Fail mode on valve 1

[alienitmeca]

Hello everyone,

I am elborating a datasheet for a control valve to be used in fire fighting project. While researching, I came across the term "fail mode" which means that I should choose whether the valve is going to be either open or closed in case of power failure (according to process).
After discussing with some suppliers, I understood that adding this cretria to the valve will increase its cost.

My question is as follows:

- Is it obligatory to have a fail mode for the valve in this appllication (fire fighting project) ?

- Does any NFPA code specifies it or at least mention it ?

 

[GAL02]

Dear Alienitmeca, Good morning!
Its easy to choose valve (position either close or open) to you but pl. check what's process requirement?
they may want this valve either close or open at any plant upset, so it depends.
Moreover valve cost is not more important than production loss or plant at risk.

With regards

 

[LittleInch]

Is it obligatory - yes.

This is fire service ( water line?). What do you think would be the best solution in case power failed during a fire situation? To close down the supply of water or increase it?

It's not common to have such a valve on a critical service like this precisely for these reasons. Most fire systems are required to work without relying on outside power supply - why is it there? What are the pressures in operation? what happens if it fails open?



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

 

[alienitmeca]

Thanks for your replies,

this has been very helpful

 

[zdas04]

Failure mode is a bit more complicated than you've presented it. There are two broad areas of concern: (1) what failed? and (2) what you want the valve to do on the failure.

What failed can be: (1) power supply; (2) control signal; or (3) piping/fittings. The one that most people are most concerned about today is loss of power as in "what do I want a pneumatic actuated valve to do on loss of air". this is the concern that is slowing the inevitable transition from pneumatic control using process gas to electric control (which so-called "greenhouse gas" concerns are forcing on the Oil & Gas industry). If you lose electric power unless you have some sort of backup (like batteries or capacitors) you can't move the valve any more. Loss of control signal is a lot easier because you still have motive force and you just need enough logic in the valve itself to cause the valve to move one time. It is common to put a fail position on loss of process pressure (e.g. a pipe rupture), and for that you have to identify the instrument(s) that will represent pipe failure and how to differentiate a broken pipe from a failed transducer. This one can be tough to implement.

Fail position is something that people don't think about nearly enough. I often hear people talking about "fail safe" as though that always meant "fail shut" but it doesn't. I used to work in Nuclear power and we had a motor valve on the main steam generator outlet. If it was to fail open or closed, there is a real chance that the rapid increase in steam demand could cause a reactor failure, therefore "safe" was "fail as is" or "fail in place". Often there is an automated vent or blow down valve on a compressor that is required to be open while the compressor is off line so the "safe" position might be "open". The system designer needs to look at every automated valve and decide for each possible failure mode what position he wants to leave a valve in. I've seen failure matrices for plants that had hundreds of entries and almost all of them were "shut" (e.g., Valve 063-093 Loss of power--shut; Loss of air--shut; Loss of pressure--shut) and then there will be a dozen valves marked "as is" and 3-4 marked "open" and a few with different answers for different conditions.

Remember that this discussion only applies to automated valves. Manual valves can only fail as is. Specifying a fail position on a manual gate valve will get you some questions.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist