Process Control
The integral term
?????????builds over time, as long as an error exists. When the error becomes zero, the ???????? holds its value. When a system upset occurs, the proportional term provides the initial response. Over time, the ?????????? dominates and the proportional term becomes zero.
Moving or relocating instruments can have an effect on control system response due to:
- Increased uncertainty in the control system signal , Changes in the system response time
continuous control
A common example of ???????? is cruise control on a vehicle. A cruise controller senses the speed of the vehicle, compares this speed to a desired reference, and then accelerates or decelerates the vehicle, as required. The cruise controller can vary the rate of acceleration depending on how far the vehicle's speed is from the setpoint. While the proportional control function responds to the magnitude of the error, an integral control function is needed to restore the actual value to the setpoint
On/Off Control
A control signal that can only be in either the on or the off state; no intermediate state is possible.
Very large digital systems typically replace many analog devices and control loops with centralized control and display hardware/software. For these systems, the implementation process is much more complex. In addition to the items discussed previously, areas that must be addressed for the implementation of large digital systems include:
A detailed hardware and software specification must be developed. An extensive test plan and software QA plan must be written. Cyber-security countermeasures may be required. A full 10CFR50.59 Evaluation will be required and in some cases may require prior regulatory approval. Technical Specification changes and extensive changes to the SAR may be required. Drawing, procedure, training, and equipment database changes will be required.
Primary Element
A detector or sensor that is exposed to the process variable.
DP: Vented vessel level
A device that measured the difference in head of tank vs atmospheric
Ultrasonic level
A device that uses a high-frequency sound wave to determine the distance of a liquid from a fixed point
Pitot Tube
A dp-producing device used to measure air flow in ventilation systems, consisting of a small tube inserted into the flow stream, with one opening or part facing the flow steam (impact port) and the other port perpendicular to the flow stream (static port). Both ports sense static pressure in the system but only the impact port senses a pressure increase due to flowrate
Fieldbus Technology
A fieldbus is a control network typically used in process control and industrial automation. Fieldbuses are often used to provide two way communications between a PLC (or other device) and an end device. Modern fieldbus technologies allow for multiple connections of a control cable to be made within a control loop. This provides a substantial savings in cost and labor for installation and maintenance of single cables being run to individual components.
RTD
A fine wire metal element coiled and supported on an electrically non-conductive form. The device measures the change in resistance proportional to the change in temperature.
Ladder Logic
A highlevel graphical programming method that uses "coils" and "contacts" in software to emulate relay operations.
Update Times
A measure of how long it takes a digital control system to complete a cycle and update all inputs and outputs. Update times are an important parameter for digital controls used for fastchanging process
Process control
A method to monitor and adjust specific process parameters to maintain desired values.
Thermocouple
A pair of dissimilar conductors joined together to form a sensing junction. When temperature of the junction changes, a small voltage develops at the output.
Electrostatic Discharge
A potentially damaging highvoltage discharge of static electricity to or from an electronic device. ESD can cause latent damage that is not immediately apparent and is particularly dangerous to lowvoltage digital components.
Protective action
A process control specifically designed to prevent damage to systems and equipment.
Stable Control
A stable process will exhibit decreasing amplitude oscillations in response to system upsets.
Graphical User Interface
A system, known as a GUI or "gooey", that uses graphics to represent various elements, which can be manipulated by the user to execute functions in the machine. Windows is a familiar GUI.
Flow
A venturi is used in the measurement of which process variable?
SCR phase controller
An electronic circuit that varies the firing angle of a Silicon Controlled Rectifier (SCR). This is done to change the portion of the AC waveform used to energize a heater in order to control the heat output
Vacuum
An expression of pressure, usually as an absolute pressure or possibly with relation to atmosphere. Condenser pressure is an example of negative pressure relative to atmosphere or an absolute pressure, usually expressed in inches of mercury.
