Pressure Relief
“Grace under pressure”
– Ernest Hemingway
Harry J. Toups LSU Department of Chemical Engineering with
significant material from SACHE 2003 Workshop presentation
by Scott Ostrowski (ExxonMobil)
and Professor Emeritus Art Sterling
1/51
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
2/51
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
3/51
Potential Lines of Defense
 Inherently Safe Design
– Low pressure processes
 Passive Control
– Overdesign of process equipment
 Active Control
– Install Relief Systems
4/51
What is a Relief System?
 A relief device, and
 Associated lines and process
equipment to safely handle the material
ejected
5/51
Why Use a Relief System?
 Inherently Safe Design simply can’t
eliminate every pressure hazard
 Passive designs can be exceedingly
expensive and cumbersome
 Relief systems work!
6/51
Pressure Terminology
 MAWP
 Design pressure
 Operating




pressure
Set pressure
Overpressure
Accumulation
Blowdown
7/51
Code Requirements
General Code requirements include:
– ASME Boiler & Pressure Vessel Codes
– ASME B31.3 / Petroleum Refinery Piping
– ASME B16.5 / Flanges & Flanged Fittings
8/51
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
9/51
Relief Design Methodology
LOCATE
RELIEFS
CHOOSE
TYPE
DEVELOP
SCENARIOS
SIZE RELIEFS
(1 or 2 Phase)
CHOOSE
WORST CASE
DESIGN RELIEF
SYSTEM
10/51
Locating Reliefs – Where?
 All vessels
 Blocked in sections of cool liquid lines
that are exposed to heat
 Discharge sides of positive
displacement pumps, compressors,
and turbines
 Vessel steam jackets
 Where PHA indicates the need
LOCATE
RELIEFS
11/51
Choosing Relief Types
 Spring-Operated Valves
 Rupture Devices
CHOOSE
TYPE
12/51
Spring-Operated Valves
 Conventional Type
CHOOSE
TYPE
13/51
Picture: Conventional Relief
Valve
Conventional
Relief Valve
CHOOSE
TYPE
14/51
Superimposed Back
Pressure
 Pressure in
discharge header
before valve opens
 Can be constant or
variable
CHOOSE
TYPE
15/51
Built-up Back Pressure
 Pressure in
discharge header
due to frictional
losses after valve
opens
 Total =
Superimposed +
Built-up
CHOOSE
TYPE
16/51
Spring-Operated Valves
 Balanced Bellows Type
CHOOSE
TYPE
17/51
Picture: Bellows Relief
Valve
Bellows
Relief Valve
CHOOSE
TYPE
18/51
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 is
too high
CHOOSE
TYPE
19/51
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
20/51
Rupture Devices
 Rupture Disc
 Rupture Pin
CHOOSE
TYPE
21/51
Conventional
Metal Rupture Disc
CHOOSE
TYPE
22/51
Conventional
Rupture Pin Device
CHOOSE
TYPE
23/51
When to Use a SpringOperated 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
24/51
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
25/51
When to Use Both Types
 Need a positive seal (toxic material,
material balance requirements)
 Protect safety valve from corrosion
 System contains solids
CHOOSE
TYPE
26/51
Relief Event Scenarios
 A description of one specific relief event
 Usually each relief has more than one relief
event, more than one scenario
 Examples include:
–
–
–
–
Overfilling/overpressuring
Fire
Runaway reaction
Blocked lines with subsequent expansion
 Developed through Process Hazard Analysis
(PHA)
DEVELOP
SCENARIOS
27/51
An Example: Batch Reactor
 Control valve on
nitric acid feed line
stuck open, vessel
overfills
 Steam regulator to
jacket fails, vessel
overpressures
 Coolant system
fails, runaway
reaction
DEVELOP
SCENARIOS
Raw
Material
Feeds
Organic substrate
Catalyst
Nitric Acid
Reactor ~ 100 gallons
Product
28/51
Sizing Reliefs

Determining relief rates

Determine relief vent area
SIZE RELIEFS
(Single Phase)
29/51
Scenarios Drive Relief Rates
 Overfill (e.g., control valve failure)
– Maximum flow rate thru valve into vessel
 Fire
– Vaporization rate due to heat-up
 Blocked discharge
– Design pump flow rate
SIZE RELIEFS
(Single Phase)
30/51
Overfill Scenario Calcs
 Determined maximum flow thru valve
(i.e., blowthrough)
 Liquids:
Qm  Cv A 2 g c P
( 1)/( 1)



g
M


c  2 
 Gases: Qm 
 Cv APo

choked
RgTo  1
SIZE RELIEFS
(Single Phase)
31/51
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)
32/51
Determine Wetted Area
B  cos11 2 E D 


