Normally Closed Proximity Switch

Looks like you've totally analyzed this situation. There is a question here somewhere????

Its adjusted max to the desired level of closing and opening , When valve is open we have taken close limit switch for open indication . As we know our standard method is when vale open ZLH become 1 and ZLL is 0 ( NO switch ). here we have NC switch so close switch become 1 to 0 and open switch ll become 0 to 1 . this case we are considering two switche status . Our issue is during vale open as i said ZLL is 0 and ZLH is 1 . Momently valve is physcially closed but ZLL wire is broken as it is already 0 and ZLH also become 0 usually mismatch alarm will be generated If close status is used for activating a Emergency depressureization system nothing is going to happen . I need a solution for this with using same switch .

Trying to get my head around your explanation and requirements, but if it is simply that you need the open condition to output a 0 instead of a 1, then consider using a NOT gate to invert that signal. Any competent electronics tech could build you one quite easily if they know the circuit parameters.

Can you adjust the switch to only react to "100% closing"?

Consider inter phasing with some signal conditioners. It's just a logic (not, yes) conversion

Attempting fail safe operation with a single NC switch at each end is doomed to failure. Replace switches at both ends with change over contacts. Use the NO at each end for position indication. Wire both NCs in series and create an "intermediate" signal. Provide indicators for all three signals so the plant operator and maintenance can see what is happening. Feed all three signals into an Exclusive OR circuit so if you get no signal (broken wire) or two at once (shorted wire) you know you are in fault mode. You will need to add a pulse delay to cater for the brief interruption during switch changeover. Add a timer to the intermediate signal set to just longer than the time needed for the valve to change status. If the timer trips the valve has stopped/slowed due to an obstruction in the half way position which should trigger a separate fault mode. Set back the limits from the extreme at each end so they trip early, then add delays so the actuator continues to drive the valve onto it's seat. Setting a limit right on the cusp risks the possibility of not tripping and causing a false positive alarm condition. Alarm signals should be latched and only reset after a competent person has ascertained the reason for the fault and resolved the problem. Alarms should be divided into categories of warning, serious and critical. Warning can in most circumstances allow the plant to continue running and leaves decisions about shut down to a trained operator.. Serious should shut down the plant in a controlled manner. Critical should generate an Emergency Stop shutdown. Data logging of all alarms is a good idea. In complex plants one fault can cause other parts of the system to behave erratically. Being able to check which fault was logged first saves a lot of time when diagnosing problems.

Please to be noted that its not a mechanical micro limit switch . It is normally closed proximity switch . Here i have attached the present logic and proposed logic to be implemented .

Proximity switches can and do fail high as well as fail low. If cables are damaged a short circuit is just as likely as an open circuit. When the valve is in transit from open to closed or closed to open you are not even looking for a failure, but this is the commonest time for a failure to occur. Fail safe operation implies guarding against all possible failure modes, not just the ones you choose. I never suggested that the limits should not be proximity switches, just that the proximity switches that you currently have installed are inadequate.

Stick a travel timer on the valve. If it hasn't changed state by [time] after asking it to move, then you've got a failure, so alarm.

Actually it sounds like all you need is a pull up resistor that is held to ground when you want the value to be zero. When it is 1, the pull up resistor is redundant and does not do anything.

By putting a pull up resistor on the output side of the circuit, it provides a signal when there should be none due to a broken cable. Make sense?