Digital transmitters, converters, and recorders are often specified to directly replace existing analog components. The most common reasons for replacement are:
Analog replacements are no longer available or have become obsolete.Digital components are more reliable, accurate, stable, and easier to calibrate.Digital components often perform selfdiagnostics to aid in maintenance.Digital controls can be made much more fault-tolerant than analog control systems and can be designed to continue to operate with one or more failures.Inventories of spares can be reduced, as a single piece of hardware can be configured to do many different functions and replace a large number of different devices, as pictured in the graphic at right.
Maintenance Strategies
Analog systems required skilled technicians to maintain the system in an appropriate working fashion by completing frequent, recurring instrument calibrations to ensure the equipment is operating appropriately. While digital systems do not need this level of recurring maintenance activities, they still require an individual capable of working with the control system to change system parameters and functions. This infrequently requires a technician who can work with the coded language of the system, but more often the technician must be trained to use the specific logic blocks provided in the digital system.
Disturbance
Any influence that tends to drive the controlled variable away from the desired, reference, or setpoint conditions.
Network Architecture
As the complexity of digitally controlled systems increases, it may be necessary for numerous PLCs or other microprocessor based devices to work together on a single network. Network Architecture is the design of the network; it is the framework for the specification of a network's physical components and their functional organization, configuration, and operation. This often includes graphical user interfaces (GUIs), PLCs, the end devices, and interconnecting fieldbus
Orifice plate cons
Causes a relatively high permanent pressure drop, the sharp edges can erode, particles can collect upstream of the orifice plate
Modification impacts
Changing from analog to digital control systems can be a challenge. New failure modes can be created when moving to the new system. Additionally, digital equipment is more sensitive to EMI/RFI and susceptible to software programming errors. The new functions have to be verified and tested, which requires additional time and resources. Modifications to the system being controlled will also require evaluation of the control system. For example, replacement of a control valve with one that has different flow characteristics over its stroke length will affect the control system. Installation of dampeners in instrument sensing lines to minimize the effects of pulsations can adversely affect response times. Such installations should be evaluated under the design process.Valve accessories such as volume boosters shouldn't be added or deleted without evaluation of the effects on control system responses.
Bourdon Tube Pressure
Consist of a curved metal tube with an oval cross-section that is closed at one end and attached to the pressure source at the other end. As pressure is applied the tube tends to straighten, which operate a pointer or transducer proportional to the measure pressure
Reject To Manual
Continuous control systems may also be designed to "reject to manual" if parameters are exceeded or an input is lost. An alarm alerts the operator to take manual control.
Filled system cons
Distance between measure point and indication must be limited, Typically does not have transmitter capability
Ultrasonic level cons
Doppler type requires particulates or bubbles in fluid to work correctly, due to their complexity, not as reliable as other devices
For the activities listed below, the need for I&C expert involvement may not be recognized, since the potential impact is indirect and often rather subtle.
EMI/RFI Impacts, Environmental Impacts, Changes to instrument locations, Piping/Tubing Changes, Valve Design Changes
EMI/RFI
Electromagnetic Interference and Radio Frequency Interference (EMI/RFI) is electronic interference that is either radiated or carried by signal or power wiring. This interference can come from other digital or analog devices, handheld radios or telephones, or from components like relays and large motors.
on/off control
Examples of ??????? are sump pumps, air compressors, and heaters.
Venturi cons
Expensive, more difficult to install and maintain than other devices
Primary Sensor Location
Flow meters are sensitive to their piping location. A long, continuous, straight run of pipe is required ahead of the primary flow element.
Low Flow Rates
Flow meters using differential pressure transmitters require the output signal to be processed by a square root extractor to convert the differential pressure signal to flow rate. An inaccurate reading can result at low flow rates.
