What is positive displacement flow meter?

What is positive displacement flow meter?

Positive displacement (PD) circulate meters are move meters that present high accuracy (±0.1% of precise flow in some cases) and good repeatability (up to 0.05% of reading). These meters require no power provide to function and no direct upstream or downstream piping for set up. They are well suited to circulate measurement of viscous fluids, similar to oils, fuels and solvents, that are troublesome to measure with other kinds of circulate meters.
PD flow meters are most widely used in home water meters, with millions of these models produced every year. Volumetric circulate meters can be used for clear, sanitary and corrosive fluids such as water and meals, in addition to some gases. They are sometimes most applicable when excessive accuracy at an affordable price is required.
How does a optimistic displacement circulate meter work?

Positive displacement move meter technology is the one circulate measurement know-how that immediately measures the volume of fluid passing through the meter. Volumetric flowmeters do this by repeatedly intercepting fluid to measure its flow rate. This process could be considered repeatedly filling a bucket with fluid earlier than pouring the contents downstream. The number of instances the bucket is stuffed and emptied signifies the circulate rate via the flowmeter. There are many volumetric move meter geometries out there.
Retention is often achieved using rotating components that form a shifting seal between one another and/or the flowmeter physique. In most designs, the rotating elements have tight tolerances so that these seals prevent fluid from passing via the flowmeter with out being measured (sloshing). In some volumetric flowmeter designs, bearings are used to support the rotating parts. Rotation can be sensed mechanically or by detecting the motion of the rotating components. As more fluid flows, the rotating elements rotate proportionally quicker. The transmitter processes the sign generated by the rotation to determine the circulate price of the fluid. Some volumetric move meters have mechanical registers that present the entire circulate price on a local show.
Features

Suitable for smaller line sizes, low move charges, high viscosities and long durations, particularly for oils. The disadvantages are that there are shifting components to put on, upkeep required, extra impurities, and never as updated as other applied sciences with new protocols, and so on.
Accuracy: One of the primary benefits of utilizing PD move meters is the high accuracy they offer, the excessive accuracy of the internal components means maintaining the clearances between the sealing surfaces to a minimum. The smaller these clearances are, the higher the accuracy related to them. Only fluids that may bypass this seal usually are not accounted for, that is known as “bypass” or “sloshing”.
Range and repeatability: Another profit is that circulate meters can deal with a variety of viscosities, and it is not uncommon to expertise greater ranges of accuracy when coping with excessive viscosity fluids, simply as a result of reduction of bypasses. When contemplating and comparing flowmeter accuracy, you will want to understand “linearity”; i.e., the flexibility of the meter to precisely measure the complete regulation ratio, and “repeatability”, i.e., the ability to remain accurate over multiple cycles. This is another area where PD circulate meters excel, with zero.02% repeatability and 0.5% linearity being commonplace.
Reliability: If the right move meter is chosen for an application, it could be anticipated to function correctly for many years. Meters are often sent for restore and recalibration that have been in the area for 10 and generally 20 years of steady use. This reliability is due largely to the fact that the identical confirmed technology has been in use for more than 60 years, allowing main advances to be centered on tribology and attaining the required accuracy at an inexpensive value.
Low upkeep: The recommended maintenance degree is strongly influenced by the application. For instance, if the flow meter is dealing with a fluid with lubricating properties (i.e. oil), routine maintenance could be just about eradicated. However, if the fluid has poor lubrication properties, then it is best to debate upkeep necessities along with your distributor.
It could be very uncommon that any upkeep on a optimistic displacement move meter is extra frequent than other tools within the similar system and can be scheduled on the identical time, thus minimizing downtime.
Types of displacement flow meters

The operation of a volumetric (PD) flow meter consists of separating the liquid into exactly measured increments and persevering with to move. Each section is counted by a connection register. Because each increment represents a discrete volume, optimistic displacement items are in style in automated batching and accounting functions. Volumetric circulate meters are best for measuring the move of viscous liquids or where a simple mechanical instrumentation system is required.
Positive displacement circulate meters for liquids or chapter-action disc meters

