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PROCESS CONTROL APPLET SERIES II
Basics of Flow Measurement by DP Cell, Instrumentation and Noise filtering
Flow Measurement by Differential Pressure
A flow rate can be measured by measuring the pressure difference between two points in the a pipline that has flow restriction between the two points. Thus a differential pressure flowmeter consists of a differential pressure (DP) producer and a DP transducer. The DP producer, e.g. orifice, imposes a restricion in a pipeline so that the pressures before and after restriction are different depending on the flow rate. The DP transducer, called DP cell, measures the pressure drop across the producer. The correct selection and installation of the DP transducer plays an important part in determining the accuracy of the flow rate measurement. Over 40% of all liquid, gas, and steam measurements made in industry are still accomplished using common types of differential pressure flowmeter, that is, the orifice plate, Venturi tube, and nozzle. The operation of these flowmeters is based on the observation made by Bernoulli that if an annular restricion is placed in a pipeline, then the velocity of the fluid through the restriction is increased.
Objectives of This Applet
This applet is designed to illustrate the followings :
- - Flow measurement by orifice and DP cell
- - Instrumentation for measuring devices
- - Selection of orifice plate
- - Calibrating components in instrumentation
- - Noise filtering
This applet consists of 5 parts. The horizontal thin panels on top and bottom are the title panel and status panel, respectively. The title panel is permanent one and the status panel is changing depending on the situation to give the explanation to users. In the Middle, there are three panels. The leftmost panel shows the instrumentation and circuit diagram. The right-upper panel shows the displays of each instrument unit. The right-lower panel shows the Option panel and control panel fo each unit. In this help, only these three panels will be described.
It shows the schematic diagram of flow measurement by DP cell and the connection diagram of components for the flow measurement. The DP cell will generate 4-20mA with power source and the signal will be converted to voltage signal between 1-5volts. There is nothing to adjust.
This panel has seven displays. Each display has an analog and a digital meter. The actual flow can be changed by dragging the red triangle flow indicator. Except the actual flow, by clicking the digital displays, the control panel below will be switched to the corresponding instrument unit. In the lower left corner, there is a small chart icon which can toggle the strip chart for the signals.
This strip chart in the middle shows the acutal flow, analog signal and the filtered signal. There are also two triangles to adjust the magnification of the plot. The change in magnification will recenter the plot based on the value of actual flow except the full size(x1). The maximum magnification is x20.
The orifice size, noise size and filtering can be selected from the listboxes in this panel. If a noise is added, the signals except acutal flow will be corrupted by noise and the blue indicators will show the noisy signal. In the bottom, an arbitrary value can be specified for the parameter values when the green # keys in the parameter panels are clicked. If you start typing in the input field the background color will change to red. After type the desired value, press 'ENTER' to reflect the change to the applet. Then, the background color will come back to black and the corresponding parameter value will be updated.
When the digital displays in the display panel are clicked, this control panel will be switched to the corresponding panel. The user-specifiable parameters depending on the selection of the instrument such as zero, span, view range and etc. can be altered by clicking adjustment triangles or green # keys. The different control panels are shown below. In the control panel for DP cell, there is an option for square root extractor. If this option is selected, the output of the DP cell is proprtional to the square root of the input signal.
Explain the concept of DP Cell Implementation (Java Applet will help you interactively.)
Flow Measurement by Differential Pressure
A flow rate can be measured by measuring the pressure difference between two points in the a pipline that has flow restriction between the two points. Thus a differential pressure flowmeter consists of a differential pressure (DP) producer and a DP transducer. The DP producer imposes a restricion in a pipeline so that the pressures before and after restriction are different depending on the flow rate. The DP transducer, called DP cell, measures the pressure drop across the producer. The correct selection and installation of the DP transducer plays an important part in determining the accuracy of the flow rate measurement. Over 40% of all liquid, gas, and steam measurements made in industry are still accomplished using common types of differential pressure flowmeter, that is, the orifice plate, Venturi tube, and nozzle. The operation of these flowmeters is based on the observation made by Bernoulli that if an annular restricion is placed in a pipeline, then the velocity of the fluid through the restriction is increased. The increase in velocity at restriction causes the static pressure to decrease at this section, and a pressure difference is created across the element. The pressure difference is related to the rate of fluid flowing through the pipe.
One of the major advantages of the orifice plate, Venturi tube, or nozzle is that the measurement uncertainty can be predicted without the need for calibration, if it is manufactured and installed in accordance with one of the international standards covering these devices. In addition, this type of differential pressure flowmeter is simple, has no moving parts, and is therefore reliable. The main disadvantages of these devices are their limited ranges (typically 3:1), the permanent pressure drop they produce in the pipeline (which can results in higher pumping costs), and their sensitivity to installation effects (which can be minimized using straight length of pipe before and after the flowmeter). The combined advantages of this type of flowmeter are still quite hard to beat, and although it has limitations, these have been well investigated and can be compensated for in most circumstances. Unless very high accuracy is required, or unless the application makes a nonintrusive device essential, the differnetial pressure flowmeter should be considered. [J. G. Webster, Measurement, Instrumentation, and Sensors Handbook]
Measuring device of Differential Pressure
Differentail Pressure Transducer
The main factors that should be considered when choosing a differential pressure transducer for a flow measurement application are the differential pressure range to be covered, the accuracy required, the maximum pipeline pressure, and the type and temperature range of the fluid being metered. The most modern DP transducers consist of a pressure capsule in which their capacitance, strain gauge, or resonant wire techniques are used to detect the movement of diaphragm. Using these techniques, a typical accuracy of +/-0.1% of full scale is possible.
