Theflowinopenchannelscanalsobedetectedwith-out using flumes, weirs, or any other primary devices. Onesuchdesigncomputesflowinroundpipesoropenchan-nels by ultrasonically measuring the depth, calculating theflowing cross-sectional area on that basis, and multiplyingtheareabythevelocitytoobtainvolumetricflow(seeFigure7.6.16).Anotheropen-channelflowmeterthatdoesnotneedaprimaryelementusesaroboticallyoperatedmagneticflowmeterprobetoscanthevelocityprofileintheopenchannel(seeFigure7.6.17).Inthisdesign,thecomputer1999CRCPressLLCFI G.7.6.15 Dual-rangeParshallflume.(Reprinted,withper-mission,fromFischer&PorterCo.)FI G.7.6.16 Volumetricflowcomputermeasuringdepthandvelocity in an open channel without a primary device. (Reprinted,withpermission,fromMontedoro-WhitneyCorp.)FI G. 7.6.17 Robotically operated magmeter probe sensor usedtocomputechannelflow.(Reprinted,withpermission,fromMSRMagmeterMfg.Ltd.)algorithm separately calculates and adds the flow segmentsthrougheachsliceofthevelocityprofileasthevelocitysensormovesdowntothebottomofthechannel.Level SensorsMost level sensors used in the wastewater industry do notneed to be very accurate; reliable operation, rugged design,andlowmaintenancearemoreimportant.Fortheserea-sons,thenewerleveldetectordesigns(laser,microwave,radar,gammaradiation,andtime-domainreflectometrytypes) are seldom used. Similarly, the designs that use me-chanical motion (float, displacer, or tape designs) are usedinfrequentlysincethesolid-stateorforce-balancedesignsaremoremaintenancefree.On clean water level applications for local level indica-tion, reflex-type level gauges, resistance tapes, and bubblergaugesareusedmostoften.Forhigh-andlow-levelswitches, conductivity, capacitance, vibrational, ultrasonicandthermallevelswitchesareused.Forleveltransmitterapplications,d/pandultrasonicdesignsareoftenused.Fordirtyorsludge-typelevelmeasurement,extended-diaphragm-type or purged d/p sensors, capacitance probes,and ultrasonic detectors are usually used. Lately, electronicloadcellshavealsobeenusedtodetectthelevelonthebasisofweightmeasurementinsomelargertanks.Forsludge or oil interface detection, ultrasonic, optical, vibra-tional,thermal,andmicrowavelevelswitchesworkwell.Table7.6.3providesanoverallsummaryofthefeaturesandcapabilitiesofalllevelmeasuringdevices.I NTERFACEMEASUREMENTWhen detecting the interface between two liquids, the mea-surementcanbebasedonthedifferenceofdensities,di-electric constants, electric or thermal conductivities, opac-ity,orthesonicandultrasonictransmittanceofthetwofluids.Environmentalengineersshouldbasetheirmea-surementontheprocesspropertywiththelargeststepchangebetweentheupperandlowerfluids.If,insteadofa clean interface, a rag layer (a mix of the two fluids) ex-istsbetweenthetwofluids,theinterfacedetectorcannotchangethatfact(itcannoteliminatetheraglayer);butifproperlyselected,theinterfacedetectorcansignalitsbe-ginningandendandtherebymeasureitsthickness.Interfacelevelswitchesareusuallyultrasonic,optical(Figure7.6.18),capacitance,float,conductivity,thermal,microwave, or radiation designs. The ultrasonic switch de-scribed in Figure 7.6.19 uses a gap-type probe installed ata10-degreeanglefromthehorizontal.Atoneendofthegap is the ultrasonic source, at the other end is the receiver.Aslongastheprobeisintheupperorlowerliquid,thedetectorreceivestheultrasonicpulse.When the interface enters the gap, the pulse is deflected,andtheswitchisactuated.Thisswitchcandetectthein-terface between water and oil or other hydrocarbons, suchasvinyl-acetate.Ifthethicknessofthelightlayerratherthan the location of the interface in the tank is of interest,theultrasonicgapsensorcanbeattachedtoafloatasshowninFigure7.6.20.Continuous measurement of the interface between twoliquids can be detected by d/p transmitters if P1is detectedintheheavyliquidandP2inthelightliquid.Inatmos-phericvessels,threebubblertubescanachievethesameinterface measurement. The configuration shown in Figure7.6.21 is appropriate for applications where the density ofthe light layer is constant and the density of the heavy liq-uid is variable. In these differential pressure-type systems,the movement of the interface level must be large enoughtocauseachangethatsatisfiestheminimumspanofthed/ptransmitters.Ifthedifferencebetweenthedielectricconstants is substantial, such as in crude oil desalting, ca-pacitance probes can also serve as continuous interface de-tectors.On clean services, float- and displacer-type sensors canalso be used as interface level detectors. For float-type unitsa float density heavier than the light layer but lighter thanthe heavy layer must be selected. In displacer-type sensors,thedisplacermustalwaysbeflooded,theupperconnec-tion of the chamber must be in the light liquid layer, andthe lower connection must be in the heavy liquid layer. Inthisarrangement,thedisplacerbecomesadensitysensor.Therefore, the smaller the difference between the densitiesofthefluidsandthesmallertherangewithinwhichtheinterfacecanmove,thelargerdisplacerdiameterwillberequired. Displacer density can be the same or more thanthatoftheheavylayer.BubblersAPPLICATIONSUsuallylocalindicatoronopentankscontainingcorrosive,slurry,orviscousprocessliquids.Canalsobeusedonpressurizedtanksbutonlyuptothepressureoftheairsupply.OPERATINGPRESSUREUsuallyatmospheric.OPERATINGTEMPERATURELimitedonlybypipematerial;purginghasalsobeenusedonhigh-temperature,fluidized-bedcombustionprocessestodetectlevels.MATERIALSAnypipematerialavailable.COSTS$100to$500dependingonaccessories.INACCURACYDependsonreadibilityofpressureindicator,usually 0.5%to2%offullscale.RANGEUnlimited.1999CRCPressLLC1999CRCPressLLCTABLE 7.6.3ORIENTATION TABLE FOR LEVEL DETECTORSLevel RangeApplicationsAvailableCostDesignsLiquidsSolidsTypeLimitationsAir bubblersUL12% FS

