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