Chapter-action disc meters are the commonest kind of PD meter. They are used as residential water meters around the globe. As water flows through the metering chamber, it causes the disc to oscillate (chapter motion), which turns the spindle, which rotates the magnet. This magnet is coupled to a mechanical register or pulse transmitter. Because the meter captures a set amount of fluid each time the spindle rotates, the circulate fee is proportional to the speed of rotation of the spindle.
Oscillating piston flowmeter

Oscillating piston move meters are typically used in viscous fluid providers, such as oil metering on engine take a look at stands, the place the regulation ratio is not critical. These meters are also used in residential water service and may cross a restricted quantity of filth, similar to pipe scale and fantastic sand (i.e. -200 mesh or -74 micron), but not giant particles or abrasive solids.
Elliptical Gear and Convex Angle Gauges

The elliptical gear partial discharge meter makes use of two fine-tooth gears, one mounted horizontally and the other vertically, with the gears meshing at the tip of the vertical gear and the center of the horizontal gear. The two rotors rotate relative to one another, thereby creating interception within the crescent-shaped hole between the housing and the gears. These gauges may be very correct if the slippage between the housing and equipment is kept small. If the process fluid viscosity is larger than 10 centipoise and the circulate price is bigger than 20% of rated capability, an accuracy of zero.1% AR could be obtained. At decrease move rates and lower viscosities, slippage will increase and accuracy decreases to 0.5% AR or less.
Helical meter

The Helical Meter is a volumetric device that uses two radially inclined helical gears to continuously entice the process fluid because it flows. The circulate forces the helical gears to rotate in the airplane of the pipe. Optical or magnetic sensors are used to encode a sequence of pulses proportional to the rotation velocity of the helical gears. The force required to rotate the helix is comparatively low, leading to a relatively low stress drop in comparability with other PD devices. The greatest achievable accuracy is roughly ±0.2% or price.
Metering Pumps

Metering pumps are PD devices that additionally transfer kinetic power to the method fluid. There are three basic designs: peristaltic, piston and diaphragm.
Peristaltic pumps run by systematically squeezing plastic tubing through a finger or cam into a housing, which can also be used to position the tubing. These metering pumps are utilized in laboratories, varied medical purposes, most environmental sampling systems, and for dishing out hypochlorite solutions. The tubing could be silicone rubber or, if a extra corrosion resistant materials is required, PTFE tubing can be used.
The piston pump delivers a fixed quantity of fluid on every “out” stroke and a fixed volume of fluid into the chamber on every “in” stroke. A examine valve prevents fluid from flowing backwards. As with all positive displacement pumps, piston pumps generate pulsating move. To decrease pulsation, a number of pistons or pulsation dampening vessels are put in. Because of the tight tolerances of the piston and cylinder liner, a flushing mechanism should be provided in abrasive applications. The measurement of the piston pump depends on the piston displacement as nicely as the required flow and discharge stress. Select a verify valve (or in critical purposes, a double examine valve) to prevent backflow.
Diaphragm metering pumps are the commonest industrial PD pumps. A typical configuration features a diaphragm, a chamber, and suction and discharge check valves to prevent backflow. The piston can be related on to the diaphragm or can pressure hydraulic fluid to actuate the diaphragm. Maximum output pressure is approximately a hundred twenty five psig. variants embrace bellows diaphragms, hydraulically actuated double diaphragms, and pneumatically reciprocating double diaphragms.
Gas optimistic displacement circulate meters

PD fuel flow meters operate by calculating the cut-off volume of the gas passing through, similar to the greatest way PD move meters function on liquids. The major difference is that the fuel is compressible.
Diaphragm gas meters are most commonly used to measure the flow of natural gas, especially for household metered consumption. The meter is made from solid aluminum with a cloth-backed rubber diaphragm. The meter consists of 4 chambers: two diaphragm chambers on the inlet and outlet aspect and an inlet and outlet chamber in the meter physique. The passage of the gasoline by way of the meter creates a strain distinction between the 2 diaphragm chambers through the diaphragm chamber on the compression inlet facet and the diaphragm chamber on the growth outlet side. This motion alternately empties and fills the 4 chambers. A slide valve on the top of the instrument alternately acts on the chambers and synchronizes the motion of the diaphragms, in addition to the crank mechanism that operates the instrument register.
High Precision Positive Displacement Systems

A excessive accuracy gas meter is often a hybrid that combines a normal constructive displacement meter with a motor drive that eliminates pressure drop throughout the meter. Balancing inlet and outlet pressures eliminates slip, leakage and gas fugitive. In a high-precision fuel meter set up, a high-sensitivity vane is used to detect differential pressure and a displacement sensor is used to measure vane deflection.
How to make use of volumetric move meters?

