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E. Ultrasonic Flowmeter

The principle behind an ultrasonic meter is that the speed of a sound wave is decreased if directed against a flowing fluid and increased if directed with the flow. In its simplest form, one sound transducer, mounted in the pipe wall, generates a sound which is received by another transducer located a known distance away. Flow is mathematically inferred from the velocity measured.

1. Transit Time

Principle of Operation: Transit time decreases in the downstream
direction and increases in the upstream direction. Flow is proportional to

Where values of are the measured transit times.

Acceptable fluids must:
• support the passage of sound from the transmitting transducer to
the receiving
• be in a full pipeline
• be continuous, not pulsating flow
• contain no material to deposit on the wall

Advantages include:
• highest accuracy ultrasonic meter
• fast response time


Limitations:
• usually limited to single-phase fluids

2. Reflection or Doppler

Principle of Operation: Sound waves are reflected from moving particles
in the flow path. These reflections undergo frequency shifts proportional
to the Mach number (V/C where V is the fluid speed and C is the speed
of sound).

Acceptable fluids must:
• support the passage of sound
• contain sufficient scatterers or other disturbances to provide a
Doppler reflection but not contain so many scatterers that the sound cannot penetrate into the flow
• be in a full pipeline
• be continuous, not pulsating flow
• contain no material to deposit on the wall

Advantages include:
• suitable for liquids with entrained gases or undissolved solids
• easy to install
• low price

Limitations:
• not recommended for single-phase fluids
• accuracies between 3% & 5% when installed properly

3. Beam Drift

Principle of Operation: The ultrasonic transmitter utilizes 2 acoustic
beams separated by a short distance to send sonic signals downstream
with the fluid. When turbulent flow occurs, the movement of the eddy
causes a change to the acoustic signal which produces a unique
signature. The transmitter looks for the second identical acoustic signal.
When found, the difference in time is calculated to determine the
velocity.

Advantages include:
• suitable for highly turbulent flow

Limitations:
• will not work in laminar flow and some swirl flows


4. Surface Height In Open or Partially Filled Channels

Principle of Operation: The height of a liquid in an open weir or flume or
a partially filled duct is a function of the flow velocity. The height can be
found using ultrasonic air-or-liquid path time-of-flight measurements.

Advantages include:
• Widely used for water flow applications
• Accuracy about 3%

Limitations:
• Compensation usually needed for the acoustic velocity in the
beam path

5. Noise
Principle of Operation: Noise increases with flow rate.

Advantages include:
• Suitable for low cost flow switches, leak detectors, and boundary
layer acoustic monitors to detect the transition from laminar to
turbulent flow

Limitations:
• Not accurate for flow measurements
• Highly non-linear
• Sensitive to errors caused by ambient vibration or sound


5.2.3 Mass Flowmeters

The continuing need for more accurate flow measurements in mass-related
processes (chemical reactions, heat transfer, etc.) has resulted in the
development of mass flowmeters. Various designs are available, but the one
most commonly used for liquid flow applications is the Coriolis meter. Its
operation is based on the natural phenomenon called the Coriolis force,
hence the name.

A. Coriolis Mass Flowmeter

Coriolis meters come in many shapes and sizes, but all function basically the same way. Each coriolis meter consists of one or more flow tubes. As fluid enters a vibrating coriolis tube, the particles accelerate (due to the vibration) exerting a force on the inlet side of the tube. As fluid leaves the vibrating coriolis tube, the particles decelerate and exert a force in the opposite direction from the inlet. The resulting forces angularly deflect the tube(s) an amount that is inversely proportional to the mass flow rate within the tube. The angular deflection is optically measured. Coriolis mass flow measurement is not affected by changing process conditions.

The Coriolis meter is insensitive to operating conditions of viscosity, density, type of fluid, and slurries.

B. Thermal Mass Flowmeter

Thermal flowmeters can be divided into two types. The first type measures the current required to maintain a fixed temperature across a heated element. The greater the flow, the more current required to maintain a constant temperature. The current required is proportional to the mass flow rate.

The second type of thermal flowmeter is the “hot wire”. The hot wire method measures the temperature at two points on an element or “hot wire”. As the fluid or gas flows over the element, heat is dissipated. The upstream side of the hot wire will be hotter than the downstream side. The change in temperature is proportional to the mass flow.


5.2.4 Open Channel Flow

The "open channel" refers to any conduit in which liquid flows with a free surface.
Included are tunnels, non-pressurized sewers, partially filled pipes, canals,
streams, and rivers. Of the many techniques available for monitoring open-
channel flows, depth-related methods are the most common. These techniques
presume that the instantaneous flow rate may be determined from a
measurement of the water depth, or head. Weirs and flumes are the oldest and
most widely used primary devices for measuring open-channel flows.

A. Weirs

Weirs and flumes are distinguished as “rate meters” in that they are used in open pipe or channels that do not flow full. They fall into the general category of “head-area” meters and are used extensively in the measurement of irrigation water as well as the primary device for municipal and industrial wastewater applications.

Weirs are dam-like structures placed across the flowing stream. A notch of predetermined size and shape, cut out of the upper edge, creates a path for the flow. The sheet of liquid falling over the weir through the notch is called the nappe. When air has free access beneath the nappe, the flow is considered free; otherwise, it is considered submerged. The degree of submergence can significantly reduce the flow over the weir. Most weirs are sharp crested. Common weir profiles are V-Notch (triangular), Rectangular, or trapezoidal (Cipolletti).

