What are the three 3 types of flow meters?

02 Sep.,2024

 

11 Flow Meter Types and Their Advantages and Disadvantages

So far, there are as many as 60 Flow Meter Types available for industrial use. The reason why there are so many varieties is that so far we have not found a flow meter that is suitable for any fluid, any range, any flow state and any use conditions.

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Each of these 60 kinds of flow meters has its specific applicability and limitations. According to the measurement principles, there are mechanical principles, thermal principles, acoustic principles, electrical principles, optical principles, atomic physics principles, etc. According to the most popular and extensive classification method at present, it is divided into: volumetric flowmeter, differential pressure flowmeter, float flowmeter, turbine flowmeter, electromagnetic flowmeter, vortex flowmeter, ultrasonic flowmeter, mass flowmeter wait.

Here we have selected the 11 most commonly used flow meters to introduce and analyze their advantages and disadvantages.

We have introduced the definition of flow measurement and flow meter in the blog &#;Flow Measurement 101&#;. If you are a newbie, you can refer to it.

Next, let&#;s take a look at 11 Flow Meter Types and Their Advantages and Disadvantages.

Electromagnetic Flowmeter

working principle

The electromagnetic flowmeter is an instrument for measuring conductive liquids based on Faraday&#;s law of electromagnetic induction. Electromagnetic flowmeters have a series of excellent characteristics that can solve problems that are difficult to apply with other flowmeters, such as the measurement of dirty flows, mud, and corrosive flows.

advantages and disadvantages

Advantage:

  • The measurement channel is a smooth straight tube and will not be blocked. Suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, mud, sewage, etc.;
  • There is no pressure loss caused by flow detection, and the energy saving effect is good;
  • The measured volumetric flow rate is virtually unaffected by changes in fluid density, viscosity, temperature, pressure and conductivity;
  • Large flow range and wide caliber range;
  • Corrosive fluids can be used.

Disadvantages:

  • Liquids with very low conductivity, such as petroleum and oil products, cannot be measured;
  • Cannot measure gas, steam and liquids containing large bubbles;
  • Cannot be used at higher temperatures.

Applications

Electromagnetic flowmeters have a wide range of applications.

Large-diameter instruments are mostly used in water supply and drainage projects.

Small and medium diameters are often used in high-demand or difficult-to-measure situations, such as blast furnace tuyere cooling water control in the steel industry, measurement of pulp liquid and black liquor in the papermaking industry, strong corrosive liquids in the chemical industry, and slurry in the nonferrous metallurgical industry.

Small diameter and micro diameter are often used in the pharmaceutical industry, food industry, biochemistry and other places with hygienic requirements.

Electromagnetic flowmeter can also be used for Partially Filled Pipe flow measurement.

Learn more about Magnetic Flowmeter Technology and choose Magnetic Flow Meters for your applications.

Turbine flowmeter

working principle

Turbine flowmeter is the main type of velocity flowmeter. It uses a multi-blade rotor (turbine) to sense the average flow velocity of the fluid and derive the flow rate or total amount.

Generally, it consists of two parts: sensor and display, and can also be made into an integral type.

advantages and disadvantages

Advantages:

  • High precision, among all flow meters, it is the most accurate flow meter;
  • Good repeatability;
  • Zero point drift, good anti-interference ability;
  • wide range;
  • Compact structure.

Disadvantages:

  • Unable to maintain calibration characteristics for a long time;
  • Fluid physical properties have a great influence on flow characteristics.

Applications

Turbine flowmeters are widely used in the following measurement objects: petroleum, organic liquids, inorganic liquids, liquefied gases, natural gas and cryogenic fluids. In Europe and the United States, turbine flowmeters are second only to orifice flowmeters in terms of natural measurement meter.

Learn more about Turbine Flowmeter Technology and choose Turbine Flow Meters for your applications.

Vortex flowmeter

working principle

The vortex flowmeter is an instrument in which a non-streamlined vortex generator is placed in the fluid. The fluid alternately separates and releases two series of regularly staggered vortexes on both sides of the generator.
Vortex flowmeters can be divided according to frequency detection methods: stress type, strain type, capacitive type, thermal type, vibration type, photoelectric type and ultrasonic type, etc.

advantages and disadvantages

Advantages:

  • Simple and solid structure;
  • Applicable to many types of fluids;
  • Higher accuracy;
  • Wide range;
  • The pressure loss is small.

