Measuring liquid move is a important requirement for so much of industrial crops. In some operations, the ability to make accurate move measurements is so essential that it might possibly make the distinction between a profit or a loss. In different circumstances, inaccurate move measurements or failure to make them can result in severe (or even catastrophic) results.
For most liquid circulate measurement devices, flow is set inferentially by measuring the change in velocity or kinetic vitality of the liquid. The velocity depends on the strain difference that forces the liquid via the pipe or conduit. Since the cross-sectional area of the pipe is known and stays constant, the typical flow rate is an indicator of the move price. In this case, the essential relationship for figuring out the circulate fee of a liquid is
Q = V x A
Q = Flow price of liquid via the pipe
V = Average flow price
A = Cross-sectional area of the pipe
Other elements that affect the move fee of a liquid embody the viscosity and density of the liquid, and the friction of the liquid involved with the pipe.
Liquid circulate could be measured immediately using optimistic displacement circulate meters. These models divide the liquid into specific increments and proceed to maneuver. The total move fee is the accumulation of the measured increments and may be counted mechanically or electronically.
Table of Contents
Reynolds number
Types of liquid circulate meters
Differential Pressure move meters
Orifice plate circulate meters
Venturi circulate meters
Flow tubes
Flow nozzle
Pitot tube flowmeter
Elbow flowmeter
Target meter
Variable space move meter
Positive displacement move meter
Reciprocating piston flowmeter
Oval Gear Flow Meters
Disc sort meter
Rotary vane gauges
Spiral circulate meter
Velocity meters
Turbine circulate meters
Vortex flowmeters
Electromagnetic move meters
Ultrasonic flow meters
Mass circulate meter
Coriolis move meters
Thermal mass flow meters
Open channel move meter
Select a right liquid flow meter
Using move meters
Calibration
Maintenance
Reynolds quantity
The performance of a flow meter can additionally be influenced by the dimensionless unit called Reynolds quantity. It is defined as the ratio of the inertial drive of a liquid to its resistance.
The method is
R = 3160 x Q x Gt
D x ต
R = Reynolds number
Q = Flow fee of the liquid, gpm
Gt = Specific gravity of the liquid
D = Internal diameter of the pipe, in.
ต = Viscosity of the liquid, cp
Flow rate and specific gravity are the inertial forces, pipe diameter and viscosity are the resistance. For most liquid functions, the pipe diameter and particular gravity stay fixed. At very low velocities or high viscosities, R may be very low and the liquid flows in a easy layer with the very best velocity at the heart of the pipe, the place viscous forces at the pipe wall restrict it to very low velocities. This sort of flow is called laminar circulate. the R value is beneath about 2000. a attribute of laminar flow is the parabolic shape of its velocity distribution.
However, most applications contain turbulent move with R-values above 3000. turbulent move occurs at high speeds or low viscosities. The flow breaks down into turbulent eddies that move through the pipe with the identical average velocity. The fluid velocity is much less vital and the velocity distribution is extra uniform in shape. A transition zone exists between turbulent and laminar circulate. Depending on the pipe configuration and other set up situations, the flow in this zone may be turbulent or laminar.
Types of liquid flow meters
Many forms of circulate meters can be found to be used in closed pipe techniques. In basic, units could be categorized as differential strain meters, positive displacement meters, velocity meters, and mass meters. Differential strain devices (also known as head gauges) embody orifice plates, venturi, flow tubes, move nozzles, pitot tubes, elbow sort move meters, goal kind circulate meters and variable area move meters.
Positive displacement flow meters include piston, elliptical-gear, chapter-actuated disk and rotary vane varieties. Velocity meters embrace turbine, vortex shedding, electromagnetic, and acoustic designs. Mass meters embrace Coriolis and thermal sorts. Measurement of liquid move in open channels often involves weirs and flumes.
Differential Pressure move meters
The use of differential stress as an inferred measurement of the flow rate of a liquid is well-known. By far, differential strain flow meters are essentially the most commonly used unit today. It is estimated that more than 50% of all liquid circulate measurement functions use this sort of unit.
The fundamental operating precept of differential strain circulate meters is predicated on the premise that the stress drop across the meter is proportional to the square of the move fee. The circulate price is obtained by measuring the differential stress and extracting the sq. root.
Like most circulate meters, differential strain move meters have a main element and a secondary element. The major factor causes a change in kinetic vitality, which creates a differential strain in the pipe. The device should be properly matched to the pipe dimension, flow conditions and liquid characteristics. And, the accuracy of the component measurement should stay good within affordable limits. The secondary factor measures the differential strain and supplies a sign or studying that is converted to an precise move value.
Orifice plate move meters
Orifice plates are the most well-liked liquid flow meters in use at present. An orifice is simply a flat piece of metallic with a specific dimension gap drilled in it. Most orifices are concentric, however eccentric, conical (quadrant) and segmented designs are additionally out there.
In practice, the orifice is put in within the pipe between two flanges. As the first gadget, the orifice restricts the flow of fluid, thereby making a differential pressure across the plate. Pressure measurement ports on either side of the plate are used to detect the difference. The major benefits of orifice plates are that they haven’t any transferring parts and that their cost doesn’t increase considerably with the dimensions of the pipe.
Tapered and quadrant orifices are comparatively new. These models were developed primarily for measuring liquids with low Reynolds numbers. An essentially constant circulate coefficient can be maintained at an R-value under 5000. Tapered orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each software.
The phase wedge is a variant of the section orifice. It is a throttle orifice designed primarily to measure the circulate of liquids containing solids. The device is able to measuring move at low Reynolds numbers and still sustaining the required square root relationship. Its design is simple and the wedge gap has just one important dimension. The stress drop via the system is only about half that of a traditional orifice plate.
The one-piece wedge assembly combines the wedge component and pressure measurement fitting into a one-piece pipe fitting bolted to a traditional pressure transmitter. No particular piping or fittings are required to install the gadget within the pipeline.
The metering accuracy of all orifice circulate meters is decided by the set up circumstances, the orifice plate area ratio, and the bodily traits of the liquid being measured.
Venturi flow meters
The advantage of a venturi is its capacity to handle giant flows at low pressure drops. A venturi is basically a section of pipe with a conical inlet and a straight throat. As the liquid passes by way of the throat, it increases in velocity, resulting in a pressure distinction between the inlet and outlet regions.
