Flowmeters Course

7/30/2019 Flowmeters Course

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Flow Measurements

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Definitions and UnitsFlow rate corrections

Differential Pressure Flow Transmitters

Differential Pressure Methods

Orifice Plates

Venturi Tubes

Flow Nozzles

Pitot Tubes

Vortex Type Flow Elements

Target Flowmeter

Turbine Flowmeter

Positive Displacement Flowmeter

Ultrasonic Flowmeter

Coriolis Flowmeter

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Itis the art and science of:

1. applying instruments2. to sense a chemical or physical process

condition.

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Measurement of a given quantity is an act or

the result of comparison between the

quantity and a predefined standard.

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In order that the results are meaningful,

there are two basic requirements:

1. The standard used for comparison

purposes must be accurately defined andshould be commonly accepted.

2. The apparatus used and the methodadopted must be proved.

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The advancement of science and technology

is dependent upon a parallel progress in

measurement techniques.

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There are two major functions in all

branches of engineering:

1. Design of equipment and processes.

2. Proper operation and maintenance of

equipment and processes.

Both functions require measurements.

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Direct Method: The unknown quantity is directly

compared against a standard.

Indirect Method: Measurement by direct methods

are not always possible, feasible and practicable.

Indirect methods in most of the cases areinaccurate because of human factors.

They are also less sensitive.11


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In simple cases, an instrument consists of a

single unit which gives an output reading or

signal according to the unknown variable

applied to it.

In more complex situations, a measuring

instrument consists of several separateelements.

Instruments

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These elements may consist of:

Transducer elements which convert the

measurand to an analogous form.

The analogous signal is then processed by

some intermediate means and then fed to

The end devices to present the results forthe purposes of display and or control.

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These elements are:

A detector.

An intermediate transfer device.An indicator.

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The history of development of instruments

encompasses three phases:

Mechanical.

Electrical.

Electronic.

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Reaching corporate economic goals

Controlling a process

Maintaining safety

Providing product quality

Purpose of Process

Measurement

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No matter how advanced or sophisticated the

distributed control system,

the control system is only as effective as the

process measurement instruments it isconnected to;

therefore, successful process control is

dependent on successful instrument

application.

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To correctly apply instrumentation, an

engineer must clearly understand the

operations and limitations of the instrument,

as well as understanding the chemical and

physical properties of the process.

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Fundamental to applying process

instrumentation is interpreting the instruments

performance envelope.

Every field measurement device has its own

distinct envelope that constitutes the process andenvironmental conditions it can perform to.

Likewise, every application has a characteristic

envelope that represents the application's

process and environmental conditions.

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It is the science of measurement.

As a science, metrology uses terminology and

definitions that the process measurementengineer must be familiar with.

He must and have a clear understanding of,

because vendors may vary in the use of a term.

Metrology

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The instrument engineer must consider the

following dynamic conditions that affect

process measurement:

Temperature Effects

Static Pressure Effects Interference

Instrumentation Response

Noise Damping and Digital Filtering

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These dynamic conditions cause the presence ofuncertainty in measuring systems.

No measurement, however precise or repeated,

can ever completely eliminate this uncertainty.

The uncertainty of measuring systems is

exemplified in the effects temperature variationscan have on measurements.

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Temperature influences can exhibit some of the

most severe effects on a process measurement,

both in the process media itself and the

measurement instrument.

Temperature Effects

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Some obvious examples of severe temperatureinfluences include temperature-induced phase

transitions.

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It would be hard to determine what would happen to an

orifice plate, differential pressure measurement if theprocess suddenly changed from a liquid to a solid or gas.


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Other temperature induce dynamic changes

include:

Change in the dimensions of the measuring

element,

Modification of a resistance of a circuit, or

Temperature-induced change in the flux density

of a magnetic element.

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Similar to temperature effects, pressure changes canalso trigger phase transitions, especially in gas

applications.

Pressure effects seen in differential pressure (DP)

devices are an example.

Because the differential pressure devices are used in

flow and level applications, the importance of pressure

effects should not be underestimated.

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The goal is to minimize the total error that pressure

effects can cause.

To illustrate this, consider a differential pressure

instrument that is calibrated in a lab at zero static

pressure.

The transmitter is re-zeroed after installation by

opening an equalizing valve in the process under

pressure to eliminate zero shifts;

however, variations inline pressure are not accounted

for during normal operations.

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Interference, in process measurement terms, refers toeither external power or electrical potential that can

interfere with the reception of a desired signal or the

disturbance of a process measurement signal.

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The dynamic characteristic of instrumentation

response refers to how quickly a measuring instrument

reacts or responds to a measured variable.

An ideal, perfect instrument would have an

instantaneous response, which in effect, is called zero

lag.

In general, with modern electronic instrumentation,the response time is adequate for most applications.

Instrumentation Response

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Noise is often described as a signal that doesnot represent actual process measurement

information.

Noise can originate internally within the process

measuring system or externally from the process

condition.

It makes up part of the total signal from which

the desired signal must be read.32


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Damping is defined as the progressive reductionor suppression of oscillation in a device or system.

In more practical terms, damping describes theinstruments performance in the way a pointer or

indicator settles into a steady indication after a

change in the value of the measured quantity.

Damping and Digital Filtering

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A response is not damped at all, oscillation

continues.

A response is underdamped or periodic, as is the

case when overshoot occurs.

A response is overdamped or aperiodic, when the

response is slower than an ideal or desired

condition.

A response is critically damped, when the

response represents an ideal or desired condition.

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Range

It is defined as the region between the limits

within which a quantity is measured, received,or transmitted, expressed by stating the lower

and upper range values.

Measurement Terminology

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Upper Range Value (URV) is defined as the highest

quantity that an instrument is adjusted to measure.

Lower Range Value (LRV) is defined as the lowest

quantity that an instrument is adjusted to measure.

Upper Range Limit (URL) is defined as the maximumacceptable value that a device can be adjusted to

measure.

Lower Range Limit (LRL) is defined as the minimum

acceptable value that a device can be adjusted to

measure.

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Rangeability is the ratio of the maximum

measurable value to the minimum measurable

value.

Turndown is defined as the ratio of the normal

maximum measured variable through the

measuring device to the minimum controllablemeasured variable.

In a conventional differential pressure transmitter,if the maximum pressure is 7.45 kPa and the

minimum pressure is 1.24 kPa, the span turndown

is 6 to 1 (6:1).38


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These terms are often interchanged, confused and

misunderstood.


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Zero Elevation Range is defined as a range wherethe zero value of the measured variable is greater

than the lower range value.

The zero value can be between the lower range

value and the upper range value, at the upper range

value, or above the upper range value.

