1、raising standards worldwideNO COPYING WITHOUT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAWBSI Standards PublicationBS ISO 4126-10:2010Safety devices for protectionagainst excessive pressurePart 10: Sizing of safety valves for gas/liquidtwo-phase flowBS ISO 4126-10:2010 BRITISH STANDARDNationa
2、l forewordThis British Standard is the UK implementation of ISO 4126-10:2010.The UK participation in its preparation was entrusted to TechnicalCommittee PSE/18/6, Industrial valves, steam traps, actuators andsafety devices against excessive pressure - Safety devices againstexcessive pressure.A list
3、of organizations represented on this committee can beobtained on request to its secretary.This publication does not purport to include all the necessaryprovisions of a contract. Users are responsible for its correctapplication. BSI 2010ISBN 978 0 580 57618 8ICS 13.240Compliance with a British Standa
4、rd cannot confer immunity fromlegal obligations.This British Standard was published under the authority of theStandards Policy and Strategy Committee on 31 December 2010.Amendments issued since publicationDate Text affectedBS ISO 4126-10:2010Reference numberISO 4126-10:2010(E)ISO 2010INTERNATIONAL S
5、TANDARD ISO4126-10First edition2010-10-01Safety devices for protection against excessive pressure Part 10: Sizing of safety valves for gas/liquid two-phase flow Dispositifs de scurit pour protection contre les pressions excessives Partie 10: Dimensionnement des soupapes de sret pour les dbits diphas
6、iques gaz/liquide BS ISO 4126-10:2010ISO 4126-10:2010(E) PDF disclaimer This PDF file may contain embedded typefaces. In accordance with Adobes licensing policy, this file may be printed or viewed but shall not be edited unless the typefaces which are embedded are licensed to and installed on the co
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9、he Central Secretariat at the address given below. COPYRIGHT PROTECTED DOCUMENT ISO 2010 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without per
10、mission in writing from either ISO at the address below or ISOs 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 copyrightiso.org Web www.iso.org Published in Switzerland ii ISO 2010 All rights res
11、ervedBS ISO 4126-10:2010ISO 4126-10:2010(E) ISO 2010 All rights reserved iiiContents Page Foreword iv Introduction.v 1 Scope1 2 Normative references1 3 Terms and definitions .1 4 Symbols and abbreviated terms 7 5 Application range of the method .12 5.1 General .12 5.2 Limitations of the method for c
12、alculating the two-phase mass flux in safety valves12 5.3 Limitations of the method for calculating the mass flow rate required to be discharged.13 6 Sizing steps14 6.1 General outline of sizing steps 14 6.2 Step 1 Identification of the sizing case.15 6.3 Step 2 Flow regime at safety valve inlet.16
13、6.4 Step 3 Calculation of the flow rate required to be discharged 21 6.5 Step 4 Calculation of the dischargeable mass flux through a safety valve28 6.6 Step 5 Proper operation of safety valves connected to inlet and outlet lines34 Annex A (informative) Identification of sizing scenarios38 Annex B (n
14、ormative) Sizing of a safety valve 39 Bibliography44 BS ISO 4126-10:2010ISO 4126-10:2010(E) iv ISO 2010 All rights reservedForeword ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International
15、 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, a
16、lso 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 commit
17、tees 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 pos
18、sibility 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 4126-10 was prepared by Technical Committee ISO/TC 185, Safety devices for protection against excessive pressure. ISO 4126 con
19、sists 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 saf
20、ety 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
21、flow BS ISO 4126-10:2010ISO 4126-10:2010(E) ISO 2010 All rights reserved vIntroduction Well-established recommendations exist for the sizing of safety devices and the connected inlet and outlet lines for steady-state, single-phase gas/vapour or liquid flow. However, in the case of a two-phase vapour
22、/liquid flow, the required relieving area to protect a system from overpressure is larger than that required for single-phase flow when the same vessel condition and heat release are considered. The requirement for a larger relief area results from the fact that, in two-phase flow, the liquid partia
23、lly blocks the relieving area for the vapour flow, by which most of the energy is removed by evaporation from the vessel. This part of ISO 4126 includes a widely usable engineering tool for the sizing of the most typical safety valves in fluid services encountered in various industrial fields. It is
24、 based on the omega parameter method, which is extended by a thermodynamic non-equilibrium parameter. Without this extension for considering non-equilibrium, the proposed method is in accordance with API RP 520. A balance is attempted between the accuracy of the method and the unavoidable uncertaint
25、ies in the input and property data under the actual sizing conditions. There are other sizing methods available, which are referred to in this part of ISO 4126. In case of two-phase flow, the fluid state and, hence, the mass flow rate required to be discharged are dependent on the size of the safety
26、 valve. Furthermore, the two-phase mass flow rate through a safety valve essentially depends on the mass flow quality (mass fraction of vapour) of the fluid at the inlet of the valve. Because these parameters are, in most cases, not readily at hand during the design procedure of a relief device, thi
27、s part of ISO 4126 also includes a comprehensive procedure that covers the determination of the fluid-phase composition at the safety valve inlet. This fluid-phase composition depends on a scenario that leads to the pressure increase. Therefore, the recommended sizing procedure starts with the defin
28、ition of the sizing case and includes a method for the prediction of the mass flow rate required to be discharged and the resulting mass flow quality at the inlet of the safety valve. If flow is confirmed to be single-phase up to the narrowest flow cross-section, it is appropriate to use ISO 4126-1.