GFPFIntroduces foreign substance to process; high maintenanceCapacitance2000

12% FS

GFGFGLPFFPProblem with interface between conductivelayers and detection of foamConductivityPoint sensor18001/8in

FPFLLLLLCan detect interface only between switchconductive and nonconductive liquids; field effect design for solidsDiaphragm3500.5% FS

GFFFFPSwitches only for solids serviceDifferential12000.1% AS

EGEGPPlugging eliminated by only extended pressurediaphragm seals or repeaters. Purging and sealing legs also usedDisplacer8500.5% FS

EPPFGNot recommended for sludge orslurry serviceFloat5001% FS

GPPFMost designs limited by moving partsto clean service. Only preset densityfloats following interfacesLaserUL

0.5 in

LGGFFFFLimited to cloudy liquids or bright solidsin tanks with transparent vapor spacesLevel gauges7000.25 in

GFPFGlass not allowed in some processesMicrowave switchPoint sensor400

0.5 in

GGFGGGFThick coatingOptical switchesPoint sensor260

0.25 in

GFEFGFFPFRefraction-type for clean liquids only; reflection-type requires clean vapor spaceRadar450

0.12 in

GGFPPFPInterference from coating, agitator blades, spray, or excessive turbulenceRadiationUL

0.25 in

GEEGFGEERequires a Nuclear Regulatory Commission(NRC) licenseM a x i m u m T e m p e r a t u r e ( F ) C ( F 3 2 ) / 1 . 8A v a i l a b l e a sN o n c o n t a c tI n a c c u r a c y( 1 i n 2 5 . 4 m m )U n d e r $ 1 0 0 0$ 1 0 0 0 $ 5 0 0 0O v e r $ 5 0 0 0S w i t c hL o c a l I n d i c a t o rT r a n s m i t t e rC l e a nV i s c o u sS l u r r y / S l u d g eI n t e r f a c eF o a mP o w d e rC h u n k yS t i c k yIn feet1 3 6 12 24 48 96100 150 2000.31 2 4 8 1632 34 5067In meters1999CRCPressLLCResistance tape2250.5 in

GGGLimited to liquids under near-atmosphericpressure and temperature conditionsRotating paddlePoint sensor5001 in

GFPLimited to detection of dry, noncorrosive,switchlow-pressure solidsSlip tubes2000.5 in

FPPAn unsafe manual deviceTape-type3000.1 in

EFPGGFFOnly the inductively coupled float suited forlevel sensorsinterface measurement. Float hangup apotential problem with most designsThermal8500.5 in