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Response Time is defined as the time taken for the

system output to rise from 0% to the first crossover

point of 100% of the final steady state value.

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Accuracy is sometimes referred to as the maximum

uncertainty or limit of uncertainty.

In practical terms, accuracy qualitatively represents the

freedom from mistake or error.

In metrological terms, accuracy represents the degreeof conformity of an indicated value to an accepted

standard value, or ideal value.

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Precision is confused with accuracy.

Precision, by definition, is the reproducibility withwhich repeated measurements of the same measured

variable can be made under identical conditions.

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Reproducibility is the same as precision.

The close agreement among repeatedmeasurements of the output for the same value

input that are made under the same operating

conditions over a period of time, approaching from

both directions.

If the measuring instrument is given the same

inputs on a number of occasions and the results lieclosely together, the instrument is said to be of high

precision.

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Repeatability

It is same as reproducibility except thatrepeatability represents the closeness of agreement

among a number of consecutive measurements of

the output for the same value of input under thesame operating conditions over a period of time

(approaching from the same direction).

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Linearity is the closeness to which a curve

approximates a straight line.

Independent LinearityTerminal Linearity

Zero-based Linearity

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Hysteresis

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Deadband

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Drift

It represents an undesired slow change or

amount of variation in the output signal over a

period of time (days, months, or years), with a

fixed reference input.

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Zero Drift represents drift with zero input signal.

In practical terms, the zero of the measuringinstrument shifts.

In a mechanical instrument, it is usually caused by a

slipping linkage. The correction is to re-zero the

instrument.

In an electronic instrument, zero shift is usually causedby environmental changes. The correction is to re-zero

the electronic instrument.

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Span Drift represents drift or gradual change in

calibration as the measurement moves up the scale

from zero.

In a mechanical instrument, it is usually caused by

changes in the spring constant of the instrument, or by

the linkage.

In a electronic instrument, span shift is usually caused

by changes in the characteristics of a component.

The correction can be to adjust the span of the display

element.

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Partial Drift represents drift on only a portion of the

instruments span.

In a mechanical instrument, it is usually caused by an

overstressed part of the measuring instrument.

In an electronic instrument, partial shift is usually

caused by drift in an electronic component.

The correction is periodic inspection and calibration.

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R li bili


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Reliability

It represents a measuring devicesability to perform a

measurement function without failure over a specified

period of time or amount of use.

Usually reliability data is extrapolated.

Reliability is often expressed as (MTTF) specification.

After failure, repair must take place.

MTTF + MTTR = MTBF

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Process measurement suppliers tend to follow severalrules when designing equipment to achieve reliability.

Keep the design simple,

Avoid using glass as a structural material,

Keep electronics cool as possible,

Provide easy serviceability.

Overview of Typical Design Criteria

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Housing

Metals

Gasket Considerations

Seal Considerations Associated Hardware Options

Process Connections Options

Installation Orientation Effects of Vibration

Environment and Hardware Materials

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Environment and Hardware Materials

Reliability Quality

Accuracy

Cost

Repeatability

Previous acceptance

Availability of spares

Compatibility with existing equipments

Flexibility of use

Compatibility with the environments Ease of maintenance

Ease of operation

Application suitability

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l l d d


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Electrical design and instrument

loop wiring considerations

Power Requirements

Power Consumption

Wiring Terminations Output Signal

RFI Effects

Grounding of Instruments Shielding Considerations

Lightning Protection

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SAFETY CONSIDERATIONS

limiting the energy level

keeping sparks away from flammable mixtures

containing an explosion

diluting the gas level protecting against excessive temperature

Probability that a hazardous gas is present

Quantity of a hazardous gas

Nature of the gas (is it heavier or lighter than air) The amount of ventilation

The consequences of an explosion

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Pressure values themselves are essential data

for monitoring.

Often, the values of process variables other

than pressure are derived from (inferred from)

the values that are measured for pressure.

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P i f M i R l i P


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Properties of Matter in Relation to Pressure

Measurement

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Pressure Equation

Pressure is defined as the amount of force per unitarea.

P =F/A

where:

P = pressureF = force =ma

A = area

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G Ab l t Diff ti l d V


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Gauge, Absolute, Differential, and Vacuum

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P M i D i


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Pressure Measuring Devices

Categories of pressure measuring devices :

Gravitational gauges

Deformation sensors and switches

Transducers and transmitters

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Gravitational Gauges


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Gravitational Gauges

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Deformation (Elastic) Sensors and Switches


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Deformation (Elastic) Sensors and Switches

Bourdon Tube

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Advantages

They are available in a wide variety of pressure ranges. They are proven and suitable for many pressure

applications.

They have good accuracy.

Disadvantages

Vibration and shock could be harmful to mechanicallinkage.

They are susceptible to hysteresis as they age.

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Bellows

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Diaphragm


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Pressure Transducer


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Pressure Transducer

It is a device that provides an electrical output signal that is

proportional to the applied process pressure.

The output signal is specified as either a volt, current, or

frequency output.

A pressure transducer always consists of two elements:

A force summing element, such as a diaphragm, converts

the unknown pressure into a measurable displacement or

force.

A sensor, such as a strain gauge, converts the displacement

or force into a usable, proportional output signal.

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Strain Gauge


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Strain Gauge

The sensor changes its electrical resistance when it

stretches or compresses.

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Potentiometer Element


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Potentiometer Element

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Capacitive Sensor


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Capacitive Sensor

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Performance Advantages


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They have good rangeability and response time.

They have very good accuracy.

Typical accuracies are about 0.1% of reading or 0.01 % of

full scale.

Typical transducers support a very wide pressure range.

High vacuum and low differential pressure ranges are

supported.

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Inductance-Type Transducer


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yp

Changing the spacing between two magnetic devices causes a

change in the reluctance.

The change in reluctance then represents the change in pressure.

One type of reluctance pressure transducer is the linear variable

differential transformer (LVDT).

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Piezoelectric Gauge


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Piezoelectric Gauge

Materials that create an electrical voltage when a force is

applied.

They measure rapidly changing pressures.

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They provide a self generated output signal.

They have high speed of response.

They have good accuracy, about 1% of full

scale is typical.

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Design of Pressure Transmitters


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Design of Pressure Transmitters

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Meter Body Designs


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Meter Body Designs

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Transmitter Process Locations


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Purpose of Flow Measurement


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Monitor and control the flow rates.

Develop material and energy balances.

Sustain the efficiency and to minimize

waste.

Purpose of Flow Measurement

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Importance of Accurate


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Material balances in separation processes.

Pumps and compressor operations.

Custody transfer operations.

