GE Masoneilan 4700E, Руководство по обслуживанию

Страница: 16 / 80

© 2015 General Electric Company. All rights reserved
.
16 | = GE Oil & Gas
Pilot
The pilot is essentially a three-way sliding valve. The plug regulates supply air flow to and from
the actuator to the exhaust port. The position of this plug, governed by the diaphragm, deter-
mines the output pressure of the positioner. The 4700P/4800P pneumatic positioner action may
be reversed by interchanging the supply and exhaust connections and changing cam lobe and
lever arm orientation.
Direct Action
Increasing instrument signal pressure produces an increase in output pressure.
Reverse Action
Increasing instrument signal pressure produces a decrease in output pressure.
Cam
The cam is the intermediate element in the feedback mechanism between the actuator and the
feedback spring. Its profile determines the relationship between the valve plug position and the
control signal.
Linear , Split Linear , or Percentage Control characteristics are available by selection
of proper lobe on cams supplied for Camflex II, Varimax, 87/88, and 87U/88U, actuators. Cams
supplied on Ball and Butterfly valves maintain the inherent valve characteristic. Custom charac-
teristics are available on special order.
Optional bypass valve (4700P, Direct Acting Model only)
During normal operation, the instrument signal is applied directly to the positioner diaphragm
and the regulated supply pressure flows through the pilot valve to or from the actuator.
The bypass valve module permits the positioner to be isolated for maintenance while operating
the valve directly with the instrument signal. By turning the nylon bypass valve to the bypass po-
sition (the raised arrow on the bypass valve aligned with the word
Bypass on the face positioner
body indicates this position), the normal positioner output pressure to the actuator is blocked
and the instrument signal is applied both to the positioner diaphragm and the actuator. The by-
pass valve does not block supply pressure to the pilot. Therefore, the supply line should be shut
off before disassembling the positioner.