Safety devices for protection against excessive pressure - Part 7: Common data (ISO 4126-7:2013)

ISO 4126-7:2013 specifies requirements for safety valves. It contains information which is common to ISO 4126-1 to ISO 4126-6 to avoid unnecessary repetition.

Sicherheitseinrichtungen gegen unzulässigen Überdruck - Teil 7: Allgemeine Daten (ISO 4126-7:2013)

Dieser Teil von ISO 4126 legt Anforderungen an Sicherheitsventile fest. Er enthält zur Vermeidung unnötiger Wiederholungen Daten, die den Teilen 1 bis 6 der ISO 4126 gemein sind.
Zu ausdampfenden Flüssigkeiten oder Zweiphasengemischen siehe ISO 4126-10.
Der Anwender wird darauf hingewiesen, dass die Verwendung der in 6.3 angegebenen allgemeinen Gasgleichung nicht empfohlen wird, wenn die Abblasetemperatur mehr als 90 % der thermodynamischen kritischen Temperatur und der Abblasedruck mehr als 50 % des thermodynamischen kritischen Drucks beträgt. Darüber hinaus wird die Kondensation nicht berücksichtigt. Bei Auftreten von Kondensation sollte das in 6.3 angegebene Verfahren nicht angewendet werden.

Dispositifs de sécurité pour protection contre les pressions excessives - Partie 7: Données communes (ISO 4126-7:2013)

L'ISO 4126-5:2013 spécifie des exigences pour les soupapes de sûreté. Elle contient des données qui sont communes aux parties 1 à 6 de l'ISO 4126 pour éviter les répétitions inutiles.
Pour les liquides auto-vaporisants ou les mélanges biphasiques, voir l'ISO 4126‑10.

Naprave za varovanje pred visokim tlakom - 7. del: Splošni podatki (ISO 4126-7:2013)

Ta del standarda ISO 4126 vsebuje informacije, ki so enake za dele od 1 do 6 standarda ISO 4126, da se ne ponavljajo brez potrebe. Uporabnika opozarjamo, da ni priporočena uporaba formule idealnega plina iz točke 6.3, ko je temperatura za razbremenitev večja od 90 % termodinamične kritične temperature in tlak za razbremenitev večji od 50 % termodinamičnega kritičnega tlaka. Poleg tega se kondenzacija ne upošteva. Če pride do kondenzacije, se metoda iz točke 6.3 ne sme uporabiti.

General Information

Status
Published
Publication Date
16-Jul-2013
Current Stage
6060 - Definitive text made available (DAV) - Publishing
Due Date
16-Aug-2011

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SLOVENSKI STANDARD
SIST EN ISO 4126-7:2013
01-september-2013
1DGRPHãþD
SIST EN ISO 4126-7:2004
SIST EN ISO 4126-7:2004/AC:2009
Naprave za varovanje pred visokim tlakom - 7. del: Splošni podatki (ISO 4126-
7:2013)

Safety devices for protection against excessive pressure - Part 7: Common data (ISO

4126-7:2013)

Sicherheitseinrichtungen gegen unzulässigen Überdruck - Teil 7: Allgemeine Daten (ISO

4126-7:2013)

Dispositifs de sécurité pour protection contre les pressions excessives - Partie 7:

Données communes (ISO 4126-7:2013)
Ta slovenski standard je istoveten z: EN ISO 4126-7:2013
ICS:
13.240 Varstvo pred previsokim Protection against excessive
tlakom pressure
SIST EN ISO 4126-7:2013 en,fr

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST EN ISO 4126-7:2013
---------------------- Page: 2 ----------------------
SIST EN ISO 4126-7:2013
EUROPEAN STANDARD
EN ISO 4126-7
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2013
ICS 13.240 Supersedes EN ISO 4126-7:2004
English Version
Safety devices for protection against excessive pressure - Part
7: Common data (ISO 4126-7:2013)

Dispositifs de sécurité pour protection contre les pressions Sicherheitseinrichtungen gegen unzulässigen Überdruck -

excessives - Partie 7: Données communes (ISO 4126- Teil 7: Allgemeine Daten (ISO 4126-7:2013)

7:2013)
This European Standard was approved by CEN on 25 April 2013.

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. Up-to-date lists and bibliographical references concerning such national

standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN member.