Importance of Accurate

Measurement

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Flowmeter Definition


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A flowmeter is defined as A device that

measures the rate of flow or quantity of a

moving fluid in an open or closed conduit.

Flowmeter Definition

It usually consists of a primary device and asecondary device.

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Primary Device

It is defined as The device mounted internally

or externally to the fluid conduit that produces a

signal with a defined relationship to the fluid

flow in accordance with known physical laws

relating the interaction of the fluid to the

presence of the primary device.

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Secondary Device

It is defined as The device that responds to the

signal from the primary device and converts it to

a display or to an output signal that can betranslated relative to flow rate or quantity.

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Some Drawing Symbols


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Some Drawing Symbols

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General Categories


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Flow instrument categorization often varies.

1. Rate or quantity type.

2. Energy usage type.

General Categories

of Flow Instruments

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1a Rate meters


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They are the most common classification of

flowmeters.

Rate meters measure the process fluidsvelocity.

Because a pipes cross sectional area is known,

the velocity is then used to calculate the flow

rate.

1a. Rate meters

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A t t ith i f th fl t


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A rate meter can either infer the flow rate or

measure the velocity of the flowing fluid to

determine the flow rate.

In differential pressure flowmeter, the flow

rate is inferred from the measureddifferential pressure.

In turbine meter, the velocity of the fluidtimes the area is used to determine the flow

rate.

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Meters that directly measure mass can alsobe considered either as

a quantity meter or as

a mass flow rate meter.

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2. Energy Approach


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A. Extractive Energy

Flowmeters take energy from the fluid

flow.

An orifice plate is an example of an

extractive-type.

gy pp

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B. Additive Energy

Flowmeters introduce some energy into

the fluid flow.

A magnetic flowmeter is an example of an

additive type.

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Volumetric Flow Rate


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It represents the volume of fluid that passes ameasurement point over a period of time.

The calculation is based on the formula:

Q = A x v

where

Q = volumetric flow rate

A = cross-sectional area of the pipe

v = average flow velocity (flow rate)

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Volumetric Flow Rate

Mass Flow Rate


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It represents the amount of mass that passes a specific

point over a period of time.

The calculation is based on the formula:

W = Q x

whereW = mass flow rate

Q = volumetric flow rate

= density108

Mass Flow Rate

Units of Measure


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Meter Run


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It is defined as The upstream and downstream length

of pipe containing the orifice flanges and orifice plate or

orifice plate with or without quick change fittings.

No other pipe connections should be made within thenormal meter tube dimensions except for pressure taps

and thermowells.

The meter tube must create an acceptable flow pattern

(velocity profile) for the fluid when it reaches the orifice

plate.

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Meter Run


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Flow Straighteners (conditioners)


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They help to provide accurate measurement when a

distorted flow pattern is expected.

They are installed in the upstream section of meter tube.

They reduce the upstream meter tube length

requirement.

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Flow Straighteners (conditioners)


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Compressible versus


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Temperature and pressure changes cause the volume

of a fluid to change.

The change in volume is much more extreme in gasesthan in liquids.

For accurate gas flow measurements, the

compressibility factor is included in the measurement.

z =PV/nRT114

p

Incompressible Flow


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Viscosity


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Viscosity is frequently described as a fluids resistance


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Viscosity is frequently described as a fluid s resistance

to flow.

It have a dramatic effect on the accuracy of flow

measurement.

Resistance to flow occurs because of internal friction

between layers in the fluid.

Water, for example, having low viscosity has less

resistance to flow.

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When a fluid is in motion, layers of fluid are


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subject to tangential shearing forces, causing the

fluid to deform.

Fluids low viscosity does not become an

influential property of the fluid upon flow

measurement.

However, when measuring the flow rate of a

fluid with high viscosity, the viscosity doesbecome an influential property in flow

measurement.118

Viscosity is often expressed in terms of the


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Viscosity is often expressed in terms of the

following:

Dynamic viscosity

Kinematic viscosity

Viscosity index

Viscosity scales

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Dynamic Viscosity (Absolute Viscosity)


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It represents a fundamental viscosity measurement of a

fluid.

Density of fluid does not play a part in the viscosity

measurement.

Absolute viscosity is a ratio of applied shear stress to

resulting shear velocity.

The measurement units for dynamic (absolute) viscosity

are centipoise, Pascal-seconds, or lb/ft-second.

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One method to measure viscosity is to rotate a


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disk in the fluid at a particular rotational speed.

The rotational torque required to keep the disk

rotating divided by the speed of rotation and by

the disk contacting surface area is a measure of

absolute viscosity.

Another viscosity measurement that can be used

for liquids and gases is the falling sphereviscometer.

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Rotational and Falling Sphere Viscometers122

Kinematic Viscosity (n)


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It represents a ratio ofdynamic (absolute) viscosity to the

density of the fluid and is expressed in stokes (n = m / r).

In liquids, an increasing temperature usually results in

lowering the kinematic viscosity.

In gases, an increasing temperature increases the

kinematic viscosity.

The measurement units for kinematic viscosity are either

centistokes, meter2/second, or ft2/second.

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Th th d f d t i i ki ti i it


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The method for determining kinematic viscosity

involves measuring the time to drain a certain

volume of liquid by gravity out of a container

through a capillary tube or some type of restriction.

The time it takes to drain a liquid is directly relatedto viscosity.

The flow rate of fluids by gravity, which is the force

causing the flow, depends upon the density of the

fluids.

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Ostwald Capillary Viscometer

Viscosity Index


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It represents the change in viscosity with respectto temperature.

It is used with reference to petroleum products.

A high viscosity index number means that the

fluids viscosity does not change very much for a

given temperature, and vice versa.

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Viscosity Scales


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It represents viscosity measurements in time units.

Commonly used viscosity scales include the following:

oSaybolt Furol scales

oRedwood scalesoEngler scales

The three scales express kinematic viscosity in time

units rather than centistokes.

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For example, if the kinematic viscosity of a fluid at 122

F is 900 centistokes, on the Saybolt Furol scale the

equivalent viscosity is expressed as 424.5 seconds

(centistokes x 0.4717).

Flow engineering reference manuals often provide

conversion formulas between centistokes and the

respective viscosity scale.

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Basic Hydraulic Equations


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Bernoulli Equation


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P = Static Pressure (pounds force per sq. ft)

r = Density (rho) (pounds mass per cubic ft)

v = Velocity (feet per second)

g = Acceleration of Gravity (feet per second2)

Z = Elevation Head Above a Reference Datum (feet)

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Continuity Equation


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The Equation of Continuity states that the volumetric

flow rate can be calculated by multiplying the cross

sectional area of the pipe at a given point by the

average velocity at that point.