GFFPFFoam and interface detection limited by the thermal conductivities involvedTDR/PDS2213 in

FFFGGFLimited performance on sticky processmaterialsUltrasonic300

1% FS

FGGGFGFFFGPresence of dust, foam, dew in vapor space; performance limited by sloping orfluffy process materialVibrating switchesPoint sensor3000.2 in

FGGFFGGOperation limited by excessive materialbuildup can preventTDRTime Domain Reflectometry PDSPhase Difference Sensors ASin % of actual span EExcellentFFairFSin % of full scale GGoodLLimitedPPoorULUnlimitedPARTIALLISTOFSUPPLIERSAutomaticSwitchCo.;ComputerInstrumentsCorp.;DavisInstrumentMfg.;DwyerInstrumentsInc.;Fischer&PorterCo.;KingEngineeringCorp.;MeriamInstrumentDiv.ofScott&Fetzer;PetrometerCorp.;ScannivalveCorp.;TimeMarkCorp.;TrimountDiv.ofCustomInstrumentComponents;UehlingInstrumentCo.;Wallace&TiernanInc.Capacitance ProbesSERVICEPointandcontinuouslevelmeasurementofsolidsandliquids(bothconductiveandnonconductive)usingboththewettedprobeandthenoncontactingproximitydesigns.DESIGNPRESSUREUpto4000psig(28MPa)DESIGNTEMPERATUREPTFEinsulationcanbeusedfrom 300to500F(185to296C).Uncoatedbareprobescanbeusedupto1800F(982C).Aluminainsulationcanbeusedupto2000F(1128C).Proximitydesignscanalsobeusedtomeasurethelevelofmoltenmetals.EXCITATIONAfewMHzMATERIALSOFCONSTRUCTIONGenerallystainlesssteelfornonconductiveandTeflon-coatedstain-lesssteelforbothconductiveandnonconductiveservices,buthigheralloys,ceramics,PVC,Kynar,andotherplasticcoatingsarealsoavailable.SPANSFrom0.25to4000picofarad(pf).Becauseofsensitivitylimita-tions,aminimumspanof10pfispreferred.INACCURACYOnoffpointsensorsusuallyactuatewithin Afin(6 mm)oftheirsetpoints.Forcontinuousleveldetection,dividingthesensitivitybythespancalculatestheminimumpercentageerrorof1to2%offullscale.SENSITIVITYANDDRIFTDependingondesign,sensitivitiesvaryfrom0.1to0.5pf,whilethedriftper100F(56C)temperaturechangecanvaryfrom0.2to5pf.RANGEProximitydevicescanbeusedfromafractionofaninchtoafewfeet;probescanbeusedupto20 ft(6 m)andcablesupto200 ft(61 m).1999CRCPressLLCFI G.7.6.21 Interface detection with bubbler tubes. (CourtesyofFischer&PorterCo.)FI G.7.6.18 Opticalorultrasonicsludgelevelorinterfaceswitch.(CourtesyofSensallInc.)FI G.7.6.19 Ultrasonicinterfacelevelswitch.(CourtesyofSensallInc.)FI G.7.6.20 Detectingthethicknessofthetoplayer.DEADBANDANDTIMEDELAYCapacitance-typelevelswitchesareusuallyprovidedwithdead-bandsettingsadjustableoverthefullspanoftheunitandtimede-laysadjustableovera0-to25-secrange.COSTFrom$600forasimplelevelswitchwithpowersupplyandoutputrelay,plus$600foracontinuousindicator.Microprocessor-basedintelligentunitswithspecialprobeconfigurationsstartat$2000.PARTIALLISTOFSUPPLIERS(*indicatesthatthesupplieralsomarketsproximityprobes.)*ADECorp.;AeroquipCorp.;AgarCorp.Inc.;AmproduxCorp.Inc.;*ArjayEngineeringLtd.;ASCComputerSystems;ASIInstrumentsInc.;BabbittInternationalInc.;BaileyControlsCo.;BedfordControlSystems;BernhardInc.;Bindicator;ControlotronCorp.;*CustomControlSensorsInc.;*DelavanInc.;DeltaControlsCorp.;*DrexelbrookEngineeringCo.;*ElectromaticControlsCorp.;Endress