(41v) Vapor Dispersion Modeling from Non-Vertical Vents

Many chemical and hydrocarbon processing facilities include vents for relieving vapor phase materials during a high-pressure event if environmental regulations permit such discharges. The vents are used to release materials to a safe location, away from personnel and areas that could cause cascading impacts – such as an ignition source.

The selection of vent style and location can have a significant impact on the success of relieving to a safe location. Vents are designed in a variety of shapes, orientations and heights. The most common vent is vertical, which allows lighter-than-air streams to dissipate upwards and away from most equipment; even heavier-than-air streams can be prevented from causing hazardous conditions at grade by promoting mixing with air at sufficient elevations. Non-vertical vents are often used to prevent rainwater ingress to the line, leading to an outlet pipe that discharges in the horizontal or downward directions, or may discharge upwards but be capped. However, non-vertical release orientations can lead to hazardous clouds being misdirected towards grade and potentially personnel.

Further, the specific design and release orientation of a vent is often not included on Piping and Instrumentation Diagrams, which instead only include a note to “vent to safe location”. Often times, the vent installation is left to a contractor, without engineering oversight. The ambiguity around the proper selection and placement can lead to vents being placed in areas that may not actually be safe and could endanger plant personnel.

API 521 discusses typical behavior of clouds released from a vertical vent, but not those that are at different angles or have obstructions – such as permanent rain caps. This paper will utilize Computational Fluid Dynamics (CFD) modeling to inform equipment vendors and engineering companies on the potential behavior of hazardous clouds relieving from non-vertical vent designs, so that proper safe locations are selected.