Measuring liquid flow is a crucial requirement for many industrial vegetation. In some operations, the ability to make accurate flow measurements is so essential that it can make the difference between a profit or a loss. In different instances, inaccurate circulate measurements or failure to make them can result in severe (or even catastrophic) results.
For most liquid circulate measurement instruments, circulate is determined inferentially by measuring the change in velocity or kinetic power of the liquid. The velocity is determined by the stress difference that forces the liquid through the pipe or conduit. Since the cross-sectional space of the pipe is known and remains fixed, the average flow rate is an indicator of the flow fee. In this case, the essential relationship for determining the circulate rate of a liquid is
Q = V x A
Q = Flow price of liquid via the pipe
V = Average flow price
A = Cross-sectional area of the pipe
Other components that affect the move fee of a liquid embody the viscosity and density of the liquid, and the friction of the liquid in contact with the pipe.
Liquid circulate may be measured instantly using optimistic displacement move meters. These models divide the liquid into specific increments and continue to maneuver. The whole move rate is the buildup of the measured increments and can be counted mechanically or electronically.
Table of Contents
Reynolds quantity
Types of liquid circulate meters
Differential Pressure flow meters
Orifice plate move meters
Venturi circulate meters
Flow tubes
Flow nozzle
Pitot tube flowmeter
Elbow flowmeter
Target meter
Variable area move meter
Positive displacement move meter
Reciprocating piston flowmeter
Oval Gear Flow Meters
Disc sort meter
Rotary vane gauges
Spiral flow meter
Velocity meters
Turbine flow meters
Vortex flowmeters
Electromagnetic move meters
Ultrasonic move meters
Mass flow meter
Coriolis circulate meters
Thermal mass flow meters
Open channel move meter
Select a right liquid circulate meter
Using circulate meters
Calibration
Maintenance
Reynolds number
The efficiency of a flow meter can additionally be influenced by the dimensionless unit called Reynolds number. It is defined as the ratio of the inertial pressure of a liquid to its resistance.
The formula is
R = 3160 x Q x Gt
D x ต
R = Reynolds number
Q = Flow fee of the liquid, gpm
Gt = Specific gravity of the liquid
D = Internal diameter of the pipe, in.
ต = Viscosity of the liquid, cp
Flow fee and specific gravity are the inertial forces, pipe diameter and viscosity are the resistance. For most liquid functions, the pipe diameter and specific gravity remain fixed. At very low velocities or high viscosities, R may be very low and the liquid flows in a smooth layer with the highest velocity on the center of the pipe, where viscous forces at the pipe wall limit it to very low velocities. This type of flow is recognized as laminar circulate. the R worth is under about 2000. a characteristic of laminar circulate is the parabolic shape of its velocity distribution.
However, most purposes contain turbulent flow with R-values above 3000. turbulent move happens at excessive speeds or low viscosities. The move breaks down into turbulent eddies that flow through the pipe with the identical average velocity. The fluid velocity is much less important and the rate distribution is more uniform in shape. A transition zone exists between turbulent and laminar flow. Depending on the pipe configuration and different installation circumstances, the circulate in this zone could additionally be turbulent or laminar.
Types of liquid move meters
Many types of circulate meters are available to be used in closed pipe techniques. In general, devices could be categorized as differential stress meters, positive displacement meters, velocity meters, and mass meters. Differential stress gadgets (also known as head gauges) embrace orifice plates, venturi, move tubes, circulate nozzles, pitot tubes, elbow type move meters, goal type flow meters and variable area circulate meters.
Positive displacement circulate meters include piston, elliptical-gear, chapter-actuated disk and rotary vane varieties. Velocity meters embody turbine, vortex shedding, electromagnetic, and acoustic designs. Mass meters embody Coriolis and thermal varieties. Measurement of liquid circulate in open channels normally involves weirs and flumes.
Differential Pressure flow meters
The use of differential strain as an inferred measurement of the circulate fee of a liquid is well-known. By far, differential pressure circulate meters are probably the most generally used unit right now. It is estimated that more than 50% of all liquid flow measurement functions use this kind of unit.
The primary working precept of differential stress flow meters is predicated on the premise that the pressure drop throughout the meter is proportional to the square of the flow fee. The circulate fee is obtained by measuring the differential strain and extracting the sq. root.
Like most circulate meters, differential strain move meters have a primary component and a secondary element. The main element causes a change in kinetic vitality, which creates a differential stress within the pipe. The system have to be properly matched to the pipe size, circulate circumstances and liquid traits. And, the accuracy of the factor measurement should remain good inside reasonable limits. The secondary element measures the differential pressure and provides a sign or reading that’s transformed to an actual flow value.
Orifice plate flow meters
Orifice plates are the preferred liquid circulate meters in use today. An orifice is simply a flat piece of metal with a specific dimension gap drilled in it. Most orifices are concentric, but eccentric, conical (quadrant) and segmented designs are additionally obtainable.
In apply, the orifice is installed within the pipe between two flanges. As the primary system, the orifice restricts the move of fluid, thereby making a differential pressure across the plate. Pressure measurement ports on either side of the plate are used to detect the difference. The main advantages of orifice plates are that they don’t have any shifting components and that their price does not increase considerably with the scale of the pipe.
Tapered and quadrant orifices are relatively new. These units were developed primarily for measuring liquids with low Reynolds numbers. An essentially fixed circulate coefficient can be maintained at an R-value beneath 5000. Tapered orifice plates have an upstream bevel, the depth and angle of which have to be calculated and machined for each utility.
The section wedge is a variant of the phase orifice. It is a throttle orifice designed primarily to measure the move of liquids containing solids. The gadget is capable of measuring flow at low Reynolds numbers and still sustaining the required sq. root relationship. Its design is simple and the wedge gap has just one important dimension. The pressure drop by way of the device is only about half that of a standard orifice plate.
