CEN ISO/TS 21805:2019

SLOVENSKI STANDARD
01-april-2019
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]DJRWDYOMDQMHVWUXNWXUQHLQWHJULWHWHQDPHVWLWYHQLKSURVWRURY]DãþLWHQLKVVLVWHPL
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Guidance on design, selection and installation of vents to safeguard the structural
integrity of enclosures protected by gaseous fire-extinguishing systems (ISO/TS
21805:2018)
Anleitung für die Konstruktion, Auswahl und Installation von Entlüftungen zur
Gewährleistung der strukturellen Integrität von Gehäusen, die durch ortsfeste
Gaslöschanlagen geschützt sind (ISO/TS 21805:2018)
Lignes directrices pour la conception, la sélection et l'installation d'évents pour préserver
l'intégrité structurelle des enceintes protégées par des systèmes fixes de lutte contre
l'incendie à gaz (ISO/TS 21805:2018)
Ta slovenski standard je istoveten z: CEN ISO/TS 21805:2019
ICS:
13.220.10 Gašenje požara Fire-fighting
91.140.30 3UH]UDþHYDOQLLQNOLPDWVNL Ventilation and air-
VLVWHPL conditioning systems
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN ISO/TS 21805
TECHNICAL SPECIFICATION
SPÉCIFICATION TECHNIQUE
January 2019
TECHNISCHE SPEZIFIKATION
ICS 91.140.30; 13.220.10
English Version
Guidance on design, selection and installation of vents to
safeguard the structural integrity of enclosures protected
by gaseous fire-extinguishing systems (ISO/TS
21805:2018)
Lignes directrices pour la conception, la sélection et Anleitung für die Konstruktion, Auswahl und
l'installation d'évents pour préserver l'intégrité Installation von Entlüftungen zur Gewährleistung der
structurelle des enceintes protégées par des systèmes strukturellen Integrität von Gehäusen, die durch
fixes de lutte contre l'incendie à gaz (ISO/TS ortsfeste Gaslöschanlagen geschützt sind (ISO/TS
21805:2018) 21805:2018)
This Technical Specification (CEN/TS) was approved by CEN on 30 November 2018 for provisional application.

The period of validity of this CEN/TS is limited initially to three years. After two years the members of CEN will be requested to
submit their comments, particularly on the question whether the CEN/TS can be converted into a European Standard.

CEN members are required to announce the existence of this CEN/TS in the same way as for an EN and to make the CEN/TS
available promptly at national level in an appropriate form. It is permissible to keep conflicting national standards in force (in
parallel to the CEN/TS) until the final decision about the possible conversion of the CEN/TS into an EN is reached.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
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. CEN ISO/TS 21805:2019 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (CEN ISO/TS 21805:2019) has been prepared by Technical Committee ISO/TC 21
"Equipment for fire protection and fire fighting" in collaboration with Technical Committee CEN/TC
191 “Fixed firefighting systems” the secretariat of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this Technical Specification : Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia,
France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta,
Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and the United Kingdom.
Endorsement notice
The text of ISO/TS 21805:2018 has been approved by CEN as CEN ISO/TS 21805:2019 without any
modification.
TECHNICAL ISO/TS
SPECIFICATION 21805
First edition
2018-12
Guidance on design, selection and
installation of vents to safeguard the
structural integrity of enclosures
protected by gaseous fire-
extinguishing systems
Lignes directrices pour la conception, la sélection et l'installation
d'évents pour préserver l'intégrité structurelle des enceintes protégées
par des systèmes fixes de lutte contre l'incendie à gaz
Reference number
ISO/TS 21805:2018(E)
©
ISO 2018
ISO/TS 21805:2018(E)
© ISO 2018
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting
on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address
below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms . 2
5 Use and limitations . 3
6 Safety . 4
6.1 Structural safety . 4
6.2 Personnel safety . 4
7 System design — Pressure-relief venting . 4
7.1 General . 4
7.2 Extinguishant characteristics . 4
7.2.1 Pressure graphs . 5
7.3 Enclosure characteristics . 7
7.4 Pressure-relief vent paths . 7
7.5 Types of pressure-relief vents . 7
7.5.1 General. 7
7.5.2 Gravity vents . 7
7.5.3 Counter weighted flap vent. 8
7.5.4 Electrically operated vents . 8
7.5.5 Pneumatically operated vent . 9
7.5.6 Vent accessories . 9
7.6 Pressure-relief vent characteristics .10
7.6.1 Vent efficiency .10
7.6.2 Minimum opening pressure .11
7.6.3 Minimum closing pressure .11
7.6.4 Fire rating.11
7.7 Vent location and mounting .11
7.7.1 Vent location .11
7.7.2 Vent mounting .12
7.8 Pressure-relief vent area calculations .13
7.8.1 Use of agent-specific equations .13
7.8.2 Vent area requirement (non-liquefiable gases and CO ) .14
7.8.3 Vent area requirements (liquefiable gases) .18
7.8.4 Leakage .22
7.9 Cascade venting calculations .23
7.9.1 Example calculation 3 . .24
7.9.2 Cascade vent arrangements .25
7.9.3 Venting into adjacent enclosures .26
8 System design — Post discharge venting .28
9 Acceptance .28
10 Service and maintenance .28
Bibliography .30
ISO/TS 21805:2018(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso
.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 21, Equipment for fire protection and fire
fighting, Subcommittee SC 8, Gaseous media and firefighting systems using gas.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/members .html.
