Logix
®
420 Digital Positioner FCD LGENIM0106-07-AQ – 10/15
28
12 REQUIREMENTS FOR SAFETY
INTEGRITY
This section provides information and additional user responsibilities in
order to meet Safety Integrity Level 2 (SIL 2) per IEC 61508.
The safety function of the positioner is to go to the fail-safe state (vent
air from the actuator) given a low power condition to the 4 to 20 mA
input terminal.
12.1 Fail Safe State
The fail safe state is when the relay valve is at less than 5% of full stroke
such that output port is venting.
NOTE: The fail safe state above represent the fail safe state of the posi-
tioner. The valve fail safe state may be different depending on spring
configuration and tubing. Ensure the valve fail-safe state is appropriate
for your application.
12.2 Safety Function
The Logix 420 positioner moves to fail-safe state upon the removal of
analog input power (less than 3.6 mA)
12.3 Fail Safe State Response Time
Test to find the final valve assembly response time to ensure it meets
application-specific requirements.
Response times will vary widely with actuator size, the use of boosters,
stroke length, starting position, fail-safe direction, tubing size, supply
pressure, and temperature. The air flow capacity also affects the
response time. See section 2.4 PNEUMATIC OUTPUT for air flow
capacity.
Typically, the Logix 420 can de-energize a 122 cm^2 (19 in^2)
diaphragm actuator from fully open to fully closed in under 2 seconds.
This test was performed at 22 °C, using 4.1 bar supply, quarter inch
tubing.
The typical response times for a relay to travel from fully energized to
fully de-energized (exhausted) state are:
• 4180 ms at -52 °C;
• 650 ms at -40 °C;
• 172 ms at 22 °C;
• 214 ms at 85 °C;
NOTE: During the stroke calibration (Quick-Cal), stroke times are
measured and recorded in the positioner. See tuning parameters on the
positioner menu or in the DTM.
12.4 Installation
Ensure installation of the positioner is properly performed according
to this manual. Ensure tubing is configured to the actuator so that the
fail-safe state of the positioner matches the desired fail-safe state of the
valve.
12.5 Required Configuration Settings
The following user settable options must be properly configured for the
individual application in order to provide the designed safety integrity
for that application.
• Calibrate the analog input (command). The fail safe state of the
valve must correspond to the analog input command at less than
3.6 mA.
• Set the desired PST settings using the DTM.
• It is recommended to lock the local interface to prevent unintended
adjustments of the settings by an unauthorized user.
12.6 Maximum Achievable SIL
The Flowserve 420 Valve Positioner covered by this safety manual
is suitable for use in low demand mode of operation Safety Integrity
Functions (SIF) up to SIL 2 in simplex (1oo1) configurations and up to
SIL 3 in redundant configurations with a HFT of at least 1. The achieved
SIL for a particular SIF needs to be verified by PFDavg calculation for
the entire SIF including the failure rates of the associated sensors and
valves that are also part of the SIF.
Use of the Flowserve 420 Valve Positioner in a redundant (1ooN)
configurations is also limited to SIL 2.
For details, contact your Flowserve representative for Failure Mode,
Effects, and Diagnostics Analysis (FMEDA) report for Logix 420.
12.7 Reliability data
For reliability data, a detailed Failure Mode, Effects, and Diagnostics
Analysis (FMEDA) report has been prepared and is available from
Flowserve with all failure rates and failure modes for use in SIL verifica-
tion. See FMEDA report for Logix 420.
Note that the failure rates of the associated actuator need to be
accounted for in the Safety Instrumented Function (SIF) level Probability
of Failure High over Average Probability of Failure on Demand (PFH /
PFDAVG) calculation.
12.8 Lifetime limits
The expected lifetime of the Flowserve 420 positioner is approximately
10 years. The reliability data listed the FMEDA report is only valid for
this period. The failure rates of the Flowserve 420 Valve Positioner may
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