11
Logix
®
420 Digital Positioner FCD LGENIM0106-07-AQ – 10/15
flowserve.com
6 TUBING
After mounting has been completed, tube the positioner to the actuator
using the appropriate compression fitting connectors. For best perfor-
mance, use 10 mm (3/8 inch) tubing for 645 square cm (100 square
inch) actuators or larger.
6.1 Determine Air Action
The port labeled “Out” delivers air when an air supply is present and
the relay is energized. This port should be tubed to the pneumatic side
of the actuator (the side that would result in the air compressing the
actuator spring). When tubed this way, the spring is designed to return
the valve to the fail safe state should supply air or power to the unit fail.
If air from the output should open the valve, set the Air Action
configuration switch on the positioner to Air-to-Open, otherwise set it to
Air-to-Close.
The Air-to-Open and Air-to-Close selection is determined by the actuator
assembly, not the software.
6.2 Connect Supply Port
The positioner ports are threaded with 1/4 NPTF. The direct mount
output is 1/16 NPTF.
In order to maintain the recommended air quality, a coalescing filter
should always be installed in the supply gas line. An air filter is highly
recommended for all applications where dirty air is a possibility. The
positioner passage ways are equipped with small filters, which remove
medium and coarse size dirt from the pressurized air. If necessary, they
are easily accessible for cleaning.
In applications where the supply pressure is higher than the maximum
actuator pressure rating a supply regulator is required to lower the
pressure to the actuator’s maximum rating.
6.3 Vented Design
A standard Logix 420 positioner is vented directly to the atmosphere.
When supply air is substituted with sweet natural gas, piping must be
used to route the exhausted natural gas to a safe environment.
The exhaust port is located on the bottom of the positioner. The port
is tapped with either 1/4 NPTF threads and covered with a protective
cap. To control vented gas, remove the cap and connect the necessary
tubing/piping to this port.
This piping system may cause some positioner back pressure.
The maximum allowable back pressure from the exhaust port is 0.14
barg (2.0 PSIG). For output flow rates, see section 2.4 Pneumatic
Output.
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CAUTION: The back pressure in the main housing must never
rise above 0.14 barg (2.0 PSIG). This could cause the positioner to
become unresponsive under some circumstances.
6.4 Purging
Purging is intended to supply the non-pressurized side of a single
acting actuator with instrument air. This helps prevent air from the
environment (which may be salty, dirty or humid) from corroding the
springs and other actuator components. Purging uses exhaust air from
the positioner to flush the spring side of the actuator.
Tubing Configuration - Tube the Exhaust port with a “T” where one line
goes to the non-pressurized side of the positioner and the second line
goes to the atmosphere. Install an exhaust plug on the second line to
prevent debris from entering the tubing.
Figure 5: Pneumatic Ports
Exhaust (Piezo and Actuator)
(0.14 barg (2 psi) Max)
In (Supply)
Out (Control)
Out
(Direct Mount Control)