A   DE   L  D B /180
wet




SIZE RELIEFS
(Single Phase)
33/51
Determine Heat Absorption
 Prompt fire-fighting & adequate
0.82
drainage:
Q
 21,000 F  A
wet
Btu/hr




 Otherwise:
Q
 34,500 F  A
wet
Btu/hr




where








0.82
Q is the heat absorption (Btu/hr)
F is the environmental factor
– 1.0 for a bare vessel
– Smaller values for insulated vessels
SIZE RELIEFS
(Single Phase)
Awet is the wetted surface area (ft2)
34/51
Determine Vaporization
Rate
where
W Q /Hvap
W = Mass flow, lbs/hr
Q = Total heat absorption to
the wetted surface, Btu/hr
Hvap = Latent heat of
vaporization, Btu/lb
SIZE RELIEFS
(Single Phase)
35/51
Determine Relief Vent Area












2(psi)1/2
Qv
in
A
38.0 gpm CoKvKpK
Service
 Liquid
where
SIZE RELIEFS
(Single Phase)









(  )
ref
1.25Ps  P
b
b
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/in2)
Pb is the gauge backpressure (lbf/in2)
36/51
Determine Relief Vent Area
Qm
Tz
A
CoK P M
Service
b
 Gas
P  Pmax 14.7
 A is the computed relief area (in2)
where P Qm isthe
1Ps for unfired
pressure
flow thru the
relief (lbmvessels
/hr)
max 1.discharge
 Co is the discharge coefficient
P K is the
1.backpressure
2Ps for vessel
s exposed to fire
 max
correction
b
P T is the
temperature
1absolute
.33Ps for
pipingof the discharge (°R)
max
 z is the compressibility factor
P is the set pressure for the relief valve
 sM is average molecular weight of gas (lbm/lb-mol)


SIZE RELIEFS
(Single Phase)
P is maximum absolute discharge pressure (lbf/in2)
 is an isentropic expansion function
37/51
Determine Relief Vent Area
 Gas






( 1)/( 1)






2


519
.
5

Service
 1
where   is an isentropic expansion
function
  is heat capacity ratio for the gas
 Units are as described in previous
slide
SIZE RELIEFS
(Single Phase)
38/51
A Special Issue: Chatter



Spring relief devices require 25-30%
of maximum flow capacity to maintain
the valve seat in the open position
Lower flows result in chattering,
caused by rapid opening and closing
of the valve disc
This can lead to destruction of the
device and a dangerous situation
SIZE RELIEFS
(Single Phase)
39/51
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)
40/51
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 overall
set 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
41/51
Design Relief System
 Relief System is more than a safety
relief valve or rupture disc, it includes:
–
–
–
–
–
–
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
DESIGN RELIEF
SYSTEM
42/51
Installation, Inspection, and
Maintenance

To undermine all the good efforts of a
design crew, simply …
1. Improperly install relief devices
2. Fail to regularly inspect relief devices,
or
3. Fail to perform needed/required
maintenance on relief devices
43/51
?? Reduced Inlet Piping
Reduced
Inlet Piping
Anything wrong
here?
44/51
?? Plugged Bellows, Signs
Failed
of
Anything wrong
Maintenance
Inspection,
Maintenance
here?
Issues
Bellows plugged
in spite of sign
Failed
Inspection
Program
45/51
?? Discharges Pointing
Anything
Discharges
wrong
Anything wrong
Down
Pointing
here?Down
here?
46/51
?? Long MomentLongArm
Moment Arm
Anything wrong
here?
47/51
?? Will these bolts hold in a
Will these
relief
event
bolts hold
in a
relief event?
Anything wrong
here?
48/51
Mexico City Disaster
Major Contributing Cause:
Missing Safety Valve
49/51
Summary
 Pressure Relief
– Very Important ACTIVE safety element
– Connected intimately with Process Hazard
Analysis
– Requires diligence in design, equipment
selection, installation, inspection and
maintenance
 Look forward to …
– Two-phase flow methodology/exercise
50/51
References
 Crowl and Louvar – Chemical Process
Safety, Chapters 8 and 9
 Ostrowski – Fundamentals of Pressure
Relief Devices
 Sterling – Safety Valves: Practical
Design, Practices for Relief, and Valve
Sizing
51/51
END OF
PRESENTATION