Process Safety Flashcards
Relief and Blowdown (42 cards)
why is it important to depressurise hazardous systems?
to minimise severity of potential loss of containment incident
what do blowdown systems allow
They enable emergency depressurisation of process system, using actuating valves
what are blowdown valves used in conjunction with, and what does this allow
blowdown valves in conjunction with orifice plates, to control rate at which system depressurises.
what are the two reasons that blowdown systems are important
- In fire situation, metal temperatures reach a level at which stress rupture could occur, even though pressure doesn’t exceed design pressure- depressurisation avoids vessel rupture.
- hazard severity is function of hazardous inventory, if inventory can be reduced following incident, severity of potential consequences significantly reduced.
what is a key aspect in blowdown system design
segregation within plant, more sections, easier to control, BUT more expensive.
what does pressure reduction lead to?
cooling, which can result in brittle fracture of materials as well as cause liquid/solid formation (ice, hydrate, wax, solid CO2)
How can blowdown loads exceed vent/flare capacity or design limits?
- simultaneous blowdown, staggered blowdown etc.
- high velocities - vibration
In the case of a heat and mass transfer process, if there is no heat from the wall, what are the temperature conditions
adiabatic and reversible, constant pressure - isentropic
In the case of a heat and mass transfer process, if heat transfer from wall, what type of expansion exists
polytropic (heat crosses system boundary0
In the case of a heat and mass transfer process, what does depressurisation mean for the system
- expansion > cooling
- gas will condense
- liquid will boil
In the case of a heat and mass transfer process, what will happen to the temperature of the system as a result of depressurisation
- temperature decreases due to Joule-Thomson effect in gas expansion and auto refrigeration of boiling liquid.
what are atmospheric vents used for
low pressure storage tanks, containing non-toxic, non corrosive liquids
what should an atmospheric vent account for?
maximum filling rate
what should be considered for use with an atmospheric vent when contents are flammable, why?
a flame arrestor to prevent flashback
what is a major consideration with flame arrestors
cleanliness, arrestor principle relies on many small passages, small passages can become blocked leading to a a hazard.
What are flare systems used to ensure?
safe operation of facilities
what two things does the design of the system take account of with regards to hydrocarbon release
- flaring caused by normal operating modes (start up, background flaring from water degasser, glycol drum)
- flaring caused by blowdown
what are the different types of flaring system
- HP relief (10 bara)
- LP relief (systems lower than 10 bara)
- Low temperature relief
- wet systems relief - systems containing water
- Dry relief - fluid water dried
what are the important flare system design considerations
- stable burning
- flame radiation
- pressure drop
- air infiltration
- liquid removal (flaming rain, angled boom offshore)
- smoke suppression
- noise
- combustion efficiency
what is the diameter of a flare stack dependent on
type of flare to be used, overall system backpressure
- open pipeflare
- cold vent
- flare employing a propriety flare tip
what two scenarios should a flare be designed for?
- continuous/ semi-continuous flaring where smokeless flaring is required.
- emergency flaring
what does calculating height of flare stack or length of boom ensure for personnel?
they are not subjected to excessive radiation levels, and prevents flammable gas clouds enroaching on ignition sources.
what does the accepted method for flare stack/boom sizing account for?
- composition of exit gas
- cross wind
- exit velocity
effect on flame length
what does the method assume?
radiation from the flame is concentrated as point source located at centre of flame