The one-piece wedge meeting combines the wedge factor and strain measurement becoming right into a one-piece pipe fitting bolted to a standard pressure transmitter. No particular piping or fittings are required to install the gadget in the pipeline.
The metering accuracy of all orifice flow meters is decided by the installation conditions, the orifice plate area ratio, and the physical characteristics of the liquid being measured.
Venturi circulate meters
The benefit of a venturi is its capacity to deal with large flows at low stress drops. A venturi is essentially a bit of pipe with a conical inlet and a straight throat. As weksler ea14 through the throat, it increases in velocity, resulting in a stress difference between the inlet and outlet regions.
Flow meters haven’t any shifting components. They may be installed in giant diameter pipes using flanged, welded or threaded end connections. The unit is often fitted with four or more strain measuring ports to average the measured pressure. Venturi tubes can be used for many liquids, together with those with high solids content.
Flow tubes
Flow tubes are somewhat much like venturi, except that they do not have an entrance cone. They have a tapered throat, however the outlet is elongated and smooth. The distance between the entrance and the tip is about one-half the diameter of the tube. The strain measurement port is situated about one-half the diameter of the tube downstream and one diameter upstream.
Flow nozzle
At excessive velocities, the flow nozzle can deal with approximately 60% of the liquid flow in comparison with an orifice plate with the identical pressure drop. Liquids containing suspended solids may also be metered. However, these devices are not recommended for prime viscosity liquids or liquids containing large amounts of viscous solids.
Pitot tube flowmeter
The Pitot tube senses two types of stress concurrently, shock pressure and static stress. The shock system consists of a tube with one end of the tube bent at proper angles to the path of move. The finish of the static tube is closed, however has a small slot in the side of the gadget. These tubes can be installed individually within the pipe or mixed in a housing.
Pitot tubes are typically 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 gadgets are prone to clogging by overseas matter within the liquid. The advantages of Pitot tubes are low price, no transferring components, simple installation and minimal stress drop.
Elbow flowmeter
Elbow circulate meters work on the principle that centrifugal forces are exerted along the outer edges because the liquid strikes along a round path. Thus, as the liquid flows by way of the pipe elbow, the drive 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 pressure 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 diploma point) for the manometer faucet. A strain sensing line can be related to the tap utilizing any convenient method.
Target meter
The goal meter senses and measures the drive attributable to the liquid putting the target or a drag disk suspended in the liquid stream. By measuring the pressure applied to the target the liquid flow fee could be immediately indicated. In its easiest kind, the meter consists only of an articulated oscillating plate that moves outward with the liquid flow. In this case, the device is used as a move indicator.
More subtle variations use sophisticated low-level pressure sensor sensing parts. The goal pressure caused by the liquid move is sensed by a pressure gauge. The output signal of the meter indicates the circulate price. The goal meter can be utilized to measure the flow price of soiled or corrosive liquids.
Variable area move meter
Variable space meters consist primarily of a conical tube and a float. Although categorised as differential stress devices, they are actually constant stress devices. Flange finish connections present a simple method to install them within the pipe. Working precept of rotameter is when there isn’t a liquid move, the float is free to rest on the bottom of the pipe. When the liquid enters the underside of the pipe, the float begins to rise. The position of the float varies immediately with the circulate fee. Its exact position is at the point where the strain distinction between the upper and decrease surfaces balances the burden of the float.
Because the move rate can be read instantly on a scale mounted subsequent to the tube, there is no need for an auxiliary circulate reading gadget. However, if desired, an computerized sensing system can be utilized to sense the float level and transmit the circulate signal. Variable space flow meter tubes are manufactured from glass, steel or plastic. Tube diameters range from 1/4 to higher than 6 inches.
Positive displacement move meter
The operation of these units consists of separating the liquid into exactly measured increments and continuing to maneuver. Each phase is counted by a connection register. Because every increment represents a discrete quantity, positive displacement models are in style in computerized dosing and accounting functions. Volumetric move meters are perfect for measuring the flow 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 versions. The specific alternative depends on the vary of flow charges required in a selected application. Piston flow meters can be used to deal with a wide range of liquids. The fluid by no means 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 intently synchronized tooth. A fixed quantity of fluid passes by way of the meter with every revolution. The shaft rotation could be monitored to obtain a selected circulate rate.
Disc type meter
Chapter transferring disc kind meters have a movable disc mounted on a concentric sphere that is located in a spherical sidewall chamber. The stress 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 moving part in the measurement chamber.
A pin extending vertically from the disk is related to a mechanical counter, which monitors the oscillating motion of the disk. Each cycle is proportional to a particular flow rate. As with all constructive displacement meters, modifications in viscosity under a given threshold will have an effect on the measurement accuracy. A big selection of sizes and capacities can be found. These items could be produced from quite lots of development supplies.
Rotary vane gauges
Rotary vane meters are available in quite a lot of designs, however all of them operate on the identical precept. The primary unit consists of an equally divided rotating impeller (containing two or extra compartments) mounted contained in the instrument housing. The impeller is in constant contact with the housing. As the impeller rotates, a set quantity of liquid is swept from every compartment to the outlet of the meter. The variety of impeller revolutions is calculated and recorded in quantity items.
Spiral move meter
The helical circulate meter consists of two radially inclined helical rotors that are gear pushed together with minimal clearance between the rotors and the housing. The two rotors switch the liquid from one end of the chamber to the other alongside the axial course.
Velocity meters
These devices function linearly with respect to the volumetric move rate. Because there is no sq. root relationship (as with differential stress devices), they have a much bigger vary. Velocity meters are least delicate to adjustments 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 connected instantly into the piping.
