Olav R. Hansen

CFD calculations of gas leak dispersion and subsequent gas explosions: validation against ignited impinging hydrogen jet experiments.

Computational fluid dynamics (CFD) tools are increasingly employed for quantifying incident consequences in quantitative risk analysis (QRA) calculations in the process industry. However, these tools must be validated against representative experimental data, involving combined release and ignition scenarios, in order to have a real predictive capability. Forschungszentrum Karlsruhe (FZK) has recently carried out experiments involving vertically upwards hydrogen releases with different release rates and velocities impinging on a plate in two different geometrical configurations. The dispersed cloud was subsequently ignited and resulting explosion overpressures recorded. Blind CFD simulations were carried out prior to the experiments to predict the results. The simulated gas concentrations are found to correlate reasonably well with observations. The overpressures subsequent to ignition obtained in the blind predictions could not be compared directly as the ignition points chosen in the experiments were somewhat different from those used in the blind simulations, but the pressure levels were similar. Simulations carried out subsequently with the same ignition position as those in the experiments compared reasonably well with the observations. This agreement points to the ability of the CFD tool FLACS to model such complex scenarios even with hydrogen as a fuel. Nevertheless, the experimental set-up can be considered to be small-scale. Future large-scale data of this type will be valuable to confirm ability to predict large-scale accident scenarios.

Predicting deflagration to detonation transition in hydrogen explosions

Because of the development in computational resources, Computational Fluid Dynamics (CFD) has assumed increasing importance in recent years as a tool for predicting the consequences of accidents in petrochemical and process industries. CFD has also been used more and more for explosion predictions for input to risk assessments and design load specifications. The CFD software FLACS has been developed and experimentally validated continuously for more than 25 years. As a result, it is established as a tool for simulating hydrocarbon gas deflagrations with reasonable precision and is widely used in petrochemical industry and elsewhere. In recent years the focus on predicting hydrogen explosions has increased, and with the latest release the validation status for hydrogen deflagrations is considered good.

CFD-based risk assessment for hydrogen applications

Computational fluid dynamics calculations for gas explosion safety have been widely used for doing risk assessments within the oil and gas industry for more than a decade. On the basis of predicted consequences of a range of potential accident scenarios a risk level is predicted. The development of applications using hydrogen as a clean energy carrier has accelerated in recent years, and hydrogen may be used widely in the future. Because of the very high reactivity of hydrogen, safe handling is critical. For most applications it is not realistic to perform an extensive risk assessment similar to what is done for large petrochemical installations. On the other hand, simplified methods, like venting guidelines, may have a questionable validity for hydrogen. The use of simple methods, if these actually are conservative, will in general predict too high consequences for the majority of scenarios, as these are not able to represent actual geometry and physics of the explosion.

Impact of DDT on FPSO explosion risk assessment

In recent explosion accidents on onshore petrochemical facilities, it has been concluded that a deflagration‐to‐detonation‐transition (DDT) took place, both with Liquefied Petroleum Gas (LPG) and gasoline vapor. DDT has also been observed in a number of large‐scale experiments. DDT leads to very high pressures (16–20 barg) and flame speeds (1,600–2,000 m/s) even outside congested regions and has a significant impact on the severity of the near‐field and far‐field explosion loads. It has not been generally accepted that DDT may be a threat at offshore petrochemical facilities and prediction tools have not been available. Thus, the potential effect of DDT is seldom considered. For Floating production, storage and offloading vessels (FPSOs), and even more Floating liquified natural gas vessels (FLNGs), due to larger dimensions and inventories, operators should be concerned that major explosion scenarios can lead to DDT phenomena.

Case Study Summary of Dryer Explosion and Venting Design

A recent explosion occurred in a single burner, recirculating solids ring dryer. The explosion caused significant damage to the dryer. All of the explosion panels opened, with two of the doors on the external ring duct were separated from their hinges, with one landing on the upper roof section, while the other fell back through the roof of the facility in the area of workers. The facility owners thought the accident was caused by a dust explosion, because they recently converted to a new process with finer solid particulates. However, process data also showed that the explosion could have been the result of a gas explosion. A computational fluids dynamic model was used to help evaluate the consequences associated with a dust or gas explosion during the incident. The simulated results showed that this was a gas explosion and not a dust explosion. Preventative recommendations are included.