Q = A x v

where

Q = volume flow rate (cubic feet per minute)A = pipe cross-sectional area (square feet)

v = average fluid velocity (feet per minute)

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Reynolds Number


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It is a major distinctive quality of fluid flow

as

The ratio of Inertial Forces to Viscous

Forces.

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Laminar flow is defined by low Reynoldsnumbers with the largest flowing fluid

moving coherently without intermixing.

Turbulent flow is defined by high Reynolds

numbers with much mixing.

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Turbulent flow is best when high heat transfer is

wanted,


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while laminar flow is best when flowing fluid is to bedelivered through a pipe with low friction losses.

Flow is considered laminar when the Reynolds number

is below 2,000.

Turbulent flow occurs when the Reynolds number is

above 4,000.

Between these numbers, the flow characteristics have

not been defined.140


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Newtonian versus

N t i Fl id


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In Newtonian fluids, the resistance to deformation

when subjected to shear (consistency of fluid) is

constant if temperature and pressure are fixed.

Whereas in a non-Newtonian fluid, resistance to

deformation is dependent on shear stress even

though the pressure and temperature are fixed.

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non-Newtonian Fluids


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Rheograms


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It can be used to determine the characteristics of any fluid.

Rheograms evolved from the science of rheology, which

studies flow.

(Rheo, derived from the Greek language, means a

flowing.)

Rheograms are useful as an aid to interpret viscositymeasurements.

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Non-Newtonian Fluids

Fluids that do not show a constant ratio of shear stress to


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Fluids that do not show a constant ratio of shear stress to

shear rate are defined as non-Newtonian fluids.

Fluids exhibit different viscosity at different shear rates.

In non-Newtonian fluids, there is a nonlinear relation between

the magnitude of applied shear stress and the rate of angulardeformation.

Non-Newtonian fluids, which have different classifications,

tend to be liquid mixtures of suspended particles.

Thick hydrocarbon fluids are considered non-Newtonian

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FLOW MEASURING DEVICE

SELECTION CRITERIA


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SELECTION CRITERIA

Application fundamentals

Specifications

Safety considerations

Metallurgy Installation considerations

Maintenance and calibration

Compatibility with existing process instrumentation

Custody transfer concerns

Economic considerations

Technical direction150


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Application Fundamentals Flowchart151


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Flowmeter Applications


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Flowmeter Applications (Continued)


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Strainers are used to protect meters from debris in a

liquid stream


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liquid stream.

Strainers are not intended for filtering a liquid.

Strainers should be carefully selected to ensure that they

have a low pressure drop when used with high velocityflowmeters.

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Deaerators are air elimination devices that protect the

f i i l l f i


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meter from receiving a large slug of air.

The air elimination device separates that air from the

liquid through the use of special baffles.

In the case of some positive displacement meters, a largeslug of air can completely damage the meter.

In the case of a turbine meter, air may not cause

damage, but will cause errors in readings (registrations).

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Isolation Valves are typically provided at a meter inlet to

permit meter repairability without shutting down the


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permit meter repairability without shutting down the

process.

Block and Bleed Valves are used in meter runs to

provide a means for calibration. These valves divert the

flow to the meter prover loop.

Control Valves provide a means of controlling flowrate

and/or back pressure.

For example, flowrate control is necessary to prevent a

positive displacement meter from over-speeding.

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Typical Maintenance Concerns by Flowmeter Type


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Accuracy Reference


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Accuracy is measured in terms of maximum positive

and negative deviation observed in testing a device

under a specified condition and specified procedure.

The accuracy rating includes the total effect ofconformity, repeatability, dead-band, and hysteresis

errors.

An accuracy reference of simply 2% is incomplete.

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Percent of Rate Accuracy: It applies to meters such as turbine meters, DC

magnetic meters, vortex meters, and Coriolis meters.


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Percent of Full Scale Accuracy: It refers to the accuracy of primary meters such

as rotameters and AC magnetic meters.

Percent of Maximum Differential Pressure: It applies to differential pressure

flow transmitters.

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Totalization


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It represents the process of counting the amount of

fluid that has passed through a flowmeter.

Its purpose is to have periodic (daily or monthly)

readings of the material usage or production.

The totalization data is used for billings for material

usage or production.

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Multivariable Transmitters


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In measuring flow, temperature is required to

compensate for changes in density.


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A multivariable transmitter is essentially fourtransmitters in one package.

A multivariable transmitter measures differential

pressure, absolute pressure, and process temperature.

The multivariable transmitter also calculates the

compensated flow.

Traditionally, three separate transmitters and flow

calculation were required for this measurement.164

The multivariable transmitter incorporates

microprocessor based technology which provides the


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advantages of better readability and tighter

integration.

Additionally, the multivariable transmitter reduces

installation costs, spares inventories, andcommissioning times.

The transmitter has the flexibility to be used in

applications such as custody transfer, energy andmaterial balances, and advanced control and

optimization.

165


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166

Custody Transfer


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167


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168

Flow measurement for custody transfer, where

ownership of a product transfers is on occasion


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ownership of a product transfers, is on occasion

regarded as a separate flow measurement topic.

There are two types of custody transfer in flow

measurement:

1. Legal, which falls under weight and measure

requirements.

2. Contract, which is a mutual agreement between

seller and buyer.

169

In process control applications, the accuracy

i t b l t


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170

requirement may be several percent,

but for custody transfer operations the accuracy

requirement may be in tenths of a percent.

Reasons for metering hydrocarbons

Custody Transfer Concerns


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Reasons for metering hydrocarbons.

Classifications of custody transfer

measurements.

Meter provers required.

171

Reasons for Metering Hydrocarbons

In typical oil processing plants liquid hydrocarbons are


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In typical oil processing plants, liquid hydrocarbons are

metered at each custody transfer point and often at pointswhere custody does not change.

Several reasons for the metering are:

Corporate accounting requires data.

Billing is dependent upon accurate measurements.

Losses are detectable.

Business decisions are based on the measurement data. Assist negotiations, if necessary

Provide auditable, historical records.

172

Classification of Custody Transfer Measurements


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For a custody transfer measurement of a liquid

hydrocarbon, a contract requires a volumetric measurement

at standard conditions of temperature and pressure.

The techniques to do this are broadly categorized as static

and dynamic.

Static measurements are accomplished through automatic

tank gauging.

Dynamic measurements are accomplished through liquid

metering methods.

173


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174


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175


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176

Meter Provers Required


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Any flowmeters indication of a volumerepresents an unknown volume unless the

volume can be compared to a known

volume.

The known volumes are called

meter provers

177


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For a meter to be considered accurate, the

meter must be proved at the same

conditions of flowrate, temperature

pressure, and product viscosity.

