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Fundamentals of Flow Meter 1.0 Overview

Flow measurement is essential in many industries such as the oil, power, chemical, food, water, and waste treatment industries. These industries require the determination of the quantity of a fluid, either gas, liquid, or steam, that passes through a check point, either a closed conduit or an open channel, in their daily processing or operating. The quantity to be determined may be volume flow rate, mass flow rate, flow velocity, or other quantities related to the previous three.

The instrument to conduct flow measurement is called flowmeter. The development of a flowmeter involves a wide variety of disciplines including the flow sensors, the sensor and fluid interactions through the use of computation techniques, the transducers and their associated signal processing units, and the assessment of the overall system under ideal, disturbed, harsh, or potentially explosive conditions in both the laboratory and the field.

1.1 Categorization

Flowmeter By Technology Employed

Technology Instrument Measure Result1. Coriolis Miscellaneous Acceleration Mass 2. Differential Pressure Elbow Pressure Pressure Volume3. Flow Nozzle Pressure Pressure Volume4. Orifice Pressure Pressure Volume5. Pitot Tube Pressure Pressure Volume6. Pitot Tube (Averaging) Pressure Pressure Volume7. Venturi Pressure Pressure Volume8. Wedge Pressure Pressure Volume9. Magnetic Electronic Electromagnetic Field Velocity

10. Positive Displacement Nutating Disc Mechanical Volume Volume11. Oscillating Piston Mechanical Volume Volume12. Oval Gear Mechanical Volume Volume13. Roots Mechanical Volume Volume14. Target Mechanical Force Velocity15. Thermal Miscellaneous Heat Transfer Velocity16. Turbine Mechanical Volume Volume17. Ultrasonic Doppler Electronic Acoustic Waves Velocity18. Transit Time Electronic Acoustic Waves Velocity19. Variable Area Movable Vane Pressure Pressure Volume20. Rotameter Pressure Pressure Volume21. Weir, Flume Pressure Pressure Volume22. Vortex Mechanical Frequency Velocity

1.2 Installation

Flowmeters need to be integrated into existing/planning piping system to be useful. There are two types of flowmeter installation methods: inline and insertion. The inline models include connectors to the upstream and downstream pipes while the insertion models insert the sensor probe into the pipes.

Most flowmeters need to be installed at a point where the pipes on both sides remain straight for a certain distance. For inline models, the inner diameter of the pipes have to be the same as the flowmeter's line size. Between the flowmeter and the pipes, there are two types of mostly used connecting methods: flanged and wafer.

Among different types of connection methods, insertion design is more flexible and more economical in larger line sizes while inline design is more confined and usually easier to calibrate. The wafer connection is usually less expansive than flanged connection. However, it may require extra parts to allow the threading with pipes at both ends.

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2.0 Coriolis flowmeters

Coriolis flowmeters are relatively new compared to other flowmeters. They were not seen in industrial applications until 1980's. Coriolis meters are available in a number of different designs. A popular configuration consists of one or two U-shaped, horseshoe-shaped, or tennis-racket-shaped (generalized U-shaped) flow tube with inlet on one side and outlet on the other enclosed in a sensor housing connected to an electronics unit.

The flow is guided into the U-shaped tube. When an osillating excitation force is applied to the tube causing it to vibrate, the fluid flowing through the tube will induce a rotation or twist to the tube because of the Coriolis acceleration acting in opposite directions on either side of the applied force. For example, when the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube. On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist. When the tube is moving downward during the second half of the vibration cycle, it twists in the opposite direction. This twist results in a phase difference (time lag) between the inlet side and the outlet side and this phase difference is directly affected by the mass passing through the tube.

A more rescent single straight tube design is available to measure some dirty and/or abrasive liquids that may clog the older U-shaped design.

An advantage of Coriolis flowmeters is that it measures the mass flow rate directly which eliminates the need to compensate for changing temperature, viscosity, and pressure conditions. Please also note that the vibration of Coriolis flowmeters has very samll amplitude, usually less than 2.5 mm (0.1 in), and the frequency is near the natural frequency of the device, usually around 80 Hz. Finally, the vibration is commonly introduced by electric coils and measured by megnetic sensors.

