Measuring liquid circulate is a important requirement for many industrial crops. In some operations, the flexibility to make correct circulate measurements is so necessary that it could make the difference between a profit or a loss. In different cases, inaccurate flow measurements or failure to make them can lead to serious (or even catastrophic) results.
For most liquid circulate measurement instruments, flow is set inferentially by measuring the change in velocity or kinetic vitality of the liquid. The velocity depends on the stress difference that forces the liquid by way of the pipe or conduit. Since the cross-sectional area of the pipe is thought and stays constant, the average circulate price is an indicator of the flow fee. In this case, the fundamental relationship for determining the circulate fee of a liquid is
Q = V x A
Q = Flow fee of liquid via the pipe
V = Average flow rate
A = Cross-sectional space of the pipe
Other elements that affect the move price of a liquid embody the viscosity and density of the liquid, and the friction of the liquid in contact with the pipe.
Liquid circulate can be measured immediately using constructive displacement move meters. These items divide the liquid into specific increments and continue to maneuver. The whole flow rate is the buildup of the measured increments and may be counted mechanically or electronically.
Table of Contents
Reynolds number
Types of liquid flow meters
Differential Pressure move meters
Orifice plate circulate meters
Venturi circulate meters
Flow tubes
Flow nozzle
Pitot tube flowmeter
Elbow flowmeter
Target meter
Variable space move meter
Positive displacement circulate 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 move meters
Ultrasonic move meters
Mass circulate meter
Coriolis circulate meters
Thermal mass flow meters
Open channel move meter
Select a right liquid move meter
Using flow meters
Calibration
Maintenance
Reynolds number
The performance of a flow meter can also be influenced by the dimensionless unit known as Reynolds quantity. It is defined because the ratio of the inertial drive of a liquid to its resistance.
The method is
R = 3160 x Q x Gt
D x ต
R = Reynolds quantity
Q = Flow price of the liquid, gpm
Gt = Specific gravity of the liquid
D = Internal diameter of the pipe, in.
ต = Viscosity of the liquid, cp
Flow price and particular gravity are the inertial forces, pipe diameter and viscosity are the resistance. For most liquid applications, the pipe diameter and particular gravity stay fixed. At very low velocities or high viscosities, R may be very low and the liquid flows in a clean layer with the best velocity on the middle of the pipe, the place viscous forces at the pipe wall restrict it to very low velocities. This sort of flow is identified as laminar circulate. the R value is under about 2000. a attribute of laminar flow is the parabolic form of its velocity distribution.
However, most functions contain turbulent flow with R-values above 3000. turbulent move occurs at high speeds or low viscosities. The circulate breaks down into turbulent eddies that flow via the pipe with the same common velocity. The fluid velocity is much less vital and the speed distribution is more uniform in form. A transition zone exists between turbulent and laminar circulate. Depending on the pipe configuration and other set up situations, the flow on this zone could additionally be turbulent or laminar.
Types of liquid move meters
Many kinds of flow meters can be found to be used in closed pipe techniques. In general, units could be categorised as differential pressure meters, optimistic displacement meters, velocity meters, and mass meters. Differential stress gadgets (also generally known as head gauges) embody orifice plates, venturi, move tubes, move nozzles, pitot tubes, elbow type move meters, target type circulate meters and variable area circulate meters.
Positive displacement circulate meters include piston, elliptical-gear, chapter-actuated disk and rotary vane varieties. Velocity meters embrace turbine, vortex shedding, electromagnetic, and acoustic designs. Mass meters include Coriolis and thermal types. Measurement of liquid circulate in open channels usually entails weirs and flumes.
Differential Pressure flow meters
The use of differential pressure as an inferred measurement of the circulate fee of a liquid is well-known. By far, differential pressure circulate meters are probably the most generally used unit at present. It is estimated that more than 50% of all liquid move measurement functions use this type of unit.
The basic working precept of differential strain circulate meters relies on the premise that the pressure drop across the meter is proportional to the square of the circulate rate. The flow price is obtained by measuring the differential pressure and extracting the square root.
Like most move meters, differential pressure circulate meters have a major element and a secondary element. The major factor causes a change in kinetic vitality, which creates a differential stress within the pipe. The system must be properly matched to the pipe dimension, move conditions and liquid characteristics. And, the accuracy of the factor measurement should stay good inside cheap limits. The secondary factor measures the differential strain and supplies a sign or reading that’s transformed to an precise move value.
Orifice plate circulate meters
Orifice plates are the preferred liquid flow meters in use today. An orifice is simply a flat piece of metal with a specific size hole drilled in it. Most orifices are concentric, but eccentric, conical (quadrant) and segmented designs are also out there.
In apply, the orifice is installed within the pipe between two flanges. As the first gadget, the orifice restricts the circulate of fluid, thereby creating a differential strain across the plate. Pressure measurement ports on each side of the plate are used to detect the difference. The major advantages of orifice plates are that they have no shifting parts and that their price does not increase significantly with the scale of the pipe.
Tapered and quadrant orifices are comparatively new. These models were developed primarily for measuring liquids with low Reynolds numbers. An essentially constant circulate coefficient could be maintained at an R-value under 5000. Tapered orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each 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 system is able to measuring flow at low Reynolds numbers and nonetheless sustaining the required sq. root relationship. Its design is simple and the wedge gap has only one crucial dimension. The strain drop through the gadget is just about half that of a conventional orifice plate.
The one-piece wedge meeting combines the wedge factor and stress measurement fitting into a one-piece pipe becoming bolted to a traditional stress transmitter. No particular piping or fittings are required to install the system in the pipeline.