iv © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
Introduction
The guidance presented here is based on the results of a joint research program conducted in 2006 and
2007 by several fire protection system manufacturers and interested parties. The program of work
consisted of several series of tests to evaluate the peak pressure response and pressure-relief vent
area effects for each agent addressed in this document. The key data used in the development of this
document were the values of peak enclosure pressure response (PMAX) at each value of the volume-
normalized pressure-relief vent area of the test enclosure, hereinafter referred to as the “leakage-to-
volume ratio” or LVR. Other test parameters (enclosure temperature, agent quantity, discharge time,
and humidity) were held constant or varied in a specified manner. For each test series employing a
single agent, the several pairs of LVR and resultant PMAX values were graphically analysed and a best-
fit correlation curve was determined.
The LVR vs. PMAX correlation curve for each agent or system forms the basis of the associated equations
in cases where discharge of the agent results in cooling the air temperature below its dew point. (See
Humidity effects and humidity correction factor below.) In most cases, only halocarbon agents cause
sufficient cooling to cause humidity related effects on the peak enclosure pressure. Thus, a correction
for humidity effects is included in the equations for estimating vent area and maximum pressure on the
discharge of the following agents:
— FK-5-1-12
— HFC-23
— HFC-125
— HFC-227ea
The humidity corrections used in this document are based on the results of tests conducted with HFC-
227ea at different conditions of humidity.
The resulting values for humidity correction will be assumed to be equally applicable to the agents FK-
5-1-12, HFC-125 and HFC-23 until further data or analysis indicates otherwise.
The correlations of LVR to maximum negative pressure and maximum positive pressure were based
on test work performed in a test chamber at a relative humidity (RH) of approximately 38 %. If the RH
in a protected enclosure differs from 38 % then a correction to the estimated maximum negative and
positive pressures may be required. See 7.8 and 7.9 for further information on the effect of humidity. The
temperature of the test enclosure was 21°C (nominal) for all tests that form the basis of the estimating
methods given in this document.
In conducting the research program, described above, a large number of different venting arrangements
were created in the test enclosure. The equivalent leakage area (ELA) for each test was determined by
a “door fan test” and data analysis. The average enclosure pressure in effect during the many door fan
tests varied from test to test. All values of ELA were normalized to an equivalent enclosure differential
pressure of 125 Pa. The resulting enclosure correlations of peak pressure vs. LVR, and any resulting
estimate of enclosure pressure-relief vent area, reflect a pressure-relief vent area calculated at an
effective enclosure pressure of 125 Pa for a vent with discharge coefficient of 0,61.
The effectiveness of a gaseous total flooding firefighting system depends, in part, on retention of the air-
extinguishant mixture within the protected volume for a period of time. Retention of the extinguishant-
air mixture requires that gas exchange (“leakage”) between the enclosure and the ambient environment
be restricted. In order to limit the rate of gas exchange the enclosure boundary should have a high
degree of integrity. To put it another way, the sum total of the areas of the various penetrations in an
enclosure’s bounding surfaces should be low, at least during the gas-retention period (hold time) after
the end of extinguishant discharge.
Addition of a gaseous firefighting extinguishant to an enclosure having limited pressure-relief vent area
will naturally result in a change of pressure therein. If the enclosure is sealed too tightly during the
extinguishant discharge, i.e., too little pressure-relief vent area, the pressure change could exceed the
ISO/TS 21805:2018(E)
structural strength of one or more of its bounding surfaces — windows, doors, walls, ceiling. Conversely,
if the enclosure has too much pressure-relief vent area then gas exchange with the ambient atmosphere
will occur rapidly, leading to short retention time of the extinguishant within the protected volume.