Pros and Cons • Pros:

- Higher accuracy than most flowmeters

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- Can be used in a wide range of liquid flow conditions

- Capable of measuring hot (e.g., molten sulphur, liquid toffee) and cold (e.g., cryogenic helium, liquid nitrogen) fluid flow

- Low pressure drop

- Suitable for bi-directional flow

• Cons:

- High initial set up cost

- Clogging may occur and difficult to clean

- Larger in over-all size compared to other flowmeters

- Limited line size availability 3.0 Differential pressure flowmeters

Differential pressure flowmeters (in most cases) employ the Bernoulli equation that describes the relationship between pressure and velocity of a flow. These devices guide the flow into a section with different cross section areas (different pipe diameters) that causes variations in flow velocity and pressure. By measuring the changes in pressure, the flow velocity can then be calculated.

Many types of differential pressure flowmeters are used in the industry:

• Orifice Plate: A flat plate with an opening is inserted into the pipe and placed perpendicular to the flow stream. As the flowing fluid passes through the orifice plate, the restricted cross section area causes an increase in velocity and decrease in pressure. The pressure difference before and after the oriffice plate is used to calculate the flow velocity. A calculator for the orifice plate flowmeters can be found in the fluid mechanics section.

• Venturi Tube: A section of tube forms a relatively long passage with smooth entry and exit. A Venturi tube is connected to the existing pipe, first narrowing down in diameter then openning up back to the original pipe diameter. The changes in cross section area cause changes in velocity and pressure of the flow. A calculator for the venturi tube flowmeters can be found in the fluid mechanics section.

• Nozzle: A nozzle with a smooth guided entry and a sharp exit is placed in the pipe to change the flow field and create a pressure drop that is used to calculate the flow velocity.

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• Segmental Wedge: A wedge-shaped segment is inserted perpendicularly into one side of the pipe while the other side remains unrestricted. The change in cross section area of the flow path creates pressure drops used to calculate flow velocities.

• V-Cone: A cone shaped obstructing element that serves as the cross section modifier is placed at the center of the pipe for calculating flow velocities by measuring the pressure differential.

• Pitot Tube: A probe with an open tip (Pitot tube) is inserted into the flow field. The tip is the stationary (zero velocity) point of the flow. Its pressure, compared to the static pressure, is used to calculate the flow velocity. Pitot tubes can measure flow velocity at the point of measurement. See Pitot Tube Flowmeters and Pitot Static Tubes for more details.

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• Averaging Pitot Tube: Similar to Pitot tubes but with multiple openings, averaging Pitot tubes take the flow profile into consideration to provide better over all accuracy in pipe flows.

• Elbow: When a liquid flows through an elbow, the centrifugal forces cause a pressure difference between the outer and inner sides of the elbow. This difference in pressure is used to calcuate the flow velocity. The pressure difference generated by an elbow flowmeter is smaller than that by other pressure differential flowmeters, but the upside is an elbow flowmeter has less obstruction to the flow.

• Dall Tube: A combination of Venturi tube and orifice plate, it features the same tapering intake portion of a venturi tube but has a 'shoulder' similar to the orifice plate's exit part to create a sharp pressure drop. It is usually used in applications with larger flow rates.

Differential pressure flowmeters, although simple in construction and widely used in industry, have a common drawback: They always create a certain amount of pressure drop, which may or may not be tolerated in a particular application.

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Pros and Cons • Pros:

- Low to medium initial set up cost

- Can be used in wide ranges of fluid phases and flow conditions

- Simple and sturdy structures

• Cons:

- Medium to high pressure drop 4.0 Magnetic flowmeters

Magnetic flowmeters, also known as electromagnetic flowmeters or induction flowmeters, obtain the flow velocity by measuring the changes of induced voltage of the conductive fluid passing across a controlled magnetic field.