The metering accuracy of all orifice flow meters depends on the installation circumstances, the orifice plate space ratio, and the physical traits of the liquid being measured.
Venturi circulate meters
The benefit of a venturi is its ability to deal with large flows at low pressure drops. A venturi is basically a piece of pipe with a conical inlet and a straight throat. As the liquid passes by way of the throat, it will increase in velocity, resulting in a stress distinction between the inlet and outlet areas.
Flow meters have no shifting elements. They could be installed in large diameter pipes utilizing flanged, welded or threaded finish connections. The unit is often fitted with four or more stress measuring ports to average the measured pressure. Venturi tubes can be utilized for many liquids, including these with high solids content.
Flow tubes
Flow tubes are somewhat much like venturi, except that they don’t have an entrance cone. They have a tapered throat, but 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 move nozzle can handle roughly 60% of the liquid move compared to an orifice plate with the same pressure drop. Liquids containing suspended solids can also be metered. However, these devices are not beneficial for top viscosity liquids or liquids containing massive amounts of viscous solids.
Pitot tube flowmeter
The Pitot tube senses two types of stress concurrently, shock strain and static stress. The shock system consists of a tube with one finish of the tube bent at right angles to the course of move. The finish of the static tube is closed, however has a small slot within the aspect of the system. These tubes could be installed individually in the pipe or mixed in a housing.
Pitot tubes are sometimes installed by welding a coupling to the pipe and inserting the probe by way of the coupling. The use of most Pitot tubes is proscribed to single point measurements. These devices are susceptible to clogging by international matter in the liquid. The advantages of Pitot tubes are low price, no transferring elements, easy set up and minimal stress drop.
Elbow flowmeter
Elbow circulate meters work on the precept that centrifugal forces are exerted alongside the outer edges because the liquid strikes alongside a round path. Thus, as the liquid flows via the pipe elbow, the pressure on the inner surface 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’s required are two small holes placed on the midpoint of the elbow (45 degree point) for the manometer faucet. A pressure sensing line can be related to the tap utilizing any handy technique.
Target meter
The goal meter senses and measures the force caused by the liquid putting the goal or a drag disk suspended in the liquid stream. By measuring the drive utilized to the goal the liquid circulate fee could be immediately indicated. In its simplest form, the meter consists only of an articulated oscillating plate that moves outward with the liquid flow. In this case, the device is used as a move indicator.
More refined versions use subtle low-level pressure sensor sensing components. The target force caused by the liquid move is sensed by a pressure gauge. The output signal of the meter indicates the move rate. The goal meter can be utilized to measure the move rate of soiled or corrosive liquids.
Variable space move meter
Variable space meters consist primarily of a conical tube and a float. Although classified as differential pressure units, they’re really fixed stress units. Flange end connections present a simple approach to install them in the pipe. Working precept of rotameter is when there is no liquid circulate, the float is free to rest on 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 price. Its exact place is on the level where the stress distinction between the upper and decrease surfaces balances the burden of the float.
Because the circulate fee could be learn instantly on a scale mounted next to the tube, there is not a need for an auxiliary move reading device. However, if desired, an automatic sensing gadget can be used to sense the float stage and transmit the move signal. Variable space circulate meter tubes are manufactured from glass, steel or plastic. Tube diameters vary from 1/4 to larger than 6 inches.
Positive displacement move meter
The operation of those units consists of separating the liquid into precisely measured increments and persevering with to move. Each segment is counted by a connection register. Because each increment represents a discrete volume, constructive displacement units are in style in computerized dosing and accounting functions. Volumetric flow meters are good for measuring the move of viscous liquids or where a easy mechanical move meter system is required.
Reciprocating piston flowmeter
Reciprocating piston flow meters are available in single and multi-piston variations. The specific alternative is dependent upon the range of circulate rates required in a selected utility. Piston circulate meters can be used to handle a broad range of liquids. The fluid never comes 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 carefully synchronized enamel. A mounted amount of fluid passes through the meter with each revolution. The shaft rotation could be monitored to acquire a specific flow rate.
Disc kind meter
Chapter shifting disc kind meters have a movable disc mounted on a concentric sphere that’s positioned in a spherical sidewall chamber. The strain of the fluid passing via the measurement chamber causes the disc to oscillate in its circulation path without rotating about its personal axis. It is the only transferring part within the measurement chamber.
A pin extending vertically from the disk is connected to a mechanical counter, which displays the oscillating motion of the disk. Each cycle is proportional to a particular circulate fee. As with all constructive displacement meters, adjustments in viscosity under a given threshold will have an result on the measurement accuracy. A wide selection of sizes and capacities are available. These items could be created from quite so much of building materials.
Rotary vane gauges
Rotary vane meters are available in a wide selection of designs, but all of them function on the same principle. 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 fixed contact with the housing. As the impeller rotates, a fixed quantity of liquid is swept from each compartment to the outlet of the meter. The number of impeller revolutions is calculated and recorded in volume models.
Spiral move meter
The helical move meter consists of two radially inclined helical rotors which would possibly be gear driven along with minimal clearance between the rotors and the housing. The two rotors switch the liquid from one end of the chamber to the opposite alongside the axial direction.
Velocity meters
These instruments function linearly with respect to the volumetric flow price. Because there is no square root relationship (as with differential pressure devices), they’ve a a lot larger range. Velocity meters are least delicate to modifications in viscosity when the Reynolds number exceeds 10,000. Most velocity meter housings are geared up with flanges or fittings to permit them to be related directly into the piping.