The V-Notch Weir is especially recommended for metering flows less than 1 ft3 /s (cfs) equivalent to 0.65 million gal/day (mgd) and is suitable for measuring slowly changing flows up to 10 cfs. Extensive experiments have been made to determine the calibration data for v-notch weirs with 60 and 90 degree angles. Accuracy is limited to about 3%.

The Rectangular Weir is capable of high capacity metering and is simple and inexpensive to construct. Accuracy is limited to about 3%.

The Trapezoidal Weir is similar to the rectangular weir except for sloping sizes (1 horizontal to 4 vertical) of the notch. An advantage of this design is a simplified discharge formula that is more convenient to work with. Accuracy is limited to about 5%.

Weirs are capable of wide ranges. Triangular devices can operate over ranges of 50:1 and rectangular models offer ranges of 15:1. One shortcoming of weirs is that they introduce substantial head loss and may increase pumping cost. Cleanouts are needed in wastewater applications because debris and solids may become trapped at the upstream face of the weir.

B. Flumes

Flumes are primary elements that restrict the width of the channel. These are more expensive than weirs but have advantages of minimal head loss and self-cleaning design.

Venturi flumes are the forerunners of current practice. Parshall flumes are now the most common. A number of specialized configurations are also used. Leopold- lagco Lagco and palmerPalmer-bowless Bowlus designs are suited for partially filled circular pipe often used in sewer systems.


Figure 10 – Parshall Flume
Parshall Flumes consist of a converging upstream section, a downward sloping throat and an upward sloping, and a diverging downstream section. It is usually constructed of concrete, but may be constructed of wood. Stainless steel and fiberglass reinforced plastic liners have been used for metering corrosive solutions. Parshall flumes have been constructed in sizes with throat widths ranging from 3 in. to 40 ft. for measuring flows up to 1500 mgd.


FREE-FLOW CONDITION
Free flow discharge, the condition under which the rate of discharge is dependent only on the width of the throat and the depth of water at gage point Ha in the converging section, can occur at two different stages:

1. Where the liquid moves at high velocity in a thin sheet conforming
closely to the dip at the lower end of the throat
2. Where the back water raises the water surface to elevation Hb, causing
a ripple or standing wave to form at or just downstream from the end of
the throat.

Under the latter condition, the flume operates under partial submergence, but the free flow rate of discharge is not impeded as long as the ratio of Hb to Ha does not exceed the values given below.

SUBMERGED FLOW
When the ratio of Hb to Ha exceeds the values given below, the flume is operating under submerged flow and the rate of discharge is reduced. Operation of flumes under submerged flow conditions is not recommended since two gage points are required to determine the negative correction factor to apply to the free flow calibration data. There are no instruments available for direct and accurate measurement of submerged flow.

FLUME THROAT Hb / Ha
3 – 9 in. 0.6
1 – 8 ft. 0.7
10 –50 ft. 0.8


SECONDARY ELEMENTS
Float operated devices are widely used with cables or rigid rods to convert vertical float motion into rotation in the transducer.

Bubbler or purge systems are also popular as secondary elements in open channel flowmeters. A constant regulated flow of air or water is forced through a dip tube. The back pressure on the tube is monitored. This is proportional to the immersed depth of the tube, and therefore to the level. Signal conditioners are needed to obtain linear flow output.

Capacitance and other electronic probes are available for level measurement in open-channel flow applications. These are used almost exclusively for transmission and incorporate signal conditioning circuits for calibration and linearization.

Sonic or radar measurements are also being applied to level sensing for open-channel flow in non foaming applications. The transducers are mounted above the stream where they are not subject to fouling or solids accumulation.

Secondary elements, especially bubblers and ball type sensors, are often used with stilling wells to nullify effects of stream velocity. The wells also provide protection from accumulations of solids and plugging the bubbler tubes. Cleanout provisions or purging should be specified to prevent particles or debris from settling on the bottom and interfering with the measurement.


Figure 11 – Representative Secondary Elements

5.2.5 Positive Displacement Meters

Positive Displacement (PD) meters measure flow directly in quantity terms
instead of indirectly or inferentially. PD meters operate by trapping a known
volume of fluid, passing it from inlet and to outlet and counting the number of fluid
“packets” that pass. An output shaft drives through gearing to a local counter.
By selection of suitable gearing, readout in the required volumetric units can be
obtained. A pulse generator (either optical or electromagnetic) may be fitted for
transmission to a remote device. Because of production tolerances, meters must
be individually calibrated. Temperature compensation can be provided to
convert the measured flow to standard units.

PD meters are extremely accurate and repeatable if adequately maintained. PD
meters offer unequalled accuracy and flow range capabilities on high viscous
liquids. As liquid viscosity increases, the slippage and hence the error is
reduced. Measurement accuracies of 0.001 gallon per pulse are available.

PD meters are widely used for flow measurement of fuel oils and other chemicals and hydrocarbon products in small pipe sizes. The basic limitation of a PD meter is that it has moving parts with close tolerances and clearances. This limits its use to clean liquids and necessitates regular maintenance. High temperatures and pressures also can distort the output signal.

For large size PD meters, physical size and weight will require special mounting pads. Also a device to eliminate air and vapor from the liquid is required, since the meter will measure air along with the liquid. PD meters must not be left dry kept flooded at all times. If PD meters are left dry, the meter will be “hammered” every time the pump is turned on.