FprEN 764-7
(Main)Pressure equipment - Part 7: Safety systems for unfired pressure equipment
Pressure equipment - Part 7: Safety systems for unfired pressure equipment
This document specifies the requirements for safety systems which protect a vessel, a system of vessels, piping, accessories or assemblies from exceeding operating conditions.
It is also applicable to safety related indicators and alarms, signals and warning devices when used in safety systems.
Equipment connected together by piping of adequate capacity, free from potential blockages and which does not contain any valve that can isolate any part from the safety system, may be considered as a single pressure system when considering the requirements for overpressure protection.
Safety systems include the interconnections between the equipment to be protected and any discharge location. This location can either be an outlet to atmosphere or the entry into a closed disposal system.
NOTE The scope of this document and its relationship to the safety accessories and other protective devices described in the Pressure Equipment Directive are shown in Annex E.
Druckgeräte - Teil 7: Sicherheitseinrichtungen für unbefeuerte Druckgeräte
Dieses Dokument legt die Anforderungen an Sicherheitseinrichtungen zum Schutz von Druckbehältern, Systemen von Druckbehältern, Rohrleitungen, Ausrüstungsteilen und Baugruppen vor unzulässigen Betriebsbedingungen fest.
Sie beinhaltet auch sicherheitsrelevante Anzeige- und Alarmeinrichtungen, Signale und Warneinrichtungen, sofern diese in Sicherheitseinrichtungen verwendet werden.
Geräte, die über Rohrleitungen ausreichenden Querschnitts, ohne eventuelle Sperren und ohne irgendein Ventil, das Bereiche des Sicherheitssystems abtrennen kann, mit Systemen verbunden sind, dürfen in Bezug auf die Anforderungen an die Absicherung gegen Drucküberschreitung als einfaches Drucksystem betrachtet werden.
Sicherheitseinrichtungen schließen die Verbindungen zwischen dem zu schützenden Gerät und der gesamten Abblaseleitung ein. Es kann sich dabei um eine Ableitung in die Atmosphäre oder in ein geschlossenes Entsorgungssystem handeln.
ANMERKUNG Der Anwendungsbereich dieses Dokuments, bezogen auf die in der Druckgeräterichtlinie beschriebenen Ausrüstungsteile mit Sicherheitsfunktion und weitere Einrichtungen mit Schutzfunktion, ist in Anhang E dargestellt.
Équipements sous pression - Partie 7 : Systèmes de sécurité pour équipements sous pression non soumis à la flamme
Le présent document prescrit les exigences applicables aux systèmes de sécurité qui protègent un récipient, un système de récipients, une tuyauterie, des accessoires ou des ensembles vis-à-vis de dépassements des conditions de fonctionnement.
Il est également applicable aux indicateurs et alarmes liés à la sécurité, aux dispositifs de signalisation et d’avertissement lorsqu’ils sont utilisés en tant que systèmes de sécurité.
Les équipements raccordés les uns aux autres dans un système par des tuyauteries de débit adéquat, exempts de blocages potentiels et ne comprenant pas d’appareil de robinetterie susceptible d'isoler une partie quelconque, peuvent être considérés comme un système sous pression unique eu égard aux exigences de protection vis à vis des surpressions.
Les systèmes de sécurité comprennent les interconnexions entre les équipements devant être protégés et tout point de décharge. Cet emplacement peut être un orifice de sortie à l'atmosphère ou le raccordement à un système de récupération fermé.
NOTE Le domaine d’application du présent document et ses relations avec les accessoires de sécurité et les autres dispositifs de protection de la Directive Équipements sous pression sont indiqués en Annexe E.
Tlačna oprema - 7. del: Varnostni sistemi za nekurjeno tlačno opremo
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-november-2019
Tlačna oprema - 7. del: Varnostni sistemi za nekurjeno tlačno opremo
Pressure equipment - Part 7: Safety systems for unfired pressure equipment
Druckgeräte -Teil 7: Sicherheitseinrichtungen für unbefeuerte Druckgeräte
Équipements sous pression - Partie 7 : Systèmes de sécurité pour équipements sous
pression non soumis à la flamme
Ta slovenski standard je istoveten z: prEN 764-7
ICS:
23.020.32 Tlačne posode Pressure vessels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
DRAFT
EUROPEAN STANDARD
prEN 764-7
NORME EUROPÉENNE
EUROPÄISCHE NORM
October 2019
ICS 23.020.30 Will supersede EN 764-7:2002
English Version
Pressure equipment - Part 7: Safety systems for unfired
pressure equipment
Équipements sous pression - Partie 7 : Systèmes de Druckgeräte - Teil 7: Sicherheitseinrichtungen für
sécurité pour équipements sous pression non soumis à unbefeuerte Druckgeräte
la flamme
This draft European Standard is submitted to CEN members for second enquiry. It has been drawn up by the Technical
Committee CEN/TC 54.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 764-7:2019 E
worldwide for CEN national Members.