Turbine flow meters
Turbine move meters have been extensively used for exact liquid measurement functions. The system consists of a multi-bladed rotor mounted in a pipe, perpendicular to the circulate of the liquid. The rotor rotates because the liquid passes via the vanes. Rotational pace is a direct function of move rate and may be sensed by magnetic sensors, photocells or gears. Electrical pulses may be counted and totalized.
The number of electrical pulses counted in a given time interval is proportional to the flow rate. A tachometer could be added to measure the turbine velocity and decide the liquid move price. A correctly specified and put in turbine circulate meter has good accuracy, especially for low viscosity liquids.
A main problem with turbine flowmeters is bearing wear. A “bearingless” design has been developed to keep away from this drawback. The liquid entering the flow meter passes via the spiral vanes of the stator, causing the liquid move to rotate. The move acts on the spheres, inflicting 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 move rate.
Vortex flowmeters
Vortex circulate meters benefit from the natural phenomenon that occurs when a liquid flows via a blunt object. Vortex or vortex circulate alternates downstream of the object. The frequency of the vortex shedding is proportional to the rate of the liquid flowing via the flowmeter.
The three major components of the move meter are the blunt object strut mounted on the meter bore, the sensor that detects the presence of vortices and generates an electrical pulse, and the sign amplification and conditioning transmitter whose output is proportional to the move fee.
Electromagnetic circulate meters
Magnetic move meters can handle most liquids and slurries, offered that the material to be metered is electrically conductive. The major part is the circulate tube (primary element), which is installed immediately in the pipe. The stress drop across the meter is similar as the strain drop via the equal size of pipe, since there are no transferring parts or move obstructions. The voltmeter can be related directly to the move tube or remotely mounted and connected to the circulate tube via a shielded cable.
Electromagnetic circulate meters work in accordance with Faraday’s regulation of electromagnetic induction, which states that a voltage is induced when a conductor passes via a magnetic subject. The liquid acts because the conductor; the magnetic area 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 factor.
Electromagnetic flowmeters supply main advantages: they will measure tough and corrosive liquids and slurries; they’ll measure ahead and reverse circulate 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 enhancements have eradicated these problems. The pulse-type excitation approach reduces power consumption as a end result of excitation happens solely half the time within the unit. A zero setting is no longer required.
Ultrasonic move meters
Ultrasonic flowmeters could be divided into doppler meters and journey time (or transverse) meters. Doppler meters measure the frequency shift caused by the flow of a liquid. Two sensors are mounted in a housing linked to one aspect of the pipe. A signal of recognized frequency is distributed to the liquid to be measured. Solids, bubbles or any discontinuities within the liquid cause the heartbeat to be mirrored to the receiver factor. Since the liquid inflicting the reflection is shifting, the frequency of the returned pulse is shifted. The frequency shift is proportional to the speed of the liquid.
There can also be a transportable doppler meter that can be run on AC power or a rechargeable power pack. The instrument can be used by merely clamping the sensing head to the outside of the pipe. A set of 4 to 20 mA output terminals allows the unit to be related to a strip chart recorder or different distant device.
The travel time meter’s sensors are mounted on each side of the pipe. The configuration allows the sound waves to journey between the devices at an angle of forty five levels. angle to the course of fluid flow. The speed of the signal propagating between the sensors increases or decreases with the course of transmission and the speed of the liquid being measured. By transmitting the signal in both instructions alternately, a time-differential relationship proportional to the flow fee may be obtained. A limitation of the travel time meter is that the liquid being measured have to be relatively free of entrained gases or solids to attenuate signal scattering and absorption.
Mass move meter
Mass flowmeters the continued need for more accurate move measurement in mass associated processes (chemical reactions, warmth transfer, etc.) has led to the development of mass circulate meters. A number of designs are available, but the most commonly used for liquid flow applications is the Coriolis meter. It operates on the basis of a pure phenomenon often known as the Coriolis drive, therefore the title.
Coriolis circulate meters
Coriolis meters are true mass meters that directly measure mass move as an alternative of quantity move. Since the mass is fixed, 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 pressure conditions. The meter is particularly appropriate for measuring liquids whose viscosity varies with velocity at a given temperature and stress.
Coriolis meters are also obtainable in a big selection of designs. A popular unit consists of a U-shaped move tube encapsulated in a sensor housing linked to an electronic unit. The sensing unit can be mounted immediately into any course of. The electronics unit may be positioned as much as 500 feet away from the sensor.
Thermal mass move meters
Thermal mass flowmeters are traditionally used for gasoline measurements, however are also available in designs for liquid flow measurements. These mass meters also operate impartial of density, pressure and viscosity. Thermal mass meters use a heated sensing component that’s isolated from the fluid move path. The flowing stream conducts warmth from the sensing component. The heat transferred is proportional to the mass move fee. The sensor is rarely in direct contact with the fluid. The digital meeting features a flow analyzer, temperature compensator, and sign conditioner that gives a linear output proportional to the mass circulate fee.
Open channel flow meter
An “open channel” is any pipe by which liquids flow on a free floor. This includes tunnels, unpressurized sewers, partially crammed pipes, canals, streams and rivers. Of the numerous methods that can be utilized to observe flow in open channels, depth-related methods are the most common. These methods assume that instantaneous move could be determined from a measurement of water depth or head. Weirs and flumes are the oldest and most widely 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 fee, and therefore the move fee through the system. The weir consists of a vertical plate with a pointed high. The top of the plate may be straight or notched. Weirs are classified by the form of the notch. The basic types are V-notched, rectangular and trapezoidal.
