AUTOMATIC
CONTROLLER :
It is a device
which measured the value of variable quantity or condition and operates to
correct or lie it deviation of this measured value from a selected reference.
AUTOMATIC
CONTROL SYSTEM :
It is any
operable arrangement of one or more automatic controllers in closed loops with
one or more processes.
SELF
OPERATED CONTROLLER :
It is one in
which all the energy needed to operate the final control element is derived
from the controlled medium through the primary element.
RELAY
OPERATED CONTROLLER :
It is one in
which the energy transmitted through the primary element is either supplemented
or amplified for operating the final control element by employing energy from
another sources.
PROCESS :
A process
comprises the collective function performed in and by the equipment in which a
variable is to be controlled.
SELF
REGULATION :
It is an
inherent characteristic of the process which aids in limiting the deviation of
the controlled variable.
CONTROLLED
VARIABLE :
The controlled
variable is that quantity and condition which is measured and controlled.
CONTROLLED MEDIUM :
It is that
process energy or material in which a variable is controlled. The controlled
variable is a condition or characteristic of the controlled medium. For e.g.
where temperature of water in a tank is automatically controlled, the
controlled variable is temperature and controlled medium is water.
MANIPULATED
VARIABLE :
It is that
quantity or condition which is varied by the automatic controller so as to
affect the value of the controlled variable.
CONTROL
AGENT :
It is that
process energy or material of which the manipulated variation is a condition or
characteristic. The manipulated variable is a condition or characteristic of
the control agent. For e.g. when a final control element changes the fuel gas
flow to burner the manipulated variable is flow the control agent is fuel gas.
ACTUATING
SIGNAL :
The actuating
signal is the difference at anytime between the reference input and a signal
related to the controlled variable. This basically known as error signal.
DEVIATION :It is the difference between the actual value of the controlled
variable and the value of the controlled variable corresponding with set point.
OFFSET :
It is the
steady state difference between the control point and the value of the
controlled variable corresponding with set point
CORRECTIVE
ACTION :
It is the
variation of the manipulated variable produced by the controlling means. The
controlling means operates the final control element ( control value ) which in
turn varies the manipulated variable.
REFERENCE
INPUT :
It is the
reference signal in an automatic controller.
SET POINT :
It is the
position to which the control point setting mechanism is set.
CONTROL
POINT :
It is the
value of the controlled variable which under any fixed set of conditions the
automatic controller operates to maintain.
PRIMARY
FEEDBACK :
It is the
signal which is related with the reference input to obtain the actuating
signal. Simply stated primary feedback is the actual measurement of the
controlled variable which when compared with the desired measurement of the
controlled variable produces the actuating signal.
POSITIONING
ACTION :
It is that in
which there is a predetermined relation between the value of the controlled
variable and the position of the final control element.
PROPORTIONAL
ACTION :
It is that in
which there is a continuous linear relationship between the value of the actual
measurement of the controlled variable and the value position.
FLOATING
ACTION :
It is that in
which there is a predetermined relation between the deviation and speed of
final control element.
DERIVATIVE
ACTION :
It is that in
which there is a predetermined relation between a time derivative of the
controlled variable and position of final control element.
REST ACTION
:
It is the
value movement at a speed proportional to the magnitude of deviation.
RATE ACTION
:
It is that in
which there is a continuos linear relation between the rate of change of
controlled variable and position of final control element. Rate action produces
value motion proportional to the rate of change of actual measurement.
PROPORTIONAL
BAND :
It is the
range of values of the controlled variable which correspond to the full
operating range of the final control element.
RESET RATE
:
It is the
number of times/minute that the effect of proportional position action upon the
final control element is repeated by proportional speed floating action.
There are two
ways of expressing reset action :
1. Reset time
and 2. Reset Rate
1. Reset
Rate : It is commonly expressed as a number of "repeats" per
minute. It is determined by dividing.
a) Travel of
final control element ( Value stroke ) in one minute as a result of the effect
of proportional speed floating action.
b) The travel
as a result of the effect of proportional position action with the same
deviation in both cases.
2. Reset
Time : It is the time interval by which the rate is commonly expressed in
minutes. It is determined by subtracting.
a) The time
required for a selected motion of the final control element resulting from
combined effect of the proportional position plus rate action.
b) The time
required for the same motion as a result
of the effect of proportional position action alone with the same rate of
change of controlled variable in both cases or expressed in another way. It is
the time lead in terms of air pressure on the control value produced by rate
action compared with proportional position action for the same rate of change
of actual measurement in both cases.