Disadvantages:

  • Not suitable for low Reynolds number measurements;
  • A longer straight pipe section is required;
  • Lower instrument coefficient (compared to turbine flowmeter);
  • The instrument still lacks application experience in pulsating flow and multi-phase flow.

applications

Vortex flowmeter is suitable for measuring various liquid media, such as water, petroleum, chemicals, solutions, etc.
At the same time, it is also suitable for measuring various gas media, such as air, natural gas, nitrogen, etc.
In addition, vortex flowmeters can also be used to measure steam flow.

Vortex flowmeters can measure over a wide temperature and pressure range. Generally, vortex flow meters can adapt to the temperature range from -200°C to +400°C and the pressure range from vacuum to high pressure.

Learn more about Vortex Flowmeter Technology and choose Vortex Flow Meters for your applications.

Ultrasonic flow meter

working principle

Ultrasonic flowmeter is an instrument that measures flow by detecting the effect of fluid flow on ultrasonic beams (or ultrasonic pulses). According to the principle of signal detection, ultrasonic flowmeters can be divided into propagation velocity difference methods (direct time difference method, time difference method, phase difference method and frequency difference method), beam offset method, Doppler method, cross-correlation method, and spatial filtering method. and noise method, etc.

Ultrasonic flowmeters are the same as electromagnetic flowmeters. Because there are no obstructions in the flow channel of the instrument, they are both unobstructed flowmeters. They are a type of flowmeter suitable for solving difficult problems of flow measurement, especially in large-diameter flow measurement. The advantages.

advantages and disadvantages

Advantages:

  • Non-contact measurement is possible;
  • Measurement without flow obstruction and no pressure loss;
  • It can measure non-conductive liquids and is a supplement to the electromagnetic flowmeter for non-obstruction measurement.

Disadvantages:

  • The travel time method can only be used for clean liquids and gases; while the Doppler method can only be used to measure liquids containing a certain amount of suspended particles and bubbles;
  • The measurement accuracy of Doppler method is not high.

Application

The transit time method is applied to clean, single-phase liquids and gases. Typical applications include tap water, diesel, etc.
In terms of gas applications, we have good experience in the field of high-pressure natural gas;
The Doppler method is suitable for two-phase fluids with a low heterogeneous content. For example: raw sewage, factory effluents, dirty process fluids; generally not suitable for very clean liquids.

Learn more about Ultrasonic Flow Meter Technology and choose Ultrasonic Flow Meters for your applications.

In addition to being used in flow measurement, ultrasonic technology has also been applied to liquid level measurement, providing an excellent liquid level measurement solution!

Coriolis mass flow meter

working principle

Coriolis mass flow meter This flow meter is a novel instrument that directly and precisely measures fluid mass flow. The main structure uses two side-by-side U-shaped tubes, and the bent parts of the two tubes vibrate slightly toward each other, and the straight tubes on both sides will vibrate accordingly. That is, they will move closer or open at the same time, that is, the vibrations of the two tubes are synchronous and symmetrical.

If the fluid is introduced into the tube and flows forward along the tube while the tube is vibrating synchronously, the tube will force the fluid to vibrate up and down with it.

advantages and disadvantages

Advantages

  • Coriolis mass flow meters directly measure mass flow and have high measurement accuracy.
  • A wide range of measurable fluids. Various liquids including high viscosity liquids, slurries containing solids, liquids containing trace amounts of gas, and medium and high pressure gases with sufficient density.
  • The vibration amplitude of the measuring tube is small and can be regarded as an inactive part. There are no obstructions and movable parts in the measuring pipeline.
  • It is not sensitive to the upstream flow velocity distribution, so there is no requirement for upstream and downstream straight pipe sections.
  • The measured value is insensitive to fluid viscosity, and changes in fluid density have little impact on the measured value.
  • Can perform multi-parameter measurements. Just like measuring density, temperature, and derived from this measure the concentration of solute contained in a solution.

