• What is  a vortex flow meter?
• These meters work on the phenomenon of vortex shedding that takes place when fluid or gas meets an obstacle – termed as a bluff body.
• When a fluid is passed it meets an obstacle  and creates  vortices on either side of the bluff body in the low pressure region. These vortices are swept downstream to produce Karman Vortex Street.

• At low velocities ,the fluid flows very close to the body.
• When the velocity is increased, a low pressure region is created behind the bluff body. These relieve pressure on one side of the body and forms vortex and are swept downwards.
• The low pressure region then shifts towards another side of the bluff body and hence another vortex is formed.  
• Some examples  of vortices formation are whistling of  a tone, waving of a flag pole.

• Shedder are of various sizes each manufactures claiming their superiority one over other.

• Vortex flow meters work relatively well with clean low viscosity fluids, gases and steam. Higher viscosity fluids can be measured but at the expense of range ability and head loss
• Vortex flow meters cannot measure  at zero flows because vortex shedding becomes  highly irregular.
• Vibrations can cause the meters to read high  from the nearby pumps, compressors etc.
• Orientations:
• Vortex meters can be installed horizontally or vertically.
• However, for liquid measurements the meter must be full at all times.
• For steam,air and gases  applications, upstream 20 d  and downstream 5d is used. For steam application the head must be kept downwards and for gases upward orientation can be used.

• Thermal flowmeters use the thermal properties of the fluid to measure the flow of a fluid flowing in a pipe or duct. In a typical thermal flowmeter, a measured amount of heat is applied to the heater of the sensor. Some of this heat is lost to the flowing fluid.
• As flow increases, more heat is lost. The amount of heat lost is sensed using temperature measurement(s) in the sensor. The transmitter uses the heat input and temperature measurements to determine fluid flow.
• Most thermal flowmeters are used to measure gas flows. Thermal flowmeters represent 2% of global flowmeter sales.
• In many applications, the thermal properties of the fluid can be dependent upon fluid composition. In these applications, varying composition of the fluid during actual operation can affect the thermal flow measurement. Therefore, it is important for the thermal flowmeter supplier to know the composition of the fluid so that the proper calibration factor can be used to determine the flow rate accurately.
• Due to this constraint, thermal flowmeters are commonly applied to measure the flow of pure gases. Suppliers can provide appropriate calibration information for other gas mixtures, however the accuracy of the thermal flowmeter is dependent on the actual gas mixture being the same as the gas mixture used for calibration purposes. In other words, the accuracy of a thermal flowmeter calibrated for a given gas mixture will be degraded if the actual flowing gas has a different composition.

• Thermals are middling cost and they are good for low pressure gas. They are well suited for stack flow measurement and emissions monitoring uses. Insertion models are a very good choice for large pipe sizes if used as insertion meters. The best attribute is that if the gas is known, the meter reads true mass flow without needing to include pressure in a calculation. The accuracy is medium only and they are used primarily for gas. Not good for steam flow.

• Thermal flowmeters are most commonly used to measure the mass flow of clean gases, such as air, nitrogen, hydrogen, helium, ammonia, argon, and other industrial gases.
• Mixtures, such as flue stack flow and biogas flow, can be measured when their composition is known. An advantage of this technology is its dependence upon thermal properties that are almost independent of gas density. Be careful when using thermal flowmeters to measure the flow of gases with unknown and/or varying composition, such as hydrogen-bearing off-gases and other mixtures that can disproportionately affect the thermal flowmeter measurement.

• Thermal flowmeters can be applied to clean, sanitary, and corrosive gases where the thermal properties of the fluid are known.

• Thermal flowmeters are most commonly applied to measure pure gases, such as would be used for laboratory experiments, and in semi-conductor production. They can also used in chemical and petrochemical plants when the thermal properties of the gas are known. With proper attention to materials of construction, the flow of corrosive gases, such as hydrogen chloride and hydrogen sulfide can be measured.

• In order of magnitude, these are used in oil and gas, power, chemical, water and waste, metals and mining, food and beverage, pulp and paper, pharmaceutical and textile industries.

• Application Cautions for Thermal Flowmeters

• Thermal flowmeters should not be applied to abrasive fluids because they can damage the sensor. Fluids that coat the sensor can alter the relationship between the thermal properties of the fluid and the measurement and adversely affect the flow measurement. Extensive coating can render the sensor inoperable unless the sensor is routinely cleaned. This can increase maintenance associated with these flowmeters.