Positive displacement move meters measure the volumetric circulate of fluids in pipelines, such as water, hydrocarbons, cryogenic liquids and chemical substances. Some designs can measure fuel move, although liquid flow applications are more common. In liquid service, rising viscosity reduces slip and will increase the pressure drop via the flowmeter. Surprisingly, beneath low flow conditions for a given volumetric flow meter, accuracy can really enhance when viscosity increases and slip decreases.
Larger strain drops throughout the move meter can prematurely wear and/or damage bearings and/or seals. Therefore, most positive displacement move meters have a most stress drop specification designed to limit optimistic displacement circulate meter bearing put on to an affordable level. Operating the meter above the meter pressure drop limit can outcome in untimely bearing wear and catastrophic meter failure. Note that the flowmeter dimension could be elevated to cut back the strain drop in these purposes. This might significantly increase the cost, but failure to adjust to this specification could also be costlier in some applications.
Be cautious, as broken sealing surfaces can improve slippage and reduce measurement accuracy. Using a optimistic displacement flowmeter in abrasive or dirty fluids can cause upkeep problems due to the potential for injury to sealing surfaces, injury to bearings, and/or clogging of the flowmeter. A filter could additionally be required to take away dust.
When utilizing a volumetric flowmeter, be positive that air bubbles are faraway from the fluid stream. Flow measurements made within the presence of air bubbles shall be greater than the true liquid move because the bubble volumes are measured as in the occasion that they were a sure quantity of liquid. Therefore, the presence of air bubbles and (especially) the presence of various quantities of air bubbles can adversely affect the move measurement related to a volumetric circulate meter. Gas eliminators may be required to take away air bubbles and mitigate this drawback.
The flowmeter can be used for clean, sanitary, corrosive liquids such as water and food, as nicely as some gases. Materials of development are necessary as a end result of small quantities of corrosion or put on can harm sealing surfaces and adversely have an result on measurement accuracy. In addition, all wetted components must be thought-about, including the valve physique, rotating parts, bearings and gaskets.
Municipal water districts use many volumetric move meters to measure residential water utilization. Considering the installed base of hundreds of thousands of homes and apartments with metered water service, this software most likely represents one of many largest numbers of volumetric flowmeter functions worldwide.
Corrosive liquid functions are common in chemical industrial processes and chemical feed techniques used in most industries. However, other flowmeter technologies could additionally be extra suitable for these providers.
Industries used

Oil and gasoline

Water and Wastewater

Chemical

Power

Pharmaceuticals

Food and Beverage

Pulp and Paper

Metals and Mining

Aerospace

Application concerns for volumetric move meters

Avoid using Positive displacement flowmeters in soiled fluids until fouling could be effectively removed upstream of the flowmeter. Operating these flowmeters in soiled fluids can lead to clogging and elevated upkeep prices. Be careful when choosing bearings, as the non-lubricated nature of certain fluids, impurities and dust can enhance bearing put on and upkeep prices. Note that bearings often do not at all times fail catastrophically; they may slow down and adversely have an result on accuracy before they stop working.
Avoid utilizing fluids with air bubbles unless they can be effectively eliminated. As viscosity increases, make certain that the pressure drop by way of the meter is suitable. Make sure the viscosity of the working fluid is similar to the viscosity of the calibration fluid, as completely different fluids exhibit different amounts of slip that may result in measurement errors.
More articles on flow meters:
Working principle of rotameter

Ultrasonic circulate meter working principle

What is doppler circulate meter?

Relation between flow and pressuree
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What is optimistic displacement flow meter?