Disadvantages

  • The zero point of the Coriolis mass flowmeter is unstable and causes zero point drift, which affects the further improvement of its accuracy. As a result, many models of instruments have to divide the total error into two parts: basic error and zero-point instability measurement.
  • Coriolis mass flow meters cannot be used to measure low-density media and low-pressure gases. The gas content in the liquid above a certain limit (varies by model) can significantly affect the measured values.
  • Coriolis mass flow meters are sensitive to external vibration interference. In order to prevent the influence of pipeline vibration, most models of Coriolis mass flow meters have relatively high requirements for installation and fixation of flow sensors.
  • It cannot be used for larger pipe diameters, currently limited to less than 200mm.
  • Wear, corrosion or sedimentation of the inner wall of the measuring tube will affect the measurement accuracy. This is especially true for Coriolis mass flow meters with thin-walled tube measuring tubes.
  • Most models of Coriolis mass flow meters are heavy and bulky.
  • expensive. The ordinary price is 5,000 to 10,000 US dollars for a set, which is about 2 to 8 times that of electromagnetic flowmeters of the same caliber.

Applications

Coriolis mass flow meters are the best choice for applications like:

  • Food & Beverage
  • Pulp and paper
  • Petrochemical
  • Oil & Gas
  • Power
  • High Viscosity
  • Paint
  • Mining
  • Process Control
  • Water and wastewater
  • And numerous other industries

Learn more about Coriolis Mass Flow Meter Technology and choose Mass Flow Meters for your applications.

Thermal mass flow meter

working principle

Thermal flow meter sensors contain two sensing elements, a speed sensor and a temperature sensor. They automatically compensate and correct for gas temperature changes.

The electric heating part of the instrument heats the speed sensor to a certain value higher than the working temperature, so that a constant temperature difference is formed between the speed sensor and the sensor that measures the working temperature. When the temperature difference is kept constant, the energy consumed by electric heating, which can also be said to be the heat dissipation value, is proportional to the mass flow rate of the gas flowing through it.

advantages and disadvantages

Advantages

  • High reliability
  • Good repeatability
  • High measurement accuracy
  • Small pressure loss
  • No moving parts
  • Range ratio wide
  • quick response
  • No temperature and pressure compensation required

Disadvantages

  • Due to changes in cp value and thermal conductivity, the measured value will change greatly and cause errors;
  • If the gas being measured is scaled on the tube wall, it will affect the measured value, and it has the disadvantage of being easily blocked;
  • Thermal mass flow meters for pulsating flows and liquids will be limited in their use for viscous liquids.
  • The gas is required to be dry and pollution-free: Gas with high humidity affects the heat exchange on the sensor surface, which in turn affects the output results. Particulate matter also has an impact on sensor output.
  • Not suitable for high viscosity gases: For high viscosity gases, the performance of the thermal gas mass flow meter may be affected. Because the heat transfer characteristics of the gas will change.

Applications

Thermal gas mass flow meter is a new instrument used to measure and control gas mass flow.

  • Gas mass flow measurement in industrial pipelines
  • Measurement of flue gas flow rate from chimney
  • Calciner flue gas flow measurement
  • Air flow measurement in gas process
  • Compressed air flow measurement
  • Gas flow measurement during half-channel body chip manufacturing process
  • Gas flow measurement in sewage treatment
  • Gas flow measurement in heating ventilation and air conditioning systems
  • Flux recovery system gas flow measurement
  • Combustion gas flow measurement in combustion boilers
  • Gas flow measurement of natural gas, flare gas, hydrogen and other gases
  • Carbon dioxide gas flow measurement during beer production
  • Gas mass flow measurement in the production process of cement, cigarette and glass factories

Learn more about Thermal Mass Flow Meter Technology and choose Thermal Mass Flow Meters for your applications.

Volumetric flow meter

working principle

Positive displacement flowmeter, also known as fixed displacement flowmeter, or PD flowmeter for short, is the most accurate type of flow meter.

It uses mechanical measuring elements to continuously divide the fluid into a single known volume part. The total volume of the fluid is measured based on the number of times the measuring chamber is filled and discharged with the fluid in this volume part.

Volumetric flowmeters are classified according to their measuring components and can be divided into oval gear flowmeters, scraper flowmeters, double rotor flowmeters, rotating piston flowmeters, reciprocating piston flowmeters, circular gear flowmeters, and liquid-sealed rotary drum flowmeters. , wet gas meter and membrane gas meter, etc.

advantages and disadvantages

Advantages:

  • High measurement accuracy;
  • Installation pipeline conditions have no impact on measurement accuracy;
  • Can be used for measurement of high viscosity liquids;
  • wide range;
  • The direct-reading instrument can directly obtain the accumulation and total quantity without external energy, which is clear and easy to operate.

