The self-actuated PCV to be FAIL CLOSED.

[DISCLAIMER 0: Please note that this post is about pressure control valves, self-actuated type,used in process and utility systems, and therefore NOT about machinery's "positive cranckcase ventilation" valves]


I take note of Your request, and believe me - I am taking it seriously. BUT please note that a self-actuated PCV - or "pressure reducer"  is - as per definition - self-actuated.

Therefore, as per the definition of FAIL ACTION = "position of the trim when the moving power is missing", albeit "the status of the valve - open or close - when You receive it in Your workshop inside a carton box - and You sneak into the box and see that it is open / or closed" - the PCV gets increasingly closed by the action of the increasing downstream pressure.

So, inside the beautiful carton box, the valve is getting not so much downstream pressure, so You will see it OPEN.

This is the FAIL POSITION for this valve. Simple. No actuation power = no pressure downstream = OPEN. So, they are FAIL OPEN.

Sorry, I can't change this, I am just a humble engineer.

But then You, dear Client, come up with this definition of Yours, about "FAIL ACTION": "You know, the position of the trim when there is a rupture inside".

Ah, ok. So...could You be more specific? What kind of rupture?

And then You're like..."You know, the spring, the diaphragm..."

Ok. The most diffused ones have a diaphragm in the "hat" that moves the stem - the diaphragm gets pressure from downstream and pushes down the trim, to close.
A spring opposes to the diaphragm motion, trying to open the valve trim.

Remember the carton box in the workshop? No pressure = no weapon for the diaphragm = the spring wins = OPEN.

So. Please specify better:
1) do You distrust so much the poor spring?? You are so afraid that the bastard will let You down, right? So You want to know the valve action when the spring breaks. Ok, simple. Spring breaks, diaphragm wins, it gets CLOSED. Do You even want it written on the p&id instead of the normal "F.C." ? Ok, You can call it "u.S.R.C": "upon Spring Rupture: Closed". Shivering.
2) do You happen to hate the diaphragm instead? Ok, long story short: spring wins, valve OPENS.

Unless, the bastards get crummed together somewhere between open and closed position. We'll call 911 and NASA to determine if it is more OPEN or CLOSED.

(Actually, some nice and complex and expensive but very good FISHER valves can have reversed or totally configurable "rupture actions", though they add more and more moving parts and therefore apparently more possibility for rupture...and $$$...)

Are You sure You wanna get sucked into this, dear Client?

Then You, Client, solve it all.
"Well, we will take some instrument air and a control cable to the valve, and so now we can finally have it FAIL CLOSE".


Now You have one more cable - which can fail. One more tubing and instrument air - services that can fail. And then You set the outlet pressure...and what should the valve do when there is not pressure enough downstream? "Open". And when there is enough pressure downstream? "Close". Ok right, and when the DIAPHRAGM that is included in its actuator breaks ?

"Why should I care?" - You say. Well, You cared a lot with the self-actuated PCV...isn't it? Is this a magic diaphragm which never breaks?

And what about the SPRING inside the actuator? Is this a sacred spring made by elves? No. Surprise, it breaks too. Does this valve CLOSE upon any rupture?

Well, NO. It closes when the diaphragm breaks. It tends to OPEN when the spring(s) break(s).

OR, it gets all crummed together and an autopsy will eventually determine which part intruded which other and everything got stuck.

Is Your new valve more reliable? Hard to say for sure, without talking about vendors, but probably not much.
Is Your new valve more expensive? Yes, for sure. AND added up cable and tubing and instrument air consumption.

What do I intend to say?

Dear Client(s), please give us a break. Do You ever go to Wolkswagen to buy a car and want to suggest how big the injectors have to be, or which thickness they should use for the chassis? NO, You just go there and get a car with the Wolkswagen guarantee that it will be reliable and corresponding to Your needs.
So really, give us a break. You need a certain performance, with a certain consumption, at a certain cost, with a specified reliability. Well, You know what? We know what to do. It is our job to make it right, it is what we study in every working hour, it is what we work for. It will be right, and the parts which will not be right, will be corrected by us. 

So, please, RELAX.  And You'll see, oil and gas will keep on flowing, forever.

A good Yule to all our dear Clients around the world.

(disclaimer 1: I do not own the picture hereby linked. They are so nice and I hope the owners won't ask me to remove them)

(disclaimer 2: This story is "freely inspired" to a real one happened to me about 8 months ago in a nice north african country)

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