Que. : Explain the application of proportional
integral and derivative.
Ans. :
PROPORTIONAL
CONTROL ONLY :
Proportional
control only attempts to return a measurement to the set point after a load
upset has occurred. How ever it is impossible for a proportional controller to
return the measurement exactly to the set point.
USE : It is
normally used for level controls. It reduces the effect of a load change but it
can not eliminate it.
PROPNT
RESET CONTROL :
Reset action
is introduced to eliminate offset. It will integrate any difference between
measurement and set point and cause the controller's output to change until the
difference between the measurement and set point is zero. Reset will act as
long as the error exists.
USE:
Proportional + Reset controllers are by far the common types used in industrial
process control and where predominate dead times occur.
PROPNT
RESET + DERIVATIVE :
Derivative or
rate action helps the controller overcome system inertia and result in faster,
more precise control. Derivative action occurs whenever the measurement signal
changes. Under study conditions the rate action does not act. Derivative allows
the controller to inject more corrective action.
USED : On
temperature controls.
Que. : What is difference gap control ?
Ans. : Differential gap control is similar to on off
control except that a band or gap exists around the control point.
USE : In
industry differential gap control is often found in non critical level control
applications where it is desirable only to prevent a tank from flooding or
drying. When a measured variable exceeds the upper gap the control valve will
open fully or be closed fully. Similarly when it exceeds the lower gap it will
open or close fully.
Que. : Where is on off control used ?
Ans. : On off control is used when
1. Precise
control is not needed.
2. Processes
that have sufficient capacity to allow the final operator to keep up with the
measurement cycle.
3. It is
mainly used in refrigeration and are conditioning systems.
Que. :
Ans. : When reset action is applied in controllers
Where the measurement is away from the set point for long periods the rest may
drive the output to its maximum resulting in rest wind up. When the process
starts again the output will no come off its maximum until the measurement
crosses the so point causing large overshoots. This problem can be avoid by
including antireset wind up circuit which eliminates the problem of output
saturation.
Que. : Why is reset called integral and Rate
derivative ?
Ans. : RESET is called integral because of the
mathematical relationship to the output.
RATE is called
derivative because
Oi = rd (e) /
dt +O0
Oi = Output at
any instant
e = error signal
O0 = Output at
zero error.
t = time.
PROPORTIONAL
ACTION :
Oi = 100 / PB x
e + O0
Que. : Explain tuning of controllers.
Ans. : Tuning basically involves adjustment of
proportional. Integral and derivative
parameters to achieve good control. The gain, time constants, and dead times
around the loop will dictate the settings of various parameters of the
controller.
Tuning methods
are broadly classified into two :
1. Closed Loop Method : e.g. Ultimate Gain
Method.
2. Open Loop
Method : e.g. process Reaction curve.
ULTIMATE
GAIN METHOD :
The term
ultimate gain was attached to this method because its use require the
determination of the ultimate gain (sensitivity) and ultimate period. The ultimate
sensitivity Ku is the maximum allowable value of gain (for a controller with
only Proportional mode) for which the system is stable. The ultimate period is
the period of the response with the gain set at its ultimate value.
PROCESSORS REACTION CURVE :
To deter mine
the process reaction curve, the following steps are recommended. :
1. Let the
system come to steady state at the normal load level.
2. Place the
controller on manual.
3. Manually
set the output of the controller at the value at which it was operating in the
automatic mode.
4. Allow the
system to reach the steady state.
5. With
controller on manual, impose a step changes in the output of controller, which
is an signal to value.
6. Record the
response of controlled variable.
7. Return the
controller output to its previous value and return the controller to auto
operation.
Que. : Explain the working of an electronic P.I.D.
controller.
Ans. :
Input from the
measurement transmitter is compared with the set point voltage to produce a
deviation signal. The deviation signal is combined with a characterized feed
back signal to provide the input for the function generator amplifier. This
amplifiers output is delivered to the feed back network, and to the final
output which is a 10-50m.a. do signal for actuation of final operators.
PROPN
ACTION : It is a obtained by
adjusting the magnitude of feed back signal. An increase in negative feed back
means less effective gain and thus a broader proportional band.