Flow meters don’t have any transferring elements. They can be installed in massive diameter pipes utilizing flanged, welded or threaded end connections. The unit is often fitted with four or more stress measuring ports to common the measured pressure. Venturi tubes can be used for many liquids, including these with high solids content.
Flow tubes
Flow tubes are somewhat much like venturi, besides that they don’t have an entrance cone. They have a tapered throat, but the outlet is elongated and smooth. The distance between the entrance and the tip is about one-half the diameter of the tube. The stress measurement port is positioned about one-half the diameter of the tube downstream and one diameter upstream.
Flow nozzle
At high velocities, the move nozzle can deal with approximately 60% of the liquid flow in comparability with an orifice plate with the same stress drop. Liquids containing suspended solids may also be metered. However, these devices are not beneficial for high viscosity liquids or liquids containing giant amounts of viscous solids.
Pitot tube flowmeter
The Pitot tube senses two forms of stress concurrently, shock stress and static pressure. The shock gadget consists of a tube with one finish of the tube bent at proper angles to the direction of flow. The finish of the static tube is closed, but has a small slot in the facet of the device. These tubes may be installed individually in the pipe or combined in a housing.
Pitot tubes are usually installed by welding a coupling to the pipe and inserting the probe via the coupling. The use of most Pitot tubes is restricted to single level measurements. These units are susceptible to clogging by international matter in the liquid. The advantages of Pitot tubes are low price, no shifting components, simple set up and minimal stress drop.
Elbow flowmeter
Elbow flow meters work on the principle that centrifugal forces are exerted along the outer edges as the liquid moves along a round path. Thus, because the liquid flows via the pipe elbow, the pressure on the inside surface of the elbow is proportional to the density of the liquid multiplied by the sq. of its velocity. In addition, the force is inversely proportional to the radius of the elbow.
Arbitrary ninety degrees. Elbows can be utilized as liquid move meters. All that is required are two small holes positioned at the midpoint of the elbow (45 degree point) for the manometer faucet. A stress sensing line can be connected to the tap using any handy methodology.
Target meter
The goal meter senses and measures the force attributable to the liquid hanging the target or a drag disk suspended in the liquid stream. By measuring the pressure applied to the goal the liquid circulate fee could be immediately indicated. In its easiest form, the meter consists solely of an articulated oscillating plate that moves outward with the liquid move. In this case, the device is used as a move indicator.
More sophisticated variations use sophisticated low-level drive sensor sensing elements. The target force caused by the liquid circulate is sensed by a pressure gauge . The output signal of the meter signifies the circulate price. The target meter can be utilized to measure the circulate rate of dirty or corrosive liquids.
Variable area flow meter
Variable area meters consist primarily of a conical tube and a float. Although categorised as differential strain units, they’re truly constant pressure units. Flange finish connections provide a simple approach to set up them in the pipe. Working principle of rotameter is when there is no liquid circulate, the float is free to relaxation on the backside of the pipe. When the liquid enters the bottom of the pipe, the float begins to rise. The place of the float varies directly with the move price. Its precise position is at the level where the strain difference between the upper and lower surfaces balances the weight of the float.
Because the flow price can be learn directly on a scale mounted subsequent to the tube, there isn’t a need for an auxiliary circulate studying gadget. However, if desired, an computerized sensing system can be utilized to sense the float stage and transmit the move signal. Variable space circulate meter tubes are manufactured from glass, metallic or plastic. Tube diameters vary from 1/4 to greater than 6 inches.
Positive displacement move meter
The operation of these items consists of separating the liquid into precisely measured increments and persevering with to move. Each section is counted by a connection register. Because each increment represents a discrete quantity, optimistic displacement items are popular in automatic dosing and accounting functions. Volumetric flow meters are perfect for measuring the circulate of viscous liquids or the place a easy mechanical flow meter system is required.
Reciprocating piston flowmeter
Reciprocating piston move meters are available in single and multi-piston versions. The specific selection is dependent upon the vary of move charges required in a selected application. Piston move meters can be used to deal with a extensive range of liquids. The fluid never comes in contact with gears or different components that could clog or corrode.
Oval Gear Flow Meters
Oval gear flowmeters have two rotating oval gears with carefully synchronized enamel. A mounted quantity of fluid passes by way of the meter with each revolution. The shaft rotation could be monitored to obtain a specific flow rate.
Disc type meter
Chapter moving disc sort meters have a movable disc mounted on a concentric sphere that’s located in a spherical sidewall chamber. The strain of the fluid passing by way of the measurement chamber causes the disc to oscillate in its circulation path without rotating about its personal axis. It is the one moving half within the measurement chamber.
A pin extending vertically from the disk is related to a mechanical counter, which displays the oscillating movement of the disk. Each cycle is proportional to a selected circulate price. As with all constructive displacement meters, adjustments in viscosity beneath a given threshold will have an result on the measurement accuracy. A wide selection of sizes and capacities can be found. These models can be made from a wide range of building supplies.
Rotary vane gauges
Rotary vane meters are available in a variety of designs, but all of them operate on the identical precept. The primary unit consists of an equally divided rotating impeller (containing two or more compartments) mounted inside the instrument housing. The impeller is in constant contact with the housing. As the impeller rotates, a set quantity of liquid is swept from each compartment to the outlet of the meter. The variety of impeller revolutions is calculated and recorded in quantity units.
Spiral flow meter
The helical flow meter consists of two radially inclined helical rotors which might be gear pushed along with minimal clearance between the rotors and the housing. The two rotors transfer the liquid from one end of the chamber to the other along the axial course.
Velocity meters
These devices function linearly with respect to the volumetric flow fee. Because there is not a sq. root relationship (as with differential stress devices), they have a much bigger vary. Velocity meters are least sensitive to adjustments in viscosity when the Reynolds quantity exceeds 10,000. ไดอะแฟรม are equipped with flanges or fittings to allow them to be connected directly into the piping.
Turbine circulate meters
Turbine circulate meters have been extensively used for precise liquid measurement applications. The device consists of a multi-bladed rotor mounted in a pipe, perpendicular to the circulate of the liquid. The rotor rotates because the liquid passes by way of the vanes. Rotational speed is a direct function of flow fee and could be sensed by magnetic sensors, photocells or gears. Electrical pulses may be counted and totalized.