178


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179

FLOW METER CALIBRATION:

IMPORTANCE AND TECHNIQUES


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Calibration is typically performed in a laboratorysetting at several different flow rates, and uses

conditions such as changing densities, pressure,

and temperatures.

Proving differs from calibration in that it is done

in the field, typically under a single set of

conditions.

180


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The calibration can be defined as the

comparison of a measuring instrument with

specified tolerance but an undetermined

accuracy, to a measurement standard with

known accuracy

181

The use of non-calibrated instruments creates

potentially incorrect measurement and erroneous


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p y

conclusions and decisions.

It is calibration that:

provides assurance and confidence in

measurement.

maintains product in specified ranges.

182

Calibration can be a simple dimensional check to

d t t t i bl


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detect measurement variables.

Before starting calibration, a decision must be

made for the following:

Which variables should be measured.

What accuracy must be maintained.

183

S l t f i t i ll t


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Some element of error exists in all measurements

no matter how carefully they are conducted.

The magnitude of the error can never be easily

determined by experiments;

the possible value of the error can be calculated.

184

Method of Calibrations

I l th fl t d i lib t d


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In general the flow measurement devices are calibrated

by three methods:

Wet calibration uses the actual fluid flow.

Dry calibration uses flow simulation by means of an

electronic or mechanical signal.

A measurement check of the physical dimensions anduse of empirical tables relating flow rate to these

dimensions is another form of calibration.

185

Wet Calibration

It t l fl id fl


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It uses actual fluid flow.

Generally it provides high accuracy for a flowmeter

and is used when accuracy is a prime concern.

Precision flowmeters are usually wet calibrated at

the time of manufacture.

Wet calibration for flowmeters is usually performed

with water, air, or hydrocarbon fuels.

186

Dry Calibration

I i f d fl i h h


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It is performed on a flowmeter without the

presence of a fluid medium.

The input signal is Hz, mV, or P.

It is much more uncertain than wet calibration.

The overall accuracy of the flow device isinferred because the flow transducer is bypassed.

187

The input signal for a dry calibration must


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p g y

be provided by a measurement standard.

The value of the output signal requires use

of other measurement standard.

Follow the manufacturers guideline and

procedures for dry calibration.

188

Provers

The proving operation verifies the meters


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The proving operation verifies the meter s

performance and assurance.

The necessity for proving depends on how accurate

the measurement must be for the product being

handled.

Prover is considered part of the metering stations

cost and is permanently installed at the facilities.

For low value products, portable provers are used.189

Methods of Meter Proving

Pipe provers are one of the most common


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Pipe provers are one of the most common

types of provers in industry today.

The process does not have to be shut down

when proving a meter.

Two types of pipe provers:

Unidirectional prover,Bidirectional prover.

190

Unidirectional Provers


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191

It displaces a known volume by means of a

displacer traveling in only one direction inside

the prover.

The displacers travel is detected by detector

switches within the prover.


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192

Bidirectional Provers


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It requires a displacer to travel in both directionsto complete one prover run.

After stabilizing pressure and temperature, the

displacer is put into the system.

It will slow down flow in the system for a time

until the displacer picks up speed.

193


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194

Small Volume Provers


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They can accommodate a wide range of flow

rates.

They are compact in size and have less volume

than conventional unidirectional and

bidirectional pipe provers.

The time to obtain a meter factor is significantlydecreased.

195

Master Meter Method


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It is used when a pipe prover is unavailable.

The master meter method uses a known reliable

meter configured in series with the meter to beproved.

The meter measurements are then compared.

196

Weight and Volume Methods


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Weight and Volume Methods

Static calibration

Dynamic calibration

197

Static Calibration

The flow is quickly started to begin the test held


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The flow is quickly started to begin the test, held

constant during the test, and then shut off at theend of the test.

The totalized flow reading from the flowmeters iscompared with the weight or volume collected and

the performance of the meter is calculated.

The static calibration system operates best withflowmeters that have low sensitivity to low flow

rates.198

Dynamic Calibration

The flow is kept at a constant rate before the


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The flow is kept at a constant rate before the

beginning of the test.

The flow reading from the flow meter and initial

weight or volume are read together to start the testand after the desired collection period to end the

test.

The dynamic calibration systems are limited by the

meters speed of the response.

199


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200


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201


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202

Basic Equations


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203


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204

As long as the fluid speed is sufficiently subsonic

(V < mach 0.3),

the incompressible Bernoulli's equationdescribes the flow reasonably well.


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205

It is recommended that location 1 be positioned

one pipe diameter upstream of the orifice, and

location 2 be positioned one-half pipe diameter


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p p p

downstream of the orifice.

206


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For flow moving from 1 to 2, the pressure at 1

will be higher than the pressure at 2;

the pressure difference as defined will be a

positive quantity.

207

From continuity, the velocities can be replaced

by cross-sectional areas of the flow and the

volumetric flowrate Q,


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,

208

Solving for the volumetric flowrate Q gives,


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209

For real flows (such as water or air), viscosity

and turbulence are present and act to convert

kinetic flow energy into heat.


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gy

To account for this effect, a discharge coefficient

Cd is introduced into the above equation to

marginally reduce the flowrate Q,

210

Since the actual flow profile at location 2

downstream of the orifice is quite complex,


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thereby making the effective value of A2uncertain, the following substitution introducing

a flow coefficient Cf is made,

where Ao is the area of the orifice.

211

As a result, the volumetric flowrate Q for real

flows is given by the equation,


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212

The flow coefficient Cf is found from experiments

and is tabulated in reference books;


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It ranges from 0.6 to 0.9 for most orifices.

Since it depends on the orifice and pipe diameters

(as well as the Reynolds Number), one will oftenfind Cf tabulated versus the ratio of orifice

diameter to inlet diameter, sometimes defined as,

213

Most Common P flowmeters

Orifice plates


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Venturi

Flow nozzles

pitot tube / annubar

Elbow or wedge meter

214


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215

Meter Tube Assembly Example

Orifice Plate

It is the main element within an orifice meter


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tube.

It is the simplest and most economical type of

all differential pressure flowmeters.

It is constructed as a thin, concentric, flat metal

plate.

The plate has an opening or orifice.216


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An orifice plate is installed perpendicular to the

fluid flow between the two flanges of a pipe.

As the fluid passes through the orifice, the

restriction causes an increase in fluid velocity and

a decrease in pressure.

217

The potential energy (static pressure) is


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The potential energy (static pressure) is

converted into kinetic energy (velocity).

As the fluid leaves the orifice, fluid velocity

decreases and pressure increases as kineticenergy is converted back into potential energy

(static pressure).