Disadvantages:

  • The results are complex and bulky;
  • The type, caliber, and working status of the medium to be measured have great limitations;
  • Not suitable for high and low temperature situations;
  • Most instruments are only suitable for clean single-phase fluids;
  • Produces noise and vibration.

Applications

Volumetric flowmeters, differential pressure flowmeters and float flowmeters are among the three most commonly used flowmeters and are often used for total volume measurement of expensive media (oil, natural gas, etc.).

Learn more about Volumetric Flow Meters: Comprehensive Guide and Product List.

Differential pressure flow meter

working principle

A differential pressure flowmeter is an instrument that calculates flow based on the differential pressure generated by the flow detection component installed in the pipeline, the known fluid conditions, and the geometric dimensions of the detection component and the pipeline.

The differential pressure flow meter consists of a primary device (detection component) and a secondary device (differential pressure conversion and flow display instrument). Differential pressure flowmeters are usually classified in the form of test pieces, such as orifice flowmeters, Venturi flowmeters, velocity-averaging tube flowmeters, etc.

The secondary devices are various mechanical, electronic, electromechanical integrated differential pressure gauges, differential pressure transmitters and flow display instruments.

The detection parts of differential pressure flow meters can be divided into several categories according to their working principles: throttling device, hydraulic resistance type, centrifugal type, dynamic head type, dynamic head gain type and jet type.

advantages and disadvantages

Advantages:

  • The most widely used orifice flow meter has a solid structure, stable and reliable performance, and long service life;
  • It has such a wide range of applications that no other type of flow meter can compare with it so far;
  • The detection parts, transmitters, and display instruments are produced by different manufacturers to facilitate production with economies of scale.

Disadvantages:

  • Measurement accuracy is generally low;
  • The range is narrow, generally only 3:1~4:1;
  • The requirements for on-site installation conditions are high;
  • Large pressure loss (referring to orifice plate, nozzle, etc.).

Applications

  • Differential pressure flow meters have a particularly wide range of applications. Various objects have applications in flow measurement of closed pipes.
  • For example, in terms of fluids: single phase, mixed phase, clean, dirty, viscous flow, etc.;
  • Working status: normal pressure, high pressure, vacuum, normal temperature, high temperature, low temperature, etc.;
  • In terms of pipe diameter: from several mm to several meters;
  • Flow conditions: subsonic, sonic, pulsating flow, etc.

Learn more about Differential Pressure Flow Meter Technology and choose Differential Pressure Flow Meters for your applications.

Float flowmeter

working principle

The float flowmeter, also known as the rotor flowmeter, is a type of variable area flowmeter. In a vertical tapered tube that expands from bottom to top, the gravity of the float with a circular cross-section is borne by the liquid power, so that the float Can rise and fall freely within the conical tube.

The float flowmeter plays a decisive role in small and micro flow.

advantages and disadvantages

Advantages:

  • The glass cone tube float flowmeter has a simple structure and is easy to use.
  • Suitable for small pipe diameters and low flow rates;
  • Pressure loss is low.
  • Good stability: Since the measuring element of the float flowmeter is not in direct contact with the fluid, it will not be affected by changes in the physical properties of the fluid and has good stability.

Disadvantages are low pressure resistance and a greater risk of the glass tube being fragile.

Metal rotor flowmeter

working principle

The float of the metal tube rotor flowmeter is in the measuring tube. As the flow rate changes, the float moves upward. At a certain position, the buoyancy force on the float and the gravity of the float reach a balance.

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At this time, the flow annular area between the float and the orifice plate (or tapered tube) remains constant. The annulus area is proportional to the rising height of the float. That is, the rising position of the float in the measuring tube represents the flow rate. The changing position of the float is transmitted to the external indicator by the internal magnet, allowing the indicator to correctly indicate the flow value at this time. .

