SPiCESchool of Process instrumentation & Control
EducationFrom Indias leading Process Control Enterprise
Yokogawa
GRADUATE TRAINING PROGRAMPROCESS MEASUREMENT AND CONTROL
APPLICATION
INTRODUCTION TO YOKOGAWA PCI PRODUCTS FIELDBUS ENGINEERING
YIL TRAINING CENTER
PROCESS MEASUREMENT AND CONTROL APPLICATIONOBJECTIVE DURATION :
This course enables participants to learn the terminologies of
Instrumentation, measurement techniques and concepts of control
system. : 6 Days
COURSE CURRICULUM :
DAY 1 2 3 4 5 6
CONTENTINTRODUCTION & BASICS OF MEASUREMENT CALIBRATION
& CONVERSION TABLES LEVEL MEASUREMENT FLOW MEASUREMENT PRESSURE
MEASUREMENT TEMPERATURE MEASUREMENT FEEDBACK ,FEED FORWARD AND
CASCADE CONTROL TUNING OF CONTROLLER
INTRODUCTION TO YOKOGAWA PCI PRODUCTSOBJECTIVE : This course
enables the participant to understand the latest sensor technology
for industrial measurement including the recent advances in process
instrumentation. : 5 Days
DURATION
COURSE CURRICULUM :
DAY
CONTENTINTRODUCTION TO FIELD INSTRUMENTS
1
EVOLUTION OF PCI PRODUCTS (YOKOGAWA) TRANSMITTERS - EJA , EJX ,
YTA FLOW METERS VORTEX
2
ULTRASONIC FLOW METERS MAGNETIC FLOW METERS MASS FLOW METERS
3 4 5
RECORDERS / DAQWORKS CONTROLLERS (YS170 / UT SERIES) CONTROLLERS
- US1000
FIELDBUS ENGINEERINGOBJECTIVE DURATION : This course is designed
to provide participants an overall understanding of Fieldbus
Technology and the Asset Management tool. : 4 Days
COURSE CURRICULUM :
DAY1 OFFLINE ENGINEERING ONLINE ENGINEERING 2 DEVICE
REGISTRATION
CONTENTINTRODUCTION TO FIELD BUS CONCEPT
OPERATION OF FF SHADOW BLOCKS
3 4
PRM INSTALLATION PRM FEATURES & OPERATION
PROCESS MEASUREMENT AND CONTROL APPLICATION
o INTRODUCTION o CALIBRATION TECHNIQUE o LEVEL MEASUREMENT o
FLOW MEASUREMENT o PRESSURE MEASUREMENT o TEMPERATURE MEASUREMENT o
CONTROL LOOPS AND TUNING o ALARM ANNUNCIATORS
INTRODUCTION TO INSTRUMENTATIONAUTOMATION: You are well aware
that the different Industrial sectors like Information technology,
Telecom, Automobiles, Textiles all play a major role in our life.
Likewise, Automation is another important core sector, which is
virtually controlling our life. Today, automation solutions are
required right from agriculture to space technology and Plant
Automation has become absolute necessity for the manufacturing /
process industries to survive in todays global market. Automation
is simply the delegation of human control function to process
equipments for increasing productivity, Quality, Cost reduction,
Plant equipment safety A CONTROL SYSTEM, which takes cares the
various operations involved in a process, in an automated way with
minimal human intervention, is generally known as AUTOMATION.
CONTROL SYSTEM: A Control system is a combination of various
devices that are integrated as a system used to sense, measure,
indicate and control the process variables, which in turn controls
the process to achieve the desired results. PROCESS CONTROL
INSTRUMENTATION As part of a control system various measurements
and controls are generally involved in a process, to achieve the
desired process conditions. PROCESS CONTROL INSTRUMENTATION
MEASUREMENT When we want to quantify something, Measurement is
required. Examples: At what SPEED the train is going?. What is the
TEMPERATURE in the furnace? What is the PRESSURE exerted by the
System? What is the WEIGHT of the parcel? What is the water LEVEL
in the tank? How much water is FLOW ing through the pipe?
CONTROL
Now, these parameters like Pressure, Tempeature, Flow, Level
which are measured are called PROCESS VARIABLES
Page 1 of 11
PROCESS VARIABLES: The most commonly used measurements (Process
variables) are: Pressure Temperature Flow Level Speed Weight
Humidity Density Vibration Conductivity Ph Current Voltage Power
Torque Position
MEASUREMENT
INDICATION What is INDICATION? When the measured value is
presented in a readable form, we call that the value of the
parameter is INDICATED. As the value changes the indication changes
and the earlier readings are lost.
RECORDING What is RECORDING? When the measured value is RECORDED
in a readable form, we call that the value of the parameter is
RECORDED. As the value changes since the earlier readings are
recorded we can refer the previous readings.
MEASUREMENTS: Measurements are made available either in Local OR
in RemoteLOCAL
Examples:
PI 18
When it is required to know the Pressure in the pipeline or
level in the tank, one has to go to that place of installation
(local) to know the values. These types of measurement are known as
LOCAL MEASUREMENT.
LI 24 24
Page 2 of 11
REMOTE: The Parameter to be measured is sensed in the field area
and the signal is transmitted to a remote place. (Central Control
Room) for readable & control purpose and this type of
measurement is known as REMOTE MEASUREMENT. Sitting in Control
room, one can know the values. Fig 1 & 2 represents Remote
measurement. Examples: FIELD AREA CONTROL ROOM
PS H
FIG. 1ReceiverFT 13FIC 13
Transmitter Primary (sensing) elementFE 13
FIG. 2
Two types of Signals: DIGITAL & ANALOG DIGITAL: ANALOG: The
output represents anyone of the two states, that is 0 or 1, ON/OFF,
OPEN / CLOSE (FIG. 1) The output is continuous, representing 0 to
100% value of the Measurement (FIG. 2).
Page 3 of 11
The Plant Instrumentation can be divided as FIELD
IINSTRUMENTATION and Control room INSTRUMENTATIION. PLANT
INSTRUMENTATION
FIELD
CONTROL ROOM DCS PLC SCADA Industrial PCs Marshalling Racks UPS
Control Panel
INSTRUMENTSSensors Pressure Instruments Temperature Instruments
Flow Instruments Level Instruments Speed Instruments Density
Instruments Weight Instruments Analytical Instruments Control
Valves Actuators
OTHER ITEMSLocal Panels Junction Boxes Cable trays Cable duct
Cables Impulse Lines SS / Copper tubes
BASICS ON MEASUREMENTUNITS:The measurements made to quantify any
thing has to be expressed in UNITS. Example: The The The The The
The The SPEED of the train : kilometers per hour. TEMPERATURE in
the furnace: Deg. C PRESSURE : psi WEIGHT of the parcel: kgs. LEVEL
in the tank: meters AREA of the plot: Sq. Feet FLOW of water in the
pipeline: lts/hr.(LPH)
Page 4 of 11
INDICATION / READING: Examples : Measurements are generally
expressed in Percentage (%) Example: The pressure in a chamber is
between 0 and 200 psi. psi 0% 10 % 20 % 0 20 40
100%
-
200 TRANSMITTER
PNEUMATIC Output 3 to 15 psi 0.2 to 1 kg/cm2 XR Output 3 psi 6
psi 9 psi 12 psi 15 psi % 0 25 50 75 100L PH
ELECTRONIC Output 4 to 20 mA XR Output 4 mA 8mA 12 mA 16 mA 20
mA % 0 25 50 75 100 Kg/cm2 0 100 200 300 400
0 50 100 150 200
LINEAR AND SQUARE ROOT SCALES: Recorder charts for Pressure,
Temperature, Level, Specific Gravity, etc. generally have a linear
scale whereas flow charts have a square root scale. This is because
the rate of flow is proportional to the sq. Root of the
differential head. Whereas linear charts have a uniform
calibration, Sq.root charts have a sq. root calibration as shown in
the figure below. It will be noted that 50% of the flow is actually
marked on 25% of the linear chart, 70% of the flow near the 50% of
linear scale and 90% of the flow very near 80% of the linear
scale.
0
1
2
3
LINEAR SCALE
4
5
6
7
8
9
10
02
3
4
5
SQ. ROOT SCALE
6
7
8
9
10 Page 5 of 11
Consequently flow scales are cramped at the bottom (near zero)
and expanded near maximum. Accuracy of flow meter reading against
such a scale can be had only above 25% of the linear scale. LINEAR
AND SQUARE ROOT CALIBRATION TABLE: TRANSMITTER OUTPUT mA psi Kg/
cm2 0.2 0.4 0.6 0.8 1 READING LINEAR SCALE SQUARE ROOT SCALE 0% 50%
70.71% 86.60% 100% MEASURED VALUE FLOW (m3 / Hr.)
4 8 12 16 20
3 6 9 12 15
0% 25% 50% 75% 100%
0 100 200 300 400
BASIC DELINITIONS
ELEVATED ZERO: A range where the zero value of the measured
signal is greater than the lower range value. Zero lies between LRV
and URV Range: (-) 25 to 100
(-)25 LRVSUPPRESSED ZERO:
0
100 URV
A range where the zero value of the measured signal is less than
the lower range value. Zero does not appear in the scale. Page 6 of
11
Example: 20 to 100
20Typical Ranges Name Range Lower Range Value
100Upper range Value +100 +100 Span
ILLUSTRATIONS OF THE USE OF RANGE AND SPAN TERMINOLOGY:
0 20
+100 +100
Suppressed Zero Range
0 to 100 0 20 to 100 20
100 80
-25
0
+100 Elevated Zero -25 to Range +100 0
-25
+100
125
-100
Elevated Zero -100 to 0 -100 Range
0
100
ILLUSTRATIONS OF THE USE OF TERMS MEASURED VARIABLE &
MEASURED SIGNAL: TYPICAL RANGES THERMOCOUPLE 0 2000 F TYPE K T/C
-0.68 +44.91mV TYPE OF RANGE RANGE Measured 0 to Variable 2000F
Measured -0.68 to Signal +44.91mV LOWER RANGE VALUE 0 F -0.68mV
UPPER RANGE VALUE 2000 F SPAN
2000 F
+44.91mV 45.59mV 10,000Ib/h 100in H2O 500 rpm 5V 10,000 Ib/h
100in H2O 500 rpm 5V
FLOWMETER 0 10,000Ib/h 0 100in H2O
Measured 0 to 0 Ib/h Variable 10,000Ib/h Measured 0 to 100 0in
H2O Signal in H2O Measured 0 to Variable rpm Measured 0 Signal 500
0 rpm 0V
TACHOMETER 0 500 rpm 0 5V
to 5V
is defined as the closeness with which the reading approaches an
ACCURACY accepted standard value or true value. Accuracy is often
quoted as a percentage of the full scale value. Ex : accuracy : +/-
1 % fsd ERROR - The algebraic difference between the indicated and
the true value of the measured signal.