Positive displacement (PD) circulate meters are move meters that provide high accuracy (±0.1% of precise move in some cases) and good repeatability (up to 0.05% of reading). These meters require no energy provide to operate and no direct upstream or downstream piping for installation. They are nicely fitted to circulate measurement of viscous fluids, such as oils, fuels and solvents, which are troublesome to measure with different types of flow meters.
PD flow meters are most generally utilized in house water meters, with tens of millions of these models produced every year. Volumetric flow meters can be utilized for clear, sanitary and corrosive fluids corresponding to water and meals, as well as some gases. They are often most relevant when excessive accuracy at a reasonable price is required.
How does a positive displacement circulate meter work?

Positive displacement circulate meter know-how is the only circulate measurement know-how that instantly measures the quantity of fluid passing by way of the meter. Volumetric flowmeters do that by repeatedly intercepting fluid to measure its move rate. This process can be thought of as repeatedly filling a bucket with fluid before pouring the contents downstream. The number of times the bucket is crammed and emptied signifies the flow price by way of the flowmeter. There are many volumetric circulate meter geometries available.
Retention is often achieved utilizing rotating components that type a shifting seal between each other and/or the flowmeter body. In most designs, the rotating parts have tight tolerances in order that these seals prevent fluid from passing via the flowmeter without being measured (sloshing). In some volumetric flowmeter designs, bearings are used to support the rotating elements. Rotation can be sensed mechanically or by detecting the motion of the rotating components. As more fluid flows, the rotating elements rotate proportionally sooner. The transmitter processes the signal generated by the rotation to determine the circulate price of the fluid. Some volumetric flow meters have mechanical registers that show the total circulate fee on a neighborhood display.
Features

Suitable for smaller line sizes, low move rates, high viscosities and long durations, particularly for oils. The disadvantages are that there are transferring parts to wear, upkeep required, more impurities, and never as updated as other technologies with new protocols, and so on.
Accuracy: One of the main advantages of using PD flow meters is the high accuracy they provide, the high accuracy of the interior components means preserving the clearances between the sealing surfaces to a minimal. The smaller these clearances are, the upper the accuracy associated with them. Only fluids that can bypass this seal are not accounted for, this is referred to as “bypass” or “sloshing”.
Range and repeatability: Another benefit is that move meters can deal with a wide range of viscosities, and it’s not unusual to expertise larger ranges of accuracy when coping with excessive viscosity fluids, simply due to the reduction of bypasses. When contemplating and evaluating flowmeter accuracy, it is necessary to understand “linearity”; i.e., the power of the meter to precisely measure the entire regulation ratio, and “repeatability”, i.e., the ability to remain accurate over a number of cycles. This is one other area the place PD move meters excel, with 0.02% repeatability and zero.5% linearity being normal.
Reliability: If the proper flow meter is chosen for an utility, it can be anticipated to function correctly for many years. Meters are sometimes sent for restore and recalibration which were in the subject for 10 and sometimes 20 years of continuous use. This reliability is due in massive part to the reality that the same proven technology has been in use for more than 60 years, allowing major advances to be targeted on tribology and attaining the required accuracy at a reasonable cost.
Low maintenance: The recommended maintenance stage is strongly influenced by the appliance. For example, if the circulate meter is dealing with a fluid with lubricating properties (i.e. oil), routine upkeep could be nearly eliminated. However, if the fluid has poor lubrication properties, then it is best to debate maintenance necessities together with your distributor.
It may be very rare that any maintenance on a positive displacement move meter is more frequent than other equipment throughout the identical system and can be scheduled on the same time, thus minimizing downtime.
Types of displacement circulate meters

The operation of a volumetric (PD) circulate meter consists of separating the liquid into exactly measured increments and persevering with to move. Each section is counted by a connection register. Because each increment represents a discrete volume, positive displacement models are in style in automated batching and accounting functions. Volumetric circulate meters are excellent for measuring the circulate of viscous liquids or where a easy mechanical instrumentation system is required.
Positive displacement circulate meters for liquids or chapter-action disc meters