The number of electrical pulses counted in a given time interval is proportional to the circulate price. A tachometer may be added to measure the turbine pace and decide the liquid move fee. A correctly specified and put in turbine flow meter has good accuracy, especially for low viscosity liquids.
A main downside with turbine flowmeters is bearing wear. A “bearingless” design has been developed to avoid this downside. The liquid entering the move meter passes via the spiral vanes of the stator, causing the liquid circulate to rotate. The circulate acts on the spheres, causing them to travel in the space between the first stator and the spiral-like second stator. The orbital movement of the sphere is detected electronically. The frequency of the generated pulse output is proportional to the move rate.
Vortex flowmeters
Vortex flow meters take benefit of the pure phenomenon that happens when a liquid flows via a blunt object. Vortex or vortex move alternates downstream of the item. The frequency of the vortex shedding is proportional to the rate of the liquid flowing via the flowmeter.
The three major elements of the move meter are the blunt object strut mounted on the meter bore, the sensor that detects the presence of vortices and generates an electrical pulse, and the signal amplification and conditioning transmitter whose output is proportional to the flow rate.
Electromagnetic flow meters
Magnetic circulate meters can handle most liquids and slurries, supplied that the material to be metered is electrically conductive. The primary element is the circulate tube (primary element), which is installed immediately in the pipe. The stress drop throughout the meter is identical as the strain drop via the equal size of pipe, since there are no transferring elements or circulate obstructions. The voltmeter can be linked on to the flow tube or remotely mounted and related to the circulate tube via a shielded cable.
Electromagnetic flow meters work in accordance with Faraday’s regulation of electromagnetic induction, which states that a voltage is induced when a conductor passes by way of a magnetic area. The liquid acts as the conductor; the magnetic subject is generated by an energized coil outside the move tube. The quantity of voltage generated is proportional to the move price. Two electrodes mounted on the wall of the tube detect the voltage measured by the secondary element.
Electromagnetic flowmeters supply main benefits: they will measure tough and corrosive liquids and slurries; they can measure ahead and reverse move with equal accuracy. The disadvantages of early designs had been excessive power consumption and the necessity to obtain a full tube with no move to set the meter to zero initially. Recent enhancements have eliminated these problems. The pulse-type excitation approach reduces energy consumption as a result of excitation occurs only half the time within the unit. A zero setting is no longer required.
Ultrasonic flow meters
Ultrasonic flowmeters may be divided into doppler meters and travel time (or transverse) meters. Doppler meters measure the frequency shift caused by the move of a liquid. Two sensors are mounted in a housing related to one side of the pipe. A signal of identified frequency is shipped to the liquid to be measured. Solids, bubbles or any discontinuities in the liquid cause the pulse to be reflected to the receiver element. Since the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the speed of the liquid.
There can be a portable doppler meter that can be run on AC energy or a chargeable power pack. The instrument can be used by simply clamping the sensing head to the skin of the pipe. A set of 4 to 20 mA output terminals allows the unit to be related to a strip chart recorder or different distant gadget.
The travel time meter’s sensors are mounted on both sides of the pipe. The configuration permits the sound waves to journey between the gadgets at an angle of 45 degrees. angle to the direction of fluid circulate. The speed of the sign propagating between the sensors increases or decreases with the course of transmission and the pace of the liquid being measured. By transmitting the signal in both instructions alternately, a time-differential relationship proportional to the circulate rate could be obtained. A limitation of the journey time meter is that the liquid being measured must be comparatively free of entrained gases or solids to attenuate signal scattering and absorption.
Mass move meter
Mass flowmeters the continued want for more accurate flow measurement in mass related processes (chemical reactions, warmth transfer, etc.) has led to the event of mass move meters. A number of designs can be found, but the most generally used for liquid flow functions is the Coriolis meter. It operates on the basis of a natural phenomenon known as the Coriolis drive, hence the identify.
Coriolis move meters
Coriolis meters are true mass meters that instantly measure mass circulate as a substitute of quantity flow. Since the mass is constant, the meter is linear and does not need to be adjusted for modifications in liquid characteristics. It also eliminates the necessity to compensate for changing temperature and strain circumstances. The meter is particularly appropriate for measuring liquids whose viscosity varies with velocity at a given temperature and stress.
Coriolis meters are also available in a wide range of designs. A popular unit consists of a U-shaped flow tube encapsulated in a sensor housing connected to an digital unit. The sensing unit may be mounted directly into any process. The electronics unit may be located up to 500 toes away from the sensor.
Thermal mass circulate meters
Thermal mass flowmeters are traditionally used for fuel measurements, but are additionally out there in designs for liquid circulate measurements. These mass meters additionally function independent of density, stress and viscosity. Thermal mass meters use a heated sensing factor that is isolated from the fluid circulate path. The flowing stream conducts heat from the sensing element. The heat transferred is proportional to the mass move fee. The sensor is never in direct contact with the fluid. The electronic assembly includes a flow analyzer, temperature compensator, and signal conditioner that provides a linear output proportional to the mass circulate rate.
Open channel flow meter
An “open channel” is any pipe during which liquids flow on a free surface. This contains tunnels, unpressurized sewers, partially stuffed pipes, canals, streams and rivers. Of the numerous strategies that can be used to observe flow in open channels, depth-related methods are the commonest. These techniques assume that instantaneous move may be determined from a measurement of water depth or head. Weirs and flumes are the oldest and most generally used main devices used to measure circulate in open channels.
Weirs work on the principle that an obstruction in a channel causes water to circulate backwards, making a high water level (head) behind the obstruction. The head is a function of the circulate fee, and therefore the flow rate through the device. The weir consists of a vertical plate with a pointed prime. The high of the plate could be straight or notched. Weirs are categorized by the shape of the notch. The fundamental types are V-notched, rectangular and trapezoidal.
The discharge through the weir and flume is a perform of the liquid degree, so the system must use degree measurement strategies to find out the circulate fee. Staff gauges and floating working models are the simplest gadgets to make use of for this purpose. Various digital sensing, totalizing and recording systems are also out there.