Page 7 of 11
ERROR = Indicated (measured) value True value. LAG : When the
quantity being measured changes, a certain time might have elapsed
before the measuring instrument responds to the change. It is said
to show LAG.
DEAD SPACE / THRESHOLD: When the quantity being measured is
gradually increased from zero, a certain minimum level might have
reached before the instrument responds and gives a detectable
reading. This is called the threshold. It is just a dead space that
happens to occur when the Instrument is used from a zero value. For
example, a pressure gauge might not respond until the pressure has
risen to some value. This may be due to friction and other factors
of the gauge. . REPEATABILITY: The repeatability of an instrument
is its ability to display the same reading for repeated
applications of the same value of the quantity being measured. OR
The closeness of agreement among a number of consecutive
measurements of the output for the same value of the input under
the same operating approaching from the same direction, for full
range traverses. SENSITIVITY: The ratio of a change in output
magnitude to the change of input which causes it after the steady
state has been reached. RESOLUTION : The least interval between two
adjacent discrete details, which can be distinguished one from the
other.
Page 8 of 11
BASIS ON CONTROLMANUAL PERCEPTION & CONTROL
MANUAL FEEDBACK CONTROL WITH SENSOR & INDICATOR
Page 9 of 11
SKETCH FOR AUTOMATIC CLOSED LOOP FEEDBACK CONTROL
MEASUREMENTS & CONTROLS:
A control loop can broadly be divided into four functional
categories: to sense or detect the variable to be measured
transforms the detected ( sensed )signal to an interpretable stage
where it can either be read or used for further control
applications . to compare the measured signal with the desired
conditions and perform the necessary. Carries out the corrections
required so that the variable is controlled within the specified
limits.
1 2
Primary Element
Secondary Element Manipulating Element gPrimary Final
Control
3
4
Page 10 of 11
CONTROLLER: What is Control? The Process of achieving the actual
measurement at a predetermined DESIRED VALUE is known as the
CONTROL of that variable. Example: The flow of water through a pipe
line has to be controlled at a particular flow, say - 40 litres /
hr; We know that we want to control the flow at a specific value.
(SET POINT) We have to know how much water is flowing. So, we have
to measure the flow (MEASUREMENT). The difference is known as
Error. Based on the error, a suitable OUTPUT from the controller
goes to the valve to regulate in such a way to get the desired
flow.
CONTROLLER BLOCK DIAGRAM: SET POINT OUTPUT
FEED FORWARD CONTROL & FEEDBACK CONTROL Feed forward control
involves making an estimate of the quantity of action necessary to
accomplish a desired objective. Its basis is in prediction. There
is NO feedback. Eg : Washing Machine In Feedback control,
measurement (MV) of the variable to be controlled is compared with
a reference point (SP). If the difference or error exits between
the actual measurement and the set point, the automatic controller
takes the necessary action by sending the Increased / decreased
output (O/P) to the final control element to achieve the desired
control.
Page 11 of 11
CALIBRATION PROCEDURES FOR CONTROL &
INSTRUMENTATIONCALIBRATION Calibration refers to the process of
determining the relation between the output (or response) of a
measuring instrument and the value of the input quantity or
attribute, a measurement standard. Calibration is often regarded as
the process of adjusting the output or indication on a measurement
instrument to agree with value of the applied standard, within a
specified accuracy GENERAL INSTRUCTIONS FOR CALIBRATION Before
calibrating the instrument, o Check for any Physical Damage to the
Instrument o Check whether the Instrument is working or not in the
following manner For Digital Instruments, switch on the power. For
Analog Instruments, see the pointer deflection. o Clean the switch
contacts, Potentiometers, if any, by cleaning agent. o Give at
least half an hour warm-up time for all Power-On Instruments and
for Regulated Power Supply before starting Calibration. o For
Analog Instruments ensure Mechanical zero before starting the
Calibration. o Parallax error is to be avoided. o Instruments used,
as masters for Calibration must be calibrated from Govt. approved
Laboratory. ENVIRONMENTAL CONDITIONSTEMPERATURE RELATIVE
HUMIDITY
For Mechanical Instruments For Electrical Instruments
CALIBRATION POINTS o o o o
20 +/- 2.5C 25 +/- 2.5C
35 to 65 % 35 to 65 %
Calibration area should be adequately free from dust, shocks and
vibrations
The instruments should be Calibrated for all ranges. Ranges
which cannot be Calibrated or for which accuracy of the instrument
is not as per requirement must be indicated on the instrument
itself as well in records. For Analog Instruments, Calibration of
an Instrument should be performed at 25%, 50%, 75% and 100% of the
range being calibrated, Readings should be recorded at same point
while increasing and decreasing. For Digital Instruments,
Calibration should be performed at 25%, 50%, 75%, 90% of the range
being calibrated.
TABLE FOR SELECTION OF MASTER (REFERENCE STANDARD) FOR
CALIBRATION BASED ON ACCURACY DESIREDDESIRED % ACCURACY OF THE
INSTRUMENT RANGE TO BE CALIBRATED MIN. RECOMMENDED ACCURACY OF
REFERENCE STANDARD
0.05% 0.01% 0.1% 0.02% 0.2% 0.04% 0.5% 0.1% 1.0% 0.2% 2.5% 0.3%
Accuracy of Master Instrument required for Calibrating Mechanical
Instruments is recommended to be 10 times higher than the accuracy
desired Accuracy. Page 1 of 9
TRACEABLITY CHAINNPL NPL National Physical Laboratory DGSTQC
Directorate General of Standardization, Testing and Quality
Certification
DGSTQC
GOVT. APPROVED
INDUSTRY /USER
TECHNICAL INFORMATION 1. DESCRIPTION OF THE MEASUREMENT PROCESS
Standards used along with traceability information. Brief
Description of measurement method (could include measurement
scheme, measurement time frame etc.) State the number of
measurement made. Explain how the data were analyzed to obtain
measured values Include an explanation of equations, algorithms or
formula used. Definition of acronyms used in report. 2. REPORTING
MEASUREMENT RESULTS Report the measured values for the measurement.
Where item is found to be out of tolerance, both the incoming and
outgoing data should be reported. Measurement uncertainties
Influence quantities - Quantities which are not the subject of
measurement but which influences the measured values Example
1.Frequency of AC Voltage 2. Temperature & Resistance 3.
Temperature & length
3. TEST CONDITIONS LABORATORY ENVIRONMENTAL CONDITIONS
Temperature Humidity Pressure ABNORMAL CONDITIONS Stability Erratic
readings Excessive wear Noticeable physical change Repairs
performed on the calibrated item
4. PRESENTING THE DATA Units of measurement should be stated
along with associated measured values Units of Uncertainty
Uncertainty stated in the same units as the measured value 0.1% *
1000 PPM USE % 0.01% * 100 PPM USE% 0.001% * 10 PPM USE PPM 0.0001%
* 1 PPM USE PPM Tables Graphs
Page 2 of 9
5. TRACEABILITY CONTENTS OF CERTIFICATE OF CALIBRATION 1.
Calibration Organization. 2. Certificate Title 3. Item
Identification 4. Requester 5. Calibration Due 6. Due Date 7.
Certificate Number 8. Signature 6. MAINTAINING RECORDS FOR THE
EQUIPMENTS Make Type Serial Number or other ID Measurement
Capability Calibration Certificates Date of Calibration Calibration
Results After and, if necessary, before Recalibration Date
Identification of Calibration Procedure Limits of permissible error
Source of Calibration Traceability Environmental conditions during
Calibration Uncertainties Details of servicing, Adjustment, repairs
or modifications Any limitations in use Persons performing
Calibration Persons responsible for ensuring correctness Unique ID
of Calibration Report / certificate Retain Records
7. NON CONFORMING MEASURING Suffered Damage. Mishandled or
Overloaded. Shows Malfunction. Calibration Overdue. Such Equipment
shall not returned to service until reasons for nonconformity have
been eliminated and again calibrated Calibration Level Intervals of
Calibration
8. SEALING FOR INTEGRITY Access to Adjustable Devices on
Measuring Equipment whose setting affects the performance shall be
sealed to prevent tampering by unauthorized personnel.
Sub-Contracting or use of outside products and services Storage and
Handling
Page 3 of 9
9. CALIBRATION LAB EVALUATION MAJOR POINTS Adequate Records
Adequate Recall system Proper Cal. Intervals Proper Labeling Proper
Procedures Traceability Adequacy of Standards Cal. Quantity
Adequate Environmental Control 10. LABELLING Label shall include
Date or Usage time due for Recal ID of the Person who performed the
cal ID of the Agency Visibility Cal labels 11. CERTIFICATE OF
CALIBRATION Identifies the item being Calibrated and the
specification used for Calibration, includes a Traceability
statement, and certifies that the calibration was performed.
CALIBRATION PROCEDURE FOR PRESSURE INSTRUMENTS LIKE PRESSURE &
DP TRANSMITTERS, PRESSURE GAUGES, TRANSMITTERS, ETC. VISUAL
INSPECTION For any type of Physical Damage LEAK TEST Apply
full-scale pressure and check the leakage if any in the external
lines and fittings. EXERCISE MOVEMENT Three pressure cycles should
be applied to the uuc to exercise the movement DATA RECORDING
Appropriate pressure will be applied to the UUC and readings will
be recorded. Calibrate by starting at zero and continue applying
appropriate pressure increments to full range and back to zero.
CALIBRATION PROCEDURE FOR RTD/ T/ C Read the temp. Range and select
the set temp. at 10%, 50% & 90% of FS. Adjust the temp. Control
in the oil bath at the temp. Corresponding to 10% of FS Allow the
oil bath to stabilize for 30 minutes. Dip the RTD Thermocouple into
the oil bath and connect it to the multimeter. Record the
corresponding temp. The measured value as indicated value and set
temp. In the oil bath as true value. Repeat the steps no.2 to no.5
for other set temp. LEVEL
Page 4 of 9
CALIBRATION PROCEDURE FOR TEMP. INDICATORS & TRANSDUCERS
Feed 0 mV to the UUC (IUC) and observe the display. Find out the
corresponding mV from IPTS chart for the observed temperature (RT)
Select the temperature at 10%, 50% and 90% of FS and take the
corresponding mV. Subtract the mV as taken in step 2 from the mV
taken from step 3 Feed the mV (obtained from step 4) and observe
the temp. In the indicator. Record the display in the temp.
Indicator as indicated value and the selected temperature as the
true value. UUC IUC IPTS UNIT UNDER CALIBRATION INSTRUMENT UNDER
CALIBRATION INTERNATIONAL PRACTICAL TEMP. SCALE.
