Flow Measurements

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Definitions and Units Flow 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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It is the art and science of: 1. applying instruments 2. 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 and

should be commonly accepted.

2. The apparatus used and the method

adopted 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 are

inaccurate because of human factors.

They are also less sensitive. 11

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 separate

elements.

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 for

the 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 is connected to; •therefore, successful process control is dependent on successful instrument application. 17

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 instrument’s performance envelope. •Every field measurement device has its own distinct envelope that constitutes the process and environmental 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 measurement engineer 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 of uncertainty 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 variations can 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 temperature influences 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 the process suddenly changed from a liquid to a solid or gas.

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 can also 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 to either 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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•Engineers should concern themselves with response time performance.

•Although fast speed of response is an attribute of high quality instrumentation,

•some applications with rapidly changing processes would not benefit from fast responding devices and could even result in instrument damage.

•Depending on the application, some measurement lag is placed on the measuring device.

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•Noise is often described as a signal that does not 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

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

•In more practical terms, damping describes the instrument’s 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 maximum acceptable 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 controllable measured 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 where the 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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Zero Suppression Range is defined as a range where the zero value of the measured variable is less than the lower range value. In that case, the zero value does not appear on the range scale.

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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 degree of 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 with which 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 repeated measurements 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 lie closely together, the instrument is said to be of high precision.

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Repeatability It is same as reproducibility except that repeatability represents the closeness of agreement among a number of consecutive measurements of the output for the same value of input under the same 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 Linearity Terminal 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 measuring instrument 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 caused by 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 instrument’s 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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Reliability •It represents a measuring device’s ability 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 several rules 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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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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Properties of Matter in Relation to Pressure Measurement

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Pressure Equation •Pressure is defined as the amount of force per unit area.

P =F/A where: P = pressure F = force =ma A = area

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Gauge, Absolute, Differential, and Vacuum

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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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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 mechanical linkage. • They are susceptible to hysteresis as they age.

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Bellows

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Diaphragm

Other Types of Deformation Sensors 77

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 •The sensor changes its electrical resistance when it stretches or compresses.

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Potentiometer Element

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Capacitive Sensor

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Performance Advantages • 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 •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 •Materials that create an electrical voltage when a force is applied.

•They measure rapidly changing pressures.

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Performance Advantages • 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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Meter Body Designs

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Transmitter Process Locations

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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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• Material balances in separation processes. • Pumps and compressor operations. • Custody transfer operations.

Importance of Accurate Measurement

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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 a secondary 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 be translated relative to flow rate or quantity. ”

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Some Drawing Symbols

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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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•They are the most common classification of flowmeters.

•Rate meters measure the process fluid’s velocity.

•Because a pipe’s cross sectional area is known, the velocity is then used to calculate the flow rate.

1a. Rate meters

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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 measured differential pressure. •In turbine meter, the velocity of the fluid times the area is used to determine the flow rate.

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•They divide the flowing material into predetermined volume segments.

•Quantity meters count and keep track of the number of these volume segments.

•An example of a quantity meter is a positive displacement meter.

1b. Quantity meters

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Meters that directly measure mass can also be considered either as a quantity meter or as a mass flow rate meter.

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A. Extractive Energy

•Flowmeters take energy from the fluid flow.

•An orifice plate is an example of an extractive-type.

2. Energy Approach

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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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•It represents the volume of fluid that passes a measurement 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

•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 where W = mass flow rate Q = volumetric flow rate = density

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Mass Flow Rate

Units of Measure

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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 the normal 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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•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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•Temperature and pressure changes cause the volume of a fluid to change.

•The change in volume is much more extreme in gases than in liquids.

•For accurate gas flow measurements, the compressibility factor is included in the measurement.

z =PV/nRT 114

Compressible versus Incompressible Flow

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Viscosity

•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 subject to tangential shearing forces, causing the fluid to deform.

•Fluid’s 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 does become an influential property in flow measurement.

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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) •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 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 sphere viscometer.

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Rotational and Falling Sphere Viscometers 122

Kinematic Viscosity (n) •It represents a ratio of dynamic (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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•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 related to 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 •It represents the change in viscosity with respect to temperature.

•It is used with reference to petroleum products.

•A high viscosity index number means that the fluid’s viscosity does not change very much for a given temperature, and vice versa.

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Viscosity Scales •It represents viscosity measurements in time units.

