Relieve system design problems and
DIERS activity
Experience of Simulis Thermodynamics
usage for development of standards
Leonid Korelstein
Russian standards
on Safety Relieve Systems
 GOST 12.2.085-2002
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Vessels working under pressure. Safety valves. Safety
requirements.
GOSR 24570-81
Safety valves of stream and hot-water boilers. Technical
requirements.
U TB 06-90
Recommendations on safety relief valve selection, sizing
and installation (in 3 parts). Minneftehimprom USSR. 1991
STP 12-07-01 (PSRE, part 1)
SA 03-005-07, PB 09-540-03, PB 03-576-03, PB 10-573-03,
PB 10-574-03, RD 153-34.1-26.304-98, RD 51-0220570-2-93,
IPMK-2005 and other documents for specific industries
GOST 31294-2005
Direct-acting safety valves. General specifications.
PB 03-583-03
Rupture disks design, manufacturing and usage rules
Russian standards are
out of date!
 Parameters of protected system and relieve
systems are mixed
 Set pressure is not necessary equal to MAWP !
 Relief valve sizing can be for accumulation pressure
which is larger than overpressure
 Allowable loss value for discharge piping is not
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defined
Backpressure influence (for backpressure >30%)
on flow rate for balanced valves isn’t described
Rupture disks influence before and/or after relief
valve isn’t taken into account
No temperature correction for spring selection
(no such thing as “cold differential test pressure”
defined)
Many common problems are not addressed
Russian standards are
out of date!
 Many algorithms or calculation method are
missing or poorly described
 Relieving requirements - capacity (except old UTB
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and its clones)
Valve sizing for two-phase gas-liquid relieve or
flashing/condensing flow
Pressure and heat losses for discharge piping
– Heat exchange model to use (Isothermic?
Adiabatic – Fanno flow?)
– Multiple choked flow (some misty tips in
GOST 31294-2005)
Fluid temperature change calculation
Viscosity correction for valve capacity
Reactive force calculation
Noise and vibration estimation
International standards
 ISO 23251:2006 (or ANSI/API STD 521 8 edition).
Petroleum, petrochemical and natural gas
industries. Pressure-relieving and depressuring
systems
 ISO 4126
Safety devices for protection against excessive
pressure
 Part 1: Safety valves
 Part 2: Bursting disc safety devices
 Part 3: Safety valves and bursting disc safety devices
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in combination
Part 4: Pilot-operated safety valves
Part 5: Controlled safety pressure relief systems
(CSPRS)
Part 6: Application, selection and installation of
bursting disc safety devices
Part 7: Common data
Part 9: Application and installation of safety devices
excluding stand-alone bursting disc safety devices
Part 10 : Sizing of safety valves and connected inlet
and outlet lines for gas/liquid two-phase flow
International standards
 API RP 520. Sizing, Selection, and Installation
of Pressure-Relieving Devices in Refineries.
Part 1. Sizing and Selection. 7th edition, 2000.
8th edition, 2007
 API RP 520. Sizing, Selection, and Installation
of Pressure-Relieving Devices in Refineries.
Part 2. Installation. 5th edition. 2003
 API std 526. Flanged Steel Pressure Relief
Valves. 5th edition. 2002.
 API STD 2000. Venting Atmospheric and LowPressure Storage Tanks Nonrefrigerated and
Refrigerated. 5th edition, 1998
 EN 764-7:2006. European standard. Pressure
equipment – Part 7. Safety systems for
unfired pressure equipment.
What is DIERS?
 Design Institute for Emergency Relief Systems of The
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American Institute of Chemical Engineers
Formed in 1976 as a consortium of 29 companies to
develop methods for the design of emergency relief
systems to handle runaway reactions
Became DIERS User Group – DUG - in 1985
Presently, over 160 companies
European DIERS User Group (EDUG) is working
develop new techniques which will improve the design of
emergency relief systems
 Working in special Committees
 Discuss problem on the meetings (spring and fall meetings
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each year)
Cross-testing of methods and software (Round-Robins)
Conferences, book publication
Participate in standard and RAGAGEP documents
training
 PSRE Co is DUG member from 2009
International documents and
methods from DIERS

