Pneumatic Control Valve Fail Safe Position (When Instrument Air Fails)

You have to be smarter than the valve.

Depending on the valve and how it is configured, it could go either way.

It won't normally stay in an intermediate position.

I really don't know anything about valves.

Good luck.

Switch off the air and watch very carefully which way the valve stem moves - Bob's you uncle.

If instrument air fails you want the valve to fail in the safest position. Depending on the service this could be to fail open or fail closed. This is marked on the P&ID drawings, it is also mentioned in the control narrative.

It is also possible in many case to look at the valve and see on which side of the diaphragm the air is acting.

In some plants Green ones fail closed and red ones fail open (or the other way round or something like that.)

If that is not clear then go back to the first sentence.

This is marked on the P&ID drawings

You hope so. As a control engineer for a systems fabricator, I have seen this not be true even in the field, with some dire consequences. Shouldn't happen, but it does. A NO valve and a NC valve usually look the same, and this can be over looked. The P&ID is a guide. It is up to the person verifying the system to ascertain it is true. So the comments on how to ascertain NO versus NC apply.

This depends on the fail safe action the valve shall take in case of loss of instrument air for example in you power plant the gas or oil fuel valve is fail close as well as the feed water valve and main steam valve to the turbine on the contrary any vent valve or drain or blow down valve shall be fail open

as Kaisan mentioned you can know the type of valves by many ways like P&ID you will see under the control valve FC (fail close),FO (fail open) or FL (fail locked) also if the valve is air to open so it is (FC) otherwise if it is air to close it is FO

HI,

Valve action is determined by the process, for example a valve in a flare line needs to be FO fail open because if the power went out in a plant the flare line needs to stay open to depressurise equipment into the flare.

You can find the action of some valves by looking at them and using common sense, for example a sliding stem diaphragm pneumatic valve with air supply in the bottom of the actuator will be a FC valve, say if a butterfly valve with pneumatic positioner is 25% open with 6 psi air signal coming in the positioner will be a FC valve, if the same valve is 75% open with 6 psi air signal to the positioner it will be a FO valve.

I hope this helps...Have a nice day

If you are the engineer of the system, you should know. You want the valves to go to a safe position when the valve fails, the solenoid fails, or the power fails. This is always the same. If you have a spring center or a spring return valve, the fail mode is in the sprung position (center or return above.) As engineer, it is YOUR responsibility (and yours only) to be sure they are specified accordingly and called out properly on the PID dwgs. As mentioned above that responsibility is often shirked.

If you are maintenance, you CAN'T guess. You either have to know, have to follow the PID, or have to call the engineer. If none of those are an option, then you either become the engineer, or you hire one. In most cases this is not so critical that you need to hire in an engineer just for one valve, but if you have a pile of them in a steam system (high pressures and explosion danger), you would be best to do so. Otherwise, think of it as: "What is the best place for the valve to be if the power is dropped, an e-stop is hit, or the crap hits the fan?"

Usually clear logic will tell you so, but if there is a question, consult a higher power that UNDERSTANDS the system and the ramifications of failure.

Best of luck and don't cut corners on this one...

Hello Marbz,

You have not supplied enough information for an educated answer with the following guidelines: The fail position of control valves may be of critical importance. Human lives or the economic life of the plant may hang on the fail position of a control valve. For example if the process is manufacturing explosives in an exothermic reactor cooled by the water cooling valve you are repairing, failing in the open position might be a good idea. If you are replacing a control valve, do not arbitrarily change the fail position of that valve! A life and your job depend on your thoughtful and careful work. If you do not know the answer to the question you have posed, you should consider hiring a Professional Engineer to assist.

Regards,

Luther M

thanks....I will still check the valves...the plant is so old that i cannot find the PID's anymore...thanks a lot!

The other post pretty much explain the issues with fail-open, fail-closed, and fail-in-place considerations.

Fail in place actuators require a balanced diaphram (piston) in which motive air is supplied to both sides of the piston. I cant remember exactly how the control works, but a qualified instrument/control engineer can tell you how it is done.

To help decide which control action you need, conduct a failure mode and effect analysis of the system in which the valve is installed. The correct failure mode should then be obvious. But, even then, sound judgment is required.

The valve manufacturer may be able to assist in selecting the right failure mode for your application.

I would also like to point out that once the selected valve is installed, regardless of the failure mode, one must confirm the failure mode through testing. Simply shut off the actuator power source and observe the valve action. Write the results into a logbook to record the test results. As a Quality Control manager told me, "If you don't write it down, it didn't happen."

Even with the right failure mode, frequently the valve action is not fast enough to accomplish a fail safe condition. For this reason, spring loaded or power assist actuators may be necessary. For example, in an application with which I worked a 12 inch, 1500 psia rated, steam admission valve on a power turbine had to achieve tight shut-off in less than 0.5 seconds. For this application we used a spring loaded, fail-closed actuator with a pressure boosted air supply. Start-up and post repair testing was mandatory to confirm fail safe operation.

Carry out a formal HazOp Study on the plant. Consider air failure at each valve. The outcome will determine the need for air-fail-open, air-fail-closed, or air-fail-stay-put.

What about to see the pneumatic actuator itself or the actuator nameplate. If actuators are diaphragm type and knowing the valve type, looking at the position of the return spring (over or under the diaphragm) and the air inlet, may be enough to know which direction the stem will move upon air failure.

Kind regards