•Commonly used viscosity scales include the following: oSaybolt Furol scales oRedwood scales oEngler 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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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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Bernoulli 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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Continuity Equation

It is a major distinctive quality of fluid flow as

The ratio of Inertial Forces to Viscous Forces.

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Reynolds Number

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•Laminar flow is defined by low Reynolds numbers 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,

•while laminar flow is best when flowing fluid is to be delivered 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.

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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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Newtonian versus non-Newtonian Fluids

Hagen-Poiseuille Law •It defines viscosity in more practical terms.

•Newton’s definition of viscosity is the ratio of shear stress divided by shear rate.

•Hagen-Poiseuille defines it as the ratio of shear stress divided by shear rate at the wall of a capillary tube.

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Rheograms •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 viscosity measurements.

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Newtonian Fluids •It exhibits the constant ratio of shear stress to shear rate (flow velocity) when subjected to shear and continuous deformation.

•When a fluid’s temperature is fixed, the fluid exhibits the same viscosity through changing shear rates. Viscosity is not affected by shear rate (flow velocity).

•The relationship is linear between the shear stress (force) and velocity (resulting flow).

•Newtonian fluids are generally homogeneous fluids. Gasoline, kerosene, mineral oil, water and salt solutions in water are examples of Newtonian fluids.

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Non-Newtonian Fluids •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 angular deformation.

•Non-Newtonian fluids, which have different classifications, tend to be liquid mixtures of suspended particles.

•Thick hydrocarbon fluids are considered non-Newtonian fluids. 147

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FLOW MEASURING DEVICE SELECTION CRITERIA

• Application fundamentals • Specifications • Safety considerations • Metallurgy • Installation considerations • Maintenance and calibration • Compatibility with existing process instrumentation • Custody transfer concerns • Economic considerations • Technical direction

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Application Fundamentals Flowchart 151

Application Fundamentals Checklist of Selection Criteria

• Flow stream conditions: – volume – temperature – pressure – density – viscosity – flow velocity

• Flow measurement goals. • Accuracy requirements. • Range requirements. • Acceptable pressure drops. • Display and system requirements. • Potential problems (i.e., vibration). • Flow stream erosive/corrosive materials, entrained gases and solids (if any). • Available installation space and pipe geometry. • Economic factors (cost of ownership).

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Flowmeter Applications

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Flowmeter Applications (Continued)

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Providing Protection to the Flowmeter

Strainers

De-aerators

Isolation Valves

•Strainers are used to protect meters from debris in a 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 velocity flowmeters.

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•Deaerators are air elimination devices that protect the 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 large slug 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 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 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 of conformity, repeatability, dead-band, and hysteresis errors.

•An accuracy reference of simply “2%” is incomplete.

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Accuracy Reference

Percent of Rate Accuracy: It applies to meters such as turbine meters, DC magnetic meters, vortex meters, and Coriolis meters. 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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•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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Totalization

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Multivariable Transmitters

•In measuring flow, temperature is required to compensate for changes in density.

•A multivariable transmitter is essentially four transmitters 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 advantages of better readability and tighter integration.

•Additionally, the multivariable transmitter reduces installation costs, spares inventories, and commissioning times.

•The transmitter has the flexibility to be used in applications such as custody transfer, energy and material balances, and advanced control and optimization. 165

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Custody Transfer

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•Flow measurement for custody transfer, where 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.

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In process control applications, the accuracy requirement may be several percent, but for custody transfer operations the accuracy requirement may be in tenths of a percent.

• Reasons for metering hydrocarbons. • Classifications of custody transfer measurements. • Meter provers required.

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Custody Transfer Concerns

Reasons for Metering Hydrocarbons In typical oil processing plants, liquid hydrocarbons are metered at each custody transfer point and often at points where 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.

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Classification of Custody Transfer Measurements •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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Meter Provers Required •Any flowmeter’s indication of a volume represents an unknown volume unless the volume can be compared to a known volume.

The known volumes are called “meter provers”

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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.

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•Calibration is typically performed in a laboratory setting 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.

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FLOW METER CALIBRATION: IMPORTANCE AND TECHNIQUES

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

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•The use of non-calibrated instruments creates potentially incorrect measurement and erroneous conclusions and decisions.

•It is calibration that: provides assurance and confidence in measurement.

maintains product in specified ranges.

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•Calibration can be a simple dimensional check to detect measurement variables.

•Before starting calibration, a decision must be made for the following: Which variables should be measured.