Digital components are usually configurable to perform a wide variety of functions beyond those available from analog components. These include:
Implementing logical and mathematical functions (including square roots) internal to the controllerProviding multiple control loops, as well as additional inputs and outputs for indication, control, and alarmsProviding auto-tuning, where the controller uses the response of the process to determine appropriate tuning values for gain, reset, and derivative to make optimization of the process much easierAllowing ladder logic, function block, and menu-driven configuration, which allows set up to be done by engineers, technicians, or operators with little additional training
Digital components are more reliable, accurate, stable, and easier to calibrate
In a comparison of analog control systems and digital control systems, which one of the following statements is correct?
Input/Output
In digital control systems, Input/Output (I/O) typically refers to field signals coming into or going out of the system. It can also refer to all communications between a processor and the environment.
Orifice plate pros
Inexpensive, easy to install
closed-loop control
Information concerning the current state of the process is fed back to the control system from the measuring element - the detector
ElectroMagnetic Interference (EMI)
Instrument output signals use very low voltages and current, making them susceptible to interference from operating equipment and high power lines. Shielded cable can be used to prevent interference from affecting the signals.
Vapor Pressure
Instruments used to measure vessel level can be affected by the vapor pressure inside the vessel. If the vessel is open to the atmosphere, level is directly proportional to pressure. If the tank is pressurized, a reference leg is used to cancel out the effect of tank pressure changes.
More Complex Control
More stringent requirements may require a specialized system, such as the 3-element feedwater control system. Another variation could be a simple PID control system with protective overrides at high and/or low process values. Some continuous control systems transition to a different mode or simply a different setpoint after a plant trip. Remotely-calculated setpoints rather than setpoints selected manually are an example of adding sophistication to a simple PID scheme. Cascade, feedforward control, and other complex control schemes are becoming more common.
Venturi Pros
Most accurate flow measuring device, causes only a small pressure drop
Thermocouple pros
No resistance lead wire problems, can be connected directly to indicator without external power, inexpensive, provides fastest response, supports high-temperature operation
Derivative control
Noisy signals, such as are seen in flow measurements, can make use of ????????????? counter productive.
Orifice plate
Orifice plate with the differential pressure measured to calculate flow
Bourdon tube cons
Over pressure can rupture/deform tube, not used for differential pressure, accuracy can only be 1-2% - nuclear uses special accurate ones
Bellows cons
Overpressure can rupture bellows, accuracy is 1-2%
Protocol Management
Protocols provide a common means for interfacing two unrelated objects. As in the above example, proper protocol management allows numerous different plant systems to interface with the plant computer. Protocols provide the language through which the plant computers can communicate with other databases, applications, processes, and hardware.
RTD pros
Provide a linear output, provides excellent accuracy, sense area temperature, provides good linearity, can use standard wiring
Filled-system
Provides a local indication or switch for control applications by connecting a liquid or gas-filled sensing bulb to a receiving tube. The receiving tube expands or contracts due to pressure changes in the sensing bulb caused by related temperature changes
Bourdon tube pros
Provides both local indication and a transmitted signal, local indication does not require external power, con be used for very high pressures
Thermocouple cons
Provides non-linear output, reference junction temperature changes will affect measurement, requires special thermocouple wire, low repeatability, least sensitive
Filled-system pros
Requires no external power source for local indication, Output rand of -400 to 1000 deg F, inexpensive
Proportional
Responds according to how far the process is from the setpoint. The proportional term of a controller is the error term multiplied by gain. The proportional band is the reciprocal of gain, in percent units. A gain of two is the same as a proportional band of 50% and a proportional band of 20% represents a gain of five. Mathematically, proportional control can be represented by the (Gain) x (Error)
Derivative
Responds to how quickly the process value is moving away (or towards) the setpoint. The derivative term is calculated by multiplying the rate of change of the error by an internal constant. The derivative term is zero except when the process is changing. Mathematically, derivative control is the (Gain) x (The Derivative of the Error with Respect to Time).
Indicators
Signals from the control loop are often displayed throughout the plant on local ????????. These indicators provide a quick means of viewing a plant parameter. The most prevalent place for these ???????? is in the control room, where operators use the indications to monitor the
Radio Frequency Interference (RFI)
Some instruments are sensitive to radio frequencies. Areas surrounding this equipment must be marked as radio free zones. Twistedpair wire is commonly used to prevent RFI.
