Selection Criteria for Flow-to-Open and Flow-to-Close Control Valve

I'm confused as to exactly what you are after. Is this a hydraulic, pneumatic, or process control valve? You will need to know your desired flowrate through the valve, pressure rating of the valve, the pressure drop curve across the valve, function of the valve, etc.

Without more specific information I cannot really help much more.

I'm not sure, but the issue of interest seems to be what will happen if air pressure fails (when used to control various valves.) Trim type, flow rate, and actuator size are at most secondary considerations.

If control air pressure is lost (compressor failure, large line leak, etc.), certain valves may need to go open, and others to go closed. A "flow to open" valve will close on loss of air pressure; a "flow to close" valve will open on loss of air pressure. Select accordingly.

Really you mean flow to open or fail open?

Flow to open (or close) can be applicable to some valve types. Some butterfly control valves in low flow conditions do not open or close on flow. Balanced globe valves are similar.

Usually, control valves fail to its (supposed) safe condition. If flow (differential pressure) helps to open/close, the actuator must be selected accordingly. In some cases direct action, other reverse...

It's the same old story: How do you want your system operates?

Metla

Selection basis of flow direction is based solely on trim design. For example, cavitation trims are generally flow down while noise reduction trims are flow up.

Since flow direction is based on trim design I recommend you leave this decision to the control valve manufacturer.

Flow rate do not have direct influence on trim design. Flow and pressure diferential do infuence valve body and internal diameter. On the other hand if you have particles in your stream you must inform valve manufacturer. This is critical for valve body and trim selection.

Actuator type and size should meet the selected valve body and trim design.

Regards

David L V Rodrigues

David,

Thanks for your reply.... I have got my answer...

Depending on what you actually mean let me answer if what yuou mean is the failure mode ie does teh valve close or open if you lose the air to the control valve.

You need to consider the consequences of the valve being in a 100% position

so the liquid outlet from a vessel would normally be Fail to Close so that yopu keep liquid in the vessel and dont have the danger of gas blowing through the valve to the downstreeam operation.

But if the liquid is then routed to a furnace heater I would go fail open as its better to have some fluid in the furnace tubes than not.

Similarly generally I would hve the steam supply control valve to a steam heater as fail close to avoid excessive heating of the process fluid.

I hope this gives some idea unfortunately there are so many considerations that every case is different but you need to start from considering what is the worse option having the valve fail open or fail close.

My understanding is that every control valve can be set up to fail open or fail closed so the design of teh vlave flows etc have no impact.

A good answer for a flawed question. I think OP really means "Fail-to-Open and Fail-to-Close Control Valve". Basically it is a process requirement. In case of failure of instrument air supply to power cylinder or diaphragm of a control valve, valve can be either at full close or full open position. Most of cases process may require fail-closed. Only in few case need fail-open like furnace you have mentioned and anti-surge-valves of compressor.

Some process require fail-to-position also. Normally electric actuators are used there.