HauserInstruments;Enraf-Nonius;ETAControlInstruments;Fischer&Porter;FowlerCo.;FreeFlowInc.;*FSI/ForkStandardsInc.;GreatLakesInstrumentsInc.;HITechTechnologiesInc.;HydeParkElectronics;HydrilP.T.D.;Invalco;KDGMobreyLtd.;LumeniteElectronicCo.;Magne-Sonics;MagnetrolInternational;MonitorManufacturingCo.;*MTIInstrumentsDiv.;OmegaEngineering;PenberthyInc.;PrincoInstrumentsInc.;*RobertshawControlsCo.;RosemountInc.;SystematicControls;TransducerTechnologiesInc.;TVCInstrumentsCo.;VegaB.V.;Zi-TechInstrumentCorp.Conductivity ProbesAPPLICATIONSPointordifferentialleveldetectionofconductiveliquidsorslurrieswithdielectricconstantsof20orabove.Forelectrictypes,themaximumfluidresistivityis20,000ohm/cm;electronictypescanworkonevenmoreresistivefluids.Fieldeffectprobesareusedonbothconductiveandnonconductivesolidsandliquids.DESIGNPRESSUREUpto3000psig(21MPa)forconductivityprobesand100psig(6.9bars,or0.69MPa)forfieldconductivityprobes.DESIGNTEMPERATURESFrom15F(26C)to140F(60C)forunitswithintegralelec-tronicsandfrom 15F(26C)to1800F(982C)forunitswithremoteelectronicswhendetectingconductivity.Fieldeffectprobescanoperateupto212F(100C).MATERIALSOFCONSTRUCTIONConductivityprobesaremadeof316stainlesssteel,Hastelloy,tita-nium,orCarpenter20rodswithTeflon,Kynar,orPVCsleeves.Thehousingisusuallycorrosion-resistantplasticoraluminumforNEMA4and12service.ThefieldeffectprobehasaRytonprobeandaluminumhousing.PROBELENGTHSAf-in(6-mm)solidrodsareavailableinlengthsupto6 ft(1.8 m);aQh-in(2-mm)stainlesssteelcablescanbeobtainedinlengthsupto100 ft(30 m)forconductivityapplications.Fieldeffectprobesare8 in(200 mm)long.SENSITIVITYAdjustablefrom0to50,000ohmsforconductivityprobesINACCURACYAk in(3mm)COSTFrom$50to$400.Thetypicalpriceofanindustrialconductivityswitchisabout$300.PARTIALLISTOFSUPPLIERSBLTec.;BurtProcessEquipment;B/WControlsMagatekControls;ConaxBuffaloCorp.;ControlEngineeringInc.;DelavanInc.DivisionColtIndustries;DeltaControlsCorp.;ElectromaticControlsCorp.;Endress HauserInstruments;GreatLakesInstrumentsInc.;InvalcoInc.;LumeniteElectronicCo.;MonitorMfg.;NationalControlsCorp.;RevereCorp.ofAmerica;VegaB.V.;WarrickControlsInc.;Zi-TechInstrumentCorp.D/P CellsDESIGNPRESSURETo10,000psig(69MPa)DESIGNTEMPERATURETo350F(175C)ford/pcellsandto1200F(650C)forfilledsystems;othersto200F(93C).Standardelectronicsaregenerallylimitedto140F(60C).RANGEd/pcellsandindicatorsareavailablewithfull-scalerangesaslowas0to5 in(0to12 cm)H2O.Thehigherrangesarelimitedonlybyphysicaltanksizesinced/pcellsareavailablewithrangesover433 ftH2O(7MPaor134 mH2O).INACCURACY0.5to2%offullscaleforindicatorsandswitches.Ford/ptrans-mitters,thebasicerrorisfrom 0.1to0.5%oftheactualspan.Addedtothiserrorarethetemperatureandpressureeffectsonthespanandzero.Inintelligenttransmitters,pressureandtemperaturecorrectionisautomatic,andtheoverallerroris 0.1to0.2%ofthespanwithanalogoutputsandevenlesswithdigitaloutputs.MATERIALSOFCONSTRUCTIONPlastics,brass,steel,stainlesssteel,Monel,andspecialalloysforthewettedparts.Enclosuresandhousingsareavailableinalu-minum,steel,stainlesssteel,andfiberglasscomposites,withalu-minumandfiberglassthemostreadilyavailable.COST$200to$1500fortransmittersinstandardconstructionand$100to$500forlocalindicators.Add$400to$800forextendeddi-aphragmsand$300to$600forsmartfeaturessuchascommuni-cationsanddigitalcalibration.Experttanksystemscostapproxi-mately$1500forthebasictransmitterplus$3500to$4500fortheinterfaceunitand$1500to$4000