We are often asked about mitigating human factors in process safety management and oftentimes we reference this example to reiterate the importance of it.
The scenario
A plant operator was instructed by a CRO (control room operator) to verify the liquid level in a vessel by viewing the vessel gauge glass in the field. The plant operator could not see a liquid gas interface in the gauge glass and so he assumed the vessel was empty. He radioed the CRO informing the CRO that the vessel was empty. The plant operator’s reading of the gauge glass aligned with what the CRO was seeing on his display with the exception that the CRO was also seeing fluctuating pressure levels. However, unbeknownst to everyone was the fact that a limit switch inside the vessel had failed. Unfortunately, the limit switch did not fail in a safe mode. In other words, it did not stop the feed to the vessel when it failed. As the vessel was filling up, the pressure in the vessel increased until a PRV (pressure relief valve) downstream of the vessel opened, allowing liquids to flow through to the next stage of the process. Because of the initial high pressure and the opening of the PRV, slugs of liquid under pressure were carried downstream resulting in a process upset that caused millions of dollars in damage and downtime. How could this process upset have been prevented?
Ideally, when a plant operator checks a gauge glass, he/she will perform a process referred to in industry as “blowing down a gauge glass”. This simply means the plant operator will drain the liquid from the gauge glass and allow it to fill up again before he takes his reading. This ensures that there are no blockages and the gauge glass is providing the correct reading. This process typically takes 1 to 3 minutes. In the case described above, had the plant operator blown down the gauge glass, he would have discovered that the vessel was full and immediately radioed the CRO who, in turn, would have shut down the feed to the vessel.
The solution
SMT Learning discovered during subsequent discussions with CROs that, when they instruct a plant operator to check a gauge glass, they assume the plant operator will blow it down each time to ensure it is working properly and giving an accurate reading.
As part of risk management, it is important to validate operator’s know-how to perform various tasks competently and safely. This can be achieved through the implementation of a competency development and assurance system.