This prevents the indicator housing from being in direct contact with the measuring tube. Therefore, even if a limit switch or transmitter is installed, the instrument can be used under high temperature and high pressure working conditions.

advantages and disadvantages

Advantages:

  • Sturdy, simple, reliable, with low maintenance and long life.
  • Modular, intelligent, indicator design.
  • The requirements for the downstream straight pipe section are not high.
  • It has a wide flow range of 10:1.
  • Short stroke, small structure design.
  • Realize soft output through HART to display instantaneous and cumulative flow. Switch signal output, on-site setting and adjustment of media parameters, on-site performance compensation.
  • Multi-level correction of medium viscosity, density, temperature and pressure.
  • There are many types such as local type, remote transmission type, jacket type, explosion-proof, corrosion-resistant type, sanitary type, etc.
  • Stainless steel, Hastelloy, titanium, and PTFE material measurement systems are available.
  • Low pressure loss design.
  • Vertical, horizontal and various installation methods are more suitable for different use occasions.
  • All-metal structure, suitable for high temperature, high pressure and highly corrosive media.
  • Can be used in flammable and explosive hazardous locations.

Disadvantages:

  • The application of float flowmeter is limited to small and medium pipe diameters. It cannot be used for large pipe diameters, and the maximum diameter is generally DN150mm.
  • When the fluid used is different from the factory calibration fluid, the flow indication must be corrected. Float flowmeters for liquids are usually calibrated with water, and gases with air. If the density and viscosity of the actual fluid used are different, the flow rate will deviate from the original graduation value, and conversion corrections must be made.

Applications

Metal tube float flowmeter is a variable area flow measurement instrument commonly used in industrial automation process control.

Can be used to measure the flow of liquids, gases and steam. It is especially suitable for medium flow measurement with low flow rate and small flow rate. Commonly used are water and air measurements.

open channel flow meter

working principle

The working principle of the open channel flowmeter is to use open channel technology to measure the fluid level and then calculate the flow rate through the microprocessor inside the instrument.
Due to non-contact measurement, open channel flow meters can be used in harsher environments.

Under the control of a microcomputer, the open channel flowmeter transmits and receives the open channel, and calculates the distance between the open channel flowmeter and the measured liquid surface based on the transmission time, thereby obtaining the liquid level height. Since there is a certain proportional relationship between the liquid level and the flow rate, the liquid flow rate Q can be finally obtained according to the calculation formula.

advantages and disadvantages

Advantages:

  • The measurement range is large, and the flow measurement is not affected by the return water on the tributary surface.
  • During measurement, it is not affected by sediment, bubbles and large changes in water level floating in the water. The flow sensor produces resistance to the flow of water. It has a simple structure, small size and is easy to install.
  • Standard channels can be installed directly without modification, and the installation and construction costs are low.
  • The instrument has complete display and output functions. It can display measurement data such as water level, flow velocity, flow rate, cumulative flow rate, etc. It also has an RS-485 communication interface.
  • It has alarm functions for water level, mud level and flow rate exceeding limits.
  • It has a data saving function that can save setting parameters and flow values in the event of a long-term power outage.

Disadvantages:

  • In actual use, changes in water quality will affect the measurement results, especially when the water contains suspended matter or sediment with larger particles, which will cause the flow meter to become clogged or the readings will be inaccurate;
  • In open watercourses, channels or drainage systems this requires extensive excavation work.
  • Not suitable for high viscosity fluids.
  • Parshall troughs and weir troughs, etc. need to be installed;
  • A certain amount of installation space is required;
  • A mounting bracket is required.

Applications

Ultrasonic open channel flowmeter is suitable for measuring the flow of rectangular, trapezoidal and U-shaped open channels in reservoirs, rivers, water conservancy projects, urban water supply, sewage treatment, farmland irrigation, water administration and water resources.

Ultrasonic open channel flow meters need to be measured together with weirs and troughs. Commonly used weirs and troughs include Parshall troughs, rectangular troughs, triangular weirs, etc. The appropriate matching method can be selected according to different site environments.

Understanding the various types of flow meters is only half the battle in choosing the right one for you. You can learn more details in our flow meter selection guide to choose the best flow meter for your measurement!

More Flow Measurement Solutions

We at Sino-Inst produce and supply common industrial flow meters, including: electromagnetic flow meters, turbine flow meters, vortex flow meters, ultrasonic flow meters, Coriolis mass flow meters, oval gear flow meters, non-full tube electromagnetic flow meters, etc. More than 50 species.

