DOI: 10.3303/CET2290084 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 15 December 2021; Revised: 5 March 2022; Accepted: 25 April 2022 
Please cite this article as: Schmidt J., 2022, Strategy for ISO 4126 into 2030 – future standardization of pressure protection systems, Chemical 
Engineering Transactions, 90, 499-504  DOI:10.3303/CET2290084 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 90, 2022 

A publication of 

 
The Italian Association 

of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Aleš Bernatík, Bruno Fabiano 
Copyright © 2022, AIDIC Servizi S.r.l. 
ISBN 978-88-95608-88-4; ISSN 2283-9216 

Strategy for ISO 4126 into 2030 –  
Future Standardization of Pressure Protection Systems 

Jürgen Schmidt 
CSE Center of Safety Excellence, Joseph-von-Fraunhofer Str. 9, 76327 Pfinztal, Germany 
juergen.schmidtcse-institut.de  

ISO 4126 is the leading standard for protecting pressurized systems against excessive pressures. It includes 
nine parts for safety valves, bursting disks, pilot-operated valves, and controlled pressure relief systems. Single 
and two-phase flow is covered. The standard does not fully reflect the current state of technology applied in 
many companies, especially the zero-emission trends. For years, a direct discharge of fluids from safety valves 
or bursting disks into the environment has been tried to avoid. Liquids shall be discharged into separators or 
drums, and quenches are installed to condense discharged vapors. The sizing scenarios upstream of the safety 
system and the effluent system on the downstream line are not considered in ISO 4126. Both heavily influence 
the sizing and often the type of safety device. Nowadays, mechanical safety devices are adapted to the process 
to dynamically optimize the set pressure for economic production and to reduce the mass flow rate in case of 
an emergency to a minimum extent. Innovative overpressure protection systems are combined from mechanical 
controlled pressure protection systems, and electronic safety-related control systems (SIS) adapted to the 
process. Long and complex inlet and outlet lines direct the fluid into an effluent system. The next generation of 
intelligent devices will probably try to avoid any relief if possible, using a safety-related control of the process, 
e.g., limiting the feed stream or increasing the cooling. It is time to adapt the standard to these latest technologies 
and sizing methods applied already in Industry. The paper will be focused on the deficiencies of the existing 
standard ISO 4126 and propose a strategy for necessary changes during the upcoming years up to 2030. The 
strategy reflects the view of the CSE Center of Safety Excellence. 

1. History of ISO 4126 
A major goal in European harmonization was to separate product from application standardization. It was not 
allowed to specify a safety valve or bursting disk and give sizing recommendations in the same part. This led to 
misleading titles and curious sequences of topics like in part 7 of ISO 4126 “common data” with a part for “sizing 
of safety valves” for single-phase flow followed by a section for “minimum requirements for helical compression 
springs” (ISO4126-7, 2019). After that, an appendix entitled “Thermodynamic properties” is limited to tables of 
steam pressure coefficients, although such data have been available for a long time as equations for simulation 
tools. A second example is type testing of safety valves and bursting discs, an essential requirement of the 
pressure equipment directive, to validate the stable function of high integrity safety devices (PED, 2014).  For 
years, type tests were not standardized; this topic was spread over several parts of the standard ISO 4126. The 
current strategy is 17 years old. It is time to rethink the standardization of safety equipment in ISO 4126. 
According to the business plan from 2005, parts 3, 9, and 10 of the standards were to be created - this had long 
since been implemented. 
The high integrity of safety devices in ISO 4126 is mainly represented by specific requirements on the devices 
and sizing procedures for single and two-phase flow. Manufacturing, type testing, packaging, transport, 
installation, inspection, and other topics of the life cycle of a safety device are not considered. A complete view 
of a pressure protection system and its stages along a life cycle, comparable to topics already standardized for 
high integrity protection systems, including electrical, electronic, and programmable electronic systems (IEC 
61508), was not implemented in the strategy of ISO 4126.   
 

499



The past has shown that especially plant operators have increasingly withdrawn from the process of 
standardization in case of pressure protection. Manufacturers dominate several working groups of ISO 4126 
because plant engineers are missing; specific process knowledge and deep understanding of upcoming topics 
in pressure protection, e.g., functional safety, seem to be underrepresented in some parts. It is becoming 
increasingly difficult for plant engineers and operators to recognize the results and apply the modifications of 
periodical reviews of the current nine parts of the standard. Especially the chemical and petrochemical industry 
is invited to increase their activities.  

