ISO INTERNATIONAL STANDARD 23251 Second edition 2019-03 Petroleum, petrochemical and natural gas industries Pressure-relieving and depressuring systems Industriesdupétrole,de lapétrochimieetdugaznaturelSystemes dedépressurisationetdeprotectioncontreles surpressions Referencenumber IS023251:2019(E) so @IS0 2019 IS023251:2019(E) COPYRIGHTPROTECTEDDOCUMENT @ISO2019 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 orIso's memberbody in the countryof therequester. ISO copyright office CP 401 : Ch. de Blandonnet 8 CH-1214 Vernier, Geneva Phone:+41227490111 Fax:+4122749 09 47 Email: [email protected] Website: www.iso.org Published in Switzerland ii @ IS0 2019- All rights reserved IS023251:2019(E) Contents Page Foreword ..iv Introduction. 1 Scope.. 2 Normativereferences 3 Terms and definitions ..1 4 Supplements to APIStd 521, 6thedition (2014) .2 4.1 Generalrequirements .2 4.2 Chemical reaction .2 4.3 Fires... 2 4.4 Causesforvacuum .3 4.5 Considerations for individual PRDs 3 4.5.1 General. .3 4.5.2 Multiple PRDs 3 4.6 Piping 3 4.6.1 General 3 4.6.2 Backpressure 3 4.6.3 Line sizing. 3 4.6.4 Multiplereliefscenarios 3 4.6.5 Reactionforces. .3 4.7 Disposal to flare. 4 4.7.1 General. .4 4.7.2 Combustion properties 4 4.7.3 Combustion methods. .4 4.7.4 Flare systems designs. 4 4.7.5 Sizing. .4 4.7.6 Ignition of flare gases. .4 4.7.7 Liquid seal drum drums 4 4.8 Single PRD protecting several components in a process system ..4 Bibliography .5 IS0 2019-All rights reserved ili