prEN 764-7:2019 (E)
Contents Page
European foreword . 5
Introduction . 6
4.1 General . 14
4.2 Identification of hazards . 16
4.3 Solutions . 17
5.1 General . 17
5.2 Design . 19
5.2.1 General . 19
5.2.2 Reliability and design for functionality . 19
5.2.3 Material and design . 19
5.2.4 Internal protection tubes . 20
5.2.5 External chambers . 20
5.2.6 Electrical equipment . 20
5.2.7 Pressure equipment integrated in a safety system . 20
6.1 General . 21
6.1.1 Selection . 21
6.1.2 Operation . 22
6.1.3 Protection . 22
6.1.4 Pressure limit . 22
6.1.5 Discharge capacity . 22
6.1.6 Liquid applications (thermal expansion) . 22
6.1.7 Vacuum applications . 22
6.1.8 Failure of components . 22
6.2 Safety valves, direct loaded and pilot operated . 23
6.2.1 Design . 23
6.2.2 Selection of the set pressure . 23
6.2.3 Sizing . 24
6.3 Bursting disc safety devices . 24
6.3.1 Design . 24
6.3.2 Selection, application and installation . 25
6.4 Safety valves and bursting disc safety devices in series or parallel . 25
6.4.1 Design . 25
6.4.2 Application . 25
6.5 Controlled safety pressure relief systems (CSPRS) . 26
6.6 Safety related measuring, control and regulation systems (SRMCR) . 26
6.6.1 General . 26
6.6.2 Design . 26
6.6.3 Documentation . 27
7.1 Internal . 28
7.2 External . 28
7.2.1 Fire. 28
7.2.2 Static electricity . 28
7.2.3 Other hazards . 28
7.3 Electromagnetic compatibility . 28
8.1 General . 28
8.2 Safety valves or the main valve of a CSPRS . 29
8.2.1 General . 29
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8.2.2 Inlet piping . 30
8.2.3 Outlet systems . 31
8.3 Installation of bursting disc safety devices . 32
8.4 Safety valve and bursting disc safety devices in combination . 32
8.4.1 Combination . 32
8.4.2 Inlet piping to combination . 33
8.4.3 Outlet piping . 33
8.5 Isolation of relieving safety systems. 33
8.5.1 Basic requirement . 33
8.5.2 Isolating valves . 33
8.5.3 Simultaneous isolation of the relieving safety accessories and the pressure source . 33
8.5.4 Multiple relieving safety accessories . 34
8.5.5 Locking of isolating valves in a common discharge system . 34
8.5.6 Venting . 34
9.1 General . 34
9.2 Pressure indicators . 34
9.3 Temperature indicators . 35
9.4 Liquid level indicators . 35
9.5 Signals and warning devices . 35
Annex A (informative) Sizing of multiple safety valves . 38
Annex B (normative) Requirements for level limiters . 40
B.1 General . 40
B.2 Floating devices . 40
B.3 Level electrode devices . 40
B.4 Fault detection . 41
Annex C (informative) Selection of safety accessories for pressure limitation . 42
C.1 General . 42
C.2 Safety valve . 42
C.3 Bursting disc . 42
C.4 Pilot operated safety valves . 43
C.5 CSPRS . 43
C.6 SRMCR . 44
C.7 Bursting disc safety device upstream of a safety valve . 44
C.8 Bursting disc safety device downstream of a safety valve . 44
C.9 Safety valve and bursting disc in parallel . 44
C.10 Safety valves in parallel . 44
Annex D (normative) Bursting disc safety devices downstream of safety valves . 45
Annex E (informative) Scope of the standard . 46
Annex F (informative) Fault assessment chart . 47
Annex G (informative) Inspection of safety systems . 48
Annex H (informative) Risk assessment . 49
H.1 General . 49
H.2 Pressure equipment boundaries determination . 49
H.2.1 General . 49
H.2.2 Use limits . 49
H.2.3 Space limits . 49
H.2.4 Time limits . 50
H.3 Risk estimation . 50
H.4 Risk evaluation . 50
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Annex I (informative) Calculation of reaction forces . 52
Annex ZA (informative) Relationship between this European Standard and the essential
requirements of Directive 2014/68/EU (Pressure Equipment Directive) aimed to be
covered. 53
Bibliography . 54
prEN 764-7:2019 (E)
European foreword
This document (prEN 764-7:2019) has been prepared by Technical Committee CEN/TC 54 “Unfired
pressure vessels”, the secretariat of which is held by BSI.
