Coupling in Process Safety

Talking with Strangers by Malcolm Gladwell again. In this book Malcom further explores why the style of policing which employs the "holy fool" mindset is not appropriate in all locations. Many of us hold a strong belief that if people want to violate the rules or do a certain act which is quite final that they will do it anyway. People who speed will speed no matter what. People who don't follow procedures just will never follow procedures even if they are amazing. People who want to commit crimes as a means to "earn" their living will do it where ever they are, it is independent of the location.

Research however does not support this. Research shows that in most towns and cities 90% of the crime happens in less than 10% of the area of the city. Instinctually we know this and avoid certain neighbourhoods. It has even been tested that if it was not possible for the crimes to happen at that location (due to increased police presence and searching) they don't actually then happen elsewhere the crime rate drops.

Coupling in Engineering

We have all seen the work arounds on site. A hop up constructed to reach a valve which was inaccessible. A well worn path which is not the paved route or the sneaky bucket with a few tools used on night shift for clandestine tasks (if you've worked in operations you know the tasks).

The reason for all of these work arounds is our failures as engineers in design to anticipate what the human wants or needs to do in the situation. We have therefore created the conditions for the human failure baked into our design. Perhaps that human failure does not result in an incident today but one day. These people who deviate from the "safe practices" or the procedure are not born rule breakers, they are "can do" people finding a way to do the thing you asked them to do. As engineers we need to bring these lessons into our designs through the right attendance in meetings, the right reviews at the right time and lessons learnt from the past.

Lessons From The Past To Avoid Your Next Coupling Mistake

In some applications we use some very powerful pumps to increase the pressure of the fluids we are moving. If these are positive displacement pumps we can install pulsation dampeners on the inlet and outlet to reduce vibration of the line and pump to smooth the liquid flow. There are two approaches to this design. There is the cost effective (perceived cost effective) way of designing which is to design the suction for the suction pressure and the discharge for the discharge pressure. This works well in most applications except pulsation dampener charging. If the operator on the day mistakes the suction dampener for the discharge dampener and pressures it up to the discharge charge pressure we have a person right next to an unprotected HP/LP interface while it is being over pressured. This is not a good scenario and has led to injuries in the UK alone. The second route (which increases costs slightly) is to rate the suction and discharge to the same pressure up to the first isolation valve from the pump (or within the recycle loop). This then means that charging the dampener to a higher pressure will result in operability issues on restart rather than catastrophic failure.

In these two incidents from the CSB (Lyondell Bassell and Batton Rouge) the valve assembly itself was designed in such a way that there was confusion about which bolts to remove to remove the actuator. In both incidents the work party incorrectly removed bolts which had a pressure containing function leading to loss of containment and loss of life. We don't often get down to the details of valve assembly in our HAZOPs however at some point someone in the supply chain needs to be accountable for assessing this.

The final example is actually one where production pressures resulted in the engineers making an error (rather than the operator). In this WorkSafeBC example (Mushroom Composting) the operators were having some repeat issues. The first issue was that water was accumulating in the bunded area around the pumps which was flooding the pumps and then stopping the flow of water to the mushroom facility. This was solved by elevating the pumps above the potential water level. The second problem (introduced by this solution) was that the inlet to the pump was now a stagnant column of liquid exposed to the elements (longer than it had been before and above the bund wall) which now led to freezing of the inlet pipework. Again leading to stopping the flow of water to the mushroom compositing facility. This was resolved by installing an enclosure around the pump. A further issue which was introduced (or worsened) when elongating the pump suction was the potential for debris to block the inlet line. We now landed on the final coupling which resulted in the accident which was that the operator had a work around to try to dislodge this debris. This action released toxic gasses which would (without the shed) have been dispersed to concentrations which would not have had fatal effects. Viewing each problem in isolation was the error by the engineers. Each change only looked to solve the immediate problem without identifying potential other issues introduced.

Which examples do you have of coupled incidents?

Disclaimer: My articles are based on my engineering experience in two major operators over 11 years and consulting thereafter. They do not reflect the processes of either and are not endorsed by either nor the Safety and Reliability Society or the IChemE. If you find an error in the text please feel free to correct me as I too am prone to human error (although I like to think at a lower than average frequency). The intent of these articles are to educate. From beginners to experienced engineers. I hope that through the article or reference material you all get something from it. If you find the content too basic or too advanced again get in touch. Any improvement suggestions welcome.