Solenoids Valve Interlock Questio

[hswang2]

I have a solenoids valve actuated diagram which I don't quite understand( pls see the attached test 1 and test 2pdf)
The first type solenoid valve have two limit switch HZSC and HZSO which seems to be used as permissives initiators in the DCS interlock.
But, I don't know how to decide it is position switch or open/close limit switch.

The second type solenod valve have XZSC and XZSO limit switch which seems to be used as permissives initiators in a safety interlock.
And, it seems have two soft link commands from the DCS used as reset and open/close swtich.
But, I don't know what is the difference of the two type valve.

And, ome more question is that do I have to count these HZSC, XZSC as an input signal and put on the cause part on the cause & effect table.

Could any master in I&C and process engineering field explain detail to me?

Thank you very much.

 

[JLSeagull]

Read note 6, not shown on the image. The P&ID should show a failure position for the valve. A shutdown valve would normally be fail closed; a.k.a. de-energized to close.

The position switches appear to be DCS inputs for display only, one digital input each.

Two DCS outputs are shown. XY-B is a manual operator selection to open or close the valve from a DCS screen selection. Turn the output off to close the XV. XY-A is an interlock to a shutdown system. Once the trip is clear the DCS sends a signal to reset the shutdown logic.

Or perhpas not.

 

[JLSeagull]

The interlock on the position switches could provide an alarm that the valve did not open or close within a certain time after the command. The switches could also be a startup interlock for other actions. Note 6 could be very helpful.