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found some good info for everyone to gain knowledge about flow meters.
5.2.1 Differential Pressure
Historically, differential pressure measurements have been the most common flow rate meters. Differential pressure flowmeters employ the Bernoulli Equation that describes the pressure difference that results when a restriction is placed in a pipe. At the restriction, the flow velocity increases which in turn decreases the static pressure downstream. The pressure difference generated is a measure of the fluid flow rate through the restriction and the pipe. The two key components found in differential pressure flowmeters are a restriction to cause a pressure drop in the flow (differential producer) and a method of measuring the pressure drop across the obstruction (differential pressure transducer).

The basic principle on which differential pressure flow meters operate is the conversion of energy from one form to another. For liquid flows, only kinetic energy and the energy due to static pressure are considered. For gases or vapors, the internal energy of the compressed fluid is also involved.

The equation for mass flow rate of fluids thru the orifice, venture, flow nozzle can be calculated by the following equation:

m is the mass flow rate (lbm/s)
C0 is the DP device coefficient
A0 is the cross sectional area of the instrument (ft2)
Y is a correction factor supplied by the vendor (Y=0 for liquids)
p1 and p2 are upstream and downstream pressures (psi)
ρ is the density of the fluid (lb/ft3)
D0 and D1 (ft)
is a constant 4633.24 (in2*lbm)/(ft*lbf*s2)

A. Orifice Plates

One of the most common primary flow devices is the orifice plate. For pipes above 2 inches in diameter, orifice plates are mounted between bolted flanges. The flanges are threaded or welded to the pipe depending on pipe size and operating line pressures.

Most orifice plates have sharp, square or rounded upstream edges. Concentric plates are the most common design used since the accuracy is highly predictable and extensive data is available for a large range of flows, pressure differentials and pipe sizes.

Eccentric and segmental orifice plates may be used when the measured fluid contains suspended material or has both liquid and gaseous phases. Use of concentric plates in this application may lead to accumulations behind the plates and cause false readings.

Figure 1 – Orifice Plate Types
Beta ratios should be between 0.15 and 0.70 for flange taps. Beta ratios should be between 0.20 and 0.67 for pipe taps. Orifice plates with small Beta ratios and high pressure drops often function as restriction orifices. These plates may have a thickness up to ½” to withstand the energy that is dissipated.

Permanent pressure drop loss can be from 75% to 90% of the up stream pressure depending on the Beta.

Pressure tap locations are discussed in Guideline EG-19-222 (Flow Meter Installation).

B. Flow Nozzle

Flow nozzles are widely used for flow measurements at high flow velocities. The flow nozzle is more rugged and erosion resistant than the orifice plate. For a given diameter and a given differential pressure, the flow nozzle will pass approximately 65% more flow than the orifice plate.

Permanent pressure drop of a flow nozzle is greater than the Venturi flow element and less than the orifice flow element.

The flow nozzle, because of its streamlined contour, tends to sweep solids through the throat. For non-homogeneous fluids, the flow nozzle is preferable to the orifice plate. The flow nozzle should not be used if large percentages of solids are present.

Typical pressure measurements are at the radius tap locations of 1 pipe diameter upstream and 0-.5 pipe diameters downstream of the flow nozzle. The downstream tap should never be located beyond the end of the nozzle. As in all differential producers, its output varies as the square root of the flow rate.


C. Venturi

The Venturi tube can measure 25 to 50% more flow than orifice for comparable line size and head loss. The flow range for satisfactory measurement is usually considered to extend upward from Reynolds numbers of about 200,000.

Some advantages of Venturi tubes are:

• high flow rates
• minimal piping straight run requirements (typically 10 pipe diameters)
• good accuracy with Beta ratios greater than .75
• integral pressure connections

The purchase cost of the Venturi tube is greater than most other primary flow elements. However, the greater pressure recovery can result in significant energy savings in large pipelines.

The Venturi tube has a converging conical inlet, a cylindrical throat, and a diverging recovery cone. A standard Venturi tube is shown in Figure 3.


A number of taps are often employed around the circumference of the high and low pressure areas which are connected together in what is known as a piezometer ring. This allows multiple pressure measurements in order to get a better average of the pressures. As with all differential pressure measurements, the flow rate varies as the square root of the differential pressure.

D. Flow Tubes

Flow tubes are primary elements with converging and diverging sections, similar to Venturi, whose design is usually proprietary.

Some advantages of flow tubes are:
• high differential pressure with high recovery
• low cost

Limitations include:
• inapplicability to low flows and small pipes
• sensitivity to viscosity variations
• erroneous readings in highly viscous or dirty liquids

Flow tubes require approximately the same piping runs as an orifice plate.

E. Pitot Tubes

Pitot tubes are the simplest velocity head sensors. Models can be specified for a variety of difficult fluid services that include high temperature and high pressure. The sensor probes are often designed to be inserted into conduits without process shutdown.