29、 The equations of ISO 4126-1 are also included in this part of ISO 4126, modified to SI units, to calculate the flow rates at the limiting conditions of single-phase flow. BS ISO 4126-10:2010BS ISO 4126-10:2010INTERNATIONAL STANDARD ISO 4126-10:2010(E) ISO 2010 All rights reserved 1Safety devices fo
30、r protection against excessive pressure Part 10: Sizing of safety valves for gas/liquid two-phase flow 1 Scope This part of ISO 4126 specifies the sizing of safety valves for gas/liquid two-phase flow in pressurized systems such as reactors, storage tanks, columns, heat exchangers, piping systems or
31、 transportation tanks/containers. The possible fluid states at the safety valve inlet that can result in two-phase flow are given in Table 1. NOTE The expression “safety valve” is a synonym for valves as described in ISO 4126-1, ISO 4126-4 and ISO 4126-5. Table 1 Possible fluid state at the inlet of
32、 the safety valve that can result in two-phase flow Fluid state at valve inlet Cases Examples liquid subcooled (possibly flashing in the safety valve) saturated with dissolved gas cold water boiling water CO2/water gas/vapour near saturated vapour (possibly condensing in the safety valve) steam gas/
33、liquid vapour/liquid non-evaporating liquid and non-condensable gas (constant quality) gas/liquid mixture, when gas is desorbed or produced steam/water air/water 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, on
34、ly 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 3 Terms and definitions For the purposes of this document, the terms and
35、definitions given in ISO 4126-1 and the following apply. 3.1 pressurized system equipment such as reactors, storage tanks, columns, heat exchangers, piping systems and transport tanks/containers being protected against impermissible pressure accumulation by a safety valve BS ISO 4126-10:2010ISO 4126
36、-10:2010(E) 2 ISO 2010 All rights reserved3.2 Pressure NOTE 1 See Figures 1 a) and 1 b) for an illustration of the relationship of the pressures defined in 3.2.1 to 3.2.7. NOTE 2 In contrast to the definition used in other parts of this International Standard, for example ISO 4126-1, all pressures a
37、re absolute pressures and not gauge pressures. a) Pressure history of a typical tempered reaction system that is adequately sized Figure 1 Relationship of the defined pressures (continued) BS ISO 4126-10:2010ISO 4126-10:2010(E) ISO 2010 All rights reserved 3b) Typical pressure history for an externa
38、lly heated gas vented system Key pMAAmaximum allowable accumulated pressure pMAWmaximum allowable working absolute pressure psetopening pressure preseatreseating pressure poperatingoperating pressure pBDblowdown p0sizing pressure equal to psetas shown in Figure 1 a) and equal to poveras shown in Fig
39、ure 1 b) poveroverpressure pAmaximum allowable accumulation poverchange in overpressure Figure 1 Relationship of the defined pressures 3.2.1 maximum allowable working absolute pressure pMAWmaximum pressure permissible at the top of a pressurized system in its operating position for designated temper
40、ature 3.2.2 maximum allowable accumulated pressure pMAAsum of the maximum allowable working pressure and the maximum allowable accumulation NOTE The maximum allowable accumulation is established by applicable code for operating and fire contingencies. 3.2.3 maximum allowable accumulation pApressure
41、increase over the maximum allowable working pressure of a pressurized system during discharge through the safety valve NOTE The maximum allowable accumulation is expressed in pressure units or as a percentage of the maximum allowable working pressure. BS ISO 4126-10:2010ISO 4126-10:2010(E) 4 ISO 201
42、0 All rights reserved3.2.4 opening pressure psetpredetermined absolute pressure at which a safety valve under operating conditions at the latest commences to open NOTE 1 The opening pressure is the set pressure defined in ISO 4126-1 expressed as absolute pressure. NOTE 2 See Figures 1 a) and 1 b) fo
43、r details. 3.2.5 absolute overpressure poverpressure increase over the opening pressure of the safety valve, psetNOTE 1 The maximum absolute overpressure is the same as the maximum accumulation, pA, when the opening pressure of the safety valve is set at the maximum allowable working pressure of the
44、 pressurized system. NOTE 2 The absolute overpressure is expressed in pressure units or as a percentage of the opening pressure. 3.2.6 overpressure povermaximum pressure in the pressurized system during relief, i.e. pressure less or equal to the maximum accumulated pressure 3.2.7 sizing pressure p0p
45、ressure at which all property data, especially the compressibility coefficient, , are calculated for sizing the safety valve NOTE In the case of tempered and hybrid reactive systems, the sizing pressure shall be equal to the opening pressure. In the case of non-reactive and gassy systems, the design
46、er may choose a higher value for the sizing pressure, but it shall not exceed the maximum allowable accumulated pressure. 3.2.8 critical pressure pcritfluid-dynamic critical pressure occurring in the narrowest flow cross-section of the safety valve and/or at an area enlargement in the outlet line NO
47、TE At this pressure, the mass flow rate approaches a maximum at a given sizing condition in the pressurized system. Any further decrease of the downstream pressure does not increase the flow rate further. Usually, the critical pressure occurs in the safety valve, either in the valve seat, inlet nozz
48、le and/or valve body. In long safety valve outlet lines, multiple critical pressures can also occur. 3.2.9 critical pressure ratio critratio of critical pressure to the sizing pressure 3.2.10 back pressure pbpressure that exists at the outlet of a safety valve as a result of pressure in the discharg
49、e system NOTE Back pressure can be either constant or variable; it is the sum of superimposed and built-up back pressure. BS ISO 4126-10:2010ISO 4126-10:2010(E) ISO 2010 All rights reserved 53.2.11 built-up back pressure pressure existing at the outlet of the safety valve caused by flow through the valve and discharge system 3.2.12 superimposed back pressure pressure existing at the outlet of the safety valve at the time when the device is required to operate NOTE Superimposed back pressure