NOTE:
Page 5 of 9
CALIBRATIONCONVERSION : (GENERAL ENGG. UNITS)
Refer and use the conversion table Examples: 1) Convert 1000 mm
into inches Ans: 1000 x 0.03937 = 39.37 inches 2) Convert 500
litres into gallons (UK) Ans: 500 x 0.22009 = 110.045 gallons (UK)
3) Convert 75 pounds into kilograms Ans: 75 x .4536 = 34.02 kgs 4)
Convert 150 kgs into lbs Ans: 150 x 2.20462 = 330.693 lbs 5)
Convert 175 Cubicfeet into Cubic mtrs. Ans: 175 x 0.02832 = 4.956
Cubic mtrs. 6) Convert 150 Cubic inches into Cubic centimeters Ans:
150 x 16.3871 = 2458.065 Cubic cm 7) Convert 45 Kg/cm2 into psi
Ans: 45 x 14.22 = 639.9 psi 8)Convert 220 inches of water column
into mmHg Ans: 220 x 1.867 = 410.74 mmHg 9) Convert 175 m3/Hr into
l/Hr. Ans: 175 x 1000 = 175000l/Hr. 10) Convert 120 l/Hr. into
m3/min. Ans: 120 x 16.67 x 10 - 6 = 0.0020004 m3/min
Page 6 of 9
PROBLEM 1 :
Arrange the following in order from Highest to the lowest FLOW:
a) 10 gpm b) 10 l/min c) 10 l/Hr d) 10 Cfh e) 10 Cfm f) 10 m3 / min
g) 10 m3 / Hr.PROBLEM 2 :
Arrange the following in order from lowest to the highest
PRESSURE: a) 2 Kg/cm2 b) 2 Bar c) 14.7 psig d) 500 mmHg e) 1000 in
H2O f) 2000 mmH20 g) 300 in Hg ANSWERS: PROBLEM 1 : ( ANSWER ) f ,
e , g , a , b , d , c
Arrange the following in order from Highest to the lowest FLOW:
a) 10 gpm b) 10 l/min c) 10 l/Hr d) 10 Cfh e) 10 Cfm f) 10 m3 / min
g) 10 m3 / Hr 10 gpm 10 x 0.264 = 2.64 gpm 10 x 0.0044 = 0.044 gpm
10 x 0.1247 = 1.247 gpm 10 x 7.481 = 74.81 gpm 10 x 264.2 = 2642
gpm 10 x 4.403 = 44.03 gpm
PROBLEM 2 : ( ANSWER ) f , d , c , a , b , e , g Arrange the
following in order from lowest to the highest PRESSURE: a) 2 Kg/cm2
2 kg / cm2 b) 2 Bar 2 x 1.02 = 2.04 kg/cm2 c) 14.7 psig 14.7 x
0.07031 = 1.033 kg/cm2 d) 500 mmHg 500 x 1.36 x 10-3 = 0.68 kg/cm2
e) 1000 in H2O 1000 x 2.538 x 10-3 = 2.538 kg/cm2 f) 2000 mmH20 2 x
0.0999 = 0.1998 kg/cm2 g) 300 in Hg 300 x 25.39998 x 1.36 x 10-3 =
10.363 kg/cm2
Page 7 of 9
Refer and use the conversion table:
Temperature conversion Fahrenheit AND CentigradeC = ( F 32 ) x
5/9 F = ( C x 9/5 ) + 32 Examples: Convert 176 deg F into deg C C =
( 149 32 ) x 5/9 = 117x 5/9 = 65 Convert 46 deg C into deg F F = (
46 x 9/5 ) +32 = ( 9.2 x 9 ) + 32 = 82.8 + 32 = 114.8
Refer and use the conversion table: PAGE 5 OF 33 F 338 * C 640.4
170.44
Temperature Conversion
( Thermocouple )
T / C - TYPE K ( Ni Cr / Ni Al ) ( Chromel Alumel ) Consider the
room temp as 32 0 C Use the table For 320C . 1.285 mV Assume you
are measuring the temp. of the bath and the indicator shows 100 0
C. But when you measure directly the mV across the T / C head, you
will get 2.810 mV Use the table For 1000 C. 4.095 2.810 + 1.285
4.095
Page 8 of 9
TEMPERATURE CONVERSION ( THERMOCOUPLE ) T / C - TYPE K ( Ni Cr /
Ni Al ) ( Chromel Alumel )0
Consider the room temp as 32
C
Assume you are measuring the temp. of the bath and the indicator
shows 150 0 C. But when you measure directly the mV across the T /
C head, How much it will show ? Ans : 6.137 1.285 4.852 TEMPERATURE
CONVERSION ( R T D ) Pt 100 .. The Resistance is 100 OHMS for 0 Use
the table Find out the resistance value for 65 Ans : 125.15 or
125.16 Find out the temperature if the resistance value is 112.735
Ans : 330 0 0
C
C
C
Page 9 of 9
LEVEL MEASUREMENTINDUSTRIAL LEVEL MEASUREMENTThe Vast amount of
water used by industry, let alone all the solvents, chemicals, and
other liquids that are necessary for material processing, make the
measurement of liquid level essential to modern manufacturing.
There are two ways of measuring level: directly by using the
varying level of the liquid as a means of obtaining the
measurement; and indirectly, by using a variable, which change with
the liquid level, to actuate the measuring mechanism.INDUSTRIAL
LEVEL MEASUREMENTS
MEASUREMENT METHODS
DIRECT METHOD
INDIRECT METHOD
VISUAL LEVEL SENSOR
FLOAT TYPE LEVEL SENSOR
BUOYANT FORCE LEVEL MEASUREMENT
HEAD PRESSURE MEASUREMENT
ELECTRICAL LEVEL MEASUREMENT
DIP STICK SIGHT GLASS GAUGE GLASS
DISPLACEMENT TYPE LEVEL SENSOR
1. GUAGE PRESSURE MEASUREMENT 2. AIR BUBBLE PURGE SYSTEM 3.
DIFFERENTIAL PRESSURE MEASUREMENT
1. CAPACITANCE 2. CONDUCTIVITY 3. SONIC/ ULTRASONIC 4. RADAR
DIRECT LIQUID LEVEL MEASUREMENT
FIG -1 A bob weight and measuring tape provide measurement The
most simple and direct method of measuring liquid level
Dip stick level
BOB AND TAPE The simplest of the direct devices for liquid level
measurement is the bob and tape (fig.1). All you need is a bob (or
weight) suspended from a tape marked in feet and inches. The bob is
lowered to the bottom of the vessel containing the liquid, and the
level is determined by noting the point on the tape reached by the
liquid. The actual reading is made after the tape is removed from
the vessel. Obviously this method isnt suited to continuous
measurement.
--------------------------------------------------------------------------------------------------------------------Page
1 of 24
HIGH PRESSURE GAUGE GLASS (REFLEX GLASS)SIGHT GLASS Another
direct means of liquid level measurement is the sight glass (Fig
2). This consists of a graduated glass Tube mounted on the side of
the vessel. As the level of the liquid in the vessel changes, so
does the level of the liquid in the glass tube. Measurement is a
simple matter of reading the position of liquid level on the scale
of the sight glass tube.
FIG -2 As the level of the liquid in the vessel rises or falls,
so does the level of the liquid in the sight glass.GROOVES
REFLUX GLASS
FLOATS There are many kinds of float-operated mechanisms for
continuous direct liquid level measurement. The Primary device is a
float that by reason of its buoyancy will follow the changing level
of the liquid, and a mechanism that will transfer the float action
to a pointer (Fig 3). The float most familiar to you is the hollow
metal sphere; but cylinder-shaped ceramic floats and disc-shaped
floats of synthetic materials are also used. The float is usually
attached to a cable, which around a pulley or drum to which the
indicating pointer is attached. The movement of the float is thus
transferred to the pointer, which indicates the liquid level on an
appropriate scale.
FIG -3 The buoyancy of the float permits it to be immersed in
the liquid, and its movement is transmitted to the indicator as it
follows the changing liquid level.
In another kind of float-operated instrument, the float is
attached to a shaft, which transfers the motion of the float to an
indicator (Fig. 4). This type doesnt permit a wide range of level
measurement, but it does have mechanical advantages that make it
excellent for control and transmitter application.
--------------------------------------------------------------------------------------------------------------------Page
2 of 24
FIG -4 When the level of the liquid is low, the ball float will
be at position A. As the tank fills, the flow rises with the level
of the liquid to position B and its movement rotates the shaft
which operates the pointer.
FIG- 5 Float cable weight level indicator arrangement
Another variation uses the float to move a magnet (Fig. 6). As
this magnet moves, it attracts a following magnet connected to the
indicator, thus providing a reading of liquid level measurement.FIG
6 The doughnut-shaped float with magnets in it rises and falls with
the level of liquid. The follower magnet, suspended by cable in the
guide tube, rises and falls to maintain a corresponding position
with the float, and thus moves the cable to the indicator.
--------------------------------------------------------------------------------------------------------------------Page
3 of 24
FIG- 7 Magnetic float devices The magnetic float sensor may be
used to determine the level of single material in the vessel or to
determine the position of an interface between two materials of
different densities. For example oil will float on top of water. If
oil and water were both in this vessel the float could be
constructed so that it would sink in oil and float on the
water.
FIG 8 Magnetic type float Devices
--------------------------------------------------------------------------------------------------------------------Page
4 of 24
The displacer (Fig. 9) is similar in action to the buoyant float
described above, with the exception that its movement is more
restricted. With changes in liquid level, more or less of the
displacer is covered by the liquid. The more the displacer is
submerged, the greater is the force created by the displacer
because of its buoyancy. This force transferred through a twisting
or bending shaft to a pneumatic or Electronic system. For every new
liquid level position, there is a new force on the shaft, causing
it to assume a new position. The pneumatic or Electronic system is
so arranged that for each new shaft position there is a new signal
or indication. The displacer float has the advantage of being more
sensitive to small level changes than the buoyant float and less
subject to mechanical friction.
FIG 9 In The lower drawing the displacer, which weighs 5 lbs.,
weighs only 2 lbs. When the water level is at 7 inches, the changes
in weight are converted into Torque (See upper illustration). Which
operates the pneumatic system to provide readings on the indicator.
(Mason- Neilan Div of Worthington Corp.)
FIG- 10 Displacement Level Sensor
--------------------------------------------------------------------------------------------------------------------Page
5 of 24
UNDERSTANDING OF BUOYANCY AND DISPLACER FOR INDIRECT LEVEL
MEASUREMENTDISPLACEMENT DEVICES The displacement type level devices
are commonly used for continuous level measurement. It works on the
buoyancy principle of Archimedes, which states that a body immersed
in a liquid will be buoyed up a force equal to the weight of the
liquid displaced. The displacer body has a cylindrical shape. As a
result for each equal increment of submersion depth, an equal
increment of buoyancy change will result. This gives a linear,
proportional relationship, which is desirable. The effects of
buoyancy are illustrated in the fig 11. Although the vessels shown
are open in the atmosphere the principle desired applies to the
closed tank as well. The displacer is suspended from a scale that
indicates its weight at various depths of immersion. In the first
figure 11 A the displacer is completely out of the liquid and the
scale supports its full weight. As the scale indicates, it weighs
10 kg when suspended in air. When the level of the tank has risen
to immerse about half the displacer, the weight of the displacer is
approximately 6 kg. The displacers loss in weight is equal to the
weight of the volume of the liquid displaced. As the water
increases to fully immerse the displacer. The weight of the
displacer decreases, the displacer now weighs approximately 2 kg.