218


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Orifice plates always experience some energy

loss that is, a permanent pressure loss caused

by the friction in the plate.

219


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The Beta ratio is defined as the ratio of thediameter of orifice bore to internal pipe

diameter.

< 1

220

The most common holding system for an orifice


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plate is a pair of flanges, upstream anddownstream piping, and a pressure tap.

The pressure taps are located either on orifice

flanges or upstream and downstream of the pipe

from the orifice plate.

221

For precise measurement, various types of

fittings are used:


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junior fittings,

senior fittings, and

simplex fittings.

222

The fittings provide:


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easy installation of an orifice plate,

removal of the plate for changes in flow rate

services, and

convenient removal for inspection and

maintenance.

223

Senior Orifice Fitting

It is a dual-chamber device that reigns as the


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224

most widely used means of measurement fornatural gas.

Simplex Orifice Plate Holder

It is a single-chamber fittings that house and


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225

accurately position an orifice plate for differentialpressure measurement.

Junior Orifice Fitting

It is a single-chamber fitting, engineered and


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226

g g g

manufactured to make orifice plate changing

quick and easy at installations where line

movement from flange spreading is undesirable.


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227


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228


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Limitations of orifice plates include a high

irrecoverable pressure and a deterioration in

accuracy and long term repeatability because ofedge wear.

229

Two types of orifice plates designs areavailable:


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Paddle type and

Universal type.

230

The paddle type orifice plate

It is used with an orifice flange, has a handle for


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easy installation between flanges.

On the paddle type plate, the orifice bore,

pressure rating (flange rating), bore diameter,Beta ratio, and nominal line size are stamped on

the upstream face of the plate.

The outside diameter of a paddle plate varies

with the ANSI pressure rating of the flanges.231

The universal orifice plate

It is designed for use in quick change fittings.


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The universal plate is placed in a plate holder,

the outside diameter is the same for all pressure

ratings for any given size.

When using orifice fittings, the internal

diameter of the meter tube must be specified

because the orifice plate is held in an orifice

plate sealing unit.

232


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Weep Hole


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Some orifice plates have a small hole in theorifice plate besides an orifice bore either

above the center of the plate, or below the center of the plate.

234


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The area of the weep hole must be considered


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when sizing an orifice plate.

An orifice plate with a weep hole should not be

used when accurate measurement is required in a

flow measurement application, such as in gas

sales service.

236


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The orifice plate, although a relatively simple

element, is a precision measuring instrument and

should be treated accordingly.

238

Critical items considered when evaluating orifice

plates are the following:


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Flatness, smoothness, and cleanliness ofthe orifice plate.

The sharpness of the upstream orificeedge.

The bore diameter and thickness of the

orifice plate.

239

Orifice Plate Dimensions


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d represents the bore of the orifice plate.

D represents the pipe inside diameter.

Dam height represents the difference of pipe inner diameter and diameter of bore

divided by 2.

T represents the thickness of the plate.

e represents the orifice plate bore thickness which is 1/2 T

is called orifice plate bevel angle. It is 45 , +20 0.

240


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Types of Orifice Plates

242

Concentric Plates

The concentric orifice bore plates are used in


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general flow measurement applications.

The concentric orifice plate has an orifice bore

in the center of the plate.

The concentric bore plate is used for clean fluid

services, as well as for applications requiring

accurate flow measurement.

243

The center of bore is either


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beveled or

straight.

The beta ratio for the concentric plate is

between 0.1 to 0.75.

244

Eccentric Plates

It is similar to a concentric plate, but the


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eccentric plate has the bore in an offset position.

The eccentric orifice plate is used when dirty

fluids are measured, to avoid the tendency ofhole plugging if a concentric plate were used.

Flow coefficient data is limited for eccentric

orifices; therefore, it provides less accurate

measurement.245


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246

In an eccentric orifice plate, the hole is bored

tangent to the inside wall of the pipe or, more

l l


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commonly, tangent to a more concentric circlewith a diameter not smaller than 98% of the

pipes internal diameter.

When lacking specific process data for the

eccentric orifice plate, the concentric orifice plate

data may be applied as long as accuracy is not a

major issue.

247

Make sure that flanges or gaskets do not

interfere with the plate hole.

h l f


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The line size ranges from 4 to 14.

It can be made smaller than a 4 as long as the

orifice bore does not require a beveling edge.

Beta ratio is limited between 0.3 to 0.8.

Flange taps are recommended for eccentric

orifice plate installations.248

Segmental Plates

It looks like a segment of a circle with

d i l h l i ff f h l


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segmented circle hole in offset from the platescenter.

The orifice hole is bored tangent to the insidewall of the pipe or tangent to a more concentric

circle with a diameter not smaller than 98% of

the pipe internal diameter.

Installation is similar to eccentric type.249

Quadrant Edge Plate

I i d f l i R ld b


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It is used for lower pipe Reynolds numberswhere flow coefficients for sharp-edge orifice

plates are highly variable.

It is used for viscous clean liquid applications.

Nominal pipe size ranges between 1 to 6.

250

Orifice Plate Parameters

(1) Orifice flow rate.

(2) Pi li i d i


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(2) Pipe line size and pressure rating.(3) Thickness of orifice plate.

(4) Orifice Bore (d).

(5)Orifice plate holders: The orifice plate holder includes

orifice flanges, orifice fittings.(6) Beta Ratio.

(7) Differential Pressure (P).

(8) Temperature.

(9) Reynolds Number (Re).

(10) Pressure taps.

251

Pressure Taps


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252


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253

Flange Taps

Holes drilled into a pair of flanges.


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Flange tap holes are not recommended when

the pipe size is below 2 inches.

254

Pipe Taps

Pi t l t d t 2 5 D t d 8 D


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Pipe taps are located at 2.5 D upstream and 8 Ddownstream from the orifice plate.

Exact location of the taps is not critical.

However, the effect of pipe roughness and

dimensional inconsistencies can be severe.

255

The uncertainty of measurement is 50 % greater

ith f ll fl t th ith t l t th


256/382

with full flow taps than with taps close to the

orifice.

Pipe taps are not normally used unless it isrequired to install the orifice meter on a existing

pipe, or other taps cannot be used.

256

Corner Taps

Corner taps are a style of flange taps.


257/382

The only difference between corner and flange

taps is that the pressure is measured at the

corner between the orifice plate and the pipewall.

Corner taps are used when the pipe size is 2 or

less.

257

Vena Contracta Taps

When an orifice plate is inserted into the


258/382

flowline, it creates an increase in flow velocity

and a decrease in pressure.

The location of the vena contracta point isbetween 0.35 to 0.85 of pipe diameters

downstream of the plate, depending on the beta

ratio and Reynolds number.