REST ACTION
: It is obtained by charging the reset capacitor
at a rate determined by the value of
reset resister. The reset resister is variable, and constitutes reset
adjustment.
DERIVATIVE
ACTION : The connection of a
derivative capacitor across the feedback circuit delays feedback until the
capacitor is charged to a value approaching amplifier output. This delay is
controlled by value of derivative resister. This resister is variable and
constitutes derivative adjustment.
Que. : What is an analogue integrator and an analogue differentiators ?
Ans. :
ANALOGUE
INTEGRATOR :
ANALOGUE DIFFERENTIATORS :
Que. : What is anti reset wind up ?
Ans. : If the limit acts in the feed back section of
the control amplifiers integral circuit, the controller output will immediately
begin to drive in the opposite direction as soon as the process signal crosses
the set point. This approach is referred to as antireset wind up.
Que. : What are De saturators ?
Ans. : When, in some processes, e.g. batch process,
long transient responses are expected during which a sustained deviation is
present the controller integral action continuously drives the output to a
minimum or maximum value. This phenomenon is called "integral saturation
of the control unit". When this condition.
Que. : Explain the working of Rotameter.
Ans. : Variable area meters are special form of head
meters. Where in the area of flow restrictor is varied. So as to hold the
differential pressure constant. The rotameters consists of a vertical tapered
tube through which the metered fluid flows in upward direction. A
"float" either spherical or cone shaped, actually more dense than the
fluid being measured, creates an annular passage between its maximum
circumference and the weight of the tapered tube. As the flow varies the
"float" rises or falls to vary the area of the passage so that the
differential across it just balances the gravitational force on the
"float" i.e. the differential pressure is maintained constant. The
position of the "float" is the measured of the rate of flow.
Que. : Explain the working of a magnetic meter.
Ans. : An electric potential is developed when a
conductor is moved across the magnetic field. In most electrical machinery the
conductor is a "wire"; the principle is equally applicable to a
moving, electrically conductive liquid. The primary device of commercial
magnetic meters consists of a straight cylindrical electrically insulated tube
with a pair of electrodes nearly flush with the tube wall and located at
opposite ends of a tube diameter. A uniform a.c. magnetic field is provided at right
angles to electrode diameter and to the axis of the tube. The a.c. voltage
developed at the electrodes is proportional to the volume flow rate of fluid,
and to a magnetic field strength. This device is limited to electrically
conducting liquids. The magnetic meter is particularly suited to measurement of
slurries and dirty fluids, since there are no location for solids to collect
except the walls of the tube itself.
Que. : Explain the working of a turbine meter.
Ans. : Turbine meters consist of a straight flow tube
within which a turbine or fan is free to rotate, about its axis which is fixed
along the center line of the tube. Straightening rances upstream of the turbine
minimizes possible rotational components of fluid flow. In most units a magnetic
pick-up system senses the rotation of the rotor through the tube wall. The
turbine meter is a flow rate device, since the rotor speed is directly
proportional to flow rate. The output is usually in the form of electrical
pulses from the magnetic pick-up with a frequency proportional to flow rate.
Turbine meter are primarily applied to measurement of clean and noncorossive
hydrocarbons.
Que. : Explain the working of a Pitot tube.
Ans. : The pitot tube measures the velocity at point
in the conduct. If quantity rate measurement is desired, it must be calculated
from the ratio of average velocity to the velocity at the point of measurement.
PRINCIPLE : If
a tube is placed with its open and facing into a stream of fluid, then the
fluid impinging on the open end will be brought to rest, and the kinetic energy
converted to pressure energy. This the pressure built up in the tube will be
greater than that in the free stream by the impact pressure or pressure
produced by loss of kinetic energy. The increase in pressure will depend upon
the square of the velocity of the stream. The difference is measured between
the pressure in the tube and static pressure of the stream. The static pressure
is measured by a tapping in the wall of the main or by a tapping incorporated
in the pitot static tube itself. The difference between the pressure in the
tube and static pressure will be a measure of the impact pressure and therefore
of the velocity of the stream oil.
Que. : Where is the integral orifice used ?
Ans. : Integral orifice is used to measure small flow
rates. It is mounted directly on the secondary device. The integral orifice
diameter varies between 0.020 inch and 0.250 inch diameter. The integral
orifice finds considerable use in laboratory and pitot plants.
