A blowdown stack is an elevated vent or vertical stack that is used to vent the pressure of components of a chemical,
refinery
A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value.
Types of refineries
Different types of refineries ...
or other plant if there is a process problem or emergency. A blowdown stack can be used to complement a
flare stack
A gas flare, alternatively known as a flare stack, flare boom, ground flare, or flare pit is a gas combustion device used in places such as petroleum refineries, chemical plants and natural gas processing plants, oil or gas extraction sites havi ...
or as an alternative. The purpose is to prevent 'loss of containment' of volatile liquids and gases. Blowdown from several systems may be combined in a blowdown header prior to the stack. A
knock-out pot may be provided at the base of the stack to remove any liquids. Blowdown stacks may either be ignited (like a flare) or un-ignited (a ‘cold’ vent). The height of the blowdown stack must be tall enough to ensure the safe dispersal of vapour.
Blowdown
Blowdown is the controlled removal, safe flow and disposal of
vapour
In physics, a vapor (American English) or vapour (British English and Canadian English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature,R. H. Petrucci, W. S. Harwood, and F. G. Herr ...
from a
pressure vessel
A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.
Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
.
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.
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.
Blowdown is through a pipe connected to the vapour space of the vessel.
A normally closed actuated blowdown valve (BDV) opens and allows vapour to pass from the vessel to relief system or blowdown stack.
BDVs are configured to open in the event of a failure of the control or actuation system. A restriction
orifice plate An orifice plate is a device used for measuring flow rate, for reducing pressure or for restricting flow (in the latter two cases it is often called a ').
Description
An orifice plate is a thin plate with a hole in it, which is usually placed in ...
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.
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
Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
.
Incidents
The failure of the blowdown stack to contain
hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
s vented from a
raffinate In chemical separation terminology, the raffinate (from French ''raffiner'', to refine) is a product which has had a component or components removed. The product having the removed materials is referred to as the extract. For example, in solvent ex ...
splitter led to the catastrophic
Texas City Refinery explosion
The Texas City Refinery explosion occurred on March 23, 2005, when a vapor cloud of natural gas and petroleum ignited and violently exploded at the isomerization (ISOM) process unit at the BP Texas City refinery in Texas City, Texas, killing 15 ...
in 2005.
See also
*
Gas flare
A gas flare, alternatively known as a flare stack, flare boom, ground flare, or flare pit is a gas combustion device used in places such as petroleum refineries, chemical plants and natural gas processing plants, oil or gas extraction sites ha ...
*
Shutdown valve
*
Piping
Within industry, piping is a system of pipes used to convey fluids (liquids and gases) from one location to another. The engineering discipline of piping design studies the efficient transport of fluid.
Industrial process piping (and accompan ...
*
Oil production plant
An oil production plant is a facility which processes production fluids from oil wells in order to separate out key components and prepare them for export. Typical oil well production fluids are a mixture of oil, gas and produced water. An oil prod ...
*
Oil refinery
An oil refinery or petroleum refinery is an industrial process plant where petroleum (crude oil) is transformed and refined into useful products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, lique ...
References
External links
* ''System Failure Case Studies''. NASA. January 2008, Volume 2, Special Issue. Retrieved October 9, 2012
Chemical equipment
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