GENERAL INSTRUMENATION QUESTION AND ANSWERS

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.