DIERS (AIChE) methodology
(Design Institute for Emergency Relief Systems of The American
Institute of Chemical Engineers)
 Emergency Relief System Design Using DIERS Technology. The Design
Institute for Emergency Relief Systems (DIERS). Project Manual. NY, 1992
 Workbook for Chemical Reactor Relief System Sizing. Contract Research
Report 136/1998. HSE Books. 1998
 Guidelines for Pressure Relief and Enfluent Handling Systems (GPREH).
American Institute of Chemical Engineers. 1998.
 DUG members articles (Darby, Leung, Fisher, Melhem and others)
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The most important DIERS methodology
 Models and methods for Vessel Disengagement Dynamics and Prediction of
Two-Phase Flow Onset and Flow pattern and parameters
 Models and methods for relief system analysis and design for 2-phase, flashing
and condensing flows
 Models and methods for relief system analysis and design for systems with
chemical reactions
How PSRE Co
participates in DIERS work?
 Studies DIERS methodology to use it in
new Russian standards and RAGAGEP
documents and in “Safety Valve” software
 Participates in discussions on the
meetings
 Reports own experience (implemented in
Hydrosystem software)
 Participates in new edition of GPREH
document development
New edition of GPREH
 Work on 2nd Edition started at the end of 2006
 New edition is going to include the most modern methods
 Currently most part of the book is written
 Book structure
 Chapter 1 – Introduction
 Chapter 2 – Relief Design Criteria and Strategy (Review)
 Chapter 3 – Relief System Design and Rating Computations. The
most difficult chapter, describes methods for different
computations:
– Venting requirements (including chemical reaction cases)
– Relief valve sizing
– Inlet and Discharge Piping Analysis
– Reactive force and Vibration
 Chapter 4 – Handling Emergency Relief Effluents (Review)
 Chapter 5 – Design Methods for Handling Effluent from
Emergency Relief Systems
 We are participating in the work on chapter 3 and are
responsible for valve sizing and piping analysis method
examples
Basis of DIERS methodology
Gas-liquid flow in relief system
 What cases to consider
 Two-phase fluid discharge from protected
system
– As the result of boiling in protected
system
– Liquid with non-condensable gases
 Liquid flashing in the valve and/or
inlet/discharge piping
 Retrograde condensation in relief system
Basis of DIERS methodology
Two-phase fluid flow discharge
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When possible
 Foam product
 High viscosity liquid boiling
 Volume boiling
–
–
Chemical reactions
When boiling at the vessel walls is about the same as volume boiling
(large surface vs volume ratio)
– heating jackets
– narrow zones, channels or tubes
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Swelling of the liquid due to boiling and two-phase discharge as the result
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Flow patterns in the Vessels according DIERS
 Homogenious
 Bubble
 Churn – turbulent
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DIERS elaborated methods and equations for two phase discharge, flow
parttern, void fraction prediction
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For reactive systems DIERS proposed analysis methods on the base of
chemical kinetics equations using the lab data from adiabatic calorimeter
tests
Basis of DIERS methodology
Sizing of relief valve
 In most cases relief valve discharge rate can be
calculated on the base of combination of the
following models:
 Ideal nozzle at isentropic flow
 Homogeneous equilibrium flow model (HEM) for two-
phase flow
 Appropriate discharge coefficient correcting from ideal
to real case
 For frozen two-phase flow (liquid + non-
condensable gas) slip correlation should be
applied
 For small valves (with nozzle length < 10 cm) and
boiling liquid with quality < 0.1 there is not
enough time to establish thermodynamic
equilibrium. HEM model underestimates valve
discharge capacity (sometimes in several times).
More precise calculation demands taking into
account boiling delay (superheated liquid)
Valve discharge capacity
 v2 
 d    0

 2
dP
 From momentum
or energy equation
 By integration on pressure and assuming zero
velocity at the enter to the valve

G 2   t2

P


dP

 2 



P
1

 max
 
 Choked and non-choked flow
 From this equation analytical equations can
be developed for given state equation – for
example for ideal liquid or ideal gas
Homogeneous Direct Integration
Method (HDI - Darby)
 Direct numerical calculation of the
integral
 Density is calculated as
  G  1   L

x
x  S 1  x  G /  L
 Temperature and quality is calculated as
result of isentropic flash calculation at
given pressure (thermodynamic library
is necessary)
 This method is the most general, covers
all cases including subcooled liquid, 2phase mixture, retrograde condensation
etc
Omega-method (Leung)
 For one-component fluid far from critical
point, when thermodynamic library isn’t
available
 Density vs pressure is described as
o
 Po 
    1  1

P 
2
 2Po GLo  C pLoTo Po   GLo 
   o 1 







h

h
GL
Lo
GL
o


 o
 This allows to get analytical expressions
from the basis equation
 This approach works for subcooled
case as well
Discharge coefficients
for two-phase flow
 Prof Darby proposal
 Use gas coefficient in case of choked flow
 Use liquid coefficient in case of non-choked
flow
Two main problems currently
DIERS is working on
 How to take into account
thermodynamics non-equilibrium
 Methods of predicting relief valve
instability to escape chatter
(replacement of 3% rule!)
Thermodynamics nonequilibrium models
 Darby - HNDI (Homogenious Non-
Equillibrium Direct Integration) method
x  xo   xe  xo  L /10
 Leung – Омеga HNE method
 Diener-Schmidt omega – method
 More complex relaxation methods
 All above methods deal only with onecomponent fluid…
Relief system stability
models
 3% rule is empirical engineering
practice rule not based on firm facts or
theory
 More correct empirical rule (for
example in terms of blowdown)
 Fisher-Melhem model
 Darby model (API)