(26c) Sulfur Pit Explosion Protection by Deflagration Venting

Liquid sulfur produced by the Claus process contains dissolved H₂S, which when liberated into the vapor space of its container can form a flammable mixture with air.  Concrete, underground storage pits have been the industry standard for the collection, degassing and storage of elemental sulfur, produced in the Claus process, for many years.  Most sulfur pits include features designed to prevent deflagrations using the principle of combustible concentration reduction, whereby the concentration of H₂S is maintained below the lower flammability limit (LFL) by dilution with sweep air drawn through the vapor space of the pit using a steam ejector.  Backup ejectors and natural draft “chimneys” (steam jacketed vents to atmosphere), together with safety interlocks monitoring H₂S concentration, sweep air and/or motive steam flow, are commonly used features designed to ensure that sweep air keeps flowing at an adequate rate, and to alert operators if there is a deficiency.

Should all of these protective features fail, many sulfur pits also include deflagration vents (or “explosion hatches”) designed to relieve pressure resulting from a deflagration, thereby preventing a full-blown explosion event that could compromise the structural integrity of the pit and injure personnel.  NFPA 68 – Standard on Explosion Protection by Deflagration Venting, as well as some European standards, contain guidelines for designing deflagration vents.  However, these standards can be challenging to interpret and apply.  Something as seemingly straightforward as determining the strength of a concrete enclosure can be difficult.  There are also different types of venting devices to choose from, ranging from explosion doors or hatches to rupture diaphragms, each with different efficiency, costs, operability and maintenance concerns.

Complicating matters even further is the fact that owners, operators and engineers don’t always agree on the need for deflagration vents and often ask, “are these really required?”  This straightforward question is not always simple to answer, as some consider the sweep air and monitoring features to be sufficient, while others consider deflagration vents to be essential, similar to a relief valve on a pressure vessel.

This paper discusses some of the arguments for and against deflagration vents.   It also provides interpretation of the guidelines and design methods included in the applicable NFPA standards and reviews the different types of vent designs commonly used.  Alternative sulfur storage designs that can eliminate the need for deflagration vents altogether are also explored.

Sulfur industry literature is noticeably silent on this subject.  This paper seeks to raise the profile of this important issue within the sulfur industry.