The discharge via the weir and flume is a operate of the liquid degree, so the device must use degree measurement techniques to discover out the move price. Staff gauges and floating operating units are the simplest gadgets to make use of for this purpose. Various electronic sensing, totalizing and recording techniques are additionally out there.
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 surface of the liquid and timing the return echo. Linearized circuitry converts the peak of the liquid to a flow fee. A bar graph recorder data the circulate price and a digital totalizer data the whole number of gallons. Another recently launched microprocessor-based system uses ultrasonic or float sensors. A keypad with an interactive LCD show simplifies programming, control and calibration duties.
Select a right liquid move meter
Data reveals that more than 75% of the move meters put in in business don’t carry out nicely. And poor selection accounts for 90 % of those issues. Clearly, flowmeter selection is not a job for amateurs.
The most essential requirement is to know exactly what the instrument should do. There are numerous issues to consider 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 sign required? If a distant output is required, is it a proportional sign or a contact closure to start or cease another device? Is the fluid viscous, clear or slurry? Is it electrically conductive? What is its particular gravity or density? What flow rates are involved within the application? What is the working temperature and strain of the process? Accuracy (see glossary), range, linearity, repeatability, and piping requirements should also be thought-about.
It is equally important to grasp what a flow meter cannot do as well as what it can do earlier than making a last alternative. Every instrument has strengths and weaknesses, and the diploma of performance satisfaction is directly associated to how nicely the instrument’s options and weaknesses match the appliance necessities. Often, the user’s expectations of flowmeter performance do not match those offered by the supplier. Most suppliers are keen to assist prospects choose the right flowmeter for a selected job. Many present questionnaires, checklists and specification sheets designed to obtain the important information essential to match the correct flowmeter to the job.
Technical enhancements to the flowmeter should also be thought-about. For example, a typical mistake is to pick the design that was hottest for a given application several years ago and assume that it is still the most effective tool for the job. In latest years, many modifications and innovations may have occurred in the development of flowmeters for specific purposes, leading to a wider range of selections.
Flow meters are available in a variety of prices. Variable area flowmeters are often the least expensive, with some smaller units costing lower than $90. Mass flow meters are the costliest. They begin at about $3000. However, the total system price should all the time be thought of when selecting a flowmeter. Installation, operation and maintenance prices are also important economic elements. For a number of the extra complex designs, maintenance costs may be high.
As with many different merchandise, the plant engineer normally will get what he pays for when he buys a flow meter. But his satisfaction with the product will depend upon the care he uses in deciding on and putting in the instrument. It comes again to understanding the method, the product and the circulate metering requirements. It just isn’t unusual to “overbuy”. Plant engineers shouldn’t purchase more highly effective or complex flowmeters than they need.
Using circulate meters
Although suppliers are all the time prepared to offer flowmeter set up providers, it’s estimated that about 75% of customers install their very own equipment. But set up errors can occur. One of the most common situations is not allowing adequate upstream and downstream straight pipe for the flowmeter.
Each design has some tolerance for erratic velocity conditions in the piping, however all installations require correct piping configurations to operate effectively. Proper piping provides the proper circulate sample for the system. Without it, accuracy and performance could be adversely affected. Flow meters are generally installed backwards (especially orifice plates). Pressure sensing strains may also be reversed.
For electrical components, intrinsic security is a vital consideration in hazardous areas. Most flowmeter suppliers provide intrinsically secure designs for this sort of use.
Stray magnetic fields are current in most industrial vegetation. Power traces, relays, solenoids, transformers, motors and mills all contribute their share of interference. Users should be positive that the flowmeter they choose is not subject to such disturbances. The downside occurs primarily with the electronics in the secondary parts that have to be protected. Strict adherence to the manufacturer’s beneficial installation practices can usually forestall such issues.
Calibration
All circulate meters require an preliminary calibration. In most instances, the instrument is calibrated by the manufacturer for the required circumstances of use. However, if certified personnel are available on the factory, the person can carry out his own calibration.
The need for recalibration relies upon tremendously on how well the meter is matched to the application. Certain fluids that pass through the meter are usually abrasive, aggressive or corrosive. Over time, some components of the device will degrade sufficient to affect efficiency. Some designs are extra susceptible to wreck than others. For instance, put on on particular person turbine blades can cause efficiency adjustments. If the applying is important, the accuracy of the flow meter must be checked regularly. In other instances, recalibration will not be required for a quantity of years because the appliance just isn’t important and wouldn’t otherwise change the meter’s performance. Some flowmeters require particular tools for calibration. Most manufacturers will provide such companies at their crops or consumer amenities, and they will convey the gear with them for on-site calibration.
Maintenance
Many components can have an result on maintenance necessities and the expected life of a move meter. The primary issue, after all, is matching the proper instrument to the precise utility. Poorly selected gear will at all times cause issues early on. Flowmeters without transferring elements usually require less attention than units with shifting elements. But all flowmeters will ultimately require some sort of maintenance.
The major component in a differential pressure flowmeter requires a lot of piping, valves and fittings when connected to the secondary factor, so upkeep is often a recurring activity in such installations. Pulse strains can turn into clogged or corroded and should be cleaned or changed. And, improperly positioned secondary components can lead to measurement errors. Repositioning parts may be expensive.
Flow meters with shifting components require periodic inside inspection, especially if the liquid being measured is dirty or viscous. Installing a filter earlier than such units will help scale back fouling and wear. Accessible devices corresponding to ultrasonic or electromagnetic meters might have problems with the electronics of their secondary parts. Pressure transducers associated with secondary elements ought to be removed and inspected periodically.