forsoftwareplusahand-heldcommunicator.PARTIALLISTOFD/PCELLSUPPLIERSABBKent-Taylor;DresserIndustries;EnrafNonius;Fischer&PorterCo.;FoxboroCo.;Honeywell,Inc.;ITTBarton;JohnsonYokogawa;KingEngineeringCorp.;L&JEngineeringInc.;MajorControls,Inc.;RosemountInc.,MeasurementDiv.,VarecDiv.;SchlumbergerIndustries,StathamDiv.;SmarInternational;TexasInstruments;UehlingInstrumentPARTIALLISTOFTANKFARMPACKAGESUPPLIERSTheFoxboroCo.;KingEngineeringCorp.;L&JEngineeringInc.;SarasotaM&CInc.;TexasInstrumentsInc.;Varec,aRosemountDiv.Thelevelmeasurementdeviceusedmostoftenonslurryand sludge services is the extended-diaphragm-type differ-1999CRCPressLLCentialpressuretransmitter(seeFigure7.6.22).Thedi-aphragm extension eliminates the dead-ended cavity in thenozzle,wherematerialsaccumulate,andbringsthesens-ingdiaphragmflushwiththeinsidesurfaceofthetank.ThesensingdiaphragmissometimescoatedwithTeflonto further minimize material buildup. One of the best meth-odsofkeepingthelow-pressuresideofthed/pcellcleanis to insert another extended-diaphragm device in the up-pernozzle.Thisdevicecanbeapressurerepeater,whichcanrepeatbothvacuumsandpressureswithintherangeoftheavailablevacuumandplantorinstrumentairsup-plypressures.Whenairorvacuumisunavailableattheprocess pressures, extended-diaphragm-type chemical sealscanbeused(seeFigure7.6.23)ifproperlycompensatedforambienttemperaturevariationsandsunexposure.Level GaugesTYPESTubularglass,armoredreflex,ortransparentandmagneticgaugesDESIGNPRESSURETubulargaugeglassesareusuallylimitedto15psig(1bar).At100F(38C),armored-reflexgaugescanberatedto4000psig(270bars 27MPa);transparentgaugesto3000psig(200bars 20MPa),andbullseyeunitsupto10,000psig(690bars 69MPa).Magneticlevelgaugesareavailableupto3500psig(230bars 23MPa).DESIGNTEMPERATURETubulargaugeglassesareusuallylimitedto200F(93C).Armoredgaugescanbeusedupto700F(371C),andmagneticgaugesareavailablefrom 320to750F(196to400C).MATERIALSOFCONSTRUCTIONThewettedpartsofarmoredgaugesareavailableinsteel,stainlesssteel,andtemperedborosilicateglass.Magneticlevelgaugesareavailablewithsteelflangesandstainlesssteel,K-monel,Hastelloy-B,andsolidPVCchambers.Availablechamberandfloatlinerma-terialsincludeKynar,Teflon,andKel-F.RANGEForarmoredgauges,thevisiblelengthofasectionis10to20 in(250to500 mm).Amaximumoffoursectionspercolumnisrec-ommendedwithamaximumtotaldistancebetweengaugeconnec-tionsof5 ft(1.5 m).INACCURACYLevelgaugescanbeprovidedwithscales.Thereadingaccuracyislimitedbyvisibility(foamingandboiling),andtheheightoftheliquidcolumninthegaugecanalsodifferfromtheprocesslevel.Iftheliquidinthegaugeiswarmer,itisalsolighter,andthereforetheerrorisonthehighside;iftheliquidinthegaugeiscolder(heavier),theindicationislow.Readoutwafersizelimitsmagneticgaugedisplayaccuracyto Afin(6 mm).COSTSExcludingthecostofshutoffvalvesorpipestands,theper-ft(300mm)unitcostoftubularglassgaugesisabout$25;armored-reflexandtransparentgaugescostabout$150/ftand$200/ft,respec-tively,whilemagneticlevelgaugesinstainlesssteelconstructioncostabout$500/ft.PARTIALLISTOFSUPPLIERSDanielIndustriesInc.;EssexBrassCo.;ImoIndustriesInc.(mag-netic);JergusonGaugeandValves,Div.oftheClarkRelianceCorp.(regularandmagnetic);JoglerInc.;KencoEngineeringCo.(magnetic);KrohneAmericaInc.;K-TekCorp.