A typical magnetic flowmeter places electric coils around (inline model) / near (insertion model) the pipe of the flow to be measured and sets up a pair of electrodes across the pipe wall (inline model) or at the tip of the flowmeter (insertion model). If the targeted fluid is electrically conductive, i.e., a conductor, its passing through the pipe is equivalent to a conductor cutting across the magnetic field. This induces changes in voltage reading between the electrodes. The higher the flow speed, the higher the voltage.

The operation principle of inline magnetic flowmeters

The operation principle of insertion magnetic flowmeters

Pros and Cons

• Pros:

- Minimum obstruction in the flow path yields minimum pressure drop

- Low maintenance cost because of no moving parts

- High linearity

- Two and multi beam models have higher accuracy than other comparably priced flowmeters

- Can be used in hazardous environments or measure corrosive or slurry fluid flow

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• Cons:

- Requires electrical conductivity of fluid higher than 3 µS/cm in most cases

- Zero drifting at no/low flow (may be avoided by low flow cut-off; new designs improve on this issue)

5.0 Positive displacement flowmeters

Positive displacement flowmeters, also know as PD meters, measure volumes of fluid flowing through by counting repeatedly the filling and discharging of known fixed volumes. A typical positive displacement flowmeter comprises a chamber that obstructs the flow. Inside the chamber, a rotating/reciprocating mechanical unit is placed to create fixed-volume discrete parcels from the passing fluid. Hence, the volume of the fluid that passes the chamber can be obtained by counting the number of passing parcels or equivalently the number rounds of the rotating/reciprocating mechanical device. The volume flow rate can be calculated from the revolution rate of the mechanical device.

Many types of positive displacement flowmeters are used in the industry. They are named after the mechanical device inside the chamber. They all share the same principle of operation and are volumetric flow measuring instruments. Common positive displacement flowmeters are:

• Nutating Disc:

• Rotating Valve:

• Oscillating Piston:

• Oval Gear:

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• Roots (Rotating Lobe):

• Birotor:

• Rotating Impeller:

The accuracy of positive displacement flowmeters relies on the integrity of the capillary seal that separates incoming fluid into discrete parcels. To achieve the designed accuracy and ensure that the positiive displacement flowmeter functions properly, a filtration system is required to remove particles larger the 100 µm as well as gas (bubbles) from the liquid flow.

Positive displacement flowmeters, although simple in principle of operation and widely used in the industry, all cause a considerable pressure drop which has to be considered for any potential application.

Pros and Cons • Pros:

- Low to medium initial set up cost

- Can be used in viscous liquid flow

• Cons:

- Higher maintenance cost than other non-obstructive flowmeters

- High pressure drop due to its total obstruction on the flow path

- Not suitable for low flow rate

- Very low tolerance to suspension in flow (particles larger than 100 µm need to be filtered before the liquid enters the flowmeter)

- Gas (bubbles) in liquid could significantly decrease the accuracy

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6.0 Target flowmeters

Target flowmeters, also known as drag force flowmeters, insert a target (drag element), usually a flat disc or a sphere with an extension rod, into the flow field. They then measure the drag force on the inserted target and convert it to the flow velocity.

One major advantage of the target flowmeter over other flowmeters is, with a sphere drag element, a proper strain gage layout, and well thought-out mathematical formulas, a target flowmeter is capable of measuring sporadic and multi-directional flows.

Pros and Cons • Pros:

- Low initial set up cost

- Can be used in abrasive, contaminated, or corrosive fluid flow

- Can be made to measure flow velocity that is sporadic or multidirectional with sphere drag element designs

• Cons:

- Pressure drop is inevitable due to the rod and the drag element

- Less popular than it was before 7.0 Thermal flowmeters

Thermal flowmeters measure the heat carried away from the sensor by the passing flow to determine the mass flow rate.