This document is currently submitted to the second CEN Enquiry.
This document will supersede EN 764-7:2002.
This document has been prepared under a mandate given to CEN by the European Commission and the
European Free Trade Association, and supports essential requirements of EU Directive(s).
For the relationship with EU Directive(s), see the informative Annex ZA which is an integral part of this
document.
This European Standard consists of the following parts:
— Pressure equipment – Part 1: Vocabulary;
— Pressure equipment – Part 2: Quantities, symbols and units;
— Pressure equipment – Part 4: Establishment of technical delivery conditions for metallic materials;
— Pressure equipment – Part 5: Inspection documentation of metallic materials and compliance with the
material specification;
— Pressure equipment – Part 6: Structure and content of operating instructions [CEN/TR 764-6];
— Pressure equipment – Part 7: Safety systems for unfired pressure equipment;
— Pressure equipment and assemblies – Part 8: Proof test [CEN/TS 764-8].
For the preparation of this document, CEN/TC 54 “Unfired pressure vessels” was supported by
Technical Committee CEN/TC 267 “Industrial piping and pipelines”.
Annexes A, C, E, F, G, H and I of this document are informative. Annexes B and D are normative.
The changes compared to the previous edition are the following:
— new Annexes F, G, H, I related to Fault assessment chart, inspection processes, basics for Risk
assessment and reaction forces calculation;
— new Annexe ZA template;
— new rules of presentation taken into account;
— updated normative references.
prEN 764-7:2019 (E)
Introduction
A safety system can be the ultimate limitation to protect pressure equipment from exceeding its
allowable limits or a means to prevent a potentially hazardous situation leading to injury. These limits
consist of permissible pressure, temperature, level, flow or a combination of these which were fixed at
the design stage. Regular control and/or monitoring devices which are not a necessary part of a safety
systems are excluded from this document since they become active in advance of a safety system
(see Figure 1).
It is essential to consider not only the pressure relieving device or safety related measurement, control
and regulation system (SRMCR) but the whole of the pressure relief system so as not to reduce the
relieving capacity or adversely affect the proper operation of the pressure relieving devices. Operating
problems frequently occur in pressure relief systems because of incorrect selection of the appropriate
device or because a correctly selected device was adversely affected by improper handling, incorrect
installation or lack of maintenance.
In some cases it can be necessary to establish the basic details of the safety system before selecting the
value of the maximum allowable pressure PS for the equipment to be protected. Some safety systems
need a usual margin between the maximum operating pressure and their reseating pressure which it is
essential to consider before selecting PS.
This document does not deal explicitly with hazards specifically covered in other European Directives
like, for instance, the ATEX-Directive 2014/34/EU or the EMC-Directive 2014/30/EU. In cases where
such hazards might occur and could become relevant for the design and/or construction of the safety
system, it is important that specific standards for these hazards are used or a risk analysis is made.
NOTE The role of harmonized standards in supporting the essential safety requirements of European
Directives is described in the “Guide to the implementation of directives based on the New Approach and the
Global Approach”. It can be necessary for products to meet the requirements of more than one directive and it is
the responsibility of the manufacturer to ensure that these requirements are complied with. Annex ZA draws
attention to the essential safety requirements of EU Directive 2014/68/EU “Pressure Equipment Directive”
addressed by this document. This document also draws attention to subjects which are not covered in detail but
are relevant to safety systems.
prEN 764-7:2019 (E)
Key
1 maximum allowable pressure PS X 1,10
2 no continuous operation in this zone
3 maximum allowable pressure PS
4 regulating system reacts
5 safety accessory reacts
6 monitoring system reacts
7 normal operating range
Y pressure divided by Ps
X time
A action of a regulating system
B action of a monitoring system
C action of a safety accessory
Figure 1 — Response of regulating systems, monitoring systems and safety accessories in
relation to PS
prEN 764-7:2019 (E)
1 Scope
This document specifies the requirements for safety systems which protect a vessel, a system of vessels,
piping, accessories or assemblies from exceeding operating conditions.