Thus, the use of gaseous firefighting systems should address two performance considerations:
a) Pressure management within the protected volume during the period of extinguishant
discharge, and
b) Retention of the extinguishant-air mixture within the enclosure for a specified period of time after
the completion of the discharge.
This document provides guidance for limiting pressure extremes in an enclosure during the discharge
of a clean agent fire extinguishing system. This document does not provide the information necessary to
determine all of the requirements related to the design, installation, service, maintenance, inspection,
test, and/or requalification of fire suppression systems.
Some limitations and restrictions apply to the use of the equations contained in this document. Please
refer to the text and notes that follow.
The information in this document does not supersede the manufacturer’s guidance. The information
contained in this document is presented as supplementary to the guidance provided by the respective
system manufacturers. Guidance from the system manufacturer should always be followed and used
for purposes of system design, installation, operation and maintenance.
It has been assumed in the preparation of this document that the execution of its provisions is entrusted
to people appropriately qualified and experienced in the specification, design, installation, testing,
approval, inspection, operation and maintenance of systems and equipment, for whose guidance it has
been prepared, and who can be expected to exercise a duty of care to avoid unnecessary release of
extinguishant.
vi © ISO 2018 – All rights reserved

TECHNICAL SPECIFICATION ISO/TS 21805:2018(E)
Guidance on design, selection and installation of vents to
safeguard the structural integrity of enclosures protected
by gaseous fire-extinguishing systems
1 Scope
This document provides guidance on fulfilling the requirements contained in ISO 6183:2009, 6.4.1 and
7.4.1 and ISO 14520-1:2015, 5.2.1 h and 5.3 h, in respect to over and under pressurisation venting and
post discharge extract.
It considers the design, selection and installation of vents to safeguard the structural integrity of
enclosures protected by fixed gaseous extinguishing systems and the post discharge venting provisions
where used.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at https: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
NOTE For the purposes of this document, the term “bar” signifies “gauge”, unless otherwise indicated.
Concentrations or quantities expressed in percentages (%) signify by volume, unless otherwise indicated.
3.1
back pressure
pressure downstream of a vent
3.2
fire damper
device designed to prevent the spread of fire
3.3
free pressure-relief vent area
sum of the free pressure-relief vent areas of the pressure-relief vents provided
Note 1 to entry: This is determined by the gross pressure-relief vent area x the vent efficiency.
3.4
gross pressure-relief vent area
total area of the pressure-relief vent
3.5
negative pressure
pressure in the protected room which is lower than the pressure immediately outside the enclosure
boundary
ISO/TS 21805:2018(E)
3.6
peak pressure
The maximum pressure (positive and negative) generated within a an enclosure caused by the discharge
of the gaseous agent
3.7
positive pressure
pressure in the protected room which is higher than the pressure immediately outside the enclosure
boundary
3.8
enclosure strength
specified differential pressure limit for the protected enclosure
3.9
pressure-relief area
sum of the free pressure-relief vent area and the enclosure leakage area
3.10
pressure-relief vent
device that provides a flow path through an enclosure boundary to limit the pressure therein
4 Symbols and abbreviated terms
2 2
A pressure-relief vent area to limit negative pressure to a specified P , cm (in )
N N
2 2
A pressure-relief vent area to limit positive pressure to a specified P , cm (in )
P P
C agent design concentration, in percent by volume
M is the molecular weight of air = 0,029 (kg/mol)
AIR
M molecular weight of the agent, (kg/mol)
AGT
Q minimum design quantity of agent (kg)
P pressure (Pa)
P negative pressure, psf (Pa)
N
P positive pressure, psf (Pa)
P
P and P represent either
N P
—  Design pressure limits for estimating A or A , or
N P
—  Estimates of maximum values of P or P for given values of A or A
N P N P
R gas law constant, 8,314 (J/mol-K)
% RH relative humidity in hazard space at 21 °C (70 °F), %
s specific volume of the agent at the design temperature (m /kg)
s specific volume of the homogenous agent-air mixture (m /kg), which is the inverse of the
H
density
2 © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
t agent discharge time, s
V volume of the protected space (m )
ρ agent-air mixture density at the specified temperature and pressure (kg/m )
H
5 Use and limitations
This document is for the use by those competent in the design, installation, servicing and maintenance
of fixed gaseous firefighting systems. It also serves as guidance for those involved in the design,
construction and operation of buildings in which such systems are installed.