Turbine flow meters
Turbine circulate meters have been extensively used for precise liquid measurement purposes. The gadget consists of a multi-bladed rotor mounted in a pipe, perpendicular to the circulate of the liquid. The rotor rotates as the liquid passes through the vanes. Rotational velocity is a direct perform 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 interval is proportional to the circulate rate. A tachometer could be added to measure the turbine pace and decide the liquid flow rate. A properly specified and put in turbine move meter has good accuracy, especially for low viscosity liquids.
A main drawback with turbine flowmeters is bearing put on. A “bearingless” design has been developed to keep away from this problem. The liquid getting into the flow meter passes through the spiral vanes of the stator, inflicting the liquid flow to rotate. The flow acts on the spheres, causing them to journey within the space between the primary stator and the spiral-like second stator. The orbital movement of the sphere is detected electronically. The frequency of the generated pulse output is proportional to the move price.
Vortex flowmeters
Vortex flow meters take benefit of the natural phenomenon that occurs when a liquid flows by way of a blunt object. Vortex or vortex circulate 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 major components 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 signal amplification and conditioning transmitter whose output is proportional to the circulate fee.
Electromagnetic move meters
Magnetic move meters can deal with most liquids and slurries, offered that the material to be metered is electrically conductive. The primary component is the circulate tube (primary element), which is put in immediately in the pipe. The stress drop throughout the meter is identical because the strain drop through the equal length of pipe, since there aren’t any moving elements or move obstructions. The voltmeter can be connected directly to the flow tube or remotely mounted and related to the flow tube through a shielded cable.
Electromagnetic circulate meters work based on Faraday’s regulation of electromagnetic induction, which states that a voltage is induced when a conductor passes through a magnetic field. The liquid acts because the conductor; the magnetic area is generated by an energized coil exterior the flow tube. The amount of voltage generated is proportional to the move price. Two electrodes mounted on the wall of the tube detect the voltage measured by the secondary factor.
Electromagnetic flowmeters offer major advantages: they will measure difficult and corrosive liquids and slurries; they’ll measure ahead and reverse flow with equal accuracy. The disadvantages of early designs were excessive power consumption and the necessity to acquire a full tube with no circulate to set the meter to zero initially. Recent improvements have eliminated these issues. The pulse-type excitation technique reduces energy consumption as a end result of excitation occurs solely half the time in the unit. A zero setting is not required.
Ultrasonic circulate meters
Ultrasonic flowmeters can be divided into doppler meters and travel time (or transverse) meters. Doppler meters measure the frequency shift brought on by the circulate of a liquid. Two sensors are mounted in a housing connected to at least one side of the pipe. A sign of identified frequency is sent to the liquid to be measured. Solids, bubbles or any discontinuities in the liquid trigger the coronary heart beat to be reflected to the receiver component. Since the liquid inflicting the reflection is shifting, the frequency of the returned pulse is shifted. The frequency shift is proportional to the velocity of the liquid.
There can also be a portable doppler meter that could be run on AC power or a rechargeable power pack. The instrument can be utilized by merely clamping the sensing head to the surface of the pipe. A set of four to 20 mA output terminals allows the unit to be connected to a strip chart recorder or other distant device.
The journey time meter’s sensors are mounted on each side of the pipe. The configuration allows the sound waves to travel between the gadgets at an angle of 45 levels. angle to the direction of fluid move. The pace of the signal propagating between the sensors increases or decreases with the course of transmission and the pace of the liquid being measured. By transmitting the sign in both instructions alternately, a time-differential relationship proportional to the circulate fee may 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 sign scattering and absorption.
Mass circulate meter
Mass flowmeters the continued need for more correct flow measurement in mass associated processes (chemical reactions, warmth switch, and so on.) has led to the event of mass circulate meters. A number of designs can be found, but essentially the most generally used for liquid flow functions is the Coriolis meter. It operates on the idea of a pure phenomenon known as the Coriolis drive, therefore the name.
Coriolis circulate meters
Coriolis meters are true mass meters that directly measure mass circulate as an alternative of quantity circulate. Since the mass is constant, the meter is linear and does not have to be adjusted for changes in liquid traits. It additionally eliminates the want to compensate for changing temperature and strain situations. The meter is especially suitable for measuring liquids whose viscosity varies with velocity at a given temperature and pressure.
Coriolis meters are also out there in a wide range of designs. A in style unit consists of a U-shaped flow tube encapsulated in a sensor housing related to an electronic unit. The sensing unit could be mounted directly into any course of. The electronics unit may be located as a lot as 500 toes away from the sensor.
Thermal mass move meters
Thermal mass flowmeters are traditionally used for gasoline measurements, but are additionally out there in designs for liquid circulate measurements. These mass meters additionally function impartial of density, strain 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 component. The warmth transferred is proportional to the mass flow fee. The sensor isn’t in direct contact with the fluid. The digital assembly includes a move 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 by which liquids circulate on a free floor. This consists of tunnels, unpressurized sewers, partially crammed pipes, canals, streams and rivers. Of the many strategies that can be used to watch move in open channels, depth-related methods are the most typical. These methods assume that instantaneous flow can be determined from a measurement of water depth or head. Weirs and flumes are the oldest and most generally used main gadgets used to measure move in open channels.
Weirs work on the precept that an obstruction in a channel causes water to move backwards, making a high water degree (head) behind the obstruction. The head is a operate of the move fee, and subsequently the move rate through the device. The weir consists of a vertical plate with a pointed high. The top of the plate could be straight or notched. Weirs are categorised by the form of the notch. The basic varieties are V-notched, rectangular and trapezoidal.
