Description
For the installation of low-level limiters of “high-integrity
design” in measuring pots mounted outside of the boiler, it
is imperative that the periodic purging of the connecting lines
to the boiler be monitored properly.
To achieve this, the connecting lines are shut off one after the
other at defined intervals and the measuring pot is drained.
The logic unit SRL 63 monitors compliance with the preset
times and the sequence of valve operations; in addition, it
bypasses the low-level limiter to ensure that the system is
not shut down during purging.
The logic unit for monitoring the purging cycle consists of a
compact PLC, a safety time-lag relay and a coupling relay.
The design complies with EN 50156
Function
The logic unit monitors the following times:
? Interval time: This is the time interval at which,
depending on the mode (24h / 72h operation), the connecting lines have to be purged.
? Standby period: The purging process must be initiated
during this time. The standby period begins when the
interval time has elapsed.
? Purging time: During this period, the purging process
must be performed through actuation of the valves. Actuation of the valves is signalled by means of the limit
switches; similarly, when the electrode of the low-level
limiter is exposed, this is sensed through the output
contact of the corresponding level switch. If a signal is
not received within the purging time, the safety circuit is
opened. Since a low-level limiter may be bypassed for a
maximum of 5 minutes, monitoring of the purging time
is a safety-relevant function.
