Electrical units of measurement are based on the International (metric) System, also known asthe SI System. Units of electrical measurement include the following:
Ampere
Volt
Ohm
Siemens
Watt
Henry
Farad
Voltage
Voltage, electromotive force (emf), or potential difference, is described as the pressure or forcethat causes electrons to move in a conductor. In electrical formulas and equations, you will seevoltage symbolized with a capital E, while on laboratory equipment or schematic diagrams, thevoltage is often represented with a capital V.
Current
Electron current, or amperage, is described as the movement of free electrons through aconductor. In electrical formulas, current is symbolized with a capital I, while in the laboratoryor on schematic diagrams, it is common to use a capital A to indicate amps or amperage (amps).
Resistance
Now that we have discussed the concepts of voltage and current, we are ready to discuss a thirdkey concept called resistance. Resistance is defined as the opposition to current flow. Theamount of opposition to current flow produced by a material depends upon the amount ofavailable free electrons it contains and the types of obstacles the electrons encounter as theyattempt to move through the material. Resistance is measured in ohms and is represented by thesymbol (R) in equations. One ohm is defined as that amount of resistance that will limit thecurrent in a conductor to one ampere when the potential difference (voltage) applied to the
conductor is one volt. The shorthand notation for ohm is the Greek letter capital omega (W). Ifa voltage is applied to a conductor, current flows. The amount of current flow depends upon theresistance of the conductor. The lower the resistance, the higher the current flow for a givenamount of voltage. The higher the resistance, the lower the current flow.
Ohm’s Law
In 1827, George Simon Ohm discovered that there was a definite relationship between voltage,current, and resistance in an electrical circuit. Ohm’s Law defines this relationship and can bestated in three ways.1. Applied voltage equals circuit current times the circuit resistance. Equation (1-2) is amathematical respresentation of this concept.E = I x R or E = IR (1-2)
2. Current is equal to the applied voltage divided by the circuit resistance. Equation
3. Resistance of a circuit is equal to the applied voltage divided by the circuit current.
Equation (1-4) is a mathematical representation of this concept.
R (or W) E (1-4)
I
where
I = current (A)
E = voltage (V)
R = resistance (W)
If any two of the component values are known, the third can be calculated.
Since circuit resistance and circuit current are known, use Ohm’s Law to solve for
applied voltage.
E = IR
E = (0.5 A)(100 W) = 50 V
Conductance
The word "reciprocal" is sometimes used to mean "the opposite of." The opposite, or reciprocal,of resistance is called conductance. As described above, resistance is the opposition to currentflow. Since resistance and conductance are opposites, conductance can be defined as the abilityto conduct current. For example, if a wire has a high conductance, it will have low resistance,and vice-versa. Conductance is found by taking the reciprocal of the resistance. The unit usedto specify conductance is called "mho," which is ohm spelled backwards. The symbol for "mho"is the Greek letter omega inverted ( ). The symbol for conductance when used in a formula is
G. Equation (1-5) is the mathematical representation of conductance obtained by relating the
definition of conductance (1/R) to Ohm’s Law, G 1 (1-5)
Power
Electricity is generally used to do some sort of work, such as turning a motor or generating heat.Specifically, power is the rate at which work is done, or the rate at which heat is generated. Theunit commonly used to specify electric power is the watt. In equations, you will find powerabbreviated with the capital letter P, and watts, the units of measure for power, are abbreviatedwith the capital letter W. Power is also described as the current (I) in a circuit times thevoltage (E) across the circuit. Equation (1-6) is a mathematical representation of this concept.
P = I x E or P = IE (1-6)
Using Ohm’s Law for the value of voltage (E),
E = I x R
and using substitution laws,
P = I x ( I x R)
power can be described as the current (I) in a circuit squared times the resistance (R) of thecircuit. Equation (1-7) is the mathematical representation of this concept.
P = I2R (1-7)
Inductance
Inductance is defined as the ability of a coil to store energy, induce a voltage in itself, andoppose changes in current flowing through it. The symbol used to indicate inductance inelectrical formulas and equations is a capital L. The units of measurement are called henries.The unit henry is abbreviated by using the capital letter H. One henry is the amount ofinductance (L) that permits one volt to be induced (VL) when the current through the coil changesat a rate of one ampere per second. Equation (1-8) is the mathematical representation of the rateof change in current through a coil per unit time.inductance L. The negative sign indicates that voltage induced opposes the change in currentthrough the coil per unit time (DI/Dt).