The discharge via the weir and flume is a operate of the liquid degree, so the system must use level measurement methods to find out the circulate rate. Staff gauges and floating working items are the best devices to make use of for this function. Various electronic sensing, totalizing and recording methods are also obtainable.
A current improvement contains 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 move price. A bar graph recorder data the flow rate and a digital totalizer records the total number of gallons. Another lately launched microprocessor-based system uses ultrasonic or float sensors. A keypad with an interactive LCD display simplifies programming, management and calibration tasks.
Select a right liquid flow meter
Data shows that greater than 75% of the flow meters installed in business do not perform nicely. And poor choice accounts for 90 p.c of those issues. Clearly, flowmeter choice just isn’t a job for amateurs.
The most important requirement is to know exactly what the instrument ought to do. There are a quantity of issues to assume about right here. Will the measurement be used for course of control (repeatability is the primary concern) or for accounting or trade handover (high accuracy is important)? Is a local indication or a remote sign required? If a remote output is required, is it a proportional signal or a contact closure to begin or cease another device? Is the fluid viscous, clean or slurry? Is it electrically conductive? What is its specific gravity or density? What move 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 should also be considered.
It is equally essential to know what a move meter cannot do in addition to what it could do earlier than making a ultimate alternative. Every instrument has strengths and weaknesses, and the degree of performance satisfaction is directly associated to how properly the instrument’s features and weaknesses match the applying necessities. Often, the user’s expectations of flowmeter performance do not match those supplied by the supplier. Most suppliers are eager to help clients select the right flowmeter for a particular job. Many present questionnaires, checklists and specification sheets designed to obtain the important info essential to match the correct flowmeter to the job.
Technical enhancements to the flowmeter must also be thought of. For instance, a typical mistake is to choose out the design that was hottest for a given application a quantity of years ago and assume that it’s still one of the best tool for the job. In latest years, many changes and innovations may have occurred in the improvement of flowmeters for particular functions, resulting in a wider range of choices.
Flow meters are available in a variety of costs. Variable area flowmeters are normally the least costly, with some smaller items costing lower than $90. Mass move meters are the most expensive. They begin at about $3000. However, the total system cost should always be thought of when choosing a flowmeter. Installation, operation and maintenance prices are additionally essential financial components. For a few of the more complex designs, maintenance prices could be high.
As with many other products, the plant engineer usually gets what he pays for when he buys a circulate meter. But his satisfaction with the product will depend upon the care he makes use of in selecting and putting in the instrument. It comes again to understanding the method, the product and the move metering requirements. It just isn’t uncommon to “overbuy”. Plant engineers shouldn’t buy extra highly effective or complex flowmeters than they want.
Using circulate meters
Although suppliers are at all times ready to offer flowmeter set up providers, it is estimated that about 75% of customers set up their very own gear. But set up errors can occur. One of the commonest situations isn’t allowing enough upstream and downstream straight pipe for the flowmeter.
Each design has some tolerance for erratic velocity situations in the piping, however all installations require proper piping configurations to operate effectively. Proper piping provides the proper flow pattern for the gadget. Without it, accuracy and efficiency could be adversely affected. Flow meters are sometimes put in backwards (especially orifice plates). Pressure sensing lines may additionally be reversed.
For electrical elements, intrinsic safety is an important consideration in hazardous areas. Most flowmeter suppliers provide intrinsically protected designs for this sort of use.
Stray magnetic fields are current in most industrial vegetation. Power traces, relays, solenoids, transformers, motors and turbines all contribute their share of interference. Users should be sure that the flowmeter they choose isn’t subject to such disturbances. The drawback happens primarily with the electronics in the secondary parts that must be protected. Strict adherence to the manufacturer’s really helpful installation practices can usually forestall such issues.
Calibration
All move meters require an initial calibration. In most circumstances, the instrument is calibrated by the manufacturer for the specified situations of use. However, if certified personnel can be found on the manufacturing unit, the person can perform his own calibration.
The need for recalibration depends tremendously on how properly the meter is matched to the application. Certain fluids that pass via the meter tend to be abrasive, aggressive or corrosive. Over time, some elements of the device will degrade sufficient to have an effect on performance. Some designs are more prone to wreck than others. For example, wear on individual turbine blades may cause performance adjustments. If the application is critical, the accuracy of the flow meter must be checked incessantly. In different cases, recalibration is in all probability not required for many years as a end result of the application isn’t critical and would not in any other case change the meter’s efficiency. Some flowmeters require special gear for calibration. Most manufacturers will provide such services at their crops or consumer amenities, and they will bring the gear with them for on-site calibration.
Maintenance
Many factors can affect maintenance necessities and the expected lifetime of a circulate meter. The main factor, after all, is matching the right instrument to the precise utility. Poorly chosen equipment will always cause problems early on. Flowmeters with out shifting elements often require less attention than devices with shifting components. But all flowmeters will ultimately require some sort of maintenance.
The major component in a differential strain 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 strains can become clogged or corroded and must be cleaned or replaced. And, improperly positioned secondary elements can result in measurement errors. Repositioning parts could be expensive.
Flow meters with shifting components require periodic inside inspection, especially if the liquid being measured is dirty or viscous. Installing a filter before such units will assist scale back fouling and wear. Accessible devices such as ultrasonic or electromagnetic meters may have problems with the electronics of their secondary components. Pressure transducers related to secondary parts should be eliminated and inspected periodically.
Applications where coatings may happen are also potential problems for accessible instruments such as magnetic or ultrasonic gadgets. If the coating is insulated, the operation of the magnetic move meter can ultimately be impaired if the electrodes are insulated from the liquid. This state of affairs could be averted by regular cleaning. With ultrasonic move meters, the refraction angle may change and the acoustic energy absorbed by the coating may cause the circulate meter to not work.
