Consequence modelling: How to reduce the risk of an accidental release at your facility
October 28, 2019
October 28, 2019
New regulations highlight the importance of reexamining modelling after facility design changes
Wastewater treatment facilities. Refineries. Pipelines. Manufacturing plants. Industrial facilities safely and effectively deal with toxic or flammable materials every day. Even so, accidental releases do occur and can have negative effects on the environment and the surrounding community.
I’m an air quality scientist with expertise in consequence modelling and quantitative risk assessments. So, what is consequence modelling? It’s the process of predicting the scale of an accidental release—and it’s a critical part of emergency response planning and risk assessment for your facility.
Consequence modelling is a two-step process: First, we determine how the hazardous substance will be released. Second, we determine how far that substance will travel through the air or on the ground (or both.) This also means understanding your processes so that you can be confident that the worst-case accident is captured to help you plan accordingly. When you plan for the worst-case scenario—and you’ve designed your facility accordingly—you’ll be ready to deal with the consequences of an accidental release (as minor as they might be).
Consequence modelling has never been more important as the Canadian regulatory environment continues to mature with respect to accidental releases. For example, Environment and Climate Change Canada has released new regulations that focus more than ever on identifying worst-case scenarios and understanding their consequence to the environment. The 2019 Environmental Emergency Regulations came into effect on August 24th and cover the release of hazardous substances, including the inhalation of toxic materials and hazards from explosions, pool fires, and other forms of combustion. This complements other guidance recently released in Canada, such as the CSA-Z767—a new CSA standard for process safety management.
So, how can you reduce the risk of an accidental release at your facility? Well, it starts with your design process. But it doesn’t end there.
It’s important to remember that consequence modelling is not a one-time exercise. It should be considered throughout design, construction, and operation of your facility. While there are tools to identify when revisiting consequence modelling that may be appropriate, it can still be difficult to appreciate which changes to a process or facility will be more likely to change the predicted consequence from an accidental release. The concern is that one small change can mean a huge change in your hazard exposure. Let’s look at some real-life examples that I recently presented at the Canadian Chemical Engineering Conference, held in Halifax, Nova Scotia from October 20–23, 2019.
Our atmospheric sciences group completed consequence modelling for a wastewater treatment plant. This plant was collecting biogas to offset fuel consumption. This biogas was flammable and contained a high concentration of carbon monoxide, and our work helped confirm set-back distances for their process to avoid explosion risks. A year later, construction was scheduled to begin on a safety vent designed to collect this biogas in the event of an accidental release. Trouble was, over that time, the design of the vent had changed to include a weather hood instead of pointing the pipe sideways. Even though the rest of the design hadn’t changed, that small modification from a horizontal to a vertical pipe tripled the predicted consequence. Fortunately, proper process safety management captured the need for revised consequence modelling, which allowed our engineering team to revise the design and avoid the risk from such a release.
The design can change throughout the life cycle of a facility—and just one small change can mean a big change in your hazard exposure.
In another case, a client at an oil and gas facility contacted our team to review the venting from a safety valve on top of a storage tank. Normally, this work involves thinking about the different hazardous situations that can occur and then selecting the worst-case scenario for consequence modelling. We chose a scenario where a flow control valve feeding gas into the tank failed. The result of this failure was that gas would keep flowing into the tank even when it was full and, eventually, that gas would vent into the atmosphere. The safety vent was designed to quickly and safely reduce the pressure in the tank within a matter of a few seconds. What was not captured until after we reviewed the release scenario was that, once the vent pressure reduced, the vent would close again. Since the flow control valve failed, the tank could re-pressurize, leading to a repetitive scenario where there is a period of venting, followed by re-filling of the tank, and venting again. For toxic gases, this repeating scenario presents a substantially greater consequence than a single venting. By carefully considering the accidental release scenario, our conclusion gave our client better insight into how their facility and process design impacted worker safety.
The new Environmental Emergency Regulations remind us that there is never a bad time to reexamine if your consequence modelling still reflects your facility’s process design. As a facility owner or operator, process safety management should always be at the forefront of your facility design—from initial planning right through to operations and maintenance. Every assessment will be unique, so choose the right approach for your facility to protect employee safety, as well as the environment and surrounding community.