(magnetic);MagTechDiv.ISEofTexasInc.(magnetic);MetronTechnology(magnetic);Oil-RiteCorp.;PenberthyInc.(regularandmagnetic)Optical SensorsTYPESVisibleorinfrared(IR)lightreflection(noncontactingtypeusuallyforsolidsandlasertypeformoltenglassapplications),lighttrans-mission(usuallyforsludgelevel),andlightrefractionincleanliq-uidlevelservicesAPPLICATIONSPointsensorprobesforliquid,sludge,orsolids(somecontinuousdetectorsalsoavailable)1999CRCPressLLCFI G. 7.6.22 Schematic diagram showing the clean and cold airoutputoftherepeaterrepeatingthevaporpressure(Pv)inthetank.FI G.7.6.23 Schematic diagram that shows how the tempera-ture compensated, extended-diaphragm-type, chemical seals pro-tectthed/pcellfromplugging.DESIGNPRESSUREUpto150psig(10.3bars)withpolypropylene,polysulfone,PVDF,orTeflonandupto500psig(35bars)withstainlesssteelprobesDESIGNTEMPERATUREBetween150and200F(66to93C)withplasticprobesandupto260F(126C)withstainlesssteelprobesMATERIALSOFCONSTRUCTIONQuartzreflectorswithViton-AorRulonseals,mountedinpolypropylene,polysulfone,Teflon,polyvinylfluoride,phenolic,aluminum,orstainlesssteelprobesHOUSINGSCanbeintegralwiththeprobeorremote.Explosion-proofenclo-suresandintrinsicallysafeprobesarebothavailable.Withremoteelectronics,thefiber-opticcablecanbefrom50to250 ft(15to76m)long.DIMENSIONSRefractionprobelengthsvaryfrom1to24 in(25to600 mm),andtheprobediameterisusually0.5to1 in(12to25 mm).COSTSFiberopticlevelswitchescostfrom$100and$300.Portablesludgeleveldetectorscost$900.Continuoustransmitterstomeasuresludgedepthorsludgeinterfacecost$4000andup.PARTIALLISTOFSUPPLIERSAutomataInc.(noncontactingIR);BTGInc.(IR);ConaxBuffaloCorp.(fiberoptic);EnrafNoniusTankInventorySystemsInc.(IR);GemsSensorsDiv.IMOIndustriesInc.(fiberoptics);GenelcoDiv.ofBindicatorInc.(IRswitch);Kinematics&ControlsCorp.(switch);MarklandSpecialtyEngineeringLtd.(IRforsludge);OPWDivisionofDoverCorp.;3MSpecialtyOpticalFibers;Zi-TechInstrumentCorp.(switch)Resistance TapesAPPLICATIONSLiquidsincludingslurriesbutnotsolids.Canalsomeasuretemper-atureRESOLUTIONAk in,whichisthedistancebetweenhelixturnsACTUATIONDEPTH(AD)Thedepthrequiredtoshortoutthetapevarieswiththespecificgravity(SG)asfollows:AD(ininches) 4/(SG).Therefore,ADattheminimumSGof0.5is8 in(200 mm).TEMPERATUREEFFECTA100F(55C)changeintemperaturechangestheresistanceoftheunshortedtapeby0.1%.Temperaturecompensationisavail-able.INACCURACY0.5iniftheADiszeroedoutandbothADandtemperatureareconstant.IfSGvaries,azeroshiftbasedonAD 4/(SG)occurs.ColdtemperaturealsoraisestheAD.WETTEDMATERIALFluorocarbonpolymerfilmALLOWABLEOPERATINGPRESSUREFrom10to30psia(0.7to2.1barsabsolute)OPERATINGTEMPERATURERANGE20to225F(29to107C)COSTSResistancetapeunitcostvarieswithserviceandwithtapelength.A10 ft(3 m)tapewithbreatherandtransmitterforwaterservicecostsfrom$600to$1000.Theaddedcostforlongertapesis$25ormoreperfoot,dependingontheservice.SUPPLIERSMetritapeInc.;R-TapeCorp.;SankyoPio-TechThe resistance tape (see Figure 7.6.24) for continuous liq-uidlevelmeasurementwasinventedintheearly1960s,initiallyforwaterwellgaugingandsubsequentlyforma-rine and industrial usage. The sensor is a flat, coilable strip(ortape),rangingfrom3to100 ft(1to30 m)inlength,suspended from the top of the tank. It is small enough