Baker Hughes Masoneilan 4700P, Инструкция по эксплуатации

Страница: 11 / 66

Masoneilan 4700P/E & 4800P/E Positioners Manual | 11 © 2021 Baker Hughes Company. All rights reserved.
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,
determines 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
TM
II and 87/88 actuators. Cams supplied
on Butterfly valves maintain the inherent valve characteristic. Custom characteristics 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
position (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 bypass valve does not block supply pressure to the pilot. Therefore, the supply line should
be shut off before disassembling the positioner.
WARNING
When a valve (because of high pressure drop) uses a supply
pressure higher than 20 psig, the 3-15 psig instrument signal
may not be sufficient to operate the valve when the positioner is
bypassed. Moreover, if the positioner with a high supply pressure is
bypassed suddenly, the high pressure in the actuator may damage
the diaphragm and/or the control instrument. Therefore, the bypass
valve should be used only if the positioner supply pressure is 20
psig (140 kPa or 1, 4 bar), or at most 35 psig (240 kPa, or 2, 4 bar),
or if the controller is capable of operating the valve directly. When
bypassing a positioner, ensure that the controller output is equal to
supply required to operate valve.