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