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RELIEF SYSTEMS IN PROCESS PLANTS
ARE THEY NEEDED?
RELIEF SYSTEM
SAFETY INTERLOCK
SYSTEM
ALARM SYSTEM
BASIC PROCESS
CONTROL SYSTEM
Could
these all
fail due
to a
common
fault?
Hint: Yes!!
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RELIEF SYSTEMS IN PROCESS PLANTS
RELIEF SYSTEM
SAFETY INTERLOCK
SYSTEM
ALARM SYSTEM
BASIC PROCESS
CONTROL SYSTEM
Location
Equipment selection
Documenting on
drawings
Maximum capacity
Whats in thistopic?
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Copyrights by CCPS/American Institute of Chemical Engineers and copied with the permission of AIChE
WE SHOULD ALSO PROTECT AGAINST
EXCESSIVE VACUUM
The following example uses buckling pins
overpressure
underpressure
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WE MUST SAFELY PROCESS OR DISPLOSE
OF MATERIAL FROM RELIEF SYSTEM!
Fromrelief
To environment Vent steam, air
Holding for later processing Waste water treating
Recycle to process Fuel gas, fuel oil, solvent
Recover part to process
Immediate neutralization Flare, toxic materials
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SIZING RELIEF SYSTEMS
$ DETERMINE THE RELIEF FLOW (maximum
possible)
$ DETERMINE THE SET PRESSURE
- based on process needs and equipment materials andconstruction
$ SELECT THE RELIEF TYPE- based on advantages/disadvantages of both
$ CALCULATE THE NEEDED AREA FOR FLOW
$ SELECT THE COMMERCIAL DEVICE FROMVENDOR=S SPECIFICATIONS
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DETERMINING FLOWS FOR SIMPLE SITUATIONS
Fin Fout = ?
QFout = ?
Tsource T
Relief FlowSource
Fout = ?
Liquid or vaporfilled system
Two-phasesystem withheat input
Liquid systemwith no phasechange andwith heattransfer
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DETERMINING FLOWS FOR SIMPLE SITUATIONS
Fin Fout = ?
Q Fout = ?
Tsource T
Relief FlowSource
Fout = ?
Liquid or vaporfilled system
Two-phasesystem withheat input
Filled Liquid systemwith no phasechange andwith heattransfer
Fin
Q(Vg-Vl)/Hvap
Q/CpWhat is the mechanism for heat transfer? Radiation or
convection?
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DETERMINE THE MAXIMUM FLOW FOR THE
OVERHEAD VAPOR
Under what
conditions is the
vapor at its
maximum?F
Z
q
R
D
B
LC
LC
PC
A
XD
AXB
V
Relief
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DETERMINE THE MAXIMUM FLOW FOR THE
RELIEF SYSTEM
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DETERMINING THE AREA FOR DIAPHRAGMS IN
VAPOR SERVICE$
Basic equation for flow through an orifice venting toatmosphere
$ Rearrange to solve forarea without correction
factors
What special factor
is important in the
diaphragm relieving to
a recovery system?
Pg2AC=F
c0
To flare
Liquid
recovery
K.O.drum
Burstdiaphragm
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DETERMINING THE AREAS FOR VALVES AND
DIAPHRAGMS IN VAPOR OR GAS SERVICE
$ Often, the flow is sonic or Achoked@ - in this situation, theflow depends on the upstream pressure but not the
downstream pressure. This occurs when Ps > 0.5Pb ; this is
rough estimate, see references.
$ The defining equation is
With K* depending on T, heat capacity ratio, mol. weight, back
pressure, and compressibility
Question: How do we know if the flow is choked?
Does this mean that an increase in back pressure has no effect on flow?
PAK=Fchoked*
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DETERMINING THE AREAS FOR VALVES AND
DIAPHRAGMS
FOR EXOTHERMIC REACTORS
CAUTION!
$ Two-phase venting has proved difficult to model reliably
- DIERS, (Design Inst. for Emergency Relief Systems) formed by
companies and AIChE- goals are to determine sizing methods, verify
experimentally, and prepare program
$ Experience has shown that
- Two-phase flow occurs often- Two-phase flow during runaway
reactions requires a much larger area than predicted by one-phase
methods
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DETERMINING THE AREAS FOR VALVES AND
DIAPHRAGMS
FOR EXOTHERMIC REACTORS
CAUTION!
$ A standard experimental apparatus is used to collect adiabatic data onrunaway reactions
- small scale so little hazard - gives results for sizing relief devices (T and
P vs time; flow regime - foamy or non-foamy; and viscosity - laminar or
turbulent regime)
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SAFETY RELIEF SYSTEMS,
WHAT HAVE WE LEARNED?
These are self-actuated, no external power is required
Two major devices are relief valve and rupture disk
Relief is required for all closed volumes
We must divert material to a adequate handling
system
We are now ready to gain experience in designing and
evaluating safety relief systems.
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RELIEF SYSTEM LEARNING TASKS
1. Review the CCPS WWW material
(on course site in restricted area)
Lesson 1. Relief Required
Lesson 2. Runaway Reactions
Lesson 3. Safeguards (explosions)
2. Review this lesson and solutions.
3. Read Perrys Handbook, seventh Edition,
p. 26-26 to 26-38, 26-73 to 26-74
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