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