It is also applicable to safety related indicators and alarms, signals and warning devices when used in
safety systems.
Equipment connected together by piping of adequate capacity, free from potential blockages and which
does not contain any valve that can isolate any part from the safety system, can be considered as a
single pressure system when considering the requirements for overpressure protection.
Safety systems include the interconnections between the equipment to be protected and any discharge
location. This location can either be an outlet to atmosphere or the entry into a closed disposal system.
NOTE The scope of this document and its relationship to the safety accessories and other protective devices
described in the Pressure Equipment Directive are shown in Annex E.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN 764-1:2015+A1:2016, Pressure equipment – Part 1: Vocabulary
EN 837-1:1996, Pressure gauges – Part 1: Bourdon tube pressure gauges – Dimensions, metrology,
requirements and testing
EN 837-3:1996, Pressure gauges – Part 3: Diaphragm and capsule pressure gauges – Dimensions,
metrology, requirements and testing
EN 13480-3:2017, Metallic industrial piping – Part 3: Design and calculation
EN 50156-1:2015, Electrical equipment for furnaces and ancillary equipment – Part 1: Requirements for
application design and installation
EN 60204-1:2018, Safety of machinery – Electrical equipment of machines – Part 1: General requirements
(IEC 60204-1:2016, modified)
EN 60529:1991, Degrees of protection provided by enclosures (IP Code) (IEC 60529:1989)
EN 61508 (all parts), Functional safety of electrical/electronic/programmable electronic safety-related
systems (IEC 61508 series)
EN ISO 4126-1:2013 , Safety devices for protection against excessive pressure – Part 1: Safety valves
(ISO 4126-1:2013)
EN ISO 4126-2:2019, Safety devices for protection against excessive pressure – Part 2: Bursting disc safety
devices (ISO 4126-2:2018)
EN ISO 4126-1:2013 is currently impacted by the stand-alone amendments EN ISO 4126-1:2013/A1:2016,
Safety devices for protection against excessive pressure — Part 1: Safety valves — Amendment 1
(ISO 4126-1:2013/Amd 1:2016) and EN ISO 4126-1:2013/A2:2019, Safety devices for protection against excessive
pressure — Part 1: Safety valves (ISO 4126-1:2013).
prEN 764-7:2019 (E)
EN ISO 4126-3:2006, Safety devices for protection against excessive pressure – Part 3: Safety valves and
bursting disc safety devices in combination (ISO 4126-3:2006)
EN ISO 4126-4:2013, Safety devices for protection against excessive pressure – Part 4: Pilot-operated
safety valves (ISO 4126-4:2013)
EN ISO 4126-5:2013 , Safety devices for protection against excessive pressure – Part 5: Controlled safety
pressure relief systems (CSPRS) (ISO 4126-5:2013)
EN ISO 4126-6:2014, Safety devices for protection against excessive pressure – Part 6: Application,
selection and installation of bursting disc safety devices (ISO 4126-6:2014)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in EN 764-1:2015+A1:2016 and the
following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• ISO Online browsing platform: available at http://www.iso.org/obp
• IEC Electropedia: available at http://www.electropedia.org/
3.1
safety accessory
device designed and used to protect pressure equipment against the ultimate limits being exceeded and
to prevent allowable limits being exceeded more than momentarily (see Annex E and Figure 1) in
reasonably foreseeable conditions
Note 1 to entry: Such devices include:
— devices for direct pressure limitation, such as safety valves, bursting disc safety devices, buckling rods,
controlled safety pressure relief systems (CSPRS), and
— limiters, such as pressure switches or temperature switches or fluid level switches and ‘safety related
measurement control and regulation (SRMCR)’ devices.
Note 2 to entry: In this document “safety accessory” solely refers to the pressure risk/hazard.
