Fail Open Valves
Valves are usually either: Fail Closed, air to open or
Fail Open, air to close
• Regardless of the way the valve operates, the operator is interested in the knowing and adjusting the position of the valve, not the value of the signal. "Up is always open"
• All controllers have some means of indicating the controller output in terms of the valve position. When the operator increases the output as indicated on the controller, the valve opens.
Indication Inversion
|
The output indication is
inverted. The controller action takes the valve action into acount. The flow loop is direct acting. Most analog controllers work like this. |
Signal Inversion |
|
|
The output signal is
inverted. The controller action ignores the valve action. The flow loop is reverse acting Some distributed control systems work like this. |
