prEN 12953-8

• Abstract:

  This document specifies the requirements for safeguards against excessive pressure in shell boilers as defined in EN 12953-1:2025.

• Status: Not Published
• Publication Date: 06-Jan-2028
• ICS: 27.060.30 - Boilers and heat exchangers
• ICS: 27.100 - Power stations in general
• Technical Committee: CEN/TC 269 - Shell and water tube boilers
• Drafting Committee: CEN/TC 269/WG 2 - Shell boilers
• Current Stage: 4020 - Submission to enquiry - Enquiry
• Start Date: 03-Sep-2026
• Due Date: 21-Aug-2026
• Completion Date: 03-Sep-2026
• Directive: Harmonized Standard2014/68/EU - Directive 2014/68/EU of the European Parliament and of the Council of 15 May 2014 on the harmonisation of the laws of the Member States relating to the making available on the market of pressure equipment
• Mandate: M/601 - COMMISSION IMPLEMENTING DECISION of 1.3.2024 on a standardisation request to the European Committee for Standardization and the European Committee for Electrotechnical Standardization as regards pressure equipment and assemblies in support of Directive 2014/68/EU of the European Parliament and of the Council
• Ref Project: oSIST prEN 12953-8:2026 - Shell boilers - Part 8: Requirements for safeguards against excessive pressure

• Revises: EN 12953-8:2001/AC:2002 - Shell boilers - Part 8: Requirements for safeguards against excessive pressure
• Effective Date: 10-Jun-2026
• Revises: EN 12953-8:2001 - Shell boilers - Part 8: Requirements for safeguards against excessive pressure
• Effective Date: 08-Oct-2025

SLOVENSKI STANDARD
01-oktober-2026
Mnogovodni kotli - 8. del: Zahteve za opremo in varnostne naprave za
preprečevanje prekoračitve tlaka
Shell boilers - Part 8: Requirements for safeguards against excessive pressure
Großwasserraumkessel - Teil 8: Anforderungen an Sicherheitseinrichtungen gegen
Drucküberschreitung
Chaudières à tubes de fumée - Partie 8 : Exigences pour la protection vis à vis des
excès de pressions
Ta slovenski standard je istoveten z: prEN 12953-8
ICS:
13.240 Varstvo pred previsokim Protection against excessive
tlakom pressure
27.060.30 Grelniki vode in prenosniki Boilers and heat exchangers
toplote
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM
September 2026
ICS 27.060.30 Will supersede EN 12953-8:2001
English Version
Shell boilers - Part 8: Requirements for safeguards against
excessive pressure
Chaudières à tubes de fumée - Partie 8 : Exigences pour Großwasserraumkessel - Teil 8: Anforderungen an
la protection vis à vis des excès de pressions Sicherheitseinrichtungen gegen Drucküberschreitung
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 269.
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, Türkiye 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
© 2026 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 12953-8:2026 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
1 Scope . 4
2 Normative references . 4
3 Terms and definitions . 4
4 Requirements . 5
4.1 General requirements for safeguards (safety pressure relief devices) . 5
4.2 Discharge capacity . 5
4.2.1 Steam boilers . 5
4.2.2 Hot water boilers . 6
4.2.3 Economizer . 7
4.3 Set pressures . 7
4.4 Cross-sections and pressure losses in the inlet pipe of safety valves in accordance
with EN ISO 4126-1:2013 . 8
4.5 Discharge system . 8
Annex A (informative) Example for discharge flow calculation through an expansion safety
valve . 9
Annex B (informative) Flash vessels . 10
B.1 General . 10
B.2 Mechanical design . 10
B.3 Calculation of required volume and required diameter for steam and water outlet
for solution 1 . 11
B.4 Calculation of required volume and required diameter for steam and water outlet
for solution 2 . 17
Annex C (informative) Significant technical changes between this European Standard and
the previous edition . 19
Annex ZA (informative) Relationship between this European Standard and the Essential
Requirements of EU Directive 2014/68/EU aimed to be covered. 20
Bibliography . 21
European foreword
This document (prEN 12953-8:2026) has been prepared by Technical Committee CEN/TC 269 “Shell and
water-tube boilers”, the secretariat of which is held by DIN.
