Single valves can leak because they are not fully closed, are fouled or just leak. During working behind a single valve the valve might be accidentally opened or start leaking, releasing chemicals.
- During and after line breaking due to a repair or maintenance activity.
- When the plant is not fully deenergized.
Challenges in the Field
- Older plant design often might not provide a second barrier or full block and bleed option to isolate equipment.
- Placing a blind, turning a spectacle flange.
Options to Get it Right
- Realize when it is not possible to work behind double isolation.
- Try to remove the substance or energy in the system before start working behind a single valve.
- If isolation by a single valve cannot be avoided:
- Validate that the single valve is not leaking e.g. at a drain point downstream of the isolation, or by a pressure gauge.
- Mechanically lock the isolation valve handle to avoid accidental knocking open during the task, deactivate the actuator for automated valves after checking the valve fail-safe position.
- Mount a spade or blind flange after the single valve directly after the line break.
- Consider if emergency responders should be in place during the line break, until the blind-flange is placed.
- Wear appropriate personal protective equipment (PPE) during the task.
- Keep working time short and avoid critical process conditions during.
This page only summarises the guidance – refer to the EPSC website for further information.
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