It does not replace the need for the person responsible for the design, construction and operation of the
building to fulfil their obligations in respect to providing adequate structural provisions.
There may be other trades and services involved in the complete system and the document is limited to
providing the guidance outlined in the document and does not purport to be expert in all areas.
After applying the enclosure peak pressure and pressure-relief vent area analysis of this document, the
user may conclude that an enclosure may require additional pressure-relief vent area in order to avoid
exceeding specified maximum pressure values upon discharge of a gaseous agent system. If that is the
case, it is recommended that the user advise the supplier of a supplemental venting device that such
device be specified and selected by use of this document.
The maximum pressure developed in an enclosure on discharge of a clean agent fire extinguishing
system is affected by several characteristics of the system itself and the enclosure being protected. Of
particular importance are the thermodynamic properties of the agent and the discharge characteristics
of the hardware. Each of the following sections contains correlation equations that are specific to the
agent type and manufacturer’s hardware. The equations can be used to make estimates of the following:
a) enclosure pressure-relief vent area given a specified enclosure pressure limit;
b) maximum positive or negative pressure developed in an enclosure given a stated or calculated
pressure-relief vent area.
NOTE The formulae in this document for halocarbon agents have a limited range of applicability based on the
parametric limitations of the data from which they were derived. Table 1 indicates the applicable limits of design
concentration, discharge time, and enclosure pressure response for the use of this document. The maximum
peak pressure estimates (both positive and negative) based on data obtained for each agent are given in Table 1.
Caution — It is physically possible to develop pressures greater than those covered by this
document during system discharges.
Table 1 — Summary of equation application limits
Agent Minimum Maximum Minimum Maximum Maximum Maximum
agent agent discharge discharge over- under
time time pressure pfs pressure pfs
Conc Conc
(Pa) (Pa)
% vol % vol
FK-5-1-12 4,2 6 6 10 5 (239) 25 (1 197)
HFC-23 18 30 6 10 30 (1 437) n/a
HFC-125 8 10,5 6 10 10 (479) 10 (479)
HFC-227ea 6,25 10,5 6 10 8 (383) 20 (958)
ISO/TS 21805:2018(E)
6 Safety
6.1 Structural safety
The provision of correctly designed and engineered pressure venting of enclosures protected by
gaseous fire-extinguishing systems is essential in order to prevent the possibility of failure of structural
integrity. This is essential to mitigate forces exerted by the changes in enclosure pressure when gaseous
fighting media are discharged into an enclosure.
6.2 Personnel safety
The operation of pressure-relief vents or extract systems, requires the displacement of mixtures of air/
gaseous media from a protected enclosure to atmosphere or another area not necessarily protected. The
safety issue may arise due to exposures to the extinguishants themselves or products of combustion
and/or extinguishant breakdown products. In addition, any hazards arising from the operation of the
over/under pressurisation vents themselves should be considered.
7 System design — Pressure-relief venting
7.1 General
The basic design principle is to limit the pressure excursions imposed on the structure of the protected
enclosure by the discharge of gaseous extinguishant to that within the limits the enclosure is able to
withstand.
A room integrity test can be used to determine the equivalent leakage area, or simply” vent” area
that exists at the time of evaluation.  The methods of this document can use the known or estimated
pressure-relief vent area to estimate the maximum pressure that will be developed on discharge of
a clean agent system. In the event that the estimated maximum pressure exceeds a specified design
threshold, the methods of this document may be used to estimate a pressure-relief vent area sufficient
to limit development of pressure upon system discharge to an acceptable value.
7.2 Extinguishant characteristics
Consideration should be given to positive pressurisation created by all extinguishants and additionally
to negative pressurisation created by some extinguishants as defined in Table 2.
Table 2 — Pressure effects of gaseous extinguishant
Extinguishant name Positive pressure created Negative pressure created
FK-5-1-12 Yes Yes
HFC-125 Yes Yes
HFC-227ea Yes Yes
HFC-23 Yes No
IG 01 Yes No
IG 100 Yes No
IG 55 Yes No
IG 541 Yes No
CO Yes No
NOTE  Negative pressure has been observed, with adverse effects. It can occur in certain cases where large quantities of
CO are released into a space having low leakage to ambient.
4 © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
7.2.1 Pressure graphs
The graphs below illustrate the typical pressure excursions that would occur during discharge within
the protected area.