• Varying the percentage of certain components that have different thermal properties from calibrated values can cause thermal flowmeters to become highly inaccurate.

• In other words, thermal flowmeters are often not suitable for applications with fluids that have varying composition and unknown components. However, in some applications, thermal flowmeters could measure reasonably accurately when the flow stream contains components with similar thermal properties.

• Aerosols and gases with droplets can cause thermal flowmeters to become erratic and/or measure full scale flow. This is because the large amount of thermal energy used to heat the liquid/droplet is interpreted as a high flow signal. Operating a thermal flowmeter above its maximum flow rate will generally not damage the flowmeter but can cause measurement error because its calibration curve can become unpredictable.

• Turbine flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster. Turbine flowmeters now constitute 7% of the world market.

• Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of metal generating a pulse. Turbine flowmeter sensors are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid. Transmitters and sensing systems are available to sense flow in both the forward and reverse flow directions.

• The cost is moderate. Very good at clean, low viscosity fluids of moderate velocity and a steady rate. Turndown is very good as it can read very low compared to the maximum flow. They are reliable if put in a clean fluid especially if it has some lubricity. AGA and API approved for custody transfers. They do cause some pressure drop where that may be a factor such as gravity flows. Not reliable for steam. Bearings wear out.

• Turbine flowmeters measure the velocity of liquids, gases and vapors in pipes, such as hydrocarbons, chemicals, water, cryogenic liquids, air, and industrial gases. High accuracy turbine flowmeters are available for custody transfer of hydrocarbons and natural gas. These flowmeters often incorporate the functionality of a flow computer to correct for pressure, temperature and fluid properties in order to achieve the desired accuracy for the application.

• Be careful using turbine flowmeters on fluids that are non-lubricating because the flowmeter can become inaccurate and fail if its bearings prematurely wear. Some turbine flowmeters have grease fittings for use with non-lubricating fluids. In addition, turbine flowmeters that are designed for a specific purpose, such as for natural gas service, can often operate over a limited range of temperature (such as up to 60ºC) whereby operation at higher temperatures can damage the flowmeter.

• This flowmeter can be applied to sanitary, relatively clean, and corrosive liquids in sizes up to approximately 24 inches. Smaller turbine flowmeters can be installed directly in the piping, but the size and weight of larger turbine flowmeters may require the installation of substantial concrete foundations and supports. The flow of corrosive liquids can be measured with proper attention to the materials of construction of all wetted parts, such as the body, rotor, bearings, and fittings.

• Applications for turbine flowmeters are found in the water, petroleum, and chemical industries. Water applications include distribution systems within and between water districts. Petroleum applications include the custody transfer of hydrocarbons. Miscellaneous applications are found in the food and beverage, and chemical industries.

• In order of magnitude from largest to smallest, these are used in oil and gas, water and wastewater, gas utility, chemical, power, food and beverage, aerospace, pharmaceutical, metals and mining, and pulp and paper.

• Turbine flowmeters are less accurate at low flow rates due to rotor/bearing drag that slows the rotor. Make sure to operate these flowmeters above approximately 5 percent of maximum flow. Turbine flowmeters should not be operated at high velocity because premature bearing wear and/or damage can occur. Be careful when measuring fluids that are non-lubricating because bearing wear can cause the flowmeter become inaccurate and fail. In some applications, bearing replacement may need to be performed routinely and increase maintenance costs. Application in dirty fluids should generally be avoided so as to reduce the possibility of flowmeter wear and bearing damage. In summary, turbine flowmeters have moving parts that are subject to degradation with time and use.

• Abrupt transitions from gas flow to liquid flow should be avoided because they can mechanically stress the flowmeter, degrade accuracy, and/or damage the flowmeter. These conditions generally occur when filling the pipe and under slug flow conditions. Two-phase flow conditions can also cause turbine flowmeters to measure inaccurately.

• Propeller meters are very similar to turbine meters in technology and application. The main difference between the two is in the rotating element and it's suspension in the fluid stream. A propeller is usually made of thick injection molded plastic and faces directly into the flow, suspended from a single bearing assembly. Turbines are thinner and are usually supported on both sides by two, lighter-weight bearing assemblies.