Applications the place coatings may happen are additionally potential issues for accessible devices similar to magnetic or ultrasonic units. If the coating is insulated, the operation of the magnetic circulate meter can eventually be impaired if the electrodes are insulated from the liquid. This scenario may be prevented by common cleansing. With ultrasonic move meters, the refraction angle could change and the acoustic vitality absorbed by the coating may cause the circulate meter to not work.
More articles on move meters:
Relation between circulate and strain
Difference between flow meter and flow transmitter
Water level sensor types and works
Solution of water air pollutionn
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Measuring liquid flow is a important requirement for many industrial vegetation. In some operations, the ability to make accurate move measurements is so important that it can make the distinction between a revenue or a loss. In different cases, inaccurate circulate measurements or failure to make them can result in serious (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 determined by the strain distinction that forces the liquid through the pipe or conduit. Since the cross-sectional area of the pipe is known and stays constant, the common circulate rate is an indicator of the circulate rate. In this case, the fundamental relationship for determining the circulate price of a liquid is
Q = V x A
Q = Flow price of liquid by way of the pipe
V = Average move rate
A = Cross-sectional area of the pipe
Other elements that have an effect on the move fee of a liquid include the viscosity and density of the liquid, and the friction of the liquid involved with the pipe.
Liquid circulate may be measured directly using positive displacement flow meters. These models divide the liquid into particular increments and proceed to maneuver. The complete flow fee is the buildup of the measured increments and can be counted mechanically or electronically.
Table of Contents
Reynolds number
Types of liquid move meters
Differential Pressure 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 flow meter
Reciprocating piston flowmeter
Oval Gear Flow Meters
Disc type meter
Rotary vane gauges
Spiral move meter
Velocity meters
Turbine flow meters
Vortex flowmeters
Electromagnetic circulate meters
Ultrasonic move meters
Mass circulate meter
Coriolis circulate meters
Thermal mass flow meters
Open channel circulate meter
Select a right liquid circulate meter
Using move meters
Calibration
Maintenance
Reynolds number
The performance of a circulate meter can additionally be influenced by the dimensionless unit referred to as Reynolds quantity. It is outlined 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 fee of the liquid, gpm
Gt = Specific gravity of the liquid
D = Internal diameter of the pipe, in.
ต = Viscosity of the liquid, cp
Flow price and particular gravity are the inertial forces, pipe diameter and viscosity are the resistance. For most liquid functions, the pipe diameter and specific gravity stay fixed. At very low velocities or excessive viscosities, R may be very low and the liquid flows in a clean layer with the best velocity at the center of the pipe, the place viscous forces at the pipe wall restrict it to very low velocities. This kind of flow is recognized as laminar circulate. the R worth 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 flow occurs at excessive speeds or low viscosities. The circulate breaks down into turbulent eddies that circulate via the pipe with the identical common velocity. The fluid velocity is much less vital 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 situations, the flow in this zone may be turbulent or laminar.
Types of liquid flow meters
Many types of flow meters can be found for use in closed pipe techniques. In general, devices may be categorized as differential pressure meters, constructive displacement meters, velocity meters, and mass meters. Differential strain units (also known as head gauges) include orifice plates, venturi, circulate tubes, circulate nozzles, pitot tubes, elbow kind flow meters, target type circulate meters and variable area flow meters.
Positive displacement circulate meters embrace piston, elliptical-gear, chapter-actuated disk and rotary vane varieties. Velocity meters include turbine, vortex shedding, electromagnetic, and acoustic designs. Mass meters embrace Coriolis and thermal types. 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 price of a liquid is well known. By far, differential strain move meters are the most generally used unit right now. It is estimated that more than 50% of all liquid flow measurement functions use this sort of unit.
The fundamental working principle of differential pressure flow meters is based on the premise that the strain drop throughout the meter is proportional to the square of the flow fee. The flow rate is obtained by measuring the differential pressure and extracting the sq. root.
Like most flow meters, differential pressure flow meters have a primary component and a secondary element. The major component causes a change in kinetic vitality, which creates a differential strain in the pipe. The device must be correctly matched to the pipe measurement, flow situations and liquid characteristics. And, the accuracy of the factor measurement should remain good inside affordable limits. The secondary element measures the differential stress and provides a sign or reading that’s transformed to an precise move value.
Orifice plate circulate meters
Orifice plates are the most well-liked liquid circulate meters in use today. An orifice is just a flat piece of metal with a specific measurement hole drilled in it. Most orifices are concentric, but eccentric, conical (quadrant) and segmented designs are additionally available.
In follow, the orifice is installed in the pipe between two flanges. As the first system, the orifice restricts the flow of fluid, thereby creating a differential pressure throughout the plate. Pressure measurement ports on both sides of the plate are used to detect the difference. The main benefits of orifice plates are that they have no moving components and that their price doesn’t enhance considerably with the size of the pipe.
Tapered and quadrant orifices are comparatively new. These units had been developed primarily for measuring liquids with low Reynolds numbers. An essentially fixed flow coefficient could be maintained at an R-value beneath 5000. Tapered orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for every software.
The segment wedge is a variant of the segment orifice. It is a throttle orifice designed primarily to measure the circulate of liquids containing solids. The gadget is capable of measuring move at low Reynolds numbers and still maintaining the required square root relationship. Its design is simple and the wedge hole has only one critical dimension. The stress drop through the device is only about half that of a conventional orifice plate.
The one-piece wedge meeting combines the wedge factor and stress measurement becoming right into a one-piece pipe becoming bolted to a conventional strain transmitter. No special piping or fittings are required to install the device in the pipeline.
The metering accuracy of all orifice flow meters depends on the installation situations, the orifice plate area ratio, and the physical characteristics of the liquid being measured.
