Masoneilan SVI II AP Installation and Maintenance ... - GE Energy
Masoneilan SVI II AP Installation and Maintenance ... - GE Energy
Masoneilan SVI II AP Installation and Maintenance ... - GE Energy
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Air to Open <strong>and</strong> Air to Close Actuators<br />
B<br />
Actuator Action<br />
It is important to correctly assign the sign + or - of each control variable throughout a control<br />
system. Even the control valve subsystem can be complex. Figure 82 <strong>and</strong> Figure 83 show<br />
the action of air to open, ATO, <strong>and</strong> air to close, ATC, valves when used with <strong>SVI</strong> <strong>II</strong> <strong>AP</strong>. The<br />
figures show a direct acting positioner with linear <strong>and</strong> percentage characteristics. Some<br />
hysteresis is shown for the actuator pressure signal that is caused by friction in typical<br />
actuators. The scales are chosen to emphasize the relationships between input current <strong>and</strong><br />
actuator pressure, so that the failsafe valve position is shown at the lower left of each graph.<br />
Note that for an ATC valve, 4 mA represents 100% valve travel not the expected 0%. The<br />
controller <strong>and</strong> other human machine interfaces must correctly show that the valve is open<br />
100% at 4 mA <strong>and</strong> closed 0% at 20. The graph shows the valve movement <strong>and</strong> actuator<br />
pressure when the Tight Shut-off, T.S., option is set at about 5%, in this example. The valve<br />
movement <strong>and</strong> actuator pressure are also shown at the low current lift-off point at about 3.6<br />
mA, below which the positioner is initializing its settings until power is stabilized.<br />
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