This European Standard exists 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, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,

Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United

Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: Avenue Marnix 17, B-1000 Brussels

© 2013 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 4126-7:2013: E

worldwide for CEN national Members.
---------------------- Page: 3 ----------------------
SIST EN ISO 4126-7:2013
EN ISO 4126-7:2013 (E)
Contents Page

Foreword ..............................................................................................................................................................3

Annex ZA (informative) Relationship between this European Standard and the Essential

Requirements of EU Directive 97/23/EC (PED) ..................................................................................4

---------------------- Page: 4 ----------------------
SIST EN ISO 4126-7:2013
EN ISO 4126-7:2013 (E)
Foreword

This document (EN ISO 4126-7:2013) has been prepared by Technical Committee ISO/TC 185 "Safety

devices for protection against excessive pressure" in collaboration with the Technical Committee CEN/TC 69

“Industrial valves” the secretariat of which is held by AFNOR.

This European Standard shall be given the status of a national standard, either by publication of an identical

text or by endorsement, at the latest by January 2014, and conflicting national standards shall be withdrawn at

the latest by January 2014.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent

rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights.

This document supersedes EN ISO 4126-7:2004.

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.

For relationship with EU Directive, see informative Annex ZA, which is an integral part of this document.

According to the CEN-CENELEC Internal Regulations, the national standards organizations of the following

countries are bound to implement this European Standard: 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, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom.

Endorsement notice

The text of ISO 4126-7:2013 has been approved by CEN as EN ISO 4126-7:2013 without any modification.

---------------------- Page: 5 ----------------------
SIST EN ISO 4126-7:2013
EN ISO 4126-7:2013 (E)
Annex ZA
(informative)
Relationship between this European Standard and the Essential
Requirements of EU Directive 97/23/EC (PED)

This European Standard has been prepared under a mandate given to CEN by the European Commission

and the European Free Trade Association to provide one means of conforming to Essential Requirements of

the New Approach Directive 97/23/EC (PED).

Once EN ISO 4126-7 is cited in the Official Journal of the European Communities under that Directive and has

been implemented as a national standard in at least one Member State, compliance with the normative

clauses of EN ISO 4126-7 confers, within the limits of the scope of EN ISO 4126-7, a presumption of

conformity with the corresponding Essential Requirements of that Directive and associated EFTA regulations.

Table ZA.1 — Correspondence between this European Standard and Directive 97/23/EC (PED)

Essential Requirements of Directive 97/23/EC (PED)
Sub-clauses of this EN
Essential Requirements Annex I of PED
5 and 6 Pressure Limitations 2.11.1

WARNING — Other requirements and other EU Directives may be applicable to the product(s) falling within

the scope of EN ISO 4126-7.
---------------------- Page: 6 ----------------------
SIST EN ISO 4126-7:2013
INTERNATIONAL ISO
STANDARD 4126-7
Second edition
2013-07-15
Safety devices for protection against
excessive pressure —
Part 7:
Common data
Dispositifs de sécurité pour protection contre les pressions excessives —
Partie 7: Données communes
Reference number
ISO 4126-7:2013(E)
ISO 2013
---------------------- Page: 7 ----------------------
SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2013

All rights reserved. Unless otherwise specified, 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
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.org
Web www.iso.org
Published in Switzerland
ii © ISO 2013 – All rights reserved
---------------------- Page: 8 ----------------------
SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and units ............................................................................................................................................................................................... 3

5 Determination of safety valve performance ............................................................................................................................ 4

5.1 Determination of coefficient of discharge ....................................................................................................................... 4

5.2 Critical and subcritical flow ......................................................................................................................................................... 4

5.3 Discharge capacity at critical flow ......................................................................................................................................... 4

5.4 Discharge capacity for any gas at subcritical flow.................................................................................................... 5

5.5 Discharge capacity for non-flashing liquid as the test medium in the turbulent zone where

the Reynolds number Re is equal to or greater than 80 000 .......................................................................... 5

6 Sizing of safety valves ...................................................................................................................................................................................... 6

6.1 General ........................................................................................................................................................................................................... 6

6.2 Valves for gas or vapour relief ................................................................................................................................................... 6

6.3 Calculation of capacity...................................................................................................................................................................... 6

7 Thermodynamic properties ..................................................................................................................................................................... 8

7.1 Steam data................................................................................................................................................................................................... 8

7.2 Value of C as a function of k.......................................................................................................................................................... 8

7.3 Theoretical capacity correction factors for sub-critical flow (K ) ............................................................. 8

7.4 Estimating chart for compressibility factor, Z ...........................................................................................................19

7.5 Capacity correction factor for viscosity, K ...................................................................................................................