So, when the water level changes from 0 to 100%, the weight of the
displacer also changes proportionally. BOUYANCY EFFECT ON THE
DISPLACER Two important points to be considered here are: 1. When
the liquid level is lowered to completely uncover the displacer,
the displacer can no longer measure level. Any changes in level
below the lower end of the displacer will not be measured. 2. The
same is true when the liquid level rises to the top of the
displacer. Then, any changes in liquid level above the top of the
displacer will not be detected. The main difference between a
displacer and the float operated device are: The displacer movement
is very little compared to the float which rises or falls as per
the level. Therefore the displacer loses weight and the float gains
in height as the level rises in the tank. The displacer can also be
used for interface level measurement whereas float can only be used
for measuring the level of liquids. When the displacer is attached
to a torque tube by linkage the equivalent torque variations due to
the buoyancy effect on the displacer operates a pneumatic or
Electronic Transducer / Transmitter. This is the normal
transmission of level in closed process vessels, distillation
columns, intermediate storage tanks etc.
FIG- 11
--------------------------------------------------------------------------------------------------------------------Page
6 of 24
INDIRECT LIQUID LEVEL MEASUREMENT
There are several types of indirect level measuring devices that
are operated by Pressure. Any rise in the level causes an increase
of pressure, which can be measure by the gauge. The gauge scale is
marked in units of level measurement (feet or inches).FIG- 12 B
Liquid naturally increases. This
FIG- 12 A As the tank fills, the pressure of the
Hydrostatic level measurement in Increase of pressure can be
read on the Gauge in feet and inches of level.
an open tank
FIG- 12 C Closed tank level measurement
FIG- 12 D The pressure of air in the air trap is Expressed on
the scale in units of level
If the nature of the liquid prevents its being allowed to enter
the pressure gauge, a transmitting fluid (such as air, which is the
cheapest and handiest) must be used between the liquid and the
gauge. The air trap and the diaphragm box provide a means of
accomplishing this. The air trap consists of a box, which is
lowered into the liquid (Fig.12 D). As the liquid rises, the
pressure on the air trapped in the box increases. This air pressure
is piped through tubing to the pressure gauge, which has a scale on
which the level can be read.The diaphragm box (Fig 13), like the
air trap, transmits air pressure to a gauge, but in this case the
air is trapped inside it by a flexible diaphragm covering the
bottom of then box. As the level of the liquid rises, the pressure
on the diaphragm increases. This pressure acts on the air in the
closed system and is piped to the pressure gauge where a reading
can be taken.FIG- 13: Deflection of the flexible diaphragm by
compression, as the liquid level rises, causes the gauge to
respond.
--------------------------------------------------------------------------------------------------------------------Page
7 of 24
AIR BUBBLER SYSTEM
FIG- 14 Air Bubbler System of Level Measurement
The Air bubbler system is a system of indirect level measurement
especially suitable for liquids that are corrosive, viscous, or
contain suspended solids. BUBBLER TYPE LEVEL MEASUREMENT
FIG- 15 The air pressure to the bubbler pipe is minutely in
excess of the liquid pressure in the vessel, so that the air
pressure indicated is a measure of the level in the tank
The pressure caused by the liquid column is used in the bubbler
method of level measurement (Fig.15). A pipe is installed
vertically in the vessel with its open end at the zero level. The
other end of the pipe is connected to a regulated air supply and to
a pressure gauge. To make a level measurement the air supply is
adjusted so that the pressure is slightly higher than the pressure
due to the height of the liquid. This is accomplished by regulating
the air pressure until bubbles can be seen slowly leaving the open
end of the pipe. The gauge then measures the air pressure needed to
overcome the pressure of the liquid. The gauge is calibrated in
feet or inches of level. The methods described above can only be
used when the vessel containing the liquid is open to the
atmosphere. When the liquid is in a pressure vessel, the liquid
column pressure cant be used unless the vessel pressure is balanced
out. This is done through the use of differential pressure meters
(Fig. 16). Connections are made to the vessel at top and bottom,
and to the two column of the differential pressure meter. The top
connection is made to the low made to the low pressure column of
the meter, and the bottom connection to the high pressure column.
In this way the pressure in the vessel is balanced out, since it is
fed both column of the meter. The difference in pressure detected
by the meter will be due then only to the changing level of the
liquid.FIG- 16 When the liquid is in a closed vessel, level can be
measured using a differential pressure manometer.
--------------------------------------------------------------------------------------------------------------------Page
8 of 24
Liquid level can be measured using radioactivity or ultrasonic.
For continuous level measurement by radioactivity, one or more
radioactive source are placed on one side of a vessel with a
pick-up on other side (Fig. 17). As the level of the liquid
changes, it absorbs more or less of the radioactive energy received
by the pick-up, which is a special electronic amplifier designed to
produce enough electrical meter. The meter scale is marked in level
units-inches or feet.
FIG- 17 Radioactive system Measurement
of
Level
The ultrasonic method operates on the sonar principle (Fig. 18).
Sound waves are sent to the surface of the liquid and are reflected
back to the receiving unit. Changes in level are accurately
measured by detecting the time it takes for the waves to travel to
the surface and back to the receiver. The longer the time required
the further away is the liquid surface, providing a measurement of
how much the level has changed. These systems have been described
very simply here. Actually they are highly complicated in both
design and installation. These systems have been described very
simply here. Actually they are highly complicated in both design
and installation.
FIG- 18 Sound waves reflected back from the surface of the
liquid to the receiving unit can provide an accurate measurement of
liquid level.
Another method of determining the level of liquid materials is
to weigh the entire vessel, since the weight changes as the level
of the material varies. The vessel may be weighed on mechanical
scale (Fig.19); or it may be weighed electrically using load cells
(Fig.20). Load Cells are Specially constructed mechanical units
containing strain gauges, which provide a measurable electrical
output proportional to the stress applied by the weight of the
vessel on the load Cells. As the pressure on the cell due to the
weight of the vessel changes, the electrical resistance of the
strain gauge changes. The strain gauge is connected into a bridge
circuit containing an electrical meter graduated in unit of level
measurement. It should be noted that the weighing method is
accurate only if the density and particle size of the substance
being weighed are uniform and the moisture content remains
constant. The change in weight must be due entirely to the change
in level.
FIG- 19 Scale on which the vessel and its Liquid content weighed
mechanically
FIG- 20 Vessel weighed electrically using load cells
--------------------------------------------------------------------------------------------------------------------Page
9 of 24
LEVEL MEASUREMENT BY D.P. TRANSMITTERS
To determine the level of a liquid in an open tank, connect the
high side of the Transmitter to a tap at the bottom of the tank.
Vent the low side of the transmitter to the atmosphere. The
pressure represents the height of the liquid in the tank multiplied
by the specific gravity of the liquid; therefore, the output of the
transmitter will be proportional to the liquid level above the
transmitter. If the tank is located above the transmitter, the zero
must be readjusted to elevate the range. To determine the liquid
level in a closed tank, steps must be taken to compensate for tank
pressure generated above the top of the liquid and the top of the
tank. This is accomplished by placing a tap at the top of the tank
and connecting it to the low side of the transmitter. When this has
been done, the differential pressure measured by the Transmitter is
proportional to the height of the liquid in the tank multiplied by
the specific gravity of the liquid. If the liquid has a vapor that
could condense in the piping connected to the top of the tank, the
piping should be filled with the measured liquid. This will exert a
head pressure on the low side of the transmitter and must be zeroed
out. D.P. Transmitter with process connection for clean liquids
FIG- 21 DP Transmitters
--------------------------------------------------------------------------------------------------------------------Page
10 of 24
THREE APPLICATIONS OF LIQUID LEVEL MEASUREMENT WITH DP
TRANSMITTERAPPLICATION 1 LIQUID LEVEL IN OPEN TANKSPAN = H1x G1, in
inches w.g. if H1 is in inches G1= specific gravity of the process
liquid Lower Range Value = (H2 x G1), in inches w.g. if H2 is in
inches Upper Range Value = Lower Range Value + Span
FIG -22 APPLICATION 2 LIQUID LEVEL IN CLOSED TANK WITHOUT
CONDENSABLE VAPOURSSPAN = H1x G1, in inches w.g. if H1 is in inches
G1= specific gravity of the process liquid Lower Range Value = (H2
x G1), in inches w.g. if H2 is in inches Upper Range Value = Lower
Range Value + Span
FIG -23 APPLICATION 3 LIQUID LEVEL IN CLOSED TANK WITH
CONDENSABLE VAPOURS
SPAN = H1x G1, in inches w.g. if H1 is in inches Lower Range
Value = (H2 x G1) (H4 x Gw), in inches w.g. if H2 & H4 is in
inches Upper Range Value = Lower Range Value + Span G1= specific
gravity of the process liquid Gw= specific gravity of liquid in wet
leg
FIG -24
--------------------------------------------------------------------------------------------------------------------Page
11 of 24
ZERO SUPPRESSION/ ZERO ELEVATION TECHNIQUES FOR D.P. TYPE LEVEL
MEASUREMENTS1. ZERO SUPPRESSION Adjusting the Zero Output Signal to
produce to desired measurement. Usually used in Level Measurement
to Counteract the Zero Elevation caused by a Wet leg. 2. ZERO
ELEVATION Adjusting the Zero Output Signal to raise the Zero to a
higher starting point. Usually used in Level Measurement for
starting measurement above the Vessel Connection Point.
CALIBRATION POINTS TO BE NOTED FOR LEVEL MEASUREMENTA- VENTED /
OPEN TANK B- PRESSURISED CLOSE TANK
CALIBRATION POINTS TO BE NOTED FOR LEVEL MEASUREMENT WITH
DIAPHRAGM & REMOTE SEAL TYPE TRANSMITTERA- VENTED / OPEN TANK
B- VENTED / OPEN TANK
CALIBRATION POINTS TO BE NOTED FOR LEVEL MEASUREMENT WITH
DIAPHRAGM & REMOTE SEAL TYPE TRANSMITTERA- VENTED / OPEN TANK
B- VENTED / OPEN TANK
--------------------------------------------------------------------------------------------------------------------Page
12 of 24
CALIBRATION POINTS TO BE NOTED FOR LEVEL MEASUREMENT WITH
DIAPHRAGM & REMOTE SEAL TYPE TRANSMITTERA- PRESSURISED CLOSE
TANK B- PRESSURISED CLOSE TANK
FIG- 31
FIG- 32
CALIBRATION POINTS TO BE NOTED FOR LEVEL MEASUREMENT WITH
DIAPHRAGM & REMOTE SEAL TYPE TRANSMITTER
FIG- 33
ELECTRICAL / ELECTRONIC METHODS OF LEVEL MEASUREMENTA)
CONTINUOUS LEVEL MEASUREMENT With continuous measurement the level
is detected and converted into an Electronic / Pneumatic Signal.
