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