v 21

7.6 Properties of gases............................................................................................................................................................................22

8 Minimum requirements for helical compression springs ......................................................................................23

8.1 General ........................................................................................................................................................................................................23

8.2 Materials ....................................................................................................................................................................................................23

8.3 Marking ......................................................................................................................................................................................................23

8.4 Dimensions ..............................................................................................................................................................................................24

8.5 Spring plates/buttons ....................................................................................................................................................................24

8.6 Inspection, testing and tolerances ......................................................................................................................................24

9 Minimum requirements for safety valve disc springs ................................................................................................27

9.1 General ........................................................................................................................................................................................................27

9.2 Materials ....................................................................................................................................................................................................27

9.3 Marking ......................................................................................................................................................................................................27

9.4 Dimensions ..............................................................................................................................................................................................27

9.5 Inspection, testing and tolerances ......................................................................................................................................27

Annex A (informative) Examples of capacity calculations for various media ........................................................28

Bibliography .............................................................................................................................................................................................................................33

© ISO 2013 – All rights reserved iii
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SIST EN ISO 4126-7:2013
ISO 4126-7:2013(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.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International

Standards adopted by the technical committees are circulated to the member bodies for voting.

Publication as an International Standard requires approval by at least 75 % of the member bodies

casting a vote.

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.

ISO 4126-7 was prepared by Technical Committee ISO/TC 185, Safety devices for protection against

excessive pressure.

This second edition cancels and replaces the first edition (ISO 4126-7:2004), which has been technically

revised. It also incorporates the Technical Corrigendum ISO 4126-7:2004/Cor.1:2006.

ISO 4126 consists of the following parts, under the general title Safety devices for protection against

excessive pressure:
— Part 1: Safety valves
— Part 2: Bursting disc safety devices
— Part 3: Safety valves and bursting disc safety devices in combination
— Part 4: Pilot-operated safety valves
— Part 5: Controlled safety pressure relief systems (CSPRS)

— Part 6: Application, selection and installation of bursting disc safety devices

— Part 7: Common data

— Part 9: Application and installation of safety devices excluding stand-alone bursting disc safety devices

— Part 10: Sizing of safety valves for gas/liquid two-phase flow
— Part 11: Performance testing
1) Under preparation.
iv © ISO 2013 – All rights reserved
---------------------- Page: 10 ----------------------
SIST EN ISO 4126-7:2013
INTERNATIONAL STANDARD ISO 4126-7:2013(E)
Safety devices for protection against excessive pressure —
Part 7:
Common data
1 Scope

This part of ISO 4126 specifies requirements for safety valves. It contains information which is common

to ISO 4126-1 to ISO 4126-6 to avoid unnecessary repetition.
For flashing liquids or two-phase mixtures, see ISO 4126-10.

The user is cautioned that it is not recommended to use the ideal gas formula presented in 6.3 when the

relieving temperature is greater than 90 % of the thermodynamic critical temperature and the relieving

pressure is greater than 50 % of the thermodynamic critical pressure. Additionally, condensation is not

considered. If condensation occurs, the method presented in 6.3 should not be used.

2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 4126-1, Safety devices for protection against excessive pressure — Part 1: Safety valves

ISO 4126-2, Safety devices for protection against excessive pressure — Part 2: Bursting disc safety devices

ISO 4126-4, Safety devices for protection against excessive pressure — Part 4: Pilot operated safety valves

ISO 4126-5, Safety devices for protection against excessive pressure — Part 5: Controlled safety pressure

relief systems (CSPRS)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 4126-1, ISO 4126-2, ISO 4126-4

ISO 4126-5 and the following apply.

NOTE Pressure unit used in ISO 4126-7 is the bar (1 bar = 10 Pa), quoted as gauge (relative to atmospheric

pressure) or absolute as appropriate.
3.1
safety valve

valve which automatically, without the assistance of any energy other than that of the fluid concerned,

discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and

which is designed to re-close and prevent further flow of fluid after normal pressure conditions of

service have been restored

Note 1 to entry: The valve can be characterized either by pop action (rapid opening) or by opening in proportion

(not necessarily linear) to the increase in pressure over the set pressure. The use of the term safety valve in this

part of ISO 4126 applies to other valve types as covered in ISO 4126-1, ISO 4126-4 and ISO 4126-5.

