by Peter Foster
(New Castle, Delaware)
We use a Festo 170248 5/3 closed centre valve to actuate a double acting cylinder.
I understand that if the air or electrical supply is interrupted the cylinder will remain in position (except for some creep due to the different surface area at each end.
My question is if the valve function itself fails (spring/leak etc.) do these valves default to the centre closed position?
We are trying to answer a safety question as to what happens if something goes wrong with the valve. Thanks. I’ve attached the schematic, the valve in question is the one going to the DNG-200 cylinder.
___________________
I spent some time working as a territory rep for Festo, so know that if you contact them, they will have a distributor rep by to see you in no time, at least that’s the way it used to be.
Having said that, the best thing to do to determine the answer to your question is to look at the schematic of the valve.
I have uploaded a copy of the Festo 170248 5/3 closed centre valve schematic, and ask you to review it. In particular, look at either end of the schematic, and note the graphic that denotes a spring.
Since there is a spring on either end, if this valve fails due to loss of electricity or air pressure, it is designed to “spring center”, and in so doing, block all ports.
If there were no springs, “sticktion” would come into play, and the valve spool would likely stay in the position it was last in if the power and air disappeared, unless vibration moved the spool. That would not be a safe type of valve configuration to use.
In other words, the valve you have selected is designed to “fail safe” by blocking all ports and stopping any actuators in the event of a power or air disruption.
Hope that helps.
Cheers,
Bill
________________________________________________________
Festo comms
Hi Bill,
Thanks for the response. Festo seem to concur with what you say.
I spoke to them and got a couple of varying answers. They obviously don’t want to commit but the 1st guy said: with no electrical power it centre closes, with no air it centre closes, with a broken spring it will still centre since the springs are designed not to go past centre.
The 2nd guy agreed with the 1st but was less committed in his response and said they couldn’t say how far the spool might move with 1 spring failing due to variable friction etc. which I can also understand. I think it depends a bit on who you talk to.
So overall it looks like we are OK. The rub will come in the safety review when we talk about failure modes and whether any redundancy needs to be built into the system. I’ve seen double pilot valves for presses, lots of bucks and probably an overkill. However, the user does have his hand under this ram a lot of the time so we’ll see.
We are having a Festo rep come in to take a look also.
Thanks again, great website by the way. v. useful.
___________________
Thanks for the comment about my website. I’m sure glad you find it useful.
With person safety, the question is, how far do you go?
The valve is supposed to work. If a spring fails, it may or may not.
In order to take that out of the equation, then plumb in double redundancy so that if one valve fails, the other valve will do the job.
The odds of both valves or valve components failing at the time of a power or compressed air outage are probably more remote than you winning the power ball lottery.
Is that safe enough? That’s up to the safety committee.
Festo is a first class company selling extremely well built air components. But even at that, they cannot determine the conditions of use or how a product will ultimately fail, and the ramifications of that. That’s up to the user.
Again, thanks for your kind comments.
B.
______________________________________________
Valve failure
I don’t know your exact application, but perhaps some sort of pusher could be made/used to keep the operator’s hand completely out of the potential danger area?
________________________________________________
Good thought, Doug. The device in question is actually called an “anti-tie down” and it’s purpose is to ensure that an operator’s hands are not ever in an area where they could be impacted upon.
B.
__________________________________________________
Double redundancy
Just a follow up on this, the only thing I can see that will give that redundancy is something like one of the Ross safety valves which are usually used for presses.
http://commerce.rosscontrols.com/
These are designed to return the cylinder to a safe or retracted position if one spool doesn’t move as expected. This seems an overkill for us, I’d like to find a compromise. The problem is detecting the fault once it occurs.
_______________
A pressure sensor can tell a system if a particular valve has shifted or not, that can be tied into a blocking check valve that locks a cylinder if necessary.
There are lots of folks in the industrial pneumatic distributor business that can help you design the appropriate circuit and specify the products you need.
Circuit design is out of the purview of this site. If you want some help with that send me a message through the contact page and I’ll provide rates.