2. The State of Technology for Pressure Protection 
Current trends in pressure protection, namely, to avoid emergency relief or to prohibit hazardous substance 
dispersion into the ambient (ZERO emission), to use intelligent safety devices, and to reduce the consequences 
of relief scenarios, are not represented in the standard. Essential parts are missing here. The understanding of 
pressure relief today is quite different from the times when valves discharged directly into the atmosphere, 
Figure 1. There is hardly any safety equipment in the chemical and petrochemical industry without a downstream 
vent line and - in the case of liquid media - without an effluent system. Today, when sizing vent line systems, all 
components, i.e., piping, safety device, and effluent system, are considered as an integral unit and not 
separately each component. The safe function of each component is most often inter-dependent, and the 
dimensioning of the components has become more complex. For example, effluent systems will not work 
correctly if the safety device has been sized far too large. According to ISO 4126, safety devices are still sized 
separately without considering the other components. This is no longer up to date and does not correspond to 
state of art.   
 

 

Figure 1: the State of Technology for industrial pressure protection systems (end-of-pipe technology)   

Several groups worldwide develop more detailed and precise guidelines for pressure protection systems. Since 
2015 the European Industrial Sizing Group (EURISG) has been active at the CSE Center of Safety Excellence 
to harmonize internal guidelines and views not treated in current standards (EURISG, 2022). And typical 
industrial sizing cases where standards are not yet applicable or inaccurate are considered. Sixteen important 
chemical, petrochemical, and digital industry companies meet three times per year at the CSE to cooperate 
within the EURIG group. More than 1000 pages of reports have been worked out, often used to train young 
professionals, and validate internal sizing procedures. The intensive harmonization of internal guidelines by 
EURISG is a measure for the future need for standardization.  

3. Future requirements – strategy 2030 
Plant operators need a comprehensive solution for the stable functioning of pressure protection systems in their 
plant environment and not just the supply of a safety device. The entire life cycle of the pressure relief system, 
from planning and design to manufacturing, type testing, packaging, transportation, installation, and 

Control
P+

P+T-

TA

F

Separator Quench

Blowdown vessel

flare

N2

TE

ISO 4126
2021

Dispersion

NEW (Smart controlled pressure relief systems)

ISO 4126
2030

500



maintenance and inspection, shall be considered (Dechema, 2017). This is where ISO 4126 can make a 
significant contribution in the future.  
ISO 4126 (ISO4126, 2019), entitled “Safety devices for protection against excessive pressures” is currently 
limited to a tiny part of pressure protection, mainly requirements on piping, safety valves, and rupture discs. It 
should be much broader and more specific to fulfill today’s high integrity pressure protection requirements (see 
Figure 1. Safety devices are embedded in integral emergency overpressure protection concepts, including an 
independent layer of protection as operational measures to (1) repeatedly produce precisely certain products 
and in case of abnormal operation to (2) alarm and re-direct the pressurized system in a regular operation, and 
(3) to avoid any pressure relief by DCS measures, before any emergency relief systems come into play, Figure 
2.  
 

Figure 2: Safety devices as part of overpressure protection concepts  
based on an independent layer of protection 

Future standardization of ISO 4126 shall be thought from a perspective of measures to avoid overpressure 
protection and to realize a zero-emission concept. The likelihood of a pressure relief shall be dropped down to 
values even lower than the best of today’s concepts. Currently, the standard is limited to safety devices 
completely decoupled from the plant environment. Overlaps and interfaces to standards regarding other 
independent layers of protection, e.g., functional safety, shall be specified to match the requirements of both 
standards. All interest from manufacturers, consultancies, inspectors, and plant operators shall be equally 
balanced. Any part of the standard shall purely define requirements without fixing specific methods or 
procedures to avoid blockages of future innovations. Unfortunately, current standards are far beyond this 
requirement. Methods and procedures should be mainly implemented as informative annexes and options to 
fulfill the mandatorily given requirements of the main part. A focus on mandatory requirements also allows coping 
with national and regional regulations. 
A proposal for the future development of ISO 4126 is outlined in the following. The standard can be divided into 
three main areas, including several existing and new parts, see Figure 3: 