What accuracy must be maintained.

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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.

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Method of Calibrations 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 and use of empirical tables relating flow rate to these dimensions is another form of calibration.

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Wet Calibration •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.

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Dry Calibration •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 is inferred because the flow transducer is bypassed.

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•The input signal for a dry calibration must be provided by a measurement standard.

•The value of the output signal requires use of other measurement standard.

•Follow the manufacturer’s guideline and procedures for dry calibration.

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Provers •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 station’s 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 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.

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Unidirectional Provers •It displaces a known volume by means of a displacer traveling in only one direction inside the prover.

•The displacer’s travel is detected by detector switches within the prover.

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Bidirectional Provers •It requires a displacer to travel in both directions to 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.

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Small Volume Provers •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 significantly decreased.

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Master Meter Method •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 be proved.

•The meter measurements are then compared.

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Weight and Volume Methods •Static calibration •Dynamic calibration

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Static Calibration •The flow is quickly started to begin the test, held constant during the test, and then shut off at the end of the test.

•The totalized flow reading from the flowmeters is compared with the weight or volume collected and the performance of the meter is calculated.

•The static calibration system operates best with flowmeters that have low sensitivity to low flow rates. 198

Dynamic Calibration •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 test and after the desired collection period to end the test.

•The dynamic calibration systems are limited by the meter’s speed of the response.

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Basic Equations

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As long as the fluid speed is sufficiently subsonic (V < mach 0.3), the incompressible Bernoulli's equation describes the flow reasonably well.

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•It is recommended that location 1 be positioned one pipe diameter upstream of the orifice, and location 2 be positioned one-half pipe diameter downstream of the orifice.

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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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•Solving for the volumetric flowrate Q gives,

209

•For real flows (such as water or air), viscosity and turbulence are present and act to convert kinetic flow energy into heat.

•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,

•thereby making the effective value of A2 uncertain, 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,

212

•The flow coefficient Cf is found from experiments and is tabulated in reference books;

•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 often find Cf tabulated versus the ratio of orifice diameter to inlet diameter, sometimes defined as,

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Most Common P flowmeters • Orifice plates • Venturi • Flow nozzles • pitot tube / annubar • Elbow or wedge meter 214

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Meter Tube Assembly Example

Orifice Plate

•It is the main element within an orifice meter 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

•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.

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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 kinetic energy is converted back into potential energy (static pressure).

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Orifice plates always experience some energy loss – that is, a permanent pressure loss caused by the friction in the plate.

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

< 1

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•The most common holding system for an orifice plate is a pair of flanges, upstream and downstream 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.

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•For precise measurement, various types of fittings are used: junior fittings,

senior fittings, and

simplex fittings.

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The fittings provide: •easy installation of an orifice plate,

•removal of the plate for changes in flow rate services, and

•convenient removal for inspection and maintenance.

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Senior Orifice Fitting •It is a dual-chamber device that reigns as the most widely used means of measurement for natural gas.

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Simplex Orifice Plate Holder It is a single-chamber fittings that house and accurately position an orifice plate for differential pressure measurement.

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Junior Orifice Fitting •It is a single-chamber fitting, engineered and manufactured to make orifice plate changing quick and easy at installations where line movement from flange spreading is undesirable.

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Limitations of orifice plates include a high irrecoverable pressure and a deterioration in accuracy and long term repeatability because of edge wear.

229

•Two types of orifice plates designs are available:

•Paddle type and

•Universal type.

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The paddle type orifice plate

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

•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

•Some orifice plates have a small hole in the orifice plate besides an orifice bore either above the center of the plate, or below the center of the plate.

234

The purpose of the weep hole is to allow the: passage of any condensate in a gas application

or passage of gas in liquid service applications.

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•The area of the weep hole must be considered 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.

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Concentric Orifice Plate

The orifice plate, although a relatively simple element, is a precision measuring instrument and should be treated accordingly.

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•Critical items considered when evaluating orifice plates are the following: • Flatness, smoothness, and cleanliness of the orifice plate.

• The sharpness of the upstream orifice edge.

• The bore diameter and thickness of the orifice plate.

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Orifice Plate Dimensions

• 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°.

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•Several types of orifice bore designs are available for orifice plates: Concentric,

Segmental, and

Eccentric orifice plates.

•The plates are used for a wide range of applications.

