Baker Hughes Masoneilan SVIII AP, Руководство по установке и обслуживанию, Страница: 163 / 204

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Masoneilan SVI II AP Installation and Maintenance Manual =
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163
11. 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 64 and Figure 65 show the
action of air to open, ATO, and air to close, ATC, valves when used with SVI II AP. The figures show
a direct acting positioner with linear and percentage characteristics. Some hysteresis is shown
for the actuator pressure signal that is caused by friction in typical actuators. The scales are
chosen to emphasize the relationships between input current and actuator pressure, so that the
failsafe valve position is shown at the lower left of each graph. Note that for an ATC valve, 4 mA
represents 100% valve travel not the expected 0%. The controller and other human machine
interfaces must correctly show that the valve is open 100% at 4 mA and closed 0% at 20 mA.
The graph shows the valve movement and actuator pressure when the Tight Shut-off, T.S.,
option is set at about 5%, in this example. The valve movement and actuator pressure are also
shown at the low current lift-off point at about 3.6 mA, below which the positioner is initializing
its settings until power is stabilized.