Appendix E. Air To Open And Air To Close Actuators; Actuator Action - GE Masoneilan SVI II ESD Installation And Maintenance Manual

Appendix E. Air to Open and Air to Close Actuators

Actuator Action

It is important to correctly assign the sign + or - of each control variable throughout a control
system. Even the control valve subsystem can be complex. Figure 91 and Figure 92 show air to
open action, ATO, and air to close, ATC, valves used with SVI II ESD. The figures show a direct
acting positioner with linear and percentage characteristics. Some hysteresis is shown for
actuator pressure signal caused by friction in typical actuators. Scales are chosen to emphasize
relationships between input current and actuator pressure, so that the failsafe valve position is
shown at the lower left of each graph. For an ATC valve, 4 mA represents 100% valve travel not
the expected 0%. Controller and other human machine interfaces must correctly show that the
valve is fully open (100%) at 4 mA and fully closed (0%) at 20 mA. The graph shows valve
movement and actuator pressure when Tight Shutoff, T.S., option is set at about 5%, in this
example. Valve movement and actuator pressure are also shown at the low current lift-off point
at about 3.6 mA, below which positioner is initializing its settings until power is stabilized.
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