 Product 
 Life Cycle Operation 
 Testing 

There are already several established standard parts in the product area. Future changes can certainly be 
expected for controlled safety devices in part 5. Mechanical safety devices and functional safety protection 
systems are moving closer together. Component type testing meets statistical consideration of random failures 
– these different concepts need to be harmonized. Low-pressure valves and the topics of quality management, 
manufacturing, and packaging/transportation are currently not represented at all. Is extensive component testing 
sufficient without regulating the manufacture of pressure relief devices along these lines? Well-known 
manufacturers have already developed high-quality management guidelines in their companies to guarantee 
the quality of each device comparable to the device tested in a type test. Requirements for such quality 
management – not the management system itself – are essential features for the high integrity of safety devices 
and should be part of the standard. 
The area of operation or life cycle operation is the least developed in ISO 4126. Currently, there are only 
parts for sizing safety valves and bursting discs. Unfortunately, considering life cycle operation leads to specific 
difficulties: as soon as a safety device is installed in a plant, the specific plant environment comes into play. It is 

Measure to Reduce Consequences
Safety Devices (Valve/Rupture Disk)

PLC Measures

Organizational Measures
Process Control System, 
(Personal)Training , Alarm

Location
Design of 
Apparatus-/
Process
Chemistry

Civil Protection

Measure:
1) Protection

2) Control

3) Operability

501



decisive for the sizing and stable function of the device. Figure 3 already lists key parts of this environment. 
They start with the design scenario, the strategies to avoid any discharge of hazardous substances, and the 
effluent systems. Special topics such as property data determination (e.g., for mixtures under-sizing conditions), 
consequence analyses, maintenance and inspection, and noise emissions should be standardized. In the area 
of life cycle operation, most parts of the standard are missing. Today, every plant operator has a mostly internal 
set of guidelines, but although it has been the state of technology for years, it is not mapped in ISO 4126.  
The accuracy and complexity of state-of-the-art methods and models for sizing each of the components of a 
pressure protection system have increased and are expected to grow in the future further. Hence, there is a 
strong need for appropriate sizing tools certified against the standard’s requirements for single- and two-phase 
flow. For example, ProSaR is a web-based software application with high usability and is thoroughly adapted to 
ISO4126 (PROSAR, 2022).  
 

 

Figure 3: Proposal for future enlargement and standardization of ISO 4126 based on three main areas   

In the area of testing, unfortunately, almost nothing has been standardized in ISO 4126. For many years, the 
operators of test facilities and the valve and rupture disk manufacturers have been developing a first standard 
part 11 for type tests according to ISO 4126. However, it has not yet been possible to reach an agreement. 
Figure 3 lists eight topics in the area of testing. Testing is becoming very important, especially in times of the 
shift towards hydrogen technologies. Type testing according to ISO 4126 was considered as a specific test to 
fulfill the requirements on a sole safety device under laboratory conditions. But it is no longer acceptable for 
pressure relief devices not to be tested at their typical operating conditions. High-pressure valves have been 
type tested at low set pressures and some without springs. This is not the state of technology, and plant 
operators should no longer accept such component tests. However, the tests themselves are not useful in some 
cases and need to be reconsidered. At high pressures - hydrogen typically in the 700 - 1200 bar range - type 
tests with air are not representative for pressure relief with hydrogen. The air cools below the dew point of 
oxygen and nitrogen due to the Joule-Thomson effect. The valves become extremely cold, freeze up, and 
behave differently than when hydrogen is blown off. It is time that this is considered in ISO 4126, and a 
standardization part for the tests at high pressures is created. The same applies to very low pressures, e.g., the 
breathing devices for low-pressure storage tanks. There are currently no standards, and measurement results 
from manufacturers worldwide differ significantly in quality and are often not comparable. 
The proposed strategy 2030 of ISO 4126 corresponds to a change of perspective: life cycle operation of 
pressure protection systems from an "operator view" with its typical applications in industry. Only a 
comprehensive view of pressure protection systems in a large variety of industrial plants ensures safe operation. 
Tests must be carried out under typical operating conditions and not (only) seen as a stand-alone valve or 
bursting disk test. It is time to go forward. All interested parties are requested to participate constructively.  
The old business plan for ISO 4126 from 2005 has been slightly reformulated to fit into the proposed strategy:  
“The scope of ISO/TC 185 is the standardization of safety devices for protection against excessive pressure 
along a system life cycle. Safety devices include safety valves, bursting disk devices, pilot-operated safety 