• Propeller flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster.

• Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of metal generating a pulse. Propeller flowmeter sensors are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid. Transmitters and sensing systems are available to sense flow in both the forward and reverse flow directions.

• Propeller meters are accurate and relatively inexpensive and can be economical even when only needed for a short time.

• Propeller flowmeters measure the velocity of liquids, gases and vapors in pipes, such as hydrocarbons, chemicals, water, cryogenic liquids, air, and industrial gases. High accuracy propeller flowmeters are available for custody transfer of hydrocarbons and natural gas. These flowmeters often incorporate the functionality of a flow computer to correct for pressure, temperature and fluid properties in order to achieve the desired accuracy for the application.

• Be careful because using propeller flowmeters on fluids that are non-lubricating, because the flowmeter can become inaccurate and fail if its bearings prematurely wear. Some propeller flowmeters have grease fittings for use with non-lubricating fluids. In addition, propeller flowmeters that are designed for a specific purpose, such as for natural gas service, can often operate over a limited range of temperature (such as up to 60ºC) whereby operation at higher temperatures can damage the flowmeter.

• This flowmeter can be applied to sanitary, relatively clean, and corrosive liquids in sizes up to approximately 24 inches. Smaller propeller flowmeters can be installed directly in the piping, but the size and weight of larger propeller flowmeters may require the installation of substantial concrete foundations and supports. The flow of corrosive liquids can be measured with proper attention to the materials of construction of all wetted parts, such as the body, rotor, bearings, and fittings.

• Applications for propeller flowmeters are found in the water, petroleum, and chemical industries. Water applications include distribution systems within and between water districts. Petroleum applications include the custody transfer of hydrocarbons. Miscellaneous applications are found in the food and beverage, and chemical industries.

• Propeller flowmeters are less accurate at low flow rates due to rotor/bearing drag that slows the rotor. Make sure to operate these flowmeters above approximately 5 percent of maximum flow. Propeller flowmeters should not be operated at high velocity because premature bearing wear and/or damage can occur. Be careful when measuring fluids that are non-lubricating because bearing wear can cause the flowmeter become inaccurate and fail. In some applications, bearing replacement may need to be performed routinely and increase maintenance costs. Application in dirty fluids should generally be avoided so as to reduce the possibility of flowmeter wear and bearing damage. In summary, propeller flowmeters have moving parts that are subject to degradation with time and use.

• Abrupt transitions from gas flow to liquid flow should be avoided because they can mechanically stress the flowmeter, degrade accuracy, and/or damage the flowmeter. These conditions generally occur when filling the pipe and under slug flow conditions. Two-phase flow conditions can also cause propeller flowmeters to measure inaccurately.

• Coriolis mass flowmeters measure the force resulting from the acceleration caused by mass moving toward (or away from) a center of rotation. This effect can be experienced when riding a merry-go-round, where moving toward the center will cause a person to have to “lean into” the rotation so as to maintain balance. As related to flowmeters, the effect can be demonstrated by flowing water in a loop of flexible hose that is “swung” back and forth in front of the body with both hands. Because the water is flowing toward and away from the hands, opposite forces are generated and cause the hose to twist. They represent about 21% of all flowmeters sold.

• In a Coriolis mass flowmeter, the “swinging” is generated by vibrating the tube(s) in which the fluid flows. The amount of twist is proportional to the mass flow rate of fluid passing through the tube(s). Sensors and a Coriolis mass flowmeter transmitter are used to measure the twist and generate a linear flow signal.

• This technology has high accuracy, can handle sanitary applications, is approved for custody transfer and is highly reliable and low maintenance. Mass flow is more important than volume for fluids intended for the production of energy. These include petroleum liquids and natural gas both compressed and liquefied. The cost is high, especially for line sizes above four inches. Pressure drop can be a consideration for “U” shaped tube designs and high viscosity fluids.

