Single Acting Actuated Valve Fail Hold Position 1

[srhash]

For a single-acting actuator pneumatic valve, how does fail hold work?

For a fail safe condition (FSC or FSO) the air is released to ensure that the valve moves to the fail safe condition. Similar action happens indirectly via SOV/ Positioner ports in case of electric failure.

As i understand, for a control valve if fail hold signal comes in, the positioner isolates the input air port, and vent/ exhaust is also closed. Consequently, air remains entrapped from the out port to the actuator supply. However, in such case if electric failure of positioner takes place, does the valve remain in same position or move to fail safe?

Similarly, for On/Off valve, if fail hold signal comes in, the SOV remains energized/ de-energized and ensures that the air supply remains intact to hold the valve in the last position. However, in same case if at this time SOV power fails, does the valve move to fail safe position?

My understanding as it stands right now is that in both cases, valve will move to fail safe position and the last hold position (fail hold) will be lost.

Would like to have some feedback/ correction of my views if wrongly understood.

 

[gerhardl]

This will depend on the actual air flow control circuit with solenoid valve(s), type(s), check valves and internal flow regulation in the regulator. All possible control situations are possible to obtain when constructing/selecting components. A 3/2 or 5/2 or 5/3 solenoid valve correctly used will in most cases alone give the wished for failsafe position.

The normal general description would be:

Failsafe closed (perhaps most used)
Failsafe open
Failsafe hold (current position held)

If you post a more detailed flow chart with component types you will probably have a chance of a more direct answer.

 

[LittleInch]

The thing you need to realise is that in a single acting fail in position actuator and its controller there are no springs involved. Everything needs a positive signal or a positive action to work.

Hence the control solenoid needs a signal to open and signal to close. No signal, no change. ditto the actuator. No air pressure, no movement.

You are mixing up control valves in your OP. They are harder to get to get fail in position, but far from impossible



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[gerhardl]

Nessesary addition as given by LittleInch! Fully agree!

Generally 'fail' position of a solenoid will be defined as the position where no current is present, eg. current failing, and air is presumed still OK.

To move the solenoid into fail position when current fails, one would normally relay on a force originating from a spring and/or magnetic force to move pilot or piston within the solenoid valve itself, sometimes helped by air released from one side of membrane or piston. This again to release forces at the main valve.

'Failsafe' as this is the only position the whole assembled valve/equipment can take (when the current fails).

 

[srhash]

@LittleInch & gerhardl>> Thanks for the feedback. However, i am still a little confused.

Let me extract one case example and post here and then take up the discussion for further understanding.