This document is currently submitted to the CEN Enquiry.
This document will supersede EN 12953-8:2001 and EN 12953-8:2001/AC:2002.
This document has been prepared under a standardisation request addressed to CEN by the European
Commission. The Standing Committee of the EFTA States subsequently approves these requests for its
Member States.
For the relationship with EU Legislation, see informative Annex ZA, which is an integral part of this
document.
Annex C provides details of significant technical changes between this document and the previous
edition.
The European Standard series EN 12953 concerning shell boilers consists of the following parts:
— Part 1: General
— Part 2: Materials for pressure parts of boilers and accessories
— Part 3: Design and calculation for pressure parts
— Part 4: Workmanship and construction of pressure parts of the boiler
— Part 5: Inspection during construction, documentation and marking of pressure parts of the boiler
— Part 6: Requirements for equipment for the boiler
— Part 7: Requirements for firing systems for liquid and gaseous fuels for the boiler
— Part 8: Requirements for safeguards against excessive pressure
— Part 9: Requirements for limiting devices of the boiler and accessories
— Part 10: Requirements for boiler feed water and boiler water quality
— Part 11: Acceptance tests
— Part 12: Requirements for firing systems for solid fuels for the boiler
— Part 13: Operating instructions
Although these parts can be obtained separately, it should be recognized that the parts are inter-
dependent. As such, the design and manufacture of shell boilers requires the application of more than
one part in order for the requirements of the standard to be satisfactorily fulfilled.
Any feedback and questions on this document should be directed to the users’ national standards body.
A complete listing of these bodies can be found on the CEN website.
1 Scope
This document specifies the requirements for safeguards against excessive pressure in shell boilers as
defined in EN 12953-1:2025.
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 12952-10:2021, Water-tube boilers and auxiliary installations — Part 10 Requirements for safety
devices against excessive pressure
EN 12953-1:2025, Shell boilers — Part 1: General
EN 12953-6:2024, Shell boilers — Part 6: Requirements for equipment for the boiler
EN 12953-9:2024, Shell boilers — Part 9: Requirements for limiting devices of the boiler and accessories
EN ISO 4126-1:2013, Safety devices for protection against excessive pressure — Part 1: Safety valves
(ISO 4126-1:2013)
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 4126-7:2013, Safety devices for protection against excessive pressure — Part 7: Common data
(ISO 4126-7:2013)
EN ISO 4126-10:2024, Safety devices for protection against excessive pressure — Part 10: Sizing of safety
valves for gas/liquid two-phase flow
3 Terms and definitions
For the purposes of this document the terms and definitions given in EN 12953-1:2025,
EN 12953-6:2024, EN ISO 4126-1:2013 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/

Document impacted by A1:2016 and A2:2019.
Document impacted by A1:2016.
Document impacted by A1:2016.
4 Requirements
4.1 General requirements for safeguards (safety pressure relief devices)
4.1.1 Boilers, with the exception of open vented hot water boilers, shall have at least one safety device
sized for the rated output (see 4.2) of the boiler which shall ensure against excessive pressure. The safety
devices shall be in accordance with:
— EN ISO 4126-1:2013 for safety valves; or
— EN ISO 4126-4:2013 for pilot-operated safety valves; or
— EN ISO 4126-5:2013 for controlled safety pressure relief systems.
For combination of different safety devices, the requirements according to EN 12952-10:2021, 5.1 or 5.2
shall be fulfilled.
Additionally, every superheater shall have at least one safety device on the outlet side. The capacity of
this safety device shall be a minimum of 25 % of the maximum continuous rating. However, this
requirement can be waived in the case of non-isolatable superheaters when the maximum metal
temperature achieved does not exceed its design temperature.