A current growth includes the use of ultrasonic pulses to measure liquid degree. Measurements are made by sending an acoustic pulse from the sensor to the floor of the liquid and timing the return echo. Linearized circuitry converts the height of the liquid to a move rate. A bar graph recorder data the circulate price and a digital totalizer information the total variety of gallons. Another lately introduced microprocessor-based system makes use of ultrasonic or float sensors. A keypad with an interactive LCD show simplifies programming, management and calibration tasks.
Select a right liquid move meter
Data shows that greater than 75% of the flow meters put in in trade don’t carry out well. And poor choice accounts for ninety % of those issues. Clearly, flowmeter selection isn’t a job for amateurs.
The most essential requirement is to know precisely what the instrument should do. There are a quantity of points to consider here. Will the measurement be used for course of control (repeatability is the principle concern) or for accounting or commerce handover (high accuracy is important)? Is an area indication or a distant sign required? If a remote output is required, is it a proportional sign or a contact closure to start or cease one other device? Is the fluid viscous, clear or slurry? Is it electrically conductive? What is its particular gravity or density? What move rates are involved in the application? What is the operating temperature and strain of the process? Accuracy (see glossary), vary, linearity, repeatability, and piping necessities must even be thought of.
It is equally important to know what a flow meter can not do in addition to what it can do earlier than making a last choice. Every instrument has strengths and weaknesses, and the degree of efficiency satisfaction is immediately related to how nicely the instrument’s features and weaknesses match the appliance necessities. Often, the user’s expectations of flowmeter performance do not match these supplied by the supplier. Most suppliers are keen to assist prospects select the right flowmeter for a particular job. Many provide questionnaires, checklists and specification sheets designed to obtain the important data necessary to match the correct flowmeter to the job.
Technical enhancements to the flowmeter must also be considered. For instance, a common mistake is to pick the design that was hottest for a given software a number of years ago and assume that it is nonetheless the best device for the job. In latest years, many changes and improvements may have occurred within the development of flowmeters for particular functions, resulting in a wider range of selections.
Flow meters are available in a extensive range of prices. Variable area flowmeters are often the least costly, with some smaller items costing less than $90. Mass flow meters are the most expensive. They begin at about $3000. However, the entire system value must at all times be thought-about when choosing a flowmeter. Installation, operation and upkeep prices are also necessary financial elements. For a variety of the extra complicated designs, upkeep costs can be excessive.
As with many other merchandise, the plant engineer usually gets what he pays for when he buys a move meter. But his satisfaction with the product will depend upon the care he makes use of in deciding on and installing the instrument. It comes again to understanding the method, the product and the circulate metering requirements. It just isn’t uncommon to “overbuy”. Plant engineers should not buy extra powerful or complicated flowmeters than they need.
Using circulate meters
Although suppliers are at all times ready to offer flowmeter installation services, it’s estimated that about 75% of users set up their own gear. But set up errors can occur. One of the most common situations isn’t allowing adequate upstream and downstream straight pipe for the flowmeter.
Each design has some tolerance for erratic velocity situations within the piping, but all installations require proper piping configurations to function successfully. Proper piping supplies the proper flow sample for the system. Without it, accuracy and performance could be adversely affected. Flow meters are typically installed backwards (especially orifice plates). Pressure sensing traces can be reversed.
For electrical parts, intrinsic safety is an important consideration in hazardous areas. Most flowmeter suppliers offer intrinsically protected designs for this sort of use.
Stray magnetic fields are current in most industrial plants. Power traces, relays, solenoids, transformers, motors and mills all contribute their share of interference. Users must make positive that the flowmeter they select isn’t topic to such disturbances. The problem occurs primarily with the electronics in the secondary parts that should be protected. Strict adherence to the manufacturer’s beneficial installation practices can usually forestall such issues.
Calibration
All move meters require an preliminary calibration. In most circumstances, the instrument is calibrated by the manufacturer for the required situations of use. However, if qualified personnel can be found at the manufacturing unit, the consumer can carry out his own calibration.
The need for recalibration relies upon greatly on how properly the meter is matched to the appliance. Certain fluids that cross by way of the meter are typically abrasive, aggressive or corrosive. Over time, some parts of the device will degrade sufficient to have an result on performance. Some designs are more vulnerable to wreck than others. For example, wear on particular person turbine blades may cause efficiency adjustments. If the application is critical, the accuracy of the circulate meter should be checked incessantly. In different circumstances, recalibration is most likely not required for many years as a result of the application isn’t crucial and wouldn’t otherwise change the meter’s efficiency. Some flowmeters require particular gear for calibration. Most manufacturers will present such companies at their plants or user amenities, and they will bring the tools with them for on-site calibration.
Maintenance
Many factors can have an effect on maintenance requirements and the expected lifetime of a move meter. The primary issue, after all, is matching the proper instrument to the particular application. Poorly chosen gear will all the time cause problems early on. Flowmeters without shifting parts normally require much less consideration than units with shifting components. But all flowmeters will eventually require some kind of upkeep.
The major element in a differential strain flowmeter requires lots of piping, valves and fittings when related to the secondary element, so maintenance can be a recurring process in such installations. Pulse lines can turn into clogged or corroded and have to be cleaned or replaced. And, improperly positioned secondary components can result in measurement errors. Repositioning components could be costly.
Flow meters with transferring components require periodic inner inspection, particularly if the liquid being measured is soiled or viscous. Installing a filter earlier than such gadgets will assist scale back fouling and wear. Accessible devices corresponding to ultrasonic or electromagnetic meters may have problems with the electronics of their secondary components. Pressure transducers associated with secondary components should be eliminated and inspected periodically.
Applications where coatings might occur are also potential issues for accessible devices corresponding to magnetic or ultrasonic units. If the coating is insulated, the operation of the magnetic move meter can eventually be impaired if the electrodes are insulated from the liquid. This situation can be averted by regular cleaning. With ultrasonic flow meters, the refraction angle could change and the acoustic energy absorbed by the coating could cause the flow meter to not work.
More articles on circulate meters:
Relation between move and pressure
Difference between move meter and circulate transmitter
Water stage sensor sorts and works
Solution of water pollutionn
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Measuring liquid move is a important requirement for many industrial crops. In some operations, the flexibility to make correct move measurements is so important that it could make the difference between a profit or a loss. In other instances, inaccurate circulate measurements or failure to make them can lead to critical (or even catastrophic) results.
