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