DtInductance will be studied in further detail later in this text.
Capacitance
Capacitance is defined as the ability to store an electric charge and is symbolized by the capitalletter C. Capacitance (C), measured in farads, is equal to the amount of charge (Q) that can betored in a device or capacitor divided by the voltage (E) applied across the device or capacitorplates when the charge was stored. Equation (1-10) is the mathematical representation forcapacitance.C Q (1-10)
Friday, December 21, 2007
Wednesday, December 19, 2007
Link Active Scheduler
(LAS)
All links have one and only one Link Active Scheduler (LAS). The LAS
operates as the bus arbiter for the link. The LAS does the following:
• recognizes and adds new devices to the link.
• removes non-responsive devices from the link.
• distributes Data Link (DL) and Link Scheduling (LS) time on the
link. Data Link Time is a network-wide time periodically
distributed by the LAS to synchronize all device clocks on the
bus. Link Scheduling time is a link-specific time represented as
an offset from Data Link Time. It is used to indicate when the
LAS on each link begins and repeats its schedule. It is used by
system management to synchronize function block execution
with the data transfers scheduled by the LAS.
• polls devices for process loop data at scheduled transmission
times.
• distributes a priority-driven token to devices between scheduled
transmissions.
Any device on the link may become the LAS, as long as it is capable.
The devices that are capable of becoming the LAS are called link
master devices. All other devices are referred to as basic devices. When
a segment first starts up, or upon failure of the existing LAS, the link
master devices on the segment bid to become the LAS. The link master
that wins the bid begins operating as the LAS immediately upon
completion of the bidding process. Link masters that do not become the
LAS act as basic devices. However, the link masters can act as LAS
backups by monitoring the link for failure of the LAS and then bidding
to become the LAS when a LAS failure is detected.
Only one device can communicate at a time. Permission to communicate
on the bus is controlled by a centralized token passed between devices
by the LAS. Only the device with the token can communicate. The LAS
maintains a list of all devices that need access to the bus. This list is
called the “Live List.”
Two types of tokens are used by the LAS. A time-critical token, compel
data (CD), is sent by the LAS according to a schedule. A non-time
critical token, pass token (PT), is sent by the LAS to each device in
ascending numerical order according to address.
LAS = Link Active Scheduler
Introduction
Device Addressing
Fieldbus uses addresses between 0 and 255. Addresses 0 through 15 are
reserved for group addressing and for use by the data link layer. For all
Fisher-Rosemount fieldbus devices addresses 20 through 35 are
available to the device. If there are two or more devices with the same
address, the first device to start will use its programmed address. Each
of the other devices will be given one of four temporary addresses
between 248 and 251. If a temporary address is not available, the device
will be unavailable until a temporary address becomes available.
Scheduled Transfers Information is transferred between devices over the fieldbus using
three different types of reporting.
• Publisher/Subscriber: This type of reporting is used to transfer
critical process loop data, such as the process variable. The data
producers (publishers) post the data in a buffer that is
transmitted to the subscriber (S), when the publisher receives the
Compel data. The buffer contains only one copy of the data. New
data completely overwrites previous data. Updates to published
data are transferred simultaneously to all subscribers in a single
broadcast. Transfers of this type can be scheduled on a precisely
periodic basis.
• Report Distribution: This type of reporting is used to broadcast
and multicast event and trend reports. The destination address
may be predefined so that all reports are sent to the same
address, or it may be provided separately with each report.
Transfers of this type are queued. They are delivered to the
receivers in the order transmitted, although there may be gaps
due to corrupted transfers. These transfers are unscheduled and
occur in between scheduled transfers at a given priority.
• Client/Server: This type of reporting is used for
request/response exchanges between pairs of devices. Like Report
Distribution reporting, the transfers are queued, unscheduled,
and prioritized. Queued means the messages are sent and
received in the order submitted for transmission, according to
their priority, without overwriting previous messages. However,
unlike Report Distribution, these transfers are flow controlled
and employ a retransmission procedure to recover from
corrupted transfers.