258


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Pressure and Flow Profile

259

Vena contracta taps are located 1D upstream and at the

Vena contracta location downstream.

Vena contracta Taps are the optimum location for


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Vena contracta Taps are the optimum location formeasurement accuracy.

They are not used for pipes less than 6 in diameter.

260

Liquid Service

Tap Locations The pressure tap location in liquid

service orifice meters should be located to prevent


261/382

service orifice meters should be located to preventaccumulation of gas or vapor in the connection

between the pipe and the differential pressure

instrument.

The differential pressure instrument should be close

to the pressure taps or connected through downward

sloping connecting pipe of sufficient diameter toallow gas bubbles to flow back into the line.

261

Transmitter Installation The installation of

differential pressure transmitters should be

located below the pipe and sloping upwards

toward the pipe to prevent the collection of gas


262/382

toward the pipe to prevent the collection of gasbubbles in the impulse tubing.

Vent Holes are required for venting of any gas ina liquid service.

Location of the vent hole in a liquid service is atthe top of a pipe, above the center line.

262

Gas Services

Tap Locations Pressure tap locations in a gas

service must be installed in the top of the line


263/382

service must be installed in the top of the linewith upward sloping connections towards a pipe.

The differential pressure measuring instrumentmay be close-coupled to the pressure taps in the

side of the lines or connected through upward

sloping connecting pipe of sufficient diameter to

prevent liquid from accumulating in the line.

263



located above the pipe with the impulse tubing

sloping downward towards the pipe so that any


264/382

sloping downward towards the pipe so that anycondensate drains into the pipe.

Drain Holes A drain hole is required for drainingof any liquid in a gas service.

Location of the drain hole is below the center line

of the pipe.

264

Steam Services

Tap Locations require the use of condensing

chambers in steam or vapor applications because


265/382

chambers in steam or vapor applications becausecondensate can occur at ambient temperatures.

Generally, the pressure tap connection has adownward sloping connection from the side of

the pipe to the measuring device.

265



located above the pipe with the impulse tubing

sloping downward towards the pipe so that any


266/382

sloping downward towards the pipe so that anycondensate drains into the pipe.

Drain Holes A drain hole is required for drainingof any condensate liquid in a steam service.

The location of a drain hole is below the centerline of the pipe.

266

Standard Flow

Flow measurement of a fluid stated in volume units at base

(standard) conditions of P and T is called standard flow.


267/382

For crude petroleum and its liquid products, the vapor pressure is

atmospheric

pressure at base temperature, the base pressure is called

equilibrium vapor pressure.

The base condition for natural gases is defined as a pressure of

14.73 psia (101.56 kPa) at a temperature of 60 F (15.56C).

267

Compensated Flow

Compensated flow represents a flow under fluid conditions that

may vary.


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The conditions are measured and used along with flowmeter

signal to compute the true flow rate from the flowmeter.

The output signal from a flowmeter represents the true flow rate

value under specified fluid conditions.

For a liquid service, variations in density or viscosity can change

the meters accuracy.

For gas services, a change in temperature, pressure, and

molecular weight can ruin the accuracy of the meter.

268


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269

Computer Programs for Sizing Orifice Plates

ORICALC-2,

EA-25


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EA 25,

ORSPEC,

FLOWEL,

INSTRUCALC,

ORIFICE2, and

FLOW CONSTANT270

http://www.pipeflowcalculations.com/orifice/


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271

Common primary element errors:

Beta ratio is too large for the meter run

Orifice plate is not flat, it is concave or convex

Orifice does not have sharp edges

Orifice plate is installed backwards


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Orifice plate is installed backwards Orifice plate is damaged through poor handling

An incorrect size is used for the orifice meter tube or plate

Orifice plate is not centered in the line

Orifice meter tube is corroded

Tap locations are incorrect Contaminants build up on orifice plate

Contaminants build up on meter run

Hydrates build up on meter run and orifice plate

Flow conditioners are dislodged and move closer to plate

Leaks occur around orifice plate Pressure tap or thermowell installed upstream of meter

Welding meter supports distorts meter run

272

Common secondary element errors:

Gauge lines are too small

Gauge lines are too long

Gauge lines leak


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Gauge lines leak

Gauge lines have sags or loops that collect condensates

Gauge line slopes are not correct

Incorrect ranges are used on secondary instruments

Differential pressure transmitter was not zeroed properly Excessive dampening is used in secondary instrument

273

Other Differential Pressure Flowmeters


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274

Flow Nozzles

The flow nozzle is another type of differential-

producing device that follows Bernoullis


275/382

producing device that follows Bernoulli stheorem

The permanent pressure loss produced by theflow-nozzle device is approximately the same as

the permanent pressure loss produced by the

orifice plates.

275

The flow nozzle can handle dirty and abrasive

fluids better than can an orifice plate


276/382

fluids better than can an orifice plate.

In a flow nozzle with the same line size, flow

rate, and beta ratio as an orifice meter, thedifferential pressure is lower, and the permanent

pressure loss is less.

276

Performance and Applications

Changing a flow nozzle is more difficult than


277/382

Changing a flow nozzle is more difficult thanchanging an orifice plate when there is a change

in flow rate requirements.

Flow nozzles are used for steam, high velocity,

nonviscous, erosive fluids, fluids with some

solids, wet gases, and similar materials.

277

The flow nozzles pass 60% more flow than the

orifice plate of the same diameter and


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orifice plate of the same diameter and

differential pressure.

A flow nozzles inaccuracy of 1% of rate isstandard with 0.25% of rate flow calibrated.

278


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279

Typical Nozzle Installations

Venturi Meter

A venturi design can be described as a restriction

with a long passage with smooth entry and exit


280/382

with a long passage with smooth entry and exit.

Venturi tubes produce less permanent pressure

loss and more pressure recovery than the other

meters.

It is one of the more expensive head meters.

Low pressure drops for non-viscous fluids.280


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281


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282

Venturi Designs

Performance Advantages:

The long form venturi develops up to 89%f 0 75 b t ti d


283/382

The long form venturi develops up to 89%pressure recovery for a 0.75 beta ratio and

decreases to 86% recovery for a 0.25 beta ratio.

The short form venturi develops up to 85%

recovery at 0.75 beta ratio and decreases to 7 %

at 0.25 beta ratio.

283

A venturi meter has a low permanent pressure

loss and high recovery at higher beta ratios.


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A venturi meter can be used for dirty fluids and

slurries.

Higher accuracy (better than orifice).

284

Performance Disadvantages:

A venturi meter is a very expensive measuring


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A venturi meter is a very expensive measuring

device to use.