Our flow meters are widely used in production and processes in various industries. Including customization for high temperature, extremely low temperature, high viscosity, corrosion and other special media measurements. If you need to purchase a flow meter or have any questions, please feel free to contact us.

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    Types of Flow Meters for Different Applications

    What is a Flow Meter?

    A flow meter (or a flow sensor) is an flow instrument that is used to indicate the amount of liquid, gas, or vapor moving through a pipe or conduit by measuring linear, non-linear, mass, or volumetric flow rates. Since flow control is often essential, measuring the flow of liquids and gasses is a critical need for many industrial applications &#; and there are many different types of flow meters that can be utilized depending on the nature of the application.

    When choosing a flow meter to buy, one should consider such intangible factors as familiarity of plant personnel, their experience with c and maintenance, spare parts availability, and meant time between failure history, etc., at the particular plant site. It is also recommended that the cost of the installation be computed only after taking these steps. One of the most common flow measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly, an attempt is made to justify the use of a device because it is less expensive. Those &#;inexpensive&#; purchases can be the costliest installations.

    Flow Meter Types and Uses

    Differential Pressure Flow Meters

    The use of differential pressure as an inferred measurement of a liquid&#;s rate of flow is well known. Differential pressure flow meters are, by far, the most common units in use today. These meters, which boast high accuracy, calculate fluid flow by reading pressure loss across a pipe restriction. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.

    The basic operating principle of differential pressure flow meters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.

    Differential pressure flow meters, like most flow meters, have a primary and secondary element. The primary element causes a change in kinetic energy, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid&#;s properties. And, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.

    Orifice Flow Meters

    Orifice flow meters are the most popular liquid flow meters in use today. An orifice is simply a flat piece of metal with a specific-sized hole bored in it. Most orifices in use are of the concentric type, but eccentric, conical (quadrant), and segmental designs are also available.

    In practice, the orifice plat is installed in the pipe between two flanges. Acting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference. Major advantages of orifices are that they have no moving parts, and their cost does not increase significantly with pipe size.

    Conical and quadrant orifices are relatively new. The units were developed primarily to measure liquids with low Reynolds numbers. Essentially constant flow coefficients can be maintained at R values below . Conical orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each application.

    The segmental wedge is a variation of the segmental orifice. It is a restriction orifice primarily designed to measure the flow of liquids containing solids. The unit has the ability to measure flows at low Reynolds numbers and still maintain the desired square-root relationship. Its design is simple, and there is only one critical dimension the wedge gap. Pressure drop through the unit is only about half that of conventional orifices.

    Integral wedge assemblies combine the wedge element and pressure taps into a one-piece pipe coupling bolted to a conventional pressure transmitter. No special piping or fittings are needed to install the device in a pipeline.

    Metering accuracy of all orifice flowmeters depends on the installation conditions, the orifice area ratio, and the physical properties of the liquid being measured.

    Venturi Flow Meters

    Venturi tubes have the advantage of being able to handle large flow volumes at low pressure drops. A venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. As liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions.

    The flowmeters have no moving parts. They can be installed in large diameter pipes using flanged, welded or threaded-end fittings. Four or more pressure taps are usually installed with the unit to average the measured pressure. Venturi tubes can be used with most liquids, including those having a high solids content.

    Pitot Tube Flow Meter

    Pitot tubes are generally installed by welding a coupling on a pipe and inserting the probe through the coupling. Use of most pitot tubes is limited to single point measurements. The units are susceptible to plugging by foreign material in the liquid. Advantages of pitot tubes are low cost, absence of moving parts, easy installation, and minimum pressure drop.

    Positive Displacement Flow Meters

    Operation of these units consists of separating liquids into accurately measured increments and moving them on. Each segment is counted by a connecting register. Because every increment represents a discrete volume, positive-displacement units are popular for automatic batching and accounting applications. Positive-displacement meters are good candidates for measuring the flows of viscous liquids or for use where a simple mechanical meter system is needed.

    Reciprocating piston meters are of the single and multiple-piston types. The specific choice depends on the range of flow rates required in the particular application. Piston meters can be used to handle a wide variety of liquids. A magnetically driven, oscillating piston meter is shown in Fig. 1. Liquid never comes in contact with gears or other parts that might clog or corrode.


