Que. : Explain Cascade Control system with a diagram.
What would happier if a single controller were used ?
Ans. : Cascade means two controllers is series. One
of them is the Master or Primary and the second is the secondary of slave
controller. The output of the secondary controller operates the final control
element, that is the valve.
LOOP EXPLANATION :
The output of
the temp. transmitter goes as measurement signal to the TIC which is the master
controller. Similarly the output of pressure transmitter goes as measurement
signal to the PIC which is the secondary controller.
The output of
TIC comes at set point to PIC which is turn operates the valve. The reqd. temp.
is set on the TIC.
USE OF
CASCADE SYSTEM :
Cascade loops
are invariably installed to prevent outside disturbances from entering the
process. The conventional single controller as shown in the diagram cannot
responds to a change in the fuel gas pressure until its effect is felt by the
process temp. sensor. In other words an error in the detected temperature has
to develop before corrective action can be taken. The cascade loop in contrast
responds immediately correcting for the effect of pressure change, before it
could influence the process temperature. The improvement in control quality due
to cascading is a function of relative speeds and time lags. A slow primary
(Master) variable and a secondary (Slave) variable which responds quickly to
disturbances represent a desirable combination for this type of control. If the
slave can respond quickly to fast disturbances then these will not be allowed to enter the process and thereby will
not upset the control of primary (master) variable. It can be said that use of
cascade control on heat transfer equipment contributes to fast recovery from
load changes or other disturbances.
Que. : Explain ratio control system.
Ans. : A ratio control system is characterized by the
fact that variations in the secondary variable do not reflect back on the
primary variable. In the above diagram 0 a ratio control system the secondary
flow is hold in some proportion to a primary uncontrollable flow.
If we assume
that the output of primary transmitter is A, and the output of the secondary
transmitter is B, And that multiplication factor of the ratio relay is K, then
for equilibrium conditions which means set valve is equal to measured valve, we
find the following relation :
KA - B = 0
or B/A = K,
where 'K' is the ratio setting of the relay.
Que. : What is Furnace Draft control ?
Ans. : Balanced draft boilers are generally used
negative furnace pressure. When both forced draft and induced draft are used
together, at some point in the system the pressure will be the same as that of
atmosphere. Therefore the furnace pressure must be negative to prevent hot gas
leakage. Excessive vacuum in the furnace however produces heat losses through
air infiltration. The most desirable condition is that the one have is a very
slight ( about 0.1" H20 ) negative pressure of the top of furnace.
Que. : What is feed back control ? What is feed
forward control ? Discuss its application ?
Ans. :
FEEDBACK
CONTROL :
Feed back
control involves the detection of the controlled variable and counteracting of
charges its it’s value relative to set point, by adjustment of a manipulated
variable. This mode of control necessities that the disturbance variable must
affect the controlled variable itself before correction can take place. Hence
the term 'feedback' can imply a correction 'back' in terms of time, a
correction that should have taken place earlier when the disturbance occurred.
FEED
FORWARD CONTROL :
Feed forward
control system is a system in which corrective action is based on measurement
of disturbances inputs into the process. This mode of control responds to a
disturbance such that is instantly compensates for that error which the
disturbance would have otherwise caused in the controlled variable letter in
time.
Feed forward
control relies on a prediction. As can be seen from the figure of feed forward
control a necessary amount of input goes to the process. This measurement goes
to the controller which gives output to the control valve. The control valve
regulates the flow.
FEED BACK
CONTROL :
In feed
forward control no difference between the desired result and actual result need
exist before corrective action is taken in feed back control a difference must
exist. Hence, open loop or feed forward control is capable of perfect control,
but feed back is not. Due to economic impractality of precision , predicting
the amount of correction necessary to achieve satisfactory results with feed
forward control, feed back control is most often used. In order to properly
choose the type of feed back controller for a particular process application,
two factors time and gain must be considered.
Que. : What are Intrinsically safe system ?
Ans. : Intrinsic safety is a technique for designing
electrical equipment for safe use in locations made hazardous by the presence
of flammable gas or vapors in air.
"Defn.
:" Intrinsically safe circuit is one in which any spark or thermal effect
produce either normally or under specified fault conditions is incapable of
causing ignition of a specified gas or vapor in air mixture at the most easily
ignited concentration.
HAZARDOUS
AREAS :
The
specification of products or systems sold as intrinsically safe must state in
what hazardous areas they are infect intrinsically safe. Universal cooling of
hazardous areas has not, unfortunately, been adopted in all countries. However
two sets of codes in common use are.
Que. : What does a transmitter output start from 3-15
psi or (0.2 - 1 Kg/Cm2) or 4 - 20 ma. etc. ?
Ans. : The transmitter output stance from what is
known as "live zero". This system has specific advantages :
1. The systems
automatically alarms when the signal system becomes inoperative.
2. The output
areas is linear ( Ratio of 1 : 5 ).
DEAD ZERO
SIGNAL :
The advantage
is that it does not have to be biased to true zero. A "Live zero"
gives the computer additional information, so that it can takes appropriate
alarm action in case of a measurement failure, because it can discriminate
between a transmitter operating, but transmitting a zero measurement and a
failure, in the signal system.
Que. : What is force balance and motions balance
principle ?
Ans. :
FORCE
BALANCE PRINCIPLE :
"A
controller which generates and output signal by opposing torque’s".
The input
force is applied on the input bellows which novas the beam. This crackles
nozzle back pressure. The nozzle back pressure is sensed by the balancing
bellows which brings the beam to balance. The baffle movement is very less
about 0.002" for full scale output.
MOTION
BALANCE PRINCIPLE :
"A
controller which generates an output signal by motion of its parts".
The increase
in input signal will cause the baffle to move towards the nozzle. The nozzle
back pressure will increase. This increase in back pressure acting on the
balancing bellows, will expands the bellows, there by moving the nozzle upward.
The nozzle will move untill motion (almost) equals the input (baffle) motion.
Advantages
of force Balance :
1. Moving
parts are fever.
2. Baffle
movement is negligible.
3. Frictional
losses are less.
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