Tuning for Specific Operating Conditions
Systems that are designed to operate in more than one line-up have special tuning considerations. Imagine what would happen if a system is optimally tuned for operation at a lower pressure during test conditions. This may cause the system to be unstable when used to support its design function at higher operating pressures. Some control systems transition to a different setpoint or mode of control in the event of a plant trip.
Feedback
The constant input of the measured process parameter.
Continuous Control
The control signal response is proportional to the magnitude of the error so the final control element operates continuously within pre-defined limits.
Final Control Element
The final control element manipulates the process parameter
Temperature, pressure, flow and level
The four most commonly measured process variables are?
Simple Digital Devices
The less complex digital transmitters are basically digital substitutes for analog components that measure temperature, pressure, flow, and level. They are typically less expensive and more accurate than the analog components they replace.
Software Quality Assurance
The process of ensuring that software operates as designed, that adequate documentation is kept of changes, that potential users are identified, that backups of software and data are adequate, and that eventual retirement of the application is adequately addressed.
Controlled variable
The process variable that is being maintained at a specified value or within a specified range.
Gain
The ratio of the change in the output to the change in the input. Gain represents the strength of the proportional term. The error is multiplied by the gain to produce the proportional output (demand).
A digital control upgrade represents a significant design effort. Due to the vulnerabilities you learned about earlier, additional rigor must be applied to ensure a troublefree installation. There are also special regulatory requirements for software quality assurance and for evaluating the safety aspects of digital controls in safetyrelated systems.
The recommended replacement system must be accurately specified to ensure the needs of the plant are appropriately addressed. A formal software quality assurance (QA) plan and a formal test plan must be submitted. The susceptibility of the system to electrostatic discharge (ESD) must be evaluated.
Setpoint
The reference value that is compared to the measured process value is called the
Secondary Element
The secondary element consists of the transducer and transmitter.
Compute
This function determines if the error is positive or negative, and what output is needed to the engine (speed up or slow down).
Compare
This function determines if there is a difference (called an error) between the measured parameter (vehicle speed) and what was entered as the desired setpoint vehicle speed. For our purposes here, we will assume there is a difference, and you are either going too fast or too slow.
Correct
This function involves the effort performed by the engine to make the actual speed match the setpoint speed. The engine speeds up or slows down until the measured speed matches the setpoint speed. Feedback is the term used for the constant input of the measured process parameter (speed) to achieve the desired setpoint. Some design methods provide a varied (faster or slower) correction based on how far the measured process parameter is from the desired setpoint.
Capacitance Sensors
Unit comprised of a pressure housing, diaphragms, and capacitor plates. Capacitance is proportional to the area of the plates and inversely proportional to the distance between the plates. The applied differential pressure controls the distance between the plates, therefore the change in capacitance is proportional to the change in differential pressure
Makes a change to the selected process parameter
What is the purpose of the final control element in the operation of a process control system?
Pressure
What process variable is measured using a bourdon tube?
They ensure that optimal operating parameters are maintained through the control of plant and equipment parameters
What purpose do process control systems serve in nuclear power plant operations
Derivative
When working with PID controllers, in which operation is the term calculated by multiplying the rate of change of the error by an internal setting constant?
Linear
When working with globe-style valves, in which valve characteristic will the flow change (in percent of full flow) be the same for any percentage change in valve opening
Venturi
a device used to measure feedwater flow rates, consisting of a converging conical inlet section, a cylindrical throat and a diverging recovery cone. A different pressure is developed between the inlet and throat sections, then measured with a differential pressure transmitter
RTD cons
a little expensive, requires external power, slower response time, must have three or 4 wires to get decent accuracy and overcome the lead resistance effect.