    Figure 1: Oscillating-piston meter operates on magnetic drive principle so that liquid will not come in contact with parts. A partition plate between inlet and outlet ports forces incoming liquid to flow around a cylindrical measuring chamber and through the outlet port. The motion of the oscillating piston in the unit is transferred to a magnetic assembly in the measuring chamber, which is coupled to a follower magnet on the other side of the chamber wall.


    Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.

    Nutating-disk meters have a moveable disk mounted on a concentric sphere located in a spherical side-walled chamber. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.

    A pin extending perpendicularly from the disk is connected to a mechanical counter that monitors the disk's rocking motions. Each cycle is proportional to a specific quantity of flow. As is true with all positive-displacement meters, viscosity variations below a given threshold will affect measuring accuracies. Many sizes and capacities are available. The units can be made from a wide selection of construction materials.

    Rotary-vane meters are available in several designs, but they all operate on the same principle. The basic unit consists of an equally divided, rotating impeller (containing two or more compartments) mounted inside the meter's housing. The impeller is in continuous contact with the casing. A fixed volume of liquid is swept to the meter's outlet from each compartment as the impeller rotates. The revolutions of the impeller are counted and registered in volumetric units.

    Helix flow meters consist of two radically pitched helical rotors geared together, with a small clearance between the rotors and the casing. The two rotors displace liquid axially from one end of the chamber to the other.

    Volumetric Flow Meters

    These instruments operate linearly with respect to the volume flow rate. Because there is no square-root relationship (as with differential pressure devices), their rangeability is greater. Volumetric meters have minimum sensitivity to viscosity changes when used at Reynolds numbers above 10,000. Most velocity-type meter housings are equipped with flanges or fittings to permit them to be connected directly into pipelines.

    Turbine Flow Meters

    Turbine meters have found widespread use for accurate liquid measurement applications. The unit consists of a multiple-bladed rotor mounted with a pipe, perpendicular to the liquid flow. The rotor spins as the liquid passes through the blades. The rotational speed is a direct function of flow rate and can be sensed by magnetic pick-up, photoelectric cell, or gears. Electrical pulses can be counted and totalized, Fig. 2.


    Figure 2: Turbine flow meter consists of a multiple-bladed, free-spinning, permeable metal rotor housed in a non-magnetic stainless-steel body. In operation, the rotating blades generate a frequency signal proportional to the liquid flow rate, which is sensed by the magnetic pickup and transferred to a read-out indicator.


    The number of electrical pulses counted for a given period of time is directly proportional to flow volume. A tachometer can be added to measure the turbine's rotational speed and to determine the liquid flow rate. Turbine meters, when properly specified and installed, have good accuracy, particularly with low-viscosity liquids.

    A major concern with turbine meters is bearing wear. A "bearingless" design has been developed to avoid this problem. Liquid entering the meter travels through the spiraling vanes of a stator that imparts rotation to the liquid stream. The stream acts on a sphere, causing it to orbit in the space between the first stator and a similarly spiraled second stator. The orbiting movement of the sphere is detected electronically. The frequency of the resulting pulse output is proportional to flow rate.

    Vortex Flow Meters

    Vortex meters make use of a natural phenomenon that occurs when a liquid flows around a bluff object. Eddies or vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing through the meter, Fig. 3.


    Figure 3: Vortex meters operate on the principle that when a non-streamlined object is placed in the middle of a flow stream, a series of vortices are shed alternately downstream of the object. The frequency of the vortex shedding is directly proportional to the velocity of the liquid flowing in the pipeline.


    The three major components of the flowmeter are a bluff body strut-mounted across the flowmeter bore, a sensor to detect the presence of the vortex and to generate an electrical impulse, and a signal amplification and conditioning transmitter whose output is proportional to the flow rate, Fig. 4. The meter is equally suitable for flow rate or flow totalization measurements. Use for slurries or high viscosity liquids is not recommended.


    Figure 4


    Magmeter

    Electromagnetic meters can handle most liquids and slurries, providing that the material being metered is electrically conductive. The flow tube mounts directly in the pipe. Pressure drop across the meter is the same as it is through an equivalent length of pipe because there are no moving parts or obstructions to the flow. The voltmeter can be attached directly to the flow tube or can be mounted remotely and connected to it by a shielded cable.

    Electromagnetic flow meters operate on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube, Fig. 5. The amount of voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage, which is measured by the secondary element.


