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