Partial Valve Stroke Test on a FL Double Acting Actuator

[GrayD]

Hi

Does anyone have any ideas how to hook-up a pneumatic Fail Last Double acting actuator so that if performing a PVST that it shall return to the Fail Open position again on loss of air or power?

Thanks in advance
Graham

 

[Ashereng]

Sorry, can you state the question again?

A double acting actuator is typically air to open and air to close.

On loss of air, it is actually fail indeterminate, not necessariy fail last unless you can block in the pressure on both sides. I suppose you can do this, I just have not seen it.

If you want a fail position, you need a second motive force to "force" the valve to the position on loss of primary air. A common one is a spring. Hence, for a "fail open valve", it is air to close, spring to open.

On loss of power, the electricity to the solenoid goes to no electricity. The solenoid vents, and the air to the actuator goes out the vent of the solenoid. The spring in the actuator then opens the valve (for a fail open valve).

I don't quite understand your OP.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[GrayD]

Ashereng

Thanks for your reply. For our spring return valves the actuator's spring(s) trigger the respective FO & FC valves to their failure positions accordingly.

However my concern is that the FL actuators may fail in a semi closing position with mitigating circumstances as the FL requirement called for "open". PVST is not a process condition and there is the slight chance that process flow would close the valve and this would go against the FL requirement for "open".

In summary the automatic PVST is to be carried out on an Open Valve using a FF Positioner (not Solenoid's), therefore the valve should be returned back to the open position if the air or power failure (4-20mA or FF) occurs during the PVST.

Thanks again, I much appreciate any ideas that you may have.
Graham

 

[Ashereng]

GrayD,

I am a bit confused by your term "FL requirement called for open". A "fail last" valve means on loss of air and/or signal, the valve stays exactly where it currently is.

If FL means "open position", then it is a fail open valve.

Are you mixing your terminology here?


In my opinion, with a double acting (air to open air to close) actuator, even if it is Fieldbus, there is no such thing as a fail position.

You can not fail to a position and stay there.

You need an external motive force to drive and hold the valve to a position.

For a pneumatic actuator, I can think of a couple of plans to address the issue. Neither is elegant or easy.

However, I am still not sure - based on your previous response.

Perhaps one of my esteemed colleagues can help clear my brain, since it is a bit cloudy over here.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[GrayD]

Ashereng

Yes, it sounds confusing doesn't it? Let me try again.

A PVST isn't carried out on a closed valve, therefore on a process trip in FL position, the valve is closed and remains closed. Dito on valve failure.

However, a PVST is carried out on a valve in the open position and in the unlikely event that a PVST is carried out and there is an air or power failure to that valve + a process trip combination, then because the PVST is not a process condition I would like to return the valve back to the open position using any remaining air (via a capacity tank if need be).

My thoughts at the moment include pressure switches on the supply lines (pneumatic and electrical contacts), pneumatic relay and capacity tanks...Just wondering if anyone had come down the same road before with an easier solution.

BR
Graham