More articles on move meters:
Relation between flow and stress
Difference between move meter and circulate transmitter
Water level sensor sorts and works
Solution of water pollutionn
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Measuring liquid circulate is a crucial requirement for many industrial vegetation. In some operations, the flexibility to make accurate flow measurements is so essential that it could make the difference between a revenue or a loss. In different cases, inaccurate circulate measurements or failure to make them can result in critical (or even catastrophic) results.
For most liquid move measurement instruments, circulate is set inferentially by measuring the change in velocity or kinetic vitality of the liquid. The velocity depends on the strain distinction that forces the liquid by way of the pipe or conduit. Since the cross-sectional area of the pipe is thought and remains fixed, the typical flow rate is an indicator of the flow rate. In this case, the essential relationship for figuring out the flow price of a liquid is
Q = V x A
Q = Flow fee of liquid through the pipe
V = Average move fee
A = Cross-sectional space of the pipe
Other factors that affect the flow rate of a liquid include the viscosity and density of the liquid, and the friction of the liquid in contact with the pipe.
Liquid circulate can be measured directly using constructive displacement move meters. These models divide the liquid into particular increments and proceed to maneuver. The complete flow rate is the buildup of the measured increments and could be counted mechanically or electronically.
Table of Contents
Reynolds number
Types of liquid move meters
Differential Pressure move meters
Orifice plate circulate meters
Venturi move 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 kind meter
Rotary vane gauges
Spiral circulate meter
Velocity meters
Turbine circulate meters
Vortex flowmeters
Electromagnetic flow meters
Ultrasonic circulate meters
Mass flow meter
Coriolis move meters
Thermal mass move meters
Open channel move meter
Select a proper liquid circulate meter
Using move meters
Calibration
Maintenance
Reynolds quantity
The efficiency of a circulate meter can additionally be influenced by the dimensionless unit called Reynolds number. It is defined because 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 purposes, the pipe diameter and specific gravity stay fixed. At very low velocities or high viscosities, R could be very low and the liquid flows in a easy layer with the highest velocity at the middle of the pipe, the place viscous forces at the pipe wall restrict it to very low velocities. This sort of move known as laminar flow. the R worth is below about 2000. a attribute of laminar circulate is the parabolic form 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 move breaks down into turbulent eddies that circulate by way of the pipe with the same average velocity. The fluid velocity is less vital and the rate distribution is more uniform in shape. เกจวัดแรงดัน10bar exists between turbulent and laminar move. Depending on the pipe configuration and other set up circumstances, the flow in this zone may be turbulent or laminar.
Types of liquid flow meters
Many forms of circulate meters are available for use in closed pipe methods. In common, devices can be categorised as differential strain meters, optimistic displacement meters, velocity meters, and mass meters. Differential pressure units (also often recognized as head gauges) embody orifice plates, venturi, flow tubes, circulate nozzles, pitot tubes, elbow kind flow meters, target type flow meters and variable area circulate meters.
Positive displacement flow meters embody piston, elliptical-gear, chapter-actuated disk and rotary vane sorts. Velocity meters embrace turbine, vortex shedding, electromagnetic, and acoustic designs. Mass meters embody Coriolis and thermal varieties. Measurement of liquid flow in open channels often entails weirs and flumes.
Differential Pressure flow meters
The use of differential pressure as an inferred measurement of the circulate fee of a liquid is well-known. By far, differential strain flow meters are the most generally used unit at present. It is estimated that greater than 50% of all liquid circulate measurement applications use this kind of unit.
The fundamental operating principle of differential pressure circulate meters is predicated on the premise that the pressure drop across the meter is proportional to the sq. of the flow rate. The flow price is obtained by measuring the differential pressure and extracting the square root.
Like most move meters, differential strain move meters have a major component and a secondary element. The major component causes a change in kinetic power, which creates a differential strain within the pipe. The system must be correctly matched to the pipe dimension, flow circumstances and liquid characteristics. And, the accuracy of the factor measurement should stay good within reasonable limits. The secondary element measures the differential pressure and offers a sign or reading that’s transformed to an actual move value.
Orifice plate move meters
Orifice plates are the preferred liquid flow meters in use at present. An orifice is simply a flat piece of metallic with a particular measurement hole drilled in it. Most orifices are concentric, but eccentric, conical (quadrant) and segmented designs are additionally obtainable.
In practice, the orifice is put in in the pipe between two flanges. As the first gadget, the orifice restricts the move of fluid, thereby creating a differential stress across the plate. Pressure measurement ports on either side of the plate are used to detect the distinction. The primary benefits of orifice plates are that they don’t have any shifting parts and that their value does not enhance significantly with the size of the pipe.
Tapered and quadrant orifices are comparatively new. These items were developed primarily for measuring liquids with low Reynolds numbers. An basically fixed circulate coefficient may be maintained at an R-value beneath 5000. Tapered orifice plates have an upstream bevel, the depth and angle of which should be calculated and machined for every software.
The phase wedge is a variant of the phase orifice. It is a throttle orifice designed primarily to measure the move of liquids containing solids. The device is capable of measuring move at low Reynolds numbers and nonetheless sustaining the required square root relationship. Its design is simple and the wedge gap has just one critical dimension. The stress drop by way of the gadget is simply about half that of a traditional orifice plate.
The one-piece wedge meeting combines the wedge element and strain measurement becoming into a one-piece pipe becoming bolted to a conventional strain transmitter. No particular piping or fittings are required to install the gadget in the pipeline.
