Leak from a tube trailer during filling

The leak was located at a tube valve at the rear of the semi-trailer. It occurred at the beginning of packaging the semi-trailer, which was at a pressure of 50 bar.
The expert appraisal showed that the nut tightening torques were less than 150 Nm, while the prescribed value was 250 Nm. One of the nuts was so loose that it was possible to turn it manually.

The INITIATING CAUSE was a valve leaking because loose.

The ROOT CAUSE was the failing to execute the correct procedure when fastening the nuts responsible for the sealing of the valve. A too low torque was applied to one of them.

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The leak occured at the begin of the operation of filling a tube trailer

No injury, property loss and environmental impact.

The leak was discovered by employees which heards a hissing sound. The area was secured, and the neighbouring packaging stations, which were non in function, were locked out. The semi-trailer was moved for emptying and purging with nitrogen.
To identify the location of the leak, the operator carried out gas leak detection checks using a "thousand bubble" gas leak detector and an explosimeter.

The attribution of a root cause to this and similar events is very challenging. At first sight, it is very clear: it was a human mistake; the valve was leaking because its nuts had not been properly fixed, despite the procedures were requiring a specific target torque value. However, there could be circumstances which contributed or even determined the mistake. Ere the procedures clear and easily available? Was there time pressure, or inadequate skill and competence levels, or even road vibration causing the loosening? The attribution of the root cause to one factor is usually triggered by an absence of critical details, rather than to clarity of the analysis.