How to select the fail mode of regulating valve under fail conditions?

The selection of the pressure regulating valve fail mold:fail open or fail close,that is mainly based on the safety of the production process and the protection requirements under fail conditions. The specific selection method is as follows:

Basic definition of fail open(air close) and fail close(air-open)

Fail open(Air-close) valve: the valve opening decreases when the signal pressure increases, and it is in a fully open state when there is no signal.

Fail close(Air-open) valve: the valve opening increases when the signal pressure increases, and it is in a fully closed state when there is no signal.

Selection principles and basis

Process safety priority principle

When the signal is interrupted (such as power failure, gas failure or failure), the valve should be in the position that minimizes harm to people and equipment.

If it is safest to have the valve closed (e.g. to prevent leakage of harmful media), choose an fail close(air-to-open) valve (which automatically closes when the signal is interrupted).

If it is safest to have the valve open (such as to avoid dry burning or clogging of equipment), choose an fail open(air-to-close) valve (which automatically opens when the signal is interrupted).

Example:
The pressure regulating valve of the heating furnace fuel pipeline needs to be an fail close(air-open) valve, which cuts off the fuel when the signal is interrupted to prevent the furnace temperature from getting out of control.
The pressure regulating valve for the boiler water inlet pipeline must be an fail open(air-close) valve to maintain water supply when the signal is interrupted to prevent the boiler from burning dry.

Service Medium Characteristics Considerations

Inflammable, explosive or toxic medium: Fail close(air-open) valves are preferred, and the valves should be closed when the signal is interrupted to prevent leakage.
Medium that is easy to crystallize or solidify: It is preferred to use an fail open(air-close) valve to maintain flow when the signal is interrupted to avoid blockage.

Economical and quality assurance

When the signal is interrupted, the valve state should avoid product quality degradation or raw material waste.
If closing the valve can reduce losses (such as stopping the production of unqualified products), select fail close(air open) valve.

If opening the valve can maintain production continuity (such as preventing reaction interruption), select fail open(air-close).

Fault-safe function implementation

Pneumatic actuator type

Single-acting actuator: built-in return spring, automatically returns to position (open or closed) when the signal is interrupted.
Double-acting actuator: no spring, need to use positioner or retaining valve to achieve fault retention.

Accessory combination

Quick pressure relief valve: When the signal is interrupted, the air is quickly released to make the valve move to the preset position.
Pneumatic position-holding valve: cuts off the air supply when the signal is interrupted, maintains the current valve opening and avoids disturbance.

Mark and selection criteria

Fail Open(air-close):FO
Fail Close(Air-open):FC

Selection process

Identify key process parameters (e.g., temperature, pressure, flow).
Assess hazard scenarios when signal is interrupted.
Select fail open(air close)/fail close(air-open) based on media characteristics and safety requirements.
The rationality of the selection is verified through methods such as HAZOP (Hazard and Operability Analysis).

The service application

Fail open(air-close) valve applications:
Cooling water supply (to prevent equipment from overheating), raw material transportation (to avoid reaction interruption).
Fail close(air-open) valve applications:
Fuel control (to prevent overheating), emission of toxic media (to avoid leaks)

Notes

Positioner function

Direct acting positioner: When the input signal increases, the output pressure increases.
Reverse acting positioner: When the input signal increases, the output air pressure decreases.
The combination of actuator and positioner (such as direct acting actuator + reverse acting positioner) can realize the air-to-close function.

Structural differences

Small-diameter regulating valve: fail close(air-open)/fail open(air-close)is achieved by changing the direct/reverse action of the actuator.
Large-diameter regulating valve: fail close(air-open)/fail open(air-close) can be achieved by changing the direct/reverse installation of the valve core.

Summary

The selection of regulating valves for fail open(air close) and fail close(air open) needs to be based on process safety, medium characteristics and economy. The core logic is to ensure that the valve is in the safest state when the signal is interrupted. Through HAZOP analysis, symbol identification (FC/FO) and accessories (such as position-locking valves), fault protection can be systematically implemented. In actual selection, it is necessary to combine specific process scenarios and give priority to personal and equipment safety.