Note 3 to entry: More than one safety accessory may be necessary for these purposes.
3.2
safety system
system of safety accessories and other protective devices which prevents the process parameters
exceeding the limit values determined by the hazard analysis and ensures that the following risks are
eliminated or adequately controlled (injury to personnel, damage to the environment and damage to
property)
Note 1 to entry: The term “safety system” corresponds to the ESR 2.10 wording of “suitable protective devices”.
EN ISO 4126-5:2013 is currently impacted by the stand-alone amendments EN ISO 4126-5:2013/A1:2016,
Safety devices for protection against excessive pressure – Part 5: Controlled safety pressure relief systems (CSPRS) –
Amendment 1 (ISO 4126-5:2013/Amd 1:2016).
prEN 764-7:2019 (E)
3.3
monitoring system
system which periodically checks given parameters and draws attention to those which are outside
normal operating range but which are below the allowable values determined by the hazard analysis
Note 1 to entry: Normal operating conditions of the process can be restored either directly by the system or by
an operator.
Note 2 to entry: Within the scope of this standard, a monitoring system that is part of a safety system is
classified as adequate monitoring device in ESR 2.10 (b).
3.4
regulating system
system which adjusts or controls a particular parameter to maintain operation within predetermined
range
Note 1 to entry: Within the scope of this standard, a regulating system that is part of a safety system is classified
as adequate monitoring device in ESR 2.10 (b).
3.5
independence
ability to function as required without interference from or dependence upon other equipment
3.6
fail-safe
characteristic whereby the failure of any safety system or any energy source will leave the protected
equipment in a safe condition
3.7
hazard
potential source of harm
Note 1 to entry: The term “hazard” can be qualified in order to define its origin (for example, mechanical
hazard, electrical hazard) or the nature of the potential harm (for example, electric shock hazard, cutting hazard,
toxic hazard, fire hazard).
Note 2 to entry: The hazard envisaged by this definition:
— is either permanently present during the intended use of the pressure equipment (for example, the contained
fluid flammability, electric arc during a welding phase, noise emission, high temperature), or
— can appear unexpectedly (for example pressure and/or temperature increase, explosion, ejection as a
consequence of breakage).
[SOURCE: EN ISO 12100:2010, 3.6, modified – Note 2 to entry has been adapted to pressure equipment
and Note 3 to entry has been removed.]
3.8
risk
combination of the probability of occurrence of harm and the severity of that harm
[SOURCE: EN ISO 12100:2010, 3.12]
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3.9
residual risk
risk remaining after protective measures have been implemented
Note 1 to entry: See also Figure 2.
[SOURCE: EN ISO 12100:2010, 3.13, modified – Note 1 to entry has been removed.]
3.10
risk analysis
combination of the specification of the limits of the pressure equipment, hazard identification and risk
estimation
[SOURCE: EN ISO 12100:2010, 3.15, modified – “Machine” has been replaced with “pressure
equipment”.]
3.11
risk assessment
overall process comprising a risk analysis and a risk evaluation
[SOURCE: EN ISO 12100:2010, 3.17]
3.12
risk estimation
defining likely severity of harm and probability of its occurrence
[SOURCE: EN ISO 12100:2010, 3.14]
3.13
risk evaluation
judgement, on the basis of risk analysis, of whether the risk reduction objectives have been achieved
[SOURCE: EN ISO 12100:2010, 3.16]
3.14
reliability
ability of a system or component to perform a required function under specified conditions and for a
given period of time without failing
[SOURCE: EN ISO 12100:2010, 3.2, modified – “A machine or its component or equipment” was replaced
with “a system or component”.]
3.15
shutdown
automatically or manually initiated action to achieve a safe condition of the pressure equipment
3.16
lockout
system feature which preserves the shutdown action until it is released by a manual intervention
prEN 764-7:2019 (E)
3.17
safety related measurement, control and regulation system
SRMCR
high integrity pressure protection system
system which by means of automatic control equipment, operating independently from other process
control functions, prevents operating parameters from exceeding the ultimate limits and prevents
allowable limits being exceeded more than momentarily (see Annex E and Figure 1) in reasonably
foreseeable conditions in pressure equipment
Note 1 to entry: SRMCRs can be used to avoid exceeding of allowable limits of other parameters than Ps.
