An incident occurred at a plant with two processing units (Unit 1 and Unit 2) that shared a common line to a thermal oxidizer (Figure 1). Several instruments were due for calibration on Unit 2. Specifically, a pressure transmitter (PT) needed to be removed for calibration, so the hand valve upstream of the PT was closed and locked out.
A few days after Unit 2 was locked out, Unit 1 began to vent toxic gas through the shared line to the thermal oxidizer. The vented material from Unit 1 flowed through the line toward the thermal oxidizer. However, rather than exiting the system at the thermal oxidizer, the material made its way to Unit 2 and escaped through the valve where the pressure transmitter had been removed (Figure 2). A cloud of toxic gas gathered in the vicinity of the valve.
When moved, the valve handle “felt” like a properly operating valve. Nevertheless, an investigation discovered that the ball inside the valve was in the open position, even though the valve handle was in the closed position (Figure 3). This malfunction could have injured personnel, but fortunately, no one was harmed.
Did You Know?
- Any valve can fail in many ways, including a failure of the handle, stem, ball, or seat.
- Valve handles may not always indicate the actual position of a valve.
- The effects from one system can be felt in another and must be considered when altering a system, even temporarily.
What Can You Do?
- Use blinds, plugs, or caps whenever piping is open.
- Use accurate line breaking or opening procedures for operations involving open piping.
- Consider the effects of interconnected systems and always walk the line before transferring material.
All valves can fail. Consider double isolation.
©AIChE 2019. All rights reserved. Reproduction for non-commercial, educational purposes is encouraged. However, reproduction for any commercial purpose without express written consent of AIChE is strictly prohibited. Contact us at email@example.com or 646-495-1371.
Would you like to reuse content from CEP Magazine? It’s easy to request permission to reuse content. Simply click here to connect instantly to licensing services, where you can choose from a list of options regarding how you would like to reuse the desired content and complete the transaction.