Venturi flow meters
The benefit of a venturi is its ability to deal with massive flows at low stress drops. A venturi is basically a bit of pipe with a conical inlet and a straight throat. As the liquid passes via the throat, it will increase in velocity, leading to a pressure distinction between the inlet and outlet areas.
Flow meters haven’t any shifting parts. They could be put in in giant diameter pipes utilizing flanged, welded or threaded finish connections. The unit is usually fitted with 4 or more pressure measuring ports to common the measured strain. Venturi tubes can be used for most liquids, together with these with excessive solids content.
Flow tubes
Flow tubes are considerably much like venturi, except that they don’t have an entrance cone. They have a tapered throat, but the outlet is elongated and clean. The distance between the entrance and the tip is about one-half the diameter of the tube. The strain 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 deal with approximately 60% of the liquid circulate compared to an orifice plate with the identical strain drop. Liquids containing suspended solids can be metered. However, these devices aren’t beneficial for prime viscosity liquids or liquids containing massive amounts of viscous solids.
Pitot tube flowmeter
The Pitot tube senses two forms of stress concurrently, shock pressure and static strain. The shock system consists of a tube with one end of the tube bent at proper angles to the course of move. The end of the static tube is closed, however has a small slot within the side of the system. These tubes can be installed individually within the pipe or mixed in a housing.
Pitot tubes are sometimes put in by welding a coupling to the pipe and inserting the probe through the coupling. The use of most Pitot tubes is limited to single level measurements. These gadgets are vulnerable to clogging by foreign matter in the liquid. The advantages of Pitot tubes are low price, no transferring parts, simple installation and minimal strain drop.
Elbow flowmeter
Elbow circulate meters work on the precept that centrifugal forces are exerted alongside the outer edges as the liquid strikes along a round path. Thus, as the liquid flows by way of the pipe elbow, the force on the inner surface of the elbow is proportional to the density of the liquid multiplied by the sq. of its velocity. In addition, the force is inversely proportional to the radius of the elbow.
Arbitrary 90 levels. Elbows can be used as liquid flow meters. All that is required are two small holes positioned at the midpoint of the elbow (45 degree point) for the manometer tap. A strain sensing line could be related to the tap utilizing any convenient method.
Target meter
The target meter senses and measures the drive brought on by the liquid hanging the target or a drag disk suspended in the liquid stream. By measuring the force utilized to the target the liquid circulate rate can be instantly indicated. In its simplest kind, the meter consists only of an articulated oscillating plate that strikes outward with the liquid circulate. In this case, the gadget is used as a circulate indicator.
More sophisticated versions use refined low-level drive sensor sensing elements. The target drive brought on by the liquid move is sensed by a pressure gauge. The output sign of the meter indicates the move price. The goal meter can be used to measure the move price of dirty or corrosive liquids.
Variable space circulate meter
Variable area meters consist primarily of a conical tube and a float. Although categorised as differential pressure units, they are actually constant strain gadgets. Flange finish connections provide a simple approach to set up them within the pipe. Working principle of rotameter is when there is not any liquid move, the float is free to rest at the backside of the pipe. When the liquid enters the underside of the pipe, the float begins to rise. The place of the float varies instantly with the circulate fee. Its precise position is at the level the place the stress difference between the higher and lower surfaces balances the weight of the float.
Because the circulate rate may be read directly on a scale mounted next to the tube, there is not a need for an auxiliary move studying system. However, if desired, an computerized sensing device can be used to sense the float degree and transmit the flow sign. Variable space move meter tubes are made from glass, metal or plastic. Tube diameters range from 1/4 to larger than 6 inches.
Positive displacement flow meter
The operation of these units consists of separating the liquid into exactly measured increments and persevering with to maneuver. Each phase is counted by a connection register. Because every increment represents a discrete volume, constructive displacement units are well-liked in computerized dosing and accounting purposes. Volumetric flow meters are perfect for measuring the circulate of viscous liquids or the place a simple mechanical move meter system is required.
Reciprocating piston flowmeter
Reciprocating piston flow meters are available in single and multi-piston versions. The particular choice is determined by the range of flow charges required in a selected application. Piston move meters can be utilized to deal with a variety of liquids. The fluid by no means 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 intently synchronized tooth. A fixed amount of fluid passes via the meter with every revolution. The shaft rotation can be monitored to obtain a particular flow fee.
Disc type meter
Chapter shifting disc kind meters have a movable disc mounted on a concentric sphere that’s located in a spherical sidewall chamber. The pressure of the fluid passing through the measurement chamber causes the disc to oscillate in its circulation path without rotating about its own axis. It is the one moving half within the measurement chamber.
A pin extending vertically from the disk is linked to a mechanical counter, which screens the oscillating motion of the disk. Each cycle is proportional to a specific circulate fee. As with all positive displacement meters, changes in viscosity under a given threshold will affect the measurement accuracy. A wide selection of sizes and capacities can be found. These models can be produced from a wide selection of construction materials.
Rotary vane gauges
Rotary vane meters can be found in a variety of designs, however they all operate on the identical principle. The fundamental 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 set quantity of liquid is swept from each compartment to the outlet of the meter. The variety of impeller revolutions is calculated and recorded in volume models.
Spiral move meter
The helical move meter consists of two radially inclined helical rotors which are gear pushed along with minimal clearance between the rotors and the housing. The two rotors switch the liquid from one finish of the chamber to the opposite along the axial direction.
Velocity meters
These devices function linearly with respect to the volumetric circulate fee. Because there is not any square root relationship (as with differential strain devices), they’ve a much bigger range. Velocity meters are least delicate to adjustments in viscosity when the Reynolds number exceeds 10,000. Most velocity meter housings are equipped with flanges or fittings to permit them to be linked directly into the piping.