Chapter-action disc meters are the most typical sort of PD meter. They are used as residential water meters all over the world. As water flows via the metering chamber, it causes the disc to oscillate (chapter motion), which turns the spindle, which rotates the magnet. This magnet is coupled to a mechanical register or pulse transmitter. Because the meter captures a exhausting and fast quantity of fluid every time the spindle rotates, the flow rate is proportional to the pace of rotation of the spindle.
Oscillating piston flowmeter

Oscillating piston move meters are typically used in viscous fluid providers, such as oil metering on engine take a look at stands, where the regulation ratio isn’t crucial. These meters are also utilized in residential water service and can move a restricted amount of dirt, corresponding to pipe scale and fine sand (i.e. -200 mesh or -74 micron), but not large particles or abrasive solids.
Elliptical Gear and Convex Angle Gauges

The elliptical gear partial discharge meter uses two fine-tooth gears, one mounted horizontally and the other vertically, with the gears meshing at the tip of the vertical gear and the middle of the horizontal gear. The two rotors rotate relative to every other, thereby creating interception within the crescent-shaped hole between the housing and the gears. These gauges can be very correct if the slippage between the housing and gear is stored small. If the method fluid viscosity is larger than 10 centipoise and the move fee is larger than 20% of rated capability, an accuracy of zero.1% AR may be obtained. At decrease flow rates and lower viscosities, slippage increases and accuracy decreases to zero.5% AR or less.
Helical meter

The Helical Meter is a volumetric device that uses two radially inclined helical gears to repeatedly entice the process fluid because it flows. The circulate forces the helical gears to rotate within the airplane of the pipe. Optical or magnetic sensors are used to encode a sequence of pulses proportional to the rotation pace of the helical gears. The drive required to rotate the helix is comparatively low, resulting in a relatively low pressure drop in comparison with other PD devices. diaphragm seal is roughly ±0.2% or fee.
Metering Pumps

Metering pumps are PD instruments that also transfer kinetic energy to the method fluid. There are three primary designs: peristaltic, piston and diaphragm.
Peristaltic pumps run by systematically squeezing plastic tubing through a finger or cam into a housing, which can be used to place the tubing. These metering pumps are used in laboratories, various medical functions, most environmental sampling techniques, and for dispensing hypochlorite solutions. The tubing can be silicone rubber or, if a extra corrosion resistant materials is required, PTFE tubing can be used.
The piston pump delivers a set quantity of fluid on each “out” stroke and a hard and fast quantity of fluid into the chamber on each “in” stroke. A examine valve prevents fluid from flowing backwards. As with all constructive displacement pumps, piston pumps generate pulsating circulate. To reduce pulsation, multiple pistons or pulsation dampening vessels are put in. Because of the tight tolerances of the piston and cylinder liner, a flushing mechanism must be provided in abrasive applications. The size of the piston pump depends on the piston displacement as well as the required circulate and discharge stress. Select a verify valve (or in critical functions, a double verify valve) to prevent backflow.
Diaphragm metering pumps are the most typical industrial PD pumps. A typical configuration features a diaphragm, a chamber, and suction and discharge verify valves to forestall backflow. The piston could be linked on to the diaphragm or can force hydraulic fluid to actuate the diaphragm. Maximum output stress is roughly one hundred twenty five psig. variants embody bellows diaphragms, hydraulically actuated double diaphragms, and pneumatically reciprocating double diaphragms.
Gas constructive displacement flow meters

PD gasoline flow meters function by calculating the cut-off volume of the gasoline passing via, similar to the greatest way PD circulate meters operate on liquids. The major difference is that the fuel is compressible.
Diaphragm gas meters are most commonly used to measure the move of natural fuel, particularly for household metered consumption. The meter is made of cast aluminum with a cloth-backed rubber diaphragm. The meter consists of 4 chambers: two diaphragm chambers on the inlet and outlet facet and an inlet and outlet chamber in the meter body. The passage of the fuel by way of the meter creates a stress difference between the 2 diaphragm chambers via the diaphragm chamber on the compression inlet facet and the diaphragm chamber on the growth outlet aspect. This action alternately empties and fills the 4 chambers. A slide valve on the top of the instrument alternately acts on the chambers and synchronizes the action of the diaphragms, as well as the crank mechanism that operates the instrument register.
High Precision Positive Displacement Systems

A high accuracy gasoline meter is usually a hybrid that mixes a standard optimistic displacement meter with a motor drive that eliminates stress drop across the meter. Balancing inlet and outlet pressures eliminates slip, leakage and fuel fugitive. In a high-precision gas meter set up, a high-sensitivity vane is used to detect differential pressure and a displacement sensor is used to measure vane deflection.
How to use volumetric circulate meters?