P
ro

du
ct

Safety Valves
Bursting Disks
PRV & BD in 
combination
Pilot operated PRV
Controlled safety
pressure relief systems
Low pressure safety
relief systems
Quality management
system for
manufacturing & 
transportation

Li
fe

 C
yc

le
 

O
pe

ra
tio

n
Sizing Scenarios
Pressure Relief Prevention
Sizing of PRV
Application Selection an 
installation of PRV
Sizing of BD
Application Selection an 
installation of PRV
Sizing of PRV and BD 
for two-phase flow
Effluent systems
Consequence analysis for
dispersion from PRV and BD
Property data evaluation for
sizing conditions
Inspection, Maintainance and 
proving
Evaluation of noise emission

Te
st

in
g

Type testing of PRV
Type testing of BD
Type testing of low
pressure relief systems
Type testing of high 
pressure PRV and BD
Requirements for
authorized observer
Type testing of
pressure protection
systems during
operation
Integrity proof of PRV 
after re-installation
Component testing

502



valves, controlled safety pressure protection systems (CSPRS), combination devices, and upstream and 
downstream protection equipment. Each device type is addressed in separate and distinct parts of the standard 
ISO/TC 4126. This standardization includes general design requirements, sizing demands, type testing for 
pressure-retaining integrity, and functional testing representative of typical field applications for operating and 
flow capacity performance. The main objectives are to enhance the standard to the current state of technology 
including emission reduction considerations.” 
According to the proposed strategy for 2030, the content of ISO 4126 should be primarily extended. Many new 
parts and topics are proposed, which can only hardly be developed if the current workflow under ISO and the 
review process of the standard are not drastically increased. Parallel to the standardization process of new 
parts, research and development in process and plant safety will proceed and bring up most probably other new 
methodologies and models to be integrated into ISO 4126. A vision for ongoing R&D activities in this field is 
outlined next.  

4. Vision of future of pressure protection systems 
During the last years, several new trends in process and plant safety were identified to simultaneously increase 
the safety and productivity of plants. Classical safety devices migrate into intelligent, process adaptable, and 
online-driven pressure protection systems. The main driver is even for new safety concepts, time to market, and 
modularization of processes and plants. The new generation of safety devices will be called SmOP (smart 
overpressure protection devices), see (Schmidt C., 2022). SmOP´s enable fully open a certain cross-section of 
a pressurized system for relief at a distinct pressure but open only up to an actual necessary lift. Set pressure 
and lift are continuously changed by the hazard potential within the pressurized system. This allows maximizing 
the production window while simultaneously suppressing level swell and two-phase discharge, avoiding 
unnecessary emissions, and optimizing flow rates for downstream flares or washers. And SmOP´s close 
immediately after a hazard is removed. Periodically partial stroke tests during installation may extend inspection 
intervals or alarm when systematic errors like encrustation are detected. 
A SmOP consists of a typical actor who fasts and safely opens a relief cross-section connected to a safety-
related high integrity programmable logic control system (HPLC), Figure 4.  
 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

Figure 3: SmOP-Smart overpressure protection system developed at CSE Center of Safey Excellence   

The HPLC is linked to specific sensors for online measurement, e.g., temperatures, pressures, feed, level, etc. 
A SmOP continuously estimates the actual hazard potential in a reactor or pressurized system and limits the 
control parameter to avoid discharge from the system (zero-emission mode). In case of high energy inputs into 
the pressurized system during abnormal conditions or for low design data of older equipment, the zero-emission 
mode is economically not feasible. Here, a SmOP optimizes the opening characteristic to prepare for emergency 
relief with low flow rates under abnormal plant operation (emission-control mode).  
SmOPs will lead to a paradigm change in the future. They allow for a speedup of time-to-market because only 
very few sizes are necessary and directly connected to a particular reactor, vessel, or column. Currently, a 
safety device is sized when a reactor, vessel, or column is ordered, and all recipes for production are defined in 