Continuous measurements can be carried for all liquid and solids.
Capacitance, hydrostatic, pulse-echo (Ultrasonic Type), pulse-radar
and electromechanical principles as well as pressure measuring
sensors can be used. B) LEVEL DETECTION FOR LEVEL SWITCHING &
OVERFILL PROTECTION. Level can be detected at fixed points and
converted into switched outputs Level detection can be done for all
liquids and solids. This type of level switches work on
capacitance, microwave, radioactive, vibration and conductive
principles. The switched output can either be used for stopping and
starting filling systems (Conveyor belts, pumps, pneumatic
conveyors) or for overfill protection.
--------------------------------------------------------------------------------------------------------------------Page
13 of 24
SUMMARY OF BASIC ELECTRICAL / ELECTRONIC / LEVEL MEASURING
PRINCIPLES 1) Capacitance Type Level Measurement. 2) Pulse-Echo or
Ultrasonic Type Measurement 3) Antenna or Radar Type Level
Measurement. 4) Microwave Type Detection. 5) Electro-mechanical
Type Level Detection. 6) Vibration Level Switch. 7) Conductive
Level Switch. 1. CAPACITANCE TYPE LEVEL MEASUREMENT # MEASURING
PRINCIPLE The metal vessel wall & the measuring electrode forms
a capacitor. The product acts as the dielectric and changes the
capacitance as the level changes. An oscillator in the housing of
the electrode converts the capacitance value into a level
proportional DC current or a switched output .This universal
measuring principle is used for continuous level Measurement and
solids- even under arduous conditions.
A FIG- 34
B
C
D
# APPLICATIONS OF CAPACITANCE LEVEL MEASUREMENT 1. Capacitance
type level measurement can be used for continuous level measurement
or Hi Lo level switching for all products including solids. 2. One
of the unique capabilities is to indicate the interface between two
immiscible liquids, each having a different dielectric const.
Oil/water interface is a common application. # LIMITATIONS 1.
Calibration may be time consuming. 2. Affected by change in
dielectric constant and temperature of the material and thus
requires temperature compensation. 3. Conductive residue coating
will affect performance. 2. PULSE-ECHO OR ULTRASONIC TYPE
MEASUREMENT # MEASURING PRINCIPLE Sonic and ultrasonic sensors
consist of a transmitter that converts electrical energy into
acoustical energy and a receiver that converts acoustical energy
into electrical energy. The transmitted and return time of sonic
pulse is relayed electronically and converted to level indication.
These devices are non-contacting, reliable and accurate, no moving
parts, unaffected by changes in density, conductivity and
composition. # LIMITATIONS 1. Cannot be used for foam as the signal
may be absorbed by foam. 2. Will not work in vacuum. 3. Various
factors like instrument accuracy, vapor concentration, pressure,
temperature, relative humidity, and pressure of other gases/vapors
may affect the performance
--------------------------------------------------------------------------------------------------------------------Page
14 of 24
A
B
FIG- 35 C ULTRASONIC LEVEL MEASUREMENT IN OPEN CHANNEL FLOW
MEASURING APPLICATIONS
3. ANTENNA / GUIDED WAVE RADAR TYPE LEVEL MEASUREMENT#
TECHNOLOGY/ MEASURING PRINCIPLE Guided Wave Radar is based upon the
technology of TDR (Time Domain Reflectometry). TDR utilizes pulses
of electromagnetic energy, which are transmitted down a probe. When
a pulse reaches a liquid surface that has a higher dielectric than
air/vapor in which it is traveling, the pulse is reflected. An
Ultra high-speed timing circuit precisely measures the transit time
and provides an accurate measure of the liquid level. The
measurement requires complete mapping of the inner surfaces of the
vessel in empty conditions. The information is stored in the memory
and reflections form protrusions, shafts, agitators etc. are
compensated. Thus true level is measured. This technology is fairly
new and costly. # APPLICATIONS This measuring principle provides
non-contact continuous level measurement for liquids, solids and
slurries with high pressures and temperatures, vacuum, dust, vapour
and aggressive and toxic products
A B
DIELECTRIC ANTENNA
FIG- 36
HORN ANTENNA
--------------------------------------------------------------------------------------------------------------------Page
15 of 24
4. MICROWAVE SWITCHING
TYPE
NON-CONTACT
LEVEL
DETECTION
/
LEVEL
# MEASURING PRINCIPLE The operation of the microwave barrier is
similar to a light filter. A transmitter emits microwaves with a
frequency of 5.8 GHz to a receiver. If product is between
transmitter and receiver, the microwaves are absorbed and a damped
signal is received. The receiver signals this by a switching
command. The detection principle functions with all liquids and
solids, which reflect or absorb microwaves.
FIG- 37 A
B
C
# APPLICATIONS Non-Contact type level switching/detection in
products like oil, coal, stones and foodstuffs. 5A.
ELECTRO-MECHANICAL TYPE LEVEL DETECTION/ LEVEL SWITCHING #
MEASURING PRINCIPLE The electromechanical measuring principle is
ideal for many applications with its rugged construction. A weight
is wound off electro-mechanically on a cable. When the weight
touches the measured product, the weight is rewound to the initial
position. The measured cable length is a measure of the level. #
USER ADVANTAGES 1. Vessel heights up to 40 m 2. Accuracy better
than 0.1% 3. Adapts to product types by choice of sensing weights.
4. Full operation even with dust formation. 5. Complete separation
between cable pulley and control mechanism. 6. Easy
installation.
FIG- 38
# APPLICATIONS It is suitable for level measurement of fine and
coarse solids as well as liquids, but also for the measurement of
solids in water. Main applications are in large vessels where
solutions with other techniques are expensive or physically not
possible. # TYPICAL PRODUCTS Ore, coal, stones, sinter, plastic
powder, lime, cement, raw flour, cereals, sludge and sewage water.
--------------------------------------------------------------------------------------------------------------------Page
16 of 24
5B. MAGNETIC TILT LEVEL SWITCH # WORKING PRINCIPLE Float mounted
at one end of rigid rod moves with change in level. Magnetic
capsule at other end of rod moves accordingly within fixed limits.
Hermitically sealed switch contacts across the stainless steel case
change accordingly.FIG- 39 Nomenclature 1. Float Assy. 2. Magnet
Assy. 3. Mounting flange. 4. Housing 5. Switch Assy. 6. Terminal
Assy. 7. Cable Gland 8. Cover.
# APPLICATIONS HI-LO Level signals for Alarm Annunciation,
Safety Interlock circuits, Automatic Pump Control, Solenoid Valve
control, Prevention of tank overflows, Pump safeguard against dry
running 5C. ROTATING PADDLE LEVEL SWITCH # WORKING PRINCIPLE
Operation centers around a low torque, slow speed synchronous
motor. Absence of dry materials allows the motor to turn the
paddle. Presence of dry material tends to stall the paddle and the
motor. The resultant torque actuates a snap-action switch (es)
which in turn controls audible and visual signals and/or starts and
stops machinery such as conveyers, elevators, feeders, etc. Mounts
on top or side of bin.
FIG- 40 Rotating paddle level switch
# APPLICATIONS
1. Eliminates bin overflow, empty bins, clogged conveyors,
choked elevators and resultant damage and waste. 2. For Chemical,
food, mining, plastics, ceramics and other industries.
--------------------------------------------------------------------------------------------------------------------Page
17 of 24
5D. TILT TYPE LEVEL SWITCH# WORKING PRINCIPLE Tilt switches are
either of mercury or micro-switch type, mounted so as to hang from
the top of the storage bin. When the tilt switch hangs freely,
there is no contact between the tilt switch and control relay. As
soon as the level reaches the tilt switch, the vertical angle
changes, causing the contact to close. This creates a closed
circuit with the control relay, which activates a solenoid valve,
an alarm relay, or a motor control start/stop command.
FIG- 41 Tilt Type switches level
6A. VIBRATION TYPE LEVEL SWITCHES# MEASURING PRINCIPLE Vibration
probes for solids operate with piezoelectrically generated
vibration which is damped when the rod is covered by the product.
The integral electronics detects this damping and triggers a
switching command.
FIG- 42 Vibrating different versions. probes in mounting
# APPLICATIONS Powders and granules above a density of 0, 03 g /
cm can be detected, e.g. styropore, cement, cereals, flour, plastic
Granules etc.
--------------------------------------------------------------------------------------------------------------------Page
18 of 24
6B. TUNING FORK LEVEL DETECTION / LEVEL SWITCHING# MEASURING
PRINCIPLE The tuning fork is energized by a piezoelectric element
and vibrates at its resonant frequency of approx. 380 Hz. A second
piezoelectric element detects this frequency which is than passed
to the integral electronics. If the fork is covered by the product,
the frequency changes, and a switching command is triggered.
FIG- 43 Installation options
7. CONDUCTIVITY TYPE LEVEL SWITCHES# MEASURING PRINCIPLE When
the electrode is covered by a conductive product, a measuring
circuit is closed and a switching signal is triggered. The metallic
vessel itself is the reference electrode. In plastic vessels a
version with integral reference electrode is used. The position of
the switch point is simply determined by the electrode length. Rod
and multiple rod electrodes as well as cable and multiple cable
electrodes are available.
FIG- 44 Working Principle & Multiple rod electrode for
Min./Max. Control
--------------------------------------------------------------------------------------------------------------------Page
19 of 24
A BRIEF COMPARISON OF VARIOUS LEVEL SENSORS
--------------------------------------------------------------------------------------------------------------------Page
20 of 24
GLOSSARY OF TERMS TO BE UNDERSTOOD FOR INDUSTRIAL LEVEL
MEASUREMENTACCURACY The closeness of an Indication of reading of a
measurement device to the actual value of the quantity being
measured usually expressed as percent of full scale output or
reading. ATMOSPHERIC PRESSURE The barometric reading of pressure
exerted by the atmosphere. At sea level 14.7 lb per sq. in. or
29.92 in. of mercury. BUBBLE TUBE A length of pipe or tubing placed
in a vessel at a specified depth. To transport a gas injected into
a liquid to measure level from a hydrostatic Head. BUOYANCY The
tendency of the fluid to lift any object submerged in the body of
the fluid; the amount of force applied to the body equals the
product of fluid density and volume of fluid displaced. DENSITY 1.