Capacitance sensors pros
accuracy is 0.25%, used for differential pressure, can tolerate over pressurization ~6000 PSIG
Ultrasonic level pros
accurate while fluid density remains stable, does not contact fluid
Detector
also known as the primary element, senses change in the measured variable and provides an output proportional to the magnitude of the measured parameter. Typically the ??????? is exposed to the process being measured. For example, an orifice plate, which is a plate with a hole in it, is a primary element installed in a pipe. The differential pressure of the flow through the hole in the plate is measured to calculate the flow rate.
The difference between the safety analysis limit/protective set point
and the nominal setpoint provides a margin of safety that accounts for instrument error and process uncertainties.
Digital systems
are computer or microprocessor based equipment (controller) which is used to convert parameters to binary values for processing however, signals are still transmitted to and from the controller through analog means. Because analog and digital systems cannot interface directly, there must be some form of analog to digital conversion at the input to the controller and some form of digital to analog conversion at the output of the controller. Once the signal is converted or digitized, it is then represented by the digital device as a sequence of "1s" and "0s," referred to as binary. The signal is no longer continuous as in analog systems, as it is being sampled and replicated in binary on a repeated basis. Sampling often takes place so fast, normally in the milliseconds, that continuous control of the system is maintained.
Electric heaters in continuous control applications
are often controlled by an SCR phase controller to vary the power and heat applied to the process.
Modern paperless recorders
are specifically designed to replace aging analog devices. They fit into the same or similar panel cutouts and emulate the familiar look and feel of the equipment being replaced, while adding the capability to perform complex logic, mathematical calculations, and alarm functions
The size and configuration of the piping and tubing
associated with a given control system application is integral to the overall control system response. Therefore, changes to these parameters can potentially affect the response of the control system.
Damper
can be actuated, or moved, by compressed air cylinders, electric actuators, or hydraulic actuators
Process control systems
can be designed for manual operator control, fully automatic control, or automatic protective action
DP: Closed vessel level pros
can be mounted away from tank, not affected by tank pressure, simple, reliable
DP: Vented vessel level pros
can be mounted away from tank, not affected by tank pressure, simple, reliable
The PID controller
can perform proportional, integral, and derivative operations alone or in combination. If used in combination, the demand signals are summed to give the total demand output.
Proportional-only control
can produce acceptable results for many processes and is usually free of stability problems sometimes caused by use of integral and/or derivative control. One disadvantage of this control method is that an error will typically exist in steady state; control won't be maintained exactly at setpoint.
Temperature measurement
cannot normally be made directly, but is instead an inferred measurement based on the measured effects that a change in temperature has on the properties of other substances
A pressure transmitter
connected to the pressurizer sends a signal representing the pressurizer pressure, the measured value.
manual control
consists of an operator manually adjusting a final control element based on the indicated value of the controlled variable
Strain gauge
coverts mechanical motions to electronic signal
DP: Closed vessel level
deivce that measures the difference created in head created in wet reference leg and head created by head in the tank
Error
difference between the setpoint and measured value
In an analog control system
different DC voltages representing the measurement and setpoint values are compared using electronic components, such as operational amplifiers.
3
element control - A demand signal based on level is produced as in a typical PID scheme, but two additional measurements are also made: of steam flow from the boiler and feedwater flow into it. The difference in these flows is the flow imbalance signal. This signal is used to develop an additional bias that is added to the demand. This significantly improves the response time over a system based on level only, since the system can react before level changes appreciably.
Three
element control - What control strategy would you be likely to choose if you required accurate and stable continuous control for a large, complex system
For instruments measuring level or pressure,
elevation changes can potentially introduce errors due to pressure head effects, if not accounted for during calibration.
Equal percentage valve
for percentage of valve opening the flow change in percent of full flow will be the same throughout the range. These are used for most flow control application and in other applications where the differential pressure across the valve remains relatively constant
Linear valves
for percentage of valve opening, the flow change will be the same throughout the range, used in more flow control process
Most digital singleloop controllers
implement two or more control loops in an enclosure that is specifically designed to replace an obsolete single-loop analog controller. Process alarms, mathematical function blocks, and complex logic can be implemented in these controllers. They normally have digital communications capabilities, multiple inputs and outputs, and fast update times.