One fundamental problem with the pitot tube is that velocity measurement is made at only one point in the flow stream rather than providing integrated volumetric flow measurement. The probes must be traversed across the pipes or the velocity profiles known in advance to calculate the average flow rate. For high accuracy and consistent results, the pitot should be preceded by 50 or more diameters of straight smooth pipe. If a sufficient straight run of pipe is installed ahead of the pitot tube, an approximate average velocity reading will be obtained at a location approximately 30% of the pipe radius from the pipe wall. The basic pitot tube configuration is shown below in Figure 5.


Figure 5 – Basic Pitot Tube Configuration

One port is placed in the flowing fluid, facing upstream, and is connected through internal tubing to one side of the secondary element. This port registers the total dynamic head in the stream since the velocity is zero at the sensing tip. The static pressure is obtained from a port which faces perpendicular to the flow and is usually located in the pipe wall.

Errors are introduced in pitot tube measurements because the total and static pressures are not measured at the same point in the flow profile. This problem is eliminated in pitot static tubes, shown below in Figure 6.


Figure 6 – Pitot Static Tube


A static pitot has dual coaxial tubes. One terminates in a port facing upstream to register the total dynamic pressure at a stagnation point and the other tube is located away from the tip and faces normal to the flow or downstream to measure the static pressure at approximately the same streamline.

Combined-reversed pitot static tubes circumvent many of the problems associated with flow profiles across the channels. One design is the multi-port or annular averaging element. This element senses dynamic pressure at multiple sensing ports distributed along the diameter to provide a single indication of the average flow through the channel without a transverse. Static pressure is measured by a tube terminating in a port which faces downstream at the centerline of the conduit.

The Annubar™ manufactured by Dieterich Standard Corporation is a specialized pitot tube of this type. The ProBar Flowmeter™ manufactured by Rosemount is also a specialized pitot tube with pressure and temperature compensation. Both these flow elements are very commonly used in low pressure applications.


F. Wedge Meters

The segmental wedge is a proprietary design flow meter similar to the segmental orifice plate but with the upstream and downstream surfaces of the measuring point at a 45 degree angle to the flow stream. The wedge meter is located at the top of the pipe or conduit allowing the bottom of the pipe to be unrestricted. Wedge meters are highly linear from Reynolds numbers as low as 500 (laminar flow) to Reynolds numbers in the millions (highly turbulent flows). The wedge meter is useful when measuring slurry flow.


Figure 7 – Segmental Wedge


Some advantages of segmental wedge flow elements are:

• permanent pressure loss is approximately 50% of a similar orifice
installation
• viscous fluids measure accurately over a wide range of Reynolds
numbers
• changing viscosities can be measured
• bi-directional flow


G. Elbow Meter

The elbow meter is simply a pipe elbow that is inserted into the pipe and a pressure differential is created by the centrifugal force between the inside diameter and the outside walls of the pipe. The only pressure losses are from the elbow itself.

One limitation of the elbow is that the pressure differential produced is very small. Elbow flow meters are usually used for balancing loads for multiple compressors or pumps in multi-unit unit service. They have also been used in the nuclear industry to detect the extremely high velocities associated with pipe breaks.


H. Variable Area Meters

Variable area meters, more commonly known as rotameters, are available as indicators, transmitters, recorders, and controllers in any combination.

Variable area meters measure a wide range in of flow rates and are suitable for most fluids including high viscosity liquids and low concentration slurries. They are frequently used in low flow services such as purges where the requirements are below the range of an orifice plate.

A rotameter consists of a plummet or float within a tapered vertical tube. The force of the flowing fluid causes the float to rise until the force is balanced by gravity. Rotameter calibration suffers if the variation in viscosity or density is greater than 15%.

Rotameters can be configured to either measure liquid or gas flow. Measuring the flow of liquids and gases is a critical need in many industrial plants. Important parameters to consider when specifying rotameters include liquid volumetric flow rate, gas volumetric flow rate, operating pressure, and fluid temperature.

• Liquid volumetric flow rate applies only to those rotameters that are liquid volumetric flow sensors or meters. It is expressed as the range of flow in volume/time.
• Gas volumetric flow rate applies only to those rotameters that are gas volumetric flow sensors or meters. It is the range of flow in volume/time.
• Operating pressure is the maximum head pressure of the process media the meter can withstand.
• The maximum temperature of the media that can be monitored is usually dependent on construction and liner materials. Pipe diameter is also important to consider, especially when specifying specific mounting options.


Mounting options for rotameters include insertion types, in-line flanged, in-line threaded, and in-line clamp.
• Insertion flow meters are inserted perpendicular to flow path. They usually require a threaded hole in the process pipe or other means of access.
• In-line flanged flow meters are inserted parallel to the flow path, usually inserted between two pieces of existing flanged process pipes.
• In-line threaded flow meters are inserted parallel to the flow path, and threaded into two existing process pipes.

Rotameters are often easy to install and offer a low cost solution to flow measurement.


I. Target Flowmeter

Target elements are impact devices that are coupled to restoring mechanisms such as springs or servomotors to maintain equilibrium. Target meters are located directly in the fluid flow. The deflection of the target and force bar is proportional to the square of the flow rate (according to Bernoulli’s equation).