• Coriolis mass flowmeters measure the mass flow of liquids, such as water, acids, caustic, chemicals, and gases/vapors. Because mass flow is measured, the measurement is not affected by fluid density changes. Be particularly careful when using Coriolis mass flowmeters to measure gas/vapor flows because flow rates tend to be low in the flow range (where accuracy is degraded). Also, in gas/vapor applications, large pressure drops across the flowmeter and its associated piping can occur.
• This flowmeter can be applied to sanitary, cryogenic, relatively clean, and corrosive liquids and gases/vapors in pipes smaller than 6-12 inches. General applications are found in the water, wastewater, mining, mineral processing, power, pulp and paper, petroleum, chemical, and petrochemical industries. Materials of construction are generally limited to stainless steel and Hastelloy C. Straight-tube designs are available to measure some dirty and/or abrasive liquids.

• Many applications for Coriolis mass flowmeters are found in chemical processes where fluids can be corrosive and otherwise difficult to measure. In addition, the relative insensitivity to density allows Coriolis mass flowmeters to be applied in applications where the physical properties of the fluid are not well known. These flowmeters can also be used in chemical feed systems found in most industries.

• The industries in order of higher to lower are chemical, oil and gas, food and beverage, pharmaceutical, pulp and paper, power, metals and mining, and water and wastewater followed by all others in small amounts.

• Application Cautions for Coriolis Mass Flowmeters

• If the pressure drop is acceptable, operate a Coriolis mass flowmeter in the upper part of its flow range because operation at low flow rates can degrade accuracy. Note that high viscosity fluids increase the pressure drop across the flowmeter. For liquid flows, make sure that the flowmeter is completely full of liquid. Be especially careful when measuring gas/vapor flow with Coriolis mass flowmeters. Pay special attention to installation because pipe vibration can cause operational problems.

• Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is generated and channeled into the liquid flowing through the pipe. Following Faraday’s Law, flow of a conductive liquid through the magnetic field will cause a voltage signal to be sensed by electrodes located on the flow tube walls. When the fluid moves faster, more voltage is generated. Faraday’s Law states that the voltage generated is proportional to the movement of the flowing liquid. The electronic transmitter processes the voltage signal to determine liquid flow.

• In contrast with many other flowmeter technologies, magnetic flowmeter technology produces signals that are linear with flow. As such, the turndown associated with magnetic flowmeters can approach 20:1 or better without sacrificing accuracy. They represent about 23% of all flowmeters sold.

• Mags are intermediate in accuracy therefore not commonly used for commodity transfer except for some special cases where the fluid is not expensive like water. Can be adapted for sanitary uses. They have large line sizes available. No pressure drop induced. Dirty liquids and even slurries OK. Very reliable. On the other hand, don’t work on nonconductive fluids such as oils. Steam or gas flows don’t register. Electrodes can become coated.

• Magnetic flowmeters measure the velocity of conductive liquids in pipes, such as water, acids, caustic, and slurries. Magnetic flowmeters can measure properly when the electrical conductivity of the liquid is greater than approximately 5μS/cm. Be careful because using magnetic flowmeters on fluids with low conductivity, such as deionized water, boiler feed water, or hydrocarbons, can cause the flowmeter to turn off and measure zero flow.

• This flowmeter does not obstruct flow, so it can be applied to clean, sanitary, dirty, corrosive and abrasive liquids. Magnetic flowmeters can be applied to the flow of liquids that are conductive, so hydrocarbons and gases cannot be measured with this technology due to their non-conductive nature and gaseous state, respectively

• Magnetic flowmeters do not require much upstream and downstream straight run so they can be installed in relatively short meter runs. Magnetic flowmeters typically require 3-5 diameters of upstream straight run and 0-3 diameters of downstream straight run measured from the plane of the magnetic flowmeter electrodes.

• Applications for dirty liquids are found in the water, wastewater, mining, mineral processing, power, pulp and paper, and chemical industries. Water and wastewater applications include custody transfer of liquids in force mains between water/wastewater districts. Magnetic flowmeters are used in water treatment plants to measure treated and untreated sewage, process water, water and chemicals. Mining and mineral process industry applications include process water and process slurry flows and heavy media flows.

• With proper attention to materials of construction, the flow of highly corrosive liquids (such as acid and caustic) and abrasive slurries can be measured. Corrosive liquid applications are commonly found in the chemical industry processes, and in chemical feed systems used in most industries. Slurry applications are commonly found in the mining, mineral processing, pulp and paper, and wastewater industries.

• Magnetic flowmeters are often used where the liquid is fed using gravity. Be sure that the orientation of the flowmeter is such that the flowmeter is completely filled with liquid. Failure to ensure that the flowmeter is completely filled with liquid can significantly affect the flow measurement.