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Types of Orifice Plates

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Concentric Plates •The concentric orifice bore plates are used in 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 beveled or

straight.

•The beta ratio for the concentric plate is between 0.1 to 0.75.

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Eccentric Plates •It is similar to a concentric plate, but the eccentric plate has the bore in an offset position.

•The eccentric orifice plate is used when dirty fluids are measured, to avoid the tendency of hole 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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•In an eccentric orifice plate, the hole is bored tangent to the inside wall of the pipe or, more commonly, tangent to a more concentric circle with a diameter not smaller than 98% of the pipe’s 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.

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• Make sure that flanges or gaskets do not interfere with the plate hole.

•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.

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Segmental Plates •It looks like a segment of a circle with segmented circle hole in offset from the plate’s center.

•The orifice hole is bored tangent to the inside wall 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 •It is used for lower pipe Reynolds numbers where 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”.

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Orifice Plate Parameters (1) Orifice flow rate. (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.

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Pressure Taps

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Flange Taps •Holes drilled into a pair of flanges.

•Flange tap holes are not recommended when the pipe size is below 2 inches.

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Pipe Taps •Pipe taps are located at 2.5 D upstream and 8 D downstream from the orifice plate.

• Exact location of the taps is not critical.

•However, the effect of pipe roughness and dimensional inconsistencies can be severe.

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The uncertainty of measurement is 50 % greater with full flow taps than with taps close to the orifice. Pipe taps are not normally used unless it is required to install the orifice meter on a existing pipe, or other taps cannot be used.

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Corner Taps •Corner taps are a style of flange taps.

•The only difference between corner and flange taps is that the pressure is measured at the corner between the orifice plate and the pipe wall.

•Corner taps are used when the pipe size is 2“ or less.

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Vena Contracta Taps •When an orifice plate is inserted into the flowline, it creates an increase in flow velocity and a decrease in pressure. • The location of the vena contracta point is between 0.35 to 0.85 of pipe diameters downstream of the plate, depending on the beta ratio and Reynolds number.

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Pressure and Flow Profile

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•Vena contracta taps are located 1D upstream and at the Vena contracta location downstream.

•Vena contracta Taps are the optimum location for measurement accuracy.

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

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Liquid Service Tap Locations – The pressure tap location in liquid service orifice meters should be located to prevent accumulation 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 to allow gas bubbles to flow back into the line.

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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 bubbles in the impulse tubing. Vent Holes are required for venting of any gas in a liquid service. Location of the vent hole in a liquid service is at the top of a pipe, above the center line.

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Gas Services Tap Locations – Pressure tap locations in a gas service must be installed in the top of the line with upward sloping connections towards a pipe. The differential pressure measuring instrument may 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

Transmitter Installation – The installation of differential pressure transmitters should be located above the pipe with the impulse tubing sloping downward towards the pipe so that any condensate drains into the pipe. Drain Holes – A drain hole is required for draining of any liquid in a gas service. Location of the drain hole is below the center line of the pipe.

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Steam Services Tap Locations require the use of condensing chambers in steam or vapor applications because condensate can occur at ambient temperatures. Generally, the pressure tap connection has a downward sloping connection from the side of the pipe to the measuring device.

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Transmitter Installation – The installation of differential pressure transmitters should be located above the pipe with the impulse tubing sloping downward towards the pipe so that any condensate drains into the pipe. Drain Holes – A drain hole is required for draining of any condensate liquid in a steam service. The location of a drain hole is below the center line of the pipe.

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Standard Flow •Flow measurement of a fluid stated in volume units at base (standard) conditions of P and T is called standard flow.

•For crude petroleum and its liquid products, the vapor pressure is <= than atmospheric pressure at base temperature of 14.696 psia (101.325 kPa) at a temperature of 60°F (15.56°C).

•For a hydrocarbon liquid, when vapor pressure > 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 (15.56°C).

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Compensated Flow •Compensated flow represents a flow under fluid conditions that may vary.

•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 meter’s accuracy.

•For gas services, a change in temperature, pressure, and molecular weight can ruin the accuracy of the meter.

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269

Computer Programs for Sizing Orifice Plates •ORICALC-2,

•EA-25,

•ORSPEC,

•FLOWEL,

•INSTRUCALC,

•ORIFICE2, and

•FLOW CONSTANT 270

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

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 • 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

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Common secondary element errors: • Gauge lines are too small • Gauge lines are too long • 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

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Other Differential Pressure Flowmeters

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Flow Nozzles •The flow nozzle is another type of differential-producing device that follows Bernoulli’s theorem

• The permanent pressure loss produced by the flow-nozzle device is approximately the same as the permanent pressure loss produced by the orifice plates.