A venturi meter has limited rangeability and isonly installed when flow rates rangeability is less

than 3 to 1.

285

Pitot Tubes

The previously discussed primary differential

pressure flow metering devices utilized the


286/382

pressure flow metering devices utilized thedifference in static pressure perpendicular to the

direction of flow as a basis for inferring velocity.

The actual velocity was not measured, but was

calculated after many experimental laboratory

measurements and correlations.

286

The Pitot tube measures a fluid velocity by

converting the kinetic energy of the flow into

potential energy.


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The conversion takes place at the stagnation

point, located at the Pitot tube entrance.

287


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288

A pressure higher than the free-stream (i.e.

dynamic) pressure results from the kinematic to

potential conversion.


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This "static" pressure is measured by comparing

it to the flow's dynamic pressure with a

differential manometer

289


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290


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291

Performance Advantages:

It creates very little permanent pressure drop and, as a

result, is less expensive to operate.


292/382

A pitot tube can be installed on 4 and more.

Performance of the pitot tube is historically proven.

A pitot tubes installation and operation costs are low.

A pitot tube can be a standard differential producingdevice for all pipe sizes.

292

Performance Disadvantages:

Point-type pitot tubes require traversing the

flow stream for average velocity.


293/382

g y

Poor rangeability.

Nonlinear square root characteristic.

Difficulty of use in dirty flow streams.

293

Annubars

The sensing points are arrayed along perpendicular

diameters with the number of points in each traverse

based upon the duct size.


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p

294


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295 Annubar Design

Performance

The diamond shape annubar has long term

accuracy.


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y

The annubar has an accuracy of 1% of actual

flow and 0.1 repeatability of the actual value.

The annubar has low installation costs; a system

shutdown is not required to install the device.

296

The annubar produces a repeatable signal evenwhen the run requirements are not met.

The annubar flow sensor can handle a widerange of flow conditions with two measuring


297/382

range of flow conditions with two measuring

instruments.

The annubar should not be used if the viscosity

approaches 50 centipoise.

The annubar can be used on two phase flowmeasurements.

297

Applications

The annubar can be used for liquid and gas flow

measurement services.


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Generally, the annubar is used in clean liquid

services to avoid plugging.

The annubar can be installed for low and

medium pressure applications without shutting

down the system.

298

Wedge Type Flowmeter

The basic system consists of a cylindrical pressure

vessel into which a constriction "wedge" is

fabricated thereby leaving a open segment of a


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299

y g p g

known height.

Pressure taps which receive the sensors on either sideof the "wedge" provide the differential signal to the

Flow Transmitter which is then related, by formula, to

the rate of flow occurring through the open segment.


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300

Elbow Type Flowmeter

A differential pressure

exists when a flowing fluidchanges direction due to a


301/382

301

changes direction due to a

pipe turn.

The pressure difference

results from the centrifugal

force.

Since pipe elbows exist inplants, the cost for these

meters is very low.

However the accuracy is


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302

However, the accuracy is

very poor.

They are only applied

when reproducibility is

sufficient and other flow

measurements would bevery costly.


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303


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304


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305


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306


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307


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308

Flowing fluid forces the turbine wheels to rotate

at a speed proportional to the velocity of the

Turbine Meters


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p p p y

fluid.

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For each revolution of the turbine wheel, a

pulse is generated.

The rotational speed of shaft and frequency ofthe pulse corresponds to the volumetric flow


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the pulse corresponds to the volumetric flow

rate through the meter.

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K-factor

It is the number of pulses per unit of measurement

generated by the rotor as it turns inside the turbine.

It is usually indicted as Pulses per Gallon


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It is usually indicted as Pulses per Gallon


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312


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313Turbine Meter


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Insertion Type Turbine Meter


Excellent accuracy and good rangeability over the

full linear range of a meter.


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Low flow rate designs are available.

Some versions do not require electrical power.

Overall meter cost is not high.

Output signal from the meter is at a high

resolution rate, which helps reduce meter proving.315

Performance Disadvantages

Sensitive to a fluids increasing viscosity.

T h fl id t bl


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Two phase fluids can create usage problems.

Straight upstream piping or straighteningvanes are required in a turbine meter installation

to eliminate the flow turbulence into the meter.

316

ElectromagneticFlowmeters


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317

Faradays Law states that emf is created when a

conductive fluid moves through a magnetic

field.


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318

The axis of the conductive fluid flows at a right angle tothe magnetic field. Fluid flowing in this manner causes

a voltage that is proportional to the flow rate.


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319 Magnetic Flowmeter Principles

The voltage developed at the electrodes has an

extremely low level signal.

A signal conditioner must amplify the signal.


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There are two types of magnetic flowmeters:

AC excitation, and

DC pulse excitation.

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AC Excitation

In an AC type magnetic flowmeter, line voltage (120 or 240

V AC) is applied directly to the magnetic coils.

This generates a magnetic field in the outer body that varies


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with the frequency of the applied voltage.

An AC meters signal has a sine wave pattern.

The magnitude of the sine wave is directly proportional to

the flow velocity.

The system produces an accurate, reliable, fast responding

meter.321

DC Pulse Excitation

In a DC type magnetic flowmeter, line voltage is the

main source of power, but instead of applying it directlyto the coils, it is first applied to a magnet driver circuit.


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The magnet driver circuit sends low frequency pulses to

the coils to generate a magnetic field.

The DC pulse system eliminates the zero shift problem

that occurs in an AC system.

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323


It is non-obstructive and has no moving parts.

Pressure drop is very little.

DC pulse type power can be as low as 15 to 20 watts


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DC pulse-type power can be as low as 15 to 20 watts.

Suitable for acid, bases, water, and aqueous solutions.

Lining materials provide good electric insulation and

corrosion resistance.

The magnetic meter can handle extremely low flow.

It can be used for bidirectional flow measurements..

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Performance Disadvantages

The meters only measure conductive fluid flows.

(Hydrocarbons, gases, and pure substances cannot be measured)


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( y , g , p )

Proper electrical installation care is required.

Conventional meters are heavy and larger in size.

Meters are expensive.

325

Installation

Proper magnetic flow meter operation is very

dependent upon the installation.


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Installation considerations for a magnetic flowmeter

primarily involve the following:

Meter orientation

Minimum piping requirement

Grounding

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Magnetic Flowmeter Installation Practices

Applications

It is suited for measurement of slurries and dirty fluids

because magnetic flowmeters do not have sensors that

enter the flowing stream of fluids.


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Magnetic flowmeters are not affected by viscosity or the

consistency of Newtonian or non-Newtonian fluids.

The resulting change in flow profile caused by a change

in Reynolds number or upstream configuration piping

does no

Flowmeters Course

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