• Be especially careful when operating magnetic flowmeters in vacuum service because some magnetic flowmeter liners can collapse and be sucked into the pipeline in vacuum service, catastrophically damaging the flowmeter. Note that vacuum conditions can occur in pipes that seemingly are not exposed to vacuum service such as pipes in which a gas can condense (often under abnormal conditions). Similarly, excessive temperature in magnetic flowmeters (even briefly under abnormal conditions) can result in permanent flowmeter damage.

• In order of usage, water/wastewater industry, chemical, food and beverage, oil and gas (although not for oil and gas fluids but in support of the processes), power, pulp and paper, metals and mining, and pharmaceutical.
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• Do not operate a magnetic flowmeter near its electrical conductivity limit because the flowmeter can turn off. Provide an allowance for changing composition and operating conditions that can change the electrical conductivity of the liquid.

• In typical applications, magnetic flowmeters are sized so that the velocity at maximum flow is approximately 2-3 meters per second. Differential pressure constraints and/or process conditions may preclude application of this general guideline. For example, gravity fed pipes may require a larger magnetic flowmeter to reduce the pressure drop so as to allow the required amount of liquid to pass through the magnetic flowmeter without backing up the piping system. In this application, operating at the same flow rate in the larger flowmeter will result in a lower liquid velocity as compared to the smaller flowmeter.

• For slurry service, be sure to size magnetic flowmeters to operate above the velocity at which solids settle (typically 1 ft/sec), in order to avoid filling the pipe with solids that can affect the measurement and potentially stop flow. Magnetic flowmeters for abrasive service are usually sized to operate at low velocity (typically below 3 ft/sec) to reduce wear. In abrasive slurry service, the flowmeter should be operated above the velocity at which solids will settle, despite increased wear. These issues may change the range of the flowmeter, so its size may be different than the size for an equivalent flow of clean water.

• The closest technology to Mag that could possibly handle similar applications more cost effectively would be vortex shedding. They can handle light particulate, have a higher pressure drop, lower rangeability and are slightly less accurate.

• Paddlewheel flowmeters use the mechanical energy of the fluid to rotate a paddlewheel (just like a riverboat) in the flow stream. Paddles on the rotor are inserted into the flow to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the paddlewheel spins proportionally faster. Shaft rotation can be sensed mechanically or by detecting the movement of the paddles. Paddle movement is often detected magnetically, with each paddle or embedded piece of metal generating a pulse. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid.

• Paddlewheel flowmeters measure the velocity of liquids in pipes, such as chemicals, water and liquids. High accuracy is attainable if carefully installed. These flowmeters are measuring flow at the edge of the flow profile and thus are affected by viscosity changes. The most common use is in a system where the fluid is like water and other variables such as pH/ORP, conductivity, pressure, temperature and level are monitored. All sensors are inserted into the same pipe Ts and connect into one controller/transmitter. There are temperature and pressure limitations of this insertion system but it is very versatile. Applications for paddlewheel flowmeters are found in the water and chemical industries. Water applications include distribution systems. Miscellaneous applications are found in the food and beverage, and chemical industries.

• Paddlewheel flowmeters are less accurate at low flow rates due to rotor/bearing drag that slows the rotor. Make sure to operate these flowmeters above approximately 5 percent of maximum flow. Paddlewheel flowmeters should not be operated at high velocity because premature bearing wear and/or damage can occur. Be careful when measuring fluids that are non-lubricating because bearing wear can cause the flowmeter become inaccurate and fail. In some applications, sensor replacement may need to be performed routinely and increase maintenance costs. Applications in dirty fluids are not recommended. Easy replacement with insertion magmeters is possible when high failure rates are unacceptable and are often sold by the same vendor. In summary, paddlewheel flowmeters have moving parts that are subject to degradation with time and use.

• Abrupt transitions from gas flow to liquid flow should be avoided because they can mechanically stress the flowmeter, degrade accuracy, and/or damage the flowmeter. These conditions generally occur when filling the pipe and under slug flow conditions. Two-phase flow conditions can also cause flowmeters to measure inaccurately.

