(52bf) Overpressure Protection in Sulfur Recovery Unit (SRU)

Over the years, more stringent regulations have been imposed to reduce the sulfur contents in fuels. Most of the sulfur is recovered or produced as H₂S in most of the refining processes. However, H₂S is extremely toxic to be release to the atmosphere. Burning H₂S in an incinerator or flare will produce a large amount of SO₂, which is also subjected to strict regulation. Not only SO₂ is toxic, but it can also form acid rain, which was a big concern in the 1970s in the United States. Most of the countries have a sulfur recovery requirement of 95%-99.9%. The requirement for the United States is 99.9% for total sulfur introduced to the Sulfur Recovery Unit (SRU) as part of Clean Air Act requirement. The Sulfur Recovery Unit (SRU) is a critical unit in the refinery or gas plant to convert H₂S from the acid gas produced in the plant to valuable elemental sulfur and reduce the toxic release to the environment. Many SRUs nowadays have a Tail Gas Treating Unit (TGTU) to recover more sulfur following the Claus Plant to minimize the emission.

Throughout the years, one of the most discussed topics for SRU is the overpressure protection against the tube rupture/leak in the Waste Heater Boiler (WHB). However, there is no uniform evaluation within the industry for the tube leak case. Therefore, the responsibility has been largely placed on the owner/operator and engineering firms to complete their own analysis based on the site risk tolerance and experiences. API 521, 7^th edition, § 4.4.14.5.1 through § 4.4.14.5.3 provide the general guidance on relief protection evaluation procedures and potential mitigations, but it is still up to the users to evaluate the current existing safeguards and risk assess the consequence of WHB tube rupture. There are other potential overpressure scenarios such as a blocked outlet on the tail gas to the TGTU, catalyst/condenser plugging, or seal leg plugging aside from the tube leak.

In this paper, we will review of the unit process, identify the overpressure scenarios, discuss the challenges of overpressure protection design in SRU, and explore some of the overpressure protection measures that have been seen in the industry. The relief protection measures that will be covered in this paper are pressure relief devices, rupture disks, rupture pins, traditional seal legs, sulfur traps, and safety instrumented systems. In the conclusion of this paper, a recommendation will made on what we believe it is the best overpressure protection measure for the SRU.