1. Relieve system design problems and DIERS activity
Experience of Simulis Thermodynamics usage for development of standards
Leonid Korelstein

2. Russian standards on Safety Relieve Systems
GOST Vessels working under pressure. Safety valves. Safety requirements.
GOSR Safety valves of stream and hot-water boilers. Technical requirements.
U TB Recommendations on safety relief valve selection, sizing and installation (in 3 parts). Minneftehimprom USSR. 1991
STP (PSRE, part 1)
SA , PB , PB , PB , PB , RD , RD , IPMK and other documents for specific industries
GOST Direct-acting safety valves. General specifications.
PB Rupture disks design, manufacturing and usage rules

3. Russian standards are out of date!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 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

4. Russian standards are out of date!
Many algorithms or calculation method are missing or poorly described
Relieving requirements - capacity (except old UTB 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 )
Fluid temperature change calculation
Viscosity correction for valve capacity
Reactive force calculation
Noise and vibration estimation

5. International standards
ISO 23251:2006 (or ANSI/API STD edition). Petroleum, petrochemical and natural gas industries. Pressure-relieving and depressuring systems
ISO 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 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

6. International standards
API RP 520. Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries. Part 1. Sizing and Selection. 7th edition, th 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
API STD Venting Atmospheric and Low-Pressure Storage Tanks Nonrefrigerated and Refrigerated. 5th edition, 1998
EN 764-7:2006. European standard. Pressure equipment – Part 7. Safety systems for unfired pressure equipment.

7. What is DIERS?
Design Institute for Emergency Relief Systems of The 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 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

8. 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
DUG members articles (Darby, Leung, Fisher, Melhem and others)
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

9. 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

10. 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

11. 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

12. Basis of DIERS methodology Two-phase fluid flow discharge
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
Swelling of the liquid due to boiling and two-phase discharge as the result
Flow patterns in the Vessels according DIERS
Homogenious
Bubble
Churn – turbulent
DIERS elaborated methods and equations for two phase discharge, flow parttern, void fraction prediction
For reactive systems DIERS proposed analysis methods on the base of chemical kinetics equations using the lab data from adiabatic calorimeter tests

13. 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)

14. Valve discharge capacity
From momentum or energy equation
By integration on pressure and assuming zero velocity at the enter to the valve
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

15. Homogeneous Direct Integration Method (HDI - Darby)
Direct numerical calculation of the integral
Density is calculated as
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, 2-phase mixture, retrograde condensation etc

16. Omega-method (Leung)
For one-component fluid far from critical point, when thermodynamic library isn’t available
Density vs pressure is described as
This allows to get analytical expressions from the basis equation
This approach works for subcooled case as well

17. 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

18. 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!)

19. Thermodynamics non-equilibrium models
Darby - HNDI (Homogenious Non-Equillibrium Direct Integration) method
Leung – Омеga HNE method
Diener-Schmidt omega – method
More complex relaxation methods
All above methods deal only with one-component fluid…

20. 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)