incross-sectiontobeheldwithinaperforatedpipe(diame-terof2to3 in),whichalsosupportsthetransducerandactsasastillingpipewhentheprocessisturbulent.While resistance tapes are not widely used in the waste-water treatment industry today, their low cost, low main-tenance,andadaptabilityformultipointscanningmakesthemacandidateforuseinnewplants.Thermal SwitchesTYPESSwitchesoperateoneitherthermaldifferenceorthermaldispersion.Transmittersutilizethethermalconductivitydifferencebetweenliq-uidsandvapors.Metalmoldlevelcontrollersusedirecttempera-turedetection.APPLICATIONSLiquid,interface,andfoamleveldetection.Specialunitsareavail-ableformoltenmetallevelmeasurement.DESIGNPRESSUREUpto3000psig(207bars 20.7MPa)1999CRCPressLLCFI G.7.6.24 Schematicdiagramofresistancetapesensorop-eration.DESIGNTEMPERATUREStandardunitscanbeusedfrom 100to350F(73to177C);high-temperatureunitsoperatefrom 325to850F(198to490C).RESPONSETIME10to300 secforstandardresponseunitsand1to150 secforfastresponseunits.Thetimeconstantinmoltenmetalapplicationsisunder 1 sec.AREACLASSIFICATIONExplosion-proofandintrinsicallysafedesignsarebothavailable.MATERIALSOFCONSTRUCTION316stainlesssteel,PVC,andTeflonINACCURACYTherepeatabilityis0.25 in(6 mm)forside-mountedand0.5 in(13 mm)fortop-mountedlevelswitches.Transmittersarelessac-curate.Moltenmetallevelerrordependsonthermocouplespac-ing.COSTThecostofathermallevelswitchisabout$250.Transmitterscostabout$1000.Moldlevelsystemsarefield-installed.PARTIALLISTOFSUPPLIERSChromaloxInstrumentsandControl;DeltaMCorp.(transmitter);FluidComponentsInc.(switchandmonitor);IntekInc.RheothermDiv.(switch);ScientificInstrumentsInc.;ScullyElectronicSystems,Inc.(switch)Ultrasonic DetectorsAPPLICATIONSWettedandnoncontactingswitchandtransmitterapplicationsforliquidlevelorinterfaceandsolidslevelmeasurement.Alsousedasopen-channelflowmonitors.DESIGNPRESSUREProbeswitchesareusedupto3000psig(207bars 20.7MPa);transmittersareusuallyusedforatmosphericserviceupto7psig(0.5bar),butsomespecialunitsareavailableforuseupto150psig(10.3bars).DESIGNTEMPERATURESwitchesfrom100to300F(73to149C);transmittersfrom30to150F(34to66C)MATERIALSOFCONSTRUCTIONAluminum,stainlesssteel,titan,Monel,HastelloyB&C,Kynar,PVC,Teflon,polypropylene,PVDF,andepoxyRANGESFortanksandsilos(pulseusuallytravelsinvaporspace),upto200 ft(60 m)forsomespecialdesignsandupto25 ft(7.6 m)formoststandardsystems.Forwells(usuallysubmerged),upto2000ft(600 m)INACCURACYAk in(3mm)forahorizontalprobeswitch.Fortransmitters,theerrorvariesfrom0.25to2%offullscaledependingonthedustanddewinthevaporspaceandthequalityofthesurfacethatre-flectstheultrasonicpulse.COSTSLevelswitchescostfrom$200to$500;transmitterscostfromun-der$1000to$2500,withtheaveragecostaround$1800.PARTIALLISTOFSUPPLIERSBindicatorCo.;ContaqTechnologiesCorp.;ControltronCorp.;Crane/Pro-TechEnvironmentalInstruments;DelavanInc.ProcessInstrumentationOperations;DeltaControlsCorp.;ElectroCorp.;ElectronicSensorsInc.;Endress HauserInc.;Enterra;FischerandPorterCo.;GenelcoDiv.Bindicator;GordonProductsInc.;GreylineInstrumentsInc.;HiTechTechnologiesInc.(flyashappli-cation);HydeParkElectronicsInc.;Introkek,SubsidiaryofMagnetrolInternational;InventronInc.;KayRay/SensallInc.;KDGMobreyLtd.;Kistler-MorseCorp.;KroneAmericaInc.(sludgeinterface);MagnetrolInternational;MarklandSpecialtyEngineeringLtd.