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•The flow nozzle can handle dirty and abrasive 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, the differential pressure is lower, and the permanent pressure loss is less.

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Performance and Applications •Changing a flow nozzle is more difficult than changing 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.

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•The flow nozzles pass 60% more flow than the orifice plate of the same diameter and differential pressure.

•A flow nozzle’s inaccuracy of ± 1% of rate is standard with ± 0.25% of rate flow calibrated.

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Typical Nozzle Installations

Venturi Meter

•A venturi design can be described as a restriction 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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Venturi Designs

Performance Advantages: • 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.

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• A venturi meter has a low permanent pressure loss and high recovery at higher beta ratios. • A venturi meter can be used for dirty fluids and slurries. • Higher accuracy (better than orifice).

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Performance Disadvantages: • A venturi meter is a very expensive measuring device to use. • A venturi meter has limited rangeability and is only installed when flow rate’s rangeability is less than 3 to 1.

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Pitot Tubes •The previously discussed primary differential pressure flow metering devices utilized the difference 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.

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•The Pitot tube measures a fluid velocity by converting the kinetic energy of the flow into potential energy.

•The conversion takes place at the stagnation point, located at the Pitot tube entrance.

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•A pressure higher than the free-stream (i.e. dynamic) pressure results from the kinematic to potential conversion.

•This "static" pressure is measured by comparing it to the flow's dynamic pressure with a differential manometer

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Performance Advantages: • It creates very little permanent pressure drop and, as a result, is less expensive to operate. • A pitot tube can be installed on 4” and more. • Performance of the pitot tube is historically proven. • A pitot tube’s installation and operation costs are low. • A pitot tube can be a standard differential producing device for all pipe sizes.

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Performance Disadvantages: • Point-type pitot tubes require traversing the flow stream for average velocity. • Poor rangeability. • Nonlinear square root characteristic. • Difficulty of use in dirty flow streams.

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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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295 Annubar Design

Performance • The diamond shape annubar has long term accuracy. • 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.

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• The annubar produces a repeatable signal even when the run requirements are not met. • The annubar flow sensor can handle a wide 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 flow measurements.

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Applications •The annubar can be used for liquid and gas flow measurement services.

•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.

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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 known height.

300

•Pressure taps which receive the sensors on either side of 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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Elbow Type Flowmeter •A differential pressure exists when a flowing fluid changes direction due to a pipe turn.

•The pressure difference results from the centrifugal force.

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•Since pipe elbows exist in plants, the cost for these meters is very low.

•However, the accuracy is very poor.

•They are only applied when reproducibility is sufficient and other flow measurements would be very costly.

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Flowing fluid forces the turbine wheels to rotate at a speed proportional to the velocity of the fluid.

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Turbine Meters

•For each revolution of the turbine wheel, a pulse is generated.

•The rotational speed of shaft and frequency of 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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313 Turbine Meter

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Insertion Type Turbine Meter

Performance Advantages • Excellent accuracy and good rangeability over the full linear range of a meter. • 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. • Two phase fluids can create usage problems. • Straight upstream piping or straightening vanes are required in a turbine meter installation to eliminate the flow turbulence into the meter.

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Electromagnetic Flowmeters

Faraday’s Law states that emf is created when a conductive fluid moves through a magnetic field.

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319 Magnetic Flowmeter Principles

The axis of the conductive fluid flows at a right angle to the magnetic field. Fluid flowing in this manner causes a voltage that is proportional to the flow rate.

•The voltage developed at the electrodes has an extremely low level signal.

•A signal conditioner must amplify the signal.

•There are two types of magnetic flowmeters: AC excitation, and

DC pulse excitation.

320

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 with the frequency of the applied voltage.

•An AC meter’s 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.

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DC Pulse Excitation •In a DC type magnetic flowmeter, line voltage is the main source of power, but instead of applying it directly to the coils, it is first applied to a magnet driver circuit.

•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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Performance Advantages

• 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.

• 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. . 324

Performance Disadvantages • The meters only measure conductive fluid flows. (Hydrocarbons, gases, and pure substances cannot be measured)

• 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. 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.

•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 not change the meter’s performance or accuracy.