For fully flooded hot water boilers proportionally opening safety valves may be used as an alternative.
4.1.2 The safety devices fitted on the superheater can be considered as forming part of the total safety
device discharge capacity only when a boiler is provided with an integral superheater which cannot be
isolated.
If the superheater is equipped with a safety device, and it is necessary to prevent overheating of the
superheater, it shall be so set as to respond before the safety device on the boiler.
The difference of enthalpy of saturated steam and superheated steam shall be considered for the
requirements of total safety device discharge capacity (see also 4.1.1, 3rd paragraph).
4.1.3 There shall be no shut off devices between the safety devices and the parts of the system (e.g.
boiler, economizer, superheaters) they are protecting or between the safety devices and their points of
discharge.
4.1.4 When an economizer is fitted to a boiler which can be isolated, a safety device is required on the
economizer. The capacity of this safety device shall not be counted as forming part of the relieving
capacity of the boiler.
4.1.5 Minimum flow diameter (see EN ISO 4126-1:2013, 3.17 and 3.18) smaller than 15 mm shall not
be used.
4.1.6 It shall be possible to open safety valves with lifting gear at a pressure below the set pressure.
4.1.7 Safety devices shall be installed in the vertical position.
4.1.8 For steam boilers and hot water boilers, the safety devices shall be fitted on the top of the boiler.
4.2 Discharge capacity
4.2.1 Steam boilers
The total certified discharge capacity of the safety devices applicable to the boiler shall not be less than
the maximum continuous rating (MCR).
If the total required discharge capacity is qm and the steam space is simultaneously protected by n safety
devices (n > 1) the discharge capacity of each of them shall be at least qm/(n + 1).
The calculation of the discharge capacity of the safety devices in relation to the conditions of the steam,
shall be in accordance with EN ISO 4126-7:2013 and shall not be less than the required MCR.
Notwithstanding the requirements for safety devices herein, the safety devices fitted to any boiler
(including non isolable superheater and/or economizer) shall be capable of discharging all the steam
produced at MCR without causing a pressure rise in excess of 10 % of the maximum allowable pressure
of the boiler.
4.2.2 Hot water boilers
Dimensioning of hot water boilers safety valves shall be carried out in accordance with one of the
following methods as specified in 4.2.2.1, 4.2.2.2 and 4.2.2.3.
If the total required discharge capacity is qm and protected by n safety devices (n > 1) the discharge
capacity of each of them shall be at least qm/(n + 1).
It shall be ensured that the discharge capacity qm will not cause a pressure rise in excess of 10 % of the
maximum allowable pressure.
4.2.2.1 Dimensioning for discharge steam
For safeguarding against excessive pressure in hot water boilers, it shall be assumed that valves under
water pressure will discharge steam at that saturated steam condition which corresponds to the safety
valve set pressure.
The total certified discharge capacity of the safety devices applicable to the boiler shall not be less than
the steam mass flow corresponding to the maximum allowable heat output.
The calculation of the discharge capacity of the safety devices in relation to the conditions of the steam,
shall be in accordance with EN ISO 4126-7:2013 and shall not be less than the steam mass flow
corresponding to the maximum allowable heat output.
4.2.2.2 Dimensioning for expansion of water
Expansion safety valves for the maximum expansion of the hot water volume and for the quantity of water
to be fed by the feeding device at the maximum allowable pressure shall be used only if no possible
evaporation occurs upstream of the expansion safety valve. Therefore, the set pressure of the expansion
safety valve shall be at least 2 bar higher than the saturation pressure of the set value on the temperature
limiter.