Density changes
in a closed vessel that are caused by temperature changes must be compensated. The typical compensation method is to use multiple level transmitters calibrated for different operating conditions.
Digital logic devices
include Programmable Logic Controllers (PLCs) and Micro Controllers. These devices typically provide alarms, implement logic, and provide an operator interface. They can have as few as eight or as many as several hundred input/output points (I/O) and are normally programmed using ladderlogic symbols.
Pitot tube pros
inexpensive, causes only a small pressure drop, easy to install
Strain gauge pros
inexpensive, very accurate
Quick opening valve
inverse of equal percentage characteristic, the change in flow will be much more rapid at lower valve openings that at higher valve openings
A programmable logic controller (PLC)
is a digital computer used for automation of electromechanical processes. Within nuclear facilities PLCs are increasingly relied upon for numerous control functions throughout the plant.
A microprocessor
is a multipurpose, inexpensive, and programmable device that serves as a central processing unit of a computer, PLC, and other logic based digital devises.
Head Correction
is an offset in calibration to correct for the difference in height between the instrument and the level reference point. Head correction is not required for differential pressure flow signal.
A bistable
is an onoff device whose output depends on the relationship between the input signal and the setpoint.
DP: Closed vessel level cons
is calibrated for a particular fluid density, therefore susceptible to temperature changes
DP: Vented vessel level cons
is calibrated for a particular fluid density, therefore susceptible to temperature changes, vest must remain open
Instrument calibration
is essential to ensuring setpoint accuracy. Calibration is the process of comparing and adjusting an instrument's response, or output, to a measured variable.
An error signal
is generated if the pressurizer pressure differs from the control setpoint.
Reactor pressure in a PWR
is maintained by a vessel called the pressurizer. The pressurizer is partially filled with water, and has a saturated steam region above the water level. Pressure can be raised by heating the water with a set of variable output heaters and backup (on-off) heaters. Pressure can be lowered by opening valves to spray water through a nozzle into the steam region, condensing some of the steam. Pressure can also be reduced by opening relief valves to vent steam from the top of the pressurizer.
Measure
is normally considered the first function in Process Control. It is the actual monitoring of a parameter that is desired to be controlled. In our example, this would be the measurement of the vehicle speed.
The setpoint
is the desired value for the controlled variable.
Demand signal
is the output signal from the controller that is sent to the final control element. It is the sum of the signals calculated by the proportional, integral, and derivative sections of the controller, plus a bias term.
The nominal setpoint
is the value input in the protection system circuits. Automatic protection functions are expected to occur when the process variable reaches the nominal set point
Re
locating instruments that receive or transmit pneumatic signals - can potentially affect the time response of the control system.
Pitot tube cons
measures fluid velocity at one point, requiring the use of an averages pitot tube to obtain average flow rate, susceptible to plugging, sensitive to flow direction
Bellows pros
more sensitive to lower pressure ranges, can be used to measure differential pressure
Ultrasonic level cons
must me mounted on the tank
Ultrasonic level pros
no pressure drop, can be mounted externally of in shallow wells, high accuracy
The derivative element
of a PID controller produces a demand signal proportional to the rate of change of the error. This allows the controller to respond to system upsets before they produce an error
Process control systems
provide the capability to monitor and adjust specific process parameters to maintain desired values. In some nuclear facilities, these ?????????? are known as instrumentation and control systems.
The controller
receives and computes the error signal.
The transducer or transmitter
receives output from the detector and converts it to a standard electronic or pneumatic signal range for transmission to the controller. ????????????? are considered secondary elements because they convert the output of the primary element. Depending on the application and the instrumentation used, the function of the ???????????? (and possibly the detector) may be performed by a single device.