ISO/TS 21805:2018(E)
a)  Inert gas
b)  Inert gas (constant low)
c)  Halocarbon gas
Key
X1 positive pressure
X2 negative pressure
Y time: (a): inert gas, (b): inert gas (constant flow), (c): halocarbon gas
Figure 1 — Typical pressure excursions
6 © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
7.3 Enclosure characteristics
It is the client’s responsibility and not the fire protection system supplier to determine the room
strength. The client should advise the allowable pressure differential the protected enclosures can
withstand without sustaining damage.
It is generally accepted that that normal masonry construction can withstand 500 Pa, whilst lightweight
structures such as stud partitioning can withstand only 250 Pa. Both figures assume fixings at the
top and bottom. Certain structure types may have even lower limits, particularly suspended ceilings.
However, fire system engineers are not qualified to give guidance on room strengths, so it is up to the
client to provide this information. In the event that the client does not make clear what the allowable
pressure the enclosure will withstand, it is necessary to obtain his acceptance of the figures used.
In view of issues related to enclosures utilising suspended ceilings, it is recommended that protection is
provided to volumes above and below the suspended ceiling where practical.
7.4 Pressure-relief vent paths
It is generally assumed that positive/negative pressure-relief vent paths will lead to/from atmosphere.
Positive pressure-relief vent paths will assist in the safe transfer of the displaced air/extinguishant
volume to atmosphere in the most efficient, uncomplicated manner as well as ensuring air/extinguishant
contaminated with fire by-products also finds a safe route to outside air.
As positive pressure venting may involve the displacement of smoke the possible effect on fire detection
systems along the vent path should be considered.
Under certain circumstances it may be necessary to consider the use of adjacent spaces as the means
to dissipate the pressure condition, either directly as a function of the volume of that adjacent space or
where the adjacent space acts as transit path to atmosphere. Under the circumstances described in the
latter, special venting considerations may be required to ensure the pressure condition is not simply
transferred to that adjacent space (see 7.9).
7.5 Types of pressure-relief vents
7.5.1 General
There are various types of pressure-relief vent, which are normally closed to preserve the integrity of
the enclosure, which then open to relieve pressure impulse and close again. These pressure-relief vents
may fall into the following categories:
7.5.2 Gravity vents
The blades for these vents are generally hinged on the top edge. They have no electric or pneumatic
actuation but rely totally on the enclosure pressure change in order to move the vent blades.
This type of vent may provide a free pressure-relief vent area significantly less than the gross pressure-
relief vent area. In addition, the vent design creates turbulent flow and therefore is likely to create higher
pressure loss for any given flow. This additional pressure should be factored into the determination of
the free pressure-relief vent area required.
Vents, if not fitted with an end stop, for example ‘cat flaps’, could relieve pressures in both directions,
however, these are not recommended unless they are able to avoid compromising the enclosure fire rating.
ISO/TS 21805:2018(E)
Figure 2 — Gravity vent
7.5.3 Counter weighted flap vent
This type of vent is configured with the hinge located just off of the centre of gravity, so that when
positive pressure is exerted on the upstream side of the vent it allows the vent blades to pivot to their
fully open positions.
The vent can be designed such that there is a minimum operational release pressure, which will ensure
that nuisance movement is avoided.
Typically, these vents are more efficient (i.e. larger discharge coefficient, lower opening pressure, lower
intrinsic inertia) than gravity flap vents.
7.5.4 Electrically operated vents
This type of vent utilises blade(s) operated by an electric motor or an electromagnetic device.
This type of vent is reliant upon power at the time of the discharge, therefore if no other option is
available there should be a protected power supply to the vent motor to ensure that failure of mains
does not leave the vent in the closed position.
This type of vent generally opens more slowly than other types of vent and correct operation may be
dependent on the sequence of activation and the time allowed for the vent to open fully.
8 © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
Figure 3 — Electrically operated vent
7.5.5 Pneumatically operated vent
Pneumatically operated vents are actuated by pressure, normally of gas flowing through the pipe work
or alternatively by pilot containers or compressed air line.
7.5.6 Vent accessories
7.5.6.1 Security provisions
If the vent is located within an external wall at low level it is feasible that the client will have some
concerns regarding forced entry, therefore it is likely that security bars could be fitted across the
aperture in order to retain the building security.
7.5.6.2 Insect screen
If there is concern that insects could penetrate the building through the vent it may be necessary to
specify insect screens, however, these are fine mesh and could have a significant impact on the free
pressure-relief vent areas.
