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Pressure Relief
Grace under pressure
Ernest Hemingway
Harry J. Toups LSU Department of Chemical Engineering with
significant material from SACHE 2003 Workshop presentationby Scott Ostrowski (ExxonMobil)
and Professor Emeritus Art Sterling
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What is the Hazard?
Despite safety precautions
Equipment failures
Human error, and
External events, can sometimes lead to
Increases in process pressures beyond safe
levels, potentially resulting in
OVERPRESSURE due to a RELIEF EVENT
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What are Relief Events?
External fire
Flow from high pressure source
Heat input from associated equipment Pumps and compressors
Ambient heat transfer
Liquid expansion in pipes and surge
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Potential Lines of Defense
Inherently Safe Design
Passive Control
Active Control
Low pressure processes
Install Relief Systems
Overdesign of process equipment
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What is a Relief System?
A relief device, and
Associated lines and processequipment to safely handle the material
ejected
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Why Use a Relief System?
Inherently Safe Designsimply cant
eliminate every pressure hazard
Passive designs can be exceedingly
expensive and cumbersome
Relief systems work!
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Pressure Terminology
MAWP Design pressure
Operating
pressure Set pressure
Overpressure
Accumulation
Blowdown
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Code Requirements
General Code requirements include:
ASME Boiler & Pressure Vessel Codes
ASME B31.3 / Petroleum Refinery PipingASME B16.5 / Flanges & Flanged Fittings
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Code Requirements
Relieving pressure shall not exceed
MAWP (accumulation) by more than:
3% for fired and unfired steam boilers
10% for vessels equipped with a single
pressure relief device
16% for vessels equipped with multiple
pressure relief devices 21% for fire contingency
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Relief Design Methodology
LOCATERELIEFS
CHOOSE
TYPE
DEVELOPSCENARIOS
SIZE RELIEFS
(1 or 2 Phase)
CHOOSE
WORST CASE
DESIGN RELIEF
SYSTEM
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Locating Reliefs Where?
All vessels Blocked in sections of cool liquid lines
that are exposed to heat
Discharge sides of positivedisplacement pumps, compressors,and turbines
Vessel steam jackets
Where PHA indicates the need
LOCATE
RELIEFS
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Choosing Relief Types
Spring-Operated Valves
Rupture Devices
CHOOSE
TYPE
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Spring-Operated Valves
Conventional Type
CHOOSE
TYPE
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Picture: Conventional Relief
Valve
Conventional
Relief Valve
CHOOSE
TYPE
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Superimposed Back
Pressure
Pressure in
discharge header
before valve opens
Can be constant or
variable
CHOOSE
TYPE
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Built-up Back Pressure
Pressure indischarge headerdue to frictionallosses after valveopens
Total =
Superimposed +Built-up
CHOOSE
TYPE
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Spring-Operated Valves
Balanced Bellows Type
CHOOSE
TYPE
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Picture: Bellows Relief
ValveBellowsRelief Valve
CHOOSE
TYPE
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Pros & Cons:
Conventional Valve Advantages
+ Most reliable type if properly sized and operated
+ Versatile -- can be used in many services
Disadvantages
Relieving pressure affected by back pressure
Susceptible to chatter if built-up back pressure istoo high
CHOOSE
TYPE
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Pros & Cons:
Balanced Bellows Valve Advantages
+ Relieving pressure not affected by back pressure
+ Can handle higher built-up back pressure
+ Protects spring from corrosion
Disadvantages
Bellows susceptible to fatigue/rupture
May release flammables/toxics to atmosphere
Requires separate venting system
CHOOSE
TYPE
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Rupture Devices
Rupture Disc
Rupture Pin
CHOOSE
TYPE
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Conventional
Metal Rupture Disc
CHOOSE
TYPE
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Conventional
Rupture Pin Device
CHOOSE
TYPE
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When to Use a Spring-
Operated Valve
Losing entire contents is unacceptable
Fluids above normal boiling point
Toxic fluids Need to avoid failing low
Return to normal operations quickly
Withstand process pressure changes,including vacuum
CHOOSE
TYPE
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When to Use a Rupture
Disc/Pin
Capital and maintenance savings
Losing the contents is not an issue
Benign service (nontoxic, non-hazardous)
Need for fast-acting device
Potential for relief valve plugging High viscosity liquids
CHOOSE
TYPE
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When to Use Both Types
Need a positive seal (toxic material,
material balance requirements)
Protect safety valve from corrosion
System contains solids
CHOOSE
TYPE
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Relief Event Scenarios
A description of one specific relief event
Usually each relief has more than one reliefevent, more than one scenario
Examples include:
Overfilling/overpressuring
Fire
Runaway reaction
Blocked lines with subsequent expansion
Developed through Process Hazard Analysis(PHA)
DEVELOP
SCENARIOS
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An Example: Batch Reactor
Control valve onnitric acid feed linestuck open, vesseloverfills
Steam regulator tojacket fails, vesseloverpressures
Coolant system
fails, runawayreaction
DEVELOP
SCENARIOS
Product
Raw
Material
Feeds
Organic substrateCatalyst
Nitric Acid
Reactor ~ 100 gallons
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Sizing Reliefs
Determining relief rates
Determine relief vent area
SIZE RELIEFS
