Siemens SIPART PS2 Operating Instructions Manual page 140

Functional safety
8.2 Safety function
Situations in which it is not possible to depressurize the actuator on demand or in the case of
a fault represent a dangerous failure.
Disregarding conditions for fulfilling the safety function
Disregard can result in a malfunction of the process plant or application, e.g. process
pressure too high, maximum level exceeded.
The mandatory settings and conditions are listed in sections "Settings (Page 142)" and
"Safety characteristics (Page 142)".
• These conditions must be met in order to fulfill the safety function.
The pneumatic block of the positioner pressurizes and depressurizes the actuator. The
pneumatic block contains two pilot valves. The characteristic service life of the pneumatic
block depends on the load. On average it is approx. 200 million switching cycles for each of
the two pilot valves with symmetrical load. The number of control procedures for the
switching cycles is called in the local display or via HART communication. For more details,
see Diagnostic value '42.VENT1' / '43.VENT2' (Page 222).
Safety-instrumented system in single-channel operation (SIL 2)
Figure 8-1
The combination of transmitter, automation system and final controlling element forms a
safety-instrumented system that performs a safety function.
The transmitter generates a process-related measured value that is transferred to the
automation system. The automation system monitors this measured value. If the measured
value exceeds the range of the high or low limit, the automation system generates a
shutdown signal for the connected final controlling element, which switches the associated
valve to the specified safety position.
140
WARNING
Safety-instrumented system in single-channel operation
SIPART PS2 with and without HART
Operating Instructions, 02/2016, A5E00074631-AB