Figure 1-3 diagrams the method of scheduled data transfer. Scheduled
data transfers are typically used for the regular cyclic transfer of
process loop data between devices on the fieldbus. Scheduled transfers
use publisher/subscriber type of reporting for data transfer. The Link
Active Scheduler maintains a list of transmit times for all publishers in
all devices that need to be cyclically transmitted. When it is time for a
device to publish data, the LAS issues a Compel Data (CD) message to
the device. Upon receipt of the CD, the device broadcasts or “publishes”
the data to all devices on the fieldbus. Any device that is configured to
receive the data is called a “subscriber.”
All links have one and only one Link Active Scheduler (LAS). The LAS
operates as the bus arbiter for the link. The LAS does the following:
• recognizes and adds new devices to the link.
• removes non-responsive devices from the link.
• distributes Data Link (DL) and Link Scheduling (LS) time on the
link. Data Link Time is a network-wide time periodically
distributed by the LAS to synchronize all device clocks on the
bus. Link Scheduling time is a link-specific time represented as
an offset from Data Link Time. It is used to indicate when the
LAS on each link begins and repeats its schedule. It is used by
system management to synchronize function block execution
with the data transfers scheduled by the LAS.
• polls devices for process loop data at scheduled transmission
times.
• distributes a priority-driven token to devices between scheduled
transmissions.
Any device on the link may become the LAS, as long as it is capable.
The devices that are capable of becoming the LAS are called link
master devices. All other devices are referred to as basic devices. When
a segment first starts up, or upon failure of the existing LAS, the link
master devices on the segment bid to become the LAS. The link master
that wins the bid begins operating as the LAS immediately upon
completion of the bidding process. Link masters that do not become the
LAS act as basic devices. However, the link masters can act as LAS
backups by monitoring the link for failure of the LAS and then bidding
to become the LAS when a LAS failure is detected.
Only one device can communicate at a time. Permission to communicate
on the bus is controlled by a centralized token passed between devices
by the LAS. Only the device with the token can communicate. The LAS
maintains a list of all devices that need access to the bus. This list is
called the “Live List.”
Two types of tokens are used by the LAS. A time-critical token, compel
data (CD), is sent by the LAS according to a schedule. A non-time
critical token, pass token (PT), is sent by the LAS to each device in
ascending numerical order according to address.
LAS = Link Active Scheduler
Introduction
Device Addressing
Fieldbus uses addresses between 0 and 255. Addresses 0 through 15 are
reserved for group addressing and for use by the data link layer. For all
Fisher-Rosemount fieldbus devices addresses 20 through 35 are
available to the device. If there are two or more devices with the same
address, the first device to start will use its programmed address. Each
of the other devices will be given one of four temporary addresses
between 248 and 251. If a temporary address is not available, the device
will be unavailable until a temporary address becomes available.
Scheduled Transfers Information is transferred between devices over the fieldbus using
three different types of reporting.
• Publisher/Subscriber: This type of reporting is used to transfer
critical process loop data, such as the process variable. The data
producers (publishers) post the data in a buffer that is
transmitted to the subscriber (S), when the publisher receives the
Compel data. The buffer contains only one copy of the data. New
data completely overwrites previous data. Updates to published
data are transferred simultaneously to all subscribers in a single
broadcast. Transfers of this type can be scheduled on a precisely
periodic basis.
• Report Distribution: This type of reporting is used to broadcast
and multicast event and trend reports. The destination address
may be predefined so that all reports are sent to the same
address, or it may be provided separately with each report.
Transfers of this type are queued. They are delivered to the
receivers in the order transmitted, although there may be gaps
due to corrupted transfers. These transfers are unscheduled and
occur in between scheduled transfers at a given priority.
• Client/Server: This type of reporting is used for
request/response exchanges between pairs of devices. Like Report
Distribution reporting, the transfers are queued, unscheduled,
and prioritized. Queued means the messages are sent and
received in the order submitted for transmission, according to
their priority, without overwriting previous messages. However,
unlike Report Distribution, these transfers are flow controlled
and employ a retransmission procedure to recover from
corrupted transfers.
