Dates: May 29, 2026
An incident in Pasadena, Texas, underscored the critical need for careful evaluation of discharge locations following atmospheric releases, as even properly functioning safety devices like pressure relief valves can inadvertently lead to hazardous situations if the consequences of a discharge are not fully understood. Ensuring that safety standards, such as API Standard 521, are thoroughly integrated into operational protocols can help mitigate risks to personnel and the environment during such events.
On May 19, 2018, personnel at an ethylene vinyl-alcohol copolymers (EVOH) production plant in Pasadena, Texas, were starting up a reactor following a turnaround for maintenance. Ethylene was being introduced, and the low temperature inside the reactor caused some of the ethylene to condense. In an attempt to rectify this, coolant flow was stopped, and steam was added to increase the temperature. This caused the liquid ethylene to vaporize, increasing the pressure within the reactor. The pressure continued to rise until the pressure relief device opened, discharging flammable ethylene vapor into the atmosphere. The piping from the relief device discharged horizontally. The ethylene vapor found an ignition source, most likely nearby welding equipment, and caught fire. Twenty-one people who were at the facility at the time required medical care, including two who were life-flighted from the facility.
This incident highlights the importance of a safe discharge location. The pressure relief valve was properly sized and did its job of limiting pressure in the pressure vessel on which it was installed; however, this brings to the forefront something that should not be overlooked. By definition, a pressure relief valve moves a high pressure, potentially hazardous material out of its intended location to somewhere else. This is the last line of defense for the pressure vessel to prevent overpressure and potential failure and loss of containment at the pressure vessel, but in essence, this doesn’t solve the problem, only moves it. Therefore, the consequences of that material going elsewhere need to be thoroughly understood. The horizontal discharge piping directed the emergency relief discharge to an area where personnel were present and performing maintenance activities, including welding.
API Standard 521 provides guidance on the safe disposal to the atmosphere. If applicable environmental regulations allow it, direct discharge to the atmosphere has advantages of cost and dependability over other disposal systems, such as a flare, but hazards like flammability, toxicity, and noise must be taken into consideration. For a flammable material, an upward vertical discharge creates a flammable zone above the release location, where as the material mixes with air, there is a region containing a mixture between the lower flammability limit (LFL) and upper flammability limit (UFL). These releases need to be modeled at a range of flows to model the dynamic nature of the relief event, typically at 100% and 25% of rated capacity. The standard lists some guidance on velocity and minimum discharge distances from structures.
Ignition of a release, and the thermal effects thereof, should also be considered. There are many potential ignition sources, such as hot surfaces, electrical equipment, and even static electricity caused by friction of the release. This, in effect, turns the atmospheric release into a small flare, and the standard provides various safeguards that personnel must take for given thermal radiation exposure in the section about flaring. For example, 500 BTU/h/ft2 is the maximum thermal radiation level to which personnel with appropriate clothing can be continuously exposed.
The acute toxicity effects of a release from a pressure relief valve also need to be evaluated. API Standard 521 does not provide prescriptive guidance on the particular methodology of the dispersion analysis. Institutions such as the Center for Chemical Process Safety (CCPS) publish guidance on the various dispersion models and the conditions appropriate for their use. Since dispersion involves release into the atmosphere, knowledge of typical weather conditions at the site is essential. The conditions are often categorized in terms of stability and wind speed. Stability categorizes the vertical mixing due to the thermal gradient, often classified using the Pasquill-Gifford stability class. Stability and wind speed affect the dilution of a release and the distance it is carried. If an airport exists nearby, it often has detailed historical meteorological data that can be used. Threshold values that can be used in the analysis include Emergency Response Planning Guidelines (ERPGs) by the American Industrial Hygiene Association (AIHA) and Immediately Dangerous to Life and Health (IDLHs) by the National Institute for Occupational Safety and Health (NIOSH).
There are a few other considerations of a release to the atmosphere, including condensation, a relief stream, or the potential of liquid discharge. Liquid relief in particular introduces unique hazards such as the creation of slippery surfaces, and therefore, direct release of liquid to atmosphere is generally discouraged. Other considerations of direct release to atmosphere are corrosive effects, asphyxiation effects caused by displacement of oxygen, high temperature releases such as steam, and noise.
A proper and thorough evaluation of an atmospheric release facilitates knowledge if the release is to a safe discharge location. It ensures that the last line of defense for pressure relief does not lead to unsafe conditions elsewhere.
Process Safety Office® SuperChems® software from ioMosaic contains many models to properly and completely evaluate a relief device, including the performance of a relief device and associated piping, as well as several consequence models to evaluate the effects of an atmospheric release. These include thermal effects, toxic effects, noise, and more. This integration into one platform allows the transfer of results from one model to input into another model, which enhances execution efficiency and reduces errors.
ioMosaic understands the safety and risk processes that are required or followed around the world, and deliver practical, locally compliant solutions to our clients' safety and risk concerns. Contact us at 1.844.ioMosaic or send us a note to speak with an Engineering expert who can advise.