3.18
limiter
device which either activates the means for correction of a process parameter evolution outside its
normal operation prescribed range or provides for shutdown or shutdown and lockout
Note 1 to entry: In the scope of this standard the term “limiter” is equivalent to “limiting devices” in PED art 2
§4.
3.19
safety
freedom from risk which is not tolerable
[SOURCE: ISO/IEC Guide 51:2014, 3.14]
3.20
self-diagnosis
regular and automatic determination that all chosen components of a safety system are capable of
functioning as required
3.21
maximum allowable pressure
PS
maximum pressure for which the equipment is designed, as specified by the manufacturer
3.22
allowable temperatures
TS
maximum and minimum temperatures for which the equipment is designed, as specified by the
manufacturer
3.23
redundancy
provision of more than one device or system such that the necessary function will still be provided in
case of failure of one or more of these devices
[SOURCE: ISO 4126-9:2008, 3.5]
3.24
controlled safety pressure relief systems
CSPRS
system consisting of a main valve in combination with a control unit
[SOURCE: EN ISO 4126-5:2013, 3.1 modified – Notes to entry have been removed.]
prEN 764-7:2019 (E)
3.25
diversity
provision of different means to achieve the required function
Note 1 to entry: For example, other physical principles or other ways to resolve the same issue.
3.26
harm
injury or damage to the health of people, or damage to property or the environment
[SOURCE: ISO/IEC Guide 51:2014, 3.1]
3.27
remote scenario
exceptional load case
highly improbable situation/condition
disturbance condition that is considered not to be reasonably foreseeable, but cannot be ruled out
Note 1 to entry: “Exceptional load case” is used in EN 13445-3:2014, 5.3.2.2.
3.28
safety instrumented system
SIS
instrumented system used to implement one or more safety instrumented functions
Note 1 to entry: A SIS is composed of any combination of sensor (s), logic solver (s), and final elements(s).
[SOURCE: EN 61511-1:2004, 3.2.67, modified – Notes and Figure 1 have been deleted.]
Note 2 to entry: SIS and SRMCR are similar concepts.
3.29
allowable Limit
quantitative value of a parameter for which the pressure equipment is designed as determined by the
manufacturer through a hazard/risk analysis
Note 1 to entry: In the pressure case, PS is the allowable pressure limit.
3.30
ultimate Limit
quantitative value of a parameter that shall not be overstepped in reasonably foreseeable conditions as
determined by the manufacturer through a hazard/risk analysis
Note 1 to entry: In the pressure case 1.1PS is the ultimate pressure limit.
Note 2 to entry: Care has to be taken not to confuse the term “ultimate limit” with “limits applying to damage
limitation” used for instance in remote scenarios, external fire, and conditions not reasonably foreseeable where
the 1.1PS limit does not apply.
prEN 764-7:2019 (E)
4 Risk consideration
4.1 General
All service conditions shall be considered to select the most appropriate safety concept to ensure safe
operation. This requires a realistic assessment of risk by means of risk analysis and risk evaluation
(see Figure 2).
Annex H provides guidance related to the different steps of risk analysis and risk evaluation.
Risk analysis involves:
— determination of the boundaries of the pressure equipment including intended use and reasonably
foreseeable misuse (see Annex H);
— identification of potential hazards and characterization of hazardous situations (see 4.2 and
Annex H);
— estimation of the risk for each identified hazard (see Annex H).
Risk evaluation (see Annex H) involves the process in which, on the basis of risk analysis, judgement is
made to achieve a tolerable risk.
The manufacturer and the user should consider the most onerous conditions which can exist for
pressure and temperature within the allowable limits.
prEN 764-7:2019 (E)
NOTE Risk reduction and the selection of safety systems are not part of risk analysis and risk assessment.
Figure 2 — Process for risk assessment and risk reduction
prEN 764-7:2019 (E)
4.2 Identification of hazards
All hazards, hazardous situations and/or hazardous events related to the pressure equipment operation
shall be identified.
During operation of a piece of pressure equipment, dangerous situations may occur due, for example to:
— operational aspects including start-up, shutdown, partial load…;
— human errors;
— unreliability of some functions;
EXAMPLES Failure or trip of machines/controllers, internal mechanical failure, general or local
utility/auxiliary fluid failure.