Figure 1-3 diagrams the method of scheduled data transfer. Scheduled
data transfers are typically used for the regular cyclic transfer of
process loop data between devices on the fieldbus. Scheduled transfers
use publisher/subscriber type of reporting for data transfer. The Link
Active Scheduler maintains a list of transmit times for all publishers in
all devices that need to be cyclically transmitted. When it is time for a
device to publish data, the LAS issues a Compel Data (CD) message to
the device. Upon receipt of the CD, the device broadcasts or “publishes”
the data to all devices on the fieldbus. Any device that is configured to
receive the data is called a “subscriber.”
FUNCTION BLOCKS
OVERVIEW
This section introduces fieldbus systems that are common to all
fieldbus devices.
INTRODUCTION
A fieldbus system is a distributed system composed of field devices and
control and monitoring equipment integrated into the physical
environment of a plant or factory. Fieldbus devices work together to
provide I/O and control for automated processes and operations. The
Fieldbus FOUNDATION provides a framework for describing these systems
as a collection of physical devices interconnected by a fieldbus network.
One of the ways the physical devices are used is to perform their
portion of the total system operation by implementing one or more
function blocks.
Function Blocks Function blocks within the field bus device perform the various
functions required for process control. Because each system is different,
the mix and configuration of functions are different. Therefore, the
Fieldbus FOUNDATION has designed a range of function blocks, each
addressing a different need.
Function blocks perform process control functions, such as analog input
(AI) and analog output (AO) functions as well as
proportional-integral-derivative (PID) functions.
The standard function blocks provide a common structure for defining function block inputs,
outputs, control parameters, events, alarms, and modes, and combining
them into a process that can be implemented within a single device or
over the fieldbus network. This simplifies the identification of
characteristics that are common to function blocks.
The Fieldbus FOUNDATION has established the function blocks by
defining a small set of parameters used in all function blocks called
universal parameters.
The FOUNDATION has also defined a standard set
of function block classes, such as input, output, control, and calculation
blocks. Each of these classes also has a small set of parameters
established for it. They have also published definitions for transducer
blocks commonly used with standard function blocks. Examples include
temperature, pressure, level, and flow transducer blocks.
The FOUNDATION specifications and definitions allow vendors to add
their own parameters by importing and subclassify specified classes.
This approach permits extending function block definitions as new
requirements are discovered and as technology advances.
. When execution begins, input parameter values from other blocks are
snapped-in by the block. The input snap process ensures that these
values do not change during the block execution. New values received
for these parameters do not affect the snapped values and will not be
used by the function block during the current execution.
Once the inputs are snapped, the algorithm operates on them,
generating outputs as it progresses. Algorithm executions are
controlled through the setting of contained parameters. Contained
parameters are internal to function blocks and do not appear as normal
input and output parameters. However, they may be accessed and
modified remotely, as specified by the function block.
Input events may affect the operation of the algorithm. An execution
control function regulates the receipt of input events and the
generation of output events during execution of the algorithm. Upon
completion of the algorithm, the data internal to the block is saved for
use in the next execution, and the output data is snapped, releasing it
for use by other function blocks.
A block is a tagged logical processing unit. The tag is the name of the
block. System management services locate a block by its tag. Thus the
service personnel need only know the tag of the block to access or
change the appropriate block parameters.
Function blocks are also capable of performing short-term data
collection and storage for reviewing their behavior.
Device Descriptions
Device Descriptions are specified tool definitions that are associated
with the function blocks. Device descriptions provide for the definition
and description of the function blocks and their parameters.
To promote consistency of definition and understanding, descriptive
information, such as data type and length, is maintained in the device
description. Device Descriptions are written using an open language
called the Device Description Language (DDL). Parameter transfers
between function blocks can be easily verified because all parameters
are described using the same language. Once written, the device
description can be stored on an external medium, such as a CD-ROM or
diskette. Users can then read the device description from the external
medium. The use of an open language in the device description permits
Introduction
interoperability of function blocks within devices from various vendors.
Additionally, human interface devices, such as operator consoles and
computers, do not have to be programmed specifically for each type of
device on the bus. Instead their displays and interactions with devices
are driven from the device descriptions.