• Positive displacement flowmeter technology is the only flow measurement technology that directly measures the volume of the fluid passing through the flowmeter. Positive displacement flowmeters achieve this by repeatedly entrapping fluid in order to measure its flow. This process can be thought of as repeatedly filling a bucket with fluid before dumping the contents downstream. The number of times that the bucket is filled and emptied is indicative of the flow through the flowmeter. Many positive displacement flowmeter geometries are available.

• Entrapment is usually accomplished using rotating parts that form moving seals between each other and/or the flowmeter body. In most designs, the rotating parts have tight tolerances so these seals can prevent fluid from going through the flowmeter without being measured (slippage). In some positive displacement flowmeter designs, bearings are used to support the rotating parts. Rotation can be sensed mechanically or by detecting the movement of a rotating part. When more fluid is flowing, the rotating parts turn proportionally faster. The transmitter processes the signal generated by the rotation to determine the flow of the fluid. Some positive displacement flowmeters have mechanical registers that show the total flow on a local display. Other positive displacement flowmeters output pulses that can be used by a secondary electronic device to determine the flow rate.

• Positive displacement flowmeters can be applied to clean, sanitary, and corrosive liquids, such as water and foods, and some gases. Usually best applied when high accuracy is required at a reasonable price. PD meters represent 8% of global sales for flowmeters.

• Good for smaller line sizes, low flow rates, high viscosity and last a long time especially for oils. The downsides are there are moving parts to wear, need maintenance, snag on impurities and are not updated as much as other technologies with new protocols etc.

• Positive displacement flowmeters measure the volumetric flow of fluids in pipes, such as water, hydrocarbons, cryogenic liquids, and chemicals. Some designs can measure gas flow although liquid flow applications are much more prevalent. In liquid service, increasing viscosity decreases slippage and increases the pressure drop across the flowmeter. Surprisingly, accuracy can actually improve at low flow conditions in a given positive displacement flowmeter when viscosity increases and slippage decreases.

• A large pressure drop across the flowmeter can prematurely wear and/or damage bearings and/or seals. Therefore, most positive displacement flowmeters have a maximum pressure drop specification that is intended to limit positive displacement flowmeter bearing wear to reasonable levels. Operating the flowmeter above the pressure drop limits of the flowmeter can result in premature bearing wear and catastrophic flowmeter failure. Note that flowmeter size may be increased to reduce the pressure drop in these applications. This can increase cost significantly but failure to adhere to this specification can be even more expensive in some applications.

• Be careful because damaged sealing surfaces can increase slippage and degrade measurement accuracy. Using positive displacement flowmeters in abrasive or dirty fluids can cause maintenance problems because of potential damage to the sealing surfaces, damage to the bearings, and/or plugging of the flowmeter. A filter may be required to remove dirt.

• Be sure that gas bubbles are removed from liquid flow streams when using positive displacement flowmeters. Flow measurements taken with bubbles present will be higher than the true liquid flow because the bubble volumes will be measured as if they were a volume of liquid. Therefore, the presence of gas bubbles and (especially) the presence of a varying amount of gas bubbles can adversely affect the flow measurements associated with positive displacement flowmeters. A gas eliminator may be required to remove bubbles and mitigate this problem.

• This flowmeter can be applied to clean, sanitary, and corrosive liquids, such as water and foods, and some gases. Materials of construction are important because small amounts of corrosion or abrasion can damage the sealing surfaces and adversely affect measurement accuracy. In addition, consideration should be given to all wetted parts, including the body, rotating parts, bearings and gaskets.

• Many positive displacement flowmeters are used in municipal water districts to measure residential water consumption. Considering an installed base of millions of houses and apartments with metered water service, this application represents perhaps one of the largest number of applications of positive displacement flowmeters worldwide

• Corrosive liquid applications are commonly found in the chemical industry processes, and in chemical feed systems used in most industries. However, other flowmeter technologies may be more suitable for these services.

• The industries where they are used in descending order are oil and gas, water and wastewater, chemical, power, pharmaceutical, food and beverage, pulp and paper, metals and mining and aerospace.

• Avoid using positive displacement flowmeters in dirty fluids unless the dirt can be effectively removed upstream of the flowmeter. Operating these flowmeters in dirty fluids can cause plugging and increase maintenance costs. Be careful when selecting bearings because the non-lubricating nature of some fluids, impurities, and dirt can increase bearing wear and maintenance costs. Note that bearings usually do not necessarily fail catastrophically; they can slow down and adversely affect accuracy before they stop working.

