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