Why do we need line monitoring?
Let’s take common example of line monitoring, deluge valve. Deluge valve is part of fire suppression or F&G system which is the 3rd layer of plant protection. Consider safety and availability:
- Failsafe signal to the field
In case of spurious trip due to line fault, deluge valve shall be open even thought there is no fire and will drain the fire water system. What happen if real fire come afterward? - Non-Failsafe signal to the field
It won’t drain the fire water system due to line fault but it is fail-to-dangerous since deluge valve will not receive “activate” signal even thought there is real fire.
Based on consideration above, deluge valve requires both availability and safety. Line monitoring is one alternative for this application. If we use line monitoring, alarm will be sent to operator in case of line fault instead of directly open the deluge valve and fire water will still be available. Operator, then, should directly fix the fault.
High in-Rush Current?
This is common problem for actuator signal from PLC. The actuator requires 2A but the PLC can only provide 1A maximum. For common DO, relay can be used but for line-monitoring DO, relay on marshalling means lost the line monitoring function.
Below drawing shows the concept of high in-rush implementation (credit to Honeywell Engineers on Kerisi Project). 2 DO (at least one line-monitoring) and 1 supply were used in this implementation.
During normal condition (de-energize), line monitoring DO will monitor the connection to the actuator and 24VDC supply availability. If the supply lost or the R2 relay fault, the line will be open, thus, alarm shall be created by the line monitoring. Same case will happen if connection to the actuator disconnected on the field side.
During activate condition (ON/OFF Logic DO energize), R1 relay will switch the connection to 24VDC to drive the actuator. Line monitoring DO connection was switched to resistor to prevent line monitoring alarm.
Line-Monitoring-In-Rush
Note:
o Even thought common connection from PLC side to deluge line monitoring, common air instrument installation for deluge is failsafe.
Tags: deluge valve, F&G, High In-Rush, line monitoring, PLC
June 8, 2009 at 1:49 pm |
Valve actuators are really needed for complex tasks in the high power market.If the load requires accurate positioning, the electric actuators as well as the valve actuators has the advantage among others.That is why,to familiarize yourself in this kind of industrial application , knowing how an automation works is a wisely action and will positively keep us in track.
October 13, 2009 at 7:28 am |
It’s been 4 months ago you wrote your last posting. Come on make another one every month.