328

•The mass of the fluid is measured as opposed to the fluid volume or flow rate.

•A changing density or viscosity can affect the performance of a volumetric flowmeter,

•While a mass flowmeter would not be affected by these changes.

329

Mass Flowmeters (Coriolis Flowmeters)

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•Coriolis meters can be used on liquid and some gas applications. •The direct measurement of mass is necessary for applications where chemicals are balanced, combustion efficiencies are calculated, or production quantities must be consistent.

•If a measurement volume is desired, density corrections are required to measure the fluid at base conditions.

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•A Coriolis force is caused by flowing fluid through a tube. The Coriolis force equation is equivalent to Newton’s Second Law of Motion, where

334

•In Coriolis flowmeters, fluid typically flows through an U-shaped tube that vibrates at its natural frequency.

•As the fluid flows into the U-shaped tube, the fluid is forced to conform to the vertical momentum of the vibrating tube.

•If the U-shaped tube is moving upward during its vibration, the fluid flowing into the U-shaped tube resists and pushes downward.

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•The fluid has an upward momentum as it approaches the part of the tube where it exit.

•If that portion of the tube has a downward motion, the fluid resists the downward motion by pushing up on the tube.

•The U-shape tube then twists. The twisting is called the Coriolis effect.

•The amount of U-shaped tube twisting becomes directly proportional to the mass flow rate.

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•The detector senses the amount of tube twisting.

•The U-shaped tube can be vibrated by an oscillating driver at its natural frequency.

•Electromagnetic devices, such as velocity detectors, can be located on each side of the tube and be used to measure the velocity of the vibrating tube.

339

•When no fluid flows through the tube, all points move in sequence with the oscillating driver, forming a sine wave.

•When fluid flows in the tube, twisting occurs.

•The twisting causes a time difference to occur between the velocity detector's signals.

•The time difference is directly related to the mass flow rate.

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•The mass flow of a u-shaped Coriolis flow meter is given as:

Where: Ku is the temperature dependent stiffness of the tube, K a shape-dependent factor, d the width, the time lag, the vibration frequency and Iu the inertia of the tube. As the inertia of the tube depend on its contents, knowledge of the fluid density is needed for the calculation of an accurate mass flow rate.

342

Performance Advantage

• They can handle difficult applications.

• They are suitable for a large number of fluids.

• They have Less susceptibility to damage, wear, and maintenance.

• They can measure bidirectional flow.

• Accuracy is very good, typically ± 0.2% of rate.

• The rangeability is typically 20:1 or better.

• Their operation is independent of a fluid’s property characteristics. 343

Performance Disadvantages

• Earlier versions were susceptible to external vibrations.

• A Coriolis meter is available only up to a small size.

• Special installation requirements are followed to isolate the Coriolis meter from mechanical vibration.

• Avoid using Coriolis meters in piping or meter runs which are prone to substantial vibration, shock, or extreme temperature gradients.

• External meter piping must be well supported.

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345

Flowmeters that use sound waves to measure flow rate are called ultrasonic flowmeters.

346

Ultrasonic Flowmeters

Principles • Doppler shift (frequency shift) method • Deflecting beam method • Transit time method

Time difference

Frequency difference

347

Doppler Shift Method •It transmits a sound wave through the flowing fluid.

•The sound waves are reflected from the fluid to a receiver on the ultrasonic flowmeter.

•The frequency of the sound waves sensed at the receiver shift are affected by the Doppler effect.

348

•The frequency shift is used to determine flow rate.

•Several types of meters are available: one type requires installation of a transducer into the flowing stream,

the other is a strap-on model where installation of a transducer on the pipe is noninvasive.

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Deflecting Beam Method •The transmitter sends a sound wave that is at a right angle to the flow.

•The liquid carries the sound wave and the sound wave is “pushed” or deflected downstream.

•The deflection is directly related to the flow rate and is used to determine the flow rate.

351

Transit Time Method

•A diagonal beam is sent across the flow path.

•The beam is sent with and against the flow.

•Sound travels slower against flow.

•Most commonly used.

•Homogeneous fluids (No entrained bubbles).

•Not for heavy slurry-type applications, because of the high acoustic impedance. 352

353

Transmit Time Frequency Domain Meters

•A pulse is sent in a given direction.

•The time of pulse at the other end of sonic path is recorded.

•The same signal transmits in the opposite direction and records the time at the arrival.