For most liquid move measurement instruments, circulate is determined inferentially by measuring the change in velocity or kinetic power of the liquid. The velocity is decided by the strain distinction that forces the liquid by way of the pipe or conduit. Since the cross-sectional area of the pipe is understood and remains constant, the typical flow rate is an indicator of the circulate price. In this case, the essential relationship for determining the circulate price of a liquid is
Q = V x A
Q = Flow price of liquid through the pipe
V = Average circulate fee
A = Cross-sectional area of the pipe
Other components that affect the move price of a liquid include the viscosity and density of the liquid, and the friction of the liquid in touch with the pipe.
Liquid flow may be measured immediately using constructive displacement circulate meters. These models divide the liquid into specific increments and continue to move. The whole flow price is the buildup of the measured increments and could be counted mechanically or electronically.
Table of Contents
Reynolds quantity
Types of liquid flow meters
Differential Pressure flow meters
Orifice plate move meters
Venturi flow meters
Flow tubes
Flow nozzle
Pitot tube flowmeter
Elbow flowmeter
Target meter
Variable space flow meter
Positive displacement flow meter
Reciprocating piston flowmeter
Oval Gear Flow Meters
Disc sort meter
Rotary vane gauges
Spiral circulate meter
Velocity meters
Turbine move meters
Vortex flowmeters
Electromagnetic circulate meters
Ultrasonic circulate meters
Mass move meter
Coriolis circulate meters
Thermal mass circulate meters
Open channel move meter
Select a right liquid circulate meter
Using flow meters
Calibration
Maintenance
Reynolds quantity
The performance of a flow meter is also influenced by the dimensionless unit known as Reynolds number. It is defined because the ratio of the inertial drive of a liquid to its resistance.
The formulation is
R = 3160 x Q x Gt
D x ต
R = Reynolds number
Q = Flow price of the liquid, gpm
Gt = Specific gravity of the liquid
D = Internal diameter of the pipe, in.
ต = Viscosity of the liquid, cp
Flow price and particular gravity are the inertial forces, pipe diameter and viscosity are the resistance. For most liquid functions, the pipe diameter and particular gravity remain fixed. At very low velocities or high viscosities, R may be very low and the liquid flows in a smooth layer with the highest velocity at the center of the pipe, where viscous forces at the pipe wall limit it to very low velocities. This kind of move is called laminar flow. the R worth is under about 2000. a characteristic of laminar move is the parabolic form of its velocity distribution.
However, most functions contain turbulent flow with R-values above 3000. turbulent flow occurs at high speeds or low viscosities. The circulate breaks down into turbulent eddies that move via the pipe with the identical common velocity. The fluid velocity is less important and the velocity distribution is more uniform in shape. A transition zone exists between turbulent and laminar flow. Depending on the pipe configuration and different installation circumstances, the flow on this zone may be turbulent or laminar.
Types of liquid move meters
Many types of move meters can be found to be used in closed pipe techniques. In basic, devices may be categorised as differential stress meters, optimistic displacement meters, velocity meters, and mass meters. Differential strain gadgets (also generally recognized as head gauges) embrace orifice plates, venturi, circulate tubes, flow nozzles, pitot tubes, elbow kind flow meters, goal type move meters and variable area circulate meters.
Positive displacement move meters embrace piston, elliptical-gear, chapter-actuated disk and rotary vane varieties. Velocity meters include turbine, vortex shedding, electromagnetic, and acoustic designs. Mass meters embrace Coriolis and thermal sorts. Measurement of liquid move in open channels usually entails weirs and flumes.
Differential Pressure move meters
The use of differential pressure as an inferred measurement of the flow rate of a liquid is well known. By far, differential stress move meters are the most commonly used unit right now. It is estimated that greater than 50% of all liquid move measurement functions use this kind of unit.
The fundamental operating precept of differential strain flow meters relies on the premise that the stress drop throughout the meter is proportional to the sq. of the circulate rate. The move price is obtained by measuring the differential strain and extracting the sq. root.
Like most circulate meters, differential pressure flow meters have a primary element and a secondary element. The major factor causes a change in kinetic energy, which creates a differential strain in the pipe. The device have to be properly matched to the pipe size, flow circumstances and liquid traits. And, the accuracy of the component measurement should remain good within reasonable limits. The secondary component measures the differential pressure and provides a sign or reading that is converted to an precise move value.
Orifice plate move meters
Orifice plates are the most well-liked liquid flow meters in use right now. An orifice is solely a flat piece of metal with a selected size gap drilled in it. Most orifices are concentric, but eccentric, conical (quadrant) and segmented designs are additionally obtainable.
In apply, the orifice is installed in the pipe between two flanges. As the first system, the orifice restricts the move of fluid, thereby making a differential stress throughout the plate. Pressure measurement ports on either side of the plate are used to detect the distinction. The primary advantages of orifice plates are that they haven’t any shifting components and that their cost does not improve significantly with the scale of the pipe.
Tapered and quadrant orifices are comparatively new. These units were developed primarily for measuring liquids with low Reynolds numbers. An primarily fixed circulate coefficient can be maintained at an R-value under 5000. Tapered orifice plates have an upstream bevel, the depth and angle of which should be calculated and machined for each software.
The segment wedge is a variant of the segment orifice. It is a throttle orifice designed primarily to measure the flow of liquids containing solids. The device is able to measuring move at low Reynolds numbers and still maintaining the required sq. root relationship. Its design is simple and the wedge hole has only one important dimension. The pressure drop via the gadget is just about half that of a standard orifice plate.
The one-piece wedge meeting combines the wedge element and strain measurement fitting into a one-piece pipe fitting bolted to a traditional pressure transmitter. No particular piping or fittings are required to put in the gadget within the pipeline.
The metering accuracy of all orifice move meters is determined by the set up situations, the orifice plate space ratio, and the bodily traits of the liquid being measured.
Venturi move meters
The benefit of a venturi is its capability to deal with large flows at low strain drops. A venturi is basically a bit of pipe with a conical inlet and a straight throat. As the liquid passes through the throat, it increases in velocity, resulting in a stress difference between the inlet and outlet regions.