© ISO 2013 – All rights reserved 1
---------------------- Page: 11 ----------------------
SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
3.2
set pressure

predetermined pressure at which a safety valve under operating conditions commences to open

Note 1 to entry: It is the gauge pressure measured at the valve inlet at which the pressure forces tending to open

the valve for the specific service conditions are in equilibrium with the forces retaining the valve disc on its seat.

3.3
maximum allowable pressure, PS
maximum pressure for which the protected equipment is designed
3.4
overpressure

pressure increase over set pressure, usually expressed as a percentage of the set pressure

3.5
relieving pressure

pressure used for the sizing of a safety valve which is greater than or equal to the set pressure plus

overpressure
3.6
back pressure

pressure that exists at the outlet of a safety valve as a result of the pressure in the discharge system

Note 1 to entry: The back pressure is the sum of the superimposed and built-up back pressures.

3.7
built-up back pressure

pressure existing at the outlet of a safety valve caused by flow through the valve and the discharge system

3.8
superimposed back pressure

pressure existing at the outlet of a safety valve at the time when the device is required to operate

Note 1 to entry: It is the result of pressure in the discharge system from other sources.

3.9
flow area

minimum cross-sectional flow area (but not the smallest area between the disc and seat) between inlet

and seat which is used to calculate the theoretical flow capacity, with no deduction for any obstruction

Note 1 to entry: The symbol is A.
3.10
theoretical discharge capacity

calculated capacity expressed in mass or volumetric units of a theoretically perfect nozzle having a

cross-sectional flow area equal to the flow area of a safety valve
3.11
coefficient of discharge

value of actual discharge capacity (from tests) divided by the theoretical discharge capacity (from calculation)

3.12
certified (discharge) capacity

that portion of the measured capacity permitted to be used as a basis for the application of a safety valve

Note 1 to entry: It may, for example, equal the a)measured capacity times the de-rating factor of 0,9, or b)theoretical

capacity times the coefficient of discharge times the de-rating factor of 0,9, or c)theoretical capacity times the

certified de-rated coefficient of discharge.
2 © ISO 2013 – All rights reserved
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SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
3.13
dryness fraction
steam quality

measure of the relative vapour/liquid content of a steam quantity or stream. Expressed as the mass

fraction or percentage of vapour
4 Symbols and units
Table 1 — Symbols and their descriptions
Symbol Description Unit
A Flow area of a safety valve (not smallest area between the disc and seat) mm
C Function of the isentropic exponent, k –
K Theoretical capacity correction factor for subcritical flow –
K Coefficient of discharge –
K Certified de-rated coefficient of discharge (K × 0,9) –
dr d
K Viscosity correction factor –
k Isentropic exponent at relieving pressure and temperature –
M Molar mass kg/kmol
n Number of tests –
p Relieving pressure - absolute bar (abs)
p Back pressure - absolute bar (abs)
p Thermodynamic critical pressure - absolute bar (abs)
p Reduced pressure –
PS Maximum allowable pressure bar (abs)
Mass flow rate kg/h
q Theoretical specific discharge capacity kg/(h⋅mm )
q’ Specific discharge capacity determined by tests kg/(h⋅mm )
R Universal gas constant J/K-mol
Re Reynolds number –
T Relieving temperature K
T Thermodynamic critical temperature K
T Reduced temperature –
μ Dynamic viscosity Pa·s
v Specific volume at relieving pressure and temperature m /kg
x Dryness fraction of wet steam at the valve inlet at relieving pressure and –
temperature
k Steam pressure coefficient h⋅mm bar (abs)/
Z Compressibility factor at relieving pressure and temperature –
K and K are expressed as 0,xxx.
d dr
x is expressed as 0,xx.
© ISO 2013 – All rights reserved 3
---------------------- Page: 13 ----------------------
SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
5 Determination of safety valve performance
5.1 Determination of coefficient of discharge
The coefficient of discharge, K , is calculated from the following:
 
 
 m 
K = (1)

K shall be calculated up to three significant decimal places. Any rounding shall be down.

5.2 Critical and subcritical flow

The theoretical flow of a gas or vapour through an orifice, such as the flow area of a safety valve, increases

as the downstream pressure is decreased to the critical pressure, until critical flow is achieved. Further

decrease in downstream pressure will not result in any further increase in flow.
Critical flow occurs when
(/kk()−1 )
p 2
 
≤ (2)
 
pk+1
 
and subcritical flow occurs when
(/kk()−1 )
p 2
 
> (3)
 
pk+1
 
5.3 Discharge capacity at critical flow
5.3.1 Discharge capacity for steam
qC=0,2883 (4)

Formula (4) allows the use of steam tables to obtain the specific volume of steam at various pressures

and temperatures. The user is cautioned that the direct use of this equation can lead to an error of more

than 20 % as the temperature approaches the saturated or supercritical condition. An error of less than

1 % can only be achieved at a steam temperature at least higher than 30 °C above saturation condition or

higher than the result of 30+(p -200), in °C, using p in bar above saturation or supercritical condition.