• Avoid liquids with gas bubbles unless the bubbles can be effectively removed. As viscosity increases, be sure to ensure that the pressure drop across the flowmeter is acceptable. Make sure that the viscosity of the operating fluid is similar to that of the calibrated fluid, because the different amounts of slippage exhibited by different fluids can cause measurement error

• Differential pressure flowmeters use Bernoulli’s equation to measure the flow of fluid in a pipe. Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping routes the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow. This technology accounts for about 21% of the world market for flowmeters.

• Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the differential pressure transmitter.

• Different geometries are used for different measurements, including the orifice plate, flow nozzle, laminar flow element, low-loss flow tube, segmental wedge, V-cone, and Venturi tube

• The upside of this technology is low cost, multiple versions can be optimized for different fluids and goals, are approved for custody transfer (though it is being used less and less for this), it is a well understood way to measure flow, and it can be paired up with temperature/pressure sensors to provide mass flow for steam and other gasses. Negatives are that rangeability is not good due to a non-linear differential pressure signal (laminar flow elements excepted), accuracy is not the best and can deteriorate with wear and clogging.

• Differential pressure flowmeters inferentially measure the flow of liquids, gases and vapor, such as water, cryogenic liquids, chemicals, air, industrial gases, and steam. Be careful using differential pressure flowmeters for fluids with high viscosity, such as some hydrocarbons and foods, because their accuracy can be degraded when Reynolds number is low.

• This flowmeter can be applied to relatively clean fluids. With proper attention to materials of construction, the flow of corrosive fluids, such as are found in the chemical industry, can be measured.

• Somewhat dirty fluids can be measured by purging the impulse piping with an inert fluid. Be careful when using differential pressure flowmeters in dirty services because the dirt can plug the impulse piping and cause incorrect measurements. Diaphragm seals can sometimes be applied in these applications. However one should remember that diaphragm seals can degrade the performance of the differential pressure transmitter system, and hence, the degrade the performance of the flow measurement system.

• Differential pressure flowmeters are generally applicable to many flows in most industries, such as mining, mineral processing, pulp and paper, petroleum, chemical, petrochemical, water, and wastewater industries. Other flow measurement technologies may perform better than differential pressure flowmeters in many applications, however differential pressure flowmeters are still used extensively due to long-standing user familiarity with the technology.

• In descending order, they are used in. oil and gas, chemical, power, water and waste, pharmaceutical, metals and mining, pulp and paper, food and beverage and HVAC.

• Because of the non-linear relationship between flow and differential pressure, the accuracy of flow measurement in the lower portion of the flow range can be degraded. Plugging of the impulse piping can be a concern for many services. For slurry service, purges should be used to keep the impulse piping from plugging.

• For liquid service, impulse piping should be oriented and sloped so that it remains full of liquid and does not collect gas. For gas service, impulse piping should be oriented and sloped so that it remains full of gas and does not collect liquids. In vapor service, vapor may be allowed to condense in some of the impulse piping to form a liquid seal between the hot vapor and transmitter in order to protect the transmitter from heat.

• Be careful because the calibration of the differential pressure transmitter can be affected by the accumulation of liquid or gas in the impulse tubing. In addition, the accuracy of the flow measurement system can be degraded when varying amounts of liquid can accumulate during operation.

• Calibration issues can be important to the successful application of this technology. For example, differential pressure transmitter removal for calibration exposes the transmitter to multiple sources of potential problems that can affect the measurement, not the least of which is the extent to which the transmitter tubing is retightened after calibration. Calibration should generally be performed in-situ when possible and provisions to do so should be addressed during the design phase. For example, the differential pressure transmitter can be purchased with an integral valve manifold that allows easy calibration without disconnecting impulse tubing.

• Gas applications should be designed carefully because changes in operating pressure and operating temperature can dramatically affect the flow measurement. In other words, the gas density can vary significantly during operation. As a result, the differential pressure produced by the flowmeter can also vary significantly during operation. Failure to compensate for these effects can cause flow measurement errors of 20 percent or more in many applications. In these applications, a flow computer can be used to calculate the corrected flow measurement using actual pressure, temperature and flow measurements.