Controller
receives the transducer or transmitter signal and performs two discrete functions. First, it compares the measured variable to the setpoint and generates an error signal if they are not equal. Second, it computes the required output response based on the selected controller functions
Ladder Logic
represents the function of the controlled device by graphical symbols. The end user can quickly and easily make changes to the logic by connecting a personal computer to the PLC and reconfiguring the PLC's code. This allows the PLC to be a flexible device and adaptable to the plant if further changes or updates are required.
Capacitance sensors cons
requires external power source
Integral
responds based on the time duration an error has existed. The integral term of a controller is the error term multiplied by gain and by time, then scaled by the integral setting. The integral term will always be increasing or decreasing when an error exists. Another term for integral is "reset." Mathematically, integral control is the (Gain) x (The Integral of the Error with Respect to Time).
valve characteristic
should be selected to linearize the control response, so that the controlled variable changes approximately the same amount for a similar change in controller output throughout the range. A control valve with the wrong flow characteristic can make a process uncontrollable.
In an analog system
signals are transmitted to and from the controller through electronic or pneumatic means. The signal is either the direct value of a system parameter or a scaled value. The signal is an uninterrupted, continuously varied signal. The controller performs the compare and compute functions through analog means, as well.
The controller then sends an output
that manipulates the final control elements in an attempt to reduce the error to zero. The controller output may depend on the magnitude of the error, how long the error has existed, or how fast the error is changing.
Strain gauge cons
the low level signal is susceptible to noise if not shielded.
In a digital control system
the measurement and setpoint values are compared using a numerical calculation in the computer.
The process variable is normally less than
the setpoint and the bistable trips when the process variable is greater than the setpoint. In this configuration, the bistable is normally de-energized.
The process variable is normally less than
the setpoint and the bistable trips when the process variable is greater than the setpoint. In this configuration, the bistable is normally energized, which is the more frequently used set-up.
The process variable is normally greater than
the setpoint and the bistable trips when the process variable is less than the setpoint. In this configuration, the bistable is normally de-energized.
The process variable is normally greater than
the setpoint and the bistable trips when the process variable is less than the setpoint. In this configuration, the bistable is normally energized, which is the more frequently used set-up
automatic systems
the system output is monitored by a process sensor, which feeds the measurement signal back to a controller at the input of the system
Ultrasonic flow
transducers are used to produce ultrasonic waves from one side of the pipe to the opposite side of the pipe. The transit times and sound velocity are used to calculate flow
Over
tuning - a system with increased gain adjustments will result in a faster response, but the increased response time comes at the risk of instability. Too much gain in a controller would cause excessive overshoot and result in an unstable condition. Conversely, reducing gain to provide highly stable control under steady state conditions may make response to transients overly sluggish.
Highlyintegrated control systems
typically include a variety of digital transmitters, processors, and a sophisticated humanmachine interface (HMI). The systems connect to a network to allow remote access and monitoring. They are often redundant and distributed in order to enhance reliability. Their thousands of I/O, combined with a large amount of processing power, allow the implementation of sophisticated control algorithms..
Air operated valves (AOV)
use air pressure against a piston or diaphragm for positioning of a valve. In the event of a loss of air, air operated valves are often designed to move to their fail safe position
Differential bellows transmitters
use opposing forces on the bellows to determine pressure between the low and high connections
Pneumatic controls
utilize pressurized gas (normally air) to effect mechanical motion within a system
Integral control example
was running cold water and the operator wanted hot, they would open the hot water valve slightly. If hot water was not available after a period of time the operator would further open the valve to increase the rate at which hot water was being added
Optimum tuning
will result in quarter-wave dampening, as pictured in the graphic at right.
Head flowmeters
work on the basic principle that when a fluid flows through a restriction, its velocity increases and its pressure decreases. The change in pressure, known as differential pressure (dP), is proportional to the square of the volumetric flow rate.
Feedback
works to restore balance or decrease the effect of a disturbance to the process, as shown in the graphic below. The measurement of the controlled variable is fed to the controller. The controller then outputs to the final control element, which corrects the process