Turbine circulate meters
Turbine circulate meters have been widely used for precise liquid measurement functions. The device 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 pace is a direct operate of flow fee and can be sensed by magnetic sensors, photocells or gears. Electrical pulses can be counted and totalized.
The number of electrical pulses counted in a given time period is proportional to the flow rate. A tachometer could be added to measure the turbine pace and determine the liquid flow fee. A correctly specified and installed turbine circulate meter has good accuracy, particularly for low viscosity liquids.
A main drawback with turbine flowmeters is bearing wear. A “bearingless” design has been developed to avoid this problem. The liquid coming into the move meter passes via the spiral vanes of the stator, inflicting the liquid circulate to rotate. The circulate acts on the spheres, causing them to journey in 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 rate.
Vortex flowmeters
Vortex circulate meters reap the benefits of the natural phenomenon that happens when a liquid flows through a blunt object. Vortex or vortex circulate alternates downstream of the thing. The frequency of the vortex shedding is proportional to the speed of the liquid flowing via 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 flow fee.
Electromagnetic flow meters
Magnetic circulate meters can handle most liquids and slurries, supplied that the material to be metered is electrically conductive. The main component is the move tube (primary element), which is put in immediately within the pipe. The strain drop across the meter is identical as the pressure drop via the equivalent size of pipe, since there are not any transferring components or circulate obstructions. The voltmeter could be linked on to the move tube or remotely mounted and linked to the move tube via a shielded cable.
Electromagnetic circulate meters work according to Faraday’s regulation of electromagnetic induction, which states that a voltage is induced when a conductor passes via a magnetic field. The liquid acts as the conductor; the magnetic area is generated by an energized coil outdoors the circulate tube. The amount of voltage generated is proportional to the flow rate. Two electrodes mounted on the wall of the tube detect the voltage measured by the secondary element.
Electromagnetic flowmeters offer main advantages: they can measure troublesome and corrosive liquids and slurries; they can measure ahead and reverse move with equal accuracy. The disadvantages of early designs had been excessive energy consumption and the necessity to get hold of a full tube with no circulate to set the meter to zero initially. Recent improvements have eliminated these issues. The pulse-type excitation approach reduces energy consumption as a result of excitation happens solely half the time in the unit. A zero setting is no longer required.
Ultrasonic flow meters
Ultrasonic flowmeters could be divided into doppler meters and journey time (or transverse) meters. Doppler meters measure the frequency shift attributable to the flow of a liquid. Two sensors are mounted in a housing related to 1 facet of the pipe. A signal of recognized frequency is distributed to the liquid to be measured. Solids, bubbles or any discontinuities within the liquid cause the heartbeat to be mirrored to the receiver factor. Since the liquid inflicting the reflection is shifting, the frequency of the returned pulse is shifted. The frequency shift is proportional to the rate of the liquid.
There is also a portable doppler meter that can be run on AC energy or a rechargeable energy pack. The instrument can be used by simply clamping the sensing head to the outside of the pipe. A set of four to twenty mA output terminals allows the unit to be related to a strip chart recorder or other remote gadget.
The journey time meter’s sensors are mounted on both sides of the pipe. The configuration allows the sound waves to travel between the units at an angle of 45 degrees. angle to the path of fluid circulate. The velocity of the signal propagating between the sensors increases or decreases with the course of transmission and the speed of the liquid being measured. By transmitting the signal in each directions alternately, a time-differential relationship proportional to the circulate price may be obtained. A limitation of the travel time meter is that the liquid being measured have to be comparatively free of entrained gases or solids to attenuate sign scattering and absorption.
Mass circulate meter
Mass flowmeters the continued want for more accurate move measurement in mass associated processes (chemical reactions, heat switch, and so on.) has led to the event of mass circulate meters. A number of designs can be found, but the most commonly used for liquid circulate applications is the Coriolis meter. It operates on the premise of a natural phenomenon known as the Coriolis force, hence the title.
Coriolis flow meters
Coriolis meters are true mass meters that immediately measure mass flow as a substitute of volume flow. 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 stress circumstances. The meter is particularly suitable for measuring liquids whose viscosity varies with velocity at a given temperature and stress.
Coriolis meters are also obtainable in a variety of designs. A popular unit consists of a U-shaped circulate tube encapsulated in a sensor housing connected to an electronic unit. The sensing unit may be mounted directly into any process. The electronics unit may be situated as a lot as 500 feet away from the sensor.
Thermal mass flow meters
Thermal mass flowmeters are traditionally used for gasoline measurements, however are additionally out there in designs for liquid move measurements. These mass meters additionally operate unbiased of density, stress and viscosity. Thermal mass meters use a heated sensing component that is isolated from the fluid move path. The flowing stream conducts heat from the sensing element. The warmth transferred is proportional to the mass circulate fee. The sensor is rarely in direct contact with the fluid. The digital meeting features a move analyzer, temperature compensator, and sign 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 filled pipes, canals, streams and rivers. Of the many methods that can be used to observe move in open channels, depth-related strategies are the most common. 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 generally used main gadgets used to measure move in open channels.
Weirs work on the principle that an obstruction in a channel causes water to flow backwards, making a excessive water stage (head) behind the obstruction. The head is a operate of the move rate, and due to this fact the move fee via the gadget. The weir consists of a vertical plate with a pointed high. The high of the plate may be straight or notched. Weirs are categorized by the form of the notch. The fundamental sorts are V-notched, rectangular and trapezoidal.
The discharge by way of the weir and flume is a perform of the liquid degree, so the device must use level measurement methods to find out the move price. Staff gauges and floating operating models are the best devices to make use of for this function. Various digital sensing, totalizing and recording methods are additionally out there.