Flow meters don’t have any shifting parts. They could be put in in giant diameter pipes utilizing flanged, welded or threaded finish connections. The unit is often fitted with 4 or extra strain measuring ports to common the measured strain. Venturi tubes can be used for many liquids, together with those with excessive solids content material.
Flow tubes
Flow tubes are considerably much like venturi, besides that they don’t have an entrance cone. They have a tapered throat, but the outlet is elongated and smooth. The distance between the entrance and the tip is about one-half the diameter of the tube. The strain measurement port is positioned about one-half the diameter of the tube downstream and one diameter upstream.
Flow nozzle
At high velocities, the flow nozzle can handle roughly 60% of the liquid circulate in comparison with an orifice plate with the same strain drop. Liquids containing suspended solids can additionally be metered. However, these devices aren’t really helpful for prime viscosity liquids or liquids containing giant quantities of viscous solids.
Pitot tube flowmeter
The Pitot tube senses two types of pressure concurrently, shock pressure and static stress. The shock gadget consists of a tube with one end of the tube bent at proper angles to the direction of circulate. The end of the static tube is closed, but has a small slot within the facet of the gadget. These tubes could be put in individually in the pipe or mixed in a housing.
Pitot tubes are usually installed by welding a coupling to the pipe and inserting the probe through the coupling. The use of most Pitot tubes is restricted to single point measurements. These gadgets are vulnerable to clogging by international matter in the liquid. The benefits of Pitot tubes are low price, no shifting parts, simple set up and minimal pressure drop.
Elbow flowmeter
Elbow flow meters work on the precept that centrifugal forces are exerted alongside the outer edges as the liquid strikes alongside a round path. Thus, because the liquid flows via the pipe elbow, the pressure on the inside surface of the elbow is proportional to the density of the liquid multiplied by the sq. of its velocity. In addition, the drive is inversely proportional to the radius of the elbow.
Arbitrary ninety degrees. Elbows can be used as liquid flow meters. All that is required are two small holes positioned at the midpoint of the elbow (45 degree point) for the manometer tap. A pressure sensing line could be connected to the tap utilizing any convenient technique.
Target meter
The target meter senses and measures the pressure caused by the liquid hanging the goal or a drag disk suspended in the liquid stream. By measuring the force utilized to the goal the liquid move price may be instantly indicated. In its simplest type, the meter consists only of an articulated oscillating plate that moves outward with the liquid circulate. In this case, the device is used as a flow indicator.
More refined versions use subtle low-level drive sensor sensing parts. The target pressure caused by the liquid circulate is sensed by a strain gauge. The output signal of the meter indicates the circulate fee. The target meter can be used to measure the move rate of dirty or corrosive liquids.
Variable space circulate meter
Variable area meters consist primarily of a conical tube and a float. Although categorized as differential pressure devices, they’re truly constant strain devices. Flange end connections provide a easy method to install them within the pipe. Working precept of rotameter is when there isn’t a liquid move, the float is free to rest at the bottom of the pipe. When the liquid enters the bottom of the pipe, the float begins to rise. The position of the float varies immediately with the flow price. Its exact position is on the level where the pressure difference between the upper and decrease surfaces balances the load of the float.
Because the circulate price could be learn directly on a scale mounted subsequent to the tube, there isn’t any want for an auxiliary move reading gadget. However, if desired, an automatic sensing device can be utilized to sense the float degree and transmit the circulate sign. Variable area circulate meter tubes are made of glass, steel or plastic. Tube diameters range from 1/4 to larger than 6 inches.
Positive displacement move meter
The operation of these units consists of separating the liquid into precisely measured increments and continuing to maneuver. Each segment is counted by a connection register. Because each increment represents a discrete quantity, constructive displacement units are well-liked in computerized dosing and accounting purposes. Volumetric flow meters are ideal for measuring the circulate of viscous liquids or where a easy mechanical flow meter system is required.
Reciprocating piston flowmeter
Reciprocating piston flow meters are available in single and multi-piston variations. The particular alternative is dependent upon the vary of flow charges required in a particular utility. Piston circulate meters can be used to handle a variety of liquids. The fluid by no means is available in contact with gears or other components that could clog or corrode.
Oval Gear Flow Meters
Oval gear flowmeters have two rotating oval gears with closely synchronized teeth. A fixed quantity of fluid passes via the meter with every revolution. The shaft rotation may be monitored to obtain a particular circulate price.
Disc type meter
Chapter moving disc type meters have a movable disc mounted on a concentric sphere that’s located in a spherical sidewall chamber. The pressure of the fluid passing via the measurement chamber causes the disc to oscillate in its circulation path with out rotating about its personal axis. It is the one transferring part in the measurement chamber.
A pin extending vertically from the disk is linked to a mechanical counter, which displays the oscillating motion of the disk. Each cycle is proportional to a specific circulate price. As with all positive displacement meters, changes in viscosity beneath a given threshold will have an effect on the measurement accuracy. A wide selection of sizes and capacities are available. These units could be created from a variety of development materials.
Rotary vane gauges
Rotary vane meters can be found in a variety of designs, but they all operate on the same precept. The fundamental unit consists of an equally divided rotating impeller (containing two or extra compartments) mounted inside the instrument housing. The impeller is in constant contact with the housing. As the impeller rotates, a fixed volume of liquid is swept from every compartment to the outlet of the meter. The number of impeller revolutions is calculated and recorded in volume models.
Spiral flow meter
The helical flow meter consists of two radially inclined helical rotors which are gear pushed together with minimal clearance between the rotors and the housing. The two rotors switch the liquid from one finish of the chamber to the other alongside the axial path.
Velocity meters
These instruments operate linearly with respect to the volumetric circulate rate. Because there is no square root relationship (as with differential pressure devices), they’ve a a lot larger vary. Velocity meters are least delicate to modifications in viscosity when the Reynolds quantity exceeds 10,000. Most velocity meter housings are equipped with flanges or fittings to allow them to be linked instantly into the piping.
Turbine flow meters
Turbine flow meters have been extensively used for exact liquid measurement purposes. The device consists of a multi-bladed rotor mounted in a pipe, perpendicular to the circulate of the liquid. The rotor rotates because the liquid passes via the vanes. Rotational speed is a direct perform of circulate fee and can be sensed by magnetic sensors, photocells or gears. Electrical pulses could be counted and totalized.