The metering accuracy of all orifice move meters is dependent upon the set up conditions, the orifice plate area ratio, and the bodily characteristics of the liquid being measured.
Venturi move meters
The advantage of a venturi is its capacity to deal with giant 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 through the throat, it will increase in velocity, resulting in a stress distinction between the inlet and outlet areas.
Flow meters haven’t any moving components. They may be put in in large diameter pipes using flanged, welded or threaded finish connections. The unit is usually fitted with 4 or extra strain measuring ports to common the measured pressure. Venturi tubes can be used for many liquids, including these with excessive 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, 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 situated about one-half the diameter of the tube downstream and one diameter upstream.
Flow nozzle
At high velocities, the move nozzle can handle roughly 60% of the liquid circulate compared to an orifice plate with the same stress drop. Liquids containing suspended solids can additionally be metered. However, these devices aren’t really helpful for prime viscosity liquids or liquids containing giant amounts of viscous solids.
Pitot tube flowmeter
The Pitot tube senses two kinds of pressure concurrently, shock pressure and static pressure. The shock gadget consists of a tube with one finish of the tube bent at right angles to the course of flow. The finish of the static tube is closed, however has a small slot within the aspect of the device. These tubes could 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 proscribed to single level measurements. These units are prone to clogging by international matter within the liquid. The benefits of Pitot tubes are low price, no transferring components, simple installation and minimal strain drop.
Elbow flowmeter
Elbow circulate meters work on the precept that centrifugal forces are exerted along the outer edges as the liquid moves alongside a round path. Thus, as the liquid flows by way of 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 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 positioned on the midpoint of the elbow (45 degree point) for the manometer tap. A strain sensing line could be connected to the faucet utilizing any convenient methodology.
Target meter
The goal meter senses and measures the force attributable to the liquid hanging the target or a drag disk suspended in the liquid stream. By measuring the drive applied to the target the liquid flow fee can be immediately indicated. In its simplest type, the meter consists only of an articulated oscillating plate that strikes outward with the liquid move. In this case, the system is used as a circulate indicator.
More sophisticated versions use sophisticated low-level force sensor sensing parts. The target force attributable to the liquid move is sensed by a pressure gauge. The output sign of the meter signifies the move price. The goal meter can be utilized to measure the circulate fee of soiled or corrosive liquids.
Variable space move meter
Variable space meters consist primarily of a conical tube and a float. Although classified as differential pressure devices, they’re really fixed strain units. Flange finish connections provide a easy method to install them within the pipe. Working precept of rotameter is when there is no liquid circulate, 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 directly with the circulate fee. Its exact place is on the level where the strain difference between the higher and decrease surfaces balances the burden of the float.
Because the circulate price may be learn immediately on a scale mounted subsequent to the tube, there isn’t a need for an auxiliary circulate reading gadget. However, if desired, an automated sensing gadget can be used to sense the float degree and transmit the move signal. Variable area circulate meter tubes are made from glass, metallic or plastic. Tube diameters vary from 1/4 to greater than 6 inches.
Positive displacement flow meter
The operation of these items consists of separating the liquid into exactly measured increments and continuing to move. Each segment is counted by a connection register. Because each increment represents a discrete quantity, positive displacement units are in style in computerized dosing and accounting functions. Volumetric move meters are excellent for measuring the circulate of viscous liquids or the place a simple mechanical move meter system is required.
Reciprocating piston flowmeter
Reciprocating piston move meters are available in single and multi-piston variations. The particular alternative depends on the range of move rates required in a specific utility. Piston circulate meters can be utilized to deal with a variety of liquids. The fluid never comes in contact with gears or other components that could clog or corrode.
Oval Gear Flow Meters
Oval gear flowmeters have two rotating oval gears with intently synchronized enamel. A fastened quantity of fluid passes through the meter with every revolution. The shaft rotation may be monitored to obtain a particular move rate.
Disc sort 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 without rotating about its own axis. It is the only shifting part within the measurement chamber.
A pin extending vertically from the disk is connected to a mechanical counter, which screens the oscillating motion of the disk. Each cycle is proportional to a selected move rate. As with all constructive displacement meters, modifications in viscosity under a given threshold will affect the measurement accuracy. A big selection of sizes and capacities can be found. These units can be produced from quite so much of building materials.
Rotary vane gauges
Rotary vane meters are available in quite lots of designs, however all of them operate on the identical precept. The basic 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 onerous and fast volume of liquid is swept from every compartment to the outlet of the meter. The variety of impeller revolutions is calculated and recorded in volume models.
Spiral flow meter
The helical move meter consists of two radially inclined helical rotors which may 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 opposite along the axial course.
Velocity meters
These devices operate linearly with respect to the volumetric flow rate. Because there isn’t a sq. root relationship (as with differential strain devices), they’ve a a lot bigger vary. Velocity meters are least sensitive to changes in viscosity when the Reynolds number exceeds 10,000. Most velocity meter housings are geared up with flanges or fittings to allow them to be linked instantly into the piping.
Turbine circulate meters
Turbine flow meters have been extensively used for exact liquid measurement applications. The system consists of a multi-bladed rotor mounted in a pipe, perpendicular to the flow of the liquid. The rotor rotates because the liquid passes via the vanes. Rotational speed is a direct function of flow rate and could be sensed by magnetic sensors, photocells or gears. Electrical pulses may be counted and totalized.
The number of electrical pulses counted in a given time interval is proportional to the move fee. A tachometer can be added to measure the turbine pace and determine the liquid move fee. A correctly specified and put in turbine flow meter has good accuracy, particularly for low viscosity liquids.