7.5.6.3 Weather louvres
When fitted on exposed, external faces of a building it is possible that rain may penetrate the opening
even with the vent in the closed position. In this case a weather louvre could be fitted externally,
however this could have a significant impact on the free pressure-relief vent areas.
ISO/TS 21805:2018(E)
Figure 4 — Louvres
7.5.6.4 Decorative grilles
Where a decorative grille is used to cover the inner face of the vent assembly, however this could have
an impact on the free pressure-relief vent areas.
7.5.6.5 Limit switches
Should electrically or pneumatically operated vents be inadvertently left in the open position they
could become either a security risk or endanger the equipment within the space by the infiltration of
pollution from external sources. In this case it may be desirable to fit limit switch(es) to monitor the
position of the vent and create a warning signal, either locally, or through the building management
system, or both.
7.6 Pressure-relief vent characteristics
7.6.1 Vent efficiency
Pressure-relief vents, of whatever type (see 7.5), control the flow of air by the movement of air control
elements (blades). The design of the blades and the extent to which they open at any given pressure
determines the free pressure-relief vent area of the vent at that pressure. For example, if a vent has a
nominal area of 1,0 m and an efficiency of 50 % at 100 Pa it will provide a free pressure-relief vent area
of 0,5 m at 100 Pa. The blades of the same vent may open more at higher pressures, perhaps having
an efficiency of 80 % at 250 Pa and thus provide a free pressure-relief vent area of 0,8 m at 250 Pa.
Examples of vent efficiencies for gravity and weighted vents are shown in Figure 5. It is therefore
recommended that free pressure-relief vent areas are specified at no less than 3 pressures, for example
100 Pa, 250 Pa and 500 Pa.
Vent efficiency will be reduced by the addition of other accessories in the vent path, e.g. weather louvres,
grilles etc. Vent manufacturers should provide a safe assessment of the potential effect based on the
free pressure-relief vent area of the accessory proposed.
NOTE Vent efficiencies are provided by vent manufacturers.
10 © ISO 2018 – All rights reserved

ISO/TS 21805:2018(E)
Key
X efficiency (%)
Y pressure (Pa)
A gravity
B counter-weighted
Figure 5 — Efficiency of pressure-relief vents
7.6.2 Minimum opening pressure
The vent should be designed to have a minimum opening pressure to avoid nuisance opening. This
should be at least 50 Pa.
7.6.3 Minimum closing pressure
The vent should be designed to have a minimum closing pressure to ensure closure at the end of the
discharge. This should be at least 30 Pa.
7.6.4 Fire rating
Where vents are included in an enclosure they should not reduce the fire rating of the structure and
therefore be of equivalent fire rating.
7.7 Vent location and mounting
7.7.1 Vent location
The most favourable location for the vent is on an exterior wall of the building.
ISO/TS 21805:2018(E)
The vent should be located taking due account of the discharge nozzles and any objects both inside and
outside the enclosure in the vicinity of the vent.
The vent should be located on an area of wall within the enclosure which is devoid of all services or
other fixtures or fittings that could impede the flow path. Where available free wall space is limited,
consideration may have to be given to having a bespoke vent manufactured which will fit the available
space constraints.
The most significant hazard which arises from obstructions placed on either side of the vent are those
which are of a non-fixed or temporary arrangement, which may impede flow or prevent the vent from
functioning correctly. Such items may not be present at the time at which the gaseous firefighting
system is designed and ultimately handed over. Examples may include skips, packing boxes, filing
cabinets, etc.
Where obstruction of either side of the vent is possible, suitable warning notices or physical barriers
should be provided.
The discharge of a gaseous firefighting system causes concentrated streams of extinguishant from
the discharge nozzles, which dissipate the further the flow gets from the nozzle. It follows therefore
that placing a vent in close proximity to nozzles and directly in the path of discharge, may cause a
disproportionate quantity of extinguishant to be vented during the discharge.
Vents should be positioned taking into account the above points and any location in the enclosure
boundary may be suitable.
7.7.2 Vent mounting
The following are provided as general information which may vary between suppliers.
Vents may feature a mounting flange which is fixed to the surface to which the vent is to be mounted,
using suitable screws. The surface to which the vent is to be fitted needs to be flat and where the surface
is stepped or uneven, additional mounting frames may be required. Additional mounting frames may
also need to be utilised where the vents are being fitted in very thin enclosure walls such as GRP cabins,
or where existing window frames are used, or where the ve
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