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