Positive displacement circulate meters measure the volumetric flow of fluids in pipelines, similar to water, hydrocarbons, cryogenic liquids and chemical compounds. Some designs can measure gasoline circulate, although liquid flow purposes are extra common. In liquid service, rising viscosity reduces slip and increases the pressure drop via the flowmeter. Surprisingly, beneath low circulate circumstances for a given volumetric circulate meter, accuracy can actually improve when viscosity increases and slip decreases.
Larger pressure drops throughout the circulate meter can prematurely put on and/or injury bearings and/or seals. Therefore, most constructive displacement move meters have a most strain drop specification designed to restrict constructive displacement flow meter bearing put on to a reasonable stage. Operating the meter above the meter strain drop restrict can end result in premature bearing wear and catastrophic meter failure. Note that the flowmeter measurement could be increased to reduce the stress drop in these applications. This may significantly enhance the cost, however failure to adjust to this specification could additionally be dearer in some functions.
Be cautious, as broken sealing surfaces can enhance slippage and scale back measurement accuracy. Using a constructive displacement flowmeter in abrasive or soiled fluids can cause upkeep problems due to the potential for injury to sealing surfaces, injury to bearings, and/or clogging of the flowmeter. A filter may be required to remove dust.
When utilizing a volumetric flowmeter, make sure that air bubbles are faraway from the fluid stream. Flow measurements made within the presence of air bubbles will be higher than the true liquid move as a result of the bubble volumes are measured as in the event that they were a sure quantity of liquid. Therefore, the presence of air bubbles and (especially) the presence of varying amounts of air bubbles can adversely have an result on the move measurement related to a volumetric move meter. Gas eliminators may be required to take away air bubbles and mitigate this downside.
The flowmeter can be utilized for clean, sanitary, corrosive liquids such as water and food, in addition to some gases. Materials of development are essential because small quantities of corrosion or wear can damage sealing surfaces and adversely have an effect on measurement accuracy. In addition, all wetted parts ought to be thought of, including the valve body, rotating elements, bearings and gaskets.
Municipal water districts use many volumetric flow meters to measure residential water utilization. Considering the put in base of tens of millions of homes and apartments with metered water service, this software most likely represents one of the largest numbers of volumetric flowmeter functions worldwide.
Corrosive liquid purposes are common in chemical industrial processes and chemical feed techniques utilized in most industries. However, different flowmeter technologies may be more appropriate for these providers.
Industries used

Oil and gasoline

Water and Wastewater

Chemical

Power

Pharmaceuticals

Food and Beverage

Pulp and Paper

Metals and Mining

Aerospace

Application concerns for volumetric circulate meters

Avoid using Positive displacement flowmeters in soiled fluids unless fouling may be successfully removed upstream of the flowmeter. Operating these flowmeters in dirty fluids can lead to clogging and increased upkeep costs. Be cautious when selecting bearings, as the non-lubricated nature of sure fluids, impurities and dirt can enhance bearing wear and upkeep prices. Note that bearings normally don’t always fail catastrophically; they might decelerate and adversely have an result on accuracy earlier than they cease working.
Avoid using fluids with air bubbles until they can be effectively eliminated. As viscosity will increase, be certain that the stress drop through the meter is suitable. Make certain the viscosity of the working fluid is similar to the viscosity of the calibration fluid, as totally different fluids exhibit different quantities of slip that may result in measurement errors.
More articles on flow meters:
Working principle of rotameter

Ultrasonic move meter working precept

What is doppler move meter?

Relation between move and straine

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