(sludgelevel);Marsh-McBirneyInc.;MassaProductsCorp.;Microswitch/Honeywell,MilltronicsInc.;MonitekTechnologiesInc.;MonitorMfg.;MonitrolMfg.Co.;PanametricsInc.;Penberthy;SircoIndustrialLtd.;SORPrecisionSensors;TNTechnologiesInc.;UltrasonicArraysInc.(thickness,texture,surfacereflectivity);UnitedSensorsInc.;VegaB.V.;ZevexInc.As is shown in Figures 7.6.18 and 7.6.19, ultrasonic levelsensorsareusedwidelyonsludgelevelandsludgeinter-face detection services. Ultrasonic sludge blanket detectorscanalsobeloweredperiodicallyintothetankfortrans-mittance measurements, or they can be permanently posi-tioned for echo detection. In the newer designs, targets orsoundingpiperidgesareusedforautomaticcalibration.Evenmorerecently,flexuralsensorsareinstalledtomea-surethetransittimeorechointhetankwallinsteadofthroughtheprocessliquid.Vibrating SwitchesTYPESA.TuningforkB.VibratingprobeC.VibratingreedAPPLICATIONSLiquid,slurry,andsolidslevelswitchesDESIGNPRESSUREAandB.To150psig(10.3bars 1MPa)C.Upto3000psig(207bars 20.7MPa)DESIGNTEMPERATUREA.45to200F(43to93C)B.8to176F(10to80C)C.From150to300F(100to149C)MATERIALSOFCONSTRUCTIONAluminum,steel,andstainlesssteelMINIMUMBULKDENSITYAandC.Downto1.0lbm/ft3(16kg/m3)B.Requiresanapparentspecificgravityof0.2INACCURACYTherepeatabilityoftypeCis Akin(3 mm)1999CRCPressLLCCOSTStandardtypeA,$300;otherdesignsupto$500PARTIALLISTOFSUPPLIERSAutomationProductsInc.;BindicatorCo.;Endress HauserInc.;KDGMobreyLtd.;MonitorMfg.;MonitrolBinLevelManufacturingCo.;NohkenCo.Ltd.;VegaB.V.;ZiTechInstrumentCorp.BlaG.Liptk1999CRCPressLLC7.7pH, OXIDATION-REDUCTION PROBES (ORP) ANDION-SELECTIVE SENSORSBecausethegoalofthewastewatertreatmentindustryistopurifyandneutralizeindustrialandmunicipalwastestreams, sensors are needed to detect the activity and con-centrationofvariousionicsubstances.Animportantwa-ter parameter is the pH, which indicates the activity of thehydrogen ion and describes the acidity or alkalinity of thestream. Ion selective electrodes detect the activity of otherions,whileORPsdescribethechemicalorbiologicalprocessesinprogress.Thissectiondescribesthefeaturesandcapabilitiesofthesethreesensortypes.Probes and Probe CleanersInwastewaterapplications,environmentalengineersuseanalytical probes to detect concentrations in the sludge lay-erssituatedinthelowerpartsofscrapedbottomtanks.Theseprobesareinstalledonpivotedhingessothatthemechanicalscraperassemblycanpass(seeFigure7.7.1).Whileprobe-typein-lineanalyzerseliminatethetrans-portationlagandsampledeteriorationproblemsassoci-ated with offline analysis, they illustrate the need for effi-cientprobecleaners.Aprobecleanershouldbeplacedinside a sight glass so that clearer performance can be con-tinuouslyobservedbytheoperator(seeFigure7.7.2).Avariety of probe-cleaning devices are available. Table 7.7.1listsfeaturesandcapabilitiesfortheremovalofvariouscoatingsandTable7.7.2listssuppliers.If no sampling system is used, sample integrity is auto-maticallyguaranteed,andsensorsthatpenetratetheFI G.7.7.1 Probe-typesensorsusedtodetectthecompositionofsludgeandslurrylayersinclarifiers.(CourtesyofMarklandSpecialtyEngineeringLtd.)FI G.7.7.2 Probecleanermountedinsight-flowglassesforgoodvisibility.(CourtesyofAimcoInstrumentsInc.)