A current improvement contains using ultrasonic pulses to measure liquid degree. 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 records the circulate price and a digital totalizer records the total variety of gallons. Another recently introduced microprocessor-based system makes use of ultrasonic or float sensors. A keypad with an interactive LCD display simplifies programming, management and calibration duties.
Select a proper liquid flow meter
Data exhibits that greater than 75% of the move meters installed in business don’t perform nicely. And poor selection accounts for 90 percent of these issues. Clearly, flowmeter choice isn’t a job for amateurs.
The most essential requirement is to know precisely what the instrument ought to do. There are numerous issues to think about here. Will the measurement be used for course of control (repeatability is the principle 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 sign or a contact closure to start out or stop another device? Is the fluid viscous, clear or slurry? Is it electrically conductive? What is its particular gravity or density? What move rates are concerned in the application? What is the working temperature and strain of the process? Accuracy (see glossary), vary, linearity, repeatability, and piping necessities should even be thought-about.
It is equally essential to understand what a move meter cannot do as nicely as what it may possibly do before making a final choice. Every instrument has strengths and weaknesses, and the degree of efficiency satisfaction is instantly related to how nicely the instrument’s options and weaknesses match the appliance necessities. Often, the user’s expectations of flowmeter performance do not match these offered by the provider. Most suppliers are eager to assist customers choose the best flowmeter for a particular job. Many present questionnaires, checklists and specification sheets designed to acquire the important info essential to match the proper flowmeter to the job.
Technical enhancements to the flowmeter must also be thought of. For example, a common mistake is to select the design that was hottest for a given application a number of years ago and assume that it is nonetheless the best software for the job. In latest years, many changes and innovations might have occurred in the development of flowmeters for particular purposes, resulting in a wider range of decisions.
Flow meters are available in a variety of prices. Variable area flowmeters are normally the least expensive, with some smaller items costing lower than $90. Mass flow meters are the most costly. They start at about $3000. However, the entire system price must all the time be thought-about when choosing a flowmeter. Installation, operation and upkeep prices are also necessary economic components. For some of the extra advanced designs, maintenance costs could be high.
As with many different products, the plant engineer often will get what he pays for when he buys a flow meter. But his satisfaction with the product will depend on the care he makes use of in selecting and installing the instrument. It comes again to understanding the method, the product and the circulate metering necessities. It just isn’t uncommon to “overbuy”. Plant engineers shouldn’t buy more powerful or complicated flowmeters than they need.
Using flow meters
Although suppliers are at all times prepared to supply flowmeter installation providers, it’s estimated that about 75% of customers set up their own gear. But set up errors can occur. One of the commonest conditions is not permitting sufficient upstream and downstream straight pipe for the flowmeter.
Each design has some tolerance for erratic velocity conditions within the piping, but all installations require proper piping configurations to function effectively. Proper piping offers the proper circulate pattern for the gadget. Without it, accuracy and performance can be adversely affected. Flow meters are generally put in backwards (especially orifice plates). Pressure sensing lines can be reversed.
For electrical components, intrinsic security is a crucial consideration in hazardous areas. Most flowmeter suppliers offer intrinsically secure designs for this sort of use.
Stray magnetic fields are present in most industrial plants. Power traces, relays, solenoids, transformers, motors and generators all contribute their share of interference. Users should make positive that the flowmeter they select just isn’t topic to such disturbances. The downside happens primarily with the electronics in the secondary elements that must be protected. Strict adherence to the manufacturer’s beneficial set up practices can usually prevent such issues.
Calibration
All circulate meters require an initial calibration. In most cases, the instrument is calibrated by the producer for the desired situations of use. However, if certified personnel can be found on the manufacturing facility, the consumer can carry out his own calibration.
The need for recalibration relies upon greatly on how well the meter is matched to the application. Certain fluids that move via the meter tend to be abrasive, aggressive or corrosive. Over time, some parts of the system will degrade sufficient to have an effect on efficiency. Some designs are extra vulnerable to break than others. For instance, put on on individual turbine blades could cause performance adjustments. If the application is important, the accuracy of the flow meter must be checked frequently. In other circumstances, recalibration may not be required for many years because the applying just isn’t critical and wouldn’t otherwise change the meter’s efficiency. Some flowmeters require particular equipment for calibration. Most manufacturers will provide such providers at their crops or person amenities, and they’re going to deliver the tools with them for on-site calibration.
Maintenance
Many elements can have an result on maintenance necessities and the anticipated lifetime of a circulate meter. The major issue, in fact, is matching the right instrument to the precise software. Poorly selected equipment will all the time trigger problems early on. Flowmeters with out shifting parts normally require less attention than devices with transferring parts. But all flowmeters will finally require some kind of upkeep.
The primary element in a differential pressure flowmeter requires a lot of piping, valves and fittings when related to the secondary element, so maintenance can be 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 elements can lead to measurement errors. Repositioning elements may be costly.
Flow meters with moving parts require periodic internal inspection, especially if the liquid being measured is soiled or viscous. Installing a filter earlier than such units will help cut back fouling and put on. Accessible devices corresponding to ultrasonic or electromagnetic meters may have issues with the electronics of their secondary parts. Pressure transducers associated with secondary parts should be eliminated and inspected periodically.
Applications the place coatings could occur are additionally potential problems for accessible devices similar to magnetic or ultrasonic units. If the coating is insulated, the operation of the magnetic circulate meter can ultimately be impaired if the electrodes are insulated from the liquid. This scenario may be avoided by common cleaning. With ultrasonic flow meters, the refraction angle could change and the acoustic power absorbed by the coating could cause the move meter to not work.
More articles on circulate meters:
Relation between flow and stress
Difference between flow meter and flow transmitter
Water level sensor sorts and works
Solution of water pollutionn