Device descriptions may also include a set of processing routines called
methods. Methods provide a procedure for accessing and manipulating
parameters within a device.
BLOCK OPERATION
In addition to function blocks, fieldbus devices contain two other block
types to support the function blocks. These are the resource block and
the transducer block. The resource block contains the hardware specific
characteristics associated with a device. Transducer blocks couple the
function blocks to local input/output functions.
Function Blocks
Resource Blocks
Resource blocks contain the hardware specific characteristics
associated with a device; they have no input or output parameters. The
algorithm within a resource block monitors and controls the general
operation of the physical device hardware. The execution of this
algorithm is dependent on the characteristics of the physical device, as
defined by the manufacturer. As a result of this activity, the algorithm
may cause the generation of events. There is only one resource block
defined for a device. For example, when the mode of a resource block is
“out of service,” it impacts all of the other blocks.
Transducer Blocks
Transducer blocks connect function blocks to local input/output
functions. They read sensor hardware and write to effector (actuator)
hardware. This permits the transducer block to execute as frequently as
necessary to obtain good data from sensors and ensure proper writes to
the actuator without burdening the function blocks that use the data.
The transducer block also isolates the function block from the vendor
specific characteristics of the physical I/O.
Alerts
When an alert occurs, execution control sends an event notification and
waits a specified period of time for an acknowledgment to be received.
This occurs even if the condition that caused the alert no longer exists.
If the acknowledgment is not received within the pre-specified time-out
period, the event notification is retransmitted. This assures that alert
messages are not lost.
Two types of alerts are defined for the block, events and alarms. Events
are used to report a status change when a block leaves a particular
state, such as when a parameter crosses a threshold. Alarms not only
report a status change when a block leaves a particular state, but also
report when it returns back to that state.
This section introduces fieldbus systems that are common to all
fieldbus devices.
INTRODUCTION
A fieldbus system is a distributed system composed of field devices and
control and monitoring equipment integrated into the physical
environment of a plant or factory. Fieldbus devices work together to
provide I/O and control for automated processes and operations. The
Fieldbus FOUNDATION provides a framework for describing these systems
as a collection of physical devices interconnected by a fieldbus network.
One of the ways the physical devices are used is to perform their
portion of the total system operation by implementing one or more
function blocks.
Function Blocks Function blocks within the field bus device perform the various
functions required for process control. Because each system is different,
the mix and configuration of functions are different. Therefore, the
Fieldbus FOUNDATION has designed a range of function blocks, each
addressing a different need.
Function blocks perform process control functions, such as analog input
(AI) and analog output (AO) functions as well as
proportional-integral-derivative (PID) functions.
The standard function blocks provide a common structure for defining function block inputs,
outputs, control parameters, events, alarms, and modes, and combining
them into a process that can be implemented within a single device or
over the fieldbus network. This simplifies the identification of
characteristics that are common to function blocks.
The Fieldbus FOUNDATION has established the function blocks by
defining a small set of parameters used in all function blocks called
universal parameters.
The FOUNDATION has also defined a standard set
of function block classes, such as input, output, control, and calculation
blocks. Each of these classes also has a small set of parameters
established for it. They have also published definitions for transducer
blocks commonly used with standard function blocks. Examples include
temperature, pressure, level, and flow transducer blocks.
The FOUNDATION specifications and definitions allow vendors to add
their own parameters by importing and subclassify specified classes.
This approach permits extending function block definitions as new
requirements are discovered and as technology advances.
. When execution begins, input parameter values from other blocks are
snapped-in by the block. The input snap process ensures that these
values do not change during the block execution. New values received
for these parameters do not affect the snapped values and will not be
used by the function block during the current execution.
Once the inputs are snapped, the algorithm operates on them,
generating outputs as it progresses. Algorithm executions are
controlled through the setting of contained parameters. Contained
parameters are internal to function blocks and do not appear as normal
input and output parameters. However, they may be accessed and
modified remotely, as specified by the function block.
Input events may affect the operation of the algorithm. An execution
control function regulates the receipt of input events and the
generation of output events during execution of the algorithm. Upon
completion of the algorithm, the data internal to the block is saved for
use in the next execution, and the output data is snapped, releasing it
for use by other function blocks.
