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