Calculation of
flow rate :
Qn / Fc = Ks x
Cwi x Fa
x Fm x ( Gp
/ Ge ) hw
Que. : Explain the working of a target meter.
Ans. :
The target
meter combines in a single unit both a primary element and a force balance flow
rate transmitter. A circular disc (or target) supported concentrically in the
pipe carrying the flowing fluid results in an annular orifice configuration.
Pressure difference developed by the fluid flow through this annular orifice
produces a force on target proportional to the square of the flow rate. This
force is carried out of the pipe through a rod passing through a diaphragm
seal, and is measured by a pneumatic or electronic force balance system
identical with the mechanism of the force balance D.P. cell. The advantages of
the target meter lies primarily in its single unit construction the primary
device and responsive mechanism in a single structure. This eliminates the
diff. pressure fluid connections in most heads meters. This is particularly
used for sticky and dirty material which may plug up differential connections
and for liquids which require elevated temperatures to avoid solidification,
this elimination of liquid connection is useful.
{ Wm
} 2
F =
{------------------------------ }
{ Cst Fa
Fm Fc cf }
Que. : Where is a quadrant orifice used ?
Ans. : If the fluid is viscous and the operating
Reynolds number is low quadrant orifice is preferred
Que. : What are types of taps used for orifices ?
Ans. :
1. FLANGE
TAPS :
This are most
commonly used on pipe sizes of 2 inches or larger. They are located in the
orifice flange 2 inch from upstream and 1 inch downstream from the faces 0
orifice plate.
2. CORNER
TAPS :
On pipe sizes
less than 2 inches corner taps located directly at the face of the orifice
plate.
3. VENA CONTRACTA
AND REDIUS TAE :
Vena contracta
taps located at 1 pipe diameter upstream and at point of minimum pressure
downstream. There are mostly widely used for measurement of steam.
Radius taps
are located 1 pipe diameter upstream and 1/2 pipe diameter downstream for the
inlet face of the orifice are a close approximation to vena contracta taps upto
0.72 d / D.
4. FACE
FLOW TAPS :
Face flow taps
are located at 2 1/2 pipe diameter
upstream and B pipe diameter downstream. Full flow taps at 2 1/2 and B pipe diameter
have the same advantage as vena contracta or radius taps.
QUE. : What is Reynolds number ?
Ans. : Dynamic similarity implies a correspondence of
fluid forces in two systems. In general situation there are many classes of
forces that influence the behavior of fluids. Some of these are inertial
viscous, gravitational, compressibility, pressure and elastic forces. Certain
dimensionless ratio are developed based on fluid properties. Velocities and
dimension, which are essentially force ratio.
The more important
of these are Reynolds number
SVD
u
For most
applications in practical flow measurement the Reynolds number is taken to be
sufficient criterion of dynamic similarly. The magnitude of Reynolds number not
only indicates whether the flow is laminar or turbulent but also furnishes the
probable shape of velocity profile. Due to the strong role it plays as an
indicator of varying flow characteristics, many of the deviation from the
theoretical equations are called Reynaldo number effects.
Que. : How would you choose differential range ?
Ans. : The most common diff. range for liquid
measurement is 0-100" H20. This range is high enough to minimize the
errors caused by unequal heads in the seal chambers, differences in temps. of
load lines etc. The 100" range permits an increase in capacity upto
400" and a decrease down upto 20" by merely changing range tubes or
range adjustment.
Que. : What are positive Displacement meters ?
Ans. :
PRINCIPLE :
The principle
of measurement is that as the liquid flows through the meter it moves a
measuring element which seals off the measuring chamber into a series of
measuring compartments each holding a definite volume. As the measuring element
moves, these compartments are successively filled and emptied. Thus for each complete
of the measuring element a fixed quantity of liquid is permitted to pass from
the inlet to the outlet of the meter. The seal between measuring element and
the measuring chamber is provided by a film of measured liquid. The number of
cycle of the measuring element is indicated by means of a pointer moving over
the dial, a digital totalizer or some other form of register, driven from the
measuring element through an adjustable gearing.
The most
common forms of positive displacement meters are :
1. Reciprocating
Piston type.
2. Rotating
or Oscillating Piston type.
3. Nutating
Disc type.
4. Fluted
Spiral Rotor type.
5. Sliding
vane type.
6. Rotating
vane type.
7. Oval Gear
type.
