valve actuator and positioner

[rika kose]

Not sure if it is the right place to ask this question.

We have a butterfly valve. the valve has to be controlled at different opening positions. Valve vendor seleted double acting actuator (pneumatic). positioner (elector-pneumatic). Valve vendor guaranteed that the valve's fail safe position is "stay in the last position".

But one techincian insists that if electricity is down, the valve will not remain in position. The eletricity is only signalling on the positioner. I don't belive that positioner is acutually controlling the valve
since there is no instrumentation engineer available, I need some help from internet.

Thanks in advance.

 

[TiCl4]

You really need to discuss with the OEM to confirm the positioner is also fail-in-place. During an electrical failure, the 4-20 mA to the positioner will drop to 0 mA. You need to know how the positioner will behave at 0 mA signal.

Fail-in-place valves are normally categorized as being without spring returns so that a loss of air supply pressure generates no change in valve position. That does not mean the positioner won't do something crazy when the electrical signal drops out but air pressure remains.

 

[rika kose]

thanks for the quick reply.
The selected positioner is SMC IP8000/8100.
According to the vender the positioner is also fail in place.

 

[ashtree]

I agree with TiCl4s comments and you should also check what happens if you lose air pressure.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[TiCl4]

Rika, if the OEM also states the positioner is fail in place, the only thing left to do, if you are still concerned, is to bench test the valve. Have your I&E tech hook up power and air and send a 12 mA signal (should be 50% open), then disconnect the wire or turn the unit off to simulate power loss. You can do the same with with valving out the air supply on the bench to ensure both loss scenarios are covered.

That said, did the vendor SPECIFICALLY say the positioner was fail-in-place on loss of electrical signal, or that it was fail-in-place in general (in which case he may be referring only to loss of air)?

 

[LittleInch]

I had a look at the manual and amazingly the impact of loss of 4-20 signal is not mentioned!

So you cannot say what happens to the output air signal on loss of the inlet signal.

"I don't believe that positioner is actually controlling the valve" Errrr, what do you think is then? That's is its job in life, to control the position of the valve using a 4-20mA signal and feedback from the valve itself. Lose the signal and it is not clear at all what happens to the positioner.

Maybe it just shuts in place, maybe the springs go open or closed or maybe it goes a bit mad?

Try it and see!

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

 

[gerhardl]

With pneumatic actuators with different positions regulated by air pressure to one or both sides, the only way to ensure the valve remains in the exact position if a failure accurs, is to lock the air supply to both sides of the actuator.

The conservative (safest?) way to do this is to have separate failsafe solenoid valves (no current closed) for the air supply.

If you in addition want a locked system by low air pressure you need to cut the current to the valves through a signal from a pressure switch.

You have now following possibillities:

1. Check if the exsisting system works to the same results as above.
2. If not complain and/or add relevant safety components.
3 Or, my first choice, if suitable: change to a modern ecuipped electric actuator.