4 replies to this topic

[cammycheme]

Hi all,

 

I'm currently performing some preliminary flare sizing for a hydrogen cavern storage facility as part of the pre-FEED stage. To estimate the flaring flowrate, I've been using the HYSYS depressuring utility.

 

Following API 521 guidelines, the assumption is that the entire plant must depressurize to 6.9 kPa within 15 minutes. In HYSYS, this requires treating the plant as a single volume, although in reality, different sections operate at significantly different pressures.

 

I'm trying to determine the most appropriate initial pressure to use for this single-volume model. Options I'm considering include:

 

Using the highest pressure across the plant (which results in a conservatively high flare flowrate), or

Applying a volume-weighted average pressure based on the distribution of pressures across different sections.

Would appreciate any thoughts or recommendations on the best approach here.

 

Thanks in advance!

[Pilesar]

I question whether API 521 guidelines apply to underground hydrogen storage. If you are convinced you must depressurize within 15 minutes, then you should justify your initial pressure assumptions by calculation and documentation. Why not estimate the different sections individually with different initial pressures?

[Selvakannan]

In general, initial pressure for the emergency case (fire/confirmed leak) will be design pressure for the non-compressor system. And for the compressor system settle-out condition will be considered as initial condition for the depressurization.

[latexman]

Not a H2 expert, but have you looked at OSHA 1910.103 and NFPA 2.

[astro]

I question whether API 521 guidelines apply to underground hydrogen storage.

 

I concur.

 

cammycheme, give API STD 521 a second read.

 

At 4.6.6 Depressuring Rate (7th Ed.) the discussion revolves around the risk of vessel rupture engulfed in a pool fire, where it's important to lower the system pressure before a fire weakens the vessel wall. At a cursory level, that consideration is not relevant to cavern storage.

 

Hydrogen fires tend to lead more to jet fire risks and you'd do well to refer to Appendix A  Analytical Methodology for Fire Evaluations of API STD 521 and refine your focus to the key equipment at risk - I'm thinking surface process equipment. At the very least, I'd expect that you'll reduce the scope of the depressuring system and optimise flare size.

Edited by astro, 23 September 2025 - 03:35 AM.