Flow Meter Types And Their Principles Pdf
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Flow Meter requirements vary, from mass flow to volume flow and beyond.
Ultrasonic flow meter
March One of the most important measurements in the chemical process industries CPI is rate of flow. Flowrates can be important to measure, and in fact can be critical to measure, throughout a process — from dosing components into a reactor to material transfers in-between process steps, to discharging products at the end.
Flow metering technologies fall into four classifications: velocity, inferential, positive displacement and mass.
This article summarizes the considerations in selecting and applying these different types of flowmeters, and provides examples of flowmeters in each category. Multiplying the measured average velocity by the cross-sectional area of the meter or pipe results in volumetric flowrates.
For example, if the average fluid velocity is 2. Laminar and turbulent flow profiles. Assuming there are sufficient straight-piping runs, the cross-sectional view shown in Figure 1 illustrates two flow-profile situations: turbulent and laminar. In cases of relatively little piping-friction loss and low fluid viscosity, the flow profile of velocities is uniform across the entire cross-section of the pipe — this is called fully developed turbulent flow.
In this case, the fluid velocity at the pipe walls closely matches the fluid velocity at the center and at all points in-between. The velocity at any point is the average velocity.
This condition results when the Reynolds number is 10, or above. Calculating volumetric flowrates in this flow regime is relatively easy, as noted above.
The average velocity through a pipe becomes increasingly difficult to measure precisely when Reynolds numbers are low. Such conditions are typical of laminar flow profiles. With an insertion probe flowmeter, engineers can establish the fluid velocities across the diameter of a pipe at multiple cross-sectional locations.
Pipe geometry plays a role. Manufacturers will specify the length of straight pipe upstream and downstream of a velocity flowmeter for achieving high accuracies. But often plant piping geometries in a chemical plant will be such that sufficiently long straight pipe runs are not feasible.
The flowmeter may have to be located near an elbow, tee, valve, or change in pipe diameter. In this case the flow will not be fully developed and result in a distorted profile, such as in the example shown in Figure 2. Various flow straightening devices installed upstream of the flowmeter can help correct these distortions by creating uniform flow profiles, and thereby permitting average velocity to be inferred.
Straightening vanes are engineered, bundled tubes that are installed upstream of a meter in order to ensure a more uniform flow profile. The most practical, liquid pipeline flowrates in the CPI range from 0. Lower rates can be difficult to measure accurately and higher rates result in higher pressure drops, pumping energy costs and erosion if abrasive solids are present. Many actual applications have flowrate ranges well within these extremes. Electromagnetic flowmeters subject conductive liquids to alternating or pulsating direct-current magnetic fields.
Vortex meters place a bluff obstacle in the flow stream, which creates vortices or eddies whose frequency is proportional to flow velocity. Sensors detect and count the pressure variations produced over a fixed time. Swirl meters are similar to vortex meters, except vanes at the inlet swirl the flow, creating the pressure variations. Straightening vanes at the outlet de-swirl the flow. Turbine meters contain a turbine. A sensor detects the rotational rate. Ultrasonic meters come in two types.
The Doppler flowmeter sends an ultrasonic beam into the flow and measures the frequency shift of reflections from discontinuities in the flow. Transit-time flowmeters have an ultrasonic transmitter and receiver separated by a known distance. The difference in transit time for a signal aided by the flow versus the signal moving against the flow is a function of fluid velocity.
An inferential flowmeter calculates flowrates based on a non-flow measurement that has widely accepted correlations to rate of flow. Differential pressure. Most of these flow measurement devices depend on three principles.
First, with or without the restriction in a pipe, the overall flowrate remains the same, which pertains to the continuity equation. Third, the law of conservation of energy says the increased kinetic energy comes at the expense of fluid pressure potential energy.
The pressure drop across the restriction is a function of the fluid velocity, which can be calculated. Variables in the calculation of flowrate for differential flowmeters include the following:. When a fluid passes through a restriction in a pipe, it does not follow the contour of restriction perfectly. The smaller jet diameter results in a faster stream velocity through the restriction, resulting in higher pressure loss than if the fluid perfectly followed the contour of the bore.
Consequently, calculated flowrates from measured pressure drop and a known restriction bore diameter would tend to overstate the fluid flowrate. Therefore the rate must be corrected downward from the ideal discharge coefficient, which is equal to one.
The overall flow coefficient applied to the basic equation is often specific to both the device and the application, and depends on additional factors involving gas expansion and velocity of approach. This coefficient K factor can range from 0. They have good application flexibility since they can measure liquid, gas and steam flows, and are suitable for extreme temperatures and pressures with moderate pressure losses.
These losses depend on restriction size and type orifice, wedge, pitot, Venturi and so on and can be quite high and permanent given a low enough Beta ratio. Beta ratio is the diameter of the restrictive orifice divided by the pipe diameter. Compensation techniques can improve accuracy to 0. On the other hand, restrictive flowmeter piping elements are relatively expensive to install. Their dependence on the square root of differential pressure can severely diminish rangeability. Additionally, they require an instrument to measure differential pressure and compute a standard flow signal.