The use of expansion safety valves shall meet the following requirements apply:
a) a fast acting combustion system is used e.g. oil, gas or pulverised fuel firing systems (except slag-tap
firing or heavy brickwork in the furnace or flue-gas passes) or electrical heated system;
b) an additional pressure limiter according to EN 12953-9:2024, Clauses 4 and 6 and an additional
temperature limiter according to EN 12953-9:2024, Clauses 4 and 7 to cut-off and lock out the energy
supply. Set point of the limiters shall be the same as the foreseen limiters according to
EN 12953-6:2024, 6.5.2;
c) each pressure limiter shall have a separate connection to the boiler or the supply flow line. The
connection pipes to the limiter shall be in accordance with EN 12953-9:2024, 6.1.8;
d) safety valves shall be type approved for steam and liquid and for back pressure, when applicable;
e) minimum flow diameter (see EN ISO 4126-1:2013, 3.17 and 3.18) of the expansion safety valves shall
not be less than 25 mm.
The volume flow is the expansion volume per hour or second between hot water return temperature and
maximum hot water supply temperature (set point of temperature limiter). If applicable, the volume flow
from the pumps and the back pressure in the blow-off line shall also be considered (e.g. make up water
pump).
NOTE 1 Example for discharge flow calculation through an expansion safety valve is given in Annex A.
Alternatively, 1,25 l/h per kW may be taken as conservative discharge flow.
The calculation of the safety valve for this expansion flow may be carried out in accordance with
EN ISO 4126-10:2024. Alternatively, if the pressure p according to EN ISO 4126-7:2013, 5.2, Formula (3)
b
is higher than the saturation pressure of the set value on the temperature limiter, the calculation of the
safety valve may also be carried out for the discharge of liquid.
NOTE 2 The differential pressure to the hot water boiler, the back-pressure in the blow-off piping and the density
of the maximum hot water temperature need to be considered.
NOTE 4 Information on the maximum permissible back pressure should be indicated in the operating
instructions.
4.2.2.3 Dimensioning for two-phase flow
The applicable limits of calculation method shall comply with the requirements in EN ISO 4126-10:2024,
5.2 and 5.3.
Safety device discharge capacity shall be calculated in accordance with EN ISO 4126-10:2024 for two-
phase flow based on the maximum allowable temperature TS of the hot water boiler.
Safety valves shall be type approved for steam and liquid and for back pressure, when applicable.
4.2.3 Economizer
For safeguarding against excessive pressure in an economizer, which can be isolated from the steam
boiler or hot water boiler, dimensioning of economizer safety valves shall be carried out in accordance
with one of the following methods as specified in 4.2.3.1 and 4.2.3.2.
It shall be ensured that the discharge capacity qm will not cause a pressure rise in excess of 10 % of the
maximum allowable pressure.
4.2.3.1 Dimensioning for discharge steam
It shall be assumed that valves under water pressure will discharge steam at that saturated steam
condition which corresponds to the safety valve set pressure.
The calculation of the discharge capacity of the safety devices in relation to the conditions of the steam,
shall be in accordance with EN ISO 4126-7:2013 and shall not be less than the steam mass flow
corresponding to the maximum allowable heat output.
4.2.3.2 Dimensioning for two-phase flow
The applicable limits of calculation method shall comply with the requirements specified in
EN ISO 4126-10:2024, 5.2 and 5.3.
Safety device discharge capacity shall be calculated in accordance with EN ISO 4126-10:2024 for two-
phase flow based on the maximum allowable temperature TS of the economizer.
Safety valves shall be type approved for steam and liquid and for back pressure, when applicable.
4.3 Set pressures
4.3.1 The safety valve shall have a set pressure not higher than the maximum allowable pressure of the
unit to which it is fitted.
4.3.2 If the capacity is provided by more than one safety valve, all devices shall have a set pressure not
exceeding the maximum allowable pressure, or alternatively, if progressive opening is foreseen, the set
pressures can be set in such a way that at least one safety device shall respond if the maximum allowable
pressure is reached. The requirements of 4.2 shall not be affected hereby.
4.4 Cross-sections and pressure losses in the inlet pipe of safety valves in accordance
with EN ISO 4126-1:2013
4.4.1 The inlet line between the equipment to be protected and the safety device shall be designed and
installed to minimize dynamic effects and pressure losses that
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