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