Blowdown stack
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A blowdown stack is an elevated vent or vertical stack that is used to vent the pressure of components of a chemical,
Blowdown
Blowdown is the controlled removal, safe flow and disposal of vapour from a pressure vessel.[1] Blowdown, or depressurisation, removes hazardous inventory from a vessel, reduces the pressure in the vessel and thereby reduces the stresses in the vessel walls. Blowdown is used prior to draining of a vessel for maintenance. It is also undertaken in a plant emergency situation to remove and dispose of hazardous material to mitigate the possibility of incident escalation.[2] When pressure vessels are exposed to a fire, the stresses in the vessel walls are increased, potentially leading to rupture; blowdown reduces the stress levels.[2]
Blowdown is through a pipe connected to the vapour space of the vessel.[1] A normally closed actuated blowdown valve (BDV) opens and allows vapour to pass from the vessel to relief system or blowdown stack.[1] BDVs are configured to open in the event of a failure of the control or actuation system. A restriction orifice plate downstream of the BDV ensures the vessel is blowndown in an appropriate time period. For refinery and associated oil and gas systems the requirement is to reduce the pressure to 100 psig (6.9 barg) in 15 minutes.[2] These restrictions ensure that flowrates do not exceed the capacity of the blowdown system and that Joule-Thomson cooling does not increase the stresses in the vessel or blowdown system, potentially leading to brittle fracture.[2]
Incidents
The failure of the blowdown stack to contain hydrocarbons vented from a raffinate splitter led to the catastrophic Texas City refinery explosion in 2005.[3]
See also
- Gas flare
- Shutdown valve
- Piping
- Oil production plant
- Oil refinery
References
- ^ a b c Gas Processors Suppliers Association (2004). Engineering Data Book. Tulsa, Oklahoma: Gas Processors Suppliers Association. pp. Relief Systems Chapter 5.
- ^ a b c d American Petroleum Institute (2014). Pressure-relieving and Depressuring Systems API Standard 521. American Petroleum Institute.
- ^ "BP agrees to pay record $50.6m fine for Texas explosion". BBC News. October 7, 2012. Retrieved August 15, 2010.
External links
- "Chemical Safety Board's Preliminary Findings in BP Texas City Refinery Accident: Refinery Ablaze - 15 dead". Archived from the original on 2013-04-08. (362 kb) System Failure Case Studies. NASA. January 2008, Volume 2, Special Issue. Retrieved October 9, 2012