Question

Subject : Control System

Q : Explain the difference between direct acting system element action and reverse acting

and Explain the difference in how valve normally open and valve normally closed work

Answer 1

DIfference between Direct acting system and Reverse acting system

In the controller, the output of the controller is directly proportional to the error. The error is the difference between the measured variable set point. Therefore, if the error increases, output of controller also increase.

Direct acting system:-

i. The controller output is increase by positive error.

ii. On the other hand, we can say that Increase in error causes the increase in actuator output in the direct acting system.

Reverse acting system:-

i. The controller output is decrease by positive error.

ii. On the other hand, we can say that Increase in error causes decrease in actuator output in the reverse acting system.

* Now, Normally Open (NO) and Normally Closed (NC) valve:-

Normally Open (NO) Valve:- We can say the this valve used for safety devices and used to maintain low pressure in the system. This valve allows liquid / gas to flow through usual conditions. When system makes pressure, valve closure to maintain and regulate pressure in the system. If more pressure, valve will closed completely.

In simple word, we can say that Normally Open valves are pressure reducing valve.

Working:-

The valve closes when energized, and when that current is turned off, an isolated spring inside the valve forces it open. The magnetic force of the solenoid coil must overcome the force of the spring to close the valve. Unless acted upon, the spring force keeps the valve open. In the event of a power failure, the valve will automatically open.

Normally Closed (NC) Valve:- This valve is used for emergency. When system pressure is extreme over pressure, the valve will open for maintain safety. When pressure come to normal, this will closed automatically.

In simple word, we can say that Normally Closed valves are pressure releasing valve.

Working:-

The valve opens when energized, and when that current is turned off, an isolated spring inside the valve forces it closed. The magnetic force of the solenoid coil must overcome the force of the spring to open the valve. Unless acted upon, the spring force keeps the valve closed. In the event of a power failure, the valve will automatically close.

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