— unsafe loading conditions;
— maintenance;
— physical characteristics of the fluid (including variations):
— pressure;
— temperature;
— flow;
— level;
— fouling capability;
— adhesion;
— abrasion;
— chemical characteristics of the fluid (including foreseeable variations in composition):
— corrosivity;
— toxicity;
— flammability;
— stability;
— fouling;
— site conditions such as:
— vibration;
— temperature (e.g. freezing);
— wear;
prEN 764-7:2019 (E)
— corrosion;
— external fire (see 7.2);
— other pertinent natural or man-made phenomena.
Each of these factors as well as the combination of those can result in the normal operating parameters
range of the pressure equipment being exceeded.
Risk estimation (see Annex H) followed by risk evaluation (see Annex H) shall be performed for each of
these scenarios to determine whether risk reduction measures are to be implemented.
Exceeding the pressure equipment allowable limits in normal or reasonably foreseeable operating
conditions and misuse constitutes a risk that that shall be reduced.
Events that necessitate simultaneous occurrence of several independent causes, combination of events
that cannot normally occur simultaneously (wind and indoor equipment, etc.), are typically classified as
remote scenarios. Nevertheless, such remote scenarios may have adverse consequences that require to
be mitigated. In such cases, although the ultimate limit of the pressure equipment may be overstepped,
harm/damage limitation concerns may call for risk reduction measures.
Typical examples include:
— admitting membrane stresses higher than those of PED Annex I but still lower than those leading to
the pressure equipment failure.
— installation of a rupture disk having a minimal burst pressure higher than 1.1PS in order to avoid
breakage of a pressure equipment when accidentally submitted to external fire.
4.3 Solutions
Risk assessment shall be used to produce the basic information which is needed to correctly design the
pressure equipment and to select the most efficient safety system. The equipment shall be designed in
order to:
a) eliminate or reduce hazards and risks;
b) provide appropriate protection measures if the hazard and risk cannot be eliminated;
c) inform the user of residual hazards and risks and indicate the appropriate special measures for that
case;
d) prevent the danger from misuse.
Preference shall be given to inherently safe design solutions.
Operating instructions shall include instructions for inspection and maintenance to ensure that the
required level of protection is available.
5 Safety systems
5.1 General
A safety system is a set of safety accessories and/or other protective devices such as regulating and
monitoring systems.
Safety systems or their relevant components shall be independent of other functions, unless their safety
function is not affected by such other functions.
prEN 764-7:2019 (E)
NOTE 1 See Definition 3.5 for “independence”.
NOTE 2 Regulating and monitoring systems which are not part of a safety system are not covered by this
document.
Where the allowable limits can be exceeded, the pressure equipment shall be fitted with, or provisions
shall be made for the fitting of:
a) Regulating systems:
Where appropriate, manually or automatically operated regulating devices for the control of
operating parameters such as measurement, control and regulation devices (MCR) and switches
(pressure, temperature, level, flow…) to maintain conditions during normal operation in a pre-
determined range within the allowable limits.
Operation of regulating systems that are part of a safety system cannot be credited to demonstrate
compliance with allowable and ultimate limits but may be credited for process parameters return
within the allowable limits after actuation of monitoring systems and safety accessories.
b) Monitoring systems:
Where appropriate, adequate monitoring devices for the control of operating parameters such as
measurement, control and regulation devices (MCR), indicators, alarms, and switches (pressure,
temperature, level, flow…) which enable adequate action to be taken either automatically or
manually or activate the means for correction and/or provide for shutdown and lockout, to keep
the pressure equipment within the allowable limits.
Operation of monitoring systems that are part of a safety system can be credited for process
parameters return within the allowable limits after actuation of safety accessories or to
demonstrate compliance with allowable limits but shall not be credited to demonstrate compliance
with ultimate limits.
If the process parameter value cannot exceed the allowable limits, a monitoring system is sufficient
and a safety system does not need to include safety accessories. The process parameter can be
brought back into the normal operating range by:
— manual action - after a status signal; or
— automatically.
c) Safety accessories:
Safety accessories include:
— direct pressure limitation devices (safety valves, bursting disc safety devices, buckling rods,
CSPRS, etc.);
— limiters such as Safety Related Measurement, Control and Regulation systems (SRMCRs), and
switches (pressure, temperature, level, flow…) which serve as ultimate protection to ensure
that the process param
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