    Figure 5: Major components of obstruction-free electromagnetic flow meter&#;s flow tube include electrodes and coils.


    Electromagnetic flow meters have major advantages: They can measure difficult and corrosive liquids and slurries; and they can measure forward as well as reverse flow with equal accuracy. Disadvantages of earlier designs were high power consumption, and the need to obtain a full pipe and no flow to initially set the meter to zero. Recent improvements have eliminated these problems. Pulse-type excitation techniques have reduced power consumption, because excitation occurs only half the time in the unit. Zero settings are no longer required.

    Ultrasonic Flow Meters

    Ultrasonic flow meters can be divided into Doppler meters and Time-of-Travel (or Transit) meters. Doppler meters measure the frequency shifts caused by liquid flow. Two transducers are mounted in a case attached to one side of the pipe. A signal of known frequency is sent into the liquid to be measured. Solids, bubbles, or any discontinuity in the liquid, cause the pulse to be reflected to the receiver element, Fig. 6. Because the liquid causing the reflection is moving, the frequency of the returned pulse is shifted. The frequency shift is proportional to the liquid's velocity.


    Figure 6


    A portable Doppler meter capable of being operated on AC power or from a rechargeable power pack has recently been developed. The sensing heads are simply clamped to the outside of the pipe, and the instrument is ready to be used. Total weight, including the case, is 22 lb. A set of 4 to 20 millampere output terminals permits the unit to be connected to a strip chart recorder or other remote device.

    Time-of-travel meters have transducers mounted on each side of the pipe. The configuration is such that the sound waves traveling between the devices are at a 45 deg. angle to the direction of liquid flow. The speed of the signal traveling between the transducers increases or decreases with the direction of transmission and the velocity of the liquid being measured. A time-differential relationship proportional to the flow can be obtained by transmitting the signal alternately in both directions. A limitation of time-of-travel meters is that the liquids being measured must be relatively free of entrained gas or solids to minimize signal scattering and absorption.

    Mass Flow Meters

    The continuing need for more accurate flow measurements in mass-related processes (chemical reactions, heat transfer, etc.) has resulted in the development of mass flowmeters. Various designs are available, but the one most commonly used for liquid flow applications is the Coriolis mass flow meter. Its operation is based on the natural phenomenon called the Coriolis force, hence the name.

    Coriolis flow meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures. Coriolis flow meters are true mass meters that measure the mass rate of flow directly as opposed to volumetric flow. Because mass does not change, the meter is linear without having to be adjusted for variations in liquid properties. It also eliminates the need to compensate for changing temperature and pressure conditions. The meter is especially useful for measuring liquids whose viscosity varies with velocity at given temperatures and pressures.

    Coriolis meters are also available in various designs. A popular unit consists of a U-shaped flow tube enclosed in a sensor housing connected to an electronics unit. The sensing unit can be installed directly into any process. The electronics unit can be located up to 500 feet from the sensor.

    Inside the sensor housing, the U-shaped flow tube is vibrated at its natural frequency by a magnetic device located at the bend of the tube. The vibration is similar to that of a tuning fork, covering less than 0.1 in. and completing a full cycle about 80 times/sec. As the liquid flows through the tube, it is forced to take on the vertical movement of the tube, Fig. 7. When the tube is moving upward during half of its cycle, the liquid flowing into the meter resists being forced up by pushing down on the tube.


    Figure 7


    Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction.

    Having been forced upward, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of its vibration cycle, it twists in the opposite direction. The ammount of twist is directly proportional to the mass flow rate of the liquid flowing through the tube. Magnetic sensors located on each side of the flow tube measure the tube velocities, which change as the tube twists. The sensors feed this information to the electronics unit, where it is processed and converted to a voltage proportional to mass flow rate. The meter has a wide range of applications from adhesives and coatings to liquid nitrogen.

    Thermal-type mass flowmeters have traditionally been used for gas measurements, but designs for liquid flow measurements are available. These mass meters also operate independent of density, pressure, and viscosity. Thermal meters use a heated sensing element isolated from the fluid flow path. The flow stream conducts heat from the sensing element. The conducted heat is directly proportional to the mass flow rate. The sensor never comes into direct contact with the liquid. The electronics package includes the flow analyzer, temperature compensator, and a signal conditioner that provides a linear output directly proportional to mass flow.

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