A main downside with turbine flowmeters is bearing wear. A “bearingless” design has been developed to avoid this problem. The liquid getting into the flow meter passes by way of the spiral vanes of the stator, causing the liquid circulate to rotate. The circulate acts on the spheres, inflicting them to travel within the area between the first stator and the spiral-like second stator. The orbital movement of the sphere is detected electronically. The frequency of the generated pulse output is proportional to the flow price.
Vortex flowmeters
Vortex move meters benefit from the pure phenomenon that happens when a liquid flows via a blunt object. Vortex or vortex move alternates downstream of the item. The frequency of the vortex shedding is proportional to the velocity of the liquid flowing by way of the flowmeter.
The three main elements of the flow meter are the blunt object strut mounted on the meter bore, the sensor that detects the presence of vortices and generates an electrical pulse, and the sign amplification and conditioning transmitter whose output is proportional to the circulate price.
Electromagnetic circulate meters
Magnetic move meters can handle most liquids and slurries, provided that the fabric to be metered is electrically conductive. The major element is the move tube (primary element), which is put in directly within the pipe. The pressure drop across the meter is similar because the strain drop by way of the equal length of pipe, since there are no moving parts or move obstructions. The voltmeter may be connected on to the move tube or remotely mounted and linked to the move tube by way of a shielded cable.
Electromagnetic move meters work in accordance with Faraday’s law of electromagnetic induction, which states that a voltage is induced when a conductor passes via a magnetic subject. The liquid acts as the conductor; the magnetic field is generated by an energized coil outside the move tube. The quantity of voltage generated is proportional to the move price. Two electrodes mounted on the wall of the tube detect the voltage measured by the secondary factor.
Electromagnetic flowmeters supply major benefits: they’ll measure difficult and corrosive liquids and slurries; they can measure forward and reverse move with equal accuracy. The disadvantages of early designs have been high power consumption and the necessity to acquire a full tube with no move to set the meter to zero initially. Recent improvements have eradicated these problems. The pulse-type excitation approach reduces power consumption because excitation occurs only half the time within the unit. A zero setting is no longer required.
Ultrasonic flow meters
Ultrasonic flowmeters may be divided into doppler meters and travel time (or transverse) meters. Doppler meters measure the frequency shift brought on by the move of a liquid. Two sensors are mounted in a housing linked 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 within the liquid trigger the pulse 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 speed of the liquid.
There can also be a conveyable doppler meter that may be run on AC energy or a rechargeable power pack. The instrument can be used by simply clamping the sensing head to the outside of the pipe. A set of four to 20 mA output terminals permits the unit to be linked to a strip chart recorder or other remote device.
The travel time meter’s sensors are mounted on each side of the pipe. The configuration allows the sound waves to journey between the devices at an angle of forty five levels. angle to the path of fluid move. The speed of the sign propagating between the sensors increases or decreases with the course of transmission and the velocity of the liquid being measured. By transmitting the sign in each directions alternately, a time-differential relationship proportional to the flow fee can be obtained. A limitation of the travel time meter is that the liquid being measured must be relatively freed from entrained gases or solids to attenuate sign scattering and absorption.
Mass move meter
Mass flowmeters the continued need for extra accurate flow measurement in mass related processes (chemical reactions, warmth switch, and so on.) has led to the development of mass flow meters. A variety of designs are available, however the most commonly used for liquid move purposes is the Coriolis meter. It operates on the idea of a pure phenomenon known as the Coriolis drive, therefore the identify.
Coriolis move meters
Coriolis meters are true mass meters that immediately measure mass flow instead of quantity flow. Since the mass is constant, the meter is linear and does not have to be adjusted for changes in liquid characteristics. It additionally eliminates the want to compensate for altering temperature and strain conditions. The meter is particularly appropriate for measuring liquids whose viscosity varies with velocity at a given temperature and stress.
Coriolis meters are additionally obtainable in a wide selection of designs. A in style unit consists of a U-shaped circulate tube encapsulated in a sensor housing linked to an electronic unit. The sensing unit could be mounted immediately into any process. The electronics unit can be positioned up to 500 ft away from the sensor.
Thermal mass move meters
Thermal mass flowmeters are historically used for gas measurements, but are also obtainable in designs for liquid circulate measurements. These mass meters additionally function impartial of density, pressure and viscosity. Thermal mass meters use a heated sensing 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 price. The sensor is never in direct contact with the fluid. The electronic meeting features a flow analyzer, temperature compensator, and signal conditioner that provides a linear output proportional to the mass move rate.
Open channel circulate meter
An “open channel” is any pipe in which liquids flow on a free floor. This includes tunnels, unpressurized sewers, partially crammed pipes, canals, streams and rivers. Of the various techniques that can be used to observe circulate in open channels, depth-related strategies are the most typical. These techniques assume that instantaneous circulate could be determined from a measurement of water depth or head. Weirs and flumes are the oldest and most widely used primary units used to measure move in open channels.
Weirs work on the principle that an obstruction in a channel causes water to flow backwards, creating a excessive water degree (head) behind the obstruction. The head is a perform of the flow fee, and subsequently the circulate fee by way of 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 via the weir and flume is a perform of the liquid level, so the system should use degree measurement techniques to determine the circulate price. Staff gauges and floating operating units are the simplest devices to make use of for this function. Various digital sensing, totalizing and recording methods are additionally out there.
A current growth consists of the use 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 information the entire variety of gallons. Another lately launched microprocessor-based system makes use of ultrasonic or float sensors. A keypad with an interactive LCD show simplifies programming, control and calibration tasks.
