What is Fail Open, Fail Closed and Fail Lock in Control Valve Failure Mode

Summary

This video explains the three main failure modes in control valves: Fail-Open, Fail-Close, and Fail-Lock. Viewers will learn how each mode works, how they maintain safety and efficiency during power or signal loss, and where each mode is used in industrial systems. The video covers key components like actuators and valves, and the importance of these modes in maintaining process safety. Examples include cooling systems, fuel supply control, and tank overfill prevention. Understanding these failure modes is crucial for anyone working in process control or valve design.

Takeaways

• 😀 A control valve consists of two main components: the valve and the actuator, working together to control pressure, flow, and temperature.
• 😀 Control valves rely on external energy sources like air, electricity, or hydraulic fluid to operate effectively.
• 😀 Failure modes are the positions control valves take when power or control signals are lost, ensuring safety and equipment protection during failures.
• 😀 The Fail-Open mode causes the valve to open fully when power or signal is lost, maintaining fluid flow during power failures, such as in cooling systems.
• 😀 The Fail-Close mode causes the valve to close fully when power or signal is lost, preventing the flow of fluids in critical systems like fuel supply to heaters.
• 😀 The Fail-Lock mode holds the valve in its last known position when power or signal is lost, ensuring the valve remains in a safe state without fully opening or closing.
• 😀 P&ID symbols represent the failure modes of control valves, using abbreviations like FC (Fail-Close), FO (Fail-Open), and FL (Fail-Lock).
• 😀 A Fail-Open valve uses a direct-acting actuator where the spring force keeps the valve open, while air supply closes it.
• 😀 A Fail-Close valve uses a reverse-acting actuator where the spring force keeps the valve closed, and air supply opens it.
• 😀 Fail-Lock valves typically require additional components like positioners with lock-up valves to hold the valve in its last position, useful in systems needing manual intervention.

Q & A

What is a control valve, and what are its main components?

A control valve is a device used to regulate the flow, pressure, or temperature of a system. Its main components are the valve itself, which controls the flow, and the actuator, which drives the valve based on control signals.

What happens when a control valve loses power or control signal?

When a control valve loses power or the control signal, it enters a specific failure mode, where it moves to a predetermined position to ensure safety and prevent damage to the system.

What is the Fail-Open mode in control valves?

In the Fail-Open mode, if the control valve loses power or the control signal, the valve automatically opens to allow fluid flow. This is often used in cooling systems where continuous flow is critical.

How does the Fail-Open mode work with a direct-acting actuator?

In the Fail-Open mode with a direct-acting actuator, a spring keeps the valve open. When power is supplied, the actuator closes the valve, but if the power or signal is lost, the spring returns the valve to the fully open position.

What is the Fail-Close mode, and where is it typically used?

In the Fail-Close mode, if the control valve loses power or control signal, the valve moves to the fully closed position. This mode is used in critical applications like fuel supply to heaters or hazardous liquid systems, where stopping the flow is essential.

How does the Fail-Close mode work with a reverse-acting actuator?

In the Fail-Close mode with a reverse-acting actuator, the spring keeps the valve closed. When power is supplied, the actuator opens the valve. If power is lost, the spring forces the valve to close.

What is the Fail-Lock mode, and why is it used?

In the Fail-Lock mode, the valve remains in its last position—whether open, closed, or partially open—if the power or control signal is lost. It is used when neither fully opening nor fully closing the valve is desirable, allowing for manual intervention.

Why is Fail-Lock mode typically not shown in simple diagrams like P&ID?

The Fail-Lock mode typically requires additional components, such as positioners with lock-up valves, to maintain the valve’s position. This makes it more complex to represent in simple P&ID diagrams.

How is the failure mode of a control valve represented in a P&ID diagram?

In a P&ID diagram, the failure mode of a control valve is indicated by letters such as FO (Fail Open), FC (Fail Close), or FL (Fail Lock) beneath the valve symbol. These indicate the valve’s behavior in case of a power or signal failure.

What role does understanding control valve failure modes play in plant safety?

Understanding control valve failure modes is crucial for ensuring process safety and maintaining system integrity. By knowing how valves react to failures, operators can design systems that prevent damage, protect equipment, and ensure safe operation in emergencies.