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Instrumentation and Control Engineering Materials-Free Download

62. Define cascade control

Cascade control is defined as a control system composed of two loops where the set point of one loop (the inner loop) is the output of the control ler of the other loop (the outer loop) 

63. When cascade control will give improved performance than conventional feedback control? 

In some process the secondary variables in it introduce disturbance throughout the system is measured and controlled by a separate loop. 

64. Explain the purpose of cascade control for heat exchangers?

In heat exchangers, the control objective is to keep the exit temperature of stream. But the flow rate of the stream creates the low disturbance throughout of its a function. The secondary loop is used to compensate the flow rate of the stream. 

65. What is meant by auctioneering control?

Such control configurations select among several measurements the one with the highest value and feed it to the controller. Thus it is a selective controller which possesses several measured outputs and only one manipulated input. 

66. Give any two types of selective control system.

Override control for the protection of process equipment, auctioneering control.

67. What is limit switch?

In some cases it is necessary to change from the normal control action and attempt to prevent a process variable from exceeding an allowable upper or lower limit. This can be achieved b y the use of special t ype switches called limit switches. 

68. Mention the types of limit switches.

High Select Switch (HSS), Low Select Switch (LSS).

69. What is HSS?

High Select Switch (HSS) is a limit switch which is used whenever a variable should not exceed an upper limit.

70. What is LSS?

Low Select Switch (LSS) is a limit switch which is used whenever a variable should not exceed an lower limit.

71. What is override control?

During the operation of the plant, it is possible that some of the process variables exceed the limit. In such cases it is necessary to change from the normal control action and attempt to prevent a process variable from exceeding an allowable an allowable upper or lower limit. This can be achieved by the use of special type switches called limit switches called limit switches (HSS and LSS). This type of protective control is called override control. 

72. What is split-range control?

To control A single process output can be controlled by co-coordinating the actions of several manipulated variables all of which have same ef f ect on controlled output. Such systems are called split-range control systems. 

73. Differentiate split-range control and selective control.

Split-range control system involves one measurement and more than one manipulated variables but sele ctive control system involves one manipulated variables and several controlled outputs. 

74. Why are fuel and air sent at a specified ratio into a combustion chamber?

To obtain the most efficient combustion.

75. What are decouplers?

The special element introduced in a system with two strongly interacting loops to cancel the interaction effect between the two loops and thus render two non-interacting control loops is called decoupler. 

76. When is inferential control used?

It is used in some cases where the output of the process and the influence of the disturbance cannot be measured.

77. What are the advantages of feed forward controller

Acts before the disturbance is felt by the process. It is good for slow systems.

78. What are the disadvantages of feed forward controller

Requires identification of all possible disturbances and their direct impact. Cannot cope with unmeasured disturbances.

79. What are the advantages of feedback controller.

It does not require identification and measurement of disturbance.

80. What are the disadvantages of feed forward controller

It is unsatisfactory for slow processes with significant dead time.

81. What is flashing in control valve?

When a liquids enters a valve and the static pressure at the vena contracta less than the fluid vapour pressure and the valve outlet pressure is also less the fluid vapour pressure the condition called flashing exists. 

82. When do you use a valve positioner?

If the diaphragm actuator does not supply sufficient force to position the valve accurately and overcome any opposition that flowing conditions create a positioner may be required. 

83. Give two examples for electric actuator

Motor, Solenoids.

84. What is the need of I/P converter in a control system?

In some process loop the controller is electronic and the fin al control element is electronic one. To interconnect these two we need a device that should linearly converts electric current in to gas pressure (4-20mA-315 psi). such device is called I/P converter. 

85. Why installed characteristics of a control valve is different from inherent characteristics?

Inherent characteristics is which the valve exhibits in the laborator y condition where the pressure drop is held constant. Installed or resultant characteristics is the relationship between flow and stroke when the valve is subjected to pressure conditions of the process. 

86. Explain the function of pneumatic transmission lines.

Used to transmit the input signals into standard instrumentation pneumatic output signals (3 to 15 psi or 20 to 100 KPa).

87. What is the purpose of final control element.

Components of a control system (such as valve) is used to directly regulates the flow of energy or materials to the process. It directly determines the value of manipulated variable. 

88. What is meant by cavitations in control valve?

When a liquid enters a valve and the static pressure at the vena contracta drops to less than the fluid vapor pressure and the recovering to above fluid vapour pressure, this pressure recovery causes an implosion or collapse of the vapour bubbles formed at the vena contracta. This condition is called cavitation. 

89. What is “equal percentage” in the equal percentage valve?

For equal incre ment of stem travel at constant pressure drop an equal percentage change in existing flow occurs.

90. What are the characteristics of control valve?

Inherent characteristics, Installed characteristics.

91. Differentiate inherent characteristics and installed characteristics.

Inherent characteristics is which the valve exhibits in the laborator y condition where the pressure drop is held constant. Installed or resultant characteristics is the relationship between flow and stroke when the valve is subjected to pressure conditions of the process. 

92. What is “quick opening” control valve.

For smaller movement of the stem, there is maximum flow rate.

93. What is “Linear” control valve.

If stem position varies linearly with flow rate, then it is linear.

94. Define Control Valve sizing.

Q=Cv.sqrt(P/Sg) Q-Flow rate

Cv-Valve coefficient

P-pressure difference across valve. Sg-Specific gravity of liquid.

95. Name any one final control element.

Control Valve.

96. What is the function of control valve in a flow control system.

The function of control valve in flow control system is to regulate the flow.

97. Name one application of electrical actuator

Solenoid coil used to change gears.

98. Name the two types of plugs.

Single-seated and double-seated plug type control valves.

99. Define Rangeability

It is the ratio of maximum controllable flow to minimum controllable flow.

100. What is rotating shaft type control valves.

1. Rotating-plug valves

2. Butterfly valves

3. Louvers.

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