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Chemical Engineering > Process Safety > Flashcards

Process Safety Flashcards

Relief and Blowdown

1
Q

why is it important to depressurise hazardous systems?

A

to minimise severity of potential loss of containment incident

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2
Q

what do blowdown systems allow

A

They enable emergency depressurisation of process system, using actuating valves

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3
Q

what are blowdown valves used in conjunction with, and what does this allow

A

blowdown valves in conjunction with orifice plates, to control rate at which system depressurises.

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4
Q

what are the two reasons that blowdown systems are important

A
  • 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.
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5
Q

what is a key aspect in blowdown system design

A

segregation within plant, more sections, easier to control, BUT more expensive.

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6
Q

what does pressure reduction lead to?

A

cooling, which can result in brittle fracture of materials as well as cause liquid/solid formation (ice, hydrate, wax, solid CO2)

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7
Q

How can blowdown loads exceed vent/flare capacity or design limits?

A
  • simultaneous blowdown, staggered blowdown etc.

- high velocities - vibration

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8
Q

In the case of a heat and mass transfer process, if there is no heat from the wall, what are the temperature conditions

A

adiabatic and reversible, constant pressure - isentropic

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9
Q

In the case of a heat and mass transfer process, if heat transfer from wall, what type of expansion exists

A

polytropic (heat crosses system boundary0

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10
Q

In the case of a heat and mass transfer process, what does depressurisation mean for the system

A
  • expansion > cooling
  • gas will condense
  • liquid will boil
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11
Q

In the case of a heat and mass transfer process, what will happen to the temperature of the system as a result of depressurisation

A
  • temperature decreases due to Joule-Thomson effect in gas expansion and auto refrigeration of boiling liquid.
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12
Q

what are atmospheric vents used for

A

low pressure storage tanks, containing non-toxic, non corrosive liquids

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13
Q

what should an atmospheric vent account for?

A

maximum filling rate

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14
Q

what should be considered for use with an atmospheric vent when contents are flammable, why?

A

a flame arrestor to prevent flashback

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15
Q

what is a major consideration with flame arrestors

A

cleanliness, arrestor principle relies on many small passages, small passages can become blocked leading to a a hazard.

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16
Q

What are flare systems used to ensure?

A

safe operation of facilities

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17
Q

what two things does the design of the system take account of with regards to hydrocarbon release

A
  1. flaring caused by normal operating modes (start up, background flaring from water degasser, glycol drum)
  2. flaring caused by blowdown
18
Q

what are the different types of flaring system

A
  • 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
19
Q

what are the important flare system design considerations

A
  • stable burning
  • flame radiation
  • pressure drop
  • air infiltration
  • liquid removal (flaming rain, angled boom offshore)
  • smoke suppression
  • noise
  • combustion efficiency
20
Q

what is the diameter of a flare stack dependent on

A

type of flare to be used, overall system backpressure

  • open pipeflare
  • cold vent
  • flare employing a propriety flare tip
21
Q

what two scenarios should a flare be designed for?

A
  • continuous/ semi-continuous flaring where smokeless flaring is required.
  • emergency flaring
22
Q

what does calculating height of flare stack or length of boom ensure for personnel?

A

they are not subjected to excessive radiation levels, and prevents flammable gas clouds enroaching on ignition sources.

23
Q

what does the accepted method for flare stack/boom sizing account for?

A
  • composition of exit gas
  • cross wind
  • exit velocity
    effect on flame length
24
Q

what does the method assume?

A

radiation from the flame is concentrated as point source located at centre of flame

25
Q

what is the emergency shutdown system designed for

A

to shut down all or part of installation quickly and safely in the event of unsafe operating conditions or catastrophic situation occurring
- it completely isolates and inactivates process and utilities plant

26
Q

what action will the ESD system take

A
  • shut down running process and utilities equipment and ignition sources.
  • isolate plant from feed and product pipelines
  • isolate plant into sections ready for depressurising
  • isolates storage facility
  • transmits shutdown signal to upsteam, downstream plants as required.
27
Q

what is level 1 of ESD system?

A

total shutdown of all systems

  • manually initiated from control room, used in conjunction with abandon platform instructions.
  • a red SD shuts down entire platform (preceded by yell SD)
28
Q

what is level 2 in ESD?

A

shutdown of production systems

  • manually or automatically initiated in event of fire and gas detection/ failure of one critical systems
  • on fire and gas detectionextent of shutdown depends on area of occurence and defined by platform cause and effect diagrams
  • secures hydrocarbon handling
29
Q

what is level 3 ESD?

A

partial shutdown of production systems

30
Q

level 4 ESD?

A

individual system shutdown

31
Q

level 5 ESD?

A

shutdown of individual components

32
Q

when would level 3, 4 or 5 of ESD occur

A

if abnormal operating conditions arise in individual systems but do not require platform shutdown

33
Q

what is a zone 0 hazardous area

A

flammable atmosphere continually present or present for long periods
- normally found inside storage tanks or other vessels containing hydrocarbons/ other flammable materials

34
Q

what is a zone 1 hazardous area

A

flammable atmosphere likely to occur in. normal operation

- normally found in production or drilling areas, around tank or vessel relief or vent lines

35
Q

zone 2 hazardous area?

A

flammable atmosphere not likely to occur during normal operation, if it does, will only exist for short period
- includes most process plant areas that handle hydrocarbons

36
Q

what are the three areas that fire and gas protection covers?

A

monitoring
- fires and gas accumulations
alarm
- people alerted at all points of installation
- located throughout installation
- data at control room for nature of alarm for decisions
protection
- inventory of material reduced in event of signal from fire and gas system
- potential sources of ignition minimised
- fire protection systems initiated

37
Q

what does an active water system do?

A
  • designed to reduce risk of fire escalation and equipment, cooling external surface
38
Q

what will active protection fire system consist of?

A
  • at least two main firewater pumps (diesel engine driven, electrical motor driven)
  • at least two firewater booster pumps to maintain pressure in ring main
  • firewater ring main to distribute water
  • sprinkler and deluge systems
  • water mist systems
  • hose reels at strategic locations around installation
  • foam package
  • hand held CO2 cylinders for electrical fires
  • hand held water cylinders and fire blankets
39
Q

what are fire & blast walls designed for?

A

use between areas of different risk level to prevent passage of flame and/or smoke for minimum specific period

40
Q

what are the 3 major classifications of fire walls?

A
  • class A - 1 hour fire resistance
  • class B - 1/2 hour fire resistance
  • class H - 2 hour fire resistance
41
Q

what is the design of a firewall

A

the face of partition remote from fire is not allowed to exceed max temperature

42
Q

what is the use of blast walls?

A

used in areas with high risk of explosion and location depends on actual topsides configuration

Chemical Engineering (2 decks)

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