The number of electrical pulses counted in a given time period is proportional to the flow fee. A tachometer may be added to measure the turbine velocity and decide the liquid move rate. A properly specified and put in turbine flow meter has good accuracy, particularly for low viscosity liquids.
A major problem with turbine flowmeters is bearing put on. A “bearingless” design has been developed to keep away from this problem. The liquid entering the circulate meter passes by way of the spiral vanes of the stator, causing the liquid move to rotate. The flow acts on the spheres, inflicting them to travel in the area between the primary stator and the spiral-like second stator. The orbital movement of the sphere is detected electronically. The frequency of the generated pulse output is proportional to the flow price.
Vortex flowmeters
Vortex flow meters take benefit of the pure phenomenon that occurs when a liquid flows through a blunt object. Vortex or vortex circulate alternates downstream of the object. The frequency of the vortex shedding is proportional to the rate of the liquid flowing via the flowmeter.
The three primary elements of the move meter are the blunt object strut mounted on the meter bore, the sensor that detects the presence of vortices and generates an electrical pulse, and the sign amplification and conditioning transmitter whose output is proportional to the move rate.
Electromagnetic move meters
Magnetic circulate meters can deal with most liquids and slurries, supplied that the material to be metered is electrically conductive. The primary component is the flow tube (primary element), which is put in instantly in the pipe. The pressure drop across the meter is identical as the stress drop by way of the equivalent length of pipe, since there are no transferring parts or move obstructions. The voltmeter could be related directly to the move tube or remotely mounted and connected to the flow tube through a shielded cable.
Electromagnetic circulate meters work based on Faraday’s regulation of electromagnetic induction, which states that a voltage is induced when a conductor passes via a magnetic area. The liquid acts as the conductor; the magnetic area is generated by an energized coil outdoors the circulate tube. The quantity of voltage generated is proportional to the circulate fee. Two electrodes mounted on the wall of the tube detect the voltage measured by the secondary element.
Electromagnetic flowmeters offer main benefits: they will measure tough and corrosive liquids and slurries; they’ll measure forward and reverse flow with equal accuracy. The disadvantages of early designs were high power consumption and the necessity to get hold of a full tube with no flow to set the meter to zero initially. Recent improvements have eradicated these issues. The pulse-type excitation approach reduces energy consumption because excitation happens solely half the time in the unit. A zero setting is now not required.
Ultrasonic move meters
Ultrasonic flowmeters may be divided into doppler meters and journey time (or transverse) meters. Doppler meters measure the frequency shift brought on by the flow of a liquid. Two sensors are mounted in a housing linked to at least one side of the pipe. A signal of recognized frequency is shipped to the liquid to be measured. Solids, bubbles or any discontinuities within the liquid trigger the heartbeat to be mirrored to the receiver element. Since the liquid inflicting the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the velocity of the liquid.
There can be a transportable doppler meter that can be run on AC energy or a chargeable energy pack. The instrument can be utilized by simply clamping the sensing head to the skin of the pipe. A set of 4 to 20 mA output terminals allows the unit to be linked to a strip chart recorder or different distant gadget.
The journey time meter’s sensors are mounted on each side of the pipe. The configuration allows the sound waves to journey between the gadgets at an angle of forty five levels. angle to the direction of fluid flow. The velocity of the signal propagating between the sensors will increase or decreases with the course of transmission and the velocity of the liquid being measured. By transmitting the signal in each instructions alternately, a time-differential relationship proportional to the move rate may be obtained. A limitation of the journey time meter is that the liquid being measured should be comparatively freed from entrained gases or solids to minimize signal scattering and absorption.
Mass flow meter
Mass flowmeters the continued need for extra accurate flow measurement in mass related processes (chemical reactions, heat transfer, etc.) has led to the development of mass circulate meters. A variety of designs are available, but essentially the most generally used for liquid flow purposes is the Coriolis meter. It operates on the basis of a pure phenomenon often recognized as the Coriolis pressure, hence the identify.
Coriolis circulate meters
Coriolis meters are true mass meters that instantly measure mass circulate as an alternative of quantity flow. Since the mass is fixed, the meter is linear and doesn’t must be adjusted for adjustments in liquid traits. It also eliminates the necessity to compensate for changing temperature and pressure circumstances. The meter is particularly suitable for measuring liquids whose viscosity varies with velocity at a given temperature and pressure.
Coriolis meters are additionally out there in a big selection of designs. A popular unit consists of a U-shaped move tube encapsulated in a sensor housing related to an electronic unit. The sensing unit may be mounted immediately into any course of. The electronics unit can be located up to 500 ft away from the sensor.
Thermal mass move meters
Thermal mass flowmeters are historically used for gas measurements, but are additionally available in designs for liquid move measurements. These mass meters also operate impartial of density, pressure and viscosity. Thermal mass meters use a heated sensing element that’s isolated from the fluid circulate path. The flowing stream conducts heat from the sensing component. The heat transferred is proportional to the mass move price. The sensor isn’t in direct contact with the fluid. The digital meeting includes a move analyzer, temperature compensator, and sign conditioner that provides a linear output proportional to the mass flow fee.
Open channel flow meter
An “open channel” is any pipe during which liquids flow on a free surface. This includes tunnels, unpressurized sewers, partially filled pipes, canals, streams and rivers. Of the many techniques that can be utilized to watch circulate in open channels, depth-related methods are the most common. These strategies assume that instantaneous circulate may be determined from a measurement of water depth or head. Weirs and flumes are the oldest and most widely used main gadgets used to measure circulate in open channels.
Weirs work on the principle that an obstruction in a channel causes water to move backwards, creating a excessive water level (head) behind the obstruction. The head is a function of the flow rate, and due to this fact the move price through the system. The weir consists of a vertical plate with a pointed top. The prime of the plate may be straight or notched. Weirs are categorised by the form of the notch. The fundamental types are V-notched, rectangular and trapezoidal.
The discharge via the weir and flume is a operate of the liquid stage, so the device should use stage measurement techniques to discover out the circulate fee. Staff gauges and floating operating models are the only gadgets to make use of for this purpose. Various digital sensing, totalizing and recording systems are additionally out there.