The mass of a unit volume of a liquid at a specified temperature.
Units shall be stated as kg / m. 2. A physical property of
materials measured as mass per unit volume. DIELECTRIC CONSTANT A
material characteristic expressed as the capacitance between two
plates when the intervening space is filled with a given insulating
material divided by the capacitance of the same plate arrangement
when the space is filled with air or evacuated. DIFFERENTIAL
PRESSURE TYPE LEVEL METER/ TRANSMITTER Any of several devices
designed to measure the head of the liquid in a tank above some
minimum level and produce an indication proportional to this value;
alternately, the head below some maximum level can be measured and
similarly displayed. DISPLACER TYPE LIQUID LEVEL DETECTOR A device
for determining a liquid level by means of force measurements on
cylindrical element partly submerged in the liquid in a vessel; as
the level in the vessel rises and falls, the displacement (buoyant)
force on the cylinder varies and is measured by the lever system,
torque tube or other force measurement device. FLOAT Any component
having positive buoyancy for example, a Hollow watertight body that
rests on the surface of the liquid, partly or completely supported
by buoyant forces. FLOAT CHAMBER A vessel in which a float
regulates the liquid level. FOAMING Any of various methods of
introducing air or gas into a liquid or solid material to produce
foam. The continuous formation of bubbles which have sufficiently
high surface tension to remain as bubbles beyond the disengaging
surface.
--------------------------------------------------------------------------------------------------------------------Page
21 of 24
GAUGE GLASS A glass or plastic tubes for measuring liquid level
in a tank or pressure vessel, usually by direct sight; it is
usually connected directly to the vessel through suitable fitting
and shut off valve. GAUGE PRESSURE 1. Pressure measured relative to
ambient pressure. 2. The difference between the local absolute
pressure of the system and the atmospheric pressure at the place of
measurement. 3. Static pressure is indicated on a gauge.
HYDROSTATIC HEAD The pressure created by a height of a liquid above
a given point. MAGNETIC FLOAT GAUGE Any of several designs of
liquid level indicator that use a magnetic float to position a
pointer. PURGE 1. Increasing the sample flow above normal for the
purpose of replacing current sample-line fluid or removing
deposited or trapped materials. 2. To cause a liquid or gas to flow
from an independent source into the impulse pipe. TORQUE A rotary
force, such as that applied by a rotating shaft at any point on its
axis of rotation. WET LEG The liquid-filled low-pressure side of
the impulse line in a differential pressure level measuring system.
ZERO ELEVATION Adjusting the zero output signals to raise the zero
to a higher starting point. Usually used in level measurement for
starting measurement above the vessel connection point. ZERO
SUPPRESSION Adjusting the zero output signals to produce the
desired Measurement. Usually used in level measurement to
counteract the zero elevation caused by a wet leg.
--------------------------------------------------------------------------------------------------------------------Page
22 of 24
CALCULATION PROCEEDURESA) OPEN VESSEL BOTTOM MOUNTED TRANSMITTER
& CLOSED TANK DRY LEG METHOD20
X
ZERO SUPPRESSION
Y HP
LT LP4 90 540 WC
X- VERTICAL DISTANCE BETWEEN THE MINIMUM & MAXIMUM
MEASURABLE LEVEL
= 500 mmH2O = 100 mmH2O = 0.9
Y- VERTICAL DISTANCE BETWEEN TRANSMITTER DATUM LINE &
MINIMUM MEASURABLE LEVEL SG - SPECIFIC GRAVIY OF THE FLUID H
MAXIMUM HEAD PRESSURE TO BE MEASURED IN mmH2O E HEAD PRESSURE
PRODUCED BY Y EXPRESSED IN mmH2O RANGE = E TO E+H H = X x SG = 500
x 0.9 E = Y x SG = 100 x 0.9 = 450 mmH2O = 90 mmH2O
RANGE = E TO E+H = 90 TO (90 + 450) = 90 TO 540 mmH2O NOTE IN
CLOSED TANK DRY LEG METHOD, IF THE GAS ABOVE THE LIQUID DOES NOT
CONDENSE & THE PIPING FOR THE LOW SIDE OF THE TRANSMITTER WILL
REMAIN EMPTY. CALCULATIONS FOR DETERMINING THE RANGE WILL BE THE
SAME AS SHOWN FOR OPEN VESSEL BOTTOM MOUNTED TRANSMITTER.
-----------------------------------------------------------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------Page
23 of 24
CALCULATION PROCEEDURESB) CLOSED TANK WET LEG METHOD20 ZERO
ELEVATION
X Z Y HP LT LP- 610 WC 90 -110 4
X- VERTICAL DISTANCE BETWEEN THE MINIMUM & MAXIMUM
MEASURABLE LEVEL Y- VERTICAL DISTANCE BETWEEN TRANSMITTER DATUM
LINE & MINIMUM MEASURABLE LEVEL Z- VERTICAL DISTANCE BETWEEN
TOP OF LIQUID IN WETLEG & TRANSMITTER DATUM LINE SG1 - SPECIFIC
GRAVIY OF THE FLUID SG2 - SPECIFIC GRAVIY OF THE FLUID IN THE WET
LEG H MAXIMUM HEAD PRESSURE TO BE MEASURED IN mmH2O E HEAD PRESSURE
PRODUCED BY Y EXPRESSED IN mmH2O S HEAD PRESSURE PRODUCED BY Z
EXPRESSED IN mmH2O RANGE = (E-S) TO (H+ (E-S)) H = X x SG1 E = Y x
SG1 = = 500 x 1 50 x 1 600 x 1.1 = 500 mmH2O = 50 mmH2O = 660
mmH2O
= 500 mmH2O
= 50 mmH2O
= 600 mmH2O = 1.0 = 1.1
S = Z x SG21 =
RANGE = (E-S) TO (H+ (E-S)) = [(50-660) TO 500 + (50-660)] =
[(-610) TO 500 + (-610)] = -610 TO -110 mmH2O
--------------------------------------------------------------------------------------------------------------------Page
24 of 24
FLOW MEASUREMENTFLOW MEASURMENT BASICS
Flow is another very important process variable that has to be
measured and controlled. To understand the basic principles of flow
measurement one should be familiar with the relationship between
fluid flow and Pressure, Temperature, Viscosity, Density The two
basic properties - DENSITY & VISCOSITY play an important role
in flow measurement. Density applies to fluids in static phase.
Viscosity applies to fluids in motion.DENSITY: In simple terms,
Density is a measure of closeness of molecules in a substance.
Density is defined as Mass per unit Volume. d=m/v where d =
density, m = mass, v = volumeSPECIFIC GRAVITY
d=lbs/ft3
Another term commonly used to express density of fluid is
SPECIFIC GRAVITY (SG) SG of a liquid=density of liquid / density of
water at standard conditions. SG of a gas = density of gas /
density of air at standard conditions.VISCOSITY
Viscosity is the property that determines how freely fluids
flow. The viscosity of a fluid refers to its physical resistance to
flow. Fluids have various degrees of viscosity. Such variations
results from internal friction between particles of the substance.
A substance with a higher viscosity has a resistance to flow. For
example, two substances with different viscosities are oil and
water. Water pours freely while oil pours more slowly. Molasses is
more viscous than water, and water much more viscous than gas.
Viscosity contributes to laminar or turbulent flow characteristics.
Laminar flow is highly effected by viscosity than turbulent flow.
Viscosity reduces with the increase of temperature. For example,
when molasses is heated its viscosity will decrease. There are
several viscosity units, the most widely used being the centipoise.
The Viscosity of water at 68F is 1.0 centipoise. The viscosity of
kerosene at 68F is 2.0 centipoises. Viscosity () can be expressed
as : -1 -1 = lb . ft .s
Also one should know and be able to define the following general
flow measurement terms :Laminar flow Incompressible flow Mass flow
Static pressure Working pressure Turbulent flow Transitional flow
Compressible flow Steady flow Unsteady flow Pulsating flow Dynamic
pressure Stagnation pressure Differential pressure Pressure
loss
Page 1 of 43
VARIOUS FLOW MEASUREMENT TERMSLAMINAR FLOW :
Laminar flow is a flow characterized by the tendency of the
fluid to remain in thin parallel layers. Laminar flow occurs when
the average velocity is slow. The layers are fast moving in the
center and become slower on the outer edges of the stream. In
laminar flow fluid particles move along in parallel paths. The
laminar flow appears as several streams of liquid flowing smoothly
alongside each other.TURBULENT FLOW
Turbulent flow is a flow characterized by random motions of the
fluid particles in the transverse as well as axial directions.
Turbulent flow occurs when the average velocity is fast. The layers
disappear and the velocity is more uniform across the
stream.TRANSITIONAL FLOW
.
Transitional flow is the flow between laminar and turbulent.
Transitional flow exhibits the characteristic of both laminar and
turbulent patterns. In some cases transitional flow will oscillate
between laminar and turbulent flow.Incompressible flow is fluid
flow under conditions of constant density. Compressible flow is
fluid flow under conditions that cause significant changes in
density. Mass flow is the amount of fluid, measured in mass units,
that passes a given location per unit time. Steady flow is a flow
in which the flow rate in a measuring section does not vary
significantly with time. Unsteady flow is a flow in which the flow
rate fluctuates randomly with time and for which the mean value is
not constant. Pulsating flow is a flow rate characterized by
irregular or repeating variations. Static pressure is the pressure
of a fluid that is independent of its kinetic energy. Stagnation
pressure is a theoretical pressure that could be developed if a
flowing fluid could be brought to rest without loss of energy.
Page 2 of 43
Dynamic pressure is the increase in pressure above the static
pressure that results from complete transformation of the kinetic
energy of the fluid into potential energy. Working pressure is the
maximum allowable operating pressure for an internally pressurized
vessel, tank, or piping system. Differential pressure with respect
to flow, is the pressure drop across a restriction Pressure loss is
the decrease in pressure of a fluid as it passes through a
restriction
FLUID FLOW RELATIONSHIPSOne should know the physical laws that
apply to the flow of fluids and their measurement. 1. Pressure
across a particular point (such as an orifice) causes a flow
through the point ; the higher the pressure drop, the higher the
flow. 2. Temperature can affect flow ; higher temperatures decrease
viscosity. 3. Viscosity affects flow; a more viscous fluid flows
less easily 4. Density affects flow. The Flow decreases as the
density increases. 5. Friction affects flow ; more friction reduces
flow. 6. Specific gravity affects flow in the same way as density.