The transducer is usually part of a unit known as a transmitter, which converts differential pressure, static pressure, and ambient temperature measurements into a standardized analog or digital output signal. "Smart" transmitters use a local, dedicated microprocessor to condition signals from the individual sensors and compute volumetric or mass flow rate. These devices can be remotely configured, and a wide range of diagnostics and maintenance functions are possible using their built-in "intelligence."
As far as installation is concerned, the transmitter should be located as close to the differential pressure producer as possible. This helps ensure a fast dynamic response and reduces problems caused by vibration of the connecting tubes. The position of the pressure tappings is also important. If liquid flow in a horizontal pipe is being measured, then the pressure tappings should be located at the side of the pipe so that they cannot be blocked with dirt or filled with air bubbles. For horizontal gas flows, if the gas is clean the pressure tappings should be vertical; if steam or dirty gas is metered, then the tappings should be located at the side of the pipe. These general guidlines show that considerable care must be taken with the installation of the differential pressure transmitter if large measurement errors are to be avoided. For further details on the installation of DP transmitter, see ISO 2186. [J. G. Webster, Measurement, Instrumentation, and Sensors Handbook]
Orifice as Differential Pressure Producer
Differentail Pressure Producer
The orifice is the simplest and cheapest type of differential pressure producer. It is simply a plate with a hole of specified size and position cut in it, which can then clamped between flanges in a pipeline. (See Figure in the applet) The increase that occurs in the velocity of a fluid as it passes through the hold in the plate results in a pressure drop being developed across the plate. After passing through the restriction, the fluid flow jet continues to contract until a minimum diameter known as the vena contracta is reached. For the Venturi meter, the Bernoulli equation can be applied. However, for the orifice plate, the direct use of the Bernouli equation would result in an error.This is because the area of vena contracta is unknown and the turbulence between the vena contracta and the pipe wall results in an energy loss that is not accounted for in this equation. To overcome this problems caused by the practical application of the Bernoulli equation, two empirically determined correction factors, discharge coefficient and expansibility factor, are added. The equation for the orifice plate can be found easily in many standard textbooks on fluid dynamics. In a number of internationally recognized documents known as standards, both factors can be determined from equations and tables. These standards not only specify both factors, but also the geometry and installation conditions for the use of the most commonly used type of differential pressure flowmeter. The installation recommendations are intended to ensure that fully developed turbulent flow conditions exist within the measurement section of the flowmeter.
Using these standards, measurement performance can be confidently predicted without the need for calibration if the device is manufactured, installed, and operated in accordance with one of the international standards. In addition, the device is cheap to manufacture, has no moving parts, is reliable and can be used for metering most clean gases, liquids, and steam. The major disadvantages of the orifice plate are its limited range and sensitivity to flow disturbances. The fact that fluid flow rate is proportional to the square root of the measured differential pressure limits the range of a one plate/one differential pressure transmitter combination to about 3:1. The required diameter ratio (also known as beta ratio) of the plate depends on the maximum flow rate to be measured and the range of differential pressure transducer available. All the relationships covered in the standards assume a fully developed and stable flow profile, and so installtion of the device is critical, particularly the need for sufficient straight pipework upstream of the meter. Wear of the leading edge of the orifice plate can severely alter measurement accuracy; thus, this device is normally only used with clean fluids.
Only one type of orifice plate, the square-edged concentric, is covered by the standards. However, other types exists, having been designed for specific applications. One example is the eccentric orifice plate, which is suited for use with dirty fluids. [J. G. Webster, Measurement, Instrumentation, and Sensors Handbook]
Current-to-Voltage (I//V) Transmitter
Why do we need signal transmitter?
In a plant, the measurement is usually located in the field near the actual process equipment and the control room which centralized all the controllers and data acquisition system is located in remote place. The measurement device will convert the physical variable into electrical signals and send to control room. The most popular industrial standard signal for signal transmission is the current signal (I) of 4-20mA. The current signal can easily be sent to remote place without degradation of the quality of signal with wire. However, most of data acquisition systems accept standard voltage signal (V) of 1-5V or 0-5V. Therefore, it is required to convert the current signal to voltage signal for the data acquisition system. This can be achieved quite simply. As in th figure of applet, only a resistance (R) of 250ohms in a serial connection, which calculated from the basic equation, R=V/I, will convert the current signal of 4-20mA into voltage signal of 1-5V, linearly.
In the instrument side, if the measured signal is a voltage signal, the signal should be converted into a current signal to be transmitted to remote place. This can be achieved by use of I/V transmitter. Most modern measurement device have the function to select the kind of output signal. In these cases, the I/V transmitter is combined with measurement device and the users simply select the output option.