Pros and Cons

• Pros:

- Medium initial set up cost

- Low pressure drop

• Cons:

- Fragile, high repair cost

- For (clean) gas only

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8.0 Turbine flowmeters

Turbine flowmeters, like windmills, utilize their angular velocity (rotation speed) to indicate the flow velocity. A good turbine flowmeter requires well designed and placed aerodynamic/hydrodynamic blades that are suitable for the fluid and flow condition and bearings that are both smooth and durable to survive the sustained high-speed rotation of the turbine.

Pros and Cons

• Pros:

- Medium initial set up cost

- Reliable, time tested proven technology

• Cons:

- For clean fluid only

- Low to medium pressure drop 9.0 Ultrasonic flowmeters

Ultrasonic flowmeters measure the traveling times (transit time models) or the frequency shifts(Doppler models) of ultrasonic waves in a pre-configured acoustic field that the flow is passing through to determine the flow velocity.

Ultrasonic flowmeters can be categorized into two types based on the installation method: clamped-on and inline.

The clamped-on type is located outside of the pipe and there are no wetted parts. It can easily be installed on existing piping systems without worrying about corrosion problems. Clamped-on designs also increase the portablility of the flowmeter.

The inline type, on the other hand, requires fitting flanges or wafers for installation. However, it usually offers better accuracy and its calibration procedures are more straightforward.

Doppler Flowmeters. Doppler flowmeters are named for the Austrian physicist and mathematician Christian Johann Doppler (18031853), who in 1842 predicted that the frequencies of received sound waves depended on the motion of the source or observer relative to the propagating medium. To use the Doppler effect to measure flow in a pipe, one transducer transmits an ultrasonic beam of ~0.5 MHz into the flow stream (see Figure 2). Liquid flowing through the pipe must contain sonically reflective materials such as solid particles or entrained air bubbles. The movement of these materials alters the frequency of the beam reflected onto a second, receiving transducer. The frequency shift is linearly proportional to the rate of flow of materials in the pipe and therefore can be used to develop an analog

or digital signal proportional to flow rate.

Figure 2. Doppler ultrasonic flowmeters operate on the Doppler effect, whereby the transmitted frequency is altered linearly by being reflected from particles and bubbles in the fluid. The net result is a frequency shift between transmitter and receiver frequencies that can be directly related to the flow rate.

Transit-Time Flowmeters. Transit-time meters, as the name implies, measure the difference in travel time between pulses transmitted in the direction of, and against, the flow. This type of meter is also called time of flight and time of travel.

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Figure 3. Transit-time flowmeters measure the difference in travel time between pulses transmitted in a single path along and against the flow. Two transducers are used, one upstream of the other. Each acts as both a transmitter and receiver for the ultrasonic beam.

Pros and Cons

• Pros:

- No obstruction in the flow path, no pressure drop

- No moving parts, low maintenance cost

- Multi-path models have higher accuracy for wider ranges of Reynolds number

- Can be used to measure corrosive or slurry fluid flow

• Cons:

- Higher initial set up cost 10.0 Variable area flowmeter

A Variable area flowmeter's cross section area available to the flow varies with the flow rate. Under a (nearly) constant pressure drop, the higher the volume flow rate, the higher the flow path area.

Variable area flowmeters can further be categorized into three types

• Rotameter: The most widely used variable area flowmeter.

• Movable Vane Meter:

• Weir, Flume:

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Variable area flowmeters are robust, low cost, simple, however less accurate, compared to other more complicated flowmeters.

Pros and Cons • Pros:

- Very low initial set up cost

- Simple, robust

- Low, nearly constant, pressure drop

• Cons:

- Moderate accuracy at best

- Not suitable for low flow rate

- Some variable area flowmeters can not be used in non/low gravity environments

- Rotameters must be mounted vertically 11.0 Vortex flowmeters

Vortex flowmeters, also know as vortex shedding flowmeters or oscillatory flowmeters, measure the vibrations of the downstream vortexes caused by the barrier placed in a moving stream. The vibrating frequency of vortex shedding can then be related to the velocity of flow.

Pros and Cons

• Pros:

- Low to medium initial set up cost

- Not much maintenance needed when used in clean flow conditions

• Cons:

- Low to medium pressure drop due to the obstruction in the flow path