During routine maintenance, a process operator was cleaning a screen box (inline filter) connected to pipework below a thickener tank. He opened the drain valve to verify isolation before undoing four bolts to open the screen box door. On opening the screen box door, the operator found the screen full of material. As he pulled the screen out of its housing, a blockage upstream of the screen box dislodged. The operator was engulfed in 95°C caustic solution, receiving thermal and caustic burns to his body and face.
An emergency responder found the main isolation valve open.
Direct causes
- The main isolation valve was open, so the screen box and associated valves were not isolated.
- The process operator was in front of the screen box when the blockage dislodged.
Contributory causes
- The isolation of valves was not verified through either a second and independent method of ensuring all valves were closed, or checking the effectiveness of each isolation point by separately observing a “change of state”.
- The operator mistakenly thought he had closed the main isolation valve but its function was actually being performed by a blocked pipe.
- Excessive scale had built up inside the vessel after scheduled maintenance was delayed.
- It was difficult to determine the position of the main isolation valve (i.e. open versus closed).
- Operators were not required to lock or tag isolation points for routine work.