A block is a tagged logical processing unit. The tag is the name of the
block. System management services locate a block by its tag. Thus the
service personnel need only know the tag of the block to access or
change the appropriate block parameters.
Function blocks are also capable of performing short-term data
collection and storage for reviewing their behavior.
Device Descriptions
Device Descriptions are specified tool definitions that are associated
with the function blocks. Device descriptions provide for the definition
and description of the function blocks and their parameters.
To promote consistency of definition and understanding, descriptive
information, such as data type and length, is maintained in the device
description. Device Descriptions are written using an open language
called the Device Description Language (DDL). Parameter transfers
between function blocks can be easily verified because all parameters
are described using the same language. Once written, the device
description can be stored on an external medium, such as a CD-ROM or
diskette. Users can then read the device description from the external
medium. The use of an open language in the device description permits
Introduction
interoperability of function blocks within devices from various vendors.
Additionally, human interface devices, such as operator consoles and
computers, do not have to be programmed specifically for each type of
device on the bus. Instead their displays and interactions with devices
are driven from the device descriptions.
Device descriptions may also include a set of processing routines called
methods. Methods provide a procedure for accessing and manipulating
parameters within a device.
BLOCK OPERATION
In addition to function blocks, fieldbus devices contain two other block
types to support the function blocks. These are the resource block and
the transducer block. The resource block contains the hardware specific
characteristics associated with a device. Transducer blocks couple the
function blocks to local input/output functions.
Function Blocks
Resource Blocks
Resource blocks contain the hardware specific characteristics
associated with a device; they have no input or output parameters. The
algorithm within a resource block monitors and controls the general
operation of the physical device hardware. The execution of this
algorithm is dependent on the characteristics of the physical device, as
defined by the manufacturer. As a result of this activity, the algorithm
may cause the generation of events. There is only one resource block
defined for a device. For example, when the mode of a resource block is
“out of service,” it impacts all of the other blocks.
Transducer Blocks
Transducer blocks connect function blocks to local input/output
functions. They read sensor hardware and write to effector (actuator)
hardware. This permits the transducer block to execute as frequently as
necessary to obtain good data from sensors and ensure proper writes to
the actuator without burdening the function blocks that use the data.
The transducer block also isolates the function block from the vendor
specific characteristics of the physical I/O.
Alerts
When an alert occurs, execution control sends an event notification and
waits a specified period of time for an acknowledgment to be received.
This occurs even if the condition that caused the alert no longer exists.
If the acknowledgment is not received within the pre-specified time-out
period, the event notification is retransmitted. This assures that alert
messages are not lost.
Two types of alerts are defined for the block, events and alarms. Events
are used to report a status change when a block leaves a particular
state, such as when a parameter crosses a threshold. Alarms not only
report a status change when a block leaves a particular state, but also
report when it returns back to that state.
TIPS OF FF
Foundation H1 is intended primarily for process control, field-level interface and device integration. Running at 31.25 kbit/s, the technology interconnects devices such as transmitters and actuators on a field network. H1 is designed to operate on existing twisted pair instrument cabling with power and signal on the same wire. Fiber optic media is optional. It also supports Intrinsic Safety (IS) applications.
Foundation H1 devices comprise a function block application, act as a publisher and subscriber of process variables, transmit alarms and trends, and provide server functionality for host access and management functions. Devices can act as a scheduler and time master for regulating communication on a fieldbus segment. They are also used for bus interfaces in process control systems or in linking devices. Capable of controlling bus communications and many connections to multiple devices, they support both client and server applications. H1 technology enables field instruments and other devices to execute control functions reducing the load on plant computers and workstations. Since the H1 network is digital, I/O conversion subsystems are eliminated. The Fieldbus Foundation tests and registers the devices to ensure interoperability of registered instruments from multiple vendors. This enables the end user to select the best instruments for the application regardless of the host system supplier.Reports from leading adopters of the Foundation protocol demonstrate the advantages of control in the field with the H1 solution. For example, end users in the petrochemical industry have realized up to a 30 percent reduction in operating costs due to advanced diagnostics. Users have also seen that the all-digital H1 communications network is far less susceptible to electrical noise than traditional 4-20 mA analog systems. H1 technology enjoys widespread acceptance throughout the process industries, and is included in the international IEC standard (IEC 61158).