0 o
A method including lower temperatures is described hereafter.
Alternatively, the above equation can be rearranged as follows:
q = (5)
where k is the steam pressure coefficient.
k = (6)
0,2883C
4 © ISO 2013 – All rights reserved
---------------------- Page: 14 ----------------------
SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
R 8,3143
NOTE 1 0,2883 = = (7)
10 10

Values for the steam pressure coefficient, k , can be obtained in Table 2. See 6.3.1 for background on the

development of Table 2.

This is applicable to dry saturated and superheated steam. Dry saturated steam in this context refers to

steam with a minimum dryness fraction of 98 % where C is a function of the isentropic exponent at the

relieving conditions.
()kk+−11/( )
 
Ck=3,948 (8)
 
k +1
 
3600
NOTE 2 3, 948 = (9)
10 × R

The value of k used to determine C shall be based on the actual flowing conditions at the pressure relief

device inlet and shall be determined from Table 3.
5.3.2 Discharge capacity for any gas under critical flow conditions
qp==C 0,2883C (10)
ZT v
o o
See Figure 1 for values of Z.
()kk+−11/( )
 
Ck = 3, 948 (11)
 
k + 1
 
See Table 3 for rounded values for C.
5.4 Discharge capacity for any gas at subcritical flow
qp==CK 0,2883CK (12)
mo b b
ZT v
o o
 
k+1
 
   
p p
2k b b
 
   
   
k −1  p p 
o o 
  
 
 
K = (13)
kk+1
k−1
k +1
See Table 4 for K values.

5.5 Discharge capacity for non-flashing liquid as the test medium in the turbulent zone

where the Reynolds number Re is equal to or greater than 80 000
 
pp−
q =16, 1 (14)
 
 
 
© ISO 2013 – All rights reserved 5
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SIST EN ISO 4126-7:2013
ISO 4126-7:2013(E)
NOTE
3600 2
16, 1 = (15)
10 10
6 Sizing of safety valves
6.1 General

The certified de-rated coefficient of discharge K of the safety valve shall be not greater than 90 % of

the coefficient of discharge K determined by test:
KK≤ 09, (16)
dr d

It is not permitted to calculate the capacity with a lower overpressure than that at which the tests to

determine flow characteristics were carried out although it is permissible to calculate the capacity at a

higher relieving pressure.

Valves having a certified de-rated coefficient of discharge established on critical flow at the test back

pressure may not have the same certified de-rated coefficient of discharge at a higher back pressure; see

ISO 4126-1, ISO 4126-3, ISO 4126-4 or ISO 4126-5, as applicable, for requirements for the certification of

the coefficient of discharge of various valve types.
6.2 Valves for gas or vapour relief

No distinction is made between substances commonly referred to as vapours: the term “gas” is used to

describe both gas and vapour.

To calculate the capacity for any gas, the area and the coefficient of discharge shall be assumed to be

constant and the equations given in Clause 5 shall be used.
6.3 Calculation of capacity

The ideal gas formula presented in 6.3 should not be used when the relieving temperature is greater

than 90 % of the thermodynamic critical temperature and the relieving pressure is greater than 50 %

of the thermodynamic critical pressure. Additionally, condensation is not considered. If condensation

occurs, the method presented in 6.3 should not be used.
NOTE 1 The equation to be applied depends on the fluid to be discharged.
NOTE 2 See Annex A for example calculations.

6.3.1 Capacity calculation for (saturated, superheated or supercritical) steam at critical flow

QC=0,2883 AK (17)
mdr

Formula (17) allows the use of steam tables to obtain the specific volume of steam at various pressures

and temperatures. The user is cautioned that the direct use of this equation can lead to an error of more

than 20 % as the temperature approaches the saturated or supercritical condition. An error of less than

1 % can only be achieved at a steam temperature at least higher than 30°C above saturation condition or

higher than the result of 30+(p -200), in °C, using p in bar above saturation or supercritical condition.

0 0
A method including lower temperatures is described hereafter.
6 © ISO 2013 – All rights reserved
---------------
...

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