•The difference between two time measurements provides information on motion of the fluid in a pipe.

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Frequency Domain Meters

•The frequency domain meter uses the same type of transducers as the transit time domain meter.

•The only difference is in the processing of the signal.

•The time pulse signal is converted to a frequency signal.

•The path in each direction of flow is used,

•the sonic path generates two frequencies.

•The difference is directly proportional to flow.

Performance Advantages • Clamp-on versions are convenient for retrofits. • Usually nonintrusive. • No pressure drop. • Accuracy is comparable to orifice plates. • High rangeability; rangeability 40:1.

356

Performance Disadvantages • Limited to clean, single-phase liquids. • Straight piping for uniform flow profile. • Attenuation may limit transmission path length. • Averaging methods for large pipes are marginally cost-effective.

357

•Suitable for gas, steam, or liquid services.

•Wide flow range capability,

•Minimal maintenance,

•good accuracy, and

•Long term repeatability.

358

Vortex Shedding Meters

359

•Vortex shedding phenomenon is known as the Von Karman effect of flow across a bluff body.

•Flow alternately sheds vortices from one side to the other side of a bluff body.

•The frequency of the shedding is directly proportional to fluid velocity across the body.

360

•The output depends on the K-factor. •The K-factor relates the frequency of generated vortices to the fluid velocity.

•The K-factor varies with the Reynolds number, but is virtually constant over a broad flow range.

•The formula for fluid velocity is

Fluid velocity =Vortex frequency/K – factor

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•Positive displacement (PD) meters are used for measurement of gas and liquid. Rotating paddle meters, Oscillating piston meters, Oval gear meters, Sliding vane meters, and Bi-rotor meters.

•The term “displacement” refers to a discrete volume that is flowing through the meter.

364

Positive Displacement Meters

PD meters are mechanically driven meters and have one or more moving parts.

365

The energy required to drive the meter’s mechanical components is generated from the flow.

366

The energy to drive the meter creates a pressure loss between inlet and outlet of the meter.

367

A PD meter’s hardware can convert each unit of volume displacement into an electrical pulse.

368

Positive Displacement Meters

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•Accuracy is in terms of percentage registration:

% Registration =(actual quantity/metered quantity) x 100

•At high flow rates, the increase in pressure drop (differential pressure) increases the flow slippage rate, reducing the meter’s accuracy.

•At low flow rates, the meter has low energy because of the lower pressure drop, so the flow is under-counted, again reducing the accuracy.

•Accuracy of the meter is in the range of ± 0.1 to ± 2% of the actual flow. 371

•Rangeability of PD meters typically is 5:1, although 10:1 and greater flow ranges are possible.

•Repeatability are typically ± 0.05% or better.

•Output signals are available either in mechanical or electrical form.

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Performance Advantages • Ideal for viscous liquids • Upstream piping requirements are minimal • Some versions do not require electrical power • High rangeability in liquid and gas meter.

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Performance Disadvantages • Not ideal for liquids with suspended particles. • Mechanical wear. • Larger meters require extra installation care. • Meters can be damaged by over speeding.

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Typical PD Meter Installation

•The rotameter’s operation is based upon variable area principles.

•The flow raises a float in a tapered tube, increasing the area for passage of the flow.

•The greater the flow, the higher the float is raised.

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Variable Area Flow Meters (Rotameter)

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•A sight flow indicator is a mechanically driven device.

•Sight flow indicators are used for visual inspection of the process.

•Three types of sight flow indicators are available, which are the following: • Paddle • Flapper • Drip

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Sight Flow Indicators

Paddle Type •Its design has a propeller inside its body. •It is only used for high flow rate applications. •A pressure drop in the paddle type indicator is higher than the pressure in a drip or flapper type indicator. • It can be installed for flow directions that are horizontal or vertical upward. •It is used when dark process fluids are present.

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Flapper Type

•Bidirectional flappers are also available.

•The flapper type sight flow indicator are used for transparent or opaque solutions and gas services.

•Flow direction can be horizontal or vertically upward.

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Drip Type •Its design is used when there is a dripping of fluid in a vertically downward direction.

•The drip type design is used for vertically downward flows that are intermittent.

•Assembly consists of a chamber, glass, gasket, end covers, and bolts.

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Drag Plate •Flow produces a positive pressure on the plate.

•The force is resisted by a null-balance supporting element at the end of the support arm.

•The signal is proportional to the square of the flowrate.