Foundation H1 devices comprise a function block application, act as a publisher and subscriber of process variables, transmit alarms and trends, and provide server functionality for host access and management functions. Devices can act as a scheduler and time master for regulating communication on a fieldbus segment. They are also used for bus interfaces in process control systems or in linking devices. Capable of controlling bus communications and many connections to multiple devices, they support both client and server applications. H1 technology enables field instruments and other devices to execute control functions reducing the load on plant computers and workstations. Since the H1 network is digital, I/O conversion subsystems are eliminated. The Fieldbus Foundation tests and registers the devices to ensure interoperability of registered instruments from multiple vendors. This enables the end user to select the best instruments for the application regardless of the host system supplier.Reports from leading adopters of the Foundation protocol demonstrate the advantages of control in the field with the H1 solution. For example, end users in the petrochemical industry have realized up to a 30 percent reduction in operating costs due to advanced diagnostics. Users have also seen that the all-digital H1 communications network is far less susceptible to electrical noise than traditional 4-20 mA analog systems. H1 technology enjoys widespread acceptance throughout the process industries, and is included in the international IEC standard (IEC 61158).
COMMON FLOW
Numerous types of flowmeters are available for closed-piping systems. In general, the equipment can be classified as differential pressure, positive displacement, velocity, and mass meters. Differential pressure devices (also known as head meters) include orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meters, Fig. 2.
Positive displacement meters include piston, oval-gear, nutating-disk, and rotary-vane types. Velocity meters consist of turbine, vortex shedding, electromagnetic, and sonic designs. Mass meters include Coriolis and thermal types. The measurement of liquid flows in open channels generally involves weirs and flumes.
Space limitations prevent a detailed discussion of all the liquid flowmeters available today. However, summary characteristics of common devices are shown in Table 1. (Click here to see Table1) Brief descriptions follow.
Differential Pressure Meters
The use of differential pressure as an inferred measurement of a liquid's rate of flow is well known. Differential pressure flowmeters are, by far, the most common units in use today. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.
The basic operating principle of differential pressure flowmeters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.
Differential pressure flowmeters, like most flowmeters, have a primary and secondary element. The primary element causes a change in kinetic energy, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid's properties. And, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.
Orifices are the most popular liquid flowmeters in use today. An orifice is simply a flat piece of metal with a specific-sized hole bored in it. Most orifices are of the concentric type, but eccentric, conical (quadrant), and segmental designs are also available.
In practice, the orifice plate is installed in the pipe between two flanges. Acting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference. Major advantages of orifices are that they have no moving parts and their cost does not increase significantly with pipe size.
Conical and quadrant orifices are relatively new. The units were developed primarily to measure liquids with low Reynolds numbers. Essentially constant flow coefficients can be maintained at R values below 5000. Conical orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each application.
The segmental wedge is a variation of the segmental orifice. It is a restriction orifice primarily designed to measure the flow of liquids containing solids. The unit has the ability to measure flows at low Reynolds numbers and still maintain the desired square-root relationship. Its design is simple, and there is only one critical dimension the wedge gap. Pressure drop through the unit is only about half that of conventional orifices.
Integral wedge assemblies combine the wedge element and pressure taps into a one-piece pipe coupling bolted to a conventional pressure transmitter. No special piping or fittings are needed to install the device in a pipeline.
Metering accuracy of all orifice flowmeters depends on the installation conditions, the orifice area ratio, and the physical properties of the liquid being measured.
Venturi tubes have the advantage of being able to handle large flow volumes at low pressure drops. A venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. As liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions.
The flowmeters have no moving parts. They can be installed in large diameter pipes using flanged, welded or threaded-end fittings. Four or more pressure taps are usually installed with the unit to average the measured pressure. Venturi tubes can be used with most liquids, including those having a high solids content.