(Single Phase)
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Scenarios Drive Relief Rates
Overfill (e.g., control valve failure)
Fire
Blocked discharge
SIZE RELIEFS
(Single Phase)
Maximum flow rate thru valve into vessel
Vaporization rate due to heat-up
Design pump flow rate
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Overfill Scenario Calcs
Determined maximum flow thru valve
(i.e., blowthrough)
Liquids:
Gases:
SIZE RELIEFS
(Single Phase)
PgACQ cvm 2
)1/()1(
1
2
og
c
ovchokedm
TR
MgAPCQ
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Fire Scenario Calcs
API 520 gives all equations for
calculating fire relief rate, step-by-step
1. Determine the total wetted surface area
2. Determine the total heat absorption
3. Determine the rate of vapor or gas
vaporized from the liquid
SIZE RELIEFS
(Single Phase)
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Determine Wetted Area
SIZE RELIEFS
(Single Phase)
180/wet
BDLEDA
DEB 21cos 1
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Determine Heat Absorption
Prompt fire-fighting & adequatedrainage:
Otherwise:
where
SIZE RELIEFS
(Single Phase)
82.0
wet000,21
Btu/hr
AFQ
82.0
wet
500,34
Btu/hr
AFQ
Q is the heat absorption (Btu/hr)
F is the environmental factor
1.0 for a bare vessel Smaller values for insulated vessels
Awet is the wetted surface area (ft2)
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Determine Vaporization
Rate
vap/HQW
where
W = Mass flow, lbs/hr
Q = Total heat absorption tothe wetted surface, Btu/hr
Hvap = Latent heat ofvaporization, Btu/lb
SIZE RELIEFS
(Single Phase)
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Determine Relief Vent Area
LiquidService
where
bs25.1
)ref(
bpvovQ
gpm38.02/1)psi(2in
PPKKKCA
A is the computed relief area (in2)
Qv is the volumetric flow thru the relief (gpm) Co is the discharge coefficient
Kv is the viscosity correction
Kp is the overpressure correction
Kb is the backpressure correction
(/ref) is the specific gravity of liquid Ps is the gauge set pressure (lbf/in
2)
Pb is the gauge backpressure (lbf/in2)
SIZE RELIEFS
(Single Phase)
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Determine Relief Vent Area
GasService
where
MTzPKCAbo
mQ
A is the computed relief area (in2)
Qm is the discharge flow thru the relief (lbm/hr) Co is the discharge coefficient
Kb is the backpressure correction
T is the absolute temperature of the discharge (R)
z is the compressibility factor
M is average molecular weight of gas (lbm/lb-mol) P is maximum absolute discharge pressure (lbf/in
2)
is an isentropic expansion function
SIZE RELIEFS
(Single Phase)
valverelieffor thepressuresettheiss
pipingfors33.1max
firetoexposedsfor vessels2.1maxvesselspressureunfiredfors1.1max
7.14max
P
PP
PPPP
PP
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Determine Relief Vent Area
GasService
where
)1/()1(
125.519
is an isentropic expansionfunction
is heat capacity ratio for the gas
Units are as described in previous
slide
SIZE RELIEFS
(Single Phase)
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A Special Issue: Chatter
Spring relief devices require 25-30%of maximum flow capacity to maintainthe valve seat in the open position
Lower flows result in chattering,caused by rapid opening and closingof the valve disc
This can lead to destruction of thedevice and a dangerous situation
SIZE RELIEFS
(Single Phase)
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Chatter - Principal Causes
Valve Issues
Oversized valve
Valve handling widely differing rates
Relief System Issues
Excessive inlet pressure drop
Excessive built-up back pressure
SIZE RELIEFS
(Single Phase)
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Worst Case Event Scenario
Worst case for each relief is the event
requiring the largest relief vent area
Worst cases are a subset of the overallset of scenarios for each relief
The identification of the worst-case
scenario frequently affects relief size
more than the accuracy of sizing calcs
CHOOSE
WORST CASE
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Design Relief System
Relief System is more than a safetyrelief valve or rupture disc, it includes:
DESIGN RELIEF
SYSTEM
Backup relief device(s)
Line leading to relief device(s)
Environmental conditioning of relief device
Discharge piping/headers
Blowdown drum
Condenser, flare stack, or scrubber
I t ll ti I ti d
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Installation, Inspection, and
Maintenance
To undermine all the good efforts of a
design crew, simply
1. Improperly install relief devices2. Fail to regularly inspect relief devices,
or
3. Fail to perform needed/requiredmaintenance on relief devices
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?? Reduced Inlet Piping
Anything wrong
here?
Reduced
Inlet Piping
?? Pl d B ll F il d
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?? Plugged Bellows, Failed
Inspection, Maintenance
Bellows plugged
in spite of sign
Anything wrong
here?
Failed
Inspection
Program
Signs of
Maintenance
Issues
?? Di h P i ti
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?? Discharges Pointing
DownAnything wronghere?
Anything wrong
here?
Discharges
Pointing Down
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?? Long Moment Arm
Anything wrong
here?
Long
Moment Arm
?? Will th b lt h ld i
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?? Will these bolts hold in a
relief event
Anything wrong
here?
Will these
bolts hold
in a
relief event?
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Mexico City Disaster
Major Contributing Cause:Missing Safety Valve
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Summary
Pressure Relief
Very Important ACTIVE safety element
Connected intimately with Process Hazard
Analysis Requires diligence in design, equipment
selection, installation, inspection andmaintenance
Look forward to Two-phase flow methodology/exercise
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References
Crowl and Louvar Chemical Process
Safety, Chapters 8 and 9
Ostrowski Fundamentals of PressureRelief Devices
Sterling Safety Valves: Practical
Design, Practices for Relief, and Valve
Sizing
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END OF
PRESENTATION