Changes in temperature, pressure, and viscosity can significantly affect accuracy of differential pressure flowmeters. And while they have no moving parts, maintenance can be intensive. Variable area meters. Often called rotameters, variable area meters Figure 3 are another kind of inferential flowmeter.
Simple and inexpensive, these devices provide practical flow measurement solutions for many applications. They basically consist of two components: a tapered metering tube and a float that rides within the tube. The float position — a balance of upward flow and float weight — is a linear function of flowrate.
Operators can take direct readings based on the float position with transparent glass and plastic tubes. Rotameters with metal tubes include a magnetically coupled pointer to indicate float position. Rotameters are easy to install and maintain, but must be mounted perfectly vertical. Their flow characteristics are well documented in the literature.
Maintenance of good accuracy requires a sharp edge to the upstream side, which degrades with wear. Pressure loss is high, relative to other DP elements. Venturi meters are characterized by a gradual tapered restriction on the inlet and outlet. This element has high discharge coefficients near the ideal value of one. Pressure loss is minimal. Venturi meters find use primarily in water and wastewater applications and have limited acceptance elsewhere in the CPI.
The rangeability of about is better than orifice plates. Performance characteristics are well documented. Nozzle elements mimic the properties of the Venturi meter. They come in three standard, documented types: ISA nozzle, common outside of the U.
Wedge elements consist of a V-shaped restriction molded into the top of the meter body. This basic meter has been on the market for more than 40 years, demonstrating its ability to handle tough, dirty fluids. The slanted faces of the wedge provide self-scouring action and minimize damage from impact with secondary phases.
Wedge meter rangeability of is relatively high for a DP element. Flow tubes come in several proprietary shapes, but all tend to be more compact than the classic and short-form Venturies.
With proprietary designs, flow tubes vary in configuration, tap locations, differential pressure and pressure loss for a given flow. Manufacturers must supply test data for flow tubes. Pitot tubes are low-cost DP elements used to measure fluid flow, especially air flow in ventilation and heating, ventilating and air-conditioning HVAC systems.
They work by converting the kinetic energy of the flow velocity into potential energy pressure. Engineers can easily insert pitot tubes into existing piping, minimizing installation costs.
One type makes a measurement at a point within the pipeline or ductwork, requiring knowledge of the flow profile. Another contains multiple orifices, providing an averaging effect. Target meters. These flowmeters insert a physical target within the fluid flow. The moving fluid deflects a force bar attached to the target. The deflection depends on the target area, as well as the fluid density and velocity.
Target meters measure flows in line sizes above 0. By changing the target size and material, engineers can adapt them to different fluids and flowrate ranges. In most cases, their calibration must be verified in the field. Target meters are relatively uncommon and are found primarily in water and steam applications, as well as on wet gases.
A flow meter is used in different types of applications to measure the volumetric flowrate or mass flowrate. The specific application defines the type and capacity of the flow meter. Fluids, gases and liquids, are measured in terms of volumetric flowrate and mass flowrate. There are numerous types of flow meters depending upon the application, the specific fluid and the construction, including materials, of the flow meter. The 5 basic classification of flow meters include:. Differential pressure flow meters measure the differential pressure across an orifice where flow is directly related to the square root of the differential pressure produced. There are also primary and secondary elements in differential flow meters.
PDF | This paper intended how to measure direct mass without converting from When there is mass flow, there is some twisting of the tubes.
What Type of Flow Meter Do You Need?
This article explains a thermal mass flow meter and clarifies the working principle of thermal flow measurement. A thermal mass flow meter is a precision instrument that measures gas mass flow. The device is used in a wide range of applications in many industries.
Due to its reliance on the ability of the fluid or gas to displace the float, graduations on a given rotameter will only be accurate for a given substance at a given temperature. For all industrial process applications. These types of flow meters often measure a positive volume when dealing with small quantiles, or the volume can be totalized over time when measuring large quantiles.
flow meter types pdf
An ultrasonic flow meter is a type of flow meter that measures the velocity of a fluid with ultrasound to calculate volume flow. Using ultrasonic transducers , the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. They vary greatly in purchase price but are often inexpensive to use and maintain because they do not use moving parts , unlike mechanical flow meters. There are three different types of ultrasonic flow meters. Transmission or contrapropagating transit-time flow meters can be distinguished into in-line intrusive, wetted and clamp-on non-intrusive varieties.
Other flow meters. Fixed restriction variable head type flow meters using different sensors like orifice plate, venturi tube, flow nozzle, pitot tube, dall tube, quantity meters like positive displacement meters, mass flow meters are the popular types of mechanical flow meters.
March One of the most important measurements in the chemical process industries CPI is rate of flow. Flowrates can be important to measure, and in fact can be critical to measure, throughout a process — from dosing components into a reactor to material transfers in-between process steps, to discharging products at the end. Flow metering technologies fall into four classifications: velocity, inferential, positive displacement and mass. This article summarizes the considerations in selecting and applying these different types of flowmeters, and provides examples of flowmeters in each category.
Click here to view Vol. In case you haven't noticed, there are several types of flow meters and flow sensors. There are also a number of different operating principles. Have you ever wondered if there were other, more efficient methods than what you are currently using? If you have, you're in luck.
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