Flow tubes are somewhat similar to venturi tubes except that they do not have the entrance cone. They have a tapered throat, but the exit is elongated and smooth. The distance between the front face and the tip is approximately one-half the pipe diameter. Pressure taps are located about one-half pipe diameter downstream and one pipe diameter upstream.
Flow Nozzles, at high velocities, can handle approximately 60 percent greater liquid flow than orifice plates having the same pressure drop. Liquids with suspended solids can also be metered. However, use of the units is not recommended for highly viscous liquids or those containing large amounts of sticky solids.
Positive displacement meters include piston, oval-gear, nutating-disk, and rotary-vane types. Velocity meters consist of turbine, vortex shedding, electromagnetic, and sonic designs. Mass meters include Coriolis and thermal types. The measurement of liquid flows in open channels generally involves weirs and flumes.
Space limitations prevent a detailed discussion of all the liquid flowmeters available today. However, summary characteristics of common devices are shown in Table 1. (Click here to see Table1) Brief descriptions follow.
Differential Pressure Meters
The use of differential pressure as an inferred measurement of a liquid's rate of flow is well known. Differential pressure flowmeters are, by far, the most common units in use today. Estimates are that over 50 percent of all liquid flow measurement applications use this type of unit.
The basic operating principle of differential pressure flowmeters is based on the premise that the pressure drop across the meter is proportional to the square of the flow rate. The flow rate is obtained by measuring the pressure differential and extracting the square root.
Differential pressure flowmeters, like most flowmeters, have a primary and secondary element. The primary element causes a change in kinetic energy, which creates the differential pressure in the pipe. The unit must be properly matched to the pipe size, flow conditions, and the liquid's properties. And, the measurement accuracy of the element must be good over a reasonable range. The secondary element measures the differential pressure and provides the signal or read-out that is converted to the actual flow value.
Orifices are the most popular liquid flowmeters in use today. An orifice is simply a flat piece of metal with a specific-sized hole bored in it. Most orifices are of the concentric type, but eccentric, conical (quadrant), and segmental designs are also available.
In practice, the orifice plate is installed in the pipe between two flanges. Acting as the primary device, the orifice constricts the flow of liquid to produce a differential pressure across the plate. Pressure taps on either side of the plate are used to detect the difference. Major advantages of orifices are that they have no moving parts and their cost does not increase significantly with pipe size.
Conical and quadrant orifices are relatively new. The units were developed primarily to measure liquids with low Reynolds numbers. Essentially constant flow coefficients can be maintained at R values below 5000. Conical orifice plates have an upstream bevel, the depth and angle of which must be calculated and machined for each application.
The segmental wedge is a variation of the segmental orifice. It is a restriction orifice primarily designed to measure the flow of liquids containing solids. The unit has the ability to measure flows at low Reynolds numbers and still maintain the desired square-root relationship. Its design is simple, and there is only one critical dimension the wedge gap. Pressure drop through the unit is only about half that of conventional orifices.
Integral wedge assemblies combine the wedge element and pressure taps into a one-piece pipe coupling bolted to a conventional pressure transmitter. No special piping or fittings are needed to install the device in a pipeline.
Metering accuracy of all orifice flowmeters depends on the installation conditions, the orifice area ratio, and the physical properties of the liquid being measured.
Venturi tubes have the advantage of being able to handle large flow volumes at low pressure drops. A venturi tube is essentially a section of pipe with a tapered entrance and a straight throat. As liquid passes through the throat, its velocity increases, causing a pressure differential between the inlet and outlet regions.
The flowmeters have no moving parts. They can be installed in large diameter pipes using flanged, welded or threaded-end fittings. Four or more pressure taps are usually installed with the unit to average the measured pressure. Venturi tubes can be used with most liquids, including those having a high solids content.
Flow tubes are somewhat similar to venturi tubes except that they do not have the entrance cone. They have a tapered throat, but the exit is elongated and smooth. The distance between the front face and the tip is approximately one-half the pipe diameter. Pressure taps are located about one-half pipe diameter downstream and one pipe diameter upstream.
Flow Nozzles, at high velocities, can handle approximately 60 percent greater liquid flow than orifice plates having the same pressure drop. Liquids with suspended solids can also be metered. However, use of the units is not recommended for highly viscous liquids or those containing large amounts of sticky solids.
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