I have some question about the on-off valve (please see attachment)
N1.What is the Limit switches (in this case) used for?( Ex. permissive or trip initiators for interlock)
N2. Why does this limit switch on the on-off valve not provide any alarm in the control room?
(At least I can't see any in the attachment drawing)
N3. Our (process engineers) are ask for deciding these pneumatic hand switch (*HS A/B in the attached drawing) remote open/close from DCS is necessary or not.
However, I understand it has to be depending on the process people's apprehension for the process.
Anyone could kindly help me out, and give me suggestion that how to find any implication about this HS requiring or not from the process interlock description or any other document.

The P&ID depicts position switch contacts wired to the DCS.  The I could include some interlock logic in addition to the status display.  This example does not appear to include automatic valve actuation, only operator initiated remote activation.

BTW, I prefer the term position switch for contacts that provide open or closed position status.  The term limit switch is often used on eletrick motor operated valves describing contacts that stop the motor when the valve is at full open or travel.

The open or closed status of a valve is not necessarily and alarm situation.  This appears to be operator initiated activity to open or close the valve.  Hwoever the open status could be used as an interlock to permit starting a pump, etc.

N3. Our (process engineers) are ask for deciding these pneumatic hand switch (*HS A/B in the attached drawing) remote open/close from DCS is necessary or not.

The operating philosophy or project engineering standards might define when valves should be remotely operated.  However the P&ID reflects the automated valve actuator.  With the hardware provided it makes sense to permit the operator to open and close remotely.  Often the valve pneumatic components include a selector to permit opening or closing at the valve/ball valve.  This scheme suggests a double acting valve, no spring or failure position.  A volume bottle could provide stored energy to operate when the air supply fails.  The de-energized direction on the solenoid valve suggests that this is a spring operated valve.  The failure position of the valve is not shown.  If a blowdown valve different clients have different requirements.  Fail closed is normal for an isolation or shutdown valve.  If it is an emergency shutdown valve a selector could be for automatic or close instead of open or close.  This could prevent isolating the automatic control.

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