7. Flow and flow rate refer to the volume of fluid that passes a
given point in a pipe per unit time, as defined by the following
equations : Q = AV Where : Q = flow rate A = cross-sectional area
of pipe V = average fluid velocity The principle of the continuity
of flow is expressed by the equation: Q= Q= A1, V1, A1V1 = A2V2 =
A3V3 where flow rate A2, A3 = cross-sectional areas of pipe at
different locations 1,2,3 V2, V3 = average fluid velocity at
locations 1,2,3
VELOCITY
The Velocity of a flowing fluid is its speed in the direction of
flow. It is an important factor in flow metering because it
determines the behavior of the fluid. When the average velocity is
slow, the flow is said to be laminar. This means that the fluid
flows in layers with the fastest moving layers toward the center
and the slowest moving layers on the outer edges of the stream. As
the velocity increases, the flow becomes turbulent, with layers
disappearing and the velocity across the stream being more uniform.
In this discussion, the flow is assumed to be turbulent and the
term velocity refers to the average velocity of a particular cross
section of the stream. Rate of flow (Cubic ft. per sec) Velocity =
----------------------------------------------- = V = ft / sec Area
of pipe (Sq. feet)
Page 3 of 43
BERNOULLI'S THEOREMBernoulli's theorem relates the velocity of a
fluid at a point and the pressure of the fluid at that point. It is
just the application of work-energy theorem. According to work
energy theorem, the work done by a force acting on a system is
equal to the change in kinetic energy of the system. Consider the
streamlined flow of a.liquid through a pipe as shown in the figure.
As the liquid flows through the pipe, depending upon the position
of the liquid, there are three types of energy possessed by the
liquid during its flow.KINETIC ENERGY
Let m be the mass of liquid that-flows through the pipe with a
velocity v. Kinetic energy of the liquid = 1/2 mv 2 Kinetic energy
per unit mass of the liquid= 1/2V2POTENTIAL ENERGY
If h is the height from the ground, then the potential energy is
given by mgh. Potential energy per unit mass = ghPRESSURE
ENERGY
If p is the pressure exerted on the liquid of cross sectional
area a, then the force acting on the liquid surface is given by F =
pa (pressure = force / area)
Under the influence of this force, the liquid is driven through
a small displacement x. The work done is given by w = Fx = p.a.x w
= pV ( volume V = a x ) this work done is stored as the pressure
energy. pressure energy = pV = p m/ ( density p = mass / volume )
pressure energy per unit mass = p/
Page 4 of 43
The three types of energy possessed by the liquid at two
different points in the pipe are as follows: at A : potential
energy per unit mass = gh1 kinetic energy per unit mass = 1/2V12
pressure energy per unit mass = p1/ Total energy at A = p1/ + gh1 +
1/2V12 at B : potential energy per unit mass = gh2 kinetic energy
per unit mass = 1/2V22 pressure energy per unit mass = p2/ Total
energy at B = p2/ + gh2 + 1/2V22 Bernoulli's theorem states that
the sum of the energies possessed by a flowing liquid at any point
is constant provided the flow of liquid is steady. Total energy at
A = total energy at B p1/ + gh1 + 1/2V12 = p2/ + gh2 + 1/2V22 (ie)
p/ + gh + 1/2 V2 = constant This is known as Bernoullis equation.
From the above, it is understood that when a fluid is in motion,
the pressure within the fluid varies with the velocity of the fluid
if the flow is streamlined. The pressure within a fast moving fluid
is lower than that in a similar fluid moving slowly. This is known
as Bernoullis principle.
REYNOLDS NUMBERIn flow metering, the nature of flow can be
described by a number-the Reynolds Number, which is the average
velocity x density x internal diameter of pipe divided by
viscosity. In equation form, this is expressed as vD R = --------
Where, v = velocity D = inside diameter of pipe = fluid density =
viscosity The Reynolds Number has no dimensions of its own. From
the Reynolds Number, it can be determined whether the flow is
laminar or turbulent. Reynolds Number < 2000, the flow is
laminar Reynolds Number > 4000, the flow is turbulent. Between
these two values, the nature of the flow is unpredictable. In most
Industrial applications, the flow is turbulent. Although
measurement can be made without consideration of the Reynolds
Number, greater accuracy is possible when a correction based upon
it is made.
Page 5 of 43
METHODS OF FLOW MEASUREMENTMany different methods are used to
measure flow in a wide variety of industrial applications. These
can be divided into three broad categories as follows: 1.
Inferential type flow meters 2. Quantity flow meters 3. Mass flow
metersINFERENTIAL FLOW MEASUREMENTS
In the inferential type of flow measuring methods, the flow rate
is inferred from a characteristic effect of a related phenomenon
The following are the inferential type of flow measuring methods 1.
Variable head or differential flow meters 2. Variable area meters
3. Magnetic flow meters 4. Turbine meters 5. Target meters 6.
Thermal flow meters 7. Vortex flow meters, 8. Ultrasonic flow
meters VARIABLE HEAD OR DIFFERENTIAL FLOW METERS This is one of the
oldest and most widely used methods of industrial flow
measurements. The variable head flow meters operate on the
principle that when a restriction (or) obstruction in the line /
pipe of a flowing fluid is made ,it produces a differential
pressure across the restriction element which is proportional to
the flow rate. The proportionality is not a linear one but has a
square root relationship because the flow rate is proportional to
the square root of the differential pressure. It is simply
expressed as Qh Q=Kh where h = diff .pr (or ) P h = HP LP K
=constant DIFFERENTIAL PRESSURE METERS A differential flow meter
basically consists of two parts : Primary Elements and Secondary
Elements. The parts of the meter used to restrict the fluid flow in
the pipe line in order to produce a differential pressure are known
as primary elements They are : Orifice Plate Dall tubes Elbow Taps
Venturi Tube Pitot tubes Weir Flow nozzles Annubar tubes Flume
Secondary elements are those which measure the differential
pressure produced by the primary elements and convert them to
signals. Various secondary elements are: Manometer, Bellow/
Diaphragm Meter/ Transmitters (Mechanical/ Electrical/ Electronic/
Pneumatic).
Page 6 of 43
PRIMARY ELEMENTSOrifice Plate : The simplest and the most common
pipeline restriction used in DP method of flow measurement is the
Orifice Plate, which is a thin, circular metal plate with a hole in
it. It is held in the pipeline between two flanges called orifice
flanges . It is the easiest to install and to replace. Concentric
Orifice Plate : It is most widely used. It is usually made of
stainless steel and its thickness varies from 3.175 to 12.70mm (1/8
to 1 /2 inch.) depending on pipe line size and flow velocity. It
has a circular hole (orifice) in the middle, and is installed in
the pipe line with the hole concentric to the pipe. Eccentric
Orifice Plate : It is similar to the concentric plate except for
the offset. It is useful for measuring fluids containing solids,
oils containing water and wet steam. The eccentric orifice plate is
used where liquid fluid contains a relatively high percentage of
dissolved gases. Segmental Orifice Plate : This orifice plate is
used for the same type of services as the eccentric orifice plate.
It has a hole which is a segment of a circle. Quadrant Edge Orifice
Plate : This type of orifice plate is used for flows such as heavy
crudes, syrups and slurries, and viscous flows. It is constructed
in such a way that the edge is rounded to form a quarter-circle.
Depending on the application, it is often necessary to drill a
small drain hole usually called a weep hole. This hole is located
at the bottom when gases are measured to allow the condensate to
pass in order to prevent its building up at the orifice plate. When
the fluid is a liquid, this hole is located at the top so that
gases can pass and gas pockets cannot build up.MAXIMUM FORCE IN THE
ORIFICE PLATE INSTALLATION IS AT A. MINIMUM PRESSURE IS AT B.
BECAUSE OF LOSS OF PRESSURE , ACROSS THE PLATE, DOWNSTREAM PRESSURE
RISES ONLY AS HIGH AS POINT
Page 7 of 43
Figure gives a cross-section of a typical orifice plate
installation showing the variation in pressure that occurs across
the plate. Notice that the main flow stream takes the shape of the
venturi tube with the narrowest path slightly downstream from the
plate. This point is called the vena contracta. At this point, the
pressure is at its minimum. From this point on, the fluid again
begins to fill the pipe and the pressure rises. The pressure,
however, does not recover completely. There is a loss of pressure
across the plate. The principal consideration in selecting an
orifice plate is the ratio of its opening (d) to the internal
diameter of the pipe (D). This is often called the beta ratio. If
the d/D ratio is too small, the loss of pressure becomes too great.
If he ratio is too great the loss of pressure becomes too small to
detect and too unstable. Ratios from 0.2 to 0.6 generally provide
best accuracy. Several procedures have been developed for
calculating the correct size of an orifice to make it suitable for
measuring a particular rate of flow. The fundamental equation for
all these procedures are based as Q=EA0 2gh where, Q = flow rate
(volume per unit of time) E = Efficiency factor Ao = Area of
orifice in square feet g = acceleration due to gravity 32
feet/sec/sec h = differential pressure across orifice in feet The
efficiency factor E is required since the actual flow through an
orifice is not the same as the calculated flow. Values of E have
been determined by tests and are found in tables. It is different
for each combination of d/D radio and Reynolds Number. The letters
K or C are used to express this factor in some other equations. It
may be called as flow coefficient. This factor or coefficient has
no units since it is a ratio of the actual to the theoretical. As
stated above, values of E are found in table or on graphs. Example
: for Reynolds No. of 10,000 ratio of .6 The value of E is .678 The
orifice plate, flow nozzle, and venturi tube operate on the same
principle, and the same equation is used for the three. In addition
to the difference in flow coefficient (E), there are other factors
for each that determine which element should be used.GIVEN REYNOLDS
NO. OF 10,000 AND SELECTION0.6, THE GRAPH PLATES OF ORIFICE
INDICATES RATIO OF THE VALUE OF E AS .678
Page 8 of 43
Precision Standard Orifices are used where accounting or plant
efficiency tests are involved as per B.S. 1042 Code for Flow
Measurement. Concentric sharp edged orifice plates should be used
for, all normal applications. Eccentric or segmental orifice plates
should be used for liquids containing solids. Beveled or
rounded-edge orifice plates should be used for viscous fluids.
Plate material for industrial fluids normally should be stainless
steel or such superior material as demanded by the process
conditions. Note : Orifice plates are not generally recommended for
applications where :1.Wide variations in Flow-rate occurs.
2.Tolerance less than 3% is required. 3.Highly viscous fluids and
slurries are to be measured 4.Piping layouts do not permit adequate
straight lengths to be used. 5.System allowable pressure drop is
very small. There must be a long continuous run of straight pipe
leading up to any of these primary elements. Considerable
information is available concerning the length of straight pipe
required between such devices as elbows and valves and the primary
elements. When insufficient straight pipe is not possible, the
disturbances can be reduced or eliminated by the installation of
straightening vanes.LENGTHS OF STRAIGHT PIPE REQUIREMENT
When the beta ratio=0.6, upstream distance A must be at least 13
pipe diameters after the elbow, tee or cross. After globe or a
regulating valve upstream distance A must be at least 31 pipe
diameters. In both cases downstream distance B is 5 pipe diameters.