Select a right liquid circulate meter
Data shows that more than 75% of the move meters put in in industry do not carry out well. And poor choice accounts for ninety percent of these problems. Clearly, flowmeter selection isn’t a job for amateurs.
The most essential requirement is to know exactly what the instrument ought to do. There are a number of points to consider right here. Will the measurement be used for course of management (repeatability is the principle concern) or for accounting or trade handover (high accuracy is important)? Is a local indication or a distant signal required? If a remote output is required, is it a proportional sign or a contact closure to begin or stop one other device? Is the fluid viscous, clean or slurry? Is it electrically conductive? What is its specific gravity or density? What circulate rates are concerned in the application? What is the operating temperature and stress of the process? Accuracy (see glossary), range, linearity, repeatability, and piping requirements must even be thought-about.
It is equally important to grasp what a move meter cannot do as nicely as what it may possibly do earlier than making a final alternative. Every instrument has strengths and weaknesses, and the diploma of performance satisfaction is directly related to how nicely the instrument’s features and weaknesses match the appliance requirements. Often, the user’s expectations of flowmeter performance don’t match these supplied by the supplier. Most suppliers are keen to help customers select the best flowmeter for a selected job. Many provide questionnaires, checklists and specification sheets designed to acquire the crucial data essential to match the correct flowmeter to the job.
Technical improvements to the flowmeter must also be thought of. For instance, a typical mistake is to pick out the design that was hottest for a given application a number of years ago and assume that it’s nonetheless the best device for the job. In recent years, many adjustments and improvements could have occurred within the development of flowmeters for particular applications, leading to a wider range of decisions.
Flow meters can be found in a wide range of prices. Variable area flowmeters are usually the least expensive, with some smaller items costing less than $90. Mass move meters are the costliest. They begin at about $3000. However, the total system value must always be considered when selecting a flowmeter. Installation, operation and upkeep costs are also necessary economic elements. For some of the more advanced designs, upkeep prices can be high.
As with many other products, the plant engineer often gets what he pays for when he buys a move meter. But his satisfaction with the product will rely upon the care he uses in deciding on and putting in the instrument. It comes again to understanding the process, the product and the circulate metering necessities. It just isn’t unusual to “overbuy”. Plant engineers should not purchase extra highly effective or advanced flowmeters than they want.
Using flow meters
Although suppliers are at all times prepared to offer flowmeter installation providers, it’s estimated that about 75% of users install their very own equipment. But installation errors can happen. One of the most common conditions just isn’t permitting enough upstream and downstream straight pipe for the flowmeter.
Each design has some tolerance for erratic velocity circumstances within the piping, however all installations require correct piping configurations to function successfully. Proper piping supplies the proper move pattern for the gadget. Without it, accuracy and performance can be adversely affected. Flow meters are typically put in backwards (especially orifice plates). Pressure sensing lines can also be reversed.
For electrical components, intrinsic safety is an important consideration in hazardous areas. Most flowmeter suppliers supply intrinsically secure designs for this sort of use.
Stray magnetic fields are current in most industrial crops. Power strains, relays, solenoids, transformers, motors and turbines all contribute their share of interference. Users must be positive that the flowmeter they choose is not subject to such disturbances. The downside happens primarily with the electronics in the secondary components that should be protected. Strict adherence to the manufacturer’s recommended installation practices can often prevent such issues.
Calibration
All circulate meters require an preliminary calibration. In most circumstances, the instrument is calibrated by the producer for the desired circumstances of use. However, if certified personnel can be found on the manufacturing facility, the consumer can carry out his personal calibration.
The need for recalibration depends tremendously on how nicely the meter is matched to the application. Certain fluids that pass via the meter are typically abrasive, aggressive or corrosive. Over time, some components of the system will degrade sufficient to have an effect on performance. Some designs are extra vulnerable to damage than others. For example, wear on particular person turbine blades can cause efficiency adjustments. If the application is important, the accuracy of the move meter should be checked frequently. In other cases, recalibration will not be required for many years as a end result of the application just isn’t critical and wouldn’t otherwise change the meter’s performance. Some flowmeters require particular equipment for calibration. Most producers will provide such services at their plants or user services, and they’re going to convey the tools with them for on-site calibration.
Maintenance
Many components can affect maintenance requirements and the anticipated lifetime of a circulate meter. The major issue, of course, is matching the correct instrument to the specific software. Poorly selected tools will all the time cause problems early on. Flowmeters with out transferring parts often require less consideration than units with moving elements. But all flowmeters will eventually require some sort of maintenance.
The main factor in a differential pressure flowmeter requires lots of piping, valves and fittings when linked to the secondary element, so upkeep is normally a recurring task in such installations. Pulse strains can turn out to be clogged or corroded and should be cleaned or changed. And, improperly positioned secondary components can result in measurement errors. Repositioning parts can be costly.
Flow meters with shifting parts require periodic inside inspection, especially if the liquid being measured is soiled or viscous. Installing a filter before such units will help cut back fouling and put on. Accessible instruments such as ultrasonic or electromagnetic meters might have problems with the electronics of their secondary components. Pressure transducers associated with secondary parts must be eliminated and inspected periodically.
Applications where coatings may occur are additionally potential issues for accessible instruments similar to magnetic or ultrasonic units. If the coating is insulated, the operation of the magnetic move meter can finally be impaired if the electrodes are insulated from the liquid. This scenario may be averted by regular cleaning. With ultrasonic circulate meters, the refraction angle might change and the acoustic vitality absorbed by the coating can cause the flow meter to not work.
More articles on circulate meters:
Relation between flow and stress
Difference between circulate meter and circulate transmitter
Water degree sensor sorts and works
Solution of water air pollutionn