A current growth contains the use of ultrasonic pulses to measure liquid degree. Measurements are made by sending an acoustic pulse from the sensor to the floor of the liquid and timing the return echo. Linearized circuitry converts the height of the liquid to a move price. A bar graph recorder data the flow price and a digital totalizer information the entire number of gallons. Another just lately introduced microprocessor-based system makes use of ultrasonic or float sensors. A keypad with an interactive LCD display simplifies programming, management and calibration duties.
Select a proper liquid move meter
Data reveals that more than 75% of the flow meters put in in business do not carry out well. And poor choice accounts for 90 percent of those problems. Clearly, flowmeter choice just isn’t a job for amateurs.
The most essential requirement is to know precisely what the instrument ought to do. There are numerous issues to suppose about right here. Will the measurement be used for process management (repeatability is the principle concern) or for accounting or trade handover (high accuracy is important)? Is a local indication or a remote sign required? If a remote output is required, is it a proportional signal or a contact closure to start or cease one other device? Is the fluid viscous, clear or slurry? Is it electrically conductive? What is its particular gravity or density? What flow rates are concerned in the application? What is the working temperature and stress of the process? Accuracy (see glossary), range, linearity, repeatability, and piping requirements should even be thought-about.
It is equally essential to grasp what a move meter can not do in addition to what it could do before making a final selection. Every instrument has strengths and weaknesses, and the diploma of efficiency satisfaction is directly associated to how nicely the instrument’s features and weaknesses match the application requirements. Often, the user’s expectations of flowmeter efficiency do not match these offered by the supplier. Most suppliers are keen to assist prospects choose the right flowmeter for a particular job. Many present questionnaires, checklists and specification sheets designed to obtain the critical info necessary to match the proper flowmeter to the job.
Technical enhancements to the flowmeter must also be thought of. For example, a typical mistake is to select the design that was hottest for a given utility a number of years in the past and assume that it is still one of the best tool for the job. In current years, many changes and innovations might have occurred within the growth of flowmeters for specific functions, resulting in a wider range of selections.
Flow meters can be found in a variety of prices. Variable area flowmeters are usually the least expensive, with some smaller models costing lower than $90. Mass move meters are the costliest. They begin at about $3000. However, the whole system cost should all the time be thought of when selecting a flowmeter. Installation, operation and upkeep prices are also essential economic elements. For a variety of the extra advanced designs, upkeep costs may be excessive.
As with many other merchandise, the plant engineer usually will get what he pays for when he buys a move meter. But his satisfaction with the product will depend on the care he makes use of in choosing and installing the instrument. It comes back to understanding the method, the product and the flow metering requirements. It isn’t uncommon to “overbuy”. Plant engineers mustn’t purchase more powerful or advanced flowmeters than they want.
Using flow meters
Although suppliers are always ready to offer flowmeter set up providers, it is estimated that about 75% of customers set up their very own equipment. But set up errors can happen. One of the commonest conditions just isn’t allowing adequate upstream and downstream straight pipe for the flowmeter.
Each design has some tolerance for erratic velocity situations in the piping, but all installations require correct piping configurations to operate successfully. Proper piping offers the correct circulate sample for the gadget. Without it, accuracy and efficiency can be adversely affected. Flow meters are typically installed backwards (especially orifice plates). Pressure sensing strains may also be reversed.
For electrical parts, intrinsic safety is an important consideration in hazardous areas. Most flowmeter suppliers provide intrinsically secure designs for this sort of use.
Stray magnetic fields are present in most industrial plants. Power strains, relays, solenoids, transformers, motors and turbines all contribute their share of interference. Users should ensure that the flowmeter they choose is not subject to such disturbances. The problem occurs primarily with the electronics in the secondary parts that should be protected. Strict adherence to the manufacturer’s beneficial installation practices can usually stop such problems.
Calibration
All move meters require an initial calibration. In most cases, the instrument is calibrated by the manufacturer for the required situations of use. However, if qualified personnel are available on the manufacturing facility, the consumer can perform his personal calibration.
The want for recalibration depends significantly on how well the meter is matched to the application. Certain fluids that move through the meter tend to be abrasive, aggressive or corrosive. Over time, some elements of the device will degrade enough to have an effect on performance. Some designs are extra prone to break than others. For example, put on on particular person turbine blades could cause performance changes. If the application is important, the accuracy of the flow meter should be checked frequently. In other instances, recalibration will not be required for many years because the appliance just isn’t crucial and wouldn’t otherwise change the meter’s performance. Some flowmeters require particular gear for calibration. Most producers will present such services at their plants or person amenities, and they’ll bring the equipment with them for on-site calibration.
Maintenance
Many elements can have an result on maintenance requirements and the anticipated lifetime of a flow meter. The major issue, of course, is matching the right instrument to the particular utility. Poorly selected tools will at all times trigger issues early on. Flowmeters with out transferring elements often require much less consideration than gadgets with transferring elements. But all flowmeters will finally require some type of upkeep.
The main component in a differential pressure flowmeter requires a lot of piping, valves and fittings when linked to the secondary factor, so upkeep can be a recurring process in such installations. Pulse lines can turn into clogged or corroded and should be cleaned or changed. And, improperly positioned secondary elements can lead to measurement errors. Repositioning elements can be expensive.
Flow meters with moving components require periodic inside inspection, particularly if the liquid being measured is dirty or viscous. Installing a filter earlier than such units will assist cut back fouling and wear. Accessible devices corresponding to ultrasonic or electromagnetic meters may have problems with the electronics of their secondary elements. Pressure transducers associated with secondary parts ought to be eliminated and inspected periodically.
Applications the place coatings may occur are additionally potential problems for accessible devices corresponding to magnetic or ultrasonic gadgets. If the coating is insulated, the operation of the magnetic flow meter can finally be impaired if the electrodes are insulated from the liquid. This situation can be prevented by regular cleansing. With ultrasonic flow meters, the refraction angle could change and the acoustic energy absorbed by the coating could cause the circulate meter to not work.
More articles on circulate meters:
Relation between circulate and strain
Difference between circulate meter and move transmitter
Water degree sensor types and works
Solution of water air pollutionn