Straight run requirements become less as Beta ratio decreases. For
example, when the beta ratio is 0.4, the distance A becomes 9D
after elbows and 19D after valves. (not drawn to scale.)
STRAIGHTENING VANES ARE INSTALLED ABOVE THE ORIFICE TO REDUCE
TURBULENCE AND MAKE ACCURATE MEASUREMENT POSSIBLE (ROBERTSON MFG.
CO.)
TURBULENT FLOW OCCURS WHEN THE AVERAGE VELOCITY IS FAST. THE
LAYERS DISAPPEAR AND THE VELOCITY IS MORE UNIFORM ACROSS THE
STREAM.
Page 9 of 43
Advantages of Orifice Plates (i) low cost (ii) can be used in a
wide range of pipe sizes (3.175 to 1828.8 mm.) (iii) can be used
with differential pressure devices (iv) well-known and predictable
characteristics (v) available in many materials Disadvantages and
Limitations of Orifice Plates (i) cause relatively high permanent
pressure loss, (ii) tend to clog, thus reducing use in slurry
services (iii) have a square root characteristics (iv) accuracy
dependant on care during installation (v) changing characteristics
because of erosion, corrosion and scaling. VENTURI TUBE Another
pipeline restriction for flow metering is the venturi tube, which
is a specially shaped length of pipe resembling two funnels joined
at their smaller openings. The venturi tube is used for large
pipelines. It is more accurate than the orifice plate, but
considerably more expensive, and more difficult to install. The
Venturi tube is the most expensive but it is the most accurate
primary element. High beta ratios (above 0.75) can be used with
good results. The pressure recovery of the venturi tube is
excellent, which means that there is little pressure drop through
it. Functionally, the venturi tube is good since it does not
obstruct abrasive sediment; in fact, because of its shape it
resists wear effectively. A Venturi tube is used where permanent
pressure loss is of prime importance, and where maximum accuracy is
desired in the measurement of high viscous fluids. The pressure
taps are located one-quarter to one-half pipe diameter up-stream of
the inlet cone and at the middle of the throat section. The venturi
tube can be used to handle a fluid which is handled by an orifice
plate and fluids that contain some solids, because these venturi
tubes contain no sharp corners and do not project into the fluid
stream. It can be also used to handle slurries and dirty liquids.
Advantages (i) causes low permanent pressure loss (ii) widely used
for high flow rates (iii) available in very large pipe sizes (iv)
has well known characteristics (v) more accurate over wide flow
ranges than orifice plates or nozzles (vi) can be used at low and
high beta ratios Disadvantages (i) high cost, (ii) generally not
useful below 76.2 mm pipe size (iii) more difficult to inspect due
to its construction (iv) limitation of a lower Reynolds number of
150,000, (Some data is however available down to a Reynolds number
of 50,000 in some sizes)
Page 10 of 43
FLOW NOZZLE : compromise between the orifice plate and the
Venturi tube is the flow nozzle, which resembles the entering half
of the venturi tube .The flow nozzle is almost as accurate as the
Venturi tube, and is not so expensive to buy or as difficult to
install.
The flow nozzle is simpler and cheaper than the venturi tube. It
is slightly less accurate and does not provide as good pressure
recovery. The flow nozzle can be used with higher beta ratios
(above .75), but is not quite so wear resistant as the venturi.
Advantages (i) (ii) (iii) (iv) Disadvantages (i) (ii) permanent
pressure loss lower than that for an orifice plate available in
numerous materials for fluids containing solids that settle widely
accepted for high-pressure and temperature steam flow cost is
higher than orifice plate & limited to moderate pipe sizes
requires more maintenance (it is necessary to remove a section of
pipe to inspect or install it).
TAPPING POINTSTo obtain the pressures upstream and downstream of
the primary elements requires taps on both sides of the
restriction. The location of these pressure taps varies with the
orifice Plate. The following methods of tappings are generally
used. FLANGE TAPS, CORNER TAPS, PIPE TAPS and VENA CONTRACTA
TAPSFLANGE TAPPINGS CORNER TAPPINGS
Flange taps are located on the flanges that hold the orifice
plate in positionFLANGE TAPPING
The tapping holes are in the corners of the flange. The tappings
will be in 45o angle to the flow direction. This type of tapping is
used for pipe lines with diameter less than 2.
Page 11 of 43
PIPE TAPS
Pipe taps are located at fixed distances upstream and
down-stream of the orifice plate - the upstream pipe tap is located
2 1/2 pipe diameters from the plate & the downstream pipe tap 8
pipe diameters, from the orifice plate. NOTE : Pipe tap is also
called as FULL FLOW TAPSVENA CONTRACTA TAPS ( D & D / 2 TAPS
)
At Vena contracta point, the velocity will be max. and the
pressure will be min. The distance of vena contracta taps on the
downstream side must be calculated from application data. On the
upstream side it is located at one pipe diameter from the plate (1
D). Approx. the downstream point (Vena Contracta) is at a distance
of 1/2 D. Hence it is also known as D & D/2 taps. The pressure
taps used with the Venturi tube are located at the points of
maximum and minimum pipe diameter. The pressure taps used with the
flow nozzle are located at distances upstream and downstream of the
nozzle as designated by the manufacturer. This location is critical
and the manufacturers recommendations must be followed. Any of the
differential pressure instruments can be used for rate of flow
measurement with these primary flow elements. Since the desired
measurement is rate of flow and not differential pressure, a
conversion from differential pressure to rate of flow must be
made.FIVE WAY VALVE MANIFOLD
T1, T2 1A 1B 2A 2B 3 4A 4B 5A 5B 6
TAPPING POINTS PRIMARY ISOLATION VALVE (HP) PRIMARY ISOLATION
VALVE (LP) SECONDARY ISOLATION VALVE (HP) SECONDARY ISOLATION VALVE
(LP) EQUALISING VALVE ISOLATION VALVE TO TX (HP) ISOLATION VALVE TO
TX (LP) DRAIN VALVE (HP) DRAIN VALVE (LP) TRANSMITTER
Page 12 of 43
DALL TUBES (LO-LOSS FLOW TUBE A MODIFIED FORM OF VENTURI TUBE)
The Dall Tube is a modified form of venturi tube, a cross-section
of which is shown. It consists of two truncated cones, each with a
relatively large cone angle. The throat is formed by a
circumferential slot located between the two smaller diameters of
the truncated cones. The differential pressure produced by a dall
tube is much higher (about double) than that of venturi tube or
nozzle having the same upstream and throat diameters with the same
net head loss. Advantages i.low head loss ii.short lying length
iii.available in numerous materials of construction iv.no upper
line-size limit Disadvantages I.pressure difference is sensitive to
up-stream disturbances II.more straight pipe required in the
approach pipe length III.not considered for measuring flow of hot
feed waterOVERALL PRESSURE LOSS THROUGH OVERALL PRESSURE LOSS
THROUGH VARIOUS PRIMARY ELEMENTS VARIOUS PRIMARY ELEMENTS100
PRESSURE LOSS IN % OF ACTUAL DIFFERENTIAL 90 80 70 60 50 40 30 20
10VENTURI TUBE WITH 15 % RECOVERY CONE (SHORT) HERCHEL TYPE VENTURI
TUBE WITH LONG CONE LOW LOSS FLOW TUBE (DALL TUBE) FLOW NOZZLE
ORIFICE
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
BETA RATIO =d/D
PITOT TUBEAnother Primary flow element used to produce a
differential pressure is the pitot tube. In its simplest form, the
pitot tube consists of a tube with a small opening at the measuring
end. This small hole faces the flowing fluid (Fig). When the fluid
contacts the pitot tube, the fluid velocity is zero and the
pressure is at a maximum. This small hole, or impact opening as it
is called, provides the higher pressure for differential pressure
measurement. While the pitot tube provides the higher pressure for
differential pressure measurement, an ordinary pressure tap
provides the lower pressure reading.Page 13 of 43
The Pitot tube is an economical device for providing a
differential pressure readingP IT O T T U B E A S S E M B L Y IM P
A C T P R E S S U R E
HO LES FO R S T A T IC P R E S S U R E
S T A T IC P R E S S U R E C O M P R E S S IO N R IN G A N D G L
A N D F O R L IN E C E N T E R IN G
The pitot tube actually measures the velocity of fluid flow and
not rate of flow. However, the flow rate can be determined from the
velocity using the is formula : Q = KAV1 Where, Q = flow rate
(cubic ft. per Sec.) A = Area of flow cross section in feet. V1 =
Velocity of flowing fluid (ft.per.sec) K = flow coefficient of
pitot tube (normally about 8) There is no standardization of pitot
tubes as there is for orifice plates, venturi tubes and flow
nozzles. Each pitot tube must be calibrated for each installation.
Pitot tubes may be used where the flowing fluid is not enclosed in
a pipe or duct. For instance, a pitot tube may be used to measure
the flow of river water, or it may be suspended from an airplane to
measure the air flow. Any of the differential pressure type
instruments previously described may be used with the pitot tube.
Advantages : i. no process loss ii. economical to install iii. some
types can be easily removed from the pipe line Disadvantages i.
Have poor accuracy ii. unsuitability for dirty or sticky fluids
iii. sensitivity to upstream disturbances
ANNUBARThe Annubar is a Multiple-Ported Pitot tube that Spans
the Pipe. Pressure Ports are located at mathematically defined
positions based on Published axis symmetric Pipeline velocity
profile. These are claimed to average the differential, thereby
eliminating the need to locate average velocity point as is
necessary for pitot tubes.
Page 14 of 43
Ease of installation, low cost, very low permanent pressure
loss, and insertability into existing piping make these devices
convenient for ducts and large line size measurements.PRINCIPLE OF
OPERATION
Delta Tubes are averaging pitots designed to produce a
differential pressure output having a classical square root
relationship with flow rate. The multiported Delta Tube's
strategically located sensing ports continually sample the impact
and static pressures produced by the Delta Tube's obstruction of
the flow stream profile. Within the probe, the impact and static
pressures sensed by the upstream and downstream ports are
continually averaged in separate plenum chamber. Secondary
instruments like Switzer Differential Pressure Indicator/ Switches
(or Differential Pressure Transmitter) can be used for switching
monitoring or for direct measurement of the differential pressure
generated by the Delta Tube.
Advantages and Disadvantages of Annubar : Advantages i. It is
available for a wide range of pipe sizes ii. It is simple and
economical to install iii. It provides negligible pressure drop iv.
It can be placed in service under pressure v. It can be rotated
while in service, for cleaning action vi. It provides long-term
measurement stability. Disadvantages i. unsuitability for operating
dirty or sticky fluids ii. limited operating data
Page 15 of 43
Page 16 of 43
ELBOW TAPS
The flow measurement using elbow taps as a primary ele