Advantages of target flowmeters are:

• high accuracy
• long term repeatability
• long life
• low cost installation
• no pressure ports to plug
• dirty or clean fluids can be measured
• can be used for any type of liquid, gas, or steam cryogenics
• no moving parts such as bearings, to wear out causing failures
• high reliability where life tests have been made to 20,000,000
cycles
• can be used for any line size from 0.5 inches and up with any
type of mounting
• range/fluid changes accomplished by simply changing targets
• turndowns aprox.15:1
• can accept bi-directional flow where signal polarity indicates
direction

Limitations of target flowmeters are:
• accuracy depends on temperature (approx. 0.5% error/100C)
• much data is needed to determine the optimum size of the target
• calibration must be field verified.


5.2.2 Velocity 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. Velocity 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.

A. Magnetic Flowmeter

The electromagnetic flowmeter approaches the ideal flow measurement device for liquids because it has no restriction in the flow line, can accurately measure liquids almost impossible to handle with other meters, has a linear output that is directly proportional to flow, and has the ability to measure bi-directional flow.

The only limitation on liquid measurement is that it must meet a minimum standard of electrical conductivity.

The principle of operation for magnetic flowmeters is Michael Faraday’s Law of Electromagnetic Induction. This law states that a conductor, when moving across lines of force in a magnetic field, will induce a voltage within the conductor and the magnetic field. (For a more detailed description of this operating principle, refer to the ISA book Flow Measurement: Practical Guides for Measurement and Control, 2nd edition, D.W. Spitzer, editor).

The process fluid serves as the conductor in the flow tube. If the tube is metal it must have a lining (flourcarbon resin, polyurethane, neoprene, etc) that serves as an electrical insulator on the inside of the tube wall. A pair of electrodes, extending through the wall of the tube are flush with the inside surface of the lining. The tube end connections are usually flanged to simplify mounting in a pipeline.
Magmeter sizes range from 0.01 to 96 inches and can measure flows from 0.01 GPM to 500,000 GPM. Measurement accuracy is better than 1 percent of the flow rate.

Magmeters can measure flow rates of clean fluids, dirty fluids and slurries. The meter is sensitive to changes in density and viscosity. The meter can not be used with most hydrocarbons because of their low conductivity.

Magmeter sizing

To properly size a magmeter, the following formula should be used:

Where: = velocity in Feet/sec.
= nominal diameter of the flowmeter (inches)

The following table lists typical sizing guidelines. These guidelines are based on Rosemount Magnetic flow meters and may vary slightly by manufacturer.

Application Velocity Range (ft/s) Velocity Range (m/s)
Normal Service 2-20 0.6-6.1
Abrasive Slurries 3-10 0.9-3.1
Non-Abrasive Slurries 5-15 1.5-4.6



B. Turbine Flowmeter

A turbine flowmeter uses the moving fluid or gas to turn a turbine rotor (Figure 8). The rotational speed of the rotor varies with the flow rate. When a steady rotational speed is obtained, the speed is proportional to the fluid velocity. Flow quantity data is supplied via a precisely known number of pulses for a given volume for fluid displaced between two adjacent rotor blades. The relationship is linear within given limits for flow rate and fluid viscosity.

Figure 8 – Cut-Away of a Turbine Flowmeter

Advantages of turbine meters include:

• high accuracy and repeatability
• wide flow ranges
• many materials of construction available
• low pressure drop

Turbine flowmeters are primarily used for flow totalizing for inventory control and custody transfer, precision automatic batching for loading and batch mixing.

Turbine meters should be selected with 30% to 50% excess capacity above the maximum flow rate. Turbine meters operating below the maximum capacity provide greater reliability.


C. Vortex Flowmeter

Vortex flowmeters measure flow via a natural phenomenon known as vortex shedding (Figure 9).

Figure 9 – Principle of Vortex Shedding

When a fluid or gas flows past an obstruction, boundary layers of slow moving fluids or gases are formed along the outer surfaces. If the obstacle is streamlined, the flow cannot follow the obstacle contours on the downstream side. The separated layers become detached and roll themselves into vortices in the low pressure area behind the body. Vortices are shed from alternate sides. The frequency at which they are shed is directly proportional to velocity. The signal output from the flowmeter is generated from the action of the fluid itself, so it belongs in the “fluidic” class of flowmeters.

Advantages of vortex shedding meters are:

• high accuracy
• long term repeatability
• good rangeability
• measure liquid, gas, and two-phase flow
• calibration is independent of viscosity, density, pressure and
temperature and can be maintained for long periods of time

Limitations of vortex shedding meters are:

• not suitable for dirty or abrasive fluids
• not suitable for viscous liquids
• limited choice of materials of construction
• limited size range
• limited maximum pressure and temperature capability
• no measured flow below the low flow cut-off velocity

D. Fluidic Flowmeter (Coanda Effect)

Another type of fluidic flowmeter utilizes the Coanda effect. The Coanda effect is basically a hydraulic feed back circuit. A chamber is designed with two feedback channels on opposite sides that produce a continuous, self induced oscillation. The frequency of the oscillation relates to the fluid velocity.


Advantages of fluidic flowmeters are:

• low cost
• high accuracy
• insensitive to temperature changes

Fluidic flowmeters can be used to measure clean fluids only. They will not measure gas or slurry flows.