P
T

HPLC

DCS

F

503



detail. A worst-case consideration leads to specific device size and necessary piping. As a result, hundreds of 
different device types, sizes, and accessories are stored at manufacturers and in plants. Whenever a recipe 
changes, the capacity is increased, or new products come into play, the sizing must be repeated, and installation 
must be modified. In contrast to the classical procedure, a SmOP belonging to specific equipment, i.e., reactor, 
vessel, or column, is ordered with this equipment and needs only to be adapted by parameter inputs into the 
HPLC before a production start. Sizing is done online and on-demand. Changes in recipes or capacity are just 
a matter of hours.  
SmOPs are developed at the CSE Center of Safety Excellence (Schmidt C., 2022). Prototypes are under test. 
Due to their flexibility, they are ready for modularization of plants with production on demand.  
But a vision is too limited if SmOP´s are the final pressure protection system. New challenges came up with 
climate discussion and the need to reduce emissions. More than a million safety devices are installed and 
operated to protect pressurized systems worldwide. In the future, it will most likely not be acceptable to discharge 
any hazardous substance into ambient – neither from reactors, vessels, columns, or storage tanks. This led to 
the long-term vision of the CSE Center of Safety Excellence outlined in three steps: 

1. Smart pressure protection systems (SmOP), highly flexible and ready for modularization  
2. Zero-Emission high integrity pressure protection systems (SmartHIP) without emergency relief 
3. Inherently Safe systems based on operational measures to produce and protect simultaneously 

Zero-Emission pressure protection systems represent the next generation of protection, completely based on 
intelligent, functional safety devices (Deerberg, 1995; Biernath et al., 2021). The safety integrity systems will be 
embedded in multiple operational systems of the regular production, allowing continuous functional checks of 
any running safety system. Softsensors will be added to increase the proof depth significantly. These are 
currently based on rigorous mathematical models (white-box modeling). Further development will open the 
variety of Softsensors based on stochastic models, artificial intelligence, and a combination of them (grey-box 
and black-box modeling). Production and safety will continuously merge until the quality of operational control 
is high enough, and the multi-sensorial checks allow for proof in a sufficient depth to reach an inherently safe 
mode of any pressurized system. 

5. Conclusion 
Pressure protection needs to be re-thought. ISO 4126 does not represent the state-of-technology of highly 
interdependent emergency relief systems, including devices, piping, and effluent systems typically encountered 
in Industry. Additionally, trends like faster time-to-market, modularization of production, zero-emission, etc., are 
not yet considered. Overall, a paradigm change is necessary, and ISO 4126 needs to be updated. A proposal 
is made for a strategy of the standard up to 2030. Eighteen new parts have been identified to include the state 
of technology in the areas of product, life-cycle operation, and testing. To keep the standard updated, new 
concepts of pressure protections, represented by SmOP´s, SmartHIP´s and inherently safety-safe systems, are 
outlined in a vision of the future for pressure protection.  

References 

Biernath, J., Schmidt, C., Schmidt, J., Denecke, J., 2021. Model-based zero-emission safety concept for 
reactors with exothermal reactions for chemical plants. J. Loss Prev. Process Ind. 72, 104494. 

Schmidt C., Biernath J., Schmidt J., Denecke J., 2022, Protection of chemical reactors against exothermal 
runaway reactions with smart overpressure protection devices. CET Vol. 90, 

Dechema, 2018, Die Verfügbarkeit von mechanischen Sicherheitseinrichtungen – Sicherheitsventile, 
ProcessNet-Arbeitsausschuss „Sicherheitsgerechtes Auslegen von Chemieanlagen“, Positionspapier 

DIN EN 61508, 2011, Functional safety of safety-related electrical / electronic / programmable electronic 
systems - Part 1, General requirements. Berlin.  

DIN EN ISO 4126-1, 2019: Safety devices for protection against excessive pressure - Part 1: Safety valves 
DIN EN ISO 4126-7, 2016: Safety devices for protection against excessive pressure - Part 7: Common data 
EURISG, 2022, European Industrial Sizing Group, https://cse-engineering.de/leadership/eurisg/ 
IEC 61511-1, 2019, Functional safety - Safety instrumented systems for the process industry sector - Part 1: 

Framework, definitions, system, hardware and application programming Requirements, Beuth-Verlag. Berlin 
ISO 4126, 2019, Safety devices for protection against excessive pressure. Part 1 to 10 
PED, 2014, European Pressure Equipment Directive 2014/68/EU  
PROSAR, 2022, Sizing tool for pressure protection systems in accordance with ISO4126, https://cse-prosar.de/ 

504


	lp-